# Girvin fork



## jay_ntwr (Feb 15, 2008)

Who was looking for a Girvin fork? One of my riding buddies was joking about having one "for sale" and really does. I'm sure you've got it by now for whatever project it is, but I know it was someone on here that needed one. If you still do, I have lines on a couple it sounds like.

Later,
Jay


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## pvd (Jan 4, 2006)

One of the worst forks ever before the DUC.

I used to call those forks 'StapleGuns'. Many of the itterations had no provisions at all for rebound damping. Litterally dangerous at any speed. Other than that, they had no tortional stiffness to speak of.


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## jay_ntwr (Feb 15, 2008)

pvd said:


> One of the worst forks ever before the DUC.


Nice! Good to see your sense of humor is turned on today, Peter.


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## patpend2000 (May 11, 2004)

Walt was looking for one


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## pvd (Jan 4, 2006)

I gotta admit, the Shiver SC was amazingly bad as well. I dealt with that for too long before giving up.


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## Vlad (Feb 7, 2004)

The linkage fork? We used to ride bikes with those forks at full speed into curbs. They were kind of fun for that.


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## Schmitty (Sep 7, 2008)

I'm feeling PVD on this one. Girvin.. everything Girvin is shite. Unreal stomach turning, projectile vomit bad. Look at the dude.... bad bad bad. The only site I could find a pic of him is: 

suwonalpha.co.kr

And google told me I would die if I opened the link.

Bad.

-Schmitty-


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## Walt (Jan 23, 2004)

*That was me.*

Yes, it's a stupid idea, but I am thinking of building a "modern" version of the Girvin/Noleen/K2/whatever linkage fork. I wanted a garbage one in hand to mess around with.

Reason? I need a stupid winter project, I hate brake dive, and I think the fork could be improved with bearings instead of bushings and a 20mm axle. Not to mention an RP23 instead of gnarly elastomers.

I'll probably end up with something even crappier than the original. But hopefully it'll be fun. So Jay, if your pal wants to *give* me a fork, great. I'm not going to pay money for one, though, considering that I really only want it to help solidify my thoughts on how to go about building my own. 1 1/8" would be preferable, in the case that I actually wanted to ride around on it at any point, but it's not necessary.

-Walt


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## Schmitty (Sep 7, 2008)

Should be a good exercise in machining!

Replacing the many bushings with bearings will drive weight through the roof.. both from the bearings themselves and the needed thickness on the linkages to accomodate the bearings.. it will get very wide as well for the same reasons.

Try to use needle bearings it seems...

I think I just saw one in the bone pile in a shop here.. I'll let you know.


-Schmitty-


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## Walt (Jan 23, 2004)

*Yup!*

I'm aware that the weight will be, um, heavy. That's not particularly a concern for me, though - I regularly ride a 40 pound DH bike on XC rides, and I have no interest in selling this thing to anyone.

I'm not a great machinist and not good with computer-aided drawing/design, so I thought this would be a fun project for me to work on those skills. That's the main idea, really.

My initial thought was to recess the bearings (well, 4 sets of them, anyway) in the fork legs to keep things from getting too crazy wide, but I'm really only at the random idea stage right now. Hence my interest in an old fork to play with.

-Walt



Schmitty said:


> Should be a good exercise in machining!
> 
> Replacing the many bushings with bearings will drive weight through the roof.. both from the bearings themselves and the needed thickness on the linkages to accomodate the bearings.. it will get very wide as well for the same reasons.
> 
> ...


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## Thylacine (Feb 29, 2004)

I think there is absolutely no reason why a linkage fork couldn't be viable with modern technology (Just ignoring that German A fork for a second). In fact, think of the three main components - legs and two swing links - perfect for composite construction.

Walt, this is a cracker of a project and one I've been daydreaming about for years.

I say just ignore the Girvin and do one from scratch out of steel. Use a Turner Igus bushing kit as your pivots, grab a Fox RP23 and go for it!


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## seat_boy (May 16, 2006)

I had the coil version on my 857. I quite liked it, actually. Stiff and responsive (for the time, perhaps it wouldn't be great by today's standards). The older elastomer versions were pretty bad, though. I'd be curious to see a modernized 29er version of this.


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## Walt (Jan 23, 2004)

*Yeah...*

I'm not planning to *use* the fork for anything. I just want one to play with (and see how they did what they did) because I've never really looked at one closely. From everything I've heard, they sucked pretty badly in terms of damping and lateral/torsional stiffness. I think some bearings and a thru-axle might solve the latter two issues, and an rp23 (or whatever modern shock) would certainly help with the first.

I'm guessing that with nice composite/alloy construction, you could make a ~4# fork that worked pretty darn well. I could be wrong, and I'm sure that what *I* build will be at least a pound heavier than that and possibly quite crappy. But as I said, I like goofy projects. I'm glad *someone* doesn't think I'm an idiot...

Does Turner sell the bushing kit you're referring to? I was sort of looking at doing bearings, but I'm not married to anything at this point.

-Walt



Thylacine said:


> I think there is absolutely no reason why a linkage fork couldn't be viable with modern technology (Just ignoring that German A fork for a second). In fact, think of the three main components - legs and two swing links - perfect for composite construction.
> 
> Walt, this is a cracker of a project and one I've been daydreaming about for years.
> 
> I say just ignore the Girvin and do one from scratch out of steel. Use a Turner Igus bushing kit as your pivots, grab a Fox RP23 and go for it!


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## Walt (Jan 23, 2004)

*Just some background...*

Just as explanation - I've never ridden a mountain bike linkage fork, but I come (sort of) from a motorcycle background (mostly enduros). I used to borrow a friend's 1100gs (a BMW street/touring bike which had a linkage type front suspension setup) and just be *amazed* how much better the bike handled in any kind of braking situation. I never got around to trying out a linkage setup on a mountain bike (and in fact I'm pretty much a rigid bike dude now) but I always wondered about it. So there it is.

-Walt


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## moto367 (Nov 20, 2006)

Hey Walt, give me a history lesson here: wasn't the Girvin designed by the brother or some relation to Kevin Hines (the national enduro champ years ago) or was that Pro-Flex? I have a lengthy enduro past as well. :thumbsup:


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## Thylacine (Feb 29, 2004)

Yeah, you can buy bushing kits straight from Turner. Nice, easy, lightweight solution.

