bigredgrad01

I am presently running a full 6800 Ultegra on H+Son Archetype 23mm wide rims with 700x25c GP4000s tire.

Am I going to have any clearance issues going up to 700x28c GP4000s II tire?

Shimano doc states recommended tire width of 25mm or less for BR-6800
https://si.shimano.com/php/download.p...003-02-ENG.pdf

But I see that BMC specs GF02 with BR-6800 and Continental Ultra Sport 2 SL 700 x 28C on a Shimano WH-RS11 wheelset, but that has a narrower rim.
granfondo GF02 Ultegra compact ? road ? BMC Switzerland

Anyone running 700x28 tires on a wide rim with BR-6800(Ultegra) or similar sized BR-9000(Duraace) calipers?

Dunbar

The brakes probably won't be your problem. It's the frame clearance that could be an issue.

Domane

A 28mm GP4000s II would not fit on my Trek Domane 4.5 with Boyd Altamont rims. The front fit fine but the rear tire rubbed the bottom of the brake caliper and the brake caliper bracket. Side clearance was not a problem just the rub on the brake caliper and brake caliper bracket.

bigredgrad01

I see Domane 4.5 has 105 brakes, and Shimano actually says up to 28mm tire or less for those (BR5800), and only up to 25mm for Ultegra and DuraAce.
This (and plastic screws in Ultegra brifters) leads me to the conclusion that 105, is the new best groupset for everyday riders, and Ultegra is now a compromised groupset for racers.

grolby

Shimano is very conservative in its recommendations. There's unlikely to be any real difference in the geometry of 105 and Ultegra brakes from the same design generation, despite the different size recommendations. Historically, 28s have fit just fine under their short-reach Ultegra brakes, provided that the brake hole/brake bridge is high enough above the tire. If there's a problem, it will be due to the frame or fork, not the caliper. My own bicycle perfectly exemplifies this - a 28 would fit easily under the front Ultegra (6700) brake mounted on the Enve fork, while a 25mm tire in the back clears the caliper by at best couple of millimeters due to the low-mounted brake bridge.

Domane

To clarify I have the Dura Ace 9000 brake calipers on my Domane 4.5. My wife's Domane 4.3 has Ultregra 6800 calipers on hers. With the wide Boyd Altamont rims I can not put a Grand Prix 4000s II 28mm on the rear of our Domanes. The fronts fit fine but the rears will rub the bottom of the brake caliper and the brake caliper bracket on the seat stays.

I was really looking forward to being able to run the 28mm tires but due to the rubbing I had to go back to the 25mm Grand Prix 4000s II tires. The 4000s II measure out to 27mm on the Boyd Altamont rims so in a way I did get a 27mm tire on the bikes.

The Grand Prix 4000s II 28mm tires measure out to a 30mm tire on the Boyd Altamont rims.

On the Boyd Altamont rims the Grand Prix 4000s II are 2mm larger than the stated specs for each tire.

bigredgrad01

Thanks. I now think the underside of the fork will be an issue before the brake. It looks to have about 3mm of clearance with the 25mm tire.