Flat mount fork, 180mm post mount caliper

Is it possible?

You could of course just use a flat-post mount 160mm adaptor, with a 160-180mm post-post adaptor, though I’m not sure if that poses any safety concerns

I’d be wary – forks have to pass brake force tests but only for the rotor size and mount spec intended. FM forks are made for either 140/160 rotors or 160/180 so I’d stick to a 160 unless you’re not riding anywhere needing hard braking, traffic or steep hills etc. Or, bodge it and keep an eye on the upper mount area.

I found an adaptor that does it without bodging, ended up not using it so can’t comment on how solid it was but it seems legit. I still have it in it’s packaging if you want it for a fiver.

Or you can get them here;

https://www.bikeparts.co.uk/products/aztec-adapter-for-post-type-calliper-to-flatmount-fork-180mm-front?currency=GBP&variant=43125560279197&utm_source=google&utm_medium=cpc&utm_campaign=Google%20Shopping&stkn=a894e8283159&gad_source=1&gclid=EAIaIQobChMIsLjl-7OwhgMV34hQBh0XSQSBEAQYByABEgK8hPD_BwE

I’d be wary – forks have to pass brake force tests but only for the rotor size and mount spec intended. FM forks are made for either 140/160 rotors or 160/180 so I’d stick to a 160 unless you’re not riding anywhere needing hard braking, traffic or steep hills etc. Or, bodge it and keep an eye on the upper mount area.

@jameso, I’ve always wondered about how this works. Does the quality of the brake not have a big impact here, too? I mean, how does the stress placed by a big rotor and average brake, compare to that of a smaller rotor and better brake?

I’m running TRP Spyres, and I only have a 180mm up front as the power is a bit lacking. I wonder if it places any more force on the fork than a 160mm rotor and a decent hydraulic caliper?

@bikesandboats, great tip, that’s exactly what I was looking for, thanks

I will pass on your kind offer though, as you probably wouldn’t have made the offer if you knew I was based in Switzerland 🙂

I will pass on your kind offer though, as you probably wouldn’t have made the offer if you knew I was based in Switzerland 🙂

No I probably wouldn’t have! Hope you can find something else that works.

I’d probably not worry it with a steel fork, maybe Jameso has a point if you have a lightweight carbon fork.

@jameso, I’ve always wondered about how this works. Does the quality of the brake not have a big impact here, too? I mean, how does the stress placed by a big rotor and average brake, compare to that of a smaller rotor and better brake?

Sure, in real world use it’s all variable. I’m only talking about the fork tests, that a fork tested for a 160mm rotor might not pass when you test it with a mount adapter for 180mm, depends how the force from the brake is transferred to the blade and how that area is constructed.

Thinking that a FM140-160 fork is more likely to be a light road fork I’m just being cautious – front brakes, carbon blades bonded to Al mount areas .. I wouldn’t unless the fork is rated for the combo. If it’s a steel fork I’d be fine with it personally, I’d just keep an eye on the mount to blade join.

Thanks @jameso, currently I have a 15mm axle steel fork with an IS mount (Soma Wolverine) — I could probably use a 220mm rotor with my mechanical caliper without problems… though a 180mm is fine

I have been considering a carbon fork though, and the ones I have my eye on are all standard flat mount

Just found this thread after realising my Cascade frame & fork Im starting getting parts for to build up is flat mount and I didnt even know the standard existed until today! Plan is to build it up as a flat bar with SRAM DB8 brakes (hopefully the rear will fit in the frame).

I saw the links on the cotic site to adapters if you want to use 180mm rotors but the adapters suggest they are for 160mm adapters and this lead me to discover there is Flat Mount +20

….so this means the Cascade frame & fork are actually built for 160mm rotors and not 140mm? And if I wanted 180 front & 160 rear using PM callipers I need:

Front (180 rotor): Aztec Adapter for post type calliper to flatmount fork; 160mm front – BikeParts.co.uk

Rear (160 rotor): https://www.bikeparts.co.uk/products/aztec-adapter-for-post-type-caliper-to-flatmount-frame-140mm-rear

Is this logic right?

