Chainring wobble
 

Hi.

I have a bike that I'm trying to sell but it needs some work. The main thing I noticed was that the chainring wobbles and at one point the chain rubs against the chain cage when it's on the largest ring. Looking at the chainring I can't see any bending but I'll have to re-check. If I pull on the crank it doesn't feel like it's loose. I'm trying to find out what the problem is and where I should be looking. It's a Truvativ crankset (bike is probably around 5 years old looking at the model). I don't know anything about the history of the bike.

Any help will be appreciated.

best way to check if a chainring is flat is to remove it and place it against a sheet of glass, a mirror, or similar. If the ring is flat, then the problem lies with your crankset or BB. Don’t know an easy way to test for a bent crank, but if you remove it, you might be able to see a bent BB spindle by spinning it

Sure sort of, But this check is based on a non reality. As said the real world of a ring is as it's mounted on the crankset. So it doesn't really mater whether the ring is flat when off the crank arms. You have to deal with the world you are in.

It's pretty common to align rings while spinning the cranks while using the ft der or a piece of tape standing off the frame as an indicator. Whether one bends the ring or the spider arms is the trick. Also how.

One can bend the ring or arm in or out with various levers or impacts. Or twist the ring between the arms with two tools. Rather simple blacksmithy but takes some practice to become comfortable in doing whatever improvements can be had. And that's the take away. Not all rings are able to be perfectly aligned but a lot can be made better. Andy

Thanks for the replies. I appreciate it. I've actually watched some videos on Youtube showing how to bend a ring back to a straight position but I wasn't sure that was my problem since the ring didn't really look like it was bent. So, I was thinking maybe it is a BB problem. I guess I'm gonna have to take it off the bike and take a closer look....that's IF I can do that. I've never taken a chainring off a bike before. Wondering if I need special tools or something or it's just removing the 5 hex screws that I think are holding it in place.

Are you certain that the crank is properly torqued onto the BB?

My chain ring wobbled because the BB was not torqued correctly and was very slightly loose.

No. I don't know. I know this bike has been neglected by the previous owner. Thanks for the info. I still haven't tried taking the crank off.

So, I'm going to attempt removing the cranks but this is a first for me. I just bought the proper size hex key but I'm assuming I need a crank puller tool. Or is there a way around it ?
https://cimg7.ibsrv.net/gimg/bikefor...ee3cd79b70.jpg

So, finally got the time to get the chainring off and apart (separated the 3 rings). After examining the big ring by putting it on a flat surface there was only minor bending in it but I was able to correct that however, when I put the chainring back together and installed it on the bike the same thing still happens and the chainring still wobbles and the chain when on the large ring rubs against the chain cage every 3/4 of a turn (it rubs for the length of about 6-7 links). So, does that mean it could be the BB ? The spider doesn't look bent.

Maybe remove the crank and try the 3 remaining positions (rotate 90 degrees each time) to see whether the problem lies in asymmetric walls in the tapered holes.

Thanks for the reply and the suggestion, holytrousers. I made a second attempt yesterday to try and figure out where the problems lies but it didn't seem the crank position is the issue. I'm beginning to think maybe the spider is bent somewhere but it's hard to see just looking at it. I've noticed that the big chainring becomes curved -always in the same spot- when attached to the spider arms and the screws are tightened but when it's separated and put on a flat surface it's almost completely flat or the imperfections are very minor. I'm going to try and attach a pic. Maybe it's harder to see in the pic but the gaps between the outer edges of the big ring and the middle ring are wider than the gap in the middle. You can see in the middle the ring curves upward (looking at the pic) and that area is where the chainring moves outward and the chain robs against the chain cage when on bike. I don't know if I can put some shims or something to fix that.

Perfection is expensive , good enough is probably adequate. :innocent:

Well, it's not good enough. The chainring wobbles bad enough for the chain to rub against the chain cage for about 4-5 inches. That's a lot of chain links rubbing. What do you think ? I know the pic doesn't show much bending but when the chainring is installed the wobble is bad.

Anyway, to try and rule out it wasn't my BB (bent), I fixed a hex key to the bike to where it barely clears the 4 corners on the spindle and turned the crank (left side) and there was no touching at all and the clearance remained the same. Does that mean the BB's good ? Pic below.

Did you use a chainring bolt wrench to make sure the bolts are properly tightened? If you are certain that the chainring bolts are installed correctly, then it sounds like the issue is probably the spider arms. I've never had to fix one of those, but I would assume you could check it by removing your driveside crank, taking off the chainrings, and then laying the spider arms on a flat surface. If it doesn't lay flat, that would be a sign of mis-alignment.

If the spider arms are not lined up properly, I'm not sure the best method of fixing them. My instinct would be to take the opportunity upgrade the bike to better cranks, but YMMV.

I have used thin chain ring bolt shims to effect wobble correction with mixed results. A lot of time, trial and error and that was when thin spacer/shims were easily available. I've whacked and pried dozens (likely hundreds over the years) of rings into acceptable straightness, as I mentioned before.

Holytrousers makes a good point about rotating the mounting of the RH arm on the axle flats 90* at a time to see which is the best WRT ring straightness. Andy

It seems to me that the best place to diagnose the problem is with the crank assembly on the bike. As you slowly turn the crank you could mark the offending 4-5 inches of chain rub. Does it correspond to a spider arm? Is there a similar 4-5 inches 180 degrees from the offending section that is in the opposite direction? Do the smaller chainrings demonstrate any similar wobble or misalignment? Remember that the large diameter will magnify any imperfection of the spider or mounting. Once marked, you could see if the offending area corresponds to the slight lack of trueness when off the bike. Also, is the derailleur cage centered over the large chainring?

You said you don't know the history of the bike but is there any sign of damage? It doesn't look like it in the picture.

This is a good reference when there's precession in the crank: https://www.sheldonbrown.com/straighten-chw.html

Thank you all for the replies and the help. Well, it doesn't look like the spider arms are bent. I put the spider on a flat surface and used a nickel to slide under each arm and the clearance is the same on all four. Well, I guess it's a mystery why the chainring is flat when I put it on a flat surface but when it's attached to the spider there's a curve. Anyway, last time I tried to change the position of the crank it didn't make a difference but tried again yesterday and viola, it worked and the wobbling is much more acceptable now and the chain doesn't rub against the outer side of the cage any more HOWEVER, it now touches the other side (inside) of the cage when the chain is on the biggest ring and the biggest cog but it's not bad and I know this combination should be avoided anyway but I'll see if I can fix that too somehow.

Thanks again for the help.

I’ve had an awful time trying to align a SunRace FCM918 square tapered crankset just to find the hole was off centered. There was no solution: I’ve bought an used external bearing Shimano crank, with worn bcd-matching chainrings, exchanged the chainrings and installed a SM-BB52 bottom bracket.

Almost solved it, but still I had an occasional chain run against the outer side of the front derailleur, mainly on sprints. As you, I’ve searched ways to eliminate the problem, to no avail. U til I borrowed a GoPro, aimed it at the front chainring and take a ride.

For my surprise, I’ve found the frame (hi tensile steel one, non oversized) was flexing enough to allow some chainrub. “Good enough“ is the solution, as I don’t see any reason for a new frame right now.

By the way, I’d inspected it for cracks, it’s fine. It’s just good old (heavy?) steel flexing as it should.

FD adjustment?