A common question seems to be, can I fit”X” rotor to “Y” fork.

Often the response is that the manufacturer says no, you’ll die in a hideous fire ball.

In the same thread, someone else will point out that they can lock up the front end with the world’s smallest rotor.

Logic would suggest that if that’s true, rotor size is irrelevant so you can fill you boots. Once the rotor/caliper are static relevant to each other, it doesn’t seem to matter if the bike is moving or not.

If we accept that larger rotors stop you faster, then we must accept that they see greater peak loads.

Are these greater peak load going to rip the brake mounts off, or snap the fork crown?

Has anyone any first hand experience of either of these happening?

I’m curious as some gravel bikes offer 180mm up front but others, who claim ISO 4210 mtb frame testing, still limit them to 160mm. The claim is they only “need” 160. However, these bikes will take a 50mm/2″ tyre. It wouldn’t even enter my mind to put 160 on the front of a mountain bike.

I’m interested in the science and engineering of this, so if we can avoid the “I weight 200kg, tow a caravan and can stop my bike from 100mph down an alp with just a 140mm rotor” comments, it would be appreciated. 😉

Do we still need to bump for visibility?

Based on no knowledge whatsoever it would be the brake mounting point that I would fear for

I’ve only snapped 1 pair of forks (on a muddy fox courier), I’ve bent several pairs though, one with old school caliper brakes on an 80s cheap road bike, one with V brakes on a midrange 90s mtb, and another stronger pair on the same mtb, after hitting a car.
All bent at the crown.
I wouldn’t be worried about the brake mounts, IM(limited)E, they tend to go at the crown.
I’ve worked in several shops, and had quite a few bikes, but hardly exhaustive research eh.

A properly set up 160 mm rotor is capable of throwing you over the bars if your front tyre has the grip to do that. That’s what constrains the peak load. On a long descent, the average load might be constrained by the heat dissipation, so a larger rotor will sustain a higher average load for longer. However, that’s nothing to do with the peak load.

A larger rotor is capable of generating more torque from the same pads and caliper, but this is irrelevant if your braking is limited by tyre grip or going over the bars. The loads on the brake mounts will be the same if you are generating the same peak braking, regardless of the rotor size.

My guess is that lawyers get nervous about customers doing dumb shit so they put legal warnings. They don’t want someone tearing down the side of a volcano on a lightweight XC fork. If you’re not suicidally stupid, you aren’t going to tear your brake mounts off. Stripping the threads when fitting brakes is a much bigger problem.

The fork itself wouldn’t see greater stress*, just a higher leverage force on the caliper mounts.

*ultimately it could, but in reality, a bigger rotor just needs less lever force for a given level of braking, and better heat dissipation.

You can lock up with canti brakes off road. So that argument is null. Presumably whatever test they do is based on stopping on the road, in the dry, with you’re weight as far back as possible. Under those circumstances it’s actually very hard to lock up the front brake.

Under those circumstances it’s actually very hard to lock up the front brake.

Because you tend to fly over the bars first. IME, heavy braking on dry surfaces happens in two situations. Mostly when a car or pedestrian or something appears in front of you and you need to grab a handful of brake. I’ve had that happen twice with disk brakes, both times I stopped, but had a low-speed endo. Rotor size would not have made any difference to the peak loads on the fork or brake mounts.

The other situation is on very steep descents with good grippy surfaces. Again, despite moving weight back, the limiting factor is going over the bars. You’re typically skipping the back tyre off the ground, so all your weight and braking force is through the fork and front wheel. It won’t matter what size rotor you run.

Having said that, I’m pretty chickenshit on road descents. I don’t fancy rounding a bend at high speed and finding a car or truck blocking the road, so I’m not into the braking late and hard thing. If I was racing, it would be different, but I still don’t think fitting a larger rotor would make any difference to the peak braking forces.

If we accept that larger rotors stop you faster, then we must accept that they see greater peak loads.

