# Geometry considerations when switching to a rigid fork



## MidnightFattie (Jun 13, 2017)

I'm looking to build a fully rigid SS. When switching in a rigid fork for a bike that came with front suspension, how does this effect the Geometry? 

I had watched one of the interesting Jeff Jones videos where he was saying that setting up a bike with rigid fork that was originally designed for front suspension is not ideal because they need to be designed differently. 

I was wondering how this can be best addressed if I end up (as I most likely will) with a bike that was designed for front suspension. 

Thanks in advance


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## mack_turtle (Jan 6, 2009)

put a fork with a similar a-c measurement to what was intended for the bike. for bikes designed around forks longer than 100mm, it will be harder to find a fork that will work well. a shorter fork will drop the BB, steepen the head tube angle, and... shorten the wheelbase and lengthen the reach, I think.


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## wjphillips (Oct 13, 2008)

BB drop is the main difference. Also, handling characteristics will be altered too. For one, the wheelbase will be shorter and the front end will be lower, so you may have to accommodate that with a different stem (length and rise).

The best thing to do, is to have the bike setup with the suspension fork. Then get the bike fit dialed in the way you like it.

Then measure everything: ground to handlebar, top tube + stem to seat, etc... 

Then when you add the rigid fork, note the differences in the measurements and adjust it to try to get it back to where you had it with suspension. The first place to look is the axle to crown measurement of suspension fork and try to match that with the rigid one.


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## evenslower (Sep 26, 2005)

Figure out the A to C on your suspension fork when sagged as you would normally ride it and try to find a rigid in that ballpark. A to C on a suspension fork is a dynamic condition when riding so not 1:1 translatable to a rigid that stays constant. That said, don't over think it, get it close and give it a shot. I've run rigid on frames that previously had suspension forks (100MM) and it's worked well on some, great on others.


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## *OneSpeed* (Oct 18, 2013)

Agree with others, it's not hard, but it's good that your paying attention to details. 

Generally speaking an older frame with a 80mm suspension fork uses a 465-470mm rigid fork. 

A frame that uses a 100mm sus fork should have a 480-490mm rigid fork. 

Offset is another thing to pay attention to. Older geo with steeper HTA's usually use a 44-46mm offset fork where as more modern frames with slacker geo usually are designed around 47-51mm offset forks. (these are generalizations, not hard rules)


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## Velobike (Jun 23, 2007)

The simplest thing is to find a frame designed for rigid use. Why ride something that is compromised?


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## khardrunner14 (Aug 16, 2010)

i've had good luck just paying attention to AC measurements and running 29+ up front.


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## JakeStroganoff (Oct 22, 2013)

What is your frame and fork currently, or what frame/fork are you interested in?


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## mack_turtle (Jan 6, 2009)

Velobike said:


> The simplest thing is to find a frame designed for rigid use. Why ride something that is compromised?


not very simple, considering there are fewer than ten such options. I can only think of three right now.


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## Velobike (Jun 23, 2007)

mack_turtle said:


> not very simple, considering there are fewer than ten such options. I can only think of three right now.


You only need one...


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## bikeny (Feb 26, 2004)

mack_turtle said:


> not very simple, considering there are fewer than ten such options. I can only think of three right now.


Are there really that many? Please list whichever ones you know of! Obviously, all the Jones bikes are non-suspension corrected.


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## Addy Marx (Jul 18, 2009)

khardrunner14 said:


> i've had good luck just paying attention to AC measurements and running 29+ up front.


This. Plus a stem spacer adjustment to get my bars in the 'right' place. Has worked well for me for both 29+ front and 29" (not +) as well.


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## 2:01 (May 10, 2010)

bikeny said:


> Are there really that many? Please list whichever ones you know of! Obviously, all the Jones bikes are non-suspension corrected.


I can't think of that many, either. And most are more touring geo than MTB.

Jones
Troll/Ogre
Velo Orange Piolet
Pugsley


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## MidnightFattie (Jun 13, 2017)

2:01 said:


> I can't think of that many, either. And most are more touring geo than MTB.
> 
> Jones
> Troll/Ogre
> ...


