Thursday, October 15th 2020 – An interesting one here for the Bob Big Book of Mechanical Failures.
A bike I look after has an 11 speed rear sprocket cassette – Shimano CS M7000 XTR. Like all hyperglide Shimano cassettes 11 speed and below, it fits on the free hub splined body by sliding on to an asymmetrical groove pattern to ensure all components are correctly aligned synchronously for smooth gear changes. The whole lot is held on by a fine threaded, normally tightening lockring, driven with a special tool.
Unlike lower range cassettes, which are generally 2 or 3 piece, this arrangement turns out to be discrete sprockets for all but the largest three, and appropriate spacers which you stack on the free hub before applying the lockring. The lockring should actually tighten by precession and has grooves and a crinkle washer to stop it coming loose.
So why did this factory assembled cassette locking undo itself, allowing the ring and smallest few sprockets to tumble off the free hub and grind against the inside of the frame? I think personally because it wasn’t tightened enough in the factory.
The ring looked bad at first, until I realised that the silver ribbon was not swarf but the remnants of a foil table on the ring.
Cleaned and popped back together, all worked well. But in all my years, I’ve never seen that happen before.
Check your bikes folks. This could have been nasty.