“If the front or rear center is too short, this will reduce the bicycle’s stability whereas if the front or rear center is too long the bicycle will be less maneuverable. – UCI Technical Regulations
It is no secret how chainstays affect the handling of a bike. Even a dry, technical document like the UCI technical guidelines states very clearly what a shorter or longer chainstays does. Yet to read the forum discussions and marketing material around them, you could be forgiven for thinking that there is some dark magic going on back there.
As a whole, geometry’s evolution from old school to modern has not been a smooth-flowing process—it comes in drips and drabs. Take reach for an example. Mondraker drew a blueprint for longer reaches more than a decade ago, but it is only recently that we have been able to have intelligent discussions about it. I think Matt Beers’ recent editorial represents a growing understanding that reach isn’t some magic number for brands to compete over, but an actual functional part of bike fit and performance.
To understand the question around chainstays, you first need to fully understand what has happened to reach. Yes, bikes got longer, but by how much? It is hard to find old reach values, if for no better reason than it was not a common metric that even appeared on geo charts a decade ago. But there’s one bike I know, so I’ll use it as a benchmark: the original Ibis Ripley. Yes, it was launched in 2013, but Ibis were conservative even back then. The large had a 406mm reach and Mike Levy liked to tell everyone that all bikes should be like the Ripley. Today, the current large Ripley has a 475mm reach—a growth of more than 60mm—and there are still many size large bikes with longer reach numbers than that.
This goes a long way in explaining the industry’s fixation on short chainstays. Looking back, if your reach was 406mm, the diameter of a 29-inch wheel prohibits getting your chainstays short enough to even match the reach. With a bike this short, every millimeter closer probably felt better, and this is how dogma emerges. Unsurprisingly though, if you add 6 centimeters to the front center of a bike, the overall balance dramatically changes. Balance is the keyword in that sentence—I believe the next big step for progress is starting to have sensible discussions around chainstay length and to stop using descriptors like “nimble”, “playful” or “snappy” around them.
I’m 1.75m/5’9” – an average-sized human, more or less. Last summer I was on a 29er with 465mm reach, 64-degree head angle, and 435mm chainstays. Coming off a few years of almost exclusively running longer-chainstayed bikes, the first thing I noticed was the weight distribution. Initially, it manifested as less weight on the rear axle, so less traction.
The first consequence of me trying to mitigate this was to hang off the back of the bike to keep the rear wheel traction at the level I was used to from my other bikes. Soon I began to realize that this tactic was compromising my riding because I couldn’t be off the back and put enough weight on the front end at the same time, so the bike started to understeer. Realizing this, I turned a few dials and had a little chat with myself about riding properly.
Pushing my weight forward allowed the steering came back under control, but there was a compromise again. With my weigh forward, there now was not enough weight on the rear wheel. I could feel it in every corner, I had to delay my exit just a little longer to let the back of the bike compose itself. If I tried to be aggressive and exit early, the rear would break away and cause oversteer. Waiting like that in every corner felt like I was losing control, and with it, speed. Maybe this oversteer is what people mean by “playful” chainstays? Certainly, it makes intellectual sense – less weight on the rear axle will make it easier to pull after you, although I am not convinced there is a benefit there.
In the past I had another bike that was doing something similar. It was a 29er with a 450mm reach, 66 degree headtube angle and 435mm chainstays. I noticed the front-to-rear imbalance, although since it was a shorter travel bike I noticed it more on the climbs when the rear would slip away from me under power. On that slightly smaller bike, I could solve the issue by adding 10psi to the fork and going from a 50mm to a 40mm stem. This shifted my weight backward and I felt much more centered on the bike.
On the bike last summer, though, I was out of options. Maybe I could have found a 180mm fork, or tried a 32mm stem instead of a 40mm, but both of those came with downsides that did not appeal to me. Maybe there was more I could have done with the suspension? At the end of the day, my feeling is that if you have to go to those lengths to feel balanced, then there is a fundamental flaw with the bike’s geometry.
Reading the comments on Matt’s editorial, I saw a few people lamenting that the bike industry always seems to get the sizing wrong for X or Y-size people. There is a grain of truth in that, but the answer is far simpler and more human than people think. I believe it comes down to the size of the person developing the bike. I have always felt that I fit well on Specialized bikes, which made complete sense to me when I met their head of MTB R&D, Brandon Sloan, who happens to be about the same height as me.
After years of experimenting, I now have a preferred minimum chainstay length – 440mm for a 460mm-ish reach bike. I cannot find a downside to running chainstays at that length, in fact, I feel these slightly longer chainstays help me be more playful with my riding as I feel centered on the bike. Maybe what we need is a recalibration of perceptions? A few years ago 420mm was a short chainstay. Today it is 430mm, why couldn’t it be 440mm tomorrow? More interestingly, people like Seb Stott are starting to ask questions about what happens if we go even longer.
Implicit in all this is the idea of proportional chainstays. If there is a relationship between reach, head angle, and chainstay length, it follows that if you change one you will need to change the others. In the absence of a golden rule for proportion, how are bike companies going to make bikes that give a similar riding sensation at their smallest and largest sizes? Some brands do proportional sizing that addresses this by changing rear-centers along with reach, but most still don’t. Will the industry as a whole see the importance of balancing bikes, or with model most brands have continue to prevail?