What exactly changes in the geometry when designing for rigid front end? I'm curious to know if there are certain things to look for in what's currently out there that would be better suited to a rigid conversion than others.


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## Velobike (Jun 23, 2007)

MidnightFattie said:


> What exactly changes in the geometry when designing for rigid front end?...


Steeper head angle around 70-72º.


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## bikeny (Feb 26, 2004)

MidnightFattie said:


> What exactly changes in the geometry when designing for rigid front end? I'm curious to know if there are certain things to look for in what's currently out there that would be better suited to a rigid conversion than others.


The main thing that is different on a non-suspension corrected frame is the fork A-C distance. A suspension corrected rigid fork will have an A-C distance of 470mm or more, depending on what travel fork it's replacing. On the other hand, a Jones frame/fork uses an A-C distance of 435mm. That allows a much longer head tube on the frame, as well as shorter fork legs, which can then be made thinner and will flex less. The Jones website actually has a bunch of info on why he designs them non-suspension corrected, it's worth reading, even if you're not interested in buying one.

Edit: I just read through your post again, and you seem to be confused a bit. There a certain bikes that are designed from the ground up as rigid bikes, and they cannot run a suspension fork. Then there are bikes that are designed for suspension forks, but are also sold with rigid forks, like the Trek Stache. Or of course, you can put a rigid fork on pretty much any hard tail, you just need to find the right fork for the frame you are looking at. There is no geometry that would make a hardtail better for conversion to rigid. The geometry should be based on what you like to ride, and getting the right rigid fork will preserve that geometry.

And as others have said, the main number you need to worry about is axle to crown distance, A-C for short. That number should match the sagged number of the fork you are replacing.


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## bikeny (Feb 26, 2004)

Velobike said:


> Steeper head angle around 70-72º.


That is false. The headtube angle has nothing to do with suspension corrected vs. non-suspension corrected. For example, the Jones Plus has a 67.5 degree head tube angle.


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## Shinkers (Feb 5, 2014)

Velobike said:


> The simplest thing is to find a frame designed for rigid use. Why ride something that is compromised?


Compromised how?

If I pick a rigid fork that is the same A-C as the sagged suspension fork that the bike is spec'd with, what have I ultimately changed that the bike will notice?


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## Velobike (Jun 23, 2007)

Shinkers said:


> Compromised how?
> 
> If I pick a rigid fork that is the same A-C as the sagged suspension fork that the bike is spec'd with, what have I ultimately changed that the bike will notice?


If the bike was designed for rigid only, then you don't need such a long fork. That reduces the forces acting on the steering head, and therefore that does not need to be so heavily built. The fork can also be lighter. However there is sometimes an advantage to a longer fork in that it may have more fore and aft compliance and some people like that, others feel it affects the precision of the ride.

You'll also find that many frames designed for a suspension fork have a bend in the downtube close to the steering head to avoid contact with the suspension fork crown. A traditional rigid fork does not need this feature. The strongest structure is a triangle which you can only build with straight tubes - anything else is a heavier compromise, thus a straight downtube tube makes for a stronger frame.


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## Velobike (Jun 23, 2007)

bikeny said:


> That is false. The headtube angle has nothing to do with suspension corrected vs. non-suspension corrected. For example, the Jones Plus has a 67.5 degree head tube angle.


The question that I answered wasn't about suspension corrected - "_What exactly changes in the geometry when designing for rigid front end?_". Unless you are designing and building your own rigid front forks, you are limited by what is available. To get a decent trail figure with the offsets on commercially available forks limits your choice of head angles to the range I have mentioned.

Jones bikes have been an outlier for many years, and a miniscule % of the bikes sold, although there is now a trend is to slacker head angles because of the adoption of longer suspension forks. He also builds his own forks which is necessary because off the shelf forks did not have the offset needed for that head angle on a rigid bike.

