# Front-Center



## Clockwork Bikes (Jun 17, 2006)

In light of a recent thread with front-center talk I thought I'd bring this up. I don't design around front-center, it's just a resultant. I get the saddle and handlebars where I want them and then pick the HTA and rake to give me the ride characteristics I want.

Does anybody pick front-center first? What's your target? 

-Joel


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## Walt (Jan 23, 2004)

*I like 655mm*

I always design around front center (though I mostly talk to customers using effective toptube, since that tends to be more straightforward to explain). For me (5'11", 810mm saddle height @73 degrees, short little arms) I like around 655mm (for mountain bikes). On my old SS from 2007 or 2008, that was 71 HTA, 61cm effective toptube, 44mm rake fork for a front center at 653. My current singlespeed, to compare, is 69 HTA, 59.5 effective toptube, 44mm rake, for 655mm front center. I used an 80mmx0 stem on the old bike, and I use a 100mmx10 on the current one, for *about* the same reach.

Very different trail numbers, of course, but a very similar feel when trying to shift weight between wheels, which is what I wanted. I've always designed around front center, though for a lot of bikes with similar HTA, STA, and fork offset, it's pretty much the same as designing around effective toptube.

Customer bikes vary all over the map, but I'd say the typical XC 29er range is about 630-690mm. Just depends on the rider and the desired handling.

-Walt



Clockwork Bikes said:


> In light of a recent thread with front-center talk I thought I'd bring this up. I don't design around front-center, it's just a resultant. I get the saddle and handlebars where I want them and then pick the HTA and rake to give me the ride characteristics I want.
> 
> Does anybody pick front-center first? What's your target?
> 
> -Joel


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## j-ro (Feb 21, 2009)

Walt, thanks for chiming in. What determines FC for you? Is there a correlating physical dimension that nails it down or is it more like zones?


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## pvd (Jan 4, 2006)

I design around front center and I typically use 655 like Walt. It's great.

It isn't a resultant. It's a determining parameter. Say you choose a handlebar position and head angle, wheel diameter, and MT (via offset). You still need to choose a front center. That is adjusted using the stem geometry. That is the purpose of designing with the stem. Note that stems have gotten shorter over the years as front center has grown.

Front center is one of the more important parameters on a mountain bike.


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## pvd (Jan 4, 2006)

It's important to note that designing around front center is the exact reason that citing top tube length is such an innane thing to do. Top tube length is the result of the front center and not vis-a-vis.


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## JaquesN (Sep 14, 2009)

pvd said:


> Say you choose a handlebar position and head angle, wheel diameter, and MT (via offset).


Sorry, what's MT?


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## Live Wire (Aug 27, 2007)

JaquesN said:


> Sorry, what's MT?


mechanical trail


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## dr.welby (Jan 6, 2004)

pvd said:


> You still need to choose a front center.


How?


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## bee (Apr 7, 2008)

Clockwork Bikes said:


> In light of a recent thread with front-center talk I thought I'd bring this up. I don't design around front-center, it's just a resultant. I get the saddle and handlebars where I want them and then pick the HTA and rake to give me the ride characteristics I want.
> -Joel


I agree. The most common way to set it all up is to set the top tube length, head angle, fork length and rake. Then, the front-center is just a resultant figure.


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## MDEnvEngr (Mar 11, 2004)

*I'm like Joel here. Also, howzaabout the rear?*

Well, I've been doing it like Joel. And now I have some measuring to do to see where the FCs are.

Through trial and error, I have found that I like stems in the 100 - 110 mm range and have been designing around that. I feel that the longer stem puts more weight on the front wheel and requires less "pushing" on the inside grip to get the bike to go around corners. I've ridden some 29ers that just didn't like to turn until you pushed hard on the inside grip...then they would rail.

I'm thinking that I'll end up on the short end of Walt and PVD's 655mm number. What would you fellows say is an extremely short FC: 645? 640? I guess one needs some "boundaries" when figuring: as close to 655 without going shorter than a 50mm stem for shorties or longer than a 120mm stem for tallies.

In terms of the chainstay length, is there a correlation between saddle setback and CS length? I've been "winging" that too. Putting the riders saddle where it needs to go then adding or subtracting CS length to make the bike handle as I want it to. But perhaps going at FC backwards has made the handling random.

