# handling/geometry relative to frame size



## Tjaard (Aug 17, 2007)

This article has good information on SC Syndicate's focus on pressure distribution font and rear.
They are looking to balance the pressure front and rear for traction purposes.

When we look at larger size bikes, especially the (majority of) models that keep the same chain stay across sizes, the weight distribution changes.

As an example, I'll use my Trek Fuel ex, 23".

let's say, the bike handling was optimized for the L size.
The 'L' size bike (19.5") has a rear/wheelbase ratio of *2.85*. To get the same ratio on a 23" bike the chain stays would have to be *450mm long*.

The problem then of course is that the wheelbase would grow to be *1280mm long* as well, great for high speeds stability, but not so much for tight turns.

If, instead, you'd take a 7mm shorter offset fork, this would *reduce the front center* and *increase trail*.
Then you could *steepen the head angle* to bring trail back to where it was before, which would further *shorten the front center* and *lengthen effective reach*. Then you could *shorten the top tube*, while *maintaining the same effective reach*at the bars as the slacker bike.

New proposed geo for 23" size:
435mm chain stay (*5mm longer* than 19.5" size)
headangle *1 degree steeper*
fork offset *7mm shorter*
toptube *5mm shorter*
*reach remains* the same as current 23" size.
*trail remains* the same.
wheelbase is *10mm shorter* than current 23"
weigh distribution is similar to current 19.5" size.


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## alexbn921 (Mar 31, 2009)

Couple things here.

I want slacker HTA. This should be bike specific not size specific.

Chain stays should be longer as the bike gets bigger. Not ever going to happen with a carbon frame.

Reduced fork offset is only necessary if the rear end is too short or you are aiming for a certain speed of steering. 

You can't change real top tube measurements and keep reach the same. Steeper seat tube angle will, but your seat position is determined by BB.
Reach should increase pretty close to current standards. ie 20-25mm per size.

Why should trail remain the same? There is no magic number here. Style of riding has a lot to do with what feels comfortable.

My tallboy 3 XXL is 100mm longer than my blur XL. I kind of like the longer wheelbase and it fits me better. The blur does have unbelievable balance front to rear even if I feel on top of the bike. 

Weight distribution is key. Terrian and riding style come into place along with cockpit/suspension setup.

BB height should be variable as bigger frames run longer cranks.

I also like low handlebars so I'm glad most headtubes are short.


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## Len Baird (Aug 1, 2017)

YT uses longer chainstays starting at large size on their carbon frames. They also use short fork offsets, at least on their "Jeffsy" trail bike.


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## Tjaard (Aug 17, 2007)

alexbn921 said:


> Why should trail remain the same? There is no magic number here. Style of riding has a lot to do with what feels comfortable.


Agreed. I was going on the premise that bike designers optimize a bike for a certain riding style and that consumers choose the bike for that reason. So we are going with the premise that the amount of trail in the 'design size' was optimal. Of course, many people want something different, but that is not my point. I was talking solely about the handling differences between frame sizes.



alexbn921 said:


> I want slacker HTA. This should be bike specific not size specific.


I know YOU wanted more trail, this was speaking generically.



alexbn921 said:


> Chain stays should be longer as the bike gets bigger. Not ever going to happen with a carbon frame.


Most MTB's have alloy chain stays even on the carbon frames, so it would be possible on the 'carbon' frames just as easy as on the alloy frames. Just another reason I am bummed about all the carbon frames though...



alexbn921 said:


> Reduced fork offset is only necessary if the rear end is too short


That is exactly what I wrote above. Except that I offer reduced fork offset in larger frame sizes, along with modest rear lengthening, to keep wheelbase from growing exorbitantly, as opposed to solely lengthening rear centers.



alexbn921 said:


> Reduced fork offset is only necessary if you are aiming for a certain speed of steering.


Again, my point here is to aim for the same steering speed as the 'design size'



alexbn921 said:


> You can't change real top tube measurements and keep reach the same. \


You are right, my mistake thinking of it the wrong way. *Frame reach* would of course stay the same. However, when you steepen the head angle, the *effective reach* to the grips will increase (the effect is more pronounced the higher your grips are).



alexbn921 said:


> Why should trail remain the same? There is no magic number here. Style of riding has a lot to do with what feels comfortable.


Agreed, but again, I am discussing relative sizes. I am assuming that if a short buyer will want _XC bike Rcr_, with low trail, then so will a tall buyer. And that if a short buyer wants lot's of trail in the _new Ndro_, so will his tall buddy.

So again, my goal with this 'though experiment' was to equalize the handling of different frame sizes within a certain model.

I am not talking about modifying a bike to meet one's personal needs, I was talking about producing bikes that were 'size optimized'


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## Tjaard (Aug 17, 2007)

Len Baird said:


> YT uses longer chainstays starting at large size on their carbon frames.


Yes they do, and several other brands do lengthen the rear as well. However, they don't lengthen it enough to create the same weight distribution across sizes.



Len Baird said:


> They also use short fork offsets, at least on their "Jeffsy" trail bike.


Yep, and several other brands, like Whyte and Transition also use shorter fork offset. However, they re doing that across ALL frame sizes, simply to achieve a certain steering feel.

However, (as far as I know), no-one is doing what I was proposing above, which is that:

*Small sizes* would have *short rear centers, slack headangles and high fork offset.*

*Big sizes* would have *slightly longer rear ends, steep handangles and shorter fork offsets*.

