55 Zone Ahead: The Messy Story of Modern Mountain Bike Chainlines


55 Zone Ahead: The Messy Story of Modern Mountain Bike Chainlines

When a new standard arrives, there’s usually a backlash lasting months or even years. But the 55mm chainline got in under the radar. Though not a “standard” in the traditional sense, 55 does impact cranks, chainrings, cassettes, and frames. So, it counts. The goal is to offer more room for wider tires, sturdier frames, and bigger chainrings by bumping that (single) chainring outboard to sit 55mm from the bike’s center line. That’s 3mm further than the 52mm chainline most brands have been using since Boost 148 dropout spacing took hold. This issue may seem pretty inside-baseball, but when we learned SRAM Transmission was designed specifically around a 55mm chainline, Travis Engel figured it was a good time to take a closer look.

Most of us don’t have strong opinions about chainlines. There’s more important stuff to worry about. Like press-fit bottom brackets or headset cable routing or local elections. But chainlines have played a role in nearly every major evolution in the modern mountain bike drivetrain. As a recent example, chainlines went from approximately 49mm to approximately 52mm in 2014 when hub widths started going from 142mm to Boost 148. It’s hard to imagine, but the fate of the 29-inch wheel was still uncertain back then. The wheels and frames were flexy, chainstays were long, and tire clearance was limited. Boost widened the hub flanges by 6 mm, stiffening the rear wheel. And moving the chainring out just 3 mm allowed for a wider, stiffer frame junction, and thus, shorter chainstays.

Trek developed Boost alongside SRAM with the 1×11 drivetrain in mind, but the mountain bike industry still hadn’t completely shed the front derailleur. “The first prototype was built with three (three!) chainrings,” remembers Dylan Howes, Trek’s Senior Mountain Bike Frame Engineer. “By the time we moved close to production, though, we were building around 2x cranks.” Intertwined with this shift in chainring position was a complicated push and pull involving Q-factor, the lateral distance between the pedals. “At the time, we really wanted to stay with the ‘normal-for-the-time’ Q-factor and not move the arms out wider,” says Howes. “But one of the limitations of this was being able to have the front derailleur cage clear the normal-width crank arms.” This was quite a tightrope for Trek to walk. They knew there would be some popular resistance to Boost 148. 142mm spacing was brand new compared to 135, which had been around for two decades. Increasing Q-factors would not have helped Trek win hearts and minds, and ignoring the front derailleur altogether would have caused riots. At least in 2014.

So, they worked with what they had. And even with its delicate compromises, it is not an overstatement to say Boost made the modern 29er possible. But Howes says that, in a world free of front derailleurs and Q-factor concerns, it may have happened differently. “Had we known what 1x setups would bring, then yes, I’m quite sure we would have gone longer [than 148]. In hindsight, we should have used 145mm or 152 OLD (over-locknut-dimension) hubs.” It’s confusing, but that 7mm discrepancy involves a difference between how thru-axle and traditional quick-release hubs rest inside a dropout. Basically he means 152, which wouldn’t have been entirely new. “145 was sorta standard at the time, used on some tandems. And both Bradbury at Manitou and Charlie Cunningham, I believe, were both doing 145 long before.” Yet more ways that Charlie Cunningham saw the future, but that’s a whole other story.

The front-derailleur-free world immediately came into focus in 2016 with the introduction of Eagle 12-speed. It brought us (almost) all the range we could ever want. And although chalinlines didn’t change, the chains themselves got pushed to their limit. SRAM had to pull a dirty trick to add that extra gear without widening the freehub body. The Eagle cassette dishes its largest cog inboard by a little over 2 mm. That wasn’t part of the deal when the chainline was chosen for Boost 148. Even 1×11 made me sweat when I first saw how far the chain had to twist to reach the large cog. Sure, I spent plenty of time in the 3x days crossing a 32t chainring to a 36t cog. But if I knew I’d be climbing for a while, I could always drop into my 22t ring. In fact, I was supposed to. There used to be rules. The problem is, rules kinda suck, and the 10-50 cassette eliminated them.

Without even thinking about chainlines, most of us were hooked on what Eagle was selling. Because, despite seeming downright meme-worthy at first, Eagle actually worked. It worked pretty damn well. SRAM (and later, Shimano) 12-speed drivetrains were designed to tolerate the contortions they put their chains through. I know all this may be triggering for those of us who long for simpler times, but I, for one, welcome our new wide-range overlords. It’d be hard for most of us to ever go back. But that doesn’t mean everything is settled. Again, 148mm x 52mm was rooted in old tech. And so, new tech was inevitable.

Not long after Eagle launched, Pivot introduced 157mm hub spacing. Called “Super Boost Plus” by some and “This Shit Again?” by others, 157 pairs with a 56.5mm chainline. This did not improve the relative position between cassette and chainring (each moved outboard 4.5mm compared to Boost) but it did further improve lateral wheel stiffness and offered frame designers freedoms once prohibited by front derailleurs and q-factor concerns. Nevertheless, 157 uptake has stalled. Perhaps because it came too soon after 148. Although Evil, Knolly, Devinci and, of course, Pivot still fly the 157 flag, introducing yet another new hub standard en masse has proven to be divisive among consumers, and a headache for wheel manufacturers. So, as our industry loves to do, we came up with something in between.

Around 2020, bikes started emerging that were built around new 55mm chainlines, but with old 148mm hub spacing, and it was an enticing concept for bike brands. Like Super Boost, it offered frame designers more room, and it didn’t involve a new rear hub standard. These 55-specific bikes took advantage of the extra clearance by widening the pivot hardware around the bottom bracket, essentially to take the benefits of Boost one step further. Better strength and stiffness, and the capability to run shorter chainstays. My first experience with 55 was on the Canyon Spectral 150, which preceded the Spectral 125 I just reviewed. On the Shimano-equipped Spectral 150 I tested in 2020, I sensed a little extra noise when climbing in the largest cog. And I once had the chain drop off that cog in a hasty ratchet on a technical climb.

Then, on the SRAM-equipped 125, the chain would almost immediately derail from the largest cog if I backpedaled. To be fair, backpedaling in the granny gear is not something I do often, and both bikes performed phenomenally well otherwise. More importantly, there were tangible on-trail benefits to 55’s added rear-end stiffness and tire clearance. But something about it seemed fishy. These brands essentially want the perks of Super Boost, but don’t want to bite the bullet and adopt the new hub standard. Like they’re trying to have their cake and eat it too. I wondered if that was why Canyon, Trek, Ibis, Scott, and other brands have stayed so low-key about their 55mm chainline adoption.

“It was really quiet,” says Vernon Felton, US Director of Product Development at Canyon. “Nobody wanted to get out and actually talk about it.” I can’t really blame them. This is some nerdy shit. It’s taken me 1,000 words to get this far. And I still haven’t even touched on the factors that triggered this quest for yet another 3 mm.

“So, XX1 [11-speed] was being branded as an ‘everything drivetrain,’” Felton says. “But at the time, we were maxed out at a 42t cog, and people were still concerned about range. A lot of people went 32t on 27.5, but if you’re running 29, maybe you’re on a 30t. Then, you start expanding the cassette [with Eagle] and people were like, ‘You know what? I can go bigger.’” That was a challenge for frame designers. If they can assume we’re running a 32t ring or smaller, many will fill the space around it. And on top of that, tire preference was shifting right alongside chainring preference.

“By the time we committed to the 55mm chainline, 2.8” tires had pretty much faded from use, and 2.6” was losing out to the favored 2.5” tire size,” says Dylan Howes. “Committing to at least 34T effective sized rings and 2.5” sized tires on modern wide rims really pushed us to the 55mm chainline.”

So, that was the “push,” but it kinda started with a “pull.” Howes tells us the door to 55 was opened years earlier, when Shimano was introducing their first 1×12 groups to bike brands. “Some companies had issues with clearance on [Shimano’s] new-at-the-time cranks, and their solution was to offer slightly wider cranks with a wider spindle that moved the chainring out to a 55mm chainline.”

According to Shimano North America Mountain Bike Product Manager, Nick Murdick, a few folks at Shimano had been suggesting a move to a 55mm chainline for a while. Those clearance issues (mostly on SLX and XT cranks) were why it finally did happen. But more importantly, the reason it could happen was because the drivetrain could handle it. “From the beginning, Hyperglide+ was designed to tolerate a crank chainline that is 2mm outboard of the nominal cassette chainline,” Murdick tells us. Yes he did just say 2mm, not 3mm. Sorry to derail us over 1mm, but let me explain. Exactly what a “Boost” or “non-Boost” chainline is will vary depending on who and when you ask. That’s why I used “approximately” way back in this story’s first paragraph. Today, 52mm and 49mm are most commonly used, but that’s because of a shift that happened during some axle-widening growing pains. Murdick clarifies that, “Non-boost chain lines were mostly 50mm and boost brought that out to 53mm. During the 11 speed Hyperglide era, some crank brands found that they could eliminate chain skating when backpedaling by reducing the chain line to 52mm, and that became the standard for boost cranks.”

Bottom line, Shimano made the 55mm chainline possible. And Trek was happy with how the new configuration was working. But they couldn’t yet start taking advantage of 55’s added clearance because there weren’t enough other compatible crank options. “Around this time period, a couple of things also happened,” Howes tells us. “We began looking at SuperBoost, and SRAM started talking to us about their new future drivetrain.”

That future drivetrain was, of course, Transmission. Its direct-mount derailleur has kinda stolen the spotlight from features that may have just as much impact. One of which is that the cassette is moved outboard by 2.5mm. This was only possible because SRAM knows exactly how much clearance a Transmission-equipped bike will have on the inner surface of that drive-side dropout. So, they pushed it to the very limit. It essentially solves the derailing issue on 55mm bikes, and frees any bike brand who specs it to take advantage of the extra room behind the chainring. That room will be there whether they use it or not, because the Transmission derailleur is designed to keep its pulley cage pointed directly at 55mm from the bottom bracket’s center.

So, here we are. There is a solution in sight that may settle the chainline debate, at least on Boost bikes. But something still seems fishy. Transmission is all well and good for the high-end, but 55mm bike owners who can’t afford it are getting the short end of the stick. At least for now. There shouldn’t be a reason Transmission can’t trickle down, maybe even to the NX level. In fact, Transmission’s original patent drawings were of a cable-actuated derailleur. And speaking of patents, Shimano and TRP have applications with their own direct-mount derailleurs, so they, too, could potentially bump their cassettes out for better chain alignment. And that’s why I have faith that the industry is moving in the right direction, despite taking a few clumsy steps on the way. My point in writing this was not to say that we’re stuck with an unsolvable issue. Quite the opposite, in fact. I’m trying to illustrate that, whether or not we know it, things are always changing behind the scenes, and we eventually end up better off because of it. Except for headset cable routing.