Ask Uncle Dave
Dear Uncle Dave: Why aren't my chainstays longer?
The idea that it’s ideal to have a constant rear center across a whole size range is patently absurd. It also seems absurd that a $3,000ish frame would not have the ideal rear center.
Fortunately, more and more of the new bikes have varying the rear center across sizes. I haven’t built a spreadsheet, but the variation from small to XL seems awfully small. Thinking it might be worth an article. I know I’d be very curious to hear NSMB’s take.
What do the brands seem to be doing and how do the changes relate to the changes in front center on a percentage basis? Is that what the designers really think is best? Or, are the small not shorter because they can’t make them shorter (i.e. maybe the smalls should be 27.5)? Are the XL’s not longer because they are scared the market won’t accept them? Is the small range across sizes simply because they are not actually making different rear triangles but somehow creating some rear center variation with the linkage? Would a bigger range mean they needed to design different suspension kinematics across the sizes as well, which would add even more cost?
Cheering for Different Chainstays
(Cover photo courtesy: wakka-bicycle.com)
I often wonder what regular people would think if they knew the sorts of topics that cyclists got excited about. The way we go on about chainstay length and handlebar width we’re like kids huddled around in the locker room, whispering about the fuel economy of the latest compact sedans. This question made its way into the NSMB group Whatsapp chat, and the lid blew off. Christmas for bike nerds. I managed to kill the conversation by suggesting everybody formally participate in answering this question. It’s fascinating how often I’m able to bring a robust conversation to a crashing halt.
I’m also surprised by the way that these conversations about things like chainstay length and bar widths can swing, with absolute certainty, from year to year. Wild reactions are followed by even wilder swings back in the opposite direction. Watch a video from 10 years ago; it looks like riders stole the handlebar off a toddlers push bike, and stole their geometry from a late 90’s XC race. It’s not that long ago that most arguments about chainstays were about whether or not they were short enough. It’s not that nobody was pushing for longer chainstays, but the contrarian position tended to be a push for a few extra mm over the absolute minimum. We were certain, but we were wrong.
We also seem to have some sort of certainty that a model of bike rides the same throughout the size range, and no matter the height of the rider. It’s not unusual for a reviewer to meddle with sizes and to speak of the vast difference in handling, but we haven’t quite made the leap to extend that thinking to how a particular model might work for riders of different sizes. Digging in to such things might go some of the way to explaining why one review is glowing while another is not.
So, it’s fabulous that chainstay length has now entered the zeitgeist, but the conversation still lacks focus. Look at the companies that are playing around with size specific geometry (hat tip to Tim Coleman for summarizing this one). (Generally speaking) you’ve got (ignoring what is going on with the new Range) Norco adding 5mm as they go up a size (by moving pivot locations). Santa Cruz is moving pivot locations, but has some ranges in their jumps. Transition shares sizes between S/M, then jumps up 6mm to another shared size for L/XL (by using a different sized rear end). And then, with the aim of keeping the front/rear center ratio constant, Forbidden is jumping 14mm with each size (by moving pivot locations)! Combine that with all the other companies that do nothing, and it would be fair to say that there isn’t much of a consensus on something as basic as where one should be located relative to the two wheels on the bicycle.
With such drastic interpretations on a topic, it feels like we need to take a step back before we get too deep. A fair first question might be “what happens to a bicycle when you lengthen the chainstay?” For me, this starts with a discussion about weight distribution (which is not a simple topic). I started thinking about it seriously over the past year or two for a variety of reasons:
1 – I tend to run my rear ends a bit stiffer than recommended. I tend to run my forks at recommended, or sometimes even a bit softer. I wondered if height played into this.
2 – Cam keeps preaching the gospel of “getting centered.” He talks about riding with a more centered position a lot, as well (zing!). Again, I started to wonder if this was an easy thing for a guy with long legs and a shorter torso to say.
I’m not going to share the numbers here, because the thought of publishing an Excel sheet for everybody to pick over frightens the hell out of me, but I built a model to look at changes to weight distribution based on bike size. Assuming similar body proportions and everything else being equal (namely, the percentage of weight on the hands and feet), I proved to myself these fundamental rules of weight distribution.
1 – Shorter reach puts more weight on the front wheel compared to longer reach.
2 – As you increase stack, your hands move backwards relative to the front wheel, and less weight is placed on the front wheel. A rider on an XL frame with a 50mm stem will have his hands much, much further behind the front axle than the equivalent rider on a medium frame with the same sized stem.