This would be a great little fun project. You could make the linkages from formed laser-cut sheet, turn the bushing seats from tube.....legs would be simple.

Awesome.


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## pimpbot (Dec 31, 2003)

*Check the WW forum.*



jay_ntwr said:


> Who was looking for a Girvin fork? One of my riding buddies was joking about having one "for sale" and really does. I'm sure you've got it by now for whatever project it is, but I know it was someone on here that needed one. If you still do, I have lines on a couple it sounds like.
> 
> Later,
> Jay


IIRC, there was some guy who slapped a Fox Float in a Look linkage fork (a clone of this fork made of carbon, I think) and he said it was actually pretty good. Not sure how much I believe it. Those fork legs look like they would allow a ton of twist flex to me.


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## scuppy (Nov 10, 2007)

I have a parafork and they are stiff and responsive and handle fine at low speeds. I had a bad crash and I don't know what happened, I just lost it plowing into sand (nothing I haven't done before), I blame the fork. I haven't used it since, I just don't trust it.


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## roybatty666 (Mar 2, 2009)

I loved my old Amp linkage fork, never had any issues with it, I have been eyeing up the German linkage fork although the fact the linkage points forward not back like the Amp does look a little odd.

Go for it mate I think it will be a fantastic project, Linkage forks can be great no stiction, light weight and the ability to play with the axle path


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## PMK (Oct 12, 2004)

moto367 said:


> Hey Walt, give me a history lesson here: wasn't the Girvin designed by the brother or some relation to Kevin Hines (the national enduro champ years ago) or was that Pro-Flex? I have a lengthy enduro past as well. :thumbsup:


Yes, when Pro-Flex was in Rhode Island, Kevins brother was part of the crew.

PK


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## PMK (Oct 12, 2004)

All the discussion about using Turner bushings, these as mentioned are from IGUS.

If my memory is correct, the Girvins used IGUS bushings also.

I owned and rode and 853 ProFlex with the Girvin fork back in 93. By todays standards it was very flexible. Compared to the forks of the era like the Manitou 4 and RS Mag 21, I don't recall it being much stiffer nor softer in torsional movements.

I too have been deciding on building a new fork. Since the majority of our riding now is all tandems, the fork situation and performance is pretty slim pickins.

Currently our Cannondale MTB tandem has a Moto Freeride. Yes stiff but maintenance intensive with no new parts available. Plus these things are prone to cracking triple clamps, and tough to get enough spring rate for the tandem.

So I've been contemplating building a leading link style fork similar to that used in sidecar motocross racing. The tandem can not float or lift the front wheel, so every thing passing beneath the tire is basically an uncontrolled impact.

My thoughts for a leading link is more based on terrain following ability first, rigidity can be designed in as weight is not a primary concern, but somewhat important.

Rather than the more common two damper design, I planned to use the typical swingarm and vertical leg structure, but run a second set of verticals into a link driving a single damper. This second link would be similar to the Lawill leader type design. The swing link would dial in the leverage ratio and I'd hopefully stuff any quality 6.5" rear damper in there. This second link I'm hoping would compliment the side to side rigidity, this may allow building a slightly lighter swingarm. 

At the moment, and until I draw it on the computer coupled to a drawn frame, my greatest concern is toe clearance for the swingarms rear loop. 

Crazy or not, any thoughts. 

PK


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## pvd (Jan 4, 2006)

Walt said:


> I used to borrow a friend's 1100gs (a BMW street/touring bike which had a linkage type front suspension setup) and just be *amazed* how much better the bike handled in any kind of braking situation.


That's Telelever. It's still basically a telescoping fork, but the spring and damping is not inside the tubes. The link to the extermal shock allows the fork to get a maximum amount of support during braking. It's a very good idea. It has honest advantages for tuning and in structure. I've seen somebody doing it on bicycles. I'll need to find that.

Nothing can elliminate brake dive since the physics dictate that more weight goes to the front during braking. Any system that minimizes dive is basically shutting the fork off at a time when it's needed the most to maintain traction. Motorcycles gave up on this concept about 25 years ago.


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## pvd (Jan 4, 2006)

https://en.wikipedia.org/wiki/Motorcycle_fork

Actually, that bicycle was a Duolever configuration. I think that this would be a much better direction for you to go.










Whyte PRST-1










Whyte PRST-4

You may want to give them a call. https://www.whytebikes.com


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## pvd (Jan 4, 2006)

Here's the page of the system designer, Hossack.

http://www.hossack-design.co.uk


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## Schmitty (Sep 7, 2008)

pvd said:


> . I've seen somebody doing it on bicycles. I'll need to find that.


This it?

-Schmitty-


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## jeff (Jan 13, 2004)

*Look fork*

Used without permision from the original poster.

http://forums.mtbr.com/showthread.php?t=495308&highlight=look+fork

There are numerous threads out there regarding this fork. Cool stuff.


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## Schmitty (Sep 7, 2008)

Heavy, expensive, complicated. Motos have motors.


-Schmitty-


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## pvd (Jan 4, 2006)

If it (the Hossack) can be done in a way that is durable, structural, and close to the weight of a telescoping fork their are some huge benifits. Constant wheelbase, constant trial, or constant head angle to say the least. There is also the fact that the wheel path of the front wheel can be adjusted to follow the ground with a minimum of drag against the forward momentum of the COG.

It's all bound to the 'if'. People have been trying for over 120 years. BMW seems to have successfully brought it to street motorcycles. That means that bicycles may be able to apply a similar concept.

Most developers have pretty much given up on the idea of a leading or trailing link fork designed as a separate unit from the rest of the chassis. Building the front end into the chassis is still being persued and shows some promise. One day...one day...


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## RoyDean (Jul 2, 2007)

pvd said:


> or constant head angle to say the least.


On a full suspension bike, I don't think this can ever be possible.


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## Blaster1200 (Feb 20, 2004)

Here's another linkage fork. It was something Honda was experimenting with some time ago.


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## jay_ntwr (Feb 15, 2008)

Walt, I've sent my buddy the link to this thread to see if he'd be interested in donating it.


