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what do i risk by using a larger rotor on a fork that is not designed for it? im running a 8 inch rotor on manitou axel. premature bushing wear? fork breaking? post mounts shearing off? i cannot go to a smaller rotor. got a old hope c2 up front and the caliper has IS mounts set for a 7 in rotor(same as rear 6 in caliper). so to mount on a post mount fork i needed a +20 post fork to IS caliper adapter.
 

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Actually...

what you risk is your face! Forks are rated for a given max rotor size due to flex when the brakes are applied, a direct result of the stiffness of the fork. Plus the forces of braking tend to pull the wheel back and down. Unless the dropouts are specifically designed to counter this, the combination of braking force and flex can pull the axel of a QR hub, or even a bolt on hub, out of the drop outs even with a properly closed QR or tightened bolts. Of course this will happen at the worst possible moment, usually at high speed during hard baking in prep for a corner or something similar. The bottom line is you risk not only damage to the fork, but damage to you!

Fork manufacturers have countered this by redesigning the dropouts. If you look at an older fork and a new fork side by side the dropouts on the new fork will be canted forward rather than the old vertical style of dropout. This supports the axel from below and puts more beef directly behind as well. This counters the downward and rearward forces applied during braking. For a modern fork rotor size limitations are based solely on the stiffness and strength of the crown, stanchions, and lowers.

So I would highly recommend that you pay attnetion to the rotor size restrictions of your fork. I would further suggest that if you really want to use the Hope c2 that you get a new fork rated for the required rotor size. Or if you really want to use that Manipoop Axel, that you get different brakes that will allow you to use the correct size rotor.

Always better to be save than sorry.

Good Dirt
 

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Fellas, this one gets mis-represented often.

Larger rotors result in a REDUCED "ejection" problem compared to smaller rotors.

Quick Background for anybody unfamiliar: A fella named James Annan proposed back in 2003 that the advent of discs and existing QR dropouts were causing wheels to eject, due to the resulting forces generated.

There has been lots of debate back and forth. James' free body diagrams were pretty well vetted and his following amateurish diagram represents his ejection theory.



IF you're going to accept that his theory is correct and that this problem exists (seems the fork manufacturers are onboard now after years of foot dragging and denial) then you have to accept that LARGER ROTORS result in LESS EJECTION FORCE.

You can read James' web page here: https://www.ne.jp/asahi/julesandjames/home/disk_and_quick_release/index.html

Two factors: Ejection force is related to the ratio of wheel diameter to rotor diameter; smaller ratios increase this ratio. Larger rotors typically see their calipers mounted higher in the rotation of the rotor, resulting in a reduced vertical component of the ejection force.

Boiling it down:
- abide by fork manufacturers' recommended max rotor size
- don't stress if your fork has QR dropouts and you want to increase rotor size
- when shopping, look for QR forks with forward-oriented dropouts OR bolt-ons
- use a good quality QR: Shimano, DT's RWS clamp forcefully
 

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Squash said:
what you risk is your face! Forks are rated for a given max rotor size due to flex when the brakes are applied, a direct result of the stiffness of the fork. Plus the forces of braking tend to pull the wheel back and down. Unless the dropouts are specifically designed to counter this, the combination of braking force and flex can pull the axel of a QR hub, or even a bolt on hub, out of the drop outs even with a properly closed QR or tightened bolts. Of course this will happen at the worst possible moment, usually at high speed during hard baking in prep for a corner or something similar. The bottom line is you risk not only damage to the fork, but damage to you!

Good Dirt
I have to respectfully disagree as far as fork flex is concerned.

I know the larger rotor places the caliper in a position that the rotor has more leverage on it (the caliper) than a smaller rotor. But at the same time the larger disks don't require as much squeeze from the calipers for the same braking force. It should equal out in the end, more leverage against the fork but less clamping force on the rotor required.

I had the ultra cheap RST fork on my Hardrock and I did lots of hard braking tests when going from the stock Tectro 160mm up front to the 203 BB7. At the edge of doing an endo, the fork seemed to flex just as much with the smaller brake. Of course this is just going by eyeball and not exactly scientific. However, I would say I stop harder on average with the large brakes just because it requires so little effort. I feel so much safer with the Revelation that's on there now, it was worth it just for peace of mind.
 

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Speedub.Nate said:
Fellas, this one gets mis-represented often.

Larger rotors result in a REDUCED "ejection" problem compared to smaller rotors.

Quick Background for anybody unfamiliar: A fella named James Annan proposed back in 2003 that the advent of discs and existing QR dropouts were causing wheels to eject, due to the resulting forces generated.

There has been lots of debate back and forth. James' free body diagrams were pretty well vetted and his following amateurish diagram represents his ejection theory.



IF you're going to accept that his theory is correct and that this problem exists (seems the fork manufacturers are onboard now after years of foot dragging and denial) then you have to accept that LARGER ROTORS result in LESS EJECTION FORCE.

You can read James' web page here: https://www.ne.jp/asahi/julesandjames/home/disk_and_quick_release/index.html

Two factors: Ejection force is related to the ratio of wheel diameter to rotor diameter; smaller ratios increase this ratio. Larger rotors typically see their calipers mounted higher in the rotation of the rotor, resulting in a reduced vertical component of the ejection force.

Boiling it down:
- abide by fork manufacturers' recommended max rotor size
- don't stress if your fork has QR dropouts and you want to increase rotor size
- when shopping, look for QR forks with forward-oriented dropouts OR bolt-ons
- use a good quality QR: Shimano, DT's RWS clamp forcefully
I agree, for a given stopping force, a larger rotor transmits less force to the dropouts.

The only issue I can see is that one could produce a greater amount of overall stopping force with a larger rotor, and OTHER parts of the fork (such as the crown/steerer junction) may not be designed to handle that.
 

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How about a 220mm rotor for a Manitou Black:



I ran that thing ragged. Just don't expect the fork to last very long. Cracked steerer, worn stanchions and leaking seals in 18months. The fact that it was a Manitou fork might have had something to do with it.
 

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kapusta said:
I agree, for a given stopping force, a larger rotor transmits less force to the dropouts.

The only issue I can see is that one could produce a greater amount of overall stopping force with a larger rotor, and OTHER parts of the fork (such as the crown/steerer junction) may not be designed to handle that.
Yes. I agree, sort of. In fact, there are a number of older Fox forks that had cracked brake tabs/dropouts from being used with too large of a rotor. This is also fairly common on the Rockshox Dart forks.

Larger rotors do not always have the calipers mounted higher. I can think of a few instances where it is relatively the same or lower.

The larger rotor can generate more force. Fully locked means the rotor is not moving. It does not matter how much pressure the pads are applying. It is still more leverage.
 
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