Opinion: The Big Short - Smaller Wheels Don't Require Shorter Chainstays

Mountain bikers love meddling with dials, spacers, air pressures, …heck, anything that can be readjusted has probably been changed. Flip chips are one of those features commonly built into bikes to slacken or steepen the head angle within the manufacturers guidelines. We’ve also seen those invertible pieces used to change the suspension’s leverage rate, as seen on Trek’s Session. Another place they’re popped up more recently is on the rear triangle, allowing riders to switch between 27.5" and 29” rear wheels.

Lately, I’ve noticed a few new bike models which can accept either rear wheel sizes, but that comes with a compromise - the 29" chainstay is longer than the 27.5 setting. Take for example the new Scott Ransom and the Arc8 Extra.

You might think the rider who wants the smaller rear wheel is looking for more maneuverability, so surely they’d appreciate a shorter chainstay too. While that may be true to a degree, it drastically changes the tone of the bike. In fact, I’d argue that it potentially ruins the nature of the bike by positioning too much weight over the rear wheel. Changing the rear wheel size already changes the front to rear balance of the bike, even without altering the chainstay length.

We can look at the shift that downhill bikes underwent as they moved from dual 29" with relatively short chainstays, to mixed-wheeled configurations with chainstays that measure longer than their 29 counterparts. This long rear center and small rear wheel combo retains stability but allows the wheel to tip into corners easier.


I recently had the opportunity to put that theory to the test by riding a yet-to-be released DH bike that used multiple flip-chips, one to switch between rear wheel sizes and another to alter the chainstay length. Not every bike can offer both of these flip-chips, though. That can be due to packaging constraints, added cost, negative attributes which influence the suspension kinematics linkages, or just the brands’ intended ride characteristics.

The Giant Reign and We Are One Arrival are two enduro bikes with medium to longer chain lengths (443 and 447mm, respectively) that can accept either rear wheel size without altering the geometry. During the review period of these bikes I had a chance to try back-to-back comparisons with both wheels. Although the geometry doesn’t change on paper, there is an inherent effect in which the smaller wheel makes the bike feel like it has a shorter chainstay already.

The larger diameter, and therefore heavier, rear wheel simply brings along greater forces, in both vertical and lateral directions, meaning It can kick you around when plunging straight down the hill and requires more leverage to initiate the bike into a turn. When you switch to the smaller wheel and a shorter chainstay, the effective center of mass changes dramatically.

What’s the answer then? After noting my qualitative changes between wheel sizes, bike brands who would like to offer two rear wheel sizes should consider two chainstay length options as well. A more sensible option would be to keep the chainstay length the same when swapping to a 27.5” rear wheel if you’re looking for a more playful bike, or better yet, make the chainstay even longer if you want to retain that stability.

Is that cheap, take less resources, or make bikes less complicated? Certainly not, but simply slapping a flip-chip in there for the sake of fitting both wheel sizes could be hindering the performance of your otherwise outstanding bike.

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Reviews and Tech Opinion Matt Beer