Chainstay Length Discussion

manzanitacycles · December 10, 2023, 4:41pm

I think you meant to write the opposite. It’s the taller riders that are on more rear-biased frames since the chain stays don’t grow as the frames get larger.

I’m all for longer chain stays in general and always attempt to lengthen the chain stays as the front center increases. But we tend to want what we’re used to, so we usually end up using slightly longer stays than your average Spesh/Trek for hardtails and gravel bikes. This is mostly a marketing challenge.

The only realm where customers give me free rein is with bikepacking bikes. I haven’t gone as long as @liberationfab at 515mm but I’d love to try it out on a personal bike first.

For giggles, I mocked up a size run of 130mm travel hardtails where the front/rear ratio remained constant at 1.82. The largest size ended up with 460mm chain stays and the smallest size 412mm.

Since the larger riders are used to riding a 1.9-2 ratio frame, it’s a hard sell to get them to commit to stays longer than 430-435 even if I try to convince them of the benefits.

Once chain stays get shorter than 420mm, you run into the limits of what 12s drivetrains are designed for. Shimano says 420mm and Sram 425mm for the shortest stays allowable for their MTB drivetrains.

liberationfab · December 10, 2023, 8:08pm

Serious!!

With a shorter CS, your “breakover angle” decreases which is great for riding over techy features.

Ah yep! I got that backwards.

bushtrucker · December 10, 2023, 9:38pm

I reckon chainstay length is one of those dimensions where I honestly cant tell the difference in 10mm of change here or there. Most of my bikes are 29” and sit between 425 to 455 and they all feel “normal”. One anomaly is my longtail BFD which has CS length out to 875 and rides like a boat. But yeah I really feel like you have to get a long way outside of standard lengths before you really notice things.

I would love to try a mid-tail design (550-600) on something built for off-road riding/touring. Rick Hunter has built a few of these over the years and they always look super comfortably and capable.

I would also agree with @manzanitacycles that chainstay length really needs to be considered with front centre in terms of rider balance. And you also need to factor in STA and saddle offset there too. If you look at those latest Riv offerings they have long stays but also very slack STA’s and very upright bars. I imagine the effective position of the rider would still be quite normal in terms of balance between the front and rear wheels. In essence they’re just chasing the same off-road stability you get with the longer wheelbase of a “forward geometry” MTB. I always wondered why they don’t use the term “backward geometry” to describe this approach haha. Would be very Riv.

ben.land101 · December 11, 2023, 12:26pm

Keeping in mind, my only experience is the handful of hardtail MTB’s I’ve built. But they’ve all had the same goal, keep the rear-center as short as possible.
On Valkyrie (my latest one, it’s a 408 RC, 415 CS), this became really apparent how advantageous that is. It makes the bike so darn fun to ride. Drops & hucks are effortless, popping the front over roots to kick it with the rear feels natural. I must also note, I don’t race. My riding philosophy is “any trail is tech if you ride it wrong enough”.

Nice thing about mountain bikes like this is the seat simply doesn’t matter (going downhill). The dropper gets slammed, and we have a crazy amount of room to shift our CG drastically by moving our torso around. A lot of downhill bikes I’ve ridden are really meant for a somewhat ‘static’ body position, where you can just send it without really changing stance at all. That probably wins races, but I’ve loved how rewarding it is to lean into the front of this bike and have it ask for more. Totally different tools for different applications.

https://www.instagram.com/p/CYtZr44IQJ8/?utm_source=ig_web_copy_link&igshid=ODhhZWM5NmIwOQ==

With all that being said though, I’ve been debating this for my next bike. It’s intent will be quite different. What that means I’m not totally decided yet. So I’ll definitely be following along this thread.

terraformbicycles · December 11, 2023, 2:33pm

I’ve played around a bit with chainstay lengths and tend to prefer longer.

Notable experiments:

420mm CS on a fat bike with a long front center, sporting 27.5x3.8 tires.

it is a hoot to ride, but the front end loves to lift up on climbs.
– I wish it was more like 450mm.
— Fitting the crankset was a pain.

420mm CS on a 27.5x2.8 hardtail, 160mm fork, long front center.

the bike is really whippy. Almost feels like a bmx.
– dislike how it climbs, and disallows the user to be lazy. Your rear tire will slip on a root if you’re in the saddle.
— gonna go with 430 or 435 on the next one.

