Nerd Alert! The case for adjustable rear center lengths and a novel approach to mountain bike fit

CGPT Summarization
“This post delves into advanced mountain bike geometry and handling, focusing on the impact of chainstay length on weight distribution. The author, a long-time mountain biker with extensive experience, argues that the common practice of using static rear center lengths across different bike sizes leads to poor handling, particularly for larger bikes. They suggest that short chainstays contribute to a rearward weight bias, making it difficult to properly weight the front end, especially when bikes have longer reaches.

The author proposes that manufacturers should offer adjustable rear centers to improve weight distribution and handling for all bike sizes. They draw analogies to skiing and dirt biking, highlighting how these industries optimize balance and handling through proportional adjustments. The discussion calls for data gathering and manufacturer innovation to create bikes that can be finely tuned for individual riders, enhancing downhill performance. The need for a new approach to bike fitting is emphasized, integrating various adjustments to achieve a balanced, well-handling bike.

The author concludes by noting the growing acceptance of their ideas in the mountain biking community and expresses a desire to further develop these concepts with a progressive bike manufacturer.”

Full Post

Looking for some top tier bike geekery? This post is for you.

Mountain biking has been in my life since I was ~12 years old (I’m 39 now). I’ve worked in bike shops, written for magazines, raced as a “pro” (in quotes for a reason), been a product tester for numerous brands and have endlessly thought about ways to make the mountain biking experience better.

It’s in this vein I penned (typed?) a post back in 2016 on a popular mountain bike website with the title “The Internet Was Wrong, Short Chainstays Suck*”, where I put forth the hypothesis that short chainstay length was favoring an overly rearward weight bias as bikes have gotten longer. This made it ever increasingly difficult to properly weight the front end of the bike, especially in the larger sizes. Being it has been ~8 years since I originally wrote about this topic, I felt this was a great opportunity to update my original hypothesis here on my own blog and put forth a novel way to utilize this hypothesis in mountain bike fitting. Before anyone carries on reading this post I need to get some disclaimers and assumptions out of the way, first…

1. This link (and my entire post) assumes you have some basic understanding of bicycle geometry including reach, stack, front center, rear center, fork offset, and head tube angle.
2. This post will commonly use “chainstay length” and “rear center” interchangeably. I also often use “rear center vs front center” as part of this argument. Worth noting, front center takes into account head tube angle and offset, whereas reach does not.
3. This entire hypothesis pertains to optimizing the bike’s handling for a rider standing in the “attack” (descending) riding position. If you are looking to optimize your ride for seated/climbing you are likely a road, gravel or XC rider, and will follow a very different criteria of bike fit.

For those who don’t have time to read the above linked gazillion-page-long VitalMTB thread, my underlying idea can be distilled in the following bullet points.

1. Mountain bikes come in numerous sizes. However, to save on tooling and engineering costs, most manufacturers only vary the front triangle of the bike to accommodate bigger or smaller riders. The length of the rear end is generally static throughout a sizing run and does not have adjustment*. (yes, I know, some of this has changed, I get into this further down the post)
2. Mountain bikes have grown significantly in reach over the last 15 years. In fact, we did not even quantify “reach” until the 2010s. Instead, a derivative number which takes into account seat tube angle called “effective top tube” (see graphic above) was common to attempt to measure the bike’s perceived cockpit length. This unfortunately is meaningless once a rider stands up being it is in part driven by the seat tube angle.
3. A rider will have very different weight distribution when standing riding a size small than a size extra large. The larger the bike, the more the weight bias will be to the rear. The smaller the bike, the more the weight bias will be to the front. Again, assuming the rear center does not change. =
4. Short chainstays were all the rage. Product managers and mountain bike brands decided longer rear centers made for bikes that were “sluggish”, hard to manual and cornered poorly, hence there was a push (pun intended) for shorter and shorter rear center lengths throughout the 2000s and early 2010s. This was fueled by absolutely nothing quantitative, with the same people echoing the statement “Twenty nine inch wheels cannot corner” around the same time.
5. Legs are stronger than arms. A good handling bike will allow the rider to have a light grip on the handlebar and dynamically ride the bike through the pedals in a neutral riding position. A poor handling bike will require the rider to get “over” the front end, riding more with their hands & arms or, conversely, hang off the back to combat an overly forward weight bias (rare).
6. Measurement is easy. An objective way to measure fore/aft weight distribution is by putting a mountain bike on two scales (front/rear wheel each gets a scale), then the rider gets on the bike in the standing “attack” position and the weight distribution is measured between the two wheels. Front wheel weight / Rear wheel weight = Weight Distribution. This does not take into account the fact bikes are generally ridden in the attack position down a hill, hence we are not aiming for 50/50 but something like 48/52. More on this below.
7. Proportions matter. My final over arching point is if we want bikes to handle better, we need longer rear centers for the larger sizes. We need to be more objective as to how we establish this length, too and manufacturers should consider offering significant amounts of rear center adjustment to allow for a rider to properly fit a bike when optimizing for downhill handling.

