| Re: Rim brake heat and clincher blowoff, was Re: Potential goodnews for Mt. Washington access.
"Alfred Ryder" <nospam@nospam.com> writes:
> While we are all offering gratuitous suggestions to the one doing the work,
> let me put in mine.
>
> What about expanding Frank's suggested lab test as follows. Attach a heating
> element to the rim, either bonded to the braking surfaces or inside the rim
> somewhere. The amount of thermal energy put into the wheel would be easy to
> know. Also monitoring the temperature and pressure of the air in the tube,
> temperature of the rim, etc. would be much easier in the lab than on the
> road. Then plot the blow-off point as a function of rim temperature and air
> pressure, both being controllable variables.
>
> Such a lab experiment would not replace the road test. But I agree with
> Frank that it would add information, probably necessary information. (My
> suspicion is that the rim can get very hot for a long time before the tube
> air temperature changes very much.)
Jobst's test should reveal that; he's logging the rim temperature and
air pressure.
Something we [meaning those of us in the peanut gallery] might want
to do is ascertain whether the road test is sufficient to settle the matter.
I believe that three factors have been proposed as directly
contributing to tire blow-off:
1. tire (air) pressure (i.e. increased pressure due to heating),
2. tire movement (i.e. sidewall flexing while rolling under load).
3. tire (cord/rubber) temperature
Jobst is measuring the first while applying the second. No attempt is
being made to measure or independently vary the third factor; however,
we may be able to indirectly determine whether it could be
significant. For example, if during steady state braking, the tire
pressure quickly rises to its final value but the tire doesn't blow
off until significantly later, that would be consistent with (3) being
a factor [because the thermal mass of the tire will delay its
temperature rise].
One reason for measuring the static blowoff pressure is that it serves
as a baseline against which to compare the dynamic blowoff pressure.
As it is, we [the peanut gallery] should be able to determine whether
they are roughly the same by first measuring the static blowoff pressure,
then pumping a tire to slightly below that and just riding around to see
if doing so induces a blowoff. Any takers? Ear-protection is recommended.
Joe
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