Bicycle Mechanics - Front Derailer Question Regarding Shimming

I wanted to post this question over here where there are more mechanics and see what the general consensus is.

If you have a seat tube that calls for a 34.9 front derailer and are using the correct sized FD, how common is it to have to shim the FD to keep it from slipping down the seattube?

About as common as the number of bikes with 28.6/31.8mm seat tubes mislabeled as 34.9mm. Either the seat tube is not really 34.9mm or the front derailleur is messed up/too loose. I don't see what else it could be.

Ive mic'd a bunch of seat tubes that take 34.9 FDs and the sizing seems to run from 35.2-35.5. This seatpost is 34.8-34.9, depending on how you hold the mic.

The seat tube is a little bigger than the front derailleur clamp, so a shim would be of no use. Sounds to me like it'll be just fine.

The seat tube is a little bigger than the front derailleur clamp, so a shim would be of no use. Sounds to me like it'll be just fine.


Yea.. its not. Ive put had two shops put two different FDs on the bike and the first time you pull the chain over to the big ring, the FD slides down the seatpost.

Usually the tube is 34.9 and then you would add the paint thickness (twice) to get your [actual] mic'd 35.2-35.5. Is this frame Ti or otherwise bare?

Usually the tube is 34.9 and then you would add the paint thickness (twice) to get your [actual] mic'd 35.2-35.5. Is this frame Ti or otherwise bare?
Even if it's bare Ti, the fd shouldn't slip. I have three bare Ti Litespeed frames that all take 34.9 mm derailleur clamps and none has ever had the front derailleur slip. I've used integral clamp and braze-on adapter clamps by both Shimano and Campy and had no problems with any configuration.

JayC,

I saw your post on the group buy thread. It was never clear to me whether the clamp faces are in contact with each other eliminating additional clamping force no matter how much torque is applied to the clamp screw. If the faces are in contact, the frame and/or the clamp are defective. I bought a Pedal Force RS on the group buy in 2006. The first time I installed the FD (Shimano FD-6300, Ultegra) I did not prep it and was quite concerned about the torque applied to the screw as I feared cracking the frame. It slipped on the first ride. To correct, I took it off the frame and roughened the inside diameter of the clamp with some emery cloth. I also reviewed Shimano's technical document for the screw torque spec. The document I had did not have a reference to carbon frames. It indicated 44-60 in-lbs. for torque. The present document does indicate one should consult the frame manufacturer for tightening torque. I ended up with about 35 in-lbs of torque on the screw. The increased torque and the roughening of the clamp ID solved the problem. I have ridden over 2,000 miles with no problems. You could also roughen the clear coat on the frame slightly. I did not do so because I did not want to change the cosmetics of the seat tube. Unless the clamp faces are bottomed on each other, I believe this will solve your problem.

Usually the tube is 34.9 and then you would add the paint thickness (twice) to get your [actual] mic'd 35.2-35.5. Is this frame Ti or otherwise bare?


Its clearcoated carbon with no decals, paint, etc.. just cleared over the bare carbon.

JayC,

I saw your post on the group buy thread. It was never clear to me whether the clamp faces are in contact with each other eliminating additional clamping force no matter how much torque is applied to the clamp screw. If the faces are in contact, the frame and/or the clamp are defective. I bought a Pedal Force RS on the group buy in 2006. The first time I installed the FD (Shimano FD-6300, Ultegra) I did not prep it and was quite concerned about the torque applied to the screw as I feared cracking the frame. It slipped on the first ride. To correct, I took it off the frame and roughened the inside diameter of the clamp with some emery cloth. I also reviewed Shimano's technical document for the screw torque spec. The document I had did not have a reference to carbon frames. It indicated 44-60 in-lbs. for torque. The present document does indicate one should consult the frame manufacturer for tightening torque. I ended up with about 35 in-lbs of torque on the screw. The increased torque and the roughening of the clamp ID solved the problem. I have ridden over 2,000 miles with no problems. You could also roughen the clear coat on the frame slightly. I did not do so because I did not want to change the cosmetics of the seat tube. Unless the clamp faces are bottomed on each other, I believe this will solve your problem.

The clamp definitely isnt bottomed out. I guess I need to pick up a torque wrench in in/lbs.. I can definitely tighten the clamp more, I just didnt want to risk cracking the frame.