Cycling Forums - View Single Post - Potential good news for Mt. Washington access.

01-09-2005, 09:18 PM

Frank Krygowski wrote:

> jobst.brandt@stanfordalumni.org wrote:
>
>> Benjamin Lewis writes:
>>
>>
> [FK wrote:]
>
>>>>> When you do this, please compare with a static pressure test of the
>>>>> same tire and rim at room temperature.
>>
>>
>>
>>>> What do you mean by this. Inflation pressure is inflation pressure
>>>> whether the wheel is rotating or not.
>>
>>
>>
>>> Are you saying that tire blow off depends only on pressure and not
>>> on temperature? The test Frank proposes could confirm this.
>>
>>
>>
>> As I mentioned, rims reach their peak temperatures quickly on steep
>> descents and we don't have problems because these are transients. Rim
>> temperatures drop rapidly as soon as the brake is released if the road
>> is fairly straight and allows cooling speeds. Treacherous roads are
>> steep ones that require nearly continuous braking.
>>
>> The problem with a static test is that I believe wheel rotation may
>> have an influence on how predictably the tire releases, there being
>> constant motion between rim and tire. This is evident from the wear
>> marks in the rim and the presence of a chafing strip on the tire bead
>> at that interface.
>
>
> This is a continuation of a somewhat spirited discussion that took place
> about a month ago on r.b.tech. IIRC, at that time, several people
> pointed out that room temperature, stationary tires can withstand very
> high (over 160 psi) pressures without blowoff, and that therefore
> blowoffs must be caused by something other than _just_ the pressure
> increase from the heat of braking. I was certainly one who felt that way.
>
> Others (I'll let Jobst say who) seemed to be claiming that the only
> relevant factor was the pressure increase.
>
> Thus, after Jobst determines pressure at blowout during hard braking,
> I'd like to see the same rim subjected to a room temperature static test
> at the same pressure. We'll then know if _only_ pressure causes those
> blowouts. It's an opportunity to learn.

There still needs to be a control for temperature in the static test,
since of course compressing air into the tire raises the temperature of
the air and casing. However, if the tire is inflated slowly, actual
temperature rise will be insignificant.

In addition to the motion between the tire and rim that Jobst Brandt
mentions as possibly affecting blow-off pressure, there is the effect of
heating the tire, since the rubber will change properties with temperature.

--
Tom Sherman - Near Rock Island

01-09-2005, 09:18 PM	#95 (permalink)
Tom Sherman Guest Posts: n/a	Re: Rim brake heat and clincher blowoff, was Re: Potential good newsfor Mt. Washington access. Frank Krygowski wrote: > jobst.brandt@stanfordalumni.org wrote: > >> Benjamin Lewis writes: >> >> > [FK wrote:] > >>>>> When you do this, please compare with a static pressure test of the >>>>> same tire and rim at room temperature. >> >> >> >>>> What do you mean by this. Inflation pressure is inflation pressure >>>> whether the wheel is rotating or not. >> >> >> >>> Are you saying that tire blow off depends only on pressure and not >>> on temperature? The test Frank proposes could confirm this. >> >> >> >> As I mentioned, rims reach their peak temperatures quickly on steep >> descents and we don't have problems because these are transients. Rim >> temperatures drop rapidly as soon as the brake is released if the road >> is fairly straight and allows cooling speeds. Treacherous roads are >> steep ones that require nearly continuous braking. >> >> The problem with a static test is that I believe wheel rotation may >> have an influence on how predictably the tire releases, there being >> constant motion between rim and tire. This is evident from the wear >> marks in the rim and the presence of a chafing strip on the tire bead >> at that interface. > > > This is a continuation of a somewhat spirited discussion that took place > about a month ago on r.b.tech. IIRC, at that time, several people > pointed out that room temperature, stationary tires can withstand very > high (over 160 psi) pressures without blowoff, and that therefore > blowoffs must be caused by something other than _just_ the pressure > increase from the heat of braking. I was certainly one who felt that way. > > Others (I'll let Jobst say who) seemed to be claiming that the only > relevant factor was the pressure increase. > > Thus, after Jobst determines pressure at blowout during hard braking, > I'd like to see the same rim subjected to a room temperature static test > at the same pressure. We'll then know if _only_ pressure causes those > blowouts. It's an opportunity to learn. There still needs to be a control for temperature in the static test, since of course compressing air into the tire raises the temperature of the air and casing. However, if the tire is inflated slowly, actual temperature rise will be insignificant. In addition to the motion between the tire and rim that Jobst Brandt mentions as possibly affecting blow-off pressure, there is the effect of heating the tire, since the rubber will change properties with temperature. -- Tom Sherman - Near Rock Island