Opinion: Do We Need Size-Specific Chainstays?

A growing number of bikes are coming out with size-specific chainstays, and the effect that chainstay length has on bike handling is a major point of discussion in reviews. Size-specific chainstay lengths (where larger frames get longer chainstays) make a lot of sense intuitively - if the front-end gets longer, the back end should too, right?

But in some cases, the idea is in danger of becoming more of a box-ticking exercise than something that tangibly benefits the rider. Many of the bikes that offer this feature don't make a big enough change to noticeably affect the handling. And even for those that do, the ideal chainstay length for a given rider depends on so much more than maintaining some particular ratio of front-center to rear-center.

To understand why, we first need to talk about what effect chainstay length has on a bike's handling.


Chainstay Length 101

Also known as the rear-center, chainstay length is the horizontal distance from the rear axle to the bottom bracket. When you stand on the bike, almost all of your weight is on your pedals - only a small amount can go through your hands and the handlebars. This means the weight distribution between the front and rear wheels is largely determined by the ratio of the chainstay length to the wheelbase.


For example, in the case of the Ibis HD6 from this year's field test, the size 3 has a chainstay length of 435 mm and the wheelbase is 1,256 mm (pretty typical numbers for a size medium these days). So, the chainstay is 34.6% of the wheelbase, which means 34.6% of your weight will sit on the front wheel and 65.4% will sit on the rear wheel.* In size 5 (effectively XL), the wheelbase is 1,328 mm but the chainstay remains at 435 mm, so the weight distribution becomes 32.8% on the front.

*I'm ignoring the suspension sag here (which affects the wheelbase and chainstay length) and the weight of the bike. This is a simplification, but it's close enough to get a sense of what's happening. I'm also assuming no downward force is going through the rider's hands, both for simplicity and because it's very hard for most riders to put a significant amount of force through the handlebar consistently. Try standing on your bike with your front wheel on a bathroom scale - it's surprising how much you need to lean on the handlebar to make a significant difference to the weight on the front tire.

That may not sound like a huge difference, but if you weigh 100 kg, then on a size 3 there would be 34.6 kg of force pressing your front tire into the dirt; on a size 5, there would be 32.8 kg. That's an absolute decrease of 5.2% load on the front tire. So, if you jumped from the medium to the XL without adjusting your tire pressure, fork setup or riding style, the front wheel would be less pressed onto the trail in the latter case.

But if we fitted longer chainstays in larger sizes, we could maintain the same downward pressure on the front wheel (for a given rider and body position) across the sizes. But how much longer?


Scale Matters

Let's say we wanted every size of Ibis HD6 to have the same weight distribution as the S3 (medium). This implicitly assumes that the weight distribution is "ideal" in the size medium, which seems a little arbitrary, but let's go with it for now. Running the numbers, that would involve the S5 having a 473 mm chainstay (which is unheard of), while the S1 would need a 406 mm chainstay (which is unfeasible, as chainstays can't get much shorter than 435 mm without running into issues with tire clearance and chain angle).

In other words, the chainstay length would have to change by 15-20 mm per size to maintain a consistent weight distribution. To my knowledge, only Forbidden is getting close to that.


Looking at it another way, YT is one of the brands ticking the size-specific chainstay box with their Capra, but their chainstays only grow by 5 mm across the range (438 mm for small to large & 443 mm for XL and XXL). On the XL, the weight distribution would have been 34.4% on the front if it had the same (438 mm) chainstay as the smaller sizes, but the longer chainstay changes that to 34.7%. That's a difference of 0.3% of the rider's total weight, or the absolute amount of load on the front tire goes up by 0.7%. Despite the change in chainstay length, the small still has 7% more front tire load than the XXL.

In short, it's a token effort. But it's not just YT; most brands currently offering size-specific chainstays are only making tiny changes - the Yeti SB160 changes by 2 mm per size, which doesn't seem worth the effort to me.


How Does Chainstay Length Affect Cornering?

