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Old 05-18-09, 12:46 PM   #1
Glades2
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Tire Pressure

Per the below item from Velo News, a tire pressure inflated "to the maximum" (as indicated on the tire) is still too high, and can cause a blowout during frequent downhills:

"...I believe that this will continue to be an issue for all manufacturers of clincher rims. To explain in greater detail, with a 23C tire each 10-degree increase in rim surface temperature above an ambient temperature of 70 degrees Fahrenheit will result in a corresponding increase in tire pressure of 1psi. On longer descents with accompanying heavy or near-continuous braking, the energy transferred by braking into the rim will routinely raise the surface temperature to well over 300 degrees Fahrenheit and can under extreme conditions reach over 400 degrees. Again assuming an ambient temperature of 70 degrees, a conservative 300-degree rim surface temperature means an air pressure increase of 23psi"

Per a controversial post of mine a number of months ago on the safety board, possibly that was the cause of the blowout fatality of one of our club members, during a very steep downhill on one of the Tour de Georgia mountain roads - it makes sense since that ride requires frequent braking, per the article...

This was also proven while helping another club member earlier this year - she asked me to use my floor pump to inflate the tire to the maximum of 140, and just as it reached that mark the tire blew out, with a resounding pop (unfortunately that was her last spare, and the ride ended - it might have continued had she asked for a more conservative 100 or 110. Personally, I inflate my tires to 100, and it still gives a good hard ride, just the same)...

Last edited by Glades2; 05-18-09 at 01:02 PM.
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Old 05-18-09, 12:50 PM   #2
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On long downhills, one should alway be mindful of proper braking techniques. Tire pressure rise and the unmentioned in this article melting/blowing of tubes will definately ruin your day.
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Old 05-18-09, 12:51 PM   #3
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From noted wheel author Jobst Brandt:

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.misc
Subject: Re: Dark art of descending!
Date: 23 Aug 1999 22:03:22 GMT

Jeff Potter writes:

> Going down the top super steep half was like doing a pushup! I had to
> brake full-on on the straights not to go too fast for the curves.
> Tired hands. SUPERHOT RIMS! I had sewups on, so I stopped for a
> few minutes halfway down.

> How do you guys deal with superhot rim heating? Is there such
> a thing as too steep?

I don't know how you kept the tires from slipping on the hot glue and
piling up on the stem. This is the usual indicator that tubular tire
wheels are too hot. The next is that the tire lifts off in the
compressed area just ahead of the stem.

This is a serious problem both for tubulars and clinchers because most
clincher tires, given enough heating time on a hot rim will blow off
if inflated to the hardness that most racers like for criteriums
(hard). The faster you can go, the more power goes into wind drag and
the more air rushes over the rims. Slowing down does not help, unless
you reduce speed to a walking pace.

For steep descents where the rims stay hotter than you can bear to
touch for more than a minute, you should let some air out of the tires
to where you would normally want to re-inflate them after some disuse.
I don't mention a pressure because that depends on the tire size.
Small tires heat up faster than large ones but the blow-off pressure
is the same, it being dependent only on the opening of the rim width.

> I felt I was going too fast for the curves yet it was too steep to
> slow down enough! Well, that was when I was 'rawest,' I got a lot
> bolder at the halfway down point. It was fun to start throwing the
> body into the turn, to point the shoulders out to the exit of the
> turn, all over again. Kind of point the body, then pull the bike
> around.

All that "body English" is gratuitous gesture, much like the
motorcyclists who stick their butt out in curves while their bikes
never get down to 45 degrees (where hiking out becomes necessary to
keep hardware from dragging on the road). In fact, if you are taking a
bunch of ess bends rapidly, you'll have no time to change your
position. Just keep your weight on your (horizontally positioned)
feet, and unless the road is rough, keep light pressure on the saddle.

> The other good rule was: make half your turn by the halfway point!

I disagree, because you can slow down much faster than you can
accelerate. The trajectory is naturally asymmetric. And you can
brake all the way to the apex of the curve, but you cannot pedal at
that lean angle.

> The other good rule was: make half your turn by the halfway
> point!into the turn you're already throwing yourself down and to the
> inside and looking to the far straight, you twist your shoulders
> toward the center of the turn. It feels like you're getting yourself
> all oriented to the exit even before you've gone in!

That may look impressive but it doesn't have any positive effect on
the lean of the bicycle and its traction on the road, that being what
counts in order to corner at the limit.

> Does this at all relate to a correct way to handle switchback descents?