I've yet to build a "bad" handling bike. But a few were stunners and a bunch were fine handling. Again, some measuring to do.

This place is a fantastic resource! B


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## jfkbike2 (Feb 8, 2005)

Walt said:


> For me (5'11", 810mm saddle height @73 degrees, short little arms)
> -Walt


Damn you must have long legs for your height. I am 6'5" and use a 815mm saddle height (180mm cranks) but I know I am long in the torso. Never had a builder ask about my preferred F/C but I have always been aware of it and asked them what it worked out to be from the other dimensions... It does make a big difference.


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## Walt (Jan 23, 2004)

*Some random thoughts*

So, first off, I don't have a set formula for calculating the "right" front center. As usual, it's a mix of "science" and "intuition", which really means that I sort of fly by the seat of my pants.

Now, randomness:

-I think some folks have misunderstood me (and Pete) - 655mm front center is what I like for my *own personal (XC) bikes*. That's not what I try to make every mountain bike end up at!

-Designing around stem/TT length can work out great, if you assume every bike has the same HTA and fork offset. Once you start changing those things, if you're not taking front center into account, you could do something like this:
- Customer says "I want more stable steering at speed".
- You say "ok, we want a higher trail number. Since we're using a suspension fork with
44mm rake, we'll make the head tube angle 69 degrees instead of 71. Toptube and stem stay the same, since we want the same fit. Sweet. Good to go!"
- Bike ends up with a ~16mm longer front center - meaning that you've changed the relationship between the rider and the front wheel, made it harder to weight the front wheel when standing/descending, harder to get around switchbacks, etc. 
- If, on the other hand, you'd taken front center into account, you'd probably do something like this - slack HTA to 69 to get the trail number you want, then *shorten the effective toptube by 10-15mm* and have the customer use a longer stem to get the same rider position along with the same positioning of the wheels that you initially wanted. Bike is more stable at speed due to the higher trail number, but transferring weight works the way you wanted it to, and you haven't added 1/2"+ to the wheelbase.

-So the reason to use front center as an independent variable (along with the total reach from saddle-bars) and effective toptube/stem length as dependent variables is that it allows you to control for and understand the positioning of the front wheel relative to the rider, which is a pretty important thing in my book.

*Some general rules I use (not saying this is the "right" way to do it) for setting front center:*
-The taller the rider (saddle height is a decent proxy for how high the center of mass is going to be) *in general*, the longer the front center (and wheelbase) should be to keep from going OTB and having too much weight on the front wheel.
-Slower/twistier terrain often lends itself to shorter front centers and wheelbases. Likewise faster/less twisty can be better with a longer setup. 
-Rider style matters too - for riders who prefer to stay entirely on the ground and ride fast terrain, long is often good. For riders who want to spend a lot of time in the air and will wheelie-drop a ledge rather than roll it, shorter is usually better. 
-My own favorite "style" is to use relatively high trail numbers (85+mm) with fairly short front centers and wheelbases - you end up (IMO) with a bike that is relatively good at both high speed rough stuff and the twisties due to the shorter wheelbase. 
-In many cases these "rules" will contradict each other to some extent, so you've gotta make tradeoffs. C'est la vie.

I don't think any of those things are going to surprise anyone. Best way IMO to start *thinking* about front center so that it becomes intuitive is to look back at all your favorite bikes, and think about how they ride, and measure (or calculate) the front centers. Then you can kind of figure out a baseline (for you, anyway) and go from there.

Hopefully that awful word-salad was helpful to someone. And as I said, I'm not claiming that this is the "right" way to do a frame design. I think it's a more useful way to design than using effective TT and stem length the drivers of the front end length, but if you're mostly building bikes with very similar steering geometry, you can make that system work just fine as well. IMO if you're concerned with getting the rider on a good length chainstay (always a hot topic, and something that I think every builder considers carefully) for their size/style, you should be equally concerned with the positioning of the front wheel.

