Why's my crank wobbling?

[swee'pea99]

(Back off Fnaar...)

Just built up my new fixie and the right crank's fine, but the left one won't stay tight. I've tightened it up really tight...as tight as I dare go for fear of stripping threads...and it feels ok until you're out on the road and try a bit of reverse-braking, then you can feel it 'give', just a bit, but definitely.

Now, this happened on the crank I'd transferred over from my old fixie, where it was fine - snug as a bug. And I was well baffled, not least because the BB is brand new - a Shimano, fresh from the box. And they don't make duff gear. So I tried another crank. Same story. And another. Same story.

Two of the cranks are Shimanos one's a Sram, they're all in good nick - used, but neither old nor battered - and like I say, the first one I tried had been running fine for years on a different bike (also fitted with a Shimano BB).

Am I missing something obvious? Any suggestions much appreciated.

 

[Rickshaw Phil]

Shimano don't make duff gear, but it's not impossible that they could make a bad batch. Given that you have the same problem on 3 different cranks, my suspicions would fall back onto the new bottom bracket.

Where was the BB sourced from? Is it possible it's a fake?

 

[swee'pea99]

Thanks for that. Hmmm. Interesting. I do perhaps put too much faith in Japanese engineering! There's no way it's a fake (I'm pretty sure) - I got it from Chain Reaction Cycles, and it came in a (very cunningly/intricately folded) cardboard box that I'm sure is the real deal. But I may get back to CRC and see what they have to say. Like you say, it's not impossible.

 

[swee'pea99]

PS Update - hats off to Chain Reaction! Got back to me straightaway apologising for the duff BB and saying they'll send me a replacement straightaway - no hassle, no quibble.

 

[Rickshaw Phil]

Good result.

 

[Globalti]

Maybe they bought a batch of fakes by mistake?

 

[swee'pea99]

I suppose it's always possible.

The nightmare scenario, of course, is that I get the replacement and it's just the same...!

 

[Dayvo]

I suppose it's always possible.

The nightmare scenario, of course, is that I get the replacement and it's just the same...!

We wait with baited breath.

Let us know the outcome, SP.

 

[swee'pea99]

I will I will!

 

[swee'pea99]

Finally got round to fitting the replacement BB and..it's just the same. But - and it's a big but - I'm pretty sure I've been a numpty and got the wrong spindle length, and that's at the rot of the problem.

Here's how it goes...

I got the spindle length from the guy who sold me the frame: 110mm. When I fitted it, I found that when it was fitted tight, the drive side just rubbed up against the frame. Damn! Being a resourceful muppet, I figured 'if I move that crank out a couple of mm, it won't rub'. So I got a 2mm washer, just the right size, and it worked a dream. Clever me!

When the left crank started shifting slightly, it never occurred to me (being an idiot) that the problem could actually be on the opposite end - the drive end - of the spindle. But I think that is where it is. The washer is preventing the drive side crank properly 'wedging' itself onto the spindle, so the 'shifting right crank' is actually not shifting at all. What's happening is that the left crank, firmly fixed, is moving the spindle inside the drive-side crank, which is not properly engaged because of the washer. Not clever me.

Bottom line, I think, is I was misinformed, and the 110mm spindle I was told to get is not actually long enough.

The question now being, how much longer to go?

The 2mm washer moved the drive side crank out enough so it didn't rub the frame. So does that mean that if I go up to, say, a 115mm, I should be ok (2x2 + 1 to spare)? Or should I 'play it safe' and go up to, say, 118mm (which is only 4mm a side, when you think about it, which really doesn't seem a lot)? Or even a 122.5? Is there a downside to having a 'too long' spindle? (I can redish the wheel if necessary to ensure a straight chainline, so I'm not that bothered about that.

Any thoughts and advice appreciated. (Oh, and please don't just point me to Sheldon Brown's database...his table, as far as I can see, suggests that for a Sugino single crankset with a 70mm shell I need a 103mm - 7mm less than the 'too short' one I have already!)