Crank arm bolts keep undoing themselves

Jamey

As I mentioned in the thread I posted yesterday, I went on a maintenance course over the weekend where I took the bike to bits and then reassembled it all.

This morning, on my commute to work, I noticed the left crank arm felt funny so I stopped to check it and noticed the bolt had worked loose, so I tightened it with my multitool and carried on. Within five minutes I was doing it again and this continued all the way through the journey and is still ongoing now. for all I know the little bugger might be unscrewing itself while the bike's locked up in the car park this very second.

The right (drive side) crank arm also had the same problem but nowhere near as bad. I only had to tighten that a couple of times along the way.

So, my question...

Is it simply a case of getting it really, really, really tight (much tighter than I can manage with my little multitool)? Or have I done something fundamentally wrong, like putting far too much grease on or something?

I don't want to strip the threads by tightening it too much, that's all.

nicklouse

yes really tight. 2 grunts.

what cranks? square taper fit? or....

if you keep having problems and ride with lose cranks you will either damage the interface beyond use or you already have.

Jamey

Yes, it's a square taper fit and according to Specialized's website (my bike is a 2008 Tricross Sport) the cranks are FSA Tempo Triples.

I'm guessing threadlock's a bad idea, is it?

biondino

The crank arm bolts on my Focus are nowhere near tight - they need very little force to undo and there's a lot further I could go to tighten them. That's how they were set up and I doubt the mechanic did it wrong. Did you take them apart on your course? Can you rmemeber how tight they were when you first undid them?

Jamey

I think the bolts themselves were sort of reasonably tight when I undid them, but the crank arms needed a whole shedload of force when we were using the crank arm pulling tool and I ended up having to get someone else to help me as I broke my arms a couple of months ago and couldn't actually generate enough force to do it on my own.

nicklouse

again it all depends on the crank interface.

Jamey
thread lock can help but if the arms have been distorted (the square hole) they will move on the axle and undo any bolt.

heave on a good sized spanner/socket set. (well dont heave too much, torque figures can be found on Parktools webby).

Jamey

Yeah, just been looking at Torque figures.

I did use a proper torque wrench yesterday and tightened both sides to the recommended force (32 Newton Metres) so I dunno what happened.

balthazar

Everything about how the crank interface works here:

http://www.sheldonbrown.com/brandt/inst ... ranks.html

Jamey

balthazar wrote:

Everything about how the crank interface works here:

http://www.sheldonbrown.com/brandt/inst ... ranks.html

Ok, I've just read all of that.

So it seems that while you need to tighten the bolts securely when you first install the cranks, you should never re-tighten them afterwards unless the arm is coming loose, is that correct?

And then, if the arm is coming loose, it's probably too late regardless and you should just buy a new crank arm, right?

I read that note from Sheldon at the bottom and he says that you can just buy replacement left crank arms cheaply but I've just had a quick look around and I can't see any for sale in my usual spots. Only complete chainsets.

nicklouse

32Nm is a tad on the low side. 34-45Nm is what i would have expected.

But they will take more.

re spare arms yes the LBS will be the best place but they may need to order in.

Jamey

Ok, cheers.

I know it's difficult to say without seeing it, but do you think I've probably damaged my crank arm? It didn't get properly 'floppy' at any stage, just to the point where it felt a bit weird, sort of like it was a bit diagonal at the bottom of the circle.

And I didn't ride with any force this morning, I deliberately kept it easy (low gears) and stopped to tighten it every time it even started feeling odd.

nicklouse

50/50

just get it tight before using it again and see what happens.

balthazar

Jamey wrote:

So it seems that while you need to tighten the bolts securely when you first install the cranks, you should never re-tighten them afterwards unless the arm is coming loose, is that correct?

And then, if the arm is coming loose, it's probably too late regardless and you should just buy a new crank arm, right?

Correct. The acid test of whether or not the crank is damaged beyond repair, is to remove then refit it properly. The torque setting is only applicable at first ftting. If it keeps undoing then it should be replaced.

Generic left cranks used to be available, but perhaps these days you must buy the whole set. However, your lbs might have something unused knocking around.

Jamey

Looks like I might be buying a crank puller and a torque wrench sooner than expected then.

laughingboy

Jamey wrote:

Ok, cheers.

I know it's difficult to say without seeing it, but do you think I've probably damaged my crank arm? It didn't get properly 'floppy' at any stage, just to the point where it felt a bit weird, sort of like it was a bit diagonal at the bottom of the circle.

And I didn't ride with any force this morning, I deliberately kept it easy (low gears) and stopped to tighten it every time it even started feeling odd.

As Nicklouse says, there is a decent chance that you have damaged your crank arm, but equally, there is a chance that it'll be OK. Riding with a loose crank will distort the seating around the square taper (and you will be able to see it, if you remove the bolt). Slight damage is OK. But once the damage is enough that the distorted crank works its way loose even when tightened, you will need to replace it, in a nutshell.

Whatever you do, don't ride it until you are sure it is tightened securely. It might save you the expense of getting a new crank arm.

Jamey

Gah... Nobody at work (even the engineers) has a socket set with them so I guess I'll have to get the train home and bring my wrench in with me tomorrow.

I thought about going down to the shops and buying a torque wrench (no point buying another socket wrench - I don't need two) but you don't get any attachments with them so I'd still have to wait until I get home before I could use it.

I also thought about using my multitool to tighten it enough (like this morning) to make it to a bike shop but that probably runs too high a risk of damaging the crank arm (provided it's not damaged already).

On a related not... Square-taper triple chainsets don't seem to be too common on Wiggle and other online stores... Are they dying a death or something?

Jamey

Brought my wrench in with me today. Wish me luck.

Jamey

Both sides are now officially two grunts tight. Actually the left side might even be three grunts tight.

Time to see if it all falls apart on the journey home.

Jamey

Journey home was ok.

Journey to work this morning was ok.

That's 28 miles covered without them loosening and falling off.

With a bit of luck, I might have gotten away with this. Thanks for all your help, guys.

nicklouse

give them a check to night.

but sounds like you are fine.

Jamey

I thought I wasn't meant to tighten them again, after the first tightening? As per the Jobst Brandt article about cranks splitting because of re-tightening.

nicklouse

splitting only really happens when someone has greased the taper and then really heaves on the bolt.

If you are happy just leave them.

Jamey

Yeah, I might have greased the taper quite a bit on the maintenance course last weekend :oops:

laughingboy

Jamey wrote:

Journey home was ok.

Journey to work this morning was ok.

That's 28 miles covered without them loosening and falling off.

With a bit of luck, I might have gotten away with this. Thanks for all your help, guys.

Sounds like a success. I'm glad for you.