No, they see lower loads, because they’ve got more leverage. If you accept that even small rotors are enough to lift the back wheel on a dry road, then a large rotor will do the same, but with less force. If you don’t accept that, then the larger rotor forces are limited by lifting the back wheel, whereas the smaller ones aren’t so the larger rotor sees even lower forces 🙂

Insofar as it’s grounded in physics, I think it’s what’s been said above: although the forces are lower, you’ll be using longer adapters which increase leverage on the mounts, and a concern about people using forks intended for XC for DH.

It’s worth noting that when there was a lot of fuss about disc brakes ejecting QR wheels, at least one manufacturer said that it was down to the use of oversized rotors, which is nonsense from a physics point of view, so don’t be surprised if the warnings are not entirely grounded in physics.

It will I imagine be what was tested. If a fork was tested with a 180mm rotor, all good.
Fork design gets signed off at that. Above those specs , you’re on your own if it fails. Extra stress or not.

Not my field but I should think moving the caliper will change the leverage point and the stresses on the fork will change.

The force on the caliper is the same regardless of the rotor size. It has to be, otherwise angular momentum isn’t conservered.

A larger rotor will act as a longer lever on the caliper, but for any given wheel speed its passing through the caliper at a slower speed than a smaller rotor so the torque or reaction force is the same regardless of caliper size

My guess:
Bigger adapter brings the calliper away from the mounts so changing how the force is applied to the mount. This could change the force from being directed in to the mount as designed to being applied at an angle that potentially becomes a bending or sheer that the mount was not designed to take.
And arse covering.

The force on the caliper is the same regardless of the rotor size. It has to be, otherwise angular momentum isn’t conservered.

A larger rotor will act as a longer lever on the caliper, but for any given wheel speed its passing through the caliper at a slower speed than a smaller rotor so the torque or reaction force is the same regardless of caliper size

This is a fairly bad misunderstanding of what happens.

The larger rotor will be travelling faster, not slower. For the same clamping pressure at the caliper, it will generate greater braking force, proportional to the ratio of the rotor size. In other words, if you pull on the lever with the same force, a 200 mm rotor will generate 25% more braking force than a 160 mm rotor because of the increased leverage.

However, when you fit a larger rotor, you modulate your braking by not pulling on the lever so hard. If you are riding the same section of trail (and not hopelessly inept), you will still brake at the same point and have the same peak braking force, but pull the lever harder with the smaller rotor.

The extra leverage on the longer brake mount will be exactly cancelled by the reduced clamping force at the caliper that results from not squeezing the lever as hard. If you’re braking hard enough to lift the back wheel, the stress on the mounts should be the same.

For all the points about tyre grip and braking reality, remember that bikes have to pass lab tests that simply apply a force via a lever X number of times and the forces are higher to reduce cycles to failure (speed up testing). They apply forces differently to real world riding and to certify an item or spec as ok for sale you need to pass the tests.

I’m curious as some gravel bikes offer 180mm up front but others, who claim ISO 4210 mtb frame testing, still limit them to 160mm.

Probably about carbon forks construction – since oversized rotors need adapters it’s probably a fork mount failure risk as the test leverage on the mount increases. Carbon forks often have an Al insert for the brake mount, a likely stress point where it joins the carbon blade.

A frame can pass MTB EN but that doesn’t mean the fork also passes MTB EN, the frame test is generally done with a dummy fork. I’ve seen MTB forks fail first when tested as part of the frame test in place of the dummy fork.

Someone will have done some maths and some testing using 160mm rotors. If they haven’t done the calcs / testing at 180mm, they can’t be confident of their product. So the simplest thing is just to say it’s not tested.

Hopefully the image below shows up. See this brake with an adaptor. With a smaller rotor, the braking force would be distributed evenly between the two bolts. With the adaptor and bigger rotor, it would seem to me (an experience armchair mechanic) that the force could be directed predominantly at the top hole. Will that matter? I have my doubts. But if a brake mount is over-engineered with a safety factor of x10, that might reduce to x7 if you move the direction of the force.