-Walt


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## Walt (Jan 23, 2004)

*Yup..*

I am by far the leggiest person I have ever met - 36"+ inseam at 5'11" tall. When I used to race for Schwinn in the late 90s, I'd get the XL frames (25.5" effective toptube, some ungodly huge front center) and run like 50mm stems in order to reach the bars. All of the smaller frame sizes were impossible for me to get the saddle high enough on. It was quite a revelation when I started building bikes for myself and realized that I didn't have to be riding something that long anymore.

Humorously enough, when Schwinn folded, I managed to get Dean to give me some free bikes for the next race season, and they said "well, we can do any custom geometry you want". Guess what I told them to do? Yup, copy the Schwinn. Clueless...

-Walt



jfkbike2 said:


> Damn you must have long legs for your height. I am 6'5" and use a 815mm saddle height (180mm cranks) but I know I am long in the torso. Never had a builder ask about my preferred F/C but I have always been aware of it and asked them what it worked out to be from the other dimensions... It does make a big difference.


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## DWF (Jan 12, 2004)

You'll guys will go insane trying to argue this. Top tube length, front center, it's all pointless IMHO for anything other than reference/comparative dimensions. For the record I do what Joel does. All you really need to care about is the distance from saddle to bars, i.e., cockpit length. After you clock the rider's position relative to the BB/cranks via seat tube angle, everything else just gets moved fore & aft to achieve the weight balance & handling characteristics you want.


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## jfkbike2 (Feb 8, 2005)

Walt said:


> Humorously enough, when Schwinn folded, I managed to get Dean to give me some free bikes for the next race season, and they said "well, we can do any custom geometry you want". Guess what I told them to do? Yup, copy the Schwinn. Clueless...
> 
> -Walt


I still have my first "'custom" bike made by Columbine. The damn thing must have a TT around 23" and a stem around 160mm or longer... They, nor I had any idea of popper fit for a MTB back then (1984 ish)... So it was the opposite of what you where doing with the Schwinn. I guess we have both met in the middle since then... :thumbsup:


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## Walt (Jan 23, 2004)

*Yeah, they both work*

I think as long as you're aware of where the front wheel is ending up (part of that whole "weight balance" thing) then it doesn't matter which way you think about it. What I don't like is just setting the effective TT, HTA/fork rake, and stem length without at least considering the front center effects, which is what I think some folks do.

The bottom line for me: you can really screw up the weight balance on a bike messing with the steering geometry if you don't keep track of the front center. So it's worth paying attention to.

-Walt



DWF said:


> You'll guys will go insane trying to argue this. Top tube length, front center, it's all pointless IMHO for anything other than reference/comparative dimensions. For the record I do what Joel does. All you really need to care about is the distance from saddle to bars, i.e., cockpit length. After you clock the rider's position relative to the BB/cranks via seat tube angle, everything else just gets moved fore & aft to achieve the weight balance & handling characteristics you want.


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## DWF (Jan 12, 2004)

Walt said:


> So, first off, I don't have a set formula for calculating the "right" front center. As usual, it's a mix of "science" and "intuition", which really means that I sort of fly by the seat of my pants.
> 
> Now, randomness:
> 
> ...


I like this post and am glad you clarified this as I was definitely misunderstanding your earlier post. It does illustrate the different approaches builders take to interpret customer requests though. If a MTBer told me he want more stability at speed, I would slacken the HTA, increase trail, & lengthen his effective front center as I would want push his weight bias more to the rear while increasing the trail reduces terrain induced steering deflections as does lengthening the wheelbase. Obviously there's a balancing act going on here, but IME, increased stability on MTB's never equals a longer stem for purposes other than climbing. YMMV.


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## Walt (Jan 23, 2004)

*Nice*

I am shocked that my gibberish even made sense. I agree in general - most of the time, if someone wants stable at speed, I'll BOTH lengthen the front center and increase the trail number.

There are some cases where I want to keep the front center/wheelbase short, though (say, for a small rider on the verge of toe overlap who is already on as long/longer of a bike than they need) and in that case, I'd increase trail/slack head tube AND pull the wheel back/shorten the toptube to keep things from getting too long.

