On Fri, 14 May 2004 04:42:58 GMT, [email hidden]
Quoted message said:Carl Fogel said:Are you suggesting then that the rim heats up from braking, the hot
rim heats the air insulated by the thin rubber inner tube and any
rim strip, and the thicker outer tire acts as an insulator,
remaining much cooler?
It is more than a suggestion. I have observed enough blow-offs and
talked to tandem tourists who have had to go to drum and disc rear
brakes to avoid overheating the air in their tires.
Quoted message said:That is, where does the heat come from and where does it go? Is
just as much energy going into the pavement/contact-patch as into
the rim/brake-pad patch under steady braking? Does contact with the
ground cool the tire significantly?
As I suggested in a related message, next time you are descending a
steep road at speed, bring your bicycle to a stop using the front
brake only and grasp the tire and rim.. carefully, to appreciate the
temperature.
Quoted message said:Your example of a blow-off was at low speed, under 10 mph, but your
scenic description didn't specify whether it was the front tire. (I
assume so, since that's what does most of the braking and you mention
riding the rear brake, which might imply no front brake use.).
As I said, it was the rear tire and I merely braked with one hand over
the lever hood keeping the bicycle at just over walking speed as I
waited or my friend. It never occurred to me that this would blow
off the tire, having descended this road many times without incident
with many others.
Quoted message said:Is this the common scenario, with a hot rim rolling slowly under no
braking to blow the rim off?
Who said anything about "no braking"?????
Quoted message said:Why does the pressure rise so dramatically after no further braking
was generating heat? Or would it have blown off anyway in another
few moments if you'd kept descending?
Quoted message said:How hot does the rim get and what would the pressure increase be?
Is the original example of 100 psi rising to 120 psi what you would
expect? Would such a rise blow your tires off?
Much more than that.
Quoted message said:I still find Kinky's question interesting and still don't know the
answer, so I hope that you'll think it through and come up with a
helpful answer.
You gotta read more carefully Carl.
Jobst Brandt
[email hidden]
Dear Jobst,
"My friend stopped to take pictures so I waited for him riding the
rear brake over the lever hood at less than 10mph when BAM!"
Actually, you never do say which tire went BAM! Some of us would
assume that the front tire is the one that would heat up on a descent.
We weren't there. The lever hood is a nice detail, but you gotta write
more clearly, Jobst. (After all, you were surprised.)
As another example of how easily things may be mis-read, what does the
end of this sentence mean?
" It never occurred to me that this would blow off the tire, having
descended this road many times without incident with many others."
Many others? What others?
Many other tires? (Well, obviously. Descending without tires would be
tricky.)
Other friends? (Seems unlikely, but perhaps the point is that many
other riders had no tire trouble.)
Other brands of tires? (Possibly, but stretching.)
You know what you meant, of course, but I don't. A single word would
probably clear it up. Or maybe removing the last three words would
still say what you meant. Or it could easily be something that I
haven't guessed. Or even just a word processor editing oops.
Quick writing about vivid events is often like this--we can't believe
that the reader failed to follow us, while they're left wondering what
on earth we meant. You can't imagine how I could think it was your
front tire, while I can't figure out which one you meant.
Leaving clarity theory aside, you say that it never occurred to you
that this would blow the tire off. So what's the explanation? I seem
to recall that you've suggested that prolonged low-speed braking is
worse than high-speed descent because there's less air cooling of the
rim, but I may have misunderstood you.
Was this a prolonged low-speed descent? You say that you'd slowed down
for a friend, but for how long? A minute or so? Ten minutes? A hundred
yards, half a mile, a thousand feet of altitude?
And where does the heat generated at the tire's contact patch go on a
long descent with heavy braking? Into the pavement? The tire?
Dissipated into the air? Or am I mistaken about as much heat being
generated at the contact patch as at the brake pad?
Carl Fogel