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## mtroy (Jun 10, 2005)

Well, I have nothing constructive to add, but I find it a very interesting thread. I have often wondered while riding my SS if a better constructed version of an AMP/Girvin type fork would not be a viable fork in a lesser travel environment like 100mm and under. If...if it can be done and end up lighter and stiffer than a telescopic fork. Or why bother? I have little complaints on brake dive with a well tuned fork.

You go Walt.

Oh, and the other bike shown...the link one...wow. I remember that. I am not sure I could ride something that looked so unlike a bicycle no matter how well it worked.


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## dr.welby (Jan 6, 2004)

pvd said:


> Nothing can elliminate brake dive since the physics dictate that more weight goes to the front during braking. Any system that minimizes dive is basically shutting the fork off at a time when it's needed the most to maintain traction. Motorcycles gave up on this concept about 25 years ago.


Huh? You feed braking force into the fork/linkages to balance out some or all of the weight transfer. Nothing is getting "shut off". Same thing with anti-squat.

I think motorcycles gave up on it because it was needlessly complicated or unmarketably expensive. Definitely used in cars though.


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## pvd (Jan 4, 2006)

dr.welby said:


> Huh? You feed braking force into the fork/linkages to balance out some or all of the weight transfer. Nothing is getting "shut off". Same thing with anti-squat.


NO! The front end dives because more weight is being placed on it due to the deceleration of the bike under the COG. The shift HAS to happen. If you are doing anything that prevents the front end from compressing in this condition, then you are effectively shutting the system down.

Squat is exactly the same thing, but we use anti-squat geometry to control it. We need to get the power to the ground for maximum drive but we still need the bike to stay composed. Anti-squat slightly reduces our traction to keep the bike composed. Too much anti-squat and the rear end will completely loose traction. Very bad.

When we are braking very hard it is paramount that we keep maximum grip for control. Any anti dive mechanism will reduce grip and cause us to loose traction and control at the worst of times.


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## dr.welby (Jan 6, 2004)

pvd said:


> NO! The front end dives because more weight is being placed on it due to the deceleration of the bike under the COG. The shift HAS to happen. If you are doing anything that prevents the front end from compressing in this condition, then you are effectively shutting the system down.


BUT YES! The shift has to happen, but the fork does not need to compress. It's simple physics. Just add up your forces (positive in the direction of compression):

+ Rider weight
+ weight transfer (dive)
+ bump force
- anti-dive
---------------
= total force on suspension

If you have 100% anti-dive, the dive and anti-dive forces cancel out. So the total force on the suspension is the rider weight plus the bump force, just like it is when you're not braking.


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## Vlad (Feb 7, 2004)

I'll continue riding my hard tail and not worrying about all this stuff.


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## Blaster1200 (Feb 20, 2004)

dr.welby said:


> Definitely used in cars though.


Please explain how it's used on cars. Typical road cars?


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## 181picklz (Aug 5, 2009)

Ah, the Ribi Quadralateral. It was said to work great and was promoted by Roger Decoster(MotoX Legend). It was too complicated and heavy to produce. Same reason the Boyseen Link never saw the light of day. Even though Edvind Boyseen still uses it on his bike.



> I think motorcycles gave up on it because it was needlessly complicated or unmarketably expensive.


All anti-dive is internal on MX forks. Before they developed the technology they ran this.....










Why not try leading link instead of teloscopic?? Would be much easier to machine the parts for.


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## RoyDean (Jul 2, 2007)

dr.welby said:


> So the total force on the suspension is the rider weight plus the bump force, just like it is when you're not braking.


Wrong.

The "total force" on the suspension never changes, regardless of what type of fork you have. (well, ok, it may change very slightly due to length of the fork changing, thus affecting the realtime location of the center of gravity).

A simple free body diagram shows that the reaction force at the front wheel of a bike is same under all conditions (assuming the wheelbase and center of gravity location remains fixed).

Peter is right. Anti dive increases the wheelrate of the front wheel during activation.


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## dr.welby (Jan 6, 2004)

RoyDean said:


> A simple free body diagram shows that the reaction force at the front wheel of a bike is same under all conditions (assuming the wheelbase and center of gravity location remains fixed).


Well, could you draw it then and post it? I'm not following what you're saying.


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## smilinsteve (Jul 21, 2009)

I rented a bike with a Girvin fork, and that baby felt PLUSH. 
I guess the bushings would always wear and make the whole fork wiggle, but good bearings and beefy construction might solve that. Besides no stiction, I think the axle path helps with absorbing bumps.


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## RoyDean (Jul 2, 2007)

dr.welby said:


> Well, could you draw it then and post it? I'm not following what you're saying.


See attached image (sorry for the poor quality, I had to rush as I'm late to meet my inlaws for dinner, somebody check the math).

Basically, the final formula

RB = [F(H) + W(O)] / WB

means that the vertical reaction at the front wheel (RB) (the primary force seen by the fork, but not total as I didn't bother doing both the x and y directions and resolving them into a single force based on the head tube angle) is based solely on the braking deceleration force (simplified as "F"), the height of the center of gravity (H), the weight of the bike+rider (W), the distance of the center of gravity from the rear wheel (O), and the wheelbase of the bike (WB).

Like I said, this is simplified. Doesn't account for slight changes in the dimensions during suspension movement (but they would be minimal), and doesn't resolve x direction forces into the fork reaction force.

But the simple response is that the geometry of the CG versus the two wheel positions is the only thing that affects the force on the front wheel/fork. Slight anti-dive variations thrown in WILL affect the absolute numbers that result, but they absolutely will not "eliminate" the weight transfer phenomena.


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## dr.welby (Jan 6, 2004)

RoyDean said:


> See attached image (sorry for the poor quality, I had to rush as I'm late to meet my inlaws for dinner, somebody check the math).


Hey, wow, thanks for taking the effort! Giving it a cursory glance it all looks correct. I've always seen it in a much simpler form, where you just calculate the weight transfer as F*h/l where F is braking force, h is height of C.G., l is wheelbase.

So I fully agree with your free body diagram of the rider/bike system. In my above summing of forces, I have your two terms - the weight of the rider/bike (though techically it's the fraction of the weight on the front wheel), and the weight transfer (f*h/l).

