If i want to run 2.6 or 2.8 tyres whats the optimum rim?

I would say 32-34mm internal rim width I have a Stans Sentry & Baron but run 2.6s

35mm probably

30 for up to 2.6 on the rear, 35 for 2.8 front, 35 can be too much if you ever end up dropping back to 2.4 with some tyre profiles.
If you’ll always run big tyres then go 35.
Also depends on the brand, some 2.5 are bigger than other brand 2.6

I’ve just built some wheels. In the end I went for stans flow just in case I wanted to drop back down to 2.35. Unless you go carbon then the rims get heavier the wider you go.

Nobby nic 29er 2.6 have a nice shape on the stans flow.

Stans flow rim. 527g, 29 internal, 32.3 external, designed for 2.35 to 2.8 tyres.

https://www.notubes.com/flow-mk3-rims

Stans sentry rim. 556g, 32 internal, 35.4 external, designed for 2.5 to 3.0 tyres.

https://www.notubes.com/catalog/product/view/id/242016/s/sentry-mk3-rims/category/174/

YOu can go wider and heavier, as above I think it’s barons.

If you ride hard you will need –

35mm for 2.6

40mm for 2.8

The tires will deform on hard cornering, so you need enough width to support them at the lower pressure.

If you don’t want to push the edge of traction at warp speed then anything will probably do.

I remember running panaracer 2.1s on mavic 717s and didn’t have a problem with tyres deforming. But then I didn’t run tyres tubeless at low pressures.

I guess it all depends on how you ride, what pressures and tyres you run.

I’ve just put a 2.1 Smoke and a Dart on my winter retro fun bike. The smaller volume requires a higher pressure so they don’t get pushed sidways as much. Should be fun in the mud.

I’ve used 2.8s (DHF / DHR2) on 35mm, 40mm and 45mm rims. I could feel them deforming more in high speed turns on the 35mm rims, so I switched to 40mm and they felt more supported.

I’m currently running 2.6 MM and HD on 35mm DTSwiss XM521s and the support is great.

I’ve just built some wheels. In the end I went for stans flow

Yes, damascus, I’ll be doing the same. Currently have some OE 35mm internal rims running 2.6 to 2.8 but have also followed Stans guidance, where they suggest their rims can run slightly narrower than other recommendations. Perhaps this is specific to their design, but in any case, it’ll help save weight.

What TheGhost said.

Running 2.8s on both my main bikes now.

40mm is perfect IMO, 35mm OK, 30mm rideable but not a great shape.

Great advice thanks all.