3 – Longer chainstays place more weight on the front wheel compared to shorter chainstays.
4 – The further back your weight is on the bike, the larger the gap in weight distribution between sizes. Leaning forward there is much less of a difference than when you’re hanging off the back of the bike.
Combine this all together, and what it results in is a fair amount less weight on the front wheel for a similarly proportioned rider on an XL compared to his shorter friend on a Medium, across most situations. It also starts to explain why I need to run my rear shock a bit stiffer than a shorter rider, as I am both a) able to much more easily get further off the back due to my long legs and b) a larger percentage of weight moves to the rear when I do so. This all creates a really compelling argument for proportionally longer chainstays on larger bikes, and companies that do so are able to narrow this weight distribution gap. It also suggests that there are other parts of our bikes where it doesn’t make all that much sense that we’re all running the same length (I’m looking at you, stems).
But none of this really tells us all that much about how a bike is going to ride. If I enter a turn on a medium bike with 39.5% of my weight on the front wheel of my Medium bike, what difference will there be to the tall guy who follows me with the exact same technique but can only manage 38.1%? And how much of a change will he see if he can cut that gap in half? And what happens when he climbs a hill? There’s a whole other conversation that needs to be had about how the difference in effective seat tube vs. actual seat tube angles impacts taller riders far more than shorter riders.*
We also need to consider the changes to the bike as we start messing around with pivot locations, or changing the lengths of suspension members. It’s fairly obvious that if you throw a longer chainstay and seatstay on but keep your pivots the same, you’re going to impact the amount of travel and the leverage curves. It’s a bit less obvious what happens as you shift your pivots relative to your bottom bracket. The suspension may still move the same, but the relationship to your bottom bracket is different, which has an impact on how the bike pedals, and even on how the wheel is travelling (relative to the rider, that is). Put this all together, and you understand why a lot of companies are happy to pick one chainstay size and stay the course.
This felt like a pretty good time to reach out to some bike manufacturers and see what they had to say on the topic. Santa Cruz was where I started. They’re not generally the first to experiment with geometry, but they’re never the last. Once they start down a road, there’s a pretty good chance it’s a thing. I reached out to Senior Product Manager Josh Kissner to see what he could tell me about Santa Cruz and chainstay lengths. Here is what he had to say.
Josh - I will start with this caveat: Who the hell knows! Ha. On any given bike, there will be people who think the CS are too long and also people the same height who think they are too short. It is very much preference and terrain dependent- there's no objectively correct chainstay length. Also- none of us know what it's like to be a different height. We know what it feels like to be 5'9" or 6'2" or whatever, but have to rely on other people's feedback to try and understand what they are feeling. This makes it a very unscientific process and a difficult one to "test". That said- we do debate it quite a bit.
Somebody has obviously never seen Gattaca!
Question #1 - How is Santa Cruz sizing up chainstays as sizes change? Is it pivot/bb location changes? Different rear triangles? Something else?
Josh - We do it by moving the pivots/shock mount on the front triangle. It's MUCH easier to do this on carbon bikes since the FT tools are all unique anyway, vs trying to make different forgings etc. Still adds challenge and work to the engineering process, but it's relatively simple after they're designed.
Question #2 - Why do it? What's the end change to the handling of the bike that one experiences with longer chainstays on larger bikes. I mean, we went this long without worrying too much about it. There must be some tangible change to the handling that is achieved.
Josh - The #1 thing I think about with CS length is the wheelieability of the bike (aka the amount of weight on the front wheel), for better and worse. Sometimes it's totally awesome to pop the front wheel up and over something (encountering a wet diagonal root in the fall line), and sometimes it's not (climbing, flat corners).
The #2 attribute is just the total length of the bike and the stability thereof. When smashing through chundery junk, a long bike feels amazing. The bike is way less sensitive to body position/weight shifts and is just generally more confidence inspiring. However, when slithering through tight corners or trying to make quicker movements, it can feel awkward or difficult with a really long bike.
#1 and #2 are highly preference based, but it's tough to argue with the theory that a taller person needs a longer chainstay to get a given wheeliability or wheelbase stability. Our goal is to pick a bike's character and try to get everyone the same feeling.
Question #3 - How did you get to the jumps that you did? Looking at the new Bronson, there's some 4mm jumps, some 3mm jumps and a 5mm jump. I guess this seems a bit less arbitrary than flat 5mm jumps, but there must be some science buried in there somewhere, right?
