P
Phil, Squid-in-Training
Guest
Simon Brooke wrote:
> in message <[email protected]>, Phil,
> Squid-in-Training ('[email protected]') wrote:
>
>> Paul - *** wrote:
>>> Zilla vaguely muttered something like ...
>>>> Andrew Lee <whatsupandrewathotmaildotcom> wrote:
>>>>> "James Annan" <[email protected]> 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
> in message <[email protected]>, Phil,
> Squid-in-Training ('[email protected]') wrote:
>
>> Paul - *** wrote:
>>> Zilla vaguely muttered something like ...
>>>> Andrew Lee <whatsupandrewathotmaildotcom> wrote:
>>>>> "James Annan" <[email protected]> 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