Modern Geo Talk: Chainstays, Stack, Reach, and Bitching About It

Thanks for making a new thread! I also made one called "Argue About Chainstay Lengths Here", but I think a thread discussing geo in general is probably more useful since bike handling isn't just determined by CS length. 

I agree that having CS lengths grow by just ~20mm across all sizes is not nearly enough to cover riders of all heights, but I disagree that it's a marketing gimmick. A 20mm difference across all sizes may be small, but it still makes a significant impact on handling, and is definitely a step in the right direction. If I was in charge of a big company like Specialized, I'd also be hesitant to make any drastic changes to geo if what we have now is still selling; I'd rather make incremental changes to satisfy the growing demand for more proportional CS lengths without rocking the boat too much. Sure, most companies' CS lengths are not truly proportional, but I don't think that's a marketing gimmick as much as it is just being careful. That's why it's up to smaller brands like Forbidden to lead the charge on innovations, like how Nicolai and Kona did years ago, and wait for the more radical geo to trickle down to larger brands. 

it sounds like you are both somewhat in agreement - that truly proportional rear centers are too drastic of a change from the geo status quo to make sense for most bike brands and most riders (at least for now). whether it's a marketing gimmick is subjective, since it doesn't seem controversial to say RC should probably be longer for longer reaches, but also that a proportional 470mm+ RC on an XL isn't going to be the best for all XL riders.

talking out of my ass a bit here, but one thing I haven't heard mentioned re: front/rear center proportions is the fact that bike-body dynamics might not scale linearly in the way we would assume they do when we talk about keeping FC:RC ratios constant. given that there are observable differences in riding styles that come with larger body sizes and higher rider weights, it doesn't seem crazy to think that maybe L/XL riders don't benefit from more length between BB and rear axle in the same way S/M riders might.

personally I'm still trying to figure out what works best for me as a taller rider with a long torso. shorter (435-445) rear centers seem to be best for my lack of leg length, whereas longer rear centers don't let me get as far back over the back wheel as I'd like in some situations. I need to ride more bikes with a longer RC to get a better idea though.

https://www.youtube.com/watch?v=P18SutYYL5I
https://www.youtube.com/watch?v=i5R60JHJbxI

Once again, Steve's got you covered. 

These videos are great, I wish I had watched the second one a week ago while stressing about what size frame I should get. Luckily for me my ride experience on the new frame matches what Steve is talking about and I was right to downsize to a medium frame, from same bike in a large. I did this to in order to improve front wheel grip and cornering 'ease' as I was having a few issues on the large frame in certain situations. New frame feels great, shorter reach doesn't feel short at all and the bike turns like a dream. 

I'm here to bitch about low BBs, why the f..k are manufacturers making 160mm+ travel bikes with 330mm BBs, and worst of all publishing geo charts where they claim the bikes are 1cm higher than they end up being. I have a theory that designers test mullets vs full 29ers and realise they need to make the BB dangerously low on mullets to match the composure of a 29er with 355mm BBheight (taking my old spesh enduro as reference, along with very positive reviews of the Orbea Wild with its high BB ).

I've just seen the press release for the new Forbidden bikes, what a shame to have made them so low when the DJI motor would allow for really fun technical climbing.

"On level ground, a 100 kg rider on a bike with a 440 mm rear center and a 800 mm front center would have about 64.5 kg on the rear wheel and 35.5 kg on the front, or 64.5% of their weight on the rear and 35.5% on the front (assuming the load is at the bb)"

I know a lot of weight goes through the feet, but - at any point where handling or tyre grip is important - we are weighting the hands right?

The 100kg rider in the example above needs to put about 7kg through each hand to get a balanced front to rear. or 14kg in one hand if he is fully weighting the inside hand in a corner.

Go get your bathroom scales and push down with your hand to see what that feels like.

Can you convert Kg to freedom units?

(it's about 15 lbs)

We are also assuming that the correct “balance” is 50/50. The front and back of the bike do different things and it’s always rolling forward, and sometimes on really steep terrain (up or down). So should equally balanced even be a good thing?

Ps. I like long chainstays. 

