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

Yeah, that part is definitely a bit of a bummer. I would be curious how many design engineers and testers are above 6' in the industry. 

I ride a medium and think they're comically short. Do they get a deal only making two size stays?

It seems that the Chromag is more of a park, freeride trick bike.. The numbers make sense for that..

Rallon owner with a ratio of 1.75. It’s fantastic. Really natural and easy to figure out. Less arm pump and elbow tendonitis pain. Superb traction. Only downside is it might be slightly more likely to spin the rear wheel on heinous steep tech climbs when standing up. Maybe I just need to spend more time balancing my weight. 

re chainstays, it looks like the transition sentinel quietly went from a 448 to 442 in size large with their new colorways. WIth a 170mm fork, that puts it really close to the last generation nomad geo.

I still think they don't just because of how unbalanced the 465(!) reach will feel

I ride a large 2018 Smuggler with 430mm CS and 475mm reach and it's positively unrideable 🤦‍♂️ lol

Ok, at least one of you guys must have ridden the Reya by know, right? I’m still desperately curious about it, but I have no avenue to try one unless I buy it outright or take a flight. 

Some interesting thoughts from the guys at Spot about chainstay length in a podcast about their new bike on the Bikes & Big Ideas podcast with Simon Stewart.

https://blisterreview.com/podcasts/spots-ryan-palmer-on-their-new-lightweight-trail-bike-ep-326

Geometry talk starts around 29 minutes in but the whole podcast is a great listen!

Side note: I really dig both Simon's episodes and David Golay's episodes of the Bikes & Big Ideas podcast. I know the pacing of the speech in the David-hosted episodes is challenging for some folks but if you can hang with those episodes there's a ton of cool info and cool people to learn from/about!

Sounds like they couldn't be bothered and gave a BS answer to justify it after admitting that testing proven it made a difference. 

Not gonna lie: I had the same impression.

If it makes a difference but it costs no different because a new mold is made for every size front triangle, then I'd be making it happen. Even if that means the suspension performance changes a tiny bit. Even if for no other reason than some number of folks REALLY think it matters and the people who don't think it matters won't mind if it changes a little.

Part of performing well is believing you can perform well. If your brain expects a bike to suck, it's going to have to be OUTSTANDING to even make it out of "suck" towards "kinda ok." Handing people a "we've given consideration to your concerns" detail is probably going to predispose many more people to believe your bike will be good and believe they can perform well on it than saying "we're protecting you from your stupid selves" ever will.

I thought the other points in their considerations around frame geometry and weight were really interesting, too!

The only point I'd love to argue is that the lower and central weight placement makes the bike more maneuverable.. But, really that interview shows why you need to look at everything as a whole package,  not just one or 2 numbers. 

The whole “taller riders have more of their mass forward of the bottom bracket” thing doesn’t at all track for me for three reasons.

First, taller riders have the same limb/torso proportions are shorter riders, so their mass is going to be distributed on their body the same way.  Humans have roughly the same arm span as height and our femurs are roughly 1/4th of our height, pretty much across the board. Obviously, there is variance between individuals, but it’s not like tall people are just people with really long shins. (We’re not talking about acromegaly, I’m assuming.)

Second, every single rider, when standing on the pedals, is a) rotating around the bottom bracket and b) staying upright by leaning (or pulling) on the handlebars to some degree (i.e., reaction force from the handlebars producing a net torque of zero). The average amount of force applied to the handlebars on any ride of any meaningful length will necessarily be low because none of the muscles used are postural muscles and you can’t lock your elbows and ride a mountain bike. (See how long you can hold a 45-degree plank for in the middle of your press ROM.) So the overwhelming majority of the force being applied to your bike is being applied via your feet, which means that yes, the position of the bottom bracket between the two axles is kind of the whole game. (Yes, yes, we push on the handlebars into lips and turns, but I invite you to consider what we are bracing against when we do so, what range of motion are using, and what that means for the total amount of force produced in those situations.) If anything, taller people are more likely to want proportionally less weight on their hands, as muscle strength scales quadratically but muscle weight scales cubically.

Finally, there’s a mutual causality relation between the independent and dependent variables (i.e., rider height and weight balance) in the “experiment” that Ryan talks about here. If you put me, a taller rider, on a bike with really short chainstays, I have to move my weight forward. If I don’t, I’ll loop out on climbs and wash out on corners. That’s a measurement of how I have to adapt to a different geometry; that’s not a measurement of how taller riders should ride, or would ride naturally. It’s a theory which proves too much: tall people can ride unicycles without falling off or bursting into flames, therefore a 0mm chainstay is fine; tall people are used to riding bikes with short chainstays, therefore we don’t need to change the way we design their bikes.

Some folks like short chainstays (and some of tem are even tall!) and that’s completely fine, but I’d rather he say “it’s what our customers seem to want, it’s part of our design ethos as a company, and it’s cheaper than figuring out kinematics and rear triangle molds for each bike” than “tall people only get taller in front of the bottom bracket”.