This way, all sizes would have:
Identical Weight Distribution (for cornering)
Identical Trail (steering feel)
Identical stem length (steering feeling)
Apropriate increase in effective reach across sizes
Minimal wheelbase increase from S to XXL (to ride the same corners on the same trails)


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## alexbn921 (Mar 31, 2009)

I like where you are going with this tread.
Road bike change HTA on the larger sizes, but I don't feel that this a good way to make all bikes handle the same.
"Minimal wheelbase increase from S to XXL" I strongly disagree. Wheelbase should be proportional to the size of the frame. In fact it should grown faster then frame size to keep the balance of the bike.

Tallboy 3's grow 100mm front center and would need +30mm rear to get a similar balance.

We use longer cranks and wider bars because of our size too and these effect ground clearance and handling. Our CG is also higher which effect our leverage over the bike.

Big bikes are always going to handle differently from small bikes. I know you are trying to equalize them, but I think you are compromising the overall handling to get feel closer. Our leverage advantage might make more trail less noticeable too.


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## *OneSpeed* (Oct 18, 2013)

Geometry: There's always a delicate balance, and a trade off. 

I may have missed something but isn't that article about enduro/DH bikes? Weight distribution on that style of bike has little to do with your Fuel EX. 

I like short chainstays, long TT's, short stems, and high offset forks. I also like slightly higher BB's so I can have longer cranks. But then I'm talking about a bike that's pedal efficient and ridden on super tight single track with punchy climbs. 

It's also highly dependent on where you ride and what you want out of it. If I lived in a place that had open flow trails I would want a bike that was optimized for that terrain, likely with 29+. If I lived in a mountainous area with longer downhills and chairlifts I would want something different too. 

It's never as simple as changing just one or two measurements. I mostly agree with what Alex said. 

Every manufacturer has a different take on what is ideal for their target customers. There is no One Size Fits All. Every design change costs more money, so some manufacturers think it's worth spending the extra money to make subtle changes in geo and offer bigger size frames, and others cut costs by keeping it the same and only offering a few sizes.


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## scottzg (Sep 27, 2006)

This is totally my jam. I'm a hobbyist framebuilder with an interest in long travel hardtails. I've owned ~12 of them, 4 of which i built, one of them was a geometry mule where every aspect could be adjusted. I don't mean to sell myself as an expert, because i am NOT. Just a hobbyist with an interest.

I think if you're riding hiking trail style singletrack you start to run in to issues at around 1220mm of wheelbase. It gets harder to fit the bike around narrow sweeping turns around there, so extra length has to be added judiciously (it's not a cutoff, just when you need to start considering it). Up until 1220mm wheelbase is not worth worrying about fitting on the trail, but it's not hard to run in to that number on progressive trailbikes. For me at 6'3 i like my trail bikes ~1210 and my enduro bikes ~1240... but i could go a lot longer if the terrain warranted it or i was more concerned about carrying maximum speed... or shorter if i don't need that stability or margin for ****ups.

Longer chainstays are good, but they're on a 4:1 ratio with front center. IE 40mm of extra FC due to sizing= 10mm of chainstay length, and that's assuming you're not adding extra FC as a design feature. So... longer chainstays are good, but a little goes a long way.

Seat angle is meaningless on little bikes, but it matters for us. Same goes for shock pressure- our seated vs standing weight distribution is often wacky compared to our smaller compatriots. Don't be afraid to use volume reducers, especially on shocks where you can't tune the compression damping.

Head angle and fork offset are more about steering feel than sizing. I adapt to whatever; i have my preferences but it's not that important. It's more important in how it affects your front center and/or reach.

You haven't mentioned it, but at XL+ size we can bend the hell out of a frame. I've started building my own hardtails with comically heavy tubesets... and they're really not overstiff at all.


Tjaard, your preferences imply you think taller riders are more rear-heavy, and i don't really agree with that.




The last comment is that once you have a design that is close to optimal tiny changes are magnified. ....It may be that you have a highly refined **** design though.


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## telejefe (Mar 28, 2007)

Any thoughts on running a 46mm on a trek fuel ex 23"? Looking at the McQueen fork.


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## alexbn921 (Mar 31, 2009)

telejefe said:


> Any thoughts on running a 46mm on a trek fuel ex 23"? Looking at the McQueen fork.


Go for it. Steering will be 8% heavier when leaned over and the bike will be slightly more stable. It's easy to adapt too.


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## TooTallUK (Jul 5, 2005)

Thing is, there's little enough profit in us bigger riders anyway. Once you get into a frameset adapted at every size you get into way less return the further away from M & L frames. Around this forum, my 6'7" height isn't all that outrageous, but I am taller than 99.9% of the people one the planet and that's not a great sales point!


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## RAKC Ind (Jan 27, 2017)

Personally i dont want my chain stays longer just cause im taller. To get preferred front end feel and fit the wheelbase gets long enough for tight trails (still a bit longer than Id like) with long front end, sure dont want longer stays to match. I just move my body to adjust weight distribution. My newest bike has the shortest stays Ive ever had, longest ett but still slightly shorter wheelbase. Able to run short stem, wide bars, fits well and most nimble bike Ive had.

I hate the idea of longer stays just to make bike more "balanced". My seat position and using body english takes care of it. I like my bikes light in the front, easier for me to keep front down or use the light front to pop front tire over in short, tight, techy climbing.

But thats me.

Sent from my SM-G950U using Tapatalk


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## Ryder1 (Oct 12, 2006)

Re: the "longer stays" issue, I think choosing among 29ers largely solves the problem. Most have 17" or more. This is one of main reasons I ride 29ers. With a 36.5" inseam, I want 17-17.5 for a geared bike. If I had to choose among 27.5 bikes, my choices would be limited. I do find current 29er stays getting a bit shorter than I'd like, but my saddle position is pretty unique.

