Not sure what clamp size you need, but check with your Trek dealer. I think Trek makes a derailleur hanger for compact cranks. Be sure to use Loctite when installing it on the frame with those tiny screws.

 

You can check with a dealer, or measure it yourself.

You can use a caliper, or wind a dollar bill around it, and make a mark at the point of overlap, then unwind and measure the the distance for to get the circumferance. Divide by 3.14 (pi) to get the diameter. You don't need super precision because It has to be either 1-1/8, 1-1/4, or 1-3/8" or 28, 32, or 35mm (rounded off), so if you end up with 1.273" you know it's 1-1/4, or 31.8mm.

I'm not familiar with your frame but if it has a permantly attached "braze-on" FD mount it might be in your way when you try the clamp. You might be better off buying a Shimano braze-on FD with 2 mounting holes and using the higher one.

 

If it helps, I have a Trek 5200, 1999 and many years ago I switched to a compact 50/34 and installed a IRD front derailleur for compact. On the same compact I also run 52/38 chain rings. The derailleur works great with any of the combinations. My cassette is 12-27.
The IRD mounts of the original brase-on. No need for a clamp.

Hello,

I switched to a compact crank (48x34) and now my derailleur isnt low enough and it now shifts poorly from small to big chainring.

SOO,... I am going to install a derailleur clamp on my 1998 Trek 5200.

What size clamp do I need?

Thanks!

Gall

 

Compacts usually shift poorly. It takes quite a fine tuning to get it right.

Maybe you need a FSA specific compact derailer ?

 

Why would it shift poorly. I run a compact on my Trek 5200 with DA 7800 as well as on my Parlee Z4 with DA 7900. Shifting is crisp. Compact or no compact, if the adjustment is not right you will get poor shifting.