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Curious wobble in drivetrain... Any guesses?(8 posts)

Curious wobble in drivetrain... Any guesses?Unreasonable
Feb 10, 2003 8:35 PM
I have an oldish Klein Quantum with Shimano 600 on it, and I recently noticed something wrong with the drivetrain.

When I spin the cranks, the position of the chain relative to the front derailler cage moves side to side a few millimeters, as if the big ring were a little bent or something.

However, upon dissasembly, I do not find anything bent. The rings are not bent, the crank is not bent, and as near as I can tell, neither is the bottom bracket, although the bottom bracket was the hardest thing to check, so I'm not 100% sure on it.

Anybody have any guesses as to the source of my issue?

re: Curious wobble in drivetrain... Any guesses?Woof the dog
Feb 10, 2003 11:33 PM
did you check the spider arms?

That is probably your problem.

A Rubber mallet, or just a plain old hammer and a piece of wood will fix the problem. Just hit the spiderarm that may be a little bit bent. Its an evil procedure though... makes you think that you broke your bike. Cuase you do have to hit it hard.

re: Curious wobble in drivetrain... Any guesses?Rob Sal
Feb 11, 2003 12:25 AM
My Record cranks do that too. I don't worry about it, and I certainly won't take a hammer (rubber or otherwise) to it!
re: Curious wobble in drivetrain... Any guesses?Woof the dog
Feb 11, 2003 9:19 AM
Ha! How do you think they fix the bent ones at the factory?

Woof the dog.
re: Curious wobble in drivetrain... Any guesses?micha
Feb 11, 2003 3:04 AM
I assume you have an adjustable bottom bracket. The tiniest bit of bottom bracket bearing play will translate into a slight, but visible chainwheel wobble. Adjust the bottom bracket bearings so they have a very small amount of preload - just a tad tighter than you would adjust wheel hub bearings. The word "snug" comes to mind.

I've also seen chainwheels that didn't seat properly on one of the spider arm ramps. If you had to pry or tap the chainwheel off the spider arm ramps, this might be the case.
Crank/BB InterfaceDoubleK
Feb 11, 2003 9:33 AM
I've had this happen on one of my bikes. I'm assuming that you have a square taper BB and your chainring is true as you say it is (because this is the problem many times).

Sometimes, the crank just doesn't slide up the BB taper exactly perpendicular and you will get a slight wobble. What I ended up doing was removing the crank, rotate it 90 degrees and re-install with a VERY SLIGHT amount of grease on the BB taper. (This is will likely start a flame war.) Now check the trueness of the assembly, if it is now true you will have to move the other crankarm to the appropriate position but don't waste your time doing this until you are sure that other is set where you want it. This worked for me. I have always been a "dry taper" man but have found that sometimes I just don't get the perfect fit that I do with just a very slight amount of grease applied the the BB taper. This has been my experience and I believe that it is worth a try in your case.

Good Luck!
Crank / spindle fitCalvin
Feb 11, 2003 10:57 AM
Some wobble is common in the chainrings. A few millmeters is quite a lot, however. Bending may be required. First, you may simply try different position on the spindle. I assume you have a square spindle. You have 4 different ways to mount the arm. Each position can offer suprising results in chainring wobble. This may help minimize the problem, then bend from there.

Before bending, determine if the ring or mounting arm is the issue. If both rings move together, it is the mounting arms. If only one ring moves, then bend just that ring.
Feb 12, 2003 5:38 PM
1 - loose BB. You can check this by wiggling the end of the crank arms at several positions around the circle. No wiggle means this is not the source of your problem.

2 - "bent" spider arms (most likely source of problem). Hold a short screwdriver (or any other straight object) perpendicular to and pressed against the seat tube so that the tip just touches the chain ring bolts. Spin the cranks. You'll see if one of the spider arms is off from the rest and by how much. Repair consists of supporting the bike by resting the crank end (at the axle) it on a block of wood and banging on the offending spider with said rubber/plastic mallet (or hammer with a block of wood to protect the crank). Hit lightly, check for reduced deflection, hit harder, check, repeat.

3 - bent chain rings. If the spider arms are OK, hold the short screwdriver so that the tip just touches the large chain ring near the teeth. Spin the cranks. Straighten the offending chainring with a large adjustable wrench spaced so it just slips over the chainring. Apply gentle pressure, check, repeat.

You most likely would not find this problem without everything assembled, so your disassembly approach is flawed.