Re: BB height, we should be happy. Shorter folks should be on lower bikes with 165-170 cranks, but are sold bikes with higher BB b/c of us, IMO. Not so much anymore, but still a bit.


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## scottzg (Sep 27, 2006)

Ryder1 said:


> I do find current 29er stays getting a bit shorter than I'd like, but my saddle position is pretty unique.


FWIW, i have a knee injury that limits its range. 5 years ago i needed ~150mm of saddle setback. I've been slowly inching it forward and now i can handle about 125mm of setback before it starts to aggravate it. Since bikes are all built around steep seating positions now i'm glad i did.



RAKC Ind said:


> Personally i dont want my chain stays longer just cause im taller. To get preferred front end feel and fit the wheelbase gets long enough for tight trails (still a bit longer than Id like) with long front end, sure dont want longer stays to match. I just move my body to adjust weight distribution. My newest bike has the shortest stays Ive ever had, longest ett but still slightly shorter wheelbase. Able to run short stem, wide bars, fits well and most nimble bike Ive had.
> 
> I hate the idea of longer stays just to make bike more "balanced". My seat position and using body english takes care of it. I like my bikes light in the front, easier for me to keep front down or use the light front to pop front tire over in short, tight, techy climbing.
> 
> ...


I don't disagree with you, but have you ever had a bike with horizontal dropouts? It's striking how moving the wheel 3-5mm can affect how a bike corners and climbs without really affecting anything else, once you have the cockpit sorted.

Or like, you know how most any bike, if you sit up straight and stomp the pedals the front end comes unweighted? That is not how it is for smaller riders, i'm told. Yah, we adjust our riding style to adapt, but i only have so much range where i can shift my body and still be effective.

I don't really think it NEEDS to be longer chainstays for larger sizes, but small changes in chainstay length are more effective than pushing the rider's CoM forward... so it seems like the most sensible way to tweak that balance... or maybe use both tools. If nothing else, seeing variable length chainstays (or varying seat angles) is an indication that the designer prioritized handling over ease of production, so that's cool.


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## Pitch (Aug 9, 2008)

Not a frame builder, but a big guy (6'9") who has ridden for 25 yrs on bikes of varying geometry . My favorite handling hardtail of all time is a steel SS with 457mm (+/- due to Paragon slider dropouts) rear chain stays (measured from BB center to axle center). It carries a 68.5* HA. Wheelbase is 1232mm (+/-) depending on slider position. It is long. But, being such a tall bike, I feel the longer WB is essential to stability. The idea of a steeper HA seems completely backwards to me. Being as tall as some of us are, when riding a 100/120mm X-C style fork, that relative HA is already going to increase by 4* when I compress that fork coming downhill. Descending with a 72.5* HA now becomes super sketchy on a rider that is already markedly higher, with a center of mass that is sometimes a foot higher (and much heavier) than the average rider. This equates to exceptionally poor stability. I see your arguments for a steeper HA possibly holding more water with a road bike, but the dynamic nature of MTB suspension seems to undermine it. This bike was made with intentionally longer chain stays than any other bike I have ridden. It is the best descender of any of the 26/29" bikes I have ever ridden; hardtail or full suspension. For me, the slacker HA, longer CS's, and long WB are the reason why it is so stable. I value stability far more than short-chainstay flickability in a bike. I'd bet most taller xc riders would agree.


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## Tjaard (Aug 17, 2007)

This is great guys! So many useful experiences and interesting thoughts. I wish we could get together over drinks with a white board!


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## Tjaard (Aug 17, 2007)

alexbn921 said:


> I like where you are going with this tread.
> I strongly disagree. Wheelbase should be proportional to the size of the frame. In fact it should grown faster then frame size to keep the balance of the bike.
> 
> Tallboy 3's grow 100mm front center and would need +30mm rear to get a similar balance.
> ...


So your thought is 'scaling' where each dimension grows? That is also an interesting strategy, and one that is put into practice by Lennard Zinn, where his bikes raise the BB, but then use longer cranks to compensate. That only applies to seated climbing though: when out of the saddle(level cranks), the center of gravity is still raised, and in fact, even more, since the riders legs are also longer. 
So I agree, if you are going that route, you need to scale the other dimensions as well, to keep it from becoming unstable.

However, trails are not built by tall riders, for the 'scaling' strategy to work perfectly, the trails would need to be scaled up as well. There are of course places where the local trails are larger: wider corners, longer rollers bigger rocks etc, but most of the time whether it is purpose built trails made by smaller bikers or old hiking trails, to big of a bike is more likely than to small.



alexbn921 said:


> Big bikes are always going to handle differently from small bikes. I know you are trying to equalize them, but I think you are compromising the overall handling to get feel closer. Our leverage advantage might make more trail less noticeable too.


My main point is that I believe current bikes do a poor job of adjusting to different sizes.

My suggestion is one option, your full-on 'scaling' is another, both have their merits. I believe that my suggestion does not compromise handling, since handling is mostly determined by the following:

Proper bike fit: if the bike doesn't fit you, you can't control it well or ride it comfortably or powerfully
Weight distribution: If your weight distribution changes, that significantly changes the handling
Wheelbase: This is were the 2 strategies diverge: mine suggests keeping the handling as similar as possible, as far as tight trail features, yours to keep the stability as similar as possible.


I agree that bigger riders(with wider bars and more weight) might possibly feel better with slightly more trail.


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## Tjaard (Aug 17, 2007)

*OneSpeed* said:


> Geometry: There's always a delicate balance, and a trade off.


That is my point. Current bikes sizing, (increasing only front center) changes only toptube length, seriously changing that delicate balance.



*OneSpeed* said:


> I may have missed something but isn't that article about enduro/DH bikes? Weight distribution on that style of bike has little to do with your Fuel EX.