Josh - Definitely the subject of much debate. The different jumps in our CS length on the Bronson reflect the different jumps in the reaches between sizes. So if there's a 25mm gap between reaches, we do a 4mm CS jump. Between M&L we have a 20mm gap in reach to more precisely fit people in the thick part of the height bell curve. There we did a 3mm jump, as the difference in rider heights will be smaller between those two sizes.
We've looked at making the jumps so the FC/RC ratio remains constant. However you end up with some pretty wacky numbers if you do that- particularly at the extreme ends of the size spectrum, so you end up moderating them until they seem reasonable. And at that point, I think you're accepting that a constant ratio up the size-run is not the right metric. It is certainly a reasonable theory but not mine. In my conversations with tall people, none of them are asking for the length of chainstay necessary for this. At the other end of the spectrum, I don’t think short riders need a crazy short chainstay, either.
What we do is take feedback from riders of different heights and try to come up with some jumps. I'm 5'10" and will come up with my preferred length on a given platform through testing with dropout chips. We have other testers that are taller and shorter. We have people that have been riding our bikes with dropout flipchips for some time and giving us feedback on the various positions. We take this info into account and make our best guesses, honestly. Again- it's preference. There's no "correct" chainstay length for a size large or whatever. It's tradeoffs and opinions. There are also practical constraints that come into play. In some products, we can only go so short without interference of some kind. In this case you may see multiple sizes with the same CS length.
Question # 4 - Is there a better way? If time, money and resources weren't a concern, would you go about doing this differently? What are some of the intangible constraints that put Santa Cruz on this particular path?
Josh - Hmmm. Well if money and resources weren't a concern, we'd probably make a bunch of swingarms with different chainstay lengths so everyone could pick exactly what they want. That's not reasonable, so we've used flip-chips on some of our bikes to offer some adjustability. The problem with those are fiddly bits and bobs and a general coarseness to the adjustment. The way we're doing it on our latest bikes allows us to pick exactly the CS length we want- to the millimeter, without any extraneous parts and accessories. I think it's the best reasonable way.
This article is already tremendously long, but I reached out to Norco, as well, to get another perspective on this situation. Of the larger brands, Norco has been very progressive with geometry, and have been putting out some interesting bikes with aggressive angles and size specific geometry. I reached out to Engineering Manager, David Cox, for his perspective on this.
Question #1 - From my experience with the Shore, I assume you're accomplishing your rear centre changes the same way with your other bikes - that is moving the pivot locations relative to the bb. If not, feel free to correct me on that. It also seems like most of the other bikes are seeing a similar 5mm jump in length with each size increase. How did you make the decision that a consistent 5mm jump was the right way to go? Why not smaller/larger?
David - Yeah exactly right, we move the bottom bracket relative to the suspension pivot locations. Not many other brands even have different rear centres for different frame sizes. However, the few that do, generally achieve it by physically changing the length of the chainstay or seatstay (or both) weldments, or by using a small flip chip in the dropout. By doing this, their leverage curves will get translated up or down, to an extent that would likely require a shock re-tune. The way that we do it, by moving the bb relative to the suspension pivots, means a consistent leverage curve for all frame sizes that we match our custom shock damper tunes to.
On Shore we use 5mm jumps in rear centre between each frame size, but some of our other bikes use different jumps. Determining the jumps for each platform is part of our holistic approach to bike design that we call “Ride Aligned”: Considering how fit, geometry and kinematics interact, whilst considering the bikes intended use and ride feel we want to achieve. We use anthropometric (human measurement) data to calculate where the rider’s centre of gravity is on each frame size. This allows us to calculate the bike/rider systems weight distribution at the contact patch (where the tires meet the dirt). Through testing we validate the weight distribution and apply this across all frame sizes, so riders of different heights on their respective frame sizes all have a consistent weight distribution. We address differences in rider morphology (body shape), sex, skill and body position in bike cockpit and suspension setup through the “Ride Aligned Bike Setup Guide”, to ensure all riders experience the bike the way we intended it to ride. Simple, eh?
Question #2 - Why do it? What's the end change to the handling of the bike that one experiences with longer chainstays on larger bikes? I mean, we went this long without worrying too much about it. There must be some tangible change to the handling that is achieved for all of that effort.