However, we're interested in what's going on in the fork, and if you have anti-dive then you have that force F channeling into the fork through torque F*r, and then through some linkages or pistons into the fork. Which is why I add the anti-dive force into my sum of forces on the fork.

You mentioned something about that force increasing the wheel rate, which is where this discussion probably needs to go from here.

For me, and maybe this is just a semantic pothole, wheel rate indicates some sort of spring constant that relates a force to a displacement. My position is that the wheel rate does not change - the spring rate of the fork is the same, but the anti-dive essentially "preloads" the fork to match some or all of the weight transfer.

An interesting aside I have probably mentioned before: I was talking to a guy who was experimenting with anti-dive bike forks, and one of his first iterations had a lot of anti-dive - around 100%. Initially, he really liked it because he could run the fork much softer than he normally would, until one day he was riding a trail that had a technical switchback that required a trials- style front wheel endo move. He rolls up onto the front wheel, his forward momentum stops, the anti-dive no longer activated, the soft fork relaxes and drops down through its travel, shifting his point of balance and he goes over the bars. I remember seeing something similar playing around with those Lawill Leader forks - you could slowly roll up into an endo and watch the anti-dive reduce as you slowed down and stopped.


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## D.F.L. (Jan 3, 2004)

What shock is going to be appropriate for the front?

Seems sub-optimal to use something valved and sealed for much higher loads...

(I still like the idea)


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## RoyDean (Jul 2, 2007)

dr.welby said:


> You mentioned something about that force increasing the wheel rate, which is where this discussion probably needs to go from here.
> 
> For me, and maybe this is just a semantic pothole, wheel rate indicates some sort of spring constant that relates a force to a displacement. My position is that the wheel rate does not change - the spring rate of the fork is the same, but the anti-dive essentially "preloads" the fork to match some or all of the weight transfer.


Right, gotcha.

Ok, so "wheel rate" simply means the force vs. displacement characteristic of the wheel. In a typical bicycle fork, this will pretty much equal the spring rate of the fork, plus any misc forces involved (seals, damper effects, etc.). However, in linkage style suspensions, the rate of wheel movement does not always equal the rate of spring movement, so in those cases the wheel rate doesn't equal the spring rate. Additionally, any amount of anti-dive or anti-squat will also change the wheel rate (in a fashion that seems way more difficult to calculate, although it's definitely doable).


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## smdubovsky (Apr 27, 2007)

As RoyDean states, the force @ the front wheel does not change. If you neglect the small CG movement, both anti-dive and non anti-dive forks have the EXACT same normal force. 

W/ mechanical anti-dive the shock damping is not affected (Note: this is NOT how 'platform' damping works). Only a counteracting force is added INTERNAL to the FBD. This is how the para/telelever systems work. The dynamic spring rate doesn't change so the forks bump response is unaffected. Since the fork doesn't have to resist dive forces by using the springs, lower rate springs and damping can be used to soak up bumps better. For a big travel bike that needs high rate springs/damping to prevent bottom out there may be no benefit.

As mentioned before cars can implement anti-dive/squat. On both A-arm and Mcpherson strut suspensions can angle the front of the A-arm(s) downwards. AFAIK, its not done very often on production cars. Anti-squat in the rear can be done similarly. That's very often used on semi-trailing arm designs.


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## dr.welby (Jan 6, 2004)

smdubovsky said:


> As RoyDean states, the force @ the front wheel does not change. If you neglect the small CG movement, both anti-dive and non anti-dive forks have the EXACT same normal force.


Yes, exactly. Anti-dive does not eliminate weight transfer. It counteracts it within the suspension.


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## pvd (Jan 4, 2006)

smdubovsky said:


> This is how the para/telelever systems work.


Telelever has no anti-dive properties. It does provide additional support to the telescoping fork legs during braking. This is to fight the flex that occurs during braking. The flex on traditional telescoping forks during braking is pretty outragous.

Paralever is a rear system designed to reduce an effect known as 'shaft drive climb'. A standard shaft driven bike will jack as the gears of the shaft climb over each other. Essentially, paralever would be a pro-squat system.


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## dr.welby (Jan 6, 2004)

pvd said:


> Telelever has no anti-dive properties.


Yes, it does. Don't you own this book?


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## pvd (Jan 4, 2006)

Sorry. I'm going to have to be corrected. Thanks. But I stand by my argument against anti dive systems. I've never bought into that argument that the BMW system is done for anti-dive. I belive that it is for struture first and formost. As typically fully loaded tourers, Structure is paramount.

Under heavy braking, nearly 100% of the system weight is on the front wheel. If any less than 100% of the initial unsprung weight is not carried by the spring during braking, then there is a huge performance loss in the tracking of the front wheel. Essentially, a massive load is applied to the wheel and not the spring during braking. That blows. Since we all know BMW is not know as any kind of racing motorcycle except ancient history and 2010 MotoGP, we can accept the design as not optimal.


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## Thylacine (Feb 29, 2004)

pvd said:


> Since we all know BMW is not know as any kind of racing motorcycle except ancient history and 2010 MotoGP, we can accept the design as not optimal.


Paris-Dakar isn't racing?

So, what's the verdict on the USE S.U.B fork?


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## pvd (Jan 4, 2006)

Like I said, BMW motorcycles aren't currently known for being racebikes. Let's see what happens in 2010.

http://en.wikipedia.org/wiki/2009_Dakar_Rally#Motorcycles


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## dr.welby (Jan 6, 2004)

pvd said:


> Under heavy braking, nearly 100% of the system weight is on the front wheel. If any less than 100% of the initial unsprung weight is not carried by the spring during braking, then there is a huge performance loss in the tracking of the front wheel.


Could you explain and quantify the "huge performance loss in tracking", ideally with something other than superlatives and handwaving?


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## Thylacine (Feb 29, 2004)

Yeah, and Subaru isn't known for making Rally cars. Just look at the 2010 WRC.

You idiot.


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## pvd (Jan 4, 2006)

dr.welby said:


> Could you explain and quantify the "huge performance loss in tracking", ideally with something other than superlatives and handwaving?


I'm sure you understand what suddenly adding 150 lbs of unsprung weight to the front end would do.