I like long chainstays too. Would be curious as a tester to see what truly equal would feel and ride like. 
not saying i would jump to that as the correct set up... just would be fun. 

i like to look at the ratio (wheelbase-chainstay)/chainstay 

accounting for head angles to me is significant as well.

The interesting question is what are we really feeling?

430mm stays (about the shortest I can think any modern-ish lively quick trail bike having) vs 460mm (which is about as long as you can adjust a geometron to) is only 7% longer.

so measuring from the BB probably isn't the key thing. 

Kind of like with stems - steerer centre to bar centre is how we define the stem length, but it doesn't actually represent the feel you get - and therefore it doesnt scale linearly. 35 to 55 is a world of differnece. 50 to 70 I dont think I could tell in a blind test.

If we were talking about steep tech climbing prowess, I guess we could measure horizontally from saddle to axle. That would give a much bigger difference, especially on an upslope.

As a short person generally riding smalls or mediums and not focused on racing, I've always enjoyed shorter chainstays when choosing a bike.  Manuals, bunny hops, and diving into turns are easier for me with me.  My current Canfield Balance is one of the most dynamic bikes I've ridden with 420mm chainstays, probably the shortest you can get out there.  I really enjoy that bike, but I need to be on my toes if I'm pushing it, traction can cut out on some loose flat turns primarily.

I'd theorize that the people out there complaining about short chainstays are generally above average height folks, and that makes sense.  If you look at the Rear Center / Wheelbase ratios on bikes it's interesting.  My small canfield has a 35% RC ratio, but an XL has a 33%, so way less weight on the back.  I could see climbing being degraded, looping out easier, rear end traction reduced too much.  If considering BB as Center, to have the same ratio in XL, they would need 459mm chainstays.  I'd assume before RC's starting changing, engineers chose a size in the most sold size (probably between M and L) as optimal for their experience they wanted to have.  

The big questions comes, where should the front / rear ratio be measured from?  Is it the BB, or somewhere front of it?  Where is your center of gravity generally when you ride?  

Considering the BB as the center, Santa Cruz differs from Forbidden in how they do it.  The 5010 from Small (36.5% RC) to XL (34.5%) gets smaller at the rear proportionally which almost makes me think they are considering COG in front of the BB.  The forbidden druid remains the same across all sizes at 35.9%, making me think they assume its the BB.  

just FYI, you can easily compare bike geo in our product guide. here's a quick one i put together between a dreadnaught, firebird and sb165

https://www.vitalmtb.com/product/compare/64471,64876,50471?sizes%5B%5D=50471%2C140291&sizes%5B%5D=64471%2C268071&sizes%5B%5D=64876%2C269856

A while back someone here had mentioned they believe a FC/RC ratio of 1.6-1.7 was optimal in regards to cornering and balance. I keep a spreadsheet of frame geo and setup data going back many years and added a new row for this ratio. 

Sure enough, the bikes that cornered the best and were easy to weight the wheels correctly were right around the 1.7 mark. There was a DH bike approaching 1.9 FC/RC and it only really felt good going in a straight line through the steep chunder. You have to start with the reach number that works for you, but I'm a believer in a 1.7ish FC/RC ratio.

Steve mentions Sam Hill in the videos; in regards to frame fit/geo, Hill was known to run a tape measure from the center of the BB to the center of the grips. There was a certain number he was looking for. Similar to what Steve was talking about in regards to "spread", or the hypotenuse of the reach/stack triangle.

What bikes are sitting at 1.6 or 1.7? I believe Forbiddens are 1.78.

I believe my trail bike is 1.84(Long chainstay TB4 with cascade link) and DH bike(29 V10 with Long Chainstay) is 1.85.

Both XLs.

Im really craving a bike with a better ratio in the 1.7s or closer to. These two bikes were a step in the right direction from prior bikes but I would still love more balance. Have been considering an L front to XL rear Madonna V3 that would get me to 1.82.

15 lbs per hand. 

I lust like joking about metric vs imperial measurements since I have to switch constantly at work. 

It's especially fun to modify a prototype part that need to be welded and then machined to raise a surface by 0.3mm. 

Laser welder uses metric settings and welding rod, mill is imperial. 

