Here's some nerding on the subject, but I pulled one paragraph in particular:
"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.”"
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...
theradavist.com
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