My Spider is Untrue

[nirakaro]

No, not a country music title, but a cycling irritation. There's a millimetre or so of wobble in my outer chainring, so the chain rubs the front mech if I'm on the smaller rear cogs. I've tried rotating the chainring on the spider, without making much difference. Measuring the gap from the spider bolt-holes to my downtube, I find that two opposite ones are the same, and the other two are about 0.3mm off – one further out, one further in.

How to fix this? The options I see are –
1. Buy a new crankset. Too expensive.
2. File a bit off the spider. Dicey, and difficult to do accurately.
3. Might rotating the crank on the axle make a difference?
4. Pack the outer (and maybe the middle?) chainring out with shim washers (or possibly even with slivers of coke-can?). Seems the best option.

Am I approaching this the right way? Am I missing anything?

[nirakaro]

P.S. Alivio square taper crankset.

[Neil C]

First thing I would do is remove crank, clean up hole and axle, and re-mount 180 degrees out. Then re-measure.

If this improves matters don't forget to realign left hand crank.

(TBH the first thing I would probably do is try to adjust the front mech and ignore the wobble.)

[Samuel D]

Definitely not option #2.

Option #3 often improves matters in my experience. Don’t think you can do it by memory! Mark the spindle or this becomes a guessing game with uncertain conclusions, at least if the left crank is off.

That said, 1 mm is not a huge wobble. Careful derailleur set-up in combination with the trim function of your shifter may be sufficient although of course a true chainring is preferable.

[Cyril Haearn]

I might try washers
Was it perfect before, did it get hit and distorted somehow?

BTW I wondered whether this was a pet problem, is there a forum for that here?

[Brucey]

it is quite usual for there to be a little run-out in the chainring. It is usually possible to tweak the chainring so that it runs truer.

However this is only half the issue; the reason it bugs you is because the chain rubs on the FD and this means that there is probably scope for improving matters by attention to the FD adjustment etc.

It would help if you say what FD you have and what shifters you have too. If you can manage a photo of the FD setup this would also be useful.

cheers

[Mr Evil]

Are you sure it's the spider and not the axle that's bent?

[9494arnold]

You can usually ease a chainring with careful manipulation with an adjustable spanner after you try all the things suggested so far. (Slide it down the chainring, adjust so it's an interference fit (you might want to put a rag between jaws of spanner and chainring) and pull (or push) gently . Do it above where the chainring fixes to the spider initially.
I'd agree you could possibly adjust the front mech as a starter for 10. Make sure it's cage is parralell to the outer chainring , then adjust the outer limit screw , making sure the mech doesn' t touch the crank. Might need to adjust cable too .
It's one of those jobs that could take minutes hours days months

[hamster]

Cheaper Shimano cranksets always seem to be rather wobbly. I had a 300LX one on my first MTB that drove me nuts. I've tried M550, M650, M563, M730. With the exception of the M730 they always had a degree of wobble in the chainring. As a result I don't have a single bike with a Shimano crank: Campagnolo and Sugino seem much better.

[Cyril Haearn]

Cheaper Shimano cranksets always seem to be rather wobbly. I had a 300LX one on my first MTB that drove me nuts. I've tried M550, M650, M563, M730. With the exception of the M730 they always had a degree of wobble in the chainring. As a result I don't have a single bike with a Shimano crank: Campagnolo and Sugino seem much better.

Do you mean play/looseness or untruth/crookedness? Or both?

[hamster]

Just un-true running, they all sat tight on their tapers.

I have no idea whether the modern type with external bottom bracket run any truer.

[Keezx]

For what it's worth , I own 5 square taper cranksets (Campagnolo) and 1 external BB (SRAM Force GXP) and the SRAM is the most untrue.
Does'nt give any annoying chain rub, so I ignore it.

[nirakaro]

Sora 9x3 shifters and Suntour X-1 front mech, Brucey. I know my 48-36-22 chainrings are asking the mech to do more than it's designed for, but it works and I like it.
Cyril Haearn, I think it's been like this for as long as I've had the bike in this configuration – a small number of years - and given the tiny amount of time I spend on the big ring, it's not a major problem. I just thought, while I've got the bike stripped down for spring cleaning, it'd be nice to get it right.
I've spent many happy times tweaking the front mech, and I can stop it rubbing here, but it always ends up causing a more annoying issue somewhere else.
Good idea to remount the crank, Neil C. Marginal improvement, but not significant. The error seemed to be in the same place relative to the axle, which suggests (if my thinking skills are in order) that the issue is in the axle, not the crank, Mr. Evil. (I'm aware that I may be at the limits of my measuring skill here, even with a digital calliper, and a variation of maybe 0.3mm at 52mm radius could mean an axle out of true by a very few thou. in old money.)
I've ordered some 0.1mm and 0.2mm shim washers to play with. I'll see how it goes.