Okay, so making the chainstays much shorter on small sizes isn't feasible, and making them 5 mm longer on the biggest sizes doesn't do much, but maybe making the chainstays a lot longer on bigger sizes is worth doing. After all, head tubes can't be made any shorter than about 100mm, so many bikes use the same length head tubes on XS, S and medium, but the large and Xl still benefit from bigger head tubes. Could we see a similar approach with chainstay length, and if so, would that improve cornering traction?

Making chainstays longer in an XL will result in more pressure on the front tire for a given rider position, but how this translates to reducing the risk of washing out in a turn is more complicated than you might think.


To illustrate the point, let's take things to extremes and imagine you have a bike with a 25:75 rearward weight distribution and another with a 50:50 weight distribution. Just for context, that would be like fitting our S3 Ibis from earlier with a back-end that was either 270 mm or 820 mm long.

Again ignoring the weight of the bike, sag, and any weight on the handlebar, and assuming a 100 kg rider for easy maths, there would be 25 kg of downward force on the front tire for the first bike, while the second would have 50 kg on each wheel.

You might think that since the second bike has twice as much weight on the front tire, it will have twice as much front-wheel grip, so you can corner twice as hard without the front wheel sliding. But that's not the case.


Imagine going around a big flat turn with a constant radius at a constant speed (this is called steady-state cornering). The friction between both tires and the ground provides a lateral force towards the inside of the turn (called centripetal force) that keeps you turning and stops you from heading off in a straight line. If you have a 50:50 weight distribution, so the centre of gravity (COG) is halfway along the wheelbase, both tires provide an equal amount of this lateral force. If you have a 75:25 rearward bias, so the COG is 75% of the way along the wheelbase towards the rear, then the rear wheel must provide 75% of this lateral force, and the front wheel only provides 25%. This is because, with the COG closer to the rear wheel, there's a shorter lever arm between the tire and the COG, so the rear tire has to provide more lateral force to keep from sliding out.


But the front wheel also has 25% of the vertical load (weight) and the rear has 75%. You may remember in high school being taught that the friction between two surfaces is proportional to the force pressing them together. So, the bike with the 25/75 weight distribution has half the load on the front tire, so it can produce half the lateral force compared to the bike with a 50/50 weight distribution. But with the COG closer to the rear axle, the front wheel only needs to provide half as much lateral force to get around the corner. This means that if lateral grip is proportional to vertical load, then these factors cancel out and both wheels will be equally prone to sliding. In other words, the bike with the rearwards weight bias isn't more prone to understeering (front wheel sliding) than one with equal bias.

Does this weight distribution doesn't matter? Unfortunately not.

A weight distribution closer to 50/50 is better for cornering because of something called tire load sensitivity.


In reality (despite what my physics teacher taught me) grip usually isn't proportional to vertical load. On hard surfaces, such as asphalt, the ratio of grip to load decreases with increasing load. In other words, if you double the load on the tire, the grip it can provide is less than double. So, in our extreme case of a 75% rearward weight bias, it's the rear wheel that would be more prone to sliding, not the front, because it has to provide 75% of the lateral force but it has less than 75% of the available grip.

In contrast, on soft surfaces like mud and sand, the ratio of grip to load generally increases with increasing load, so if you double the load on the tire the lateral grip will more than double. Thanks to Steve Matthews of Vorsprung Suspension for pointing this out to me and sharing his knowledge more generally. So in this case, a bike with 25% of its weight on the front wheel will have less than 25% of the available lateral grip, so it will be most likely to slide.


Putting it All Together

So to summarise, getting closer to a 50:50 weight distribution helps the bike to handle more consistently. Otherwise, either the front or rear wheel could be more prone to sliding out, depending on the terrain. It's not as simple as saying that having too much weight on the rear wheel will always cause the front wheel to wash out more often. It's certainly not the case that getting X% more load on the front tire will allow you to corner X% harder.