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## smdubovsky (Apr 27, 2007)

pvd said:


> Under heavy braking, nearly 100% of the system weight is on the front wheel. If any less than 100% of the initial unsprung weight is not carried by the spring during braking, then there is a huge performance loss in the tracking of the front wheel. Essentially, a massive load is applied to the wheel and not the spring during braking.


Why? Over bumps very little of the force is actually carried by the actual spring in a conventional suspension anyway. Its carried by the shock shaft/fluid (damping forces are many times spring force for any reasonable sized bump velocity.) The tire sees the same force either way. It makes no difference to the wheel if the force is distributed X = Y1% spring + Z1% damping or X = Y2% spring + Z2 % damping + W% telelever. For the ultimate traction/tracking you need to keep the wheel on the ground w/ as constant a force as possible (which is what an infinitely light wheel/susp yeilding zero unsprung weight would do - you could run an air spring w/ zero damping.) Anti dive systems that decouple some of the forces and allow a more optimal spring/damper setup really can work.


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## dr.welby (Jan 6, 2004)

pvd said:


> I'm sure you understand what suddenly adding 150 lbs of unsprung weight to the front end would do.


No, I don't, because weight transfer is going to add 150 lbs of load to the front wheel whether you have anti-dive or not.

Whatever you're trying to say, I just don't think it can be said in one sentence.


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## pvd (Jan 4, 2006)

I think that if you guys don't understand the concept of unsprung weight vs.sprung weight and why we want to minimize unsprung weight, then we need to start a new threat on the basic concepts of suspension systems.


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## dr.welby (Jan 6, 2004)

pvd said:


> I think that if you guys don't understand the concept of unsprung weight vs.sprung weight and why we want to minimize unsprung weight, then we need to start a new threat on the basic concepts of suspension systems.


Look, I don't have the patience for this. You don't need to accuse others of being clueless noobs when you can't properly express your own ideas. We understand sprung and unsprung mass just fine, and that should be obvious if you look two or so posts up at what smdubovsky wrote.


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## RoyDean (Jul 2, 2007)

pvd said:


> I'm sure you understand what suddenly adding 150 lbs of unsprung weight to the front end would do.


Imagine braking hard coming into a downhill corner. You're antidive is in full effect. At the limit of your traction circle, a few feet before you start to release your front brake and turn in, you hit a small root.

The problem here is that your "suspension" front end has been tightend up (significantly). The wheelrate has been increased by double, maybe even triple. The result instead of "soaking up" that root, your front fork now feels more like a rigid than a suspension fork. Oops. Result is front tire leaves ground, you plow off the trail and into a tree.

And now your antidive fork is cracked, and you bought it used so you can't even try and pass it off as a JRA failure to the manufacturer.


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## smdubovsky (Apr 27, 2007)

RoyDean, its the exact opposite of what you say.

A coil spring fork does not change rate vs travel. Its stays X N/m. W/ a typ fork, X needs to be pretty high so that you don't bottom the fork during braking. W/ anti dive like telelever, you can actually run a LOWER spring rate, like X/2 (which means you can run less damping too.) Not only do you have more travel left to absorb the bump but it does so w/ LESS change in force to the wheel. It tracks better due to the lower(more plush) damping and doesn't run out of travel as fast. "More plush" w/o bottoming is what all DH guys ever seem to want Go read how telelever works. Or just ride a modern bmw They actually first designed it w/ zero dive but riders didn't like it because it felt too weird. Production version have a little dive left in so it feels more like other bikes.


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## dr.welby (Jan 6, 2004)

Yeah, anti-dive doesn't change the rate, it effectively adds preload. The rate stays the same.


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## pvd (Jan 4, 2006)

Their have been many anti-dive systems tried in the past. Some with geometry, some with damping, some on the spring side.


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## RoyDean (Jul 2, 2007)

smdubovsky said:


> RoyDean, its the exact opposite of what you say.
> 
> A coil spring fork does not change rate vs travel.


A: Please explain how it is "different than I say". Nowhere do I say anything about specific rates. All I state is that REGARDLESS of what your spring rate is, anti-dive increases your WHEEL rate. Spring rate and wheel rate are two totally different things.

B. Nowhere do I say a coil spring's rate changes with travel. Reread my response. And even if I did, it's not necessarily wrong. There are progressive rate coil springs in use on all types of suspensions. Tighter wound portions of the coil offer less rate at the lower travel, and after they bind the rate goes up.


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## pvd (Jan 4, 2006)

RoyDean said:


> Nowhere do I say a coil spring's rate changes with travel.


There is no such thing as a non-progressive spring. They all go progressive even if some are linear for a short period.

http://www.pvdwiki.com/index.php?title=Springs#All_springs_are_progressive


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## RoyDean (Jul 2, 2007)

pvd said:


> There is no such thing as a non-progressive spring. They all go progressive even if some are linear for a short period.


Sure there are. Your own plot shows that if you design a spring application properly, you can stay entirely within the linear range. You even state it in your wiki.

Not that I'm disagreeing with your overall statement, but in general, as an engineer who HAS designed suspensions (although for four wheeled vehicles, not bicycles), it is generally accepted that you can have linear rate springs or progressive rate springs.

Saying there is no such thing as a linear rate spring is akin to saying steel or TI don't have infinite fatigue lives.... Sure, it's true, but for most practical purposes, its safe to assume the opposite.


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## pvd (Jan 4, 2006)

Most folks have so little knowlage of what's going on with springs that they belive that Hook really wrote a Law. The schools really do a disservice on this subject. Yes, a properly designed system is going to use a spring that uses the linear portion of the springs range, but if a designer has no idea that a coil spring goes progressive past a point, they can easily pass this point and often do. You and I know that most folks have no idea that this happens. You and I both know how lame most engineers really are and how little they really know.


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## AndrewTO (Mar 30, 2005)

Nevermind. I'm tired of this. Feel free to delete my entry, Walt.


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## Walt (Jan 23, 2004)

*Stay on target...*

Guys, if you feel the need to post something unrelated to the topic, whether it's pointless chest-pounding or personal attacks, DON'T.

There's no problem with heated discussion - it's even ok if you call someone's idea(s) idiotic, as long as you're making some kind of on-topic argument about WHY. If you're not, please don't waste all our time.

This thread has been excellent so far and I know I've learned a lot by following some of these arguments. When we debate honestly, we all win in the end.