Before I continue, its worth noting I’m boiling down a very complicated system to two variables, front center and rear center. Bike handling is influenced by a myriad of variables such as headtube angle, fork offset, bottom bracket height (or bb drop), stem length, handlebar height (or stack), suspension setup, the stiffness of the frame and wheels and yes rear center/reach. However, I’d argue these variables are generally size agnostic; IE, headtube angle influences the bike’s handling the same way at the smaller end of the sizing spectrum as it does the larger size of the spectrum. To add, through much trial and error, alongside a lot of “borrowing” from the motorized side of the two wheeled world, the entire industry has converged toward a fairly tight set of numbers that work pretty dang well across these variables. Hence, static weight distribution is the most obvious “last frontier” to significantly improve mountain bike handling, and also the one with the most fertile ground to improving the on bike experience for all riders, from your flow trail intermediate to your top tier racers.

Whenever I’m looking at a problem I try and look to other industries for an analog. In this case, we have two great examples; skiing and dirt biking. Lets start with skiing.

Skis

Those who ski know every manufacturer makes a number of models of skis all in different sizing, just like mountain bikes. However, unlike mountain bikes, every ski has a different mount point (for the binding) that is proportional to the skis length and the size of the boot being utilized. Furthermore, every skier has their own personal preference largely driven off their technique and biometrics pertaining to a static mount point that should, in theory, put them in the perfect part of the ski. If done right, the skier isn’t fighting to get on top of the ski or leaning back to keep from going ass over kettle (or however the saying goes). A neutral handling ski is exactly what a skier is looking for, and exactly what mountain bike manufacturers are largely ignoring. Using this analogy, mountain bikes company’s are essentially manufacturing frames with the mount point drawn in the same spot, regardless of size, throughout the size lineup. This means one size might be very balanced, whereas others are very unbalanced.

Dirt Bikes

The next example is dirt biking. Unlike mountain biking, dirt bike manufacturers only offer one frame size across their entire model line of “adult” bikes. For example, a rider doesn’t go buy a 300cc 2 stroke in a size “medium”, they go buy a 300cc 2 stroke in the size it comes in, which is the same chassis size as every other offering in their line. This allows the manufacturer to spend immense time dialing in the handling characteristics they want in this one chassis size, which includes a lot of work pertaining to weight distribution, with and without a rider on the bike. Anyone who has ever ridden a dirt bike will attest to how balanced and neutral they handle with majority of your riding happening through your legs and the bike working “around” you.

Now for some good news – from when I originally wrote that post on Vital to now, we’ve seen rear centers, especially on race bikes, grow in length with a number of companies offering mild adjustments within the rear center and/or “size specific” rear center lengths. Awesome! Well, kind of. Outside of Paul Aston’s boutique test bikes (his site is a gem), we’re talking a modest lengthening, maybe 10-15mm. To add, those brands offering “size specific” chainstay lengths or adjustments are only doing so to the tune of 5-10mm, which is hardly enough for the type of “bike fit” I’m aiming for.

