I would like to add some clip on tri bars to my flat handled commuter bike because I’m planning on doing a 200km ride in a couple of weeks.

My current problem is that clip on tri bars don’t fit because my flat bars are “oversized” at the stem and then taper down to a narrower width at the end. They are 31.8mm in the middle and the taper starts pretty quickly so I can’t clip on the tri bars. I considered wrapping the bar in tape but I’m not sure that will be particularly safe if they start coming loose when I’m blasting down a road.

Does anyone know of a solution which would work? Is there a flat bar which is less tapered or should I think about replacing the stem so that’s it’s got a smaller diameter bar mount? Links to something compatible would be hugely appreciated.

All 31.8 bars taper down. Are you sure you can’t fit the tri-bars close enough to the stem and then adjust them to the right place?

Yeah, I’ve tried to do that and it doesn’t work. Hence I was thinking I could go with a 25.4mm stem and flat bar after which the clip on tri bars would work.

25.4 bars also taper down, to 22mm.
I have vague memories of narrow bars with a two piece shim thing that went in the 25.4 clamp. Don’t know if they’re still sold.

I considered wrapping the bar in tape but I’m not sure that will be particularly safe

Understatementcopters!

I’ve got some 22mm titanium flat bars & a 25.4mm shim for sale – email in profile

After a bit more research I think I need a flat bar which doesn’t taper for atleast an inch either side of the stem. It seems to be a thing but I don’t know what to search for to find one! If anyone has any specific links I’d appreciate it!

Don’t attempt to use tape to ‘pad-out’ a handlebar – it’ll end in pain. I working as a mechanic in a shop and a guy came in,bought some clip-ons and fitted them himself outside the shop. Returned a short while later bleeding and broken – he’d not tightened them enough, attempted to put his full weight on the very front of the bars, they slipped down, jammed the wheel and he went over-the-bars.
Most clip-ons are 31.8mm and designed to clamp-on the centre bulge of a handlebar, where the bar is strongest and exert less leverage.