Is this (disc brake adapter) going to be OK like this?

[Manc33]

I got a post-to-post disc brake adapter for my carbon fork to use a 203mm rotor, but the top left corner of the adapter was touching the fork, so I filed the corner down.

Under hard braking might the adapter and fork flex and be able to touch?

Is there a risk using a big rotor on a carbon fork?

I can always swap back to my 160mm but I'd rather have better braking.

This is on a road fork - are you even meant to use 203mm rotors on these forks at all?

The gap in the photo is only about 0.7mm. I could file even more off but then I am going to be going slightly into the screw threads on the underneath of the adapter, but it would only be taking 1 or 2 (partial) threads off the very end of the threads, it's quite possible the caliper screws will not go that far down it anyway (all the way to the end almost). Maybe it's best to shave a bit more off and get that gap about double the size to 1.5mm?

It's a genuine Shimano adapter, these things are soft metal, my guess is 6061 aluminium

The fork is a China one (Flyxii) that is full carbon, 700c, weighing 380g. Maybe I should just throw a 160mm on it eh. I have ridden hundreds of miles with a 160mm on it and it's been fine up to now. I tend to not pull the front brake hard but what if I ever need to.

[100%JR]

I think you've kind of answered your own question?
I personally wouldn't go to 200mm on a carbon road fork.The fact the adaptor touches the fork is enough to tell me it's wrong.

[Brucey]

…..The fork is a China one (Flyxii) that is full carbon, 700c, weighing 380g.

….Is there a risk using a big rotor on a carbon fork?

risk? Why yes there is. Cheap carbon forks from China are responsible for more bike recalls than any single other bike part in recent years, and that is forks that have passed through a bike manufacturer's QA process as well as the subcontractor's QA process.

The risk is (hopefully) low but since the possible consequences are so dire there are quite a lot of folks with grey beards who wouldn't do it; needless to say they didn't get to have beards that colour by acting in an overly rash fashion in the preceding years....

cheers

[reohn2]

........ the possible consequences are so dire there are quite a lot of folks with grey beards who wouldn't do it....

cheers

And I suspect a few much younger people with flat noses and no front teeth who wouldn't do it again

[amediasatex]

Regardless of the manufacturer and any concerns there, the first question to ask is is there a published maximum rotor size for your fork? Most manufacturers do specify maximum permitted, if they do not then that's another reason to err on the side of caution.

Second question... 203mm rotor on an unladen road solo* seems a tad excessive to me! Are you brakes really that bad with the 160mm?

*I'm only assuming unladen solo as it you're using a sub 400g fork for loaded riding or on a tandem then rotor size is the least of your worries!

[Manc33]

I remember reading a while ago the first carbon road disc Enve forks had problems at the brake caliper mount. Merlin are currently charging £340 for that brand of fork right now and that's the QR version, aptly named "2.0" lol. The 142mm/12mm thru axle one is so expensive, I'm not even going to post the price of it! My China one was about £57.

In Feb 2016...

ENVE has received five reports of the left leg of the fork cracking above the disc brake mount. No injuries have been reported. The forks were sold in shops and online from June 2014 through December 2015 for about $540.

And that's an expensive one that weighs 70g more than mine does.

I'll play it "safe" and just stick with a 160mm rotor. I know it's not snapped on me up to now with that size rotor on and it saves faffing about with adapters.

With carbon there's only 3 areas that worry me and this is one of them, the other two are handlebars and seatposts, I'll probably never use carbon for those. I use bar ends so imagine tightening them onto the ends of carbon bars, no chance. I have seen a China carbon seatpost (FSA knockoff) crumple the instant it was put on the bike, at about 3Nm-4Nm - that was not from a reputable seller though and its tubing was wafer thin.

Maybe what I should be doing is buying what they call a carbon "rigid" fork that's a 29er and run one of those on it. That would be a MTB fork and miles stronger than a road one, I already know they are from a Boardman one I had in the past, although that was also all carbon, it weighed around 800g - not 380g like this China road one does.