Probably only likely to be an issue for this guy:

I weight 200kg, tow a caravan and can stop my bike from 100mph down an alp with just a 140mm rotor

Intuitively that seems to be the case but unless there is some weird geometry going on then the bending force is the same regardless of how far out the mount is.

Compare 160mm to 200mm.

You have made the rotor 25% bigger, rotating at the same speed it will apply 25% more leverage on the caliper, but it doesn’t rotate at the same speed, it rotates 25% slower. The larger radius means the braking surface passes through the caliper slower for any given wheel speed. So the forces end up the same.

A frame can pass MTB EN but that doesn’t mean the fork also passes MTB EN, the frame test is generally done with a dummy fork. I’ve seen MTB forks fail first when tested as part of the frame test in place of the dummy fork.

I had noticed that. Which kind of leads to another question(s). Why go to the trouble of designing (and/or testing) a frame that is mismatched to the fork? If you choose to do that, why boast of the EN MTB testing in your marketing blurb?

Because you tend to fly over the bars first. IME,

If you accept that even small rotors are enough to lift the back wheel on a dry road

Except that’s not the peak case. That’s if you trundle down the road sat in the saddle or with your weight balanced over the BB.

Go out and find a flat road, or better yet a steep hill with a flat run out and no traffic. Then get your arse right back untill you’re worried for your undercarriage. Further than you ever would on the trail as its arms locked and actually awkward to pull yourself back up on to the bike. Then pull the brake to the bar, it won’t lock, not on any brake I’ve ever tried it with. The best I could manage is a bit of scrubbing as it gets close.

That’s the peak braking effort, not descending off road.

So you could perhapse argue that a larger brake rotor is fine, as long as you never have to brake to avoid something on a fast road descent where you’ve got enough time to really get your weight back.

As TJ points out regularly, you can’t lock the front wheel on the tandem, and you can’t raise the back either, if you get your weight far enough back then the limiting power is the power of the brake, not grip or doing an endo.

@richmtb – in one wheel revolution a 200mm rotor has more braking effect than a 160mm as 628mm of rotor circumference passes under the pad per rev vs the 503mm circ. of the 160mm. For a given wheel speed a bigger rotor has a surface which moves faster vs the pad (or more surface moves under the pad). Friction is higher per wheel rev and brakes work via friction and heat generation. Angular/rotation is the same, friction-wise it’s not the same.

You have made the rotor 25% bigger, rotating at the same speed it will apply 25% more leverage on the caliper, but it doesn’t rotate at the same speed, it rotates 25% slowerfaster.

The bigger rotor has a faster speed at the braking track, not slower.

I stand corrected – looks like I had it completely backwards.

Still its an interesting question

Why go to the trouble of designing (and/or testing) a frame that is mismatched to the fork?

I’d hope it’s designing AND testing ; ) Because most brands design a frame and buy in the fork.

If you choose to do that, why boast of the EN MTB testing in your marketing blurb?

I’m not sure tbh. You certainly can’t ride a gravel bike in the way you would an MTB. The Croix de Fer passed MTB CEN for the frame as the standards at the time didn’t clearly define a gravel bike then, there was a road and an MTB standard and part of compliance is about your marketing and intended use matching your testing in a way you can justify – ie better go a step too far than not far enough. So off-road bike = off-road testing? In hindsight it was ott but by the time the frame was stiff enough for basic loaded use, and with the short fork spec, it wasn’t that big a deal to pass MTB CEN with a common butted tube spec. Fork length is a huge factor in the tests.

I know of one gravel frame that passes MTB tests and I’m not suprised as the tubeset is ‘substantial’ for the intended use. The Croix de Fer wasn’t a whippy ‘real steel feel’ bike by any means but large OS shaped AL tubes feel (to me) like a new level of rigidity.

I don’t think it’s the brake caliper or the disc that is the risk.
If you apply the brakes as hard as possible and, all other things being equal, all of the braking force is applied through the disc to the wheel and then to the ground without locking up or you going over the bars, the front wheel will try to ‘twist’ out of the fork dropouts (back in the good old days of QR anyway). The bigger the disc the higher the leverage ratio about the axis of the wheel.

I thought this was the limiting factor of brake disc size (didn’t Cotic put the disc caliper mount on the front RH side of a rigid fork to counteract this?). maybe not so critical on bolt through (mtb and more recently road/gravel) but probably historic imperative still remains.

actually, self edit: the axis of rotation would move to the caliper bite point of the disc (i think).

Ok, but what about if you’re braking while on a treadmill?

I don’t think it’s the brake caliper or the disc that is the risk.
If you apply the brakes as hard as possible and, all other things being equal, all of the braking force is applied through the disc to the wheel and then to the ground without locking up or you going over the bars, the front wheel will try to ‘twist’ out of the fork dropouts (back in the good old days of QR anyway). The bigger the disc the higher the leverage ratio about the axis of the wheel.

I thought this was the limiting factor of brake disc size (didn’t Cotic put the disc caliper mount on the front RH side of a rigid fork to counteract this?). maybe not so critical on bolt through (mtb and more recently road/gravel) but probably historic imperative still remains.

I think (JamesO might have better details) that the issue was that repeated braking twists the fork, which stretches and relaxed the QR allowing it to “walk” (my word, there’s probably a better one) arround the dropout. The angle of the braking force on the axle tended to lift it out so it would tend to walk out. Hence we gained things like cowled/lipped dropouts where the lip extended right round the dropout to the slot.

It’s not a historical thing though, most bolt through forks are rated for 160/180
/200/220 etc.

Intuitively that seems to be the case but unless there is some weird geometry going on then the bending force is the same regardless of how far out the mount is.

Overall force, sure. But with a normal flat mount, that force is directly between the two mounting points. With the adaptor, the force is disproportionately acting at the top mount. With a long enough lever (adaptor), the lower mount could even be in tension and I’m sure it’s not designed for that.

the front wheel will try to ‘twist’ out of the fork dropouts (back in the good old days of QR anyway). The bigger the disc the higher the leverage ratio about the axis of the wheel.

I thought this was the limiting factor of brake disc size

As I understand it, a larger rotor actually reduces the problematic forces. Smaller rotors were (theoretically) more risky. That’s been eliminated now that through axles are standard (coming from a guy who owns a bunch of bikes with QR forks with 200 mm rotors).

But with a normal flat mount, that force is directly between the two mounting points. With the adaptor, the force is disproportionately acting at the top mount.

If you look at that picture, the brakes will be putting the force through the top mount at about 45 degrees, so equal parts compression and sheer forces. The bottom mount will be taking some of the sheer force. I can’t see anything there that would worry me. If they’d fitted a 220 mm rotor with a mount like that, I’d be worried, but I don’t see how that’s anything to worry about.

I would have thought typically clearance is the issue, if the fork has a bulge or angle that would or could hit the rotor under cornering deflection then it would be best avoided.

I suspect it’s the peak load and associated leverage that is the issue. Brake as hard as you can with a 140mm front rotor and it’s going to take longer to set the bike rotating forwards into a crash (before which you release the brake and ride it out). Do the same with a 200mm rotor and it happens more quickly because of the greater peak force (bear in mind that on dry tarmac it’s almost impossible to skid the front tyre in a straight line). And the caliper is further from the fork mounts so more leverage as well.

Do that repeatedly and it’ll suffer fatigue failure sooner with an oversized brake.

Do magnesium alloys fatigue more like aluminium or steel?

Magnesium is incredibly durable, it’s why you see so many of these bikes arround.