Most of the best handling rigid hardtails (as in bikes you would use for XC racing) have had HAs in the range I mentioned. In the UK that would be the On-One Scandal, Ragley TD-1, Singular Swift. I haven't ridden USA brands so I can't comment on their handling but I have raced those I have mentioned.


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## Shinkers (Feb 5, 2014)

So everyone that rides rigid should be riding one of the handful of bikes that are designed that way because if not, your bike will be too heavy and explode.

Am I getting this right?

Oh and slack geometry totally sucks because you've never raced it?


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## Velobike (Jun 23, 2007)

Shinkers said:


> So everyone that rides rigid should be riding one of the handful of bikes that are designed that way because if not, your bike will be too heavy and explode.
> 
> Am I getting this right?
> 
> Oh and slack geometry totally sucks because you've never raced it?


Try reading what I said again, only this time without your ****-tinted glasses.


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## Shinkers (Feb 5, 2014)

Oh snap! ****-tinted glasses? Alright, I'll say that I glossed over the part about bikes you'd use for XC racing.


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## seat_boy (May 16, 2006)

There is a relationship between HA and front suspension. With front suspension, the designer has to pick a HA targetting somewhere between full extension and full compression of the fork. Obviously, this involves some compromise. With a rigid design, the designer can pick the HA to match their fork offset and desired handling, knowing this number won't change as the fork compresses.

The Jones Plus does have a slack front end, but also a huge amount of fork rake--76mm. The resulting trail is pretty traditional XC, 77mm (by comparison, the Karate Monkey with its 72 degree HA has a trail of 73mm, pretty close. Trendy slack bikes are closer to 100mm trail)



bikeny said:


> That is false. The headtube angle has nothing to do with suspension corrected vs. non-suspension corrected. For example, the Jones Plus has a 67.5 degree head tube angle.


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## noisebloom (Nov 5, 2015)

I have a 2008 Gary Fisher Big Sur. A previous owner swapped out the original suspension fork with a shorter, rigid fork.

According to GF's 2008 catalog, the Big Sur originally had a Fox suspension fork with 100 mm of travel. I don't yet know the axle-to-crown length of the original Fox fork, but even an undiscriminating eye can see there is a difference between before and after.









Simple ride quality is harder to figure out. It feels quite normal for casual riding. It steers well, and as long as you don't try to lift up the front, you're none the wiser.

The shorter fork does mess with the bike's geometry. The bottom bracket is slightly lower than stock, which affects the angle I can lean into when I'm pedaling. I don't ride the bike very aggressively, so I haven't dropped a pedal yet on a turn.

More noticeable is how heavy the bike feels in front. The bike's present configuration is fairly light at <24 lbs, but with the shorter fork, my weight is farther forward. Lifting the front wheel is huge work. Wheelies don't last, simple drops require effort, and I have no confidence on one wheel at all on this bike. The front end always wants to go down down down. The weight shift toward the front is pretty obvious. I don't do nose manuals, but lifting the rear end is easier than raising the front. No surprise there.

We're probably talking an axle-to-crown difference of maybe 25-40 mm between original and aftermarket fork, but honestly it does affect how agile the bike is in front.

Down the road I see myself replacing the rigid fork with a modern 100 mm suspension fork, if I'm ever going to ride as aggressively as I want to on that bike.


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## Velobike (Jun 23, 2007)

noisebloom said:


> I have a 2008 Gary Fisher Big Sur. A previous owner swapped out the original suspension fork with a shorter, rigid fork.
> 
> According to GF's 2008 catalog, the Big Sur originally had a Fox suspension fork with 100 mm of travel. I don't yet know the axle-to-crown length of the original Fox fork, but even an undiscriminating eye can see there is a difference between before and after...


That sounds like the rigid fork has an offset not suited to the new HA - which affects the trail figure and how nimble it feels.

If you are going to put a rigid fork on a bike that was designed for suspension you should be aiming for a length that places it somewhere around the suspension fork when compressed in its working range or sag, and with the same offset. As well as preserving the handling it also avoids the low BB problem.