My proposal

Objectively dialing us into the “goldilocks” range of weight distribution is very possible. Here is how I would approach the problem…

1. Data Gathering: We first need to define what “good” looks like. As unreasonable as it may be, I propose the mountain bike industry gather as much data among a top tier group of riders/racers including weight distribution with rider on the bike, geometry of the bike, setup preferences (stem/bar) and a qualitative survey where we blindly gather feedback pertaining to how they feel the bike handles. The goal is to identify a “goldilocks” weight distribution (on the scales) front to rear we ought to shoot for, which I’d wager is going to be adjusted depending on the steepness of the terrain the rider often finds himself/herself on, biometrics and a bit of personal preference.
2. Frame Manufacturers: Next we need manufacturers to give us enough adjustment to allow the rider to dial into this goldilocks distribution. For all but a handful of boutique frames (maybe?), this is currently not happening. I’d wager we’d need upwards of 40mm of rear center adjustment for all riders to get their weight distributed properly. That said, I’d even be happy with something like 25mm, to start anyway. I want to be extra clear, in any setting where weight is a big priority (XC for instance), I do not expect this type of adjustment to be commonly (or ever) deployed. This is really aiming at your DH/enduro/longer travel trail type bikes.
3. Adjust for technique and terrain: Every rider has a different bias on the bike and we often ride in different areas with different amounts of pitch. If its steeper you may want a little shorter rear end and/or higher bars. If its flatter with lots of corners you may want to drop your bars and extend your rear end.
4. Telemetry: Though not required, it’d be great to see the weight distribution in a dynamic setting as a rider pilots their bike down a hill. Obviously, the rider can manipulate this with their body position, but if we took this data in combination with “what did you feel” type of feedback we could further triangulate toward “good”.

Turning all this into the “best bicycle fit ever”…

I’m going to assume a hyper progressive frame manufacturer listens to me and produces a frame with extensive amounts of rear center adjustment. Second, I’m going to assume, through all the data we’ve gathered, we’ve identified an appropriate range we ought to target with respect to weight distribution. What do we do with this adjustment and information? We use it as a backbone of mountain biking fit! While I’ve spent most of this post talking about rear center length, by offering the type of adjustment I’m looking for unlocks a type of fit that otherwise would not be possible (without switching frames anyway). What I envision is you as a rider fill out a questionnaire that is roughly 30 questions long. From there, you’d move to a fitting booth where your suspension is dialed to the appropriate sag and weight distribution measurements are taken. Between these measurements and what you say in your survey, we can triangulate not just on front/rear center that fit you, your body and your riding style but what type of suspension setup likely is best, handlebar height/width, stem length, handlebar height and fully dial in your bike to fit you “like a (custom) glove”.

This might sound a bit out there, but the road cycling golf community are dominated by the idea that the tool must be “fit” in order to function as intended. Mountain biking should be no different. This will keep riders from chasing their tail endlessly on suspension setup or getting on a frame that flat doesn’t work for them based on front/rear weight distribution that is required for mission at hand.

Despite what the internet may claim, we ride a bike as a collection of systems; as a bicycle. We do not ride one particular variable, such as headtube angle, in a vacuum. Headtube, rear center, front center, suspension sag, suspension progression etc all must work in symphony for your bike to work as well as it possibly can. Currently, this is not the approach we take, as bike nerds, frame manufacturers or bike shops.

So whats next? I’m not exactly sure but I do know this idea dies if it stays on my one silly blog page with nobody running with it. Hopefully we can get a frame manufacturer to buy in and start gathering data (the most important part), and testing with some of these ideas. From there? Who knows! But I sure would love to go into a bike shop in a few years to get “fit” on my hot new bike in a way that is much more likely to put me, the rider, exactly where I need to be to ride faster, be more consistent, have less fatigue and ultimately have more fun!

Thanks for reading!

Addendum

While I’m sure others have drawn the same correlation between rear center length and bike balance, it is fun to note that my after posting to VitalMTB (which has garnered 50,000 views), there have been a high number of articles posted to Pinkbike, Reddit, MTBR, Singletracks Mountain Bike News and (according to Google) a lot more. Yes, I’m partially noting this to toot my own horn, but to also note that the idea of proportional rear centers have become more and more accepted and talked about in the broader mountain bike ether.