Thanks, and good night!

-Walt


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## smdubovsky (Apr 27, 2007)

RoyDean said:


> A: Please explain how it is "different than I say". Nowhere do I say anything about specific rates. Spring rate and wheel rate are two totally different things


Sorry. You did say wheel rate not spring rate. (And by rate, I assume we're both talking about the first order differential of force/displacement.)

I understand how in mcpherson strut or A-arm how wheel and spring/shock rate are different. On a typ telescopic fork w/ springs in the fork how is the factor not 1? (unless you're talking about the HA inclination factor & using defining displacement vertical vs displacement along the travel axis - is only a ~5% difference in bikes. 200++% is possible in cars depending on the spring/shock inclination and location on the susp arms. Heck and that even changes vs travel.)

I understand that w/ telelever locating the spring on the linkage its identical to a mcpherson strut and has a wheel-spring factor. You are correct in that since we are talking about non-conventional designs we should be using wheel rate and not throwing around the terms interchangeably.



RoyDean said:


> All I state is that REGARDLESS of what your spring rate is, anti-dive increases your WHEEL rate.


Semantics may be an issue? If the rate is defined as (what I think is the more conventional) the wheels df/dx vs an external displacement then, no, telelever does not affect wheel rate. (some other designs might) If defined as the sprung masses resistance to movement then the answer would be different, but that would be a pretty non-common/unique definition.

And by 'not affect' I mean mostly negligible amts (compared to other larger changes involved). As shown in welbys side view of a bmw, the telelever arm is not exactly perpendicular to the fork (but is close) so its force contribution will vary slightly based on travel. That variance along the arc is small though. The bottom line is that the wheel rate on, say, an 1100gs since walt brought it up is _less_ than a comparable bike w/ conventional forks.

An interesting observation is that telelever is identical to a mcpherson strut. The direction of travel is only 90deg different which isn't even a consideration since vertical displacement/force is vertical no matter which way you rotate it (susp analysis is not affected by direction of travel). On telelever, the spring is not coaxially mounted on the sliding strut (coilover) but thats not required, just convienient. Porsches torsion bars work that way - applying spring force though the arm vs the strut body. Im sure someone else has mounted the spring on the arm too. Struts car very effectively resist (sideways) 'dive' based on roll center/cg heights. In fact while analyzing roll centers, roll-stiffness, cg height, etc its EXACTLY analogous to two wheel vehicles analysis. Direction of travel is only different.


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## Team Fubar Rider (Sep 3, 2003)

I had a friend that ran the Girvin forks exclusively for quite a while. I think they have a great concept, especially if you build a frame specifically for the fork. The fork had a tendency to jack the front end of the bike (or at least the cockpit) up due to the linkages. 

I have to wonder if a person tried to make a modern day "Girvin" fork with more travel if the linkages would be too long causing the fork's "J" curve axle path to be too pronounced. I am sure someone with far more knowledge than I could compensate for this...


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## Thylacine (Feb 29, 2004)

*boing!*


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## RoyDean (Jul 2, 2007)

so much complexity. So much cost. Is it really worth it?


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## dr.welby (Jan 6, 2004)

smdubovsky said:


> Semantics may be an issue?


Yeah. If wheel rate is the change in load divided by a change in wheel position, I don't see why this would change with anti-dive, at least for a simplified model of anti-dive. Consider:

Your bike is at rest with 100 lbs on the front wheel. 200 lbs on the front wheel makes the front wheel move 1 inch. The wheel rate would 100 lbs/in.

Now lets say you're braking,and anti-dive is trying to extend the fork with 50 lbs of force. You can do whatever math you want, let's say we just add that force in (even though anti-dive doesn't increase the wheel load). You'd have 150 lbs on the front, then 250 lbs, 1 inch of travel, and the wheel rate is still 100 lbs/in. The anti-dive is a constant preload, and therefore doesn't affect wheel rate.

This of course may be too much of an oversimplification. I keep thinking there's some sort of free-lunchism going on, but I keep figuring out ways to justify that there isn't. Except for one:

Let's say we are getting anti-dive strictly from wheel path. In other words, the wheel moves slightly forwards as it travels and when you brake the forces want to extend the suspension to keep it from diving. If you're braking, the anti-dive is trying to extend, and when you hit a bump the wheel has to move forwards relative to your center of gravity. Which means it has to speed up to move forwards, or you the rider has to slow down relative to it. I can see a few possible ramifications: if you're braking at your limit, could forcing the wheel to go a little bit faster cause a break in traction? Could making the front wheel go faster or slowing down the rider constitute, from the standpoint of energetics, and increase in unsprung "mass"?

And then similarly, let's say we have a perfect axle path, but we are floating the caliper and using a linkage or piston to load the suspension for ant-dive. Hitting a bump would make the caliper rotate backwards and increase the apparent speed of the disk, which would do similar things as above, I believe.

The above are loose mental models I've been thinking about. In it's simplest, most "perfect", on-paper form, it seems like anti-dive should be a winner. However, the real world doesn't seem to bare that out. Possible reasons include:

* Economics - telescoping forks are cheaper to manufacture and offer "good-enough" performance.

* Preference - Because of telescoping forks' dominance, riders are used to riding pro-dive suspensions in the front, and handling, both mental training in the rider as well as the characteristics of the bike, has been optimized around that.

* Halfway-thereism - anti-dive may be a good idea, but all implementations so far haven't been optimized correctly - for example maybe front suspension are run too soft because they can be, but that makes handling suffer in other situations.

* Free-lunchism - anti-dive looks good on paper, only if you idealize the model to get rid of certain small problems, like I mention above.

Questions, comments, napkin-sketches welcome.


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## smdubovsky (Apr 27, 2007)

Welby,
I think *Economics and *Preference are two (very) big factors. 

Telelever really works (go ride one). Its not easily adapted to a bike frame though. A DT has no real 'width' to be able to use a simple & light A-arm to resist the side forces the lower joint must take. Thats no big deal on a motorcycle though where the frame is 12+" to clear the engine/radiator. So you'd be stuck w/ a narrower link and stresses and play in the bearings are magnified. Though, I guess it wouldn't be any worse than the rear pivots on a FS bike. It also puts an odd load into the middle of the DT 

I don't know that small fore/aft movements would affect braking much. The knobs/sidewalls/spoke windup/fork deflection probably dwarf it for a long enough arm. For short travel linkages maybe it needs to be factored in.


