Update: A front-derailleur clamp for braze-on front derailleurs, like the BBB ShiftFix, might be an even better option than the seat clamp. I especially like that an FD clamp hinges open; this eliminates the risk of marring the finish of the handlebars when sliding the clamp over the thicker part. Another big plus is the lack of an inner lip that has to be filed off. I’ll definitely try one on a future project and report back.
TL;DR: Use a 31.8 mm seat clamp.
If you’ve ever tried to mount an MTB trigger shifter on modern drop bars, you’ll have been dismayed to discover that it just doesn’t fit. That’s because flat bars have a diameter of 22.2 mm, whereas drop bars are 23.8 mm thick.
One option is to file down the shifter’s clamp to fit the thicker bars. After all, you just need to take off 1.6 mm of material, which is just 0.8 mm around. This does work, but it will only fit the shifter to the thin part of the bars. This might be what you want, but I like to ride on the tops a lot, and a shifter placed there interferes too much with my grip on the bars. Ideally, I want to place the shifter as close to the stem as possible, so that I can still reach it easily with my thumb, but at the same time leave plenty of space on the tops for my hand.
Enter user StabMasta on the Mountain Bike Reviews Forum, from whom I stole the idea for this small project. I believe I’ve improved on it a bit, so I’d like to show you guys how I did it. So, without further ado, here’s the result, and below that are the instructions:
Here’s what you’ll need for this:
A SRAM shifter.
I went with a SRAM, first of all because it has a removable clamp, but also because the body of the shifter has a nice flat section that can rest against the corresponding flat surface of the seat clamp.
A 31.8 mm seat clamp.
I recommend using a quick-release clamp, since they don’t have threading inside the collar. A bolted clamp will work too, but you’ll need to bore out the thread, otherwise the clamp and the shifter will act like a couple of locknuts and tighten against each other instead of securing the clamp to the bars. I also recommend finding a clamp that has a nice flat spot perpendicular to the threading, or else you’ll need to grind out a flat surface on your own, and that’s extra work, and we hate extra work.
An M5x30 bolt.
The shifter would have come with an included bolt, but that’s much too short for us. You might need to experiment with the length here, depending on your exact seat clamp model. A longer bolt will also allow you to install some spacer washers, to fine-tune the positioning of the shifter according to your preference.
An M5x10 bolt.
This is to prevent the shifter from rotating when you press the levers. The original clamp that comes with the shifter has a channel that holds the shifter in place. We could cut out a similar channel in our own seat clamp, but what did we just say about extra work?
A 2.5 mm hex key.
A 4 mm hex key.
Something to grind off the lip inside the seat clamp. A hand file would probably work just fine, but a Dremel with a high speed cutter bit will eat it up and ask for seconds.
Replace the included bolt that covers the right-side (as the rider is looking at it; left-side bolt if you’re doing the front shifter) threaded hole with the short M5x10 bolt.
Remove the locking cam mechanism from the seat clamp. Throw it in your parts bin, despite knowing full well you’ll never need it again.
Grind out the inside lip of the seat clamp. If you’re using a Dremel, for the love of all that is holy, use eye protection! Flying metal shavings and unprotected eyes don’t mix well.
Bolt the clamp to the shifter using the remaining hole. The flat part of the clamp goes on the shifter. Don’t worry about flattening the rounded part of the clamp, where the locking cam goes, the hard steel bolt will easily dig its own groove into it as you tighten it.
Install the assembly on your handlebars, close to the stem. You may need to install the clamp first and then bolt the shifter onto it. Try to be careful not to scratch your handlebars. You can gently pry the clamp apart a bit with a screwdriver to slide it onto the thick section of handlebar without marring the finish.
That’s it. Go out there and ride!