That article is indeed about full on, world cup DH bikes. Weight distribution on the descent is still the same as for a trail bike. The only difference is the DH bike doesn't have to compromise for the climb. If anything, a DH bike needs a more rearward weight bias, since it is always ridden angled down. Trail bikes on the other hand are also ridden angled up or flat, so they normally have a more forward weight bias. 
So the point of the article was that Greg Minnaar found that he needed to lengthen the rear to keep proper front-back weight distribution to weight the front wheel on corners.



*OneSpeed* said:


> I like short chainstays, long TT's, short stems, and high offset forks. I also like slightly higher BB's so I can have longer cranks. But then I'm talking about a bike that's pedal efficient and ridden on super tight single track with punchy climbs.


Adding all those features up leads to an extreme rear-biased weight distribution. Most people would want more weight on the front for seated climbing, and everyone has better cornering traction with more weight on the front.

But anyway, my point wasn't to pick our individual preferred geometry, but to see how bike sizes should be scaled across sizes.



*OneSpeed* said:


> It's never as simple as changing just one or two measurements.


Agreed, which is what current designs (except Zinn)mostly do: they lengthen top tube only. A few lengthen the rear by a tiny bit. So that is changing 1, or maybe 2 measurements. I propose changing HA, offset, chain stays and top tube, that's 4 measurement, and changing them not randomly, but in carefully chosen amounts to balance out.


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## *OneSpeed* (Oct 18, 2013)

Tjaard said:


> Adding all those features up leads to an extreme rear-biased weight distribution. Most people would want more weight on the front for seated climbing, and everyone has better cornering traction with more weight on the front.


I'll use the Kona Honzo as an example. Yes, super short chainstays, but a quite steep Seat tube angle, which keeps your weight more forward. Also possibly the longest reach in the industry which keeps your weight forward.

I've only had one out on a trail twice, but It wasn't what I expected. I was expecting a bike that was too focused on going down, in reality it was really well balanced. The geo numbers on that frame seem to go to extremes by comparison to other bikes on the market, but when riding one it's felt great and not extreme at all.

I'm curious enough that I think I've decided to buy a Honzo ST frame this winter despite recently building a custom frame that isn't all that different. (it is different enough though) I'm not sure if it's a long term investment yet, but I know I want to spend more time with that geometry.


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## Tjaard (Aug 17, 2007)

*OneSpeed* said:


> Honzo: super short chainstays, but a quite steep Seat tube angle, which keeps your weight more forward.


Only while seated. Most of the 'interesting' riding you're out of the saddle. Seat angle doesn't affect standing weight distribution.



*OneSpeed* said:


> Also possibly the longest reach in the industry which keeps your weight forward.


Longer reach (with the same HA) only keeps your weight forward while seated, _if_ you move the saddle forward a corresponding amount. Standing weight distribution actually becomes more rear biased, since the front wheel is now further away.



*OneSpeed* said:


> I'll use the Kona Honzo as an example


Let's say that the XL Kona Honzo is the perfect geometry, well then the S is not! 
Since they only change the top tube, the weight will be much more forward on the S, and it will be much harder to loft the front wheel up, among other things.

What I mean to say is that I know there are different preferences in geometry, that is not the point. My point is that if a frame designer has optimized geometry for the intended use of the bike in size M, then how should it be changed across sizes?

So one reason for this thread was for (custom) frame builders to think about sizing. The other is to help us tall folks choose/modify a stock bike to suit our needs. 
If you can't test ride a bike (and how many of us can? Can't remember the last time there was an XXL at a demo or in a shop), then we look at reviews, but my point is to remember that* the reviewed bikes (usually M or L) will handle very differently from the sizes we end up buying*.
So hopefully this thread will give people some thought about how sizing affects handling and what they would prefer.


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## Tjaard (Aug 17, 2007)

telejefe said:


> Any thoughts on running a 46mm on a trek fuel ex 23"? Looking at the McQueen fork.


This is what led me to start this tread. Alex had posted on another thread about installing shorter offset fork on his XXL bike. I have a
Fuel EX 23" also.

So if you did this you would end up with more trail. If you feel you already had enough trail most of the time, you could run the bike in the high setting(assuming you were running low before). This would steepen the HA, bringing trail back closer to where it was with the 51mm offset in the low setting. It would also increase reach and raise the bottom bracket (so you could run longer cranks).
Combined, all these changes would also bring the front wheel a fair bit closer, increasing weight on the front wheel.


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## Tjaard (Aug 17, 2007)

Pitch said:


> ..carries a 68.5* HA. The idea of a steeper HA seems completely backwards to me...when riding a 100/120mm X-C style fork, that relative HA is already going to increase by 4* when I compress that fork coming downhill. Descending with a 72.5* HA now becomes super sketchy


Absolutely, in that case you would have negative trail*.
I was advocating steepening the HA *only* as much as to compensate for the _*decreased offset*_. So *trail*(both with extend and compressed fork) would *stay the same*. (actually reduced offset increases trail more on steep(compressed fork) head angles than slack ones, so the short offset with same trail unloaded would actually have longer trail under full compression than the slacker head angle with larger offset.

Also, I was thinking one modern bikes with head angles well under 68 degrees for bikes that will be ridden on steep terrain. Your point is more about having sufficient trail, period, not about relative geometry for sizing.



Pitch said:


> For me, the slacker HA, longer CS's, and long WB are the reason why it is so stable. I value stability far more than short-chainstay flickability in a bike. I'd bet most taller xc riders would agree.


