Max rotor size

What determines the max front rotor size? I want to use a 220mm rotor but max is listed as 203mm.

I see no issue using a +40mm adaptor, so what is the limiting factor?

Think of a car with a max towing weight. Your car will pull a caravan or trailer that’s heavier than that, but the manufacturer has stated the limit for a reason.

It will put more force through the lowers/brake mount and could break them

It will put more force through the lowers/brake mount and could break them

The force through the forks won’t change based on the rotor size. If you can lift the back tyre off the road with a 6″ rotor, the stress on the forks will be the same if you lift the back tyre off the road with an 8″ rotorl

It will be fine, 220mm would not have been around when they were certified, the force on the caliper mounts will not change much, you just end up with a longer lever (distance from hub center to caliper) and less clamping force to achieve the same retardation.

Thanks all.  Won’t risk it then just to be on the safe side.

Rockshox changed the max to 220mm and I don’t think the lowers changed from the previous max,  it’s just that 220mm weren’t a thing at the time so they just wrote what was the biggest rotor.

Long ago. I had 220 on forks before 29ers even happened.

Look, you can fit whatever you want, but the limits set by manufacturers aren’t arbitrary. I’d listen to them, personally. But you don’t have to.

Wonder where the actual issue is here.  I’m not thinking that the fork lowers are going to be ripped from the uppers… more likely the cast fork lugs need an ever longer brake adapter to space out for bigger disks and all the ones I have held or use feel very light and flimsy.

I wonder if the fork mount position will move further from the axle to enable bigger discs with reasonable sized caliper mounds…

That has already happened. Look at Zebs.

What forms it? My 32mm RS Recon is good to 220mm

I’m certain it’s nothing more than the size they were tested with.

If any bike can handle 200mm rotors but will kill you with 220mm, then there’s really not much of a safety factor in that design.

If they physically fit, you’re fine.

Yes I think fine, they just tested with what the biggest around was at the time.

Formula even make an adapter to go from the Selva/Belva 160 mount to the 220 max rotor size of the fork.

Say the 203 limit is by SRAM, they might have tested with Code RSCs. Well now put some Hayes Dominion A4s on that 203 rotor with metallic or even semi-metallic pads, it’s going to put a lot more force through everything than upping the rotor to 220. So I reckon there’s a huge margin of strength here.

Someone here running a Boxxer with 246 rotor https://www.pinkbike.com/news/dh-bike-tech-super-sized-brake-rotors.html