Wobbly crank - is it knackered now - help!

ED209

Hi All

Was out today when my left crank (Truvativ isoflow square taper) started wobbling all over the shop - not so long since I last checked the bolt but it had now worked loose so I tightened up and continued riding.

About 50m later it had got bad again, so I took the crank off and found that the crank doesn't seem to have been fitted right in the first place - it doesn't seem to have pushed far enough onto the bottom bracket, certainly compared to my girlfriends kraken (which has the same crank). From the dirt on the bolt's thread only half of it - about 7mm - was ever actually screwed into the BB. Looking into the square hole in the crank-arm, again the BB was only ever pushed partway in and the crank has started to round off inside as a result. It's impossible to keep it tight after pushing the crank on with my hands.

I was going to take the bike back to halfords for it's free check-up anyway (BTW, I 'built' the bike myself because the shop were busy so it's not their fault....it came like that from the factory) but before I do:

1) To fit a square taper crank, should I have to brace the other side and hammer it on with a rubber mallet or similar? Because I reckon that would do the trick to my one even with the slight rounding...but of course I don't want to do that if it should just slide on.

2) the BB looks fine, not deformed at all - is this likely to be the case, or might there be hidden damage?

3) I reckon halfords will fit a new crank under guarantee. But if they don't and I need to buy my own I'd prefer to upgrade while I am at it; powerspline, or stick with square taper? And what's this 'integrated BB' business like on the shimano LX cranks?


<font size="1">GT Aggressor XC2 - lovingly crafted by the artisans at Halfords with the addition of XT hydraulic brakes, M520 pedals, Ritchey Ergo foam grips and my old nice and comfy Spesh BG saddle</font id="size1">

gary_fisher3

If the crank has been rounded then you're best off with a new crank and possibly a new bottom bracket (It's hard to tell if it is right).

You don't need to use a rubber mallet. The crank bolts will push/pull the crank on as you tighten it.

There's quite a few different designs of crank/bottom bracket and each have there own advantages/disadvantages. To make the upgrade worthwhile I would go for a hollowtech 2 (or similar) like the LX crankset. Generally the external bearings seem to wear better and make the whole assembly that much stiffer.

Unknown

Should be a warranty job. Halfords are notorious for forgetting to tighten them properly, but its also something you shou;d check yourself regularly.

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ED209

Just got back in from playing in the garage. When I say the crank is slightly rounded, I mean that the corners of the hole on the inside face of the arm bulge slightly outwards, and you can see that the marks left by the BB on the inside of the square hole aren't straight as the arm had been free to pull slightly off and rotate slightly on the BB before the BB bit into it. Having just had a better look I don't really think the arm needs replacing, it's just a bit worn really. The BB still looks fine...I guess the BB is steel but the cranks are alloy (still weighs a ton though)? In which case the crank would deform first presumably.

I've tightened it much more strongly now (I was using a multitool when out but used an allen key with a length of pipe for more leverage), I can't imagine it working instantly loose again as it was before but will go for a ride tomorrow and see if I can detect any loosening. I think the problem before was that I was giving the bolt enough welly to pull the slightly deformed hole onto the taper - it would tighten, but then work loose as soon as I stood on the pedals.

I have a big torque wrench but no allen key bits for it. But, I do have some pipe, some olympic bar weights, double science GCSE and a measuring tape....so roughly how many Nm should it be tightened it too?



<font size="1">GT Aggressor XC2 - lovingly crafted by the artisans at Halfords with the addition of XT hydraulic brakes, M520 pedals, Ritchey Ergo foam grips and my old nice and comfy Spesh BG saddle</font id="size1">

Unknown

Acording to Truvativ 38-42Nm for your crankset


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skukza

if the bb side of the crank arm doesnt look right (metal should be nice a flush not buldging out ie rounded) then its had it, and will just work itself loose again. Halfords should replace it free simpley because they didnt put it on propperly. simple as.