As you can see from the comments above, many (but not all!) tall riders do value 'flickability'. It is also important to note that your bike is still pretty short, with a wheelbase of 1231 it is about 20mm shorter than my bike (which has much shorter chainsatays). So, I agree, there is not much reason for you to go shorter in wheelbase. This issue is more for bikes pushing to 1300mm, which is where we'd end up with slack HA's, high fork offsets and long chain stays to balance out weight distribution.

*Explained well here in this interview with Ian Alexander about the new Whyte S150:
"If you take the very worst-case scenario, there is a very real possibility that despite having over 100 millimeters of positive trail, a 150-millimeter-travel bike could use up 95 millimeters of trail and then exhibit negative trail in certain extreme riding circumstances. Negative trail is very disruptive, as the restoring forces acting at the tire's contact patch actually amplify the instability of the steering and the bike as a whole"​


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## alexbn921 (Mar 31, 2009)

I love geeking out on this stuff. I have 3 generation's of the best full suspension bikes made that I ride weekly.
First is an XL 1997 Cannondale super v xtr. This is my around town tow the kids bike
100mm rear, 60mm headshock, short stays, short front center, steep HTA, 170mm stem!
Weight is way to far forward and the stem is a tiller. You swing the whole front of the bike side to side and it turns from the middle. Climbing is limited as you sit up and over the rear wheel without enough weight on the front to keep it down. Decending is scetchy as the front wheel feels like it's behind your knees. Still the suspension works and the bike has a solid nimble feel to it.

Next is an XL 2008 Santa Cruz Blur XC carbon xx1. 22.5lb race bike converted to 27.5.
100mm rear 120mm front. Super short stays, average front center, balanced STA, HTA 90mm stem.
This was my main bike is extremely well sorted. Handles everything with incredible balance and composer. Bike is to small so the seat was slid back 10mm and the stem should be a 100mm. Climbing I'm to far back over the pivot of the rear axle. This makes seated climbing impossible over 23ish%. I love this bike and can drift both wheels while maintaining great feel at both ends. With all that being said I ride on top of the bike and it is too small for me.

New bike is an XXL Santa Cruz Tallboy 3. Setup as an all mountain bike with a burly build.
110mm rear. 130mm front. Short stays for a 29er. Super long front center. Same STA and slacker HTA, 50mm stem. This bike is stiff! It also fits me for the first time ever. I have the seat in the correct place for best pedaling and have the perfect reach with the 50mm stem. Climbing is limited by traction and power. Front never lifts or wanders. Decending, the steering is direct and the front wheel feels out front. Over the 10 months I've been adapting my style to shift forward and put more and more weight on the front. Still in the flater corners the front wants to push. This push reduce's the trail to zero flopping the front end. I'm also not in the perfect position with my weight shifter all the way forward. Some of this is technic, and some of it is balance of the bike. 
I recently installed a 44 offset fork to bring the front end in and it made the bike feel much shorter. So much so that I'm having to shift my weight back to the middle of the bike to compensate. It's different, but it's faster even if I'm not used to it yet.
The long wheelbase doesn't bother me at all. More later.


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## Tjaard (Aug 17, 2007)

scottzg said:


> Longer chainstays are good, but they're on a 4:1 ratio with front center. IE 40mm of extra FC due to sizing= 10mm of chainstay length, and that's assuming you're not adding extra FC as a design feature. So... longer chainstays are good, but a little goes a long way.


It's actually around a 2:1 ratio: 1200mm wheel base, 400mm chain stay, 800mm front center.



scottzg said:


> Head angle and fork offset are more about steering feel than sizing. I adapt to whatever; i have my preferences but it's not that important. It's more important in how it affects your front center and/or reach.


It's exactly because they affect all of those at once that I suggested decreasing offset, increasing head angle: This keeps trail (steering feel) the same, while shortening front center and (slightly) increasing effective reach.



scottzg said:


> Tjaard, your preferences imply you think taller riders are more rear-heavy, and I don't really agree with that.


If you mean that riders on big bikes' weight is more biased on the rear wheel than the front wheel(compared to smaller bikes), than yes, that is what I meant. That's just geometry. If you put someone on a board supported by 2 blocks, let him stand 1 foot from one end. If the board is 2' long his weight will be evenly divided on both blocks. If it's a 5' board, only 20% of their weight will be on the front block.

I did not mean that tall riders have a more rearward center of gravity relative to their feet. Maybe they do, maybe they don't no clue.

Or did you mean tall riders have fat asses ;-)


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## scottzg (Sep 27, 2006)

Tjaard said:


> It's actually around a 2:1 ratio: 1200mm wheel base, 400mm chain stay, 800mm front center.


Yah, that's how the math works, but i don't think it bears out that way in practice. Dunno why, here's some guesses-

-taller people have a relatively higher CoG (since most of weight is in our torso), which is further forward when they're in bike position.
-weight distribution in the attack position is heavily biased towards front triangle geo.
-the rider is pushed forward in the seated position and not centered over the pedals, so higher saddles push your CoG further forward.

I don't really know. One of the first frames i built CS/TT length, head/seat angle, and BB height were fairly adjustable. When i extended the front-center of the bike X, i need to extend the rear .3-.4 X, (depending on how it was achieved) to keep the bike's handling balanced. I don't really understand why, but but i've explored it a bit.

Edit- I see where you're coming from.


Tjaard said:


> If you mean that riders on big bikes' weight is more biased on the rear wheel than the front wheel(compared to smaller bikes), than yes, that is what I meant. That's just geometry. If you put someone on a board supported by 2 blocks, let him stand 1 foot from one end. If the board is 2' long his weight will be evenly divided on both blocks. If it's a 5' board, only 20% of their weight will be on the front block.
> 
> I did not mean that tall riders have a more rearward center of gravity relative to their feet. Maybe they do, maybe they don't no clue.
> 
> Or did you mean tall riders have fat asses ;-)


That's not what you're doing on a bike though; on a bike you're leaning forward and holding yourself in position with your core, with your hands supporting a little weight. No matter how tall you are you carry your weight somewhere near where the BB is.