I can’t help with the Cascade sorry nuke, but I did just come back to this and find that uberbike make 180mm adaptors:

https://www.uberbikecomponents.com/view-product/Uberbike-Flatmount-to-Post-Caliper-Disc-Brake-Adapter-180mm-Front

I had these for sale

I tried them on a focus frame but the seat stay was in the way

I’d be wary – forks have to pass brake force tests but only for the rotor size and mount spec intended.

The limiting factor on braking is that you’ll get thrown over the bars. If your brakes are powerful enough to lift the back wheel off the ground, the leverage on the fork and brake mounts is identical regardless of the rotor size, adapters, etc. However, a larger rotor will exert a smaller force on the dropouts and be less likely to eject a wheel if you have a dodgy QR skewer (this is counter-intuitive, but if you sit down and draw out how the leverage work, it’s correct.)

I’m a bit wary of making assumptions like that.

Firstly, is that limiting factor the mount breaking free of the frame or the total torque applied to the fork. If it’s the fork snapping that is the limiting factor then the less force at the mount doesn’t apply.

Secondly the argument about the peak torque in the brake being limited by the back wheel lifting is true for sustained braking. Having done a few emergency stops there are moments when you start to put the bike and rider into circular motion about front contact point. At those moments the braving forces are much larger. Obviously one has to back off the brakes pretty sharpish but it still results in more force

Now both my points above might be irrelevant but I’d be wary of exceeding the max rotor size unless i was sure

if you have a dodgy QR skewer

Flat mounts are a fairly recent thing, no recent fork should have a QR skewer and disc mounts.

….so this means the Cascade frame & fork are actually built for 160mm rotors and not 140mm? And if I wanted 180 front & 160 rear using PM callipers I need:

I don’t know about Aztec, but the Shimano part nos are here…

https://www.cotic.co.uk/product/cascade#compatibility

Post mount brakes can be fitted with 180mm rotors front and rear using Shimano adaptors SM-MA-F160P/D front and SM-MA-R160P/D rear.

Now both my points above might be irrelevant but I’d be wary of exceeding the max rotor size unless i was sure

You can’t really be ‘sure’ though, unless an individual fork has been through extensive testing with a 180mm set-up. I’ve been running a 180mm hydo disc brake on a carbon-forked Dirty Disco for several years and tens of thousands of km and – so far admittedly  – it’s been fine. My basic rationale is that to apply enough force to the mounts/forks to over-stress them, I’s most likely have to apply so much braking force that the puny 40mm front tyre had long since broken traction.

Edit: Using a DT Swiss RWS through-bolt thing rather than a QR fwiw.

Viz, the front wheel will likely break traction before enough force is applied to snap the mounts/fork leg/steerer. I’m not an engineer admittedly and I guess there maybe a scenario where a sudden peak in force applied in the instant before the tyre breaks away could cause that damage, but I’m not overly convinced. I suspect that if your fork can’t cope with the relatively small increase in braking force, then it may be a pretty marginal product to start off with.

The guy who runs Peak Torque and manufactures a very neat 180mm flat mount to flat caliper adapter on the other hand, is an engineer and his take is that – and I quote: ‘The peak braking force will increase by at least 12.5% over a 160mm disc. The increase in braking force is given by the larger disc diameter and reduction in temperatures at the pad.’ His YouTube vid goes into more detail.

180mm Disc Rotor Adapter for flat mount road/gravel forks

Overall, I’m a little more concerned about the overall strength of some of the post to flat adapters than the fork itself. I’m very definitely not an engineer however.

Having done a few emergency stops there are moments when you start to put the bike and rider into circular motion about front contact point. At those moments the braving forces are much larger.

A 160 mm rotor is quite capable of sending you over the bars. The peak stress on the fork and brake mounts will be the same regardless of the rotor size.