450-470mm CS on a 700x42mm touring bike, 70°HTA, 75°STA.

great climber, extremely planted.
– has a touch of self steer when unloaded.
— very comfortable and easy to do long days on.

I toured the austrian / slovenian / italian alps a few months ago on the touring bike and had several days where we’d climb a total of five hours and thirty minutes throughout the day. It was so stable and tracked so well I felt like I could climb forever. We climb a lot where we live and find on gravel / road / long days where you don’t want to get beaten up, that longer CS is better. We’re not speed demons, we go for 17-19km/h averages / 1,500-2,000m climbing, and comfort.

Our mtbing stuff is very rocky, sometimes slow tech, or rocky downhill stuff.

The chainstay is set to 470mm in the photo.

Daniel_Y · December 12, 2023, 2:38pm

Finally, a civil discussion about chainstay lengths on the internet. We have come so far!

As many people expressed, CSL cannot be talked about without front center, or a more generalizable variable: Rear Front Ratio (RFR):

From this thread: What are your driving dimensions and why? - #15 by Daniel_Y

rear-front ratio (RFR).

RFR = L2/L1

A quick derivation:

Rear wheel force: F1 = L2/(L1+L2) * Mg
Front wheel force: F2 = L1/(L1+L2) * Mg

The rear/front ratio is really easy to calculate because most of the terms cancel themselves out:

rear-front ratio: RFR = F1/F2 = L2/L1

I agree with a lot of the subjective and objective feelings that people have with chainstay length:

Agreed. I think there are some weird effects here too:

longer chain = more chain stretch under load
more weight on the back tire = more direct feeling with the power transfer

Rocky, techy, punchy climbs do really well with short chainstays IMO. You get more rear wheel grip and can loft the wheel easily. This is why Steve at Hardtail Party likes super short chainstays in Sedona

I think one of the funnier Poertner podcasts was about the Placebo effect. If you feel faster and go faster, it is faster!

Yup, 460mm stays are a hard sell when there is 10 years of “shortest chainstay possible” marketing to overcome! However, we are leading the charge!

Backward geometry for upduro racing!

Agreed. I think the super short chainstays makes sense with the PVD system: very long front center, super steep STA. That pulls your CG forward more than the CSL is shortened. Hardtail party Steve also sets up his very moderate Hummingbird like that:

saddle slammed forward
sized up a frame
Mid-foot position

2000m of loaded climbing, no thanks!

sunshine.fab · December 12, 2023, 3:25pm

This makes sense to me, primarily because of your free body diagram. But I really only have experience riding short CS mtbs, so I can’t make an anecdotal comparison to how longer CS feel.

Enter the Esker Hayduke LVS. John Watson isn’t the most technical or hype-proof guy out there but he has ridden a lot of bikes and he claims that one of the remarkable things about the LVS (with 600mm CS) is the increase in rear wheel traction:

“The Hayduke LVS crawled up these tight turns. With ease. Again, the tires on this demo bike were not my choice for the terrain—with a lug pattern more like a gravel bike tire than mountain bike tread—but I never lost traction and was able to swing the rear end around with ease. It left me wondering: Does a longer wheelbase increase its traction?”

This seems counterintuitive. The reasoning from the FBD would lead me to believe that an increase in rear center/decrease in F1 would result in less traction, not more. This has me wondering if the actual traction given by the rear tire is nonlinear with respect to the rear center/F1. Something like this super high tech graph:

Does this match anyone else’s experience? Or is J.W. experiencing placebo? Or something else?

liberationfab · December 12, 2023, 5:36pm

In my experience, with long-stay bikes you get extremely good traction climbing… until you shift your body weight forward. I love spinning up steep forest roads in the granny gear of my touring bike, but it’s very challenging to do any “techy” style climbs as you can’t stand up and torque without having to be very mindful of keeping your butt back toward the rear of the bike.

bushtrucker · December 12, 2023, 7:34pm

Yeah this is my experience with longer stays too. Great for seated climbing, but you can break traction pretty quick if you stand up and put in a hard stroke. I find running a more agressive than usual rear tyre helps in this regard.