Dunno if that would be much safer though, being from China again. This seller had the forethought to put 160mm on their image: https://ae01.alicdn.com/kf/HTB1Ra4VeWWs ... WUXXaw.jpg (fork is £52 and 500g).

*strokes beard that has yet to go fully grey...

Cheers folks. If I stop posting, this is why I died.

[mattsccm]

Where you are differing from the norm is the size of the disc. 160 has become the road norm and is plenty bug enough. Some people even used 140. Methinks that you are going the wrong way about this. Keeping conventional should reduce the concern about the fork although you will never remove the areguments for and against brexit/disc brakes/padded shorts/Chinese carbon.

[Gattonero]

Put it this way: most Mtb forks for XC are not rated for 200/203mm rotors, that says all!

[Manc33]

I used a 203mm rotor on an old Judy fork and to be honest the extra stopping power didn't impress me. Also the brake pads never quite lined up exactly on it, not going deep enough into the rotor, but it was usable. You get more brake rub on 203mm rotors, which is another reason to not bother with them.

Looking at various carbon forks on sale now (road or rigid MTB) they all seem to specify 160mm rotors. I'll just stick to 160mm. Pulling the brake on a 160mm can still send me over the bars anyway.

Here's another thing to consider: cable disc or hydraulic disc. If you used a 203mm rotor on a hydraulic setup, the braking forces could be extremely powerful, but on a cable setup, not so much. I remember having hydraulic discs on years ago and they are like night and day compared to cable discs, but I now run cable discs because I can't be bothered faffing about with fluid or risking a hose leaking while out riding.

The other thing is, I just don't pull my brakes very often anyway - why would you do it!

Mess with your front brake and your front brake will mess with you!

[PT1029]

203 mm rotors on our tandem (BB 7 calipers) are pretty strong brakes (never asked for the stoker's 3rd brake).
Must be over (or actual) kill on a solo bike....

[Chris Jeggo]

Am I right in thinking the torque on the fork is limited to the maximum torque the road exerts on the wheel?
If so, skidding a high-grip tyre on a high-grip surface is the worst case for the fork to withstand.

Edit: I've just realised that it's not quite as simple as that. Caliper brakes exert a torque on the fork, but the points of application of the two forces are then the fork crown and the dropouts. So applying a lower force to a bigger disc changes things a bit.

[andrew_s]

If so, skidding a high-grip tyre on a high-grip surface is the worst case for the fork to withstand.

Can't do it, not on a solo anyway. You'll go over the bars first.

[solos only]
In general, "more powerful brakes" don't stop you any quicker; they just reduce the hand grip needed.
You can't brake any harder than will lift the back wheel, and you can do that with dual pivots, cantilevers, Vs, cable discs or hydraulic discs, provided the brakes are in good condition.

It's similar with disc size and the load on the fork. For a given rate of deceleration, the force exerted on the disc mount is the same regardless of the size of the disc. A bigger disc doesn't allow you to stop faster; it gets you less hand force, and more leeway in the case of a contaminated pad or similar.

[Chris Jeggo]

If so, skidding a high-grip tyre on a high-grip surface is the worst case for the fork to withstand.

Can't do it, not on a solo anyway. You'll go over the bars first.

You can do it, but only for a split second before you go over the bars. You'd let go of the lever PDQ!

[solos only]
In general, "more powerful brakes" don't stop you any quicker; they just reduce the hand grip needed.
You can't brake any harder than will lift the back wheel, and you can do that with dual pivots, cantilevers, Vs, cable discs or hydraulic discs, provided the brakes are in good condition.

Agreed.

[Manc33]

Looking at ones that have snapped, they seem to break just above the top caliper bolt. One guy said his snapped (on a carbon fork, what else) and it was so strong he managed to still ride it back to his car again, on one fork leg.

It depends on the weight of the fork IMO, mine is only 385g which is pretty crazy, even for a road fork. Ones like the old Boardman rigid carbon fork, despite also being all carbon, are more than twice that weight and look like they could take a lot more of a hammering, which I guess includes the forces @ the caliper...