Rear end shake: warped sprocket?

[ExterminatorBike]

Does anyone know how warped is "too warped" when it comes to rear sprockets? Here is my problem:

I just got new tires (Conti twins) mounted and balanced at the local shop. I only took the wheels in, and left the rotor and sprocket off the rear wheel. Since then, I'm getting a nasty up-and-down shake at 45-50 MPH (in the chassis, not engine buzz), like I'm riding on a washboard. It comes and goes, and doesn't seem to get worse at higher speeds.

I cleaned and lubed the chain, and saw no stuck links. Thinking the rear wheel might be out of balance or the bearings might be bad, I pulled the rear wheel. It spins smoothly and and seems to be in balance (on an improvised stand), but I noticed the rear sprocket seemed not to be flat--it was slightly moving side to side as the rear wheel spun. I put a dial indicator on it, and it seems to be 32 thousands out of flat (from high point to low point). There is no service limit listed in the manual, that I can see. It didn't do this before I replaced the tires.

I don't think these are the original sprocket and chain, but I can't remember when they were last replaced. Should I play it safe and replace the set, or is something else likely to be causing my shake? I replaced the rear brake pads at the same time, but the brakes seem to work fine, not dragging that I can tell, and the rotor is not warped. I also put Hagon shocks on the rear.

Thanks in advance.
Jim

[jimcor]

I don't believe that is enough distortion in the sprocket to be noticeable and wouldn't have the washboard effect you describe. First off I would check both wheels to make sure the tires are mounted correctly and evenly. It's easy to make a mistake, and an improperly mounted tire will give exactly the effect you describe.

[ExterminatorBike]

It's at least rotating the correct way. I also measured the chain stretch, and it seems to be fine. Again, this just started with the tire/rear brake pad/shock change. The pad shims were facing the correct way, and that seemed to go smoothly. Once again, doing more than one thing at a time makes trouble shooting difficult!

[jimcor]

It's at least rotating the correct way. I also measured the chain stretch, and it seems to be fine. Again, this just started with the tire/rear brake pad/shock change. The pad shims were facing the correct way, and that seemed to go smoothly. Once again, doing more than one thing at a time makes trouble shooting difficult!

Your tires should have some type of alignment 'ring' at the bead, get those wheels off the ground, spin the tire and check for high and low spots. If they're there you must reseat the tire on the rim. It will not self correct. Sometimes it's hard to tell which tire is causing the problem.

[ExterminatorBike]

I'll do that. After I posted this, and after your reply, I'm thinking that the most likely cause for this sudden-onset problem is the tires. If I see anything fishy, or even if not, I might start by taking them back to the shop to check the mounting and balance.

[Billy Ricks]

This could be a little dangerous to do and if you try it you're on your own, don't blame me. You could pry your pads back and see if the condition goes away. You did replace pads and maybe it has something to do with it. You'll need to pump the breaks a few times when you want them back.

[ExterminatorBike]

check for high and low spots

I'm seeing a concentric ring near the rim which varies on one side from about 4 mm to 7 mm (I hate fractions). It's noticeable as you watch it spin. Would that be enough to indicate the bead is not set, or there is another mounting issue?

As for the brakes, I did have the idea of taking the pads out and being VERY careful not to step on the pedal, to see if the problem stays or goes. Like I said, doing more than one thing at a time makes it harder to track down the problem, but I figured, while the wheel was off...

[jimcor]

I'm seeing a concentric ring near the rim which varies on one side from about 4 mm to 7 mm (I hate fractions). It's noticeable as you watch it spin. Would that be enough to indicate the bead is not set, or there is another mounting issue?

There's your trouble right there.

[Billy Ricks]

Can't believe they let that wheel out of the shop like that. It's the first thing they should check after the bead pops. The first pop usually doesn't seat it all the way around.

[ExterminatorBike]

Thanks for the advice . I'll take it back to the shop tomorrow, then post back with an update when it's back together.

[chef1366]

Shop hacks. I swear :?

[ExterminatorBike]

There's your trouble right there.

Sorry for the delay in posting the results--it seems it was the tire. I swapped the old shocks back in, and ran it with no brake pads and then the old brake pads, and the problem remained. To the shop's credit, they checked the mounting and tried to make sure the bead was fully seated, and rebalanced the tire. When that didn't solve the problem, they replaced the tire and balanced it again, all at no charge to me.

Thanks for all the input!