-Walt



DWF said:


> I like this post and am glad you clarified this as I was definitely misunderstanding your earlier post. It does illustrate the different approaches builders take to interpret customer requests though. If a MTBer told me he want more stability at speed, I would slacken the HTA, increase trail, & lengthen his effective front center as I would want push his weight bias more to the rear while increasing the trail reduces terrain induced steering deflections as does lengthening the wheelbase. Obviously there's a balancing act going on here, but IME, increased stability on MTB's never equals a longer stem for purposes other than climbing. YMMV.


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## DWF (Jan 12, 2004)

Walt said:


> I am shocked that my gibberish even made sense. I agree in general - most of the time, if someone wants stable at speed, I'll BOTH lengthen the front center and increase the trail number.
> 
> There are some cases where I want to keep the front center/wheelbase short, though (say, for a small rider on the verge of toe overlap) and in that case, I'd increase trail/slack head tube AND pull the wheel back/shorten the toptube to keep things from getting too long.
> 
> -Walt


Brevity has its place but a lot of internet posters hide behind it, hence the preponderance of long drawn out discussions that end up as no more than pointless posturing. I do it myself when I'm feeling argumentative. :thumbsup:


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## shiggy (Dec 19, 1998)

Clockwork Bikes said:


> In light of a recent thread with front-center talk I thought I'd bring this up. I don't design around front-center, it's just a resultant. I get the saddle and handlebars where I want them and then pick the HTA and rake to give me the ride characteristics I want.
> 
> Does anybody pick front-center first? What's your target?
> 
> -Joel


I know how far in front of and above the BB I want my hands, and how far in front of my hands I want the front axle. Then I factor in the trail, followed by HTA and fork offset.


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## TrailMaker (Sep 16, 2007)

Walt said:


> I am shocked that my gibberish even made sense. -Walt


As a matter of fact;

I think it should be added to one or another stickie at the top of the forum. It was a very clear and comprehensive discussion of a somewhat nebulous subject that I had not seen mentioned previously. Thanks for taking the time to elucidate it in such a balanced fashion.


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## Blaster1200 (Feb 20, 2004)

Good job Walt!
Yes, I thought what you said was very easy to understand. I think this is one of the best posts I've seen from a builder explaining their design logic. When I was first looking to have a custom frame built, I knew that what I wanted out of a frame was very different than what was being done at the time, and I became very concerned that I wouldn't get out of the frame what I wanted, despite it being custom. Builders rarely ever gave any clue on what their design approach was, which I found a real turn-off. After all, as a potential custom-build customer, isn't the design what I'm really buying into? (ok, mostly) Since, at the time, I couldn't find any builder that was clear on their methodology, I turned to building my own frames (or that was one of the reasons).

Anyhow, thanks for the post and offering some valuable insight to your design process. And what a way to add value to this forum! :thumbsup:



Walt said:


> So, first off, I don't have a set formula for calculating the "right" front center. As usual, it's a mix of "science" and "intuition", which really means that I sort of fly by the seat of my pants.
> 
> Now, randomness:
> 
> ...


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## jay_ntwr (Feb 15, 2008)

TrailMaker said:


> As a matter of fact;
> 
> I think it should be added to one or another stickie at the top of the forum. It was a very clear and comprehensive discussion of a somewhat nebulous subject that I had not seen mentioned previously. Thanks for taking the time to elucidate it in such a balanced fashion.


Yeah, in all the "how do you figure out geometry threads" I've posted in/searched for/etc. this actually says something. Everyone else basically says that "it's experience" which is code for "I don't know any better than anyone else" most of the time I think.


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## Clockwork Bikes (Jun 17, 2006)

Great perspectives. Thanks, everybody.

-Joel


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## TrailMaker (Sep 16, 2007)

And so;

To carry it out a little further, I'm wondering if there could be some sort of ratio developed for TT -to- FC that would help people land where they want to with geo without having all that experience & feel for such "esoteric" ideas? This would not only help less experienced builders, but those seeking to be informed customers as well. Otherwise, in my case, I am left to simply copy another frame that I "think" works well. How many frames would one have to build to cultivate this type of understanding otherwise?

I'm not sure life will be that long for me...


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## pvd (Jan 4, 2006)

TrailMaker said:


> I'm wondering if there could be some sort of ratio developed for TT -to- FC that would help people land where they want to with geo without having all that experience & feel for such "esoteric" ideas?


