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I've got XTR M9120 cranks on my big bike. Bought new in 2020 just before Covid and running on a standard Hope BSA BB that I'd bought in 2013 with the previous frame (orginal bearings FWIW).
I noticed the last time I rode my it that they were knocking a bit. Figured the preload collar (tool-free and finger tight) had come loose a bit so nipped it up - all good. This happens maybe every 9 months to a year. Not often, but has happened before.
Pulled the bike off the rack just now to prep it for tomorrow and noticed they were still knocking and then realised I couldn't actually get rid of the play.
Popped them off to discover big wear marks on the BB axle where the crank has obviously been fretting against the bearings.
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Now I'm pretty pissed off about this. The reason I bought XTRs in the first place was that I find Shimano cranks absolutely foolproof - my hardtail is running a pair of XTRM970s that I bought s/hand off here in 2011 - again on a Hope BB and both crank and BB have been through several frames and done mega miles - indeed the only reason for buying the new ones was bloody Boost spacing and the old set not clearing the new full sus frame.
So is this a known about and regular phenomnenon? I think I have heard mention of it once before, but dismissed it as just not a thing. This is the first time I've had reason to take the cranks off the bike since I built it - the whole point of using trusted parts that *just work*. The BB bearings are still fine, so I'm not really understanding how the movement has occurred. While the bike has been used in all weathers, its not my main bike - tends to get a few hour of winch & plummet stuff rather than endless grinding treks across the moors.
The truly annoying bit is that if I want a light(ish) aluminium 24mm-axled* crankset I'm going to have to give Shimano more money, including one of their throwaway BBs with the plastic shim which hopefully stops the damage. (to be fair the newer generation BBs do seem to last OK - the original HTII ones are the reason I own a bunch of Hope BBs - but you can't service them or replace the bearings)
*because you get a sealing tube between the BB cups, which doesn't fit on 30mm axles.
I’ve got XTR M9120 cranks on my big bike.
What is the big bike?
Have you measured the bottom bracket size?
I'm running the same cranks on a hope bottom bracket and although I have marks it's not worn away like yours.
That looks like your bb might be undersized. How easy is it to fit the bb? Can you do it mostly by hand or does it get stiff and needs the tool?
When you fit the cranks, are you over tightening the tensioner
Its possible that you've just been unlucky and got some crap in there that's ground the metal away.
Looks like the bearings may have seized at some point?
Looks like seized bearings to me
Does the hope BB have nylon between bearings and crank or is it metal on metal? I've had this with SRAM 30mm cranks but that probably involved seized bearings.
The bike is a Cotic RocketMax mk2
What do you mean by the BB being undersized? As in the shell? I’ve not had the cups out since I built it, but can’t recall any problems (faced and chased by Cotic). I wouldn’t have thought a tight shell would be able to crush outboard bearings?
I’m definitely not over tightening the tensioner. A) I’ve been using HTII cranks since they first came out and understand them pretty well. B) I’m a skinny git with the power output of wet spaghetti, so I’m quite anal about minimising drag and friction.
Seized bearings a possibility I guess, but they feel nice and smooth now.
Do we reckon the cranks are scrap, or if fitting a Shimano BB which has a bigger contact area makes the play go away, they’re safe to use? I do jump occasionally, but not huge stuff and generally to nice landings - more into steep tech and rough natural stuff.
I'd definitely try it with a Shimano BB. It won't get any worse with nylon cups but probably will with metal on metal. The xt mt800 are excellent. I certainly wouldn't scrap them.
It does look like seized bearings but you say they're fine so I'd say over preloaded bearings or maybe under preloaded (doubt it) or misalignment of the bb shell that requires using a BB with plastic top hats to deal with it. I've always thought running a crank axle directly on much harder material bearing races was a bad idea.
Edit, the three replies above mine appeared before I'd finished, I got distracted mid typing.
Seized bearings a possibility I guess, but they feel nice and smooth now.
I've had this before were a bb feels rough on the bike but when you remove them they feel fine.
I have always wondered if Hope 24mm BBs would wear Shimano axles due to the steel on steel, but not seen it on my own cranks over the last 20 years. Have started using the more recent Shimano BBs eg mt800 as the newer shimano bb's seem to last a lot longer.
Still think Shimano Hollowtech 2 24mm bb design, which is now 20 years old, nailed it, and yes I have Sqr taper on some bikes as well.
To answer the OP I would agree with Damascus, maybe the bearings started to go and it maybe wasnt noticed so started to wear the cranks. Maybe go back to shimano BBs with the top hats and even bang a couple of layers of plumbers tape on. Suspect sadly those cranks are toast though.
I’ve been using Shimano cranks and Hope BBs across multiple bikes doing big miles for the best part of 15 years now. First I’ve heard of this.
You we’re right when you said you thought you’d heard of this. They look exactly like mine did. If only we’d had easy photo upload capabilities in early 2023.
I ended up ditching the Hope BB on one bike and putting Shimano in. I think the threads on the BB shell weren’t in the best shape and were allowing a tiny amount of play.
Yep, recently discovered exactly the same thing with my SLX cranks (I was running a Shimano BB). They have been in there a while to be fair (at least a decade) but its pretty annoying. You could probably take them to a machine shop but I imagine the cost of getting it fixed vs a new set of cranks would not make it worth it.
I've just left them in there and will put up with the amount of play.
Stainless steel bearing on carbon steel shaft is a combination that will produce galvanic corrosion in the right moist conditions (there's a 0.35V difference in anodic index and it needs to be less than 0.15V difference for harsh environments). The carbon steel will corrode in preference to the SS. This combined with a bit of fretting is possibly what you see here. If the shaft is assembled to the bearings with a bit of grease and regularly replenished then the fretting/corrosion would be less likely.
I run hope SS BB and Shimano HT2 but the interface is regularly regreased when stripping down to clean out the outer seal grooves.
I think the threads on the BB shell weren’t in the best shape and were allowing a tiny amount of play.
How would the threads on the shell allow play? They're either good enough to allow the cups to be tightened, or they aren't.
The alignment comes from the shell faces.