So the difference between 22.2mm and 23.8mm diameter handlebars is just that the 23.8 are thicker, with the inside diameter being the same (as far as I can tell). If I wanted to get a thumbie on a 23.8 bar, exactly how bad an idea would it be to *very carefully* file down a strip of the bar, only as wide as the thumbie needs, by 1.6mm? Theoretically, wouldn’t that part of the bar be just as safe and stable as a 22.2 diameter bar?

*Background: I’m wanting to put mustache handlebars on my bike with bar end brake levers and a Paul Comp Thumbie (w/ road shifter). Running it 1x10, so just one thumbie. As far as I can tell, Soma are the only ones who make a 22.2mm grip diameter mustache bar (their Moustache II, which is out of stock everywhere and not a single one on eBay or anywhere else I’ve looked). I have Soma’s Moustache I, which is 23.8mm but I believe the same inner diameter as the 22.2 (at least, it’s the same inner diameter as their Lauterwasser bar, which is just thinner, using less metal—the bar end brake levers I have fit great into both bars). Until Moustache II’s come back into stock (who knows when), all I’ve got is the 23.8 version, so I’d like to try to make this happen.

Let’s hear it!

Almost no risk, if you don't screw it up.
And by "almost no risk" I mean, almost no risk greater than the risk if you didn't do it. The most likely place to break will be near the stem in either case.

And here's question for you. what is a thumbie? You loved that?

As an alternative, would you be able to enlarge the thumbie ?

I assume these are clamp-on shifters.

I don't think I'd choose to file 1.6mm down from the middle of the bars. Especially if you have to buy all new parts.

I don't see the clamp range on the Microshift shifters, but they may have more flexibility than what you are finding. Steel straps?

https://www.microshift.com/

I think that the safest course would be to use 31.8mm Thumbies and shim them to fit the bar. Reducing the bar diameter will concentrate stresses at the ends of the reduced section, potentially making it weaker than a smaller but constant-diameter bar.

No idea what thumbies you are going to use, but if you can slide it over the full size end of the bar to reach the reduced section, do you need a reduced section?
Maybe a longer screw?

I run 1x11 with a thumbie on my MTB just because I can. It’s actually a few bucks more than an SLX 11 speed shifter.

I never measured anything. I just put it on and it fits. What diameter are the ends of a riser bar?

Its thick enough that it would probably handle file work without issue. I wouldn’t mess with the bar. Eating a stem while a long way from home makes me nervous.

I would not want to reduce the diameter of the bar to fit the shifter; doing so would create a stress riser, which could cause the bar to fail. With an aluminum bar, things could go from an initial crack to total failure very quickly. Failure of one or both sides of the bar can easily cause you to lose control and be injured or worse.

Rather than modify the bar to fit the shifter, I'd do as [MENTION=205349]dsbrantjr[/MENTION] suggests and buy a larger diameter shifter and shim it to fit the bar.

There is absolutely no situation that would entice me to remove .8mm off the diameter of a handlebar.

As already suggested you can open up the clamp diameter on the thumbie clamp to fit 23.8mm.

I doubt if the thumbie clamp wall thickness is thinner than the handlebar wall thickness. If the thumbie clamp wall thickness ends up too thin and fails, at least it’s not like holding part of a handlebar at 30mph.

John

Edit: I guess I was typing when JohnDThompson was posting.

You could go with a Surly Open bar. They work better with thumbies IMO. They don't sweep right back at you though, if you're dead set on that.

And all handlebar IDs are definitely not the same.

Oh, Soma Oxford is a little closer to the Mustache, and has the grip diameter you're looking for. It doesn't sweep forward as much as a mustache.

https://www.universalcycles.com/shop...&category=1678

Nope...

More like dangling a wrecking ball right next to the front wheel at 30 MPH.

"as far as I can tell" - not the best foundation for your plan - I'd buy new bars or modify the shifter mount

OP here, thanks for all the love, everyone. (Fewer snide comments than I’d expected, but that’s ok.) That’s a good point several people made about concentrating force upon the compromised section. The Soma Oxford is also a good idea, and it just so happens that there’s one on CL nearby. I’ll give that a shot and see how it feels on my (very crazy) new build.

Btw, the Paul Thumbie is an adapter. You hook up a bar end shifter to it. You can see it here: https://www.paulcomp.com/shop/compon...imanothumbies/