Also your stem looks on the long side - I can't really tell from the angle, but it might explain why the front feels heavy. A shorter stem will move your weight back and lighten the front.

You may find this discussion interesting. 
http://forums.mtbr.com/29er-bikes/what-makes-gary-fisher-g2-geometry-so-unique-559387.html


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## bikeny (Feb 26, 2004)

Velobike said:


> The question that I answered wasn't about suspension corrected - "_What exactly changes in the geometry when designing for rigid front end?_". Unless you are designing and building your own rigid front forks, you are limited by what is available. To get a decent trail figure with the offsets on commercially available forks limits your choice of head angles to the range I have mentioned.
> 
> Jones bikes have been an outlier for many years, and a miniscule % of the bikes sold, although there is now a trend is to slacker head angles because of the adoption of longer suspension forks. He also builds his own forks which is necessary because off the shelf forks did not have the offset needed for that head angle on a rigid bike.
> 
> Most of the best handling rigid hardtails (as in bikes you would use for XC racing) have had HAs in the range I mentioned. In the UK that would be the On-One Scandal, Ragley TD-1, Singular Swift. I haven't ridden USA brands so I can't comment on their handling but I have raced those I have mentioned.


He asked what changes should be considered when designing for rigid. To me that means designing from the ground up. A blanket statement saying HTA should be between 70 & 72 degrees is wrong at best, and misleading at worst. You have no idea what kind of riding he does or trails he rides. What you like for XC racing on your trails doesn't really matter.


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## Velobike (Jun 23, 2007)

bikeny said:


> He asked what changes should be considered when designing for rigid. To me that means designing from the ground up. A blanket statement saying HTA should be between 70 & 72 degrees is wrong at best, and misleading at worst. You have no idea what kind of riding he does or trails he rides. What you like for XC racing on your trails doesn't really matter.


That's right, I don't know what sort of riding he does, do you?

What we do know is that this is the single speed forum, he's looking to design a frame for rigid fork.

He did not specify a particular use, nor that he was going to build his own fork, so the question could be treated as general. The highest probability is that the bike will get XC type use, because that's how most SS rigid bikes get used.

If you get a frame built, you need a fork, so the limitations on head angles are imposed by the availability of forks unless you get that built too.

And because he did not specify a use is why I said what sort of riding I do, so he knew that I was looking at this from an XC perspective. I really do not understand why you are getting your knickers in a knot about that.

Prove me wrong by finding readily available forks he can use with other HAs without getting weird steering, because otherwise you are the one who is '_wrong at best, and misleading at worst_'.*

BTW before this becomes an endless discussion, there is a lot more to bike geometry than head angles.

*I'll be happy if you can show me a source. When I experimented with bikes with different HAs several years ago I had to bodge my own fork.


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## wjphillips (Oct 13, 2008)

noisebloom said:


> I have a 2008 Gary Fisher Big Sur. A previous owner swapped out the original suspension fork with a shorter, rigid fork.
> 
> According to GF's 2008 catalog, the Big Sur originally had a Fox suspension fork with 100 mm of travel. I don't yet know the axle-to-crown length of the original Fox fork, but even an undiscriminating eye can see there is a difference between before and after.
> 
> ...


Looks like it was turned into a big BMX bike. It would be fun to ride.


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## bikeny (Feb 26, 2004)

Velobike said:


> BTW before this becomes an endless discussion, there is a lot more to bike geometry than head angles.


Absolutely! That's exactly why saying HTA has to be between 70 and 72 is a ridiculous statement.


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## Velobike (Jun 23, 2007)

bikeny said:


> Absolutely! That's exactly why saying HTA has to be between 70 and 72 is a ridiculous statement.


I am talking about the constraints placed on the design by what is available, so it is not ridiculous, but practical.

I take it you haven't found any readily available forks that allow slacker HAs.


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## bikeny (Feb 26, 2004)

Velobike said:


> I am talking about the constraints placed on the design by what is available, so it is not ridiculous, but practical.
> 
> I take it you haven't found any readily available forks that allow slacker HAs.