Sit up on the saddle and ride no hands.... that goes out the window.

Cheers good thread!


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## Tjaard (Aug 17, 2007)

RAKC Ind said:


> My newest bike has the shortest stays Ive ever had, longest ett but still slightly shorter wheelbase.


What fork offset do you have? 'Standard' or shorter? If you've got short chain stays and a long reach, the only way to make the wheelbase shorter is to decrease fork offset or steepen head angle. If you shorten the wheelbase by steepening the head angle with ra 'standard offset' fork, trail will be reduced significantly.



RAKC Ind said:


> Personally I dont want my chain stays longer just cause I'm taller


I don't _want_ them longer because I'm taller. But I do want the same weight distribution as short riders. I actually like short chain stays for their own sake. This is why I suggested a concept that shortens front center as well, in order to *minimise* *lengthening* of rear center and wheelbase.



RAKC Ind said:


> I hate the idea of longer stays just to make bike more "balanced". My seat position and using body english takes care of it.


Using body english to correct a weight distribution issue, means you are adapting your body to fit the bike, not the other way around. The article I led with describes Greg Minnaars' request for a bike that matched his body better. I think we can safely assume that if there was a 'good' technique for riding a bike with rear weight bias, Greg Minnaar would be able to execute it.



RAKC Ind said:


> I like my bikes light in the front, easier for me to keep front down or use the light front to pop front tire over in short, tight, techy climbing


Let's assume your bike is set up prefect for the style of riding you do. What about a short friend that wants to ride with you? You could't recommend the S size of your same bike because the front tire would be to heavy right?

What I am asking is your thoughts on how to scale bikes for different frame sizes, while keeping the desired riding style the same.


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## Tjaard (Aug 17, 2007)

scottzg said:


> Yah, that's how the math works, but i don't think it bears out that way in practice. Dunno why, here's some guesses-
> 
> I don't really know. One of the first frames i built CS/TT length, head/seat angle, and BB height were fairly adjustable. When i extended the front-center of the bike X, i need to extend the rear .3-.4 X, (depending on how it was achieved) to keep the bike's handling balanced. I don't really understand why, but but i've explored it a bit.


Well I have a few ideas: First of all 0.4X increase in the rear is not that far off from my rounded of number of 1:2. The actual number for my bike is around 1:1.9 ratio. My point was just that it's not anywhere near 1:4 which was what you wrote originally. That would be 0.25X.

Then there is the fact of how you measured it: actual lengthening of chain stays vs the horizontal, effective rear center increase. Perhaps a few angles or effective reach changed on the bars, leading to a bit of weight shift right there. Any way just a few thoughts on the difference between your testing and my mathematical model.



scottzg said:


> That's not what you're doing on a bike though; on a bike you're leaning forward and holding yourself in position with your core, with your hands supporting a little weight. No matter how tall you are you carry your weight somewhere near where the BB is.
> 
> Cheers good thread!


I'm confused as to what you disagree with?
What you say above is exactly my point, that center of gravity is above the bottom bracket ('heavy feet light hands'). Sounds to me like you agree with me that weight distribution is the ratio of front:rear center, and since front center grows while rear doesn't as bikes get bigger, weight distribution moves rear ward for (tall)riders on bigger frame sizes.


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## scottzg (Sep 27, 2006)

Tjaard said:


> Well I have a few ideas: First of all 0.4X increase in the rear is not that far off from my rounded of number of 1:2. The actual number for my bike is around 1:1.9 ratio. My point was just that it's not anywhere near 1:4 which was what you wrote originally. That would be 0.25X.
> 
> Then there is the fact of how you measured it: actual lengthening of chain stays vs the horizontal, effective rear center increase. Perhaps a few angles or effective reach changed on the bars, leading to a bit of weight shift right there. Any way just a few thoughts on the difference between your testing and my mathematical model.


Durr, i can't math. Let's just say that i agree that RC needs to be lengthened along with FC, and it's less than 1:2 and more than 1:4. Within those bounds it probably comes down to the bike and rider in question.



Tjaard said:


> I'm confused as to what you disagree with?
> What you say above is exactly my point, that center of gravity is above the bottom bracket ('heavy feet light hands'). Sounds to me like you agree with me that weight distribution is the ratio of front:rear center, and since front center grows while rear doesn't as bikes get bigger, weight distribution moves rear ward for (tall)riders on bigger frame sizes.


Lol i don't know. I just know that the massive increases in CS length you're suggesting didn't work in testing. Small changes to chainstay length disproportionately affect handling.

I think it's that in the attack position the rider's CoG moves upward and forward. Seated- yes you're right, but i don't think it matters so long as the rider's CoG doesn't move too close to the rear balance point.


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## Rugerfan777 (Sep 7, 2017)

scottzg said:


> This is totally my jam. I'm a hobbyist framebuilder with an interest in long travel hardtails. I've owned ~12 of them, 4 of which i built, one of them was a geometry mule where every aspect could be adjusted. I don't mean to sell myself as an expert, because i am NOT. Just a hobbyist with an interest.
> 
> I think if you're riding hiking trail style singletrack you start to run in to issues at around 1220mm of wheelbase. It gets harder to fit the bike around narrow sweeping turns around there, so extra length has to be added judiciously (it's not a cutoff, just when you need to start considering it). Up until 1220mm wheelbase is not worth worrying about fitting on the trail, but it's not hard to run in to that number on progressive trailbikes. For me at 6'3 i like my trail bikes ~1210 and my enduro bikes ~1240... but i could go a lot longer if the terrain warranted it or i was more concerned about carrying maximum speed... or shorter if i don't need that stability or margin for ****ups.
> 
> ...


