Mountain Bike Reviews Forum banner

1 - 9 of 9 Posts

·
Wolf nipple chips
Joined
·
504 Posts
Discussion Starter #1
just built up a Blur (oh mama!) and haven't been riding it for evne a month yet, and the only real issue i have had is the XT crankset. they're brand new with a whole new design similar to BMX cranks, and the little plastic cap broke off on the non-drive side, and now that crank keeps inching its way off. no one really seems to know if that plastic cap is supposed to keep the crank on for sure, however i can't imagine that shimano would make a load-bearing piece out of f-ing plastic. any thoughts?
 

·
Registered
Joined
·
379 Posts
Cabin Fever said:
just built up a Blur (oh mama!) and haven't been riding it for evne a month yet, and the only real issue i have had is the XT crankset. they're brand new with a whole new design similar to BMX cranks, and the little plastic cap broke off on the non-drive side, and now that crank keeps inching its way off. no one really seems to know if that plastic cap is supposed to keep the crank on for sure, however i can't imagine that shimano would make a load-bearing piece out of f-ing plastic. any thoughts?
That little plastic cap only pulls the crankarms together. The holding/clamping force is provided by the two screws on the crankarm itself. Verysimilar to the top cap of the headset compresses the headset in place on the frame/fork then the stem actually holds everything together.

Duc.
 

·
Who are the brain police?
Joined
·
10,112 Posts
That cap does nothing but hold the crank together while you tighten the arm. It's not load-bearing.

I can ony imagine that the arm isn't on properly before you tightened it. Make sure that the crank is on correctly before you tighten the arm. Maybe tap it with a plastic mallet first or something?

.....ooops, didn't notice that this was already said.....
 

·
Registered
Joined
·
2,850 Posts
Actually that cap is not plastic, its aluminum, and yes it DOES play a part in keeping the arm from sliding off! That little ring does the same thing as a lockring as that's exactly what it is. Besides pulling the arms together, the the non-driveside arm will pinch it around the inner edge when the two botls are tightened keeping the ring from rattling itself loose and falling off. By being pinched agaist the arm it will not unscrew and will keep the arm settled in place so it won't slip off. If this piece is removed you basically end up with the old crank removal trick if you stripped the puller threads where you just remove the bolt and ride the crank until it slips off. Not a good thing unless you are trying to remove the cranks and possibly damage them in the process. I've seen many people push in the arms by hand and then screw in the piece not realizing how important it it to screw the piece in tight first and THEN tighten the arms. The result is it just fall off since it won't be secured in place. This part should not break however. If it did its an indication of possible installation error.
 

·
Registered
Joined
·
1,842 Posts
Hmmm.

Well, mine is sort of a plasticky aluminum...

Every one I've seen (they may have changed) is glass filled plastic. Has glass fiber in it to strengthen it. The plastic is probably a nylon.

If overtightened, it will crack, and the arm will probably work it's way off. I've heard of that happening on the new XT. I tried to run mine without the cap (loaned my good cap to a guy who overtightened his) and my non-drive arm started to work it's way off.

The cap has a torque spec of something like 6 inch pounds. Think finger-tight. Not hand, wrist and forearm tight, but fingers. The installation tool is a 1.5" diameter thumbwheel, so the cap doesn't really compress anything: just holds it where it is.

I believe that the BB bearings are non-angular contact, anyway, so sideloading them could only be bad for their lifespan. Could be wrong there, though.
 

·
Registered
Joined
·
261 Posts
Similar problem here

Riding home one night I was surprised as the crankarm slid off the axle, still attached to my shoe by the pedal\cleat. That ring cracked, and the pinch bolts apparently had worked loose over the course of a month. It took almost 2 months for the replacement part to come in (not ordered correctly by LBS, then backordered), so I probably wore the bearings by riding anyway...The pinch bolts need to be tight, and tightened evenly, once the lock ring is torqued down to move the crank arm into position. Mine have stayed tight since I replaced the ring and paid close attention to the installation.
 

·
Registered
Joined
·
2,850 Posts
Hhmmm... Is shimano using plastic on the XT's? The XTR ring is in the same material as the cassettte lockring so it won't get damaged that easily. Maybe it would be a good idea to order the XTR ring instead.
 

·
Numerous wiring problems.
Joined
·
86 Posts
Yeah the XT one is finest plastic

Hecubus said:
Hhmmm... Is shimano using plastic on the XT's? The XTR ring is in the same material as the cassettte lockring so it won't get damaged that easily. Maybe it would be a good idea to order the XTR ring instead.
I know because mine cross threaded, jammed and then broke in half. Nice, but not a major problem, I just used a rear wheel QR - gently - and a big washer to clamp everything together while I cinched up the crank bolts.

But yep, the XT is plastic. I'd assumed the XTR one was as well, apparently not then.
 
1 - 9 of 9 Posts
Top