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Left Pedal Arm Coming Loose

Posted: 2 Dec 2020, 3:28pm
by horhif
Hi

I've recently been fixing up my old mtb, a rock rider vitamin R from Decathlon. I have a 'proper' road bike and wanted to use the mtb on light trails so have been renovating it.

Anyhow, I needed to change the bottom bracket. I've done this ok. However, now my left pedal arm is coming loose and eventually falls off.

Attached are some photos. I am use if I should change the pedal, the bolt, or I've read some folks say to use lock thread. (And yes, I've also ordered full mudguards :) )

I've looked at: https://www.decathlon.co.uk/p/left-crank-arm-170mm-black/_/R-p-12285 and https://www.decathlon.co.uk/p/crank-arm-tightening-bolts/_/R-p-290

Anyone any advice, would appreciate any comments.

thanks!

Re: Left Pedal Arm Coming Loose

Posted: 2 Dec 2020, 4:39pm
by mjr
If you were doing the crank bolt up tight enough (49Nm is not unusual, says https://www.bikeride.com/torque-specifications/ ), checking it after a first short ride and it's still coming loose, then I suspect the crank arm is almost certainly worn out and the bottom bracket might be as well. I'd replace the crank arm.

Changing the pedal probably won't work, changing the bolt should do nothing and using thread lock will just slow the rate at which it falls off while allowing it to keep wearing crank and/or BB.

Re: Left Pedal Arm Coming Loose

Posted: 2 Dec 2020, 4:44pm
by alexnharvey
If you have caught it early enough threadlock can help a lot, if the eye is already quite loose then it only slows things down as mjr says..

Re: Left Pedal Arm Coming Loose

Posted: 2 Dec 2020, 5:01pm
by rjb
i recovered a worn crank by adding a shim of aluminium to the axle then tightening up the bolt tighter than intended. You risk splitting the crank if its far too tight but in my case it was an acceptable bodge which on a low cost pub bike lasted as long as the bike. I used thick aluminium foil from a pie case. :wink:

Re: Left Pedal Arm Coming Loose

Posted: 3 Dec 2020, 7:24am
by rogerzilla
You need a new crank. It's usually the left one that is affected as the interface has to cope with bending and torsion (the right one is mostly torsion).

I fit new cranks to greased tapers and ride them for 100 miles. Then I remove the bolts, clean all the threads, and refit them with threadlock. Then never touch them again until the BB needs attention. A torque wrench is useful - if you don't have one, you are unlikely to over-torque with a normal 8mm allen key. With hex head bolts/nuts, don't use full strength on a 1/2" socket driver.

Re: Left Pedal Arm Coming Loose

Posted: 3 Dec 2020, 7:46am
by Brucey
FWIW the OP's crank looks like a steel one with a plastic covering moulded onto it. If this has been moving around on the taper, there is a fair chance that both the taper and the crank are worn. By contrast with an aluminium crank which has worked loose, it is normally just the crank that is worn.

cheers

Re: Left Pedal Arm Coming Loose

Posted: 4 Jan 2021, 11:54am
by horhif
thanks for all your replies, much appreciated!

I got the pedal arm fixed at the local bike shop, seems I hadn't tightened it enough.

The arm never needed replaced, so was a quick fix.

bike is up and running grand now.

thanks!

Re: Left Pedal Arm Coming Loose

Posted: 4 Jan 2021, 12:36pm
by Steve
Good news, but keep checking it stays tight, before each ride. They sometimes work loose again and you can find yourself riding home with one leg, if you see what I mean.

Re: Left Pedal Arm Coming Loose

Posted: 4 Jan 2021, 8:02pm
by bgnukem
If there's too much wear to either the axle or crank, the crank will tend to continually work loose and will not seat properly on the BB axle. Had the same issue last year with an aluminium alloy crank and hadn't ridden it loose for very long but, by the time I realised, the damage had occurred.

If your crank is steel you might get away with it - time will tell.

Re: Left Pedal Arm Coming Loose

Posted: 6 Jan 2021, 10:10am
by [XAP]Bob
Steve wrote:Good news, but keep checking it stays tight, before each ride. They sometimes work loose again and you can find yourself riding home with one leg, if you see what I mean.


I ended up riding about 6-7 miles one sided after my left crank detached halfway to work. Rode into town at lunch to buy a replacement (that's what took it from 5 to 6-7 miles).

Very odd riding one legged, but I was glad for the cleats