David - We’ve been doing “Gravity Tune” for 11 years, originally developed for Aurum back in the dirt Norco days. So we have been worrying about it for a long time… 😉
You can look at it as longer for the bigger sizes, but in reality, most brands are designing around the “large man” and not really optimizing the bikes for people who ride other frame sizes. But we want our bikes to ride well for riders of all standing heights. Chainstay length per frame size especially benefits the more fringe sizes, for the small and tall riders; they experience the optimized “balance” of the bike that the “large man” does. So, the tangible change to handling would improve grip, agility, stability, confidence, reduce rider fatigue … all of that good stuff.
3 - Is there a better way? If time, money and resources weren't a concern, would you go about doing this differently? What are some of the intangible constraints that put Norco on this particular path?
David - The way we do it by moving in the BB relative to the suspension points is the best way to do it. However, for bikes like Range where one of the pivots rotates concentrically to the BB, we cannot use this method. So we have a different way…
For Range, we change rear centre length with bolt on dropouts. Without correction, this would change leverage for each frame size. However, we correct for this in the link arm, and each frame size has a link arm with slightly different pivot positions, allowing us to maintain the same leverage curve for all frame sizes, which is imperative when developing a shock tune to match the leverage of the bike. These size specific link arms certainly add design complexity, but the result gives consistent ride feel across all frame sizes.
So, this is two companies commenting on why they bother with size specific geometry and rear centres, and perhaps a brief glimpse into the future of what might happen with size specific bike design. It will be interesting to see how companies respond to bikes that are taking a different path (the new Norco Range with its size specific angles, and the new Dreadnought with its extreme take on size specific geometry). Heck, it will be interesting to see what reviewers think of these bikes and if it is swayed one way or another by rider size. Will this lead to a(nother) revolution in geometry, or will most companies continue to tiptoe in? Interesting times, for sure, especially for us taller riders.
*I think I might have a solution to this. Maybe somebody has done this already? Anyhow. We have reach. We have stack. And then we have the wild west of seat tube angles. What if we came up with a sort of “negative reach” measurement at different heights, rather than seat tube angles? That is at X height, the center of the top of your seatpost will be X mm behind the bottom bracket? This really gets rid of all of this dual seat angle nonsense and would start to show how much this might change depending on what height you are.
We may have a prize, but I'm not sure (sort!) but we'll let you know either way. Send us an email and we'll let you know. Also, if you have a question for Uncle Dave, even a terrible one (for shits and giggles) let him know.
Uncle Dave's Music Club
I think it is fascinating to begin to understand the contributions of each member to a band. You often can’t begin to understand until the band blows itself apart and members head in their various directions. For example, there was some sort of alchemy to the Unicorns that was never re-captured when the members moved in their own directions. Alden needed his weirdness kept in check and Nick seemed to benefit from the inspiration.
Similarly, Wolf Parade kind of sucked once Hadji left. I never thought he contributed all that much, but the quality before he left was on a different level then after.
The most interesting one though has to be Frank Black and the Pixies. It’s fair to say that Frank Black seemed to receive the lion’s share of credit for any Pixies success. Kim Deal got a bit of credit in hindsight, but David Lovering and Joey Santiago are relatively un-credited. Frank Black’s solo output (and recent Kim Deal-less Pixies…ummm…music) point to the original band being far greater than the sum of its parts.
Still, as far as Frank Black is concerned, I still get some pleasure from Teenager of the Year. I think part of it has to do with me having it on heavy rotation one time that I was busy re-reading the Hitchhikers Guide to the Galaxy series. This combination seemed to really work. They’re both a bit weird, and a bit spacey (somehow).
Headache – This has to be Frank at his poppiest. I kind of love it. So much polish. So much melody. Still weird.
Thalassocracy – Like, how is this on the same album as “Headache”? This reminds me of the tear-the-doors off Pixies moments. It could go a tiny bit further in that direction, but I’ll take it. It’s like Frank took influence from the grunge that he influenced.
Hostess with the Mostest – So wonderfully weird! Yet poppy! I always imagine this song playing in the background while a cow explains his best cut of beef to me. That could, quite literally, be the hostess with the mostest.
Superabound – If there was ever a Pixies song as a Frank Black solo object, this is it. The slow build to a small explosion is so great.
There’s a few other great songs on this album. Everything else is pretty solid. At the end of the day, it leaves me with a bit of a Trompe Le Monde feel. God, I wish he could have kept going like this.