I can tell you that taller riders are more rear heavy. I am 6'4 and 283. I hade back tire weight measured whem I was on the bike and 200lbs was on the rear wheel and 83 lbs on the front wheel.


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## jeremy3220 (Jul 5, 2017)

I've noticed some manufacturers will increase stem length based on frame size. Giant does this but keeps the same bar width across sizes. Most people consider stem length to be about handling more so than sizing and bar width more related to rider size than stem length. 

I haven't sat down and compared frame dimensions but it makes wonder if manufacturers that use a longer stem for larger bike sizes are skimping out a bit on lengthening the frame as much as they should.


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## scottzg (Sep 27, 2006)

jeremy3220 said:


> I've noticed some manufacturers will increase stem length based on frame size. Giant does this but keeps the same bar width across sizes. Most people consider stem length to be about handling more so than sizing and bar width more related to rider size than stem length.
> 
> I haven't sat down and compared frame dimensions but it makes wonder if manufacturers that use a longer stem for larger bike sizes are skimping out a bit on lengthening the frame as much as they should.


It's their way of saying they're completely out of touch. It's a holdover from 80s road bike sizing where a 42" wheelbase was a racing bike and 44" was a tourer.


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## Tjaard (Aug 17, 2007)

jeremy3220 said:


> I've noticed some manufacturers will increase stem length based on frame size. Giant does this but keeps the same bar width across sizes. Most people consider stem length to be about handling more so than sizing and bar width more related to rider size than stem length.


Agree with you that the current approach is that stem length is a big part of handling, so should not be used for sizing purposes. This is why the new 'long' trend in frame reach is so great for us tall folks, we are finally getting bikes where we can *both* run a short stem, have a steep enough seat angle _*and*_ be comfortable, not scrunched up.

There is something too be said for a longer stem though too. If the chain stays remain the same, taller riders' weight will be more biased to the rear than shorter riders' weight. Using a longer stem balances that out.
However, the difference in body size is so big that the handling would be horrible if you only used the stem to compensate.

My feeling is that a tiny little bit of stem length variation is probably useful for proper weight distribution, without having a huge effect on handling.

Maybe 35mm for the S, and 60mm for the XXL?



jeremy3220 said:


> ... it makes wonder if manufacturers that use a longer stem for larger bike sizes are skimping out a bit on lengthening the frame as much as they should.


I don't see how it would be (much) more expensive to make a frame 10mm longer, so I don't think cost saving is the reason they would do it, but maybe just a feeling that bikes would be "too big" or "too long". Designers are most likely not very tall themselves, and even if they are, they are used to seeing smaller sized bikes.

As far as keeping handlebar width the same, I agree with you that that is mostly size dependent. However, I think speccing identical(wide) bars on all sizes is a good move, as long as they can be cut down. If they can't be cut down, perhaps a wide bar on the L-XXL, and a slightly narrower one on the XS-M?


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## Tjaard (Aug 17, 2007)

YT uses short fork offset, slightly longer chain stays in larger sizes and a relatively steep head angle.

In short, they are making the bike I describe. Ironically, however, they don't make an XXL, and even their XL is super short by current standards (465mm reach on the Jeffsey 29).


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## jeremy3220 (Jul 5, 2017)

Tjaard said:


> There is something too be said for a longer stem though too. If the chain stays remain the same, taller riders weight will be more biased to the rear that shorter riders. Using a longer stem balances that out.
> However, the difference in body size is so big that the handling would be horrible if you only did that.
> 
> My feeling is that a tiny little bit of stem length variation is probably useful for proper weight distribution, without having a huge effect on handling.


Yeah, I'm sure it varies by frame. I've switched to 787 mm bars and most people running bars that wide are running around a 35 mm stem but I'm probably going to stick with 50 mm so I have enough room and can keep the front down on climbs.


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## TooTallUK (Jul 5, 2005)

Tjaard said:


> I don't see how it would be (much) more expensive to make a frame 10mm longer, so I don't think cost saving is the reason they would do it, but maybe just a feeling that bikes would be "too big" or "too long".


It's economies of scale in manufacturing. The more you make from one jig in one size the more return you get on your investment and design. Us 'big guys' are such a tiny proportion of the bike market that there just isn't the economies of scale in such nuanced frame design at the extremes of fit. Bikes are made to fit average people because there's more of them to sell to.


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## Tjaard (Aug 17, 2007)

TooTallUK said:


> It's economies of scale in manufacturing. The more you make from one jig in one size the more return you get on your investment and design. Us 'big guys' are such a tiny proportion of the bike market that there just isn't the economies of scale in such nuanced frame design at the extremes of fit. Bikes are made to fit average people because there's more of them to sell to.


Agreed, we are not the ones keeping the lights on for the industry.

I meant that when Jeremy said: _"...are manufacturers skimping on lengthening the frame as much as they should?_" I don't think it was a case that they were saving money by making the frames shorter.

I agree with you that they might be saving money by not putting as much R&D into the outlier sizes.


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## Tjaard (Aug 17, 2007)

TooTallUK said:


> Thing is, there's little enough profit in us bigger riders anyway. Once you get into a frameset adapted at every size you get into way less return the further away from M & L frames. Around this forum, my 6'7" height isn't all that outrageous, but I am taller than 99.9% of the people one the planet and that's not a great sales point!