(it’s not, get it wrong and it cracks as soon as you look at it)

Speaking of, whatever happened to Empire? They were the next big thing at one point, I remember them claiming they could go half the weight again with magnesium but didn’t want to put people off with it being too light?

Just chipping in for a reminder that chain/seat stays have the same issue.

Remember it’s not just rotor mounts that may see different stresses, the entire fork could. Bushings, steerer interface… And it doesn’t have to be a critical failure, something snapping, it could show up as accelerated fork wear, or a creak that you might not have got otherwise…

I mean, it’s definitely true that the biggest braking force is when you go over the bars. And the biggest fork load is when you ride into a tree. But it’s not just biggest loads that count, much smaller loads done often can destroy things too. So, imagine you’re braking equally hard but 10% more often, or just as often but 10% harder… Still under the maximum load but increasing the lifetime stresses.

I mean personally, I probably wouldn’t worry about it but these things tend not to be so simple especially when you’re talking about thousands of forks.

I assumed it was the fork crown that was the issue. The greater your braking force (with whatever brakes) the more force on the fork crown. And as you go bumping down steep hills with your brakes on hard the fork flexes back and fore a lot.

I’m not sure I’ve seen pictures of a brake mount failure, but I’ve seen a few fork crown failures on YouTube.

which stretches and relaxed the QR allowing it to “walk” (my word, there’s probably a better one) arround the dropout.

Mechanical precession and the left fork blade moving slightly anti-clockwise relative to the right under braking.

I assumed it was the fork crown that was the issue. The greater your braking force (with whatever brakes) the more force on the fork crown. And as you go bumping down steep hills with your brakes on hard the fork flexes back and fore a lot.

I’m not sure I’ve seen pictures of a brake mount failure, but I’ve seen a few fork crown failures on YouTube.

They can be, there are disc forks out there with lugged crowns that were designed for road caliper brake blades and some of those crowns have failed under testing.

I suspect it’s the peak load and associated leverage that is the issue. Brake as hard as you can with a 140mm front rotor and it’s going to take longer to set the bike rotating forwards into a crash (before which you release the brake and ride it out). Do the same with a 200mm rotor and it happens more quickly because of the greater peak force (bear in mind that on dry tarmac it’s almost impossible to skid the front tyre in a straight line). And the caliper is further from the fork mounts so more leverage as well.

Do that repeatedly and it’ll suffer fatigue failure sooner with an oversized brake.

I’ve never heard of brake mounts failing like this through fatigue. I’ve seen hamfisted mechanics wreck them, but not fatigue failures.

This whole issue is nothing to do with engineering. It’s do to with lawyers. An XC fork will be legally guaranteed to be safe with small rotors. A DH fork needs legal compliance for big rotors. The stress and fatigue on the brake mounts is not the problem.

Will nobody think about the poor hub ?
It has a hell of a time with all these forces pulling/pushing/twisting it this way and that.

If you look at that picture, the brakes will be putting the force through the top mount at about 45 degrees, so equal parts compression and sheer forces. The bottom mount will be taking some of the sheer force. I can’t see anything there that would worry me. If they’d fitted a 220 mm rotor with a mount like that, I’d be worried, but I don’t see how that’s anything to worry about.

Oh sure, I wasn’t saying I’d worry about a small offset like in the picture. Just that you can imagine how a larger mount offset (E.g. 220mm as you say) could cause an issue. That was the hypothetical situation I was talking about 🙂

thols2
Free Member

I’ve never heard of brake mounts failing like this through fatigue. I’ve seen hamfisted mechanics wreck them

Sure but like I say, it’s not just about the brake mount.

I think it’s partly legal/warranty based – just as frames have a fork-length limit, so forks have a disc size limit.

But the fork exerts the highest forces under compression, at which point its length and leverage is the same(ish) for all forks.

So it’s more about the type of rider and riding.