Tire pressure on hot days?

My sister was in a race in last month during which the temperature went from
about 60 degrees in the morning to 95 degrees in the mid-afternoon.

The bike techs were pumping tires up to full pressure, but a friend advised
her to drop the pressure to about 125 psi from her usual 175 psi due to the
anticipated heat. (PV=nRT, I suppose).

I'll be doing some light touring later this summer in similar conditions, and
am wondering if this is something I should watch out for? I.e. let air out of
my tires mid-day? I generally run close to the maximum listed tire
pressure. Or is this yet another tire pressure myth?

Thanks,
Sam

"Doug Huffman" writes:

"Sam Huffman" wrote in message

The bike techs were pumping tires up to full pressure, but a friend
advised
her to drop the pressure to about 125 psi from her usual 175 psi due to
the
anticipated heat. (PV=nRT, I suppose).

I'll be doing some light touring later this summer in similar conditions,
and
am wondering if this is something I should watch out for? I.e. let air out
of
my tires mid-day? I generally run close to the maximum listed tire
pressure. Or is this yet another tire pressure myth?

If you're smart enough to write PV=nRT to the group then use it. Calculate
the P-delta for your T-delta.

I can do the calculation; it looks like a roughly 10% pressure increase as a
result of temperature increasing from 60 - 95 degrees. However the question is
whether other factors mitigate or aggravate this.

I saw a reference to an experiment done on car tires in which an increase in
ambient temperature from 67 to 85 degrees (~ 5%) resulted in tire pressure
increasing from 35 to 36.5 psi (~ 4%), which is probably within experimental
error.

However the same experiment found that the tire in direct sunlight increased
from 35 to 40 psi (~ 14%), which assuming the same result applies to bicycle
tires, might become problematic if someone rides near or at maximum pressure
normally.

I was wondering if anyone had any experimental or anecdotal evidence that
might apply to cycling.

Thanks,
Sam

Sam Huffman wrote:
My sister was in a race in last month during which the temperature went from
about 60 degrees in the morning to 95 degrees in the mid-afternoon.

The bike techs were pumping tires up to full pressure, but a friend advised
her to drop the pressure to about 125 psi from her usual 175 psi due to the
anticipated heat. (PV=nRT, I suppose).



What tires is your sister that can be pumped up to 175 psi? My pro race
tires are only rated to 116 psi.

Kenny Lee

Conti. Grandprix can be pumped to 175 psi, but what about the rim, Mavic
recomend up to about 130 psi

Gustav

What tires is your sister that can be pumped up to 175 psi? My pro race
tires are only rated to 116 psi.

Kenny Lee

"Sam Huffman" wrote in message
news

"Doug Huffman" writes:

"Sam