@Daniel_Y totally agree that RFR is better than considering CS length alone but it still falls down in that it doesn’t really consider STA/rider position which is 80% of the mass of the system.

If you look at what’s happening with geo it seems pretty much set that shorter CS bikes have steeper STA’s and longer ones have slacker ones. It would be interesting to know the actually CoM for a bike set up in each configuration to figure out the true F1 and F2 values.

ElysianBikeCo · December 12, 2023, 7:41pm

I think we should all be aiming for this:

I love me some moto hill climbs.

Seriously though, I’d love to find out more about the math/physics behind this type of geo. Maybe there’s a used copy of “Hill Climbing Motos for Dummies” somewhere.

JMY · December 13, 2023, 5:40am

The steeper the hill, the shorter (read: more normal) the effective CS length will be. The rider’s weight will, on the horizontal axis, get closer and closer to the contact patch of the tyre.

It creeps rougly 11mm closer to the axle for each degree of incline (based on my setup numbers) according to the completely pointless modeling I just did based on this discussion.

I have also a feeling that the bigger purpose for the long stays on these extreme hill climb machines is to keep the bike from flipping over rather than to solely provide traction.

Robertt · December 13, 2023, 6:00am

I have very steep and short gravel hills where I sometimes train. My hardtail has 442 chainstays with xr2 tire. I dont have to stand up, but it gets slightly slippy on the steepest parts. My gravel bike has very short stays and 40 mm panaracer slicks, but I can smash it up there when standing(smooth pedalig). So Id say short stays give better traction while climbing. But there comes a point when you can,t get up with comfortable body position when the chainstays are too short. So all the geometry numbers are important.

FlatironBikes · December 13, 2023, 6:37am

Stoked to see all the science!

I went down this theoretical rabbit hole last winter and the TL;DR is that FC/RC ratio depends on intended used case, like that moto bike, but is based on the intended Center of Mass for the system, like the moto bike.

In my study, and borrowing @Daniel_Y’s freebody diagram, I assumed the center of mass for the rider/bike system was NOT located at the BB. This is against common assumption, but would provide more accuracy across different disciplines.

I played around with angles of bike slope, like @JMY’s graph, and determined that it is very possible to create a size range based around a common weight distribution value, like @manzanitacycles mentioned. The proportions of the bike would vary significantly based on intended handling characteristics (Front center driving dimensions) and intended slope gradient (XC,DH,…road?)

My focus is mountain bikes, so the saddle position is negligible. With a rigid seatpost, static CoM position between axle locations could be accounted for depending on intended riding angle, just like @JMY’s graph

My planned project for this spring is testing out a size range with a few different riders to see if there is validity to this approach.

bushtrucker · December 13, 2023, 8:49am

Ahh you beat me to it. Was thinking about this all day today and CoM is defo the cirtical piece of information that’s missing. The diagram you drew there reminds me od the one PVD did on his Forward Geometry blog post years ago. He also assumes the CoM to be slightly forward of the bottom bracket as show below. I would say on an offroad bike in a standing position this would probably be the case.

bushtrucker · December 13, 2023, 9:30am

And to add to this a little study on two bikes I had drawn in BikeCAD. The first is a Large Norco Torrent which is a pretty agreassive hardtail. I own one so the setup here is fairly accurate. It has short CS’s for a bike that clears a 2.6" tyre at 425mm and a steep STA at 77.5 degrees (sagged). I’ve arbitrarily chosen a CoM at 50mm forward of the BB which gives a CoM-R of 471mm and a CoM-F of 758mm. That’s a ratio of 1.6.

Next I’ve shown a Rivendell Wolbis/Suzie frame in 59cm which would be my size if I owned one. I’m actually very curious about the geo of these bikes for relaxed off-road riding. It has very long CS’s at 560mm and a relaxed 71.5 degree STA. I imagine this bike setup with swept bars would put your CoM behind the bottom bracket. Arbitrarily I’ve chosen 50mm. That gives a CoM-R of 506mm and a CoM-F of 808mm. Again a ratio of 1.6.

I’m not saying that this proves these bike would handle simiarly. More that the CS length of each might be considered approriate for the design intent of the bike. And that discussing CS without knowing the full setup including rider position is kinda pointless.

