Cycling UK Forum

My left crank on my singlespeed keeps coming loose. I've tried using threadlock on it, and a shop applied threadlock and assured me that it wouldn't come lose again. But, it does.

What should I do to fix this avoiding the 'good money after bad' trap? I could replace the crank. I could replace the crankset. I could replace the crankset and the bottom bracket. If I bought just a crank - cheap enough - if the problem is in either the bolt or the bottom bracket thread, then it will just come loose again.

The crankset and bottom bracket have done thousands of km. I don't think they owe me anything.

I've had this happen many moons ago - and it seems that once it's gone loose (and only been pedalled a few turns) the taper is damaged such that it won't stay put. It always seems to be the left crank. I've since made sure it is torqued up to the high figure when first assembled - that seems to be the trick (and checking after a few rides).

fastpedaller wrote:I've had this happen many moons ago - and it seems that once it's gone loose (and only been pedalled a few turns) the taper is damaged such that it won't stay put. It always seems to be the left crank. I've since made sure it is torqued up to the high figure when first assembled - that seems to be the trick (and checking after a few rides).

Would replacing just the crank fix it? It did on another bike of mine, but that was a different situation. (A new crank kept coming loose.)

I'm tempted to buy one of these: http://www.ebay.co.uk/itm/Single-Speed- ... DKrNFpKd2A and then just use the left crank, putting the right crank (including chainring) in the parts box for future use.

Annoying Twit wrote:My left crank on my singlespeed keeps coming loose. I've tried using threadlock on it, and a shop applied threadlock and assured me that it wouldn't come lose again. But, it does.

What should I do to fix this avoiding the 'good money after bad' trap? I could replace the crank. I could replace the crankset. I could replace the crankset and the bottom bracket. If I bought just a crank - cheap enough - if the problem is in either the bolt or the bottom bracket thread, then it will just come loose again.

The crankset and bottom bracket have done thousands of km. I don't think they owe me anything.

The square taper of your Lh crank may well be damaged.
Most of the times, it can be fixed by replacing the Lh crank only, if you're not too fussy you can sort it out with a fiver. Assuming is a square taper one.
If is an integrated chainset, is a different story but can be sorted too. You need to be more specific, saying "a chianset comes loose" isn;t much help to have a proper answer

well last resort is to do up very very tight and use super strong threadlock - I think you can get various types. if that dont work then as others have said damaged taper.

Annoying Twit wrote:

fastpedaller wrote:I've had this happen many moons ago - and it seems that once it's gone loose (and only been pedalled a few turns) the taper is damaged such that it won't stay put. It always seems to be the left crank. I've since made sure it is torqued up to the high figure when first assembled - that seems to be the trick (and checking after a few rides).

Would replacing just the crank fix it? It did on another bike of mine, but that was a different situation. (A new crank kept coming loose.)

I'm tempted to buy one of these: http://www.ebay.co.uk/itm/Single-Speed- ... DKrNFpKd2A and then just use the left crank, putting the right crank (including chainring) in the parts box for future use.

That would seem a correct "solution". Make sure the crankset is otherwise suitable... same length crank etc

mercalia wrote:well last resort is to do up very very tight and use super strong threadlock - I think you can get various types. if that dont work then as others have said damaged taper.

I won't recomment this, over 45-50Nm there are chances to split the crank and rounding off the bolt, if that happens one needs the angle-grinder and it's all more complicated.
Many shops do keep used cranks, sometimes you can have them for £2/3, it doesn't make sense trying to flog a dead horse, especially when the replacement is available for very little money

I think the other diagnoses are correct. You have probably damaged the taper, that is, the one in the crank.
If you look you should be able to see the damage.
Since the only other remedy is replacement you might like to try a repair method that I have used sucessfully.
With care it is possible to file out the damage. This may result in the crank going so far onto the BB axle that it hits the end of the taper before becoming tight. So file the inner face away sufficiently to enable a tight fit.
I fear this crude bodging may appal some mechanics more refined than me.
All I can say is that it worked for me and that the alternative is binning the crank.
If you replace the crank be aware that the square hole can be orientated two ways, square to the crank or diamond. Tapers can also vary, some may go on so far that the axle protrudes at the bottom of the bolt (or nut) recess. A shorter axle, from some patterns of crank, may need a crank which moves the pedal end further from the centre line of the bike.
Edited to add, I forgot Fast pedaller's caution about crank length; 170 mm. is most likely.
As Gattonero says, the replacement is cheap enough to make bodges unecessary. You might nevertheless like to try my suggestion for fun, if you share my meaness and love of messing about.

If its a square taper you could try putting a shim between axle and crank using something like an aluminium foil pie tray. I did this once as a last resort and much to my surprise it worked and lasted the full life of the chainset.

Mike Sales wrote:If you replace the crank be aware that the square hole can be orientated two ways, square to the crank or diamond.

A good point - I'd forgotten that.

Major D'oh moment. I bought a crank from a shop, then read on here about square and diamond taper

Of course I bought the wrong one.

The first square taper I had kept coming loose. Workmate suggested putting crank end in very hot water to expand it.
Refitted while hot and never came loose again.

FWIW my advice is to fit a new crank without grease on the taper, and to retorque the bolt after the first 50-100 miles.

BTW with a worn crank, I have filed them out, used threadlock, all kinds of terrible things. Sometimes this works fine, but it isn't 100% guaranteed to do anything other than take up your time. A new crank is a more effective solution.

Apologies if this is b.obvious, but cranks come in different lengths, too, so be sure to get the right one.

cheers

Thanks for the help.

This time I took my old crank to a shop, and laid it on top of the new one to check it was identical 170mm diamond tapered. The orientation of the diamond tapered hole looks identical. Hopefully nothing is wrong with this one.

There is visible damage on the old crank. This isn't the first crank I've had come loose, but after I replaced a crank on the last one (I had a spare as I'd changed the crankset) everything was then fine. I'll put plenty of torque on it, and retighten after some distance to check.

IF (and that's a big if) I get it right this time, it'll still be cheaper than if I'd taken it to the shop and asked them to 'fix it'. So, there's still the opportunity for it to not turn out to be too much of a farce.

With four bikes at home, I guess the square tapered 170mm crank will fit one of them, and hence be a useful spare. Otherwise, I'll have to replace the crankset on my single speed at some time. If I do so with a square tapered crankset, then the 'wrong' crank will become a spare for that.

Fingers crossed (that always seems a daft expression to me, and I've just used it!) everything will now be ok.