We know, there’s no such thing as a stupid question. But there are some questions you might not want to ask your local shop or riding buddies. AASQ is our weekly series where we get to the bottom of your questions – serious or otherwise. Hit the link at the bottom of the post to submit your own question.

We’re keeping it short and sweet this week, joined by Rene Krattinger, MTB Product 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.

Rene Krattinger, Scott: 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?

Rene Krattinger, Scott: 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.”.

2022-Trek-Top-Fuel-full-bike

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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15 comments

  1. B on

    soooooo, if I can read what wasn’t being said(but everyone is thinking) is that drivetrain manufacturers are moving us towards super boost

    Reply
  2. FrankTheTank on

    IDK about this. I already have an unhealthy amount of friction in my largest cog, and to a lesser extent in the 2nd largest cog on a Shimano 1×12 drivetrain. I would much rather have friction in the smallest cogs, when I am going fastest and gravity is helping me, rather than when I am working hard going uphill. I have actually been thinking of using a non-boost chainring to move the chainlink inboard further.

    Reply
  3. Chris on

    I man I hope not. But it wouldn’t surprise me if mountain bikes go to Super Boost and Gravel/CX bike go Mid-Boost. The bike industry must ensure that road, gravel/cx, and mountain wheels are not compatible across disciplines.

    Reply
  4. Gary on

    Yeah, the old “Boost” is the new “Normal” when it comes to Shimano’s latest MTB 1 crankset chain lines. 52mm is what you get now even when you choose the narrowest Q-factor option. It’s making me think twice about upgrading the drivetrain on my older hardtail with short chain stays and 135QR hubs.

    Reply
  5. alloycowboy on

    Well the bike manufacters already changed, the wheel size, bottom bracket standard, rim width, and are currently going to bike manufacter specific stem/handle bar combos. They need to change another standard so you have update your bike and drivetrain.

    Reply
  6. DOUGLASS MCCRAE on

    How long is it between when a new standard is introduced and when the parts become unavailable due to supply chain issues?

    Reply
  7. Benjo on

    0 offset (55mm) sram drivetrains have been around for ages, only Shimano is new to this game.

    The drivetrain wear increase you see with wider cranks (Vs cassette) would only really be useful if you wanted to shove something extra into the space near the BB like a through frame shock. On most designs I don’t believe the additional room it provides would be worth it for the increased drive wear and noise. Like everything it’s a trade off but I would say chainlines are too wide already.

    Reply
  8. Bron on

    So will this make it even worse to back pedal at all? On my 2021 slash if you back pedal even half a full rotation it will drop the chain in the biggest two cogs.

    Reply
  9. Matthew Hamilton on

    Why does everyone use the term standard? In reality there are no “standards” anymore. Just more headaches and hassles for bike shops and customers. How many of us weekend warriors will REALLY notice the difference? Pros may feel it pushing the extremes but for the rest of the world????

    Reply
  10. rokkitan on

    So the answer is that 55 mm chainline works and makes it easier/possible to design better bikes. Only downside is possible wider q-factor, which if I understand correctly isn’t relevant for Sram (due to offset chainrings) and Shimano is working on reducing it. I understand that they have no answer to increased wear (not relevant on the world cup circuit) and dropped chain when backpedaling, so they don’t answer that/just ignore it.

    I’m not a big fan of Cannondale, but I do wish the bike industry had adopted asymmetrical rear (and front) ends when moving to boost. Then we’d get symmetrical spokes on wheels for the best strength and durability. The chainline would supposedly be moved out by 6 mm, so a 55 mm would be one the narrow side and prioritize the large cogs on the cassette, but I assume 56,5 (used for super-boost) would work fine. Then we could forget super-boost and just use 148/110mm hubs and get all the clearance and stiffness the manufacturers want.

    55mm chainline on my latest frame (Yeti ARC) was not a nice surprise.

    Reply
  11. Tom on

    The guys who design bikes are engineers. Therefore, they are always looking to make marginal improvements in design. So this change allows more room in the critical BB area, including the opportunity to spec slightly larger, stiffer and more durable pivot bearings. In the meantime, you can continue to beat on your current bike until you need a new one

    Reply
  12. David on

    By the time the supply chains catch up with all this none of us will be alive anyway. How about let’s have a little bit of time where products are actually available before the next ones come out yeah?

    Reply

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