Crank arm dropped off

[Stu_mt]

Good ride this morning until this happened..crank arm dropped off!!Allen key bolts are still tight but it’s somehow forced itself off the spline & stripped threads off the plastic end cap.. can anyone advise??

 

[steve_sordy]

That happened to me once on a clockwork bike, but it was because the clamp bolts were not tight enough. None of the splines look damaged in the pic, so you were lucky!! Clean everything up, degrease the bolts and the threads they screw into as best you can. Then use threadlock on the ends of the bolts and fasten the bolts in stages alternating from one to the other to avoid distortion.. Use a torque wrench to tighten to the correct level.
For threadlock, I use Loctite Blue. It looks like a Pritt Stick, except it's blue and is called Loctite.

 

[Gary]

Replace the stupid plastic shimano preload cap with an Alu version and it won't happen again.
The plastic caps allow the arms to "walk" off the splines. Even when the pinch bolts are correctly torqued.

 

[Mikerb]

Why has the crank arm got what looks like a spacer in the pinch bolt gap? Surely that prevents the pinch bolt gap from compressing sufficiently. If the end cap is plastic it it merely covering the crank arm hole....not providing preload?

 

[steve_sordy]

Why has the crank arm got what looks like a spacer in the pinch bolt gap? Surely that prevents the pinch bolt gap from compressing sufficiently. If the end cap is plastic it it merely covering the crank arm hole....not providing preload?

That is a standard Shimano fitment, it even has a tiny pin that engages the spline. But I have no idea of its actual function, unless it is to stop the crank arm from being over-tightened of course.

Maybe it is cheaper overall to slot the crank arm with a standard tool and then backfill the gap with a spacer? No, I'm not convinced either, but that gap without the spacer would need a very thin circular saw. Think of how fragile the saw would be and how disruptive it would be to stop to replace it, or to deal with crank arms damaged by a broken saw...... "I don't know!" is the answer!

 

[Gary]

Why has the crank arm got what looks like a spacer in the pinch bolt gap? Surely that prevents the pinch bolt gap from compressing sufficiently. If the end cap is plastic it it merely covering the crank arm hole....not providing preload?

To stop folk overtightening the pinch bolts.
You can remove it but overtighten the bolts without the plastic spacer and the gap will close stretching the aluminium.
The reason Shimano made the crank bolts plastic AND with a specific spline AND as specific plastic tool is so folk don't overtighten the pre-load on HTII bearings hanging off an allen key or the likes. (this is irrelevant with the shimano motors as the crank arms don't preload any bearings when the cap is done up tightly.


I've absolutely no idea WTF Steve is going on about saws for in his post above.

 

[HORSPWR]

That is a standard Shimano fitment, it even has a tiny pin that engages the spline. But I have no idea of its actual function, unless it is to stop the crank arm from being over-tightened of course.

Maybe it is cheaper overall to slot the crank arm with a standard tool and then backfill the gap with a spacer? No, I'm not convinced either, but that gap without the spacer would need a very thin circular saw. Think of how fragile the saw would be and how disruptive it would be to stop to replace it, or to deal with crank arms damaged by a broken saw...... "I don't know!" is the answer!

That plastic shim has a tiny pin on it and it is designed to engage into the hole in the crank arm mounting shaft (you can see that hole in the picture above). It is to prevent the crank arm from falling off but sadly this doesn't always work. It is not designed to prevent over torquing of the crank arm pinch bolts, when torqued correctly this plastic safety shim is still slightly loose.

Shimano crank retainer/safety pin thingies | Singletrack Magazine


Crank arm pinch bolts should be torqued to 14Nm and torqued alternatively a bit at a time until the correct torque is achieved on both bolts.

 

[Mikerb]

To stop folk overtightening the pinch bolts.
You can remove it but overtighten the bolts without the plastic spacer and the gap will close stretching the aluminium.
The reason Shimano made the crank bolts plastic AND with a specific spline AND as specific plastic tool is so folk don't overtighten the pre-load on HTII bearings hanging off an allen key or the likes. (this is irrelevant with the shimano motors as the crank arms don't preload any bearings when the cap is done up tightly.


I've absolutely no idea WTF Steve is going on about saws for in his post above.

Ah ok.....dont think I have had a bike with that type of crank since my paper round bike!

 

[FOG]

Had exactly this happen on my ride yesterday. The drive side crank arm suddenly spat itself on to the road to be nearly run over by a following truck. I was a bit rattled but I had a multi tool so thought no problem to re-attach. However the multi tool 5mm allen key was too short to get into the inside pinch bolt and fouled the chain ring with the tool body. result, I could tighten the outside bolt but not the inside. I abandoned ride and rode home carefully with just the outside pinch bolt tight. As soon as I got home I used the long workshop allen keys to tighten both up but how do I stop it happening again?

 

[Gary]

It's a poor design on shimano's part. The crank arm "walks" off the axle due to

1. Not being butted up against anything pre-loading it (as per shimano's original design spec for the HTII interface.)
2. Being quite a weak flexy crank arm
3. The built in lateral movement of the motors main axle.
4. Having a pathetic plastic bolt to stop it "walking"

The solution is to replace the plastic HTII bolt with an Alu one and (blue) threadlock it. But still keep an eye on those pinchbolts and the distance between the crank and motor. If one of those bolts loses even a little bit of tightness the interface becomes compromised and it can still happen even with the stronger bolt threadlocked in there.

#UnpaidShimanoStepsBETAtester

 

[Stu_mt]

Had exactly this happen on my ride yesterday. The drive side crank arm suddenly spat itself on to the road to be nearly run over by a following truck. I was a bit rattled but I had a multi tool so thought no problem to re-attach. However the multi tool 5mm allen key was too short to get into the inside pinch bolt and fouled the chain ring with the tool body. result, I could tighten the outside bolt but not the inside. I abandoned ride and rode home carefully with just the outside pinch bolt tight. As soon as I got home I used the long workshop allen keys to tighten both up but how do I stop it happening again?

As Gary has said, the Ally cap replacement is the fix I used + checking the pinch bolts are evenly torqued after every few rides. Ebay has them around £5 a pair ??