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I have a touring bike which works fine. But when I shift to either of the two larger cogs on the front the chain hits the derailleur on both sides. Obviously it's because the cogs don't run straight but I'm unsure what my best options are. (It moves around on the smallest cog as well, but not enough to hit the derailleur)

Can I bang this around judiciously with a hammer and fix it, or is it something that is a potential danger and needs service, or should I just take the derailleur off?

If I take the derailleur off then I'll just have one front gear, but I could probably live with that. There's no bike service places in this country, nor do I have special tools.

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    Are you sure that the derailer has not gotten twisted on the seat tube? Or could it be that the right side crank arm is loose on the shaft?
    – Daniel R Hicks
    Commented Aug 16, 2016 at 11:33
  • Compare your front derailleur cage to those on other bikes - I bet its been bent narrow at the top.
    – Criggie ♦
    Commented Aug 17, 2016 at 8:21
  • @Criggie Nope it's ok, when I turn the pedal I can see it's the cogs moving out of true as a group, so something further down. I've already solved my problem though by bending the derailleur.
    – Kilisi
    Commented Aug 17, 2016 at 11:57
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    @Kilisi Politely I disagree. You've masked the symptom by your actions, but the underlying problem sounds like a bottom bracket in need of tuning or replacement. You'll need a crank puller to get the cranks off, and whatever fitment your BB needs to undo it.
    – Criggie ♦
    Commented Aug 18, 2016 at 1:53
  • @Criggie I don't really have a choice, no parts, no tools, could always send it back to NZ and complain, but bike cost me 80 NZD and shipping cost me 200 NZD, more than double the bike cost... so I'll do the best I can and smile wryly :)
    – Kilisi
    Commented Aug 18, 2016 at 2:59

2 Answers 2

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Bending the chainrings back is a possibility, but what you want to avoid doing is bending them around when what's actually causing the problem is misalignment coming from either the crank or BB spindle.

If the warble on the rings is uniform across the three rings, that's a strong but not automatic indication of a bent or poorly aligned BB spindle.

If it's the larger two only but the inner is more or less straight, that's a strong indication that the crank spider tabs are misaligned, which is often a fixable problem. Spider tabs can be "trued" by removing the chainrings, taping something flat like a popsicle stick to the FD to use as a gauge, and bending them around using an adjustable wrench so they run true with each other. This is bending aluminum, which always introduces some risk of fatigue failure. Then put it back together and if that doesn't work, you can try working the individual spots on the rings back into alignment.

Just making it a one-by may be reasonable, but it can be difficult to get it to not drop the chain if you're not using anything to help keep it on. Good chainline is a must if you're going to avoid this, but it's not necessarily enough.

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  • Yep all three have the same issue, it's the whole unit that is warbling, the BB spindle seems like a pretty robust piece of metal.
    – Kilisi
    Commented Aug 16, 2016 at 9:08
  • I guess I could force the derailleur apart a bit so the chain has more room to wobble around in?
    – Kilisi
    Commented Aug 16, 2016 at 9:17
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    If you spread the derailer a lot you can have problems with shifting and chain drop. Tweaking it a little bit to avoid rub in a certain spot can be the trick that solve problems like this though. It kind of depends on how bad it is. As for the BB, sometimes they just come this way, and every once in a while you see one that's in the process of breaking. It's possible you can make it better by trying the different positions of the crank on the spindle. Triple cranks with removable chainrings don't usually have this problem originate at the crank, unless it's been damaged in an accident.
    – Nathan Knutson
    Commented Aug 16, 2016 at 16:00
  • I've spread it about a cm and it no longer hits.
    – Kilisi
    Commented Aug 16, 2016 at 19:49
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If you take into account the time spent and the quality of the results of trying to straighten bent chain rings, you are probably better off just buying new rings. Also check the condition of the teeth: if they are badly worn, all you get for your effort is straight rings that have bad teeth.

Rings (and individual teeth, if necessary) can be straightened with an adjustable wrench. It's better done with the ring firmly attached to the crank, and the crank attached to the bike, so you can spin it and easily see where it is deviating from true.

You may be able to do the large ring, depending on how much room you have to grab it with the wrench (how much larger it is than the middle one).

The middle one will be impossible to do because the large one gets in the way. You have to remove the rings from the crank and then attach just the middle one to the crank. You will probably find that when the large ring isn't there, the bolts, which go through the crank and both rings, cannot tighten; you will need a couple of washers which have an inner diameter similar to the bolt holes to make up the missing thickness in place of the large ring.

The way the adjustable wrench is used is the obvious: just tighten it down, as firmly as you can, onto the area of the ring that you would like to bend back, and then apply leverage. Check for individual teeth being bent relative to adjacent teeth, and apply individual small bends to correct them. Teeth that are out of whack can cause nasty problems under load, like the chain riding up on the teeth and then snapping down, with each pedal revolution. It might be fine on a flat road, but as soon as you hit a hill: thunk, thunk, ...

I have experience trying to use a rubber mallet to straighten out a ring (off the bike of course). It wasn't entirely ineffective, but not very. Never tried a metal hammer; it seems like it could damage the teeth.

About special tools: if I do anything requiring removal of the rings, I remove the crank off the bottom bracket. That requires a crank puller tool. I'm not sure whether it's needed. If you can easily access the chain ring bolts without removing the crank, you're okay without the crank puller.

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  • no bike shops, no spare parts less than a month by ship away, no fancy tools. I have two secondhand identical bikes, they both look the same down there and the cogs are straight so it's something else. I've bent the derailleur to make room and happy enough with that..
    – Kilisi
    Commented Aug 17, 2016 at 11:55
  • If the chainrings are straight, but still exhibiting runout when you turn the cranks, it suggests that the drive side crank is sitting on the bottom bracket spindle on an angle.
    – Kaz
    Commented Aug 17, 2016 at 13:37

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