Seems like taller folks COG wouldn’t get pulled forward of the BB if stack increased proportionally with reach per frame size. 

A higher stack is going to be better than a lower stack, everything else being equal, but it’s not a full solution. With a higher stack, it’s harder to get your weight forward and actually weight the front wheel on techy climbs and into turns. It’s more comfortable because you’re not forced either into a deep squat or a pike plank position as much.

But it’s not going to change the basic fact that a standing rider will always have the overwhelming majority of their weight supported via their feet and thus distributed between the two contact patches according to their relative distance from the bottom bracket.

I have a bike with 440mm chainstays and a 520mm reach and a bike with 525mm chainstays and a 460mm reach and the difference is not subtle. I’m not saying the latter is the solution for everyone, but the viable design space here is actually massive and it’s frustrating to see both product development and forum conversations get blinkered into a tiny “435mm is short, 450mm is long” corner of what we could actually ride.

Are you more or less likely to spin out your back wheel on a steep standing climb with the Uber long stays? 

I brought up stack because Spots “solution” to evenly weighting the wheels gives a compromised body position, the larger frame sizes have the rider hunched over with too much weight on the hands. Stack AND chainstay length need to increase. 

What I read in what @afreak wrote and what I agree with is that a lower stack height will usually force the (taller) rider to be hunched over more and thus weight the bars more in order for the bike to work. 

It's a different story of whether that works for everybody (personal experience, it sure as hell does not), it's just an explanation of where the thinking about taller riders having more weight in front of the BB comes from. 

It's a chicken-egg question. Since going taller on the front end my riding has massively improved and my ideas about longer reach bikes needed for tall riders have been losing their support 🙂 

Damn I need to try out a long rear centre bike... 

Size M frame rider here. I’ve had numerous bikes from 438-450 mm chain stays. 

Short chainstays (Firebird) makes it easy to loop out backwards but when standing and pedalling you get good traction. 443 chainstays on a high pivot (Range) was an absolute machine climbing. Traction for days in all situations. 

450 chainstays (Rallon) tends to spin out, loose traction in extreme situations more easily. Obv there are numerous factors but it seems night and day to me. The best position so far seems to be seated with an upright body position but then you have to be careful when it gets steep. 
 

If you change nothing else you'll have less weight on the rear so you're more likely to spin out. However, longer chainstays can allow for higher bars, shorter stems and you don't have to focus as much on keeping the front down. So my real world experience going to 465 mm chainstays is that I haven't noticed an increase in spinning my rear tire on steep climbs. If anything it's probably better since I'm in a more natural position on the bike.

Waaaaaaay less. Way way way less. I think that’s less to do with weight distribution and more to do with body position. You don’t need to drape yourself over the front wheel to keep it down, you can just stand up and keep a pretty neutral body position.

The other thing I’d note is that I find I don’t actually have to get out of the saddle all that often. I can hit the steepest climbs in the area while seated. If I’m traction limited, I’m usually not power limited, and getting out of the saddle to hit sprint power isn’t going to help.

It's interesting to hear the differences in peoples' experiences.  I would love to take a spin on a long CS, moderate reach, high stack setup to feel the differences in handling.

I tackle almost every climb standing up.  I'd rather mash a higher gear than spin a lower one seated.  Honestly, I just stand almost all of the time when I'm riding my bike; it just feels better to me.  Plus, I'm a low-proficiency, high-power guy.  I started mountain biking three years ago in my forties, but I'm quite strong, so I just hammer through terrain with power rather than finessing things with grace and skill.  Haha.  

I thought the Spot interview was interesting. What he said made sense--it's a choice. It doesn't cost more to manufacture. It does change how the bike rides, which is something to consider. Some builders, Spot and Pivot for example, still believe in designs that incorporate a short CS. 

Well, no, size-specific chainstays are always going to cost more to design and manufacture. It’s more design/development/testing time, it’s more molds and layup schedules and jigs and fabricator training, it’s more SKUs to keep in inventory and manage. You can try to be clever about the packaging and minimize that blowup by moving pivot points backwards, but at the end of the day the increase in the cost is still going to be non-zero.

I do believe him that the cost isn’t why they made the decision and that their brand focus is on rear-biased, high-leverage bikes. After all, it’s not an uncommon stance, there are plenty of taller riders who appreciate the upsides and don’t mind the downsides, and they’re mostly going to be selling medium frames anyway. It’s valid.

But I can’t take “it doesn’t cost more” or “they ride the same across the sizes” seriously.

Not necessarily true. Brands like Norco and Forbidden (and many more) change the position of the pivots on the main triangle, so that the rear stays can remain the same between the sizes, but the chainstay length changes. 
 

That’s what I was referring to when I said "You can try to be clever about the packaging and minimize that blowup by moving pivot points backwards, but at the end of the day the increase in the cost is still going to be non-zero.”