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new ISIS drivetrain wobble problem(6 posts)

new ISIS drivetrain wobble problemannoyance
Jan 8, 2003 1:14 PM
so i got a whole new drivetrain, cranks (FSA carbon team), BB (FSA), derailers(Dura Ace), shifters (Dura Ace)... installation went fine (i think). but now there is a bit of a wobble, as if the chainring is slightly bent, it wobbles from about the middle of the cage on the front derailer to almost scrapping the outside of the cage. i took everything apart and made sure the chainring was flat (it is) but the only other thing i can come up with as being wrong is a bent bottom bracket. is there a way to check that or does anyone have any other ideas? i would just deal with it but i can't really adjust the front derailer correctly as it scraps if i move it enough to have good big ring shifts.

re: new ISIS drivetrain wobble problemjimmy320
Jan 8, 2003 1:38 PM
Tony--try rotating your crank and marking it where it most closely contacts the derailleur. then remove it, reinstall it 180 degrees from its original mounting orientation, and rotate it again, again noting where it most closely contacts the derailleur. If it is the same spot, you have a dent or deformed crank arm or ring. If it is 180 degrees from the original mark you have a bent bottom bracket.

(part-time wrench) Jimmy
Jan 8, 2003 3:18 PM
Make sure you are using a torque wrench and installing everything to the proper spec. It's essential to ensure that everything is nice and tight.

I'm betting that the problem is in the crank arm. It's a whole lot easier to screw up the crank arm during the manufacturing process than it is the BB, but it is possible.
9 times out ot 10, this is your problemKerry
Jan 8, 2003 4:38 PM
"bent" chainring spiders. If you hold a pencil on the seat tube so that it just touches the chain ring and spin the cranks, you'll see and mark where the wobble is. It is likely in the same place on the large and small rings, suggesting it is coming from the crank. If it only occurs on one ring, then the ring is bent, which may happen only when it is bolted to the crank. Most cranks have or develop a slight offset in one or more of the spiders. You can fix this easily by bracing the "opposite" side of the crank (at the BB axle) on a block of wood, bike horizontal. Put a wood block on the crank spider at the wobble point, and whack it with a hammer to push the spider in the desired direction. Whack lightly at first, and see how much the spider moves. Whack, check, whack, check. You'll get a feel for how much force is required. You must brace the BB or all of your hammering force will be directed into the BB bearings. The other poster's advice on checking the BB by rotating the crank on the BB is also excellent, but I'd bet that it is a slight bend in one or two crank spiders.
I've had two DA cranks with bent spider armsB2
Jan 8, 2003 5:20 PM
One came that way and the other ended up that way after a season of riding. I never got to the point where I whacked it with a hammer, but I did loose one gear combination due to chain rub on the front derailleur.

The one that came that way was replaced by Colorado Cyclist on warranty. If it's new, exchange it for a new one.

thanks all, i will try it out tonite (nm)annoyance
Jan 9, 2003 6:24 AM