Absolutely not. Front center is a very important dimention and top tube length is pretty meaningless and buried under several other important dimentions. Once you understand what is going on with front center you will never think about top tube length again.


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## TrailMaker (Sep 16, 2007)

pvd said:


> Absolutely not. Front center is a very important dimention and top tube length is pretty meaningless and buried under several other important dimentions. Once you understand what is going on with front center you will never think about top tube length again.


OK;

Given that, what is your method of arriving at your FC? What is your list of the most important dimensions, and how do you arrive at them?


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## pvd (Jan 4, 2006)

This has been covered on other threads.


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## febikes (Jan 28, 2011)

TrailMaker said:


> OK;
> 
> Given that, what is your method of arriving at your FC? What is your list of the most important dimensions, and how do you arrive at them?


It's been discussed but forum's rehash things....

In any case you can start with the fork you plan to use. The fork gives you the offset and will influence what you select for head tube angle. Once your head tube angle is locked in you think about how much wheel base you want in front and how much wheelbase you want out back. Ideally you get an idea of the rider's center of gravity and style and make logical choices that will give the bike the desired feel.


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## TrailMaker (Sep 16, 2007)

Thanks Mark;

Thanks for taking the time. Very helpful. While FC may be contained within other threads, it does not show in any thread title, and in my short time here I had never heard it mentioned. Now, it will not only come up directly, but there will be one explanation of how to create it as well.


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## vulture (Jan 13, 2004)

A full working knowledge of bicycle frame design will include the common and uncommon dimensions. I don't let any particular specific dimension drive the design. Make it fit, make it handle appropriate to its job, and use all of the tools in the box to help to that end. When you have build a lot of bikes, certain patterns appear, and results are repeatable. If you are comfortable with building using TT length STA, HTA, BB drop, and CS length, (with a given fork) you can measure FC and MT and find that they are what you want them to be. Keep it simple stupid is a great rule, if you use a measurement you don't understand, it isn't going to do you any good, but if you map your extra measurements your knowledge will increase. Walt described it best earlier, and he understands it because of the quantity of bikes he builds. PVD has a narrow but very technically correct view of most subjects but doesn't have to market his knowledge to sell bikes. I use FC and MT but I describe frames with by TTL and normal trail because thats what the public understands, and can make comparisons with.


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## DWF (Jan 12, 2004)

vulture said:


> A full working knowledge of bicycle frame design will include the common and uncommon dimensions. I don't let any particular specific dimension drive the design. Make it fit, make it handle appropriate to its job, and use all of the tools in the box to help to that end. When you have build a lot of bikes, certain patterns appear, and results are repeatable. If you are comfortable with building using TT length STA, HTA, BB drop, and CS length, (with a given fork) you can measure FC and MT and find that they are what you want them to be. Keep it simple stupid is a great rule, if you use a measurement you don't understand, it isn't going to do you any good, but if you map your extra measurements your knowledge will increase. Walt described it best earlier, and he understands it because of the quantity of bikes he builds. PVD has a narrow but very technically correct view of most subjects but doesn't have to market his knowledge to sell bikes. I use FC and MT but I describe frames with by TTL and normal trail because thats what the public understands, and can make comparisons with.


Bingo! Couldn't be better said.


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## Walt (Jan 23, 2004)

*Good points*

I find that when describing bikes to customers, it is generally easier to just talk about toptube length unless I'm doing something really unusual, as most folks aren't familiar with front center. Then again, A) I do a lot of fairly unusual stuff, B) most of my customers seem to be huge geeks, and C) I don't mind spending some time on the phone (or in person over a cold beer) explaining those sorts of details - I feel it's part of what they're paying me for, and the better they understand the variables, the better they can help me do a great design for them.

-Walt



vulture said:


> I use FC and MT but I describe frames with by TTL and normal trail because thats what the public understands, and can make comparisons with.


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## bee (Apr 7, 2008)

pvd said:


> Absolutely not. Front center is a very important dimention and top tube length is pretty meaningless and buried under several other important dimentions. Once you understand what is going on with front center you will never think about top tube length again.


