Front Mech Woe

Ok. let's hypothesise that I've been an idiot and bought a band-on front mech without checking the diameter of my seat-tube and found it is too big. Hypothetically.

If this were to have happened, is it possible to shim it out? It's a 2007 Chorus on an old steel frame (whatever the standard size is, I've checked a few steel frames of the same vintage and it is too big for any of them). Or is this likely to have been designed for a newer standard tube size (oversize alu or something). As far as I know it's just the standard chorus band-on. Or are they all designed to need shims. I've never bought a front mech 'seperate' so never had these issues.

Hope someone can be bothered with this frankly pretty dull problem.

Cheers

How big is too big?

Can't be sure without seeing, but I guess you want a 28.6mm diameter band.
I've also got a sneaking suspicion that Campagnolo only do band-on mechs in the larger sizes, and that their mechs for old + skinny steel frames tend to be braze on fitting.
You can get adapters that allow you to fit a braze-on mech to a tube that doesn't have the requisite tab, or you can get shims - I know Shimano make them, just make sure you don't get spotted by a Campy purist...

Probably around 2 mm gap all the way round, if not a little more.

If you look at Wiggle or Chain Reaction for example, you'll see that band-on front mech come in a variety of sizes; 28.6, 31.8 and 34.9mm are typical sizes. The smallest are standard for old steel frames (Inch and a quarter), the rest for the variety of sizes that is modern frame tubing.

Yes, you can shim larger one down to size. I don't know where you get such shims and it may not look very nice but it should work.

look in some light fittings these often have shims - alternatively for that much shimming i'd probably say a bit of an old tyre. i have seen bits especially for the job - but it was on someone's mech - don't know where they got it.

for variable and very tough shimming you can use the tape they use for emergency repair of car radiator hoses - can't remember name now. it should be hard enough so the mech doesn't move.

or old inner tube? not pretty but could work...

Surely for a front mech you want a solid shim? There's a lot of force pulling on that cable...

Maybe slices of a coke can, if you're determined to make it work.

You can successfully shim this with 2 sections cut from a large zip-tie - it's bloody fiddly to keep them in place while you get everything lined up, but once clamped up tight they won't go anywhere and have the added advantage of not marking the paint.

file off some material from the clamp end flats and it will just close up more

If you could get the braze-on part from another cheaper or knackered Campag front mech you could then fit it to a braze-on adapter.

go toa bike shop, they should have a bunch of mech shims around. pretty much all shimano/sram mechs that i have ever bought come with a solid shim and shops would probably have some kicking about (nice friendly local shops like mine). if its not quite right, then alter that shim, better than fucking around with bits of old tat.

^This, just find a nice shop and you're sorted. Bring them some biscuits or something, especially if they give you a shim for free.

Thanks everyone for the advice.