Yep, that is true.
There are exceptions though. On the Tarmac road bike and the new 2019 
Stumpy, Specialized touts the fact that each frame size is individually designed to address the different stiffness of short tubes under small riders and longtubes under big riders. They also make it far more (carbon!) sizes than in MTBs.

So there is a recognition, in a big brand, that the different frame sizes affect there riders experience, as well as the willingness to address that issue.


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## alexbn921 (Mar 31, 2009)

Tjaard said:


> Yep, that is true.
> There are exceptions though. On the Tarmac road bike, Specialized touts the fact that each frame size is individually designed to address the different stiffness of short tubes under small riders and big tubes under big riders. They also make it far more (carbon!) sizes than in MTBs.
> 
> So there is a recognition, in a big brand, that the different frame sizes affect there riders experience, as well as the willingness to address that issue.


They change the HTA to keep the bike shorter for larger riders too. Heavy riders need stiffer tubes to get the same feel on a road bike. Wheelbase on a road bike doesn't matter as you never negotiate hair pin turns. I bought a 61cm Roubiax for the extra length and comfort.

The whole front triangle is very easy to change from size to size as each one uses a different mold and layup. Even changing the rear length with just links wouldn't be that bad.


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## Ryder1 (Oct 12, 2006)

Tjaard said:


> So there is a recognition, in a big brand, that the different frame sizes affect there riders experience, as well as the willingness to address that issue.


I think size specific geo and tubing is widely recognized and addressed in the road world. Some are better than others. The Jamis Renegade gravel bike has size specific tubing among 6 sizes of 5 different frame materials. Plus 3 fork materials, 5 HTAs, 3 CS lengths, 3 BB drops, 3 fork offsets, 3 stem lengths, 3 bar widths, & 4 crank lengths. Complete Renegades start at $1,199 [same price as a SIR9 frame].

renegadeexile


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## Tjaard (Aug 17, 2007)

Ryder1 said:


> I think size specific geo and tubing is widely recognized and addressed in the road world. Some are better than others. The Jamis Renegade gravel bike has size specific tubing among 6 sizes of 5 different frame materials. Plus 3 fork materials, 5 HTAs, 3 CS lengths, 3 BB drops, 3 fork offsets, 3 stem lengths, 3 bar widths, & 4 crank lengths. Complete Renegades start at $1,199 [same price as a SIR9 frame].
> 
> renegadeexile


Yep, now if only we could see that commitment in mountainbikes too from the brands.


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## Tjaard (Aug 17, 2007)

So I went and got a new bike, to try and put some of this theory into practice.

My previous bike (Trek FuelEX 23") was pretty spot on for most of my riding, and professional reviews agreed with my experiences.

However, the reach was still uncomfortably short.
The wheelbase was long. Great for stability, not so great for tight turns and especially turns on skinnies.
As mentioned above, this also meant I had little weight on the front wheel.
Finally, if anything, the trail was still a bit short, especially on super high speed trails, but running the bike in the low setting shortened the reach even more and put even less weight on the front wheel.

So the new bike (Bird Aeris Am9 XL) compared to the old bike in high setting:

Same head tube angle, stem and bars.
440mm chainstays vs 333mm
518mm reach vs 502mm
37mm fork offset vs 51mm
1261mm wheelbase vs 1251m
114mm trail vs 98mm (or 102mm in low setting)
Front:rear weight ratio 1:2.87 vs 1:2.98
Front center: 821mm vs 818mm


So, Front center staid about the same, but the reach grew. In other words, my hands moved further forward (without lengthening the stem). At the same time, the trail actually increased significantly.

Oddly the shorter offset fork almost feels more lively to turn the bars. At moderate speeds it felt great, much improved. Slow speed climbing is also much improved because I can slide my saddle forward, and have the front wheel weighted.

I haven't been able to test it yet on super high speed corners, as we don't really have much of those around here. And since the old bike is off to a new home, I won't be able to compare it back to back.


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## Tjaard (Aug 17, 2007)

You have probably read this article too:

https://www.bikeradar.com/us/mtb/gear/article/pushing-the-limits-of-fork-offset-an-experiment-45343/

Coincidently, I ended up with the same headangle and offset Seb had on that bike (67 degrees and 37mm).

He writes: "_Once I got used to it, the only disadvantage I found to the shortest offset was the fact that it made my bike feel too short in the front-centre; the front axle simply was too close to the bottom bracket. Now, if the bike was longer, compensating for the short offset, that could be the recipe for a very fast bike indeed_!"

Which is exactly what I did, by adding reach, I kept my front center the same.


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## Tjaard (Aug 17, 2007)

Well, still no high speed berms, as the weather has kept some trails closed.

Overal riding is great. Climbs easy, even uphill switchbacks are easier. At moderate speeds, steering feels better. Simultaneously lighter/more responsive feel at the bars, and carves a better line.

The added weight on the front really helps, I can move around more, while still maintaining traction.

And the new Lyric RC2 170mm rocks!


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## Tjaard (Aug 17, 2007)

Well just an update to some more varied terrain testing:
I got to take the bike to Whistler and Silver MT (ID) park a bit this summer.
In the bikepark I run a 27.5x2.6 rear wheel. This drops the bottom bracket super low, and slacken the head angle back to around 66.5 degrees. Combined with the 36mm offset fork, this gives a lot of trail.
It felt get in high speed sections. With the added weight on the front, compared to my old bike), and even more trail, it eliminated the oversteering/washing that I’d sometimes had with my old bike in high speed turns. At the same time, it keeps the front wheel very avaible, allowing me to turn the bars easily to correct wheel placement mid turn and at slower speeds.

This is not really a good test, since I am not comparing things back to back, but at the very least, I would suggest very tall riders look into front and rear center ratios of their (future) bikes.


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