Different perspectives then. I think about both, but do it the way Joel does it by considering TT length, HTA, fork length, and rake first and foremost. That is what is included in most geo charts from any manufacturer. Once I have that, the FC is calculated as a resultant figure in any CAD program. I can then tweak things as necessary. Both is important, IMO.


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## MDEnvEngr (Mar 11, 2004)

Well, I've only built 13 29ers over the past few years. Going at it with Joel's method, I've come up with a range of front centers: 625 - 675 mm.

The 625 was very short, but the design had a lot of set back (an excessive amount of setback). The bike was comfortable, and was the best cornering bike ever. But it was slow. I thought that it was the excessive saddle setback that made it slow.

The 675 was noticably long in front, and took me a few rides to get used to the front wheel being out there. But once I did, this bike was a great handler too.

The frame I built that had a 654 FC wasn't anything special. It handled fine, but did not stand out in my mind.

The bike I'm riding now and loving has a FC of 666.

The Carl Strong custom that I bought 6 years ago (and blame for this building "hobby" I've got going now) had a FC of 651. Interestingly, it was not a dimension he included on his pre-build drawings he sent to me.

Of course FC is only one dimension...

Another step. B


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## TrailMaker (Sep 16, 2007)

YEEEEEEEESSSSSSS!

I will freely admit that I was hoping to provoke just such sage fleshing out of this topic. Thank you one and all for this, and do continue if there is anything else that springs to mind. Everyone does things differently, and yet everyone learns, to some extent, from someone else. When you study the history of most things, you find that there are few truly new ideas, only revisions or refinements of old ones. No matter how successful we become of our own accord, that success, to various degrees, flows from those who followed that path before us.

Thanks in particular to *Wade*. As *DWF* suggests, this is a truly insightful look at how MANY processes really work. I know these things to be true, albeit from other realms. As different as many things can be, they are the same in some respects as well. A little here, and a little there, and a recipe will produce a terrific meal. How you end up with a bike that tastes great is less important than that it does, but well rounded understanding will help.

Science, method, and perspective come from experience, yet no amount of experience can rule every day, nor will it or should it preempt further learning. Those who pay that forward are often times the best among us, in my estimation.

Thanks again.


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## pvd (Jan 4, 2006)

febikes said:


> The fork gives you the offset and will influence what you select for head tube angle.


Absolutely not. This flawed logic should never be repeated. Head angle is not influenced by the fork offset. The head angle controls the steering rate and the offset controls the mechanical trail. They are two independent parameters.


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## vulture (Jan 13, 2004)

The bike I'm riding now and loving has a FC of 666.
[/QUOTE said:


> I can't believe you wrote this in public, now everyone knows the secret handshake.


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## febikes (Jan 28, 2011)

pvd said:


> Absolutely not. This flawed logic should never be repeated. Head angle is not influenced by the fork offset. The head angle controls the steering rate and the offset controls the mechanical trail. They are two independent parameters.


My understanding is that trail is a function of head angle, fork offset or rake, and wheel size.

But what the heck, I might be wrong.... LOL You may want to correct wiki for me and everyone else...
Bicycle and motorcycle geometry - Wikipedia, the free encyclopedia


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## pvd (Jan 4, 2006)

Mechanical trail is the length of the lever acting on the head axis. It connects the contact patch to the axis. Since it is invisible, we use those other dimentions to measure it.

Mechanical Trail = Wheel Radius * Sin (90 - Head Angle) - Offset

Mechanical Trail - Pvdwiki


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## febikes (Jan 28, 2011)

I think we have been speaking the same thing the whole times although we get trapped in the wording.



pvd said:


> Absolutely not. This flawed logic should never be repeated. Head angle is not influenced by the fork offset. The head angle controls the steering rate and the offset controls the mechanical trail. They are two independent parameters.


My earlier point was that you ideally want to select head tube angle in the context of the fork you intend to use.



pvd said:


> Mechanical trail is the length of the lever acting on the head axis. It connects the contact patch to the axis. Since it is invisible, we use those other dimentions to measure it.
> 
> Mechanical Trail = Wheel Radius * Sin (90 - Head Angle) - Offset


Exactly correct. As you point out the designer needs to know both head tube angle and fork offset to determine trail. Once you lock this down you can move on to consideration of the front center, chainstay length, and bottom bracket height in the context of the weight balance you want the rider to feel on the bike. The stem serves to give the rider proper fit and is selected later.

