Bent crank/spider

Hey guys
From what I can tell I’ve bent the spider or right crank on my fixie. As the pedals go around the tension on the chain goes up and down. Is this fixable and is there a way to stop it happening again (technique for example)?

If they’re buggered can anyone recommend a replacement? They’re a nice set of shimano 600s so I wouldn’t mind something similar. Or chromed to match the rest of the bike.

Hmmn… might just be your chainring:

Or it could be a worn chain. Did it happen gradually, or as a result of some kind of impact? I recently fixed that problem on my bike with a new chain.

Measure the distance between the spider arm and chainstay for each 5 arms. If one of them is different, you’ll have a bent arm.

Although, as ndf said, a bent arm is probably not the cause of your problems.

The other option, which is more likely unless the bike has sustained a hit or crash from the side, is a bent bottom bracket spindle, which relative to a set of cranks is much cheaper to fix.

If you sit on the ground next to your bike, spin the crank and if you notice the centre going up/down or front/back then there is a fair chance that’s your prob.


thanks guys

Measure the distance between the spider arm and chainstay for each 5 arms. If one of them is different, you’ll have a bent arm.

I’ve done this. The arms of the spider are all a different distance away from the frame. As you spin the crank the distance increases and decreases which shows that they’ve all shifted (which I thought was unlikely). I then took the right crank off and spun it 180 degrees so it’s pointing the same way as the left. the arms that were furthest away from the frame are still the furthest away which to me says that it’s not the BB shell or axle.

I haven’t crashed on it either.

Seems to have been pretty sudden as far as I can tell (haven’t ridden it in a month because I took my roadie to France). Saturday the chain was loose. Tightened it. Was loose by the time I got home. Tightened it again. Super tight by the time I got to work monday. Loosened it when I got home. Tight at work tuesday. Finally on Wednesday I realise that the tension is oscillating. So to answer the question of how suddenly it came on: I think it was sudden. I don’t recall excessively slack or tight chain before I left and it was unlikely that it was ridden while I was away as I took the pedals to france with me.

I’m still wondering how the hell this would happen? Given that they’re pretty highend cranks (for their time) and I’m not exactly a racing ferret, I’m doubtful I’d have the power to bend them all. What are the chances that one of them was bent and because the chain was re-tensioned, (and the chain ring was wobbling) that the rest of the arms were pushing against the tension of the chain?

Vexing to say the least.

i’ve seen people bend chain ring arms from leaning a bike against a post.
and i’ve seen them get torqued out of alignment by an over tight chain.
most cranks have a little drift in them, and very few chainrings are perfectly round.
i guess the big question is, how much of a tight spot do you have?
one of my work bikes has a huge tight spot caused by a cheap chainring, and semi flogged chain. perfectly ridable tho.

It goes from tight (as in more tight than you’d have it normally) to loose enough that between the two cogs, the chain probably has 15-20mm of give if that makes sense. So i’d say pretty tight. I can feel the chain tighten up when i’m pedaling and can feel a fair bit of travel in the cranks before it engages when I’m track standing.

Just to drift a bit from spider problems, how tight is normal? Is there some way of setting it right every time or is a feel thing???

Dazed and confused…

It’s a feel thing. You’ll get used to it.

Tighten it up with the rear wheel straight and then spin the cranks. If they don’t spin freely or they stop spinning really soon it’s probably too tight.

If your chainline is straight then I reckon there’s a fairly wide range of chain tension adjustment that will be fine. But if it’s too tight it will wear your drivetrain and hub bearings prematurely.

The track thing is loose enough that it won’t come off, turn over the chain slowly and push sideways, if it comes off it is too loose.

Yeh, thanks peoples ,I didn’t think there was a scientific way to do it, but like you said I wasn’t really up for replacing bearings soon. :slight_smile: