Simon Brooke said:in message <[email hidden]>, Phil,
Squid-in-Training (') said:Paul - censored said:Zilla vaguely muttered something like ...
> Andrew Lee <whatsupandrewathotmaildotcom> wrote:
>> "James Annan" <[email hidden]> wrote:
>>
>>> Well, cleanER, anyway. This was forwarded to me from another
>>> front wheel victim recently. I have cut out some personal
>>> details but the reply is quoted in full:
>>>
>>> --------------
>>>
>>> (is an 8" rotor compatible with Z1 FR fork)
>>>
>>>> It will definitely fit onto the fork. It is recommended that an
>>>> 8" rotor is not used on a standard axle fork because the forces
>>>> exerted on the wheel can potentially pull the axle out of the
>>>> dropouts. So, it will physically fit, but it is not recommended
>>>> for aggressive riding.
>>
>> Is 8" big or small for a disc? Maybe I need to think through it
>> some more, but visualizing the problem, my first impression is
>> that smaller discs should create larger ejection forces than
>> larger discs. Andrew Lee
>
> Physics 101: Torque = force x distance, the larger disc has the
> larger distance from the axle.But has the same force (brakes and rider/bicycle weight) acting upon
it at that distance, so develops less force at the axle than a
smaller rotor.But you're stopping faster.
No. The limit on stopping is still the point at which you unweight the
back wheel. You can't stop a bike faster than that. The argument for
eight inch rotors, in so far as it is not just fashion, is that they
have more cooling surface area and can thus dissipate heat faster,
leading to less brake fade on very long fast downhill courses.
Ugh. If you're going downhill on a mountain bike, there are lots of bumps.
These bumps increase the normal force of the ground on the tire/fork. When
you're tired, and your lever effort is, say, X newtons, then you can hold a
higher braking force with an 8-inch rotor than if you're using 6-inch rotor.
When you have bumps, the normal force increases at what I think is a huge
magnitude, one so high that you won't lift the rear wheel (because there's
no time to get it up), and the tire won't skid, either (because there's such
a big normal force to the terrain/bump). Therefore, the braking force and
the corresponding downward dropout-busting force is higher.
With the 6-inch rotor, the bump is likely to cause the rotor to slip more
between the pads than if one was using 8-inch rotors.
Why do so many keep thinking that these issues happen on dry, flat pavement?
(including Cannondale)
--
Phil, Squid-in-Training