AASQ #142: Why are brands moving Boost frames from 52mm to 55mm chainlines?

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We’re keeping it short and sweet this week, joined by Julian Litterst, Marketing Manager at Scott Sports to discuss why more and more of the big frame manufacturers are moving their Boost (148mm x 12mm) spacing mountain bikes from the standard 52mm chainline to a 55mm chainline.

2022 Scott Spark RC & 900 XC trail mountain bikes, light fully-integrated cross-country MTB, XCO jump

Our man, Cory Benson, ripping on the new 2022 Scott Spark which now runs a 55mm chainline. Watch the full review here. c. Scott, photo by Daniel Geiger.

So, what’s up with 55mm chainlines on 148mm bikes? The industry settled on 52mm a few years back. Now Shimano says 55mm is the way to go and Trek’s all-in with the revamped Top Fuel (55mm chainline not just with Shimano cranks, but also SRAM and E13). I just can’t believe 55mm in the lowest gears can be good for a chain.

Julian Litterst, Scott Sports: 55mm chainlines present no problem at all for the chain, cassette, and chainrings. This change has been tested on the world cup circuit for almost two years now. With leading groupset manufacturers stating that the increase in width is the way forward, we are confident that their products are being designed to work perfectly in this scenario.

2022 Scott Spark RC & 900 XC trail mountain bikes, light fully-integrated cross-country MTB, complete

The 2022 Scott Spark 900 Tuned AXS XC bike’s 55mm chainline gives room for up to 2.6″ tire clearance as well as the massive 40T chainrings that Nino races on around the widely-space PF92 bottom bracket.

From a frame engineering point of view, even tiny increases in clearance in this critical area can substantially help with frame strength, stiffness, and/or tire clearance. We are always fighting for space in the area around the widest point of the tire and the chainring. To move to 55mm, therefore, presents a good opportunity for us to progress the performance of our frames.

2022 Scott Spark RC & 900 XC trail mountain bikes, light fully-integrated cross-country MTB, mudd detail

How does chainline affect Q-Factor, and is this something frame manufacturers consider?

Julian Litterst, Scott Sports: Q-factor has increased a couple of mm, but in the near future new crank development aims to adjust the Q-factor for these changes in chainline width, therefore moving back to the narrower measurements. With these changes coming through, a 55mm chainline will therefore establish itself as the standard in the future with any potential downsides having been corrected for.

hyper spark dangerholm scott sparkrc white dream built schmolke carbon seat post

Dangerholm’s staggeringly beautiful build of the “Hyper Spark”, a sub 10 kg complete build – see stratospheric spec list here.

Editor’s Note:

We also sent the reader’s question out to Travis Ott, the Marketing Manager at Trek Bikes. He said “it’s really a convention dictated by the drivetrain manufacturers. Anytime the drivetrain manufacturers can give us a couple more mm in that critical area for tire clearance and pivot junctions and bolstering chainstays, we’ll take it every time.”.


The Trek Top Fuel XC Bike (now pushing into the Trail category) has also moved to a 55mm chainline for this model year

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