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## dr.welby (Jan 6, 2004)

smdubovsky said:


> I don't know that small fore/aft movements would affect braking much. The knobs/sidewalls/spoke windup/fork deflection probably dwarf it for a long enough arm. For short travel linkages maybe it needs to be factored in.


Yeah, I don't know either. But it's the only thing that I've been able to hang any kind of criticism on, from a physics standpoint. SOMETHING must exist that makes people say "anti-dive locks out the suspension", but I don't know if it's just Internet Expert Syndrome, lousy anti-dive designs, or something real that just hasn't occured to me yet. I can find justifications in either direction for or against anti-dive.

Right now I think I understand the generalized, idealized model of what's going on. Now I'm trying to poke my understanding here and there with more specific, detailed facets of the model.


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## smdubovsky (Apr 27, 2007)

dr.welby said:


> SOMETHING must exist that makes people say "anti-dive locks out the suspension"


Thats because 99.9% of the forks out there implement anti-dive w/ a 'platform' damper (or some other old way that didn't work). Platform very much DOES lock out the suspension over small bumps. Telelever seems to be the only one thats ever gained mass acceptance into the motorcycle world. They even abandoned duolever for it so at least one company thinks its the best combination for their purposes. Many of the other leading/trailing arm designs have such short levers it prob causes other problems too.

There are probably lots of the 'internet expert' opinions out there too Same ones that say brake jack is everywhere, using your front brakes will make you endo (anyone catch the 'tip' in bicycling mag this month), etc.

What we need is a shock w/ a linear motor in the spring leg. Could vary the force and prevent dive like some of the new car suspensions are doing (though I think rover/merc/etc are using hydraulics in the sway bars for most of the anti-lean stuff). That coupled w/ rheomagnetic fluid in the other leg for active damping and you'd be good to go. Well, downhill at least. No way you could pedal all that heavy stuff up hills


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## Thylacine (Feb 29, 2004)

Hey, is 'Anti-Dive' analogous to the way in that VPP 'minimises' the effects of chain torque and rider oscillation on the suspension system? ie: the fork linkage is configured in such a way to 'minimise' the suspension compression as a result of the brakes causing a massive weight shift to the front wheel?

I'm just a lowly Industrial Designer, so the engineering is largely lost on me. 

Nobody commented on the USE S.U.B. fork, either.


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## jay_ntwr (Feb 15, 2008)

So Walt, anyone get you a Girvin yet?


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## nogod (May 30, 2009)

here just so evryone knows what a girvin looks like


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## nogod (May 30, 2009)

just because this thread exists i had to think about the concept. if you make a dual crown with linear bearings. (like the ones used for cnc machines) you could still have the fork behave the same as a telescoping fork. you could make the shock move with the linkage instead. you know if you must practice , at least make something that will work.


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## Vlad (Feb 7, 2004)

How about the battery-powered piezoelectric damper that that fork came with?


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## RoyDean (Jul 2, 2007)

lefty + girvin =

http://forums.mtbr.com/showpost.php?p=6124861&postcount=10


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## BP73 (Jun 5, 2009)

I didn't read every post, so maybe someone else has already mentioned this but....the Girvin linkage fork had an axle path that moved up and sightly back. As a result, on bigger hits, I sometimes felt like the bike would endo easier. This was especially true on large stair type drops, and any time you jumped the bike and landed front wheel first by accident.

The other thing that I remember with mine, was actually how loose the linkage assembly would become after hard riding. look at the photo above. The linkage is made up of several smaller pieces bolted together. That was the Vector model. I replaced my Vector with what I think was called the cross link model. It was pretty cool, and came with carbon fork blades, and a noleen coil shock. That fork was stiffer because I believe it used a one piece machined linkage assembly, instead of smaller pieces bolted together. However, it still felt endo prone to me because the fork compressed up AND back at the same time.

Another linkage fork from the era was the AMP Research, but it was pretty much junk, and people broke those easily on anything rougher than fire road.


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## NonConformist (Nov 19, 2004)

Not mtb, but still an interesting example of a Hassock design on a small scale.


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## Freddy Salgado (Jun 4, 2009)

John Brittens's front end worked very well to come out of nowhere and smash factory super bikes in Daytona. RIP JB. Very advanced fork tech for its time. When road bikes start taking on full suspension, this could be the way to go for short travel front end.


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## brant (Jan 6, 2004)

NonConformist said:


> Not mtb, but still an interesting example of a Hassock design on a small scale.


Is that by Dave Wrath-Sharman/Highpath?


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## pvd (Jan 4, 2006)

Freddy Salgado said:


> John Brittens's front end worked very well...


The front end was considered the worst part about the britten, I belive. Most people that I've heard from say that it was going to be ditched in favor of a traditional telescope.


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## frascati (Sep 23, 2009)

The front end of the Britten, was likely it's worst part.


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## Freddy Salgado (Jun 4, 2009)

pvd said:


> The front end was considered the worst part about the britten, I belive. Most people that I've heard from say that it was going to be ditched in favor of a traditional telescope.


 Ya the front end could have been the worst part, but it still smoked the factory rigs at Daytona! Guys like Britten think so far outside the box that who knows what that bike and front end could have been capable of?


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## pvd (Jan 4, 2006)

You may want to add the caviate that the Britten did well in BoTT. Superbike is a different story.

Don't get me wrong, Britten was amazing, but he hasn't been the onlyguy working on new things. His bikes had flaws, they never got fully developed, and we see little relection of what he did on bikes today.


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## Linnaeus (May 17, 2009)

The Britten fork looks like it could have been too stiff laterally. Sport bikes need flex designed into the chassis and suspension, b/c they require some semblance of suspension when banked at 60*. For a mtn bike, that's not necessary nor desirable.


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## mikesnowdon (Sep 25, 2009)

Little bump on the British design 'U.S.E. SUB" fork from a few years ago.










It didn't really catch on but was a nice piece of innovation in the mtb world.

Walt. Did you get anywhere with this project yet? Really interesting thread.


