So, for the 3rd time my crank arm came off, please advice

[volts]

I have lost 2 dust caps now, and I am no longer trusting the crank arms on my Vitus to stay on. I switched from the (for me) very long 175mm crank arms to some Merida 165mm arms because they didn't have shimano 170/165/160 in stock when I wanted to order.
It turned out that the plastic "security" plates for the arms didn't fit the merida cranks, but I thought to myself that surely they wouldn't be needed. They are tiny flimsy plastic things, how will they help anything?
Everything properly tightened to spec and good to go right?
Wrong. 2 dust caps lost along with all confidence in the crank arms staying on.

IMO it seems like a catastrophic design from Shimano because now I am 2 dust caps short and not very keen on going down long technical sections without triple checking the crank arms.

Does any of you have a good solution for this? I think I read something about this a while ago on this forum but after ample searching I could not find it so maybe it was somewhere else (?)

 

[Max-E]

It’s not a dust cap it’s a preload cap to pull the arm on which is then secured with the pinch bolts. There was a recent topic on this. Increase the preload with an alu cap and increase the torque on the pinch bolts and check pre and post every ride.

 

[volts]

It’s not a dust cap it’s a preload cap to pull the arm on which is then secured with the pinch bolts. There was a recent topic on this. Increase the preload with an alu cap and increase the torque on the pinch bolts and check pre and post every ride.

I tried searching but didn't come up with anything. Can you point me towards that topic?
Also, where do I buy alu caps? On ebay they look like they are sending them from china which seems a bit far from home.
But it sounds like it isn't just me having problems if people really check bolts pre and post every single ride. That's just silly stupid design? I have used what I call dust caps as preload caps actually, but since they are plastic they aren't very good for this so alu/steel seems more sensible.

 

[Shog46]

Type in search Throwing cranks ?

 

[Max-E]

I tried searching but didn't come up with anything. Can you point me towards that topic?
Also, where do I buy alu caps? On ebay they look like they are sending them from china which seems a bit far from home.
But it sounds like it isn't just me having problems if people really check bolts pre and post every single ride. That's just silly stupid design? I have used what I call dust caps as preload caps actually, but since they are plastic they aren't very good for this so alu/steel seems more sensible.

People *aren’t* checking them - that’s why they fall off! Depends how you are on maintenance. I’m old school so check things regularly.

The plastic caps are like the original Ahead plastic caps. Made of plastic to deliberately stop numptys overloading things.

Any issues with shimano 8000 motors throwing cranks ! - EMTB Forums

 

[volts]

People *aren’t* checking them - that’s why they fall off! Depends how you are on maintenance. I’m old school so check things regularly.

The plastic caps are like the original Ahead plastic caps. Made of plastic to deliberately stop numptys overloading things.

Any issues with shimano 8000 motors throwing cranks ! - EMTB Forums

Thanks.
I was actually hoping not to have to check for this every ride, that's what I try to do now and am trying to avoid. Last time it came off I had not checked it for 3 rides. I do check bolt pretty regularly and look for play but not every ride. I have 2 other bikes and I probably run them down just once per month. I didn't know it was normal to have to check for lose bolts pre and post every ride. But now I know.
And tanks for the link. Silly me searched for dust caps E8000, merida arms, metal secirity plate and a few other but not much useful came up.

 

[Jaygam]

Make sure you preload with the cap but not too much and only adjust the cap whilst the pinch bolts are loose (no wobble and it's tight enough).
Do the two pinch bolts up evenly, ie.... little by little each one (qtr turn each) untill full load, they will handle little more than 14nm of torque but again don't over do it.
It's worth getting either a 1/4" or 3/8" drive torque wrench that has a range of at least 10nm to around 25nm or more.

Its precision engineering.. If just one pinch bolt is done up to full load independently, and then the other, the pinch itself will not be entirely square and will have a tendency to come un-done as the mechanical component load at this point is being stressed from multiple axis.

If done correctly there will be a high precision friction coupling of the spline's, but unfortunately if it has been ridden loose it may have compromised the potential for a high friction coupling on the aluminium crank arm (by rounding the sharp machined edges), if it continues to happen despite this process, cleaning the crank spline and replacement of the arms may be required :/