Weighting hands sucks. Such a not fun way to ride. I will transfer a little weight to my hands, but I want to ride through my feet. I think the rise in collar bone breaks caused by front wheel wash is caused by this trend in riding (especially when paired with clipless setups that I see as being necessary for that geo/riding style.)

I like a full geo thread because most of the Long Rear Center folks are really just asking for bikes they can ride more upright and through their feet, which requires higher stacks, which lowers weight on the front of the bike, which necessitates a shorter reach or longer chainstay depending on preference/existing geo. 

There might be a place for long front center bikes and long rear center bikes to coexist, but I’m eager to promote a more balanced riding style as I believe it will lead to less injuries and more fun. Riding upright on flats has given me a completely new lease on my riding and has dramatically increased control as well as my ability to save myself when things go wrong. This approach requires a very different type of bike setup, and I believe that many of us are as passionate as we are about this geo convo because of the importance of some of these surrounding issues.

Alright, I’ll join the debate.

The idea behind “proportional” is kind of useless IMO. Sure, we can say the FC to RC ratio is maintained, but what does that mean for a rider? We are assuming that as the riders height increases, so do all body proportions. 2 riders can have the same torso length, yet be different heights due to different inseam length. Add to this arm length and shoulder width too. Which means they may prefer the same reach/span, but their body position and weight distribution will be different. The taller rider with longer arms being positioned more rearward on the bike, therefore needing/preferring longer chainstays to maintain the weight distribution between the wheels. Look at squat form/anatomy for people with longer vs shorter femurs. If you have shorter legs, shorter arms and a longer torso, you have less ability to get off the back and “behind” the rear axle which IME can cause a feeling of being pushed onto the front end of the bike. 

So I think there’s moreso an operating window of FC/RC ratios that work well, but it will be individually dependent on more than just bike size as to where you fall in that range. This is why I think adjustable chainstay length is really important, it allows you to adjust the bike to suit you, rather then you adjusting to suit the bike. 

Confused about this.  1.9 would mean a shorter rear end than a 1.7 with the same reach.  I thought the main reasons people want longer chainstays downhill is for longer wheelbase with stability through chunk.  Longer chainstays arguably make it hard to get around corners.  This ratio doesn't really match what people say?

And yes, Forbidden Druid is 1.78 FC / RC.  or 64% front.  

Not gonna lie. I didn't have a good understanding of stack (or ever really paid attention) until this winter when I swapped frames from a 2020 Slayer (xl) to a Madonna V2.2. On the Slayer, I had trimmed the steer tube on my Zeb, but still used a bunch of spacers. On the Madonna, I can barely fit the same stem without any spacers. I'm 6'4 and loved my Slayer, but the Madonna is on another level. 

A Chainstay with flip chips for length and headset with reach adjust cups would help riders dial these in I reckon.

At a 64⁰ HTA, and 160mm fork, a 1.7 F-R Ratio means an almost 1:1 reach to rear centre ratio which is further than I'd be willing to try with 475mm (ish) reach.

Long time lurker figured I’d chip in. For me at least personally the reason I tend to prefer a longer rear centre as without body position adjustment there is more weight on the front wheel. This lets me stand on my feet more in a neutral position without having to get so far forward to get traction on the front wheel and since there is less weight on the back wheel, it is easier to step the back out in turns. That, coupled with more weight on the front wheel makes cornering feel easier for me and is the benefit I notice to a long RC more than straight line stability.

Looking back at the data, for me the "good" bikes are actually 1.71-1.76. 

Spec Chisel FS (med), 130mm fork = 1.71

SC TB4 (med), cascade link, 150mm fork = 1.73

SC HT2 (med), cascade link, 170mm fork = 1.76

Spec Enduro (S2), 170mm fork, 775/442 = 1.75

Or a 1.9 ratio can also mean a longer front end with the same length rear as a bike with 1.7 ratio.  A slack DH frame coupled with a longer reach makes for a really long front center. I think that was the imbalance issue, the bike was long but only in the front if that makes sense. RC was the same as a prior frame, but the front end was much longer.

True, and I would say that perhaps this shows that the reach is too long. Or maybe it would be good with a 470mm+ RC.....😀

I have a similar bike; 64.6 HTA, 150mm fork, but the reach is only 438mm paired with a 440 RC.