The term switchback refers to mountain railroading where at the end of
each traverse, a switch is turned to back up the next traverse, after
which another switch is turned to head up the next traverse. The
appropriate term is hairpin turn and these are the ones where
trajectory asymmetry is most conspicuous, because braking can be hard
enough to raise the rear wheel when entering but one cannot accelerate
similarly. Most riders often find themselves with extra unused road
on the exit of such turns. This exemplifies the difference between
entry and exit of turns.

> Does anyone descend the top half of Flagstaff without much braking?

I'm not familiar with that road but there are many that have gradients
that requiring stops to cool the rims. There is no way of descending
them continuously unless you have insulators between the tube and rim.
Insulating rim strips are no longer offered because they were an
artifact of dirt roads that required riders to descend so slowly that
all potential energy went into the brakes and almost none into wind
drag. These rim strips were cloth tubes filled with kapok, their
insulating purpose being a mystery to most riders when they were last
offered.

Jobst Brandt <jbrandt@hpl.hp.com>
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Old 05-18-09, 01:00 PM   #4
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maddmaxx and dnvrfox,

I agree, though the very young folks on the safety board pounded the toast out of me for even mentioning it - apparently they consider that kind of thinking "unprofessional"...

Ironically I was told of the hot rim problem years ago - by a couple of racing mechanics...

Oh, well - can't please everyone!

Last edited by Glades2; 05-18-09 at 01:03 PM.
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Old 05-18-09, 01:19 PM   #5
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This all seems like a lot of hot air to me
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Old 05-18-09, 01:27 PM   #6
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Quote:
Originally Posted by Glades2 View Post
maddmaxx and dnvrfox,

I agree, though the very young folks on the safety board pounded the toast out of me for even mentioning it - apparently they consider that kind of thinking "unprofessional"...

Ironically I was told of the hot rim problem years ago - by a couple of racing mechanics...

Oh, well - can't please everyone!
That's why we have a 50+ forum. We're just a little bit smarter/wiser.
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Old 05-18-09, 01:42 PM   #7
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Quote:
Originally Posted by Glades2 View Post
"...I believe that this will continue to be an issue for all manufacturers of clincher rims. To explain in greater detail, with a 23C tire each 10-degree increase in rim surface temperature above an ambient temperature of 70 degrees Fahrenheit will result in a corresponding increase in tire pressure of 1psi. On longer descents with accompanying heavy or near-continuous braking, the energy transferred by braking into the rim will routinely raise the surface temperature to well over 300 degrees Fahrenheit and can under extreme conditions reach over 400 degrees. Again assuming an ambient temperature of 70 degrees, a conservative 300-degree rim surface temperature means an air pressure increase of 23psi".
No one denies that blowouts happen although some people will claim they don't happen unless there are other problems too (like poor mounting or bad bead).
There are a couple problems with this blurb though:

1. Just because your rims are 400F doesn't mean the air inside the tube is 400F, heat transfer takes time and air is a poor heat transfer medium so the pressure-temperature relation doesn't usually apply.

2. There are other things that happen when your rims get that hot though, for instance they expand to some degree, changing the bead-rim interface.

At any rate, blowouts are more likely to happen with higher initial pressures no matter what other factors come in to play.
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Old 05-18-09, 02:07 PM   #8
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The only tyres I have heard of having a heated rim causing a tyre to blow off the rim- have been the cause of 3 problems.

Excessive weight being carried by the tyre

Overinflation to carry that excessive weight

And long periods of braking to cause the rim to get hot.


And the only bikes that will come near the first the first two being in place are Tandems.

And for the 3rd to add to the problem It would have to be while descending a hill with a great number of turns and a long descent -----such as coming down a mountain.

I run 26x1.4 tyres on our Tandem that have a max pressure indicated on the sidewall of 120psi. We inflate to 140psi. We also have an all up weight of 400lbs. So the first two things are in place for a blowout or the tyre to jump off the rim. What we have not had though are mountain descents with lots of braking.

I also do the Un-recommended by running folders on the bike so yet another N0-No. But I can assure you that Trying to fit a tyre onto our wheels is difficult- in fact it is a 3 lever job and 4 hands to fit one.

So for the average rider that possible over-inflates his tyres- or even gets to the higher pressure recommended- I don't think there is going to be a problem.

Possibly a Heavy clydesdale running narrow tyres at or above the max pressure and then does a mountain descent will have a problem- so you have been warned.

And yes- damaged sidewalls- sloppy fit of the tyre on the rim or dragging the brakes all the way down a hill will have an adverse effect---- But we don't do that----Do we.
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