BTW, thx for PVD wiki; I use it all the time and find it to be very helpful.


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## pvd (Jan 4, 2006)

febikes said:


> My earlier point was that you ideally want to select head tube angle in the context of the fork you intend to use.


That was what I understood that you were saying. My point was that head angle is not changed to accomidate a trail that you are looking for. Head angle is selected based on the steering rate you wish. To alter the caster effect, you use offset. In the end it is far better to have the perfect head angle than to have the perfect mechanical trail.


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## DWF (Jan 12, 2004)

pvd said:


> That was what I understood that you were saying. My point was that head angle is not changed to accomidate a trail that you are looking for. Head angle is selected based on the steering rate you wish. To alter the caster effect, you use offset. In the end it is far better to have the perfect head angle than to have the perfect mechanical trail.


How do you change the offset of a suspension fork? And lacking the ability to do that, what other parameters do you have to work with? Also, what is the perfect head angle either as a combined or independent function?


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## bee (Apr 7, 2008)

pvd said:


> That was what I understood that you were saying. My point was that head angle is not changed to accomidate a trail that you are looking for. Head angle is selected based on the steering rate you wish. To alter the caster effect, you use offset. In the end it is far better to have the perfect head angle than to have the perfect mechanical trail.


I agree with febikes on this one. I assume your argument is that head angle should never be varied based on variables of the fork, such as length and offset. I don't believe that febikes is saying that the fork (length, offset, etc) completely dictates your head angle, but it does play a part in head angle. I don't think febikes would argue that head angle and variables of the fork, both, affect one another and thus, can be altered accordingly based on values of each other.


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## mack_turtle (Jan 6, 2009)

stirring the pot by reviving this thread. 10 years later, it appears that some of this discussion needs to be revisited. what wisdom has been gained on the topic in that time?


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## pvd (Jan 4, 2006)

Front center is a driving parameter.










ps. I love that my comments from 10 years ago have been proven entirely correct.


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## Ben.land101 (Dec 25, 2012)

mack_turtle said:


> stirring the pot by reviving this thread. 10 years later, it appears that some of this discussion needs to be revisited. what wisdom has been gained on the topic in that time?


The farther out we can get the front contact patch from the Center of Mass, the more stable the bike will be in the rough/steep stuff.
The only real way we can measure and compare that is by the Front-Center.

Bikes got longer, stems got shorter, head angles got slacker. All of that ultimately yields that same result.


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## stuartm2 (Mar 5, 2020)

pvd said:


> Front center is a driving parameter.


I'm sure I'm not alone in thinking it would be incredibly useful for us fledgling designers if you were to clearly list all of the driving parameters in your design process. I know you refer to them occasionally but I don't recall seeing a post on your wiki/blog or in this forum where you identify them all in a single, clear list for all to see and learn from. I guess we can start with:

Front center
...


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## pvd (Jan 4, 2006)

I did discuss them in this blog post: You can’t fix it in post. | Peter Verdone Designs

Also, again, in the documentation for the Skynet fixture: The SKYNET Master Plan | Peter Verdone Designs

The tricky bit is that I do use BikeCad to arrive at the actual seat tube angle. The saddle, seatpost, and some of the rear end geometries haven't been added to my calculations to produce the specific actual seat tube angle to result in the effective seat tube angle. It would be nice to see someone work this out as I'm pretty busy.

A recent push for a few changes in BikeCad 17.5 should make this process a lot simpler for others. I wasted a lot of time over the years for what is now automated: BikeCAD Pro 17.5 and BikeCAD FREE 17.5 | Peter Verdone Designs

Once you have that angle, the rest follows.

I am constantly refining my paradigm so little tweeks are added from time to time to improve it. I'm currently doing a full rebuilding of my process that will be complete in a few months. It probably won't look much different on the outside but the backend should be going a lot smoother. This will lead to new advances.

