Potentially warped chain ring - what are my options?
 

Hi all,

So I've got a Voodoo Limba adventure bike, with a Prowheel 48/32t chainset. On the 48t ring I can see the chain move side to side by maybe 1-2mm when pedalling on a turbo trainer, at the outermost point it hits the derailleur and causes a rubbing noise. There doesn't seem to be any play in the bracket to I suspect it's the chain ring, confirmed by a gap being visible when a straight edge is placed beside it. I've yet to take the chain rings off as I want to avoid that if possible since I haven't done it before.

Am I ok to use a disc bending tool to try and reshape it, or will it need replaced?
If I'm replacing it, is it worth getting an upgraded part? Should I be replacing both chainrings or the crankset?

A lot of old school LBS mechanics used to have a chainring bending tool which is probably quite similar to the disc bending tool you are referring to. I'm not sure if the thickness of a brake rotor is close enough to the thickness of the chainring to where you can use the disc brake straightening tool. If it doesn't fit, I would think you could use a large adjustable wrench (one of the American made ones where it can be adjusted for tight tolerances and actually hold its adjustment) might also work. If in doubt take it to a trusted bike mechanic at your LBS. If the chainring is not particular on how the chainring bolt holes should line up in relation to the crank arms, you could try orienting the chainring differently on the spider. Sometimes this will cancel out run-out and make things more in line. Worth a try. Also, you did not say there were any missing chainring bolts or anything but one obvious thing to check is that all the chainring bolts are there, that they are installed correctly with some light grease on the threads and torqued to spec.

PS: If you do try to bend the chainring straight, don't overdo it. Aluminum fatigues quite easily so only a very small bend should be attempted. If it is way out and you try to bend it back, it may well crack in which case you will then most definitely need to just replace the warped chainring.

Since this is the outer chainring it's likely it has a pin on the outside face opposite the crank arm to prevent a spilled chain from wedging between the arm and the ring. If so, you don't want to change its orientation. As noted, a good quality adjustable wrench can be used as a straightening tool.

2 mm is only .079 inches. An that is only a little more than 1/16 inch. So if 2 mm is the full range of movement from one to the other, then it's only little more than 1/32" out of kilter.

Is that causing you any problems?

It's causing the chain to hit the derailleur when in the highest gear, which I'm assuming will cause wear/damage over time as well as being annoying.
I can't adjust the derailleur out any further to avoid it either.

Does the gap between the outer and inner rings change as you rotate the pedals? Often the issue is with the crankset itself; very likely at the fit to the BB spindle. (If so, the crescent wrench fix will work to improve things, but you are only fixing the symptom. But slightly out of true cranksets have been ridden for as long as bikes have existed and generally haven't caused issues to anything except between the ears of certain people.

I have found that a small long-nose Vise-Grips® works well for truing chain rings.https://www.amazon.com/VISE-GRIP-Ori.../dp/B0000BYD9A

Before trying to bend the chainring, check to see if it runs true when not under load. Drop the chain off the crank and spin the pedals slowly to see how true the chainring is. This bike has a relatively inexpensive square taper crank that probably flexes a bit under load. If flex is causing the chain rub, bending the chainring to compensate could cause rubbing on the inside of the derailleur cage when there is no pedal load

Does it drop the chain?
if it doesn't I would try and adjust the fd first to get rid of the noise that way. No harm can be done by doing that.
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Sorry missed the bit where you tried to adjust it

If you didn't want to try bending a probably low grade aluminum chainring, Prowheel stuff is pretty cheap stuff so is not a bad candidate for replacement but you also have to consider the quality of the rest of the bike and see how much you like the bike and how much you would want to put in it.

It's definitely a cheap bike (£450 so maybe $600) so I don't want to spend a fortune on it. I'm already debating upgrading it to a Giant Contend AR 4 or a Cannondale Topstone 4 soon, so I really just want to get it to a state where I can (a) use it and (b) sell it on to someone else without feeling bad about it being damaged. It does a pretty decent for my needs though (very casual cyclist in pretty bad shape).

Then I would probably bend it slightly and carefully and be done with it save the money towards a bike you want. If it isn't perfect and you sell it just let the buyer know.

Wondering if you came to a solution for this? Bought the same bike a month ago and having the exact same issue. I wouldn't call myself much of a mechanic but had no issues in the past adjusting derailleurs, but this thing is just a #%!?& nightmare. Brought it back to the bike shop twice, where I got the answer that the gears are just 'settling in', but a 30 min ride later and there's the chain rub again. Getting pretty desperate :) not really worth it upgrading the groupset on a bike in this price range

Though temporarily reorienting the chainring will enable the OP to see whether it's the chainring itself or the BB that's responsible for the runout.

I'd straightened it out enough with an adjustable spanner that it didn't bother me unless I was in the very top gear or 2.

And then I sold it on and bought a new bike. I'd explained the rub issue to the new owners who came with a mechanic friend and didn't seem bothered about it.

You could always take it to a proper bike shop to have a look at, because Halfords are pretty hit and miss. A proper grubby small bike shop that's littered with bike parts and oil will usually give you sound advice.

You could upgrade the front cogs and it shouldn't be too bad, but I figured it was better just living with it and putting the money towards an upgraded bike.

Looks like its common problem. Bought limba last April and since then had thst problem and it became really annoying now. Been to Halfords 3 times and always got told "that's the way it is" then tried two different bike services and even if they managed to sort of fix it, problem was coming back after few days. And that's what they suggested - that chain ring is bending under pressure and only changing it could solve problem.