While this article has already gone on long enough, there's still much more to this topic, such as the ability of the rider to actively lean on the handlebar or the effect of braking to move the weight distribution closer to 50:50. Then there's the fact that most bikes use different tires on the front and rear - having a stiffer tire casing on the rear with a more aggressive tread on the front can somewhat compensate for a rearward weight bias. Also, people on smaller sizes generally have more upright riding positions and those on larger bikes have more forward-leaning riding positions due to a shorter stack height relative to reach, so the rider's weight will generally be further forward relative to the bottom bracket with a larger frame size.


The main point here is that you need to make the chainstay a lot longer to get the weight distribution a little bit closer to ideal. As a rule of thumb, you'd need to add 15-20 mm to the rear center per frame size if you want to keep the weight distribution the same. A 5-10 mm difference across the entire size range isn't doing much. Adding 10 mm to the chainstay length will only move the weight distribution of a typical bike from around 34% to 34.5% on the front (ignoring rider input).

In the past, I've tested several bikes with chainstay lengths that are adjustable by 10 mm and hastily written that I can feel a difference and prefer the longer setting. But thinking about it more deeply, extending the chainstay by 10 mm will increase rear-wheel travel by around 2.3% and reduce the spring and damping rates at the rear wheel by around 5%. If you don't adjust the shock settings, this is the main difference you'll notice. Not only that, with a 10 mm longer chainstay there will be about 1.5% more load on the front tire for a given riding position. While this difference is probably negligible, in theory, this extra load will add to the risk of tire squirm, burps and dings, so it should be compensated for by increasing the front tire pressure by a similar percentage. Otherwise, the effect is similar to running the tire softer, which will increase grip when the tire is lightly loaded, but increase the risk of squirming when it's heavily loaded. If the suspension settings and tire pressures are adjusted to compensate for these considerations, the advantage of a longer chainstay will be diminished.

In contrast, if you get your weight forward in a flat corner by leaning on the bar, you're temporarily adding load to the front tire, so it's heavily loaded relative to the tire pressure and generating good grip, but you can revert to a more neutral/rearward position in rock gardens and berms to prevent tire squirm or burping. In other words, increasing the chainstay length is not the same as riding with an aggressive body position.

With dramatically longer chainstays (such as the XL Forbidden Dreadnought with a rear center that stretches to 480 mm at sag), I think the weight distribution is noticeably better in flat turns, but there are downsides to consider too, including a shallower breakover angle (assuming the BB height stays the same); a tripping sensation on steep, technical sections; and a slightly longer overall wheelbase which requires more steering input in tight turns.

But the main drawback people notice with long chainstays is that they make it harder to manual. To lift the front wheel off the ground, you need to get your center of gravity behind the rear axle; moving the axle even 10 mm further back makes it noticeably harder to do this, and that can be a big problem for some riders.


What's the Bottom Line?

So, in an ideal world, I'd suggest picking the longest chainstay that still allows you to manual comfortably. In general, this rule of thumb would mean taller riders running longer rear ends, but it also depends on the rider's weight, skill, and cockpit setup, as well as how they prioritize consistent grip versus ease of lofting the front wheel (racing versus jibbing). It's not about hitting some magical ratio of front-center to rear-center, because there's nothing magical about that ratio in any frame size, but rather that riders who have no trouble lifting the front wheel may as well benefit from a more balanced weight distribution and the more consistent grip that goes with it.


Adjustable chainstays are better than size-specific ones because riders can choose what works for them based on many factors including terrain and personal preference - not just frame size. But remember that the 5-10 mm adjustment that's currently being offered isn't going to make a big difference to weight distribution. It would be interesting to see a larger adjustment range (at least 10 mm in either direction) become available, perhaps in combination with size-specific back ends, but I wouldn't be surprised if some tall riders still prefer a more conventional chainstay length.

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Acknowledgements

Thanks to Steve Mathews of Vorsprung Suspension, Dave Weagle and Joe McEwan of Starling Cycles for their help and advice with this article. While the opinions above are my own and don't necessarily reflect any of theirs, their insights were invaluable.

And of course, thanks to Taj Mihelich for the illustrations.

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Reviews and Tech Nerding Out Seb Stott