FYI, I just dropped a MTB frame to paint today. Hopefully, I get it back before the weekend and will have it posted next week. It's going to mess with a lot of people's heads.



stuartm2 said:


> It would be incredibly useful for us fledgling designers if you were to clearly list all of the driving parameters in your design process. I know you refer to them occasionally but I don't recall seeing a post on your wiki/blog or in this forum where you identify them all in a single, clear list for all to see and learn from.


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## stuartm2 (Mar 5, 2020)

pvd said:


> I did discuss them in this blog post: You can’t fix it in post. | Peter Verdone Designs
> 
> Also, again, in the documentation for the Skynet fixture: The SKYNET Master Plan | Peter Verdone Designs


Thanks, I'd missed those. So your list (taken from the Skynet docs) is:

Front centre
Rear centre
BB height
Head angle
ST angle & diameter
Front rim size
Front tyre height
Rear rim size
Rear tyre height, max height & gap
Fork Length, offset, travel & sag
Lower headset stack
Most of those make sense but I don't understand why the rim sizes are driving parameters - they don't appear to be 'driving' anything. Tyre height (+ width & gap on the rear) would seem sufficient. I'm also curious where fit measurements come into it. I'd assumed grip reach and stack (or RAD and RAAD if using that system) would play an important role in driving other dimensions. Rider inside leg and seatpost stickout would also seem to drive things like the seat tube length, although I suppose there's a fair bit of interplay between seatpost stickout and ST length during the design process to get the correct proportions for the bike.

And what about seat angle? Is that not driven by the need to place the rider in a comfortable seated position based on the reach and stack, rather than being a driving dimension?



pvd said:


> A recent push for a few changes in BikeCad 17.5 should make this process a lot simpler for others. I wasted a lot of time over the years for what is now automated: BikeCAD Pro 17.5 and BikeCAD FREE 17.5 | Peter Verdone Designs
> 
> Once you have that angle, the rest follows.


I was really impressed with this feature. It made a recent design I worked on so much easier to set up.



pvd said:


> FYI, I just dropped a MTB frame to paint today. Hopefully, I get it back before the weekend and will have it posted next week. It's going to mess with a lot of people's heads.


Cool. Look forward to seeing it.


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## pvd (Jan 4, 2006)

stuartm2 said:


> Front centre
> Rear centre
> BB height
> Head angle
> ...


Rim diameter and tire height are required to establish the radius of the actual wheel. That is needed for placing the base of the head tube and the seat tube offset.

There are several fit parameters that are used to establish touch points. The grip and saddle location as well as the actual crank radius. The placement of those depend on a variety of decisions and expertise but the numbers I give establish a sensible and repeatable system that define much of the bike. If you can show me a more sensible and efficient method I'd love to hear it.

You can see how that works below:









You can't use stack and reach as they are driven by other parameters. Frame reach isn’t a driving dimension | Peter Verdone Designs

I use a shorthand for my data plates as they are doing a very different job: Data Plates | Peter Verdone Designs


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## stuartm2 (Mar 5, 2020)

pvd said:


> Rim diameter and tire height are required to establish the radius of the actual wheel. That is needed for placing the base of the head tube and the seat tube offset.


Ok, I misunderstood your use of tyre height as height off the ground rather than height over the rim. We're arriving at the same dimension (tyre outer diameter) but I'm measuring it directly as I already have a pair of wheels/tyres of the size I want to run.



pvd said:


> There are several fit parameters that are used to establish touch points. The grip and saddle location as well as the actual crank radius. The placement of those depend on a variety of decisions and expertise but the numbers I give establish a sensible and repeatable system that define much of the bike. If you can show me a more sensible and efficient method I'd love to hear it.


Nope, those seem sensible.



pvd said:


> You can't use stack and reach as they are driven by other parameters. Frame reach isn’t a driving dimension | Peter Verdone Designs


I was talking about grip reach/stack, not frame reach/stack - you refer to it as grip position above. Grip and saddle positions seem like fairly immutable driving dimensions but I don't see them in either of your lists. Wondering why they're not considered 'driving' dimensions.


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## stuartm2 (Mar 5, 2020)

pvd said:


> ...


I figured this is going off-topic enough to justify starting a new thread. Join me?


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