Yes, still ridiculous. I haven't bothered to look, but most rigid forks have the same offset as their suspension fork counterparts. So why can't I ride a bike with a rigid fork and a 68 degree HTA, just like I can ride a hardtail with a suspension fork with a 68 degree HTA?

Oh wait, I do ride a bike with a 68 degree HTA and a rigid fork(with normal offset)...


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## Velobike (Jun 23, 2007)

Double post. How?


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## Velobike (Jun 23, 2007)

bikeny said:


> Yes, still ridiculous. I haven't bothered to look, but most rigid forks have the same offset as their suspension fork counterparts. So why can't I ride a bike with a rigid fork and a 68 degree HTA, just like I can ride a hardtail with a suspension fork with a 68 degree HTA?
> 
> Oh wait, I do ride a bike with a 68 degree HTA and a rigid fork(with normal offset)...


Well if you designed the frame perhaps you can tell us where you got the fork?

Presumably the bike handles well - let's assume it's not a chopper - what's the A/C and the fork offset? BTW what do you call normal offset? In the list of forks I have they vary enormously.

Also if you don't mind an eBay import from China there's a tapered fork with an offset that will work with a HA 68-9º, but I still haven't seen one in normal channels here. (I prefer to get stuff like forks from reputable distributors who back up with warranties - saves expensive dental work.)


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## noisebloom (Nov 5, 2015)

It is fun to ride. You're right, the previous owner set it up as a sort of DJ bike. It's just that with the short fork, my weight is biased too far forward, and lifting the front is a chore. I like a more centered ride. Of course, replacing the lighter, rigid fork with a heavier, suspension fork will probably not help as much as I'd like, as the weight penalty would likely offset the improvement in weight distribution.


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## bikeny (Feb 26, 2004)

Velobike said:


> Well if you designed the frame perhaps you can tell us where you got the fork?
> 
> Presumably the bike handles well - let's assume it's not a chopper - what's the A/C and the fork offset? BTW what do you call normal offset? In the list of forks I have they vary enormously.
> 
> Also if you don't mind an eBay import from China there's a tapered fork with an offset that will work with a HA 68-9º, but I still haven't seen one in normal channels here. (I prefer to get stuff like forks from reputable distributors who back up with warranties - saves expensive dental work.)


I'm on a custom frame and fork, but the fork dimensions are pretty standard so I can switch to a suspension fork if/when I want. A-C is 490mm and offset is 51mm.

Pretty much all rigid suspension corrected forks fall between 45mm and 51mm for offset these days, that's what I'd call standard.

My problem with your comments is you seem to think that HTA and offset, and thus trail have to be within a very small range of what you like. But not everyone rides like you, so geometries outside of that work too.

What do you consider acceptable offset for a bike with a 68 degree HTA?


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## Velobike (Jun 23, 2007)

bikeny said:


> I'm on a custom frame and fork, but the fork dimensions are pretty standard so I can switch to a suspension fork if/when I want. A-C is 490mm and offset is 51mm.
> 
> Pretty much all rigid suspension corrected forks fall between 45mm and 51mm for offset these days, that's what I'd call standard.
> 
> ...


I keep repeating this. My comment was based on the forks I knew to be readily available. That limits the choices of HA you can use with THOSE forks. It's nothing to do with my preferences.

It doesn't just happen with rigid forks, bike manufacturers face the same constraints with suspension forks. There are very few choices of offset, and that dictates the HA that bike manufacturers can use - unless they can convince the fork manufacturers to do a special run. There are plenty functional advantages to having slack HAs for the operation of a telescopic fork, but if you go too far that tends to be negated by the greatly increased flop.

Your bike sounds fine. With a 51mm offset a slack HA of 68º gets you a good trail figure. You have about 10mm more flop than any of my bikes, but that can be mitigated by where you position the rider's CoG within the wheelbase, ie the % weight on each wheel.

If you want to know my preferences, they are for very steep HAs (ie steeper than 72º), lots of trail and negligible flop. I established that from experimenting with various setups many years ago.


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