Oh, and back to your original post Eva (@liberationfab), I noticed you were running Bosco’s on ya mountain touring bike. Obviously this is a very Riv inspired setup. I’d be interested to know if you were able to find a way to measure your front/rear weight distribution on that bike compared to the others? Might be some interesting info to have!

ben.land101 · December 13, 2023, 12:43pm

If you all haven’t read of heard of it yet, “Motorcycle Handling And Chassis Design” by Tony Foale is definitely some required reading. Yes, it’s motorcycles, but the physics behind two-wheeled things still carries over.

Screenshot 2023-12-13 073406
The book goes into a lot of these principles, this illustration is intended for fast acceleration, but the same concept applies for climbing a hill on a mtb. Commonly referred to as the ‘looping angle’. That’s why the hill climb bikes (and drag bikes) have the stretched rear end.

PVD also did a post a while back about getting the actual CG/COM.

I’ve been wanting to setup an experiment for a while to quantify just how much the COM changes based on rider position. Some way of taking live weight measurements at the contact patches while the rider shifts around. Maybe one day I’ll get around to setting that up. Even just the horizontal measurement of COM would be interesting. I suspect it’s a much larger difference than most folks realize.

ElysianBikeCo · December 13, 2023, 2:43pm

Jeezy Creezy ya’ll make me swoon when you talk nerdy!

FlatironBikes · December 13, 2023, 3:10pm

I remember that article! It definitely inspired my rabbit hole.

Between PVD, the geometry Vorsprung video, and cadaver data for average body segment masses, I built a spreadsheet using PVD’s RAD to calculate a size run based on CoM: Tuesday Tune - COM

My neighbor (a next level bike/science nerd who likes to help) and I set up a flat land weight distribution test to validate the output of the spreadsheet. (Sorry about the photo quality, we were outside in January with a house lamp )

The assumption was by using a known geometry and fit for a bike, if the measured weight distribution matched the geometry output from the spreadsheet, then the method would be sound.

What ended up happening is the positional assumption of PVD’s RAD was the incorrect model. The spreadsheet matched the measurement data in this position, but looking at the photos, no one rides around like that. A similar method built around the neutral position would be more meaningful, but haven’t revisited the project.

This recent Pinkbike opinion got me thinking about the CoM topic again. Specifically the linked video about 50/50 weight splits in cars, but viewed from a weight shift perspective.
For suspension kinematics, anti-squat and anti-rise help to combat weight shift during acceleration and deceleration.

Can FC/RC ratio be used in tandem to create a desired weight distribution/tire grip during these shift events?

manzanitacycles · December 13, 2023, 8:02pm

This is an interesting position to take. I don’t agree or disagree with you. It’s just food for thought.

We’re out of the saddle mostly during descents. When we’re out of the saddle we have the greatest ability to affect weight distribution.

It’s when we’re seated on climbs that we have the least ability to move our weight.

For anything but park/enduro/dh bikes, should we not focus more on the position that we have least weight distribution control over?

Here’s an anecdote that probably means nothing

I’ve made hardtails for two friends. They are both advanced riders who can ride pretty much anything on a hardtail. One is 5’7" and the other is 6’3". The short dude rides a 760FC/425RC bike for a FR ratio of 1.79. The big boy rides a 838FC/430RC bike at FR ratio 1.95.

Shorty rides like a jackrabbit, bouncing off all the features. Big Boy is a bruiser going down. He tends to straight line stuff more. Even though Shorty has more weight on his front wheel, he’s still able to bunny hop and get his front wheel over stuff.

On the climbs, Shorty can cruise up everything. He’s never complained to me about rear wheel traction or the front wheel lifting. Big Boy struggles up the steep climbs because his front wheel is so eager to lift up.

I don’t know what I’m trying to get at except I think tall riders on hardtails should really consider running longer chainstays. The tall riders will have to get used to a more weighted front wheel, but they also have the length to make a greater change to weight distribution when standing.

Whether we use a consistent FR ratio OR determine a good looping angle at the CoM remains to be discussed. However, I think we shouldn’t dismiss seated CoM for trail bikes.

manzanitacycles · December 13, 2023, 8:17pm

I’ve read bits and pieces of his work online but not the book. What are your big takeaways from the book?