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## Walt (Jan 23, 2004)

*I've now got 2 of the darn things...*

First step, I think, will be to just build some new legs for one of them with a thru-axle and disc mount. We'll see how that rides. Depending on how atrocious it is, I may or may not do one from scratch.

I'll probably talk about this "project" on the blog, when I get around to doing some actual work on it.

-Walt


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## mikesnowdon (Sep 25, 2009)

I would suggest further modding the fork to incorporate some proper bearings on the linkage. Its bound to improve it.


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## nogod (May 30, 2009)

here a auction with a girvin i found on ebayhttp://cgi.ebay.com/PRO-FLEX-856-WORLD-CUP-16-FRAME-W-GIRVIN-FRONT-FORK_W0QQitemZ180416676073QQcmdZViewItemQQptZMountain_Bikes


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## NEPMTBA (Apr 7, 2007)

Ribi designed one for Roger Decoster when he was at Suzuki then revisited it again as in the pic in the thread above when R D was at Honda. Excuse was too expensive to make, but it was never said R D didn't like it!

Build away Walt you might be onto something... Just do it with 13 inches of travel to make DC happy!!!!!!!!! 

Sorry DC I had to say it cause I know you will...


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## Monkeybike (Feb 25, 2008)

look up the muddy fox interactive ... from 1996 / 1997 designed by ... dave smart (I think).


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## smdubovsky (Apr 27, 2007)

Bimota's Tesi 2D and the Vyrus 984C3 2V and the 985C3 4V have hub steering.
(hopefully the attached pic will show...)


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## ROSKO (Oct 11, 2009)

first post so please pardon any forum faux pas!

A quick intro is perhaps in order? I live in NYC and have been lurking here on MTBR for quite a while, finally broke down to register so I could see the pics here in the framebuilding area. My interests are cycle and motorcycle and I do a bit of fabrication relating to both. Sometimes one more than the other. Have not had a chance to post anything yet and this seemed like a good topic...

In most motorcycle design circles the telescoping fork is well known for having it's issues. (https://www.tonyfoale.com/Articles/Dive/DIVE.htm) The Hossack and several other FFE designs have tried to address these. One thing that seems to come up in many of these is that the "traditional" headtube placement has to be let go. I road race vintage motorcycles here in the north east and have had the opportunity to see some of the designs (both old and new) in the paddock and on the track. One local engineer has done quite an impressive job (now on version 2?):
https://www.cosentinoengineering.com/index_files/page0010.htm
I've done some interweb browsing fueled by these musings and see that there has been some experimentation, one notable example using the same a-arm type hossack is the Kimori: https://cyclesdeoro.com/events/NAHBS/2009/kimori_12.jpg

It is an interesting concept and there is a lot of info available. From all the anti-dive stuff done for both road and mx to the various linkage forks and FFE's their is a lifetime of tinkering to be done. Take a look at some of the early ELF endurance bikes with their bolt-on anti dive system(which could easily be adapted for mtb use) or the VanTech/ Yetman linkage forks of the 60's/70's which could be made to bolt right up. Now, whether they work well or not.... that's a whole 'nother can o worms.

Here's a few links for anyone interested in MC design and how some things might translate to the bicycle:

www.EuroSpares.com home to mucho photos and info
www.tonyfoale.com legendary MC designer
https://micapeak.com/mailman/listinfo/mc-chassis-design an e-mailing list on MC design and fabrication

That's all I can come with off the top of my head. Would love to see some people exploring this area. Seems like for something as new as the MTB their should be more folks trying out wacky ideas.


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## scottybinwv (Jun 29, 2010)

Bumparella for any one with info for what alloy the Girvin fork legs would be?


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## scottybinwv (Jun 29, 2010)

Myself and a frame building buddy are working on a Fat bike fork based on a Girvin. If anyone knows what the Al alloy in the fork leg is please share this info with me? Need this info for the welding and/or heat treat if its in the 6000 series. 

Seems the Proflex frames were made of 7000 and maybe the forks were too since this gets around the HT issue.

Planning to call around to some local recycle/salvage yards to see if anyone has a "sorter" as a backup but that seems remote.


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## Integrexman (Feb 9, 2013)

I built frames in the 90's and remember all the problems people had with this fork. I remember thinking it could have been done much better. A decade later and I am working on my own design kind a cross between a Proflex and the Look Fournales. This is my second post hear on MTBR when I get to ten I will start a new thread of my frames.


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## CYCLEJCE (Nov 2, 2010)

Black Sheep had their version of the Girvin fork at their booth at NAHBS. I'll post pics soon!


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## Integrexman (Feb 9, 2013)

Black sheep makes some interesting looking bikes can't wait to see what they came up with.


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## Bdabike (Jan 27, 2013)

I have a Proflex 754 in the garage, with the Girvin fork. Melted, of course.


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## CYCLEJCE (Nov 2, 2010)

https://i.imgur.com/ngw8qCIh.jpg

From the passion forum...


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## Integrexman (Feb 9, 2013)

Did they say how much travel it has?


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## jeff (Jan 13, 2004)

404'd link.


CYCLEJCE said:


> https://i.imgur.com/ngw8qCIh.jpg
> 
> From the passion forum...


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## CYCLEJCE (Nov 2, 2010)

http://forums.mtbr.com/passion/nahbs-2013-guerdonians-album-840826.html

This time?


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## patineto (Oct 28, 2005)

Some company's did make good "Girvin forks"

I'm not sure if was Nukeproof that made them..








Shock on the steer tube, very clever.
















Another alternative.








In my eyes the lack of quality control and cheap manufacturing is what kill the Girvin's but the overall concept is sound..


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## CYCLEJCE (Nov 2, 2010)

That 1st one is a head shock. No linkage. Cool though!


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## CYCLEJCE (Nov 2, 2010)

Nukeproof ! Got that headbadge on a bike...


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## TigWorld (Feb 8, 2010)

CYCLEJCE said:


> https://i.imgur.com/ngw8qCIh.jpg
> 
> From the passion forum...


A truss fork with suspension and curved structural members. Holy wacky wonderland Batman!

German-A are probably the only guys still making a linkage fork. They are supposed to be excellent.


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## CYCLEJCE (Nov 2, 2010)




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