What will make a big difference is the weight of the rider and any stuff you’re carrying on the bike. If you weigh 100 kg and have 20 kg of gear in panniers the load on the fork and brake mounts under heavy braking will be massively more than a 50 kg rider with no extra gear.

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A 160 mm rotor is quite capable of sending you over the bars. The peak stress on the fork and brake mounts will be the same regardless of the rotor size.

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That argument is too simple

Imagine the wheel is off the ground and spinning. You pull the brake and the wheel stops. A force is needed to decelerate the wheel. The faster you stop the wheel the greater the force.

When you braking there can be a huge force starting to rotate the bike and the rider. You can’t apply if for long. It seems conceivable that the max torque of the brake of the limiting factor here

Your argument is like saying that if You keep your wheels on the ground the vertical loads never exceed your weight. But in reality you can sustain huge vertical acceleration with wheels on the ground

It’s really basic physics. The maximum possible torque is when the brake locks but the tyre doesn’t skid and the rider is send over the bars. It doesn’t matter how big the rotor is, if the brakes are powerful enough to lock and send you over the bars, the peak stresses on the brake mounts and steerer will be the same.

If a smaller brake isn’t powerful enough to either skid the tyre or send you over the bars, you will generate more torque by fitting a larger rotor, but that doesn’t apply if the smaller brake is powerful enough to send you over the bars. IME, a 6″ rotor is quite capable of sending me over the bars if I grab a big handful of brake.

When I mentioned testing it was fatigue tests I had in mind rather than static torque. The point about larger rotors not necessarily adding stress in a single braking action is correct, but how that all works over a long period has to be considering fork construction etc.

In my case, I’m not too concerned about running a 180mm on a 160mm rated fork as:

(1) I’m not particularly heavy (75-80 kg) and never ride a loaded bike,

(2) the flat-post mount adaptors seem to just move the caliper further out from the fork blade, rather than out and up, so hopefully not creating some unexpeted shear, and

(3) I’m really just wanting a bigger rotor so I can one finger brake with a mechanical caliper and not overheat the rotor on long alpine descents, not to actually brake harder.

….so this means the Cascade frame & fork are actually built for 160mm rotors and not 140mm? And if I wanted 180 front & 160 rear using PM callipers I need:

@nuke

Yes that’s right. The alpaca fork is flat mount + 20 as standard, so you only need a flat to post adaptor to run a 180 mm fork. Thats what I’m doing on my cascade – used a genuine shimano adaptor for mine.

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It’s really basic physics. The maximum possible torque is when the brake locks but the tyre doesn’t skid and the rider is send over the bars. It doesn’t matter how big the rotor is, if the brakes are powerful enough to lock and send you over the bars, the peak stresses on the brake mounts and steerer will be the same.

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I agree that the max torque occurs if the brake locks without the tyre slipping, that’s a very extreme situation. But the force applied in that situation will exceed that of you of controlled braking where you are just at the point of pitching. What you describe is exactly what I’d describe in my bottom example.

You’re riding down a fat trail at 30mph. Being a riding god you pull the brake exactly hard enough to stop with a you rear tyre 1mm from the ground

Now you repeat but pull the brake harder. Because you pulled the brake harder the bike now rotates around the front contact patch and starts to pitch. The force in the brake is greater. It now has to generate enough torque to balance the moment of your weight and give the bike a rotational acceleration. The force in the brake is greater

Then you do it again and brake even harder. You pitch forward more violently with even greater rotational acceleration. The force applied to the fork by the brake is greater each time

Is the force greater by much? I don’t know? But I’m not telling anyone it’s too small to worry about until I’m sure

I’ll add one more step. This time we remove the brake and fit one of those cool 3 spoke mag wheels. As i ride past you push a metal rod through my front wheel. The spokes grab the rod and slams it into my disc mount. In your model this will apply no more force to the brake mount than braking with a 160mm rotor.

It’s pretty simple the fork is designed to pass a test with set parameters, one of those is essentially an 80mm lever applied to the LHS fork leg. You run a bigger rotor you increase that lever by 10mm (12.5% as borrowed from peak torque above) true the input load hasn’t increased, but the mechanical advantage has and therefore so has the load exerted on the fork with every brake operation. Your use case is exceeding  the available substation.

What none of you know, what the manufacturer might not even really know, is how much margin there was against the design load case it means more peak loading under hard braking or lock-ups and more input load under normal braking, thus more fatigue (in theory).

So the gamble is really down to how slim the FoS was, +10% or +200%? Realistically the odds are it was substantial, but you can never truly be sure…

I’d consider the gamble differently depending on the fork and materials TBH.

I’d consider the gamble differently depending on the fork and materials TBH.

Surely this is where the quality of brake counts too? I mean, is my mechanical TRP Spyre running a 180mm rotor going to put more or less stress on the fork than, say, a 4-pot Hope RX+ with a 160mm rotor?

Surely this is where the quality of brake counts too?

Newton’s are Newtons, whatever device you apply them with.

Newton’s are Newtons, whatever device you apply them with.

Sure, and my mechanical Spyre is capable of a lot less newtons at the rotor, relative to a decent hydraulic brake, for the same newtons applied by my fingers.

So the question is whether a weaker brake with a bigger rotor puts more stress on the fork.

You run a bigger rotor you increase that lever by 10mm (12.5% as borrowed from peak torque above) true the input load hasn’t increased, but the mechanical advantage has and therefore so has the load exerted on the fork with every brake operation. Your use case is exceeding  the available substation.

You’re thinking about it backwards. The maximum load on the fork steerer and brake mount is determined by the grip of the tyre. The wheel doesn’t change diameter so the leverage on the fork is the same when either the tyre starts to skid or the back wheel lifts off the ground (which is unchanged by increasing the rotor size). This means that the brake mounts have to cope with the same maximum torque regardless of the rotor size. So, if you run a rotor that is 25% larger in diameter, the frictional force exerted by the brake pads is reduced by the same ratio (i.e. to 80%) giving the same braking torque at the point where the rider goes OTB or the tyre skids. At the brake mount, you have a force of 80% multiplied by leverage of 125% (= 100%), so the same torque acting on the brake mount. This is because the maximum braking force is limited by either the tyre grip or rider going over the bars, it’s not limited by the brake power itself.

Has anyone here ever come across an instance of a fork failing catastrophically – or otherwise – after fitting an oversized rotor?

You’re thinking about it backwards.

If you like.

We’re only talking about changing one component of the system you described, effectively the moment arm between the dropout and the calliper.

Whatever torque is applied via the wheel just has a wee bit more mechanical advantage, if it didn’t we’d all be running 140mm rotors, there would be no benefit in going bigger and fork manufacturers would have no concerns about us changing rotors….

“Has anyone here ever come across an instance of a fork failing catastrophically – or otherwise – after fitting an oversized rotor?”

That’s the question I’m interested in. I’ve never known one fail catastrophically or otherwise by going up a rotor size.

I believe the limits imposed are more about what it’s been tested with or what size physically fits.

Remember lawyer tabs on fork dropouts? If it was that much if an issue, wouldn’t fork legs be shaped to limit rotor size?

Remember lawyer tabs on fork dropouts? If it was that much if an issue, wouldn’t fork legs be shaped to limit rotor size?

Counter-intuitively, a larger rotor actually puts less stress on the dropouts. If you don’t believe me, sit down and draw a diagram of the forces at work. The key point is that a smaller rotor means a bigger ratio of wheel diameter to rotor diameter. A larger rotor decreases the ratio of wheel size to rotor diameter, which means less leverage trying to tear the wheel out of the dropouts.

Well I’ll leave it there

Partly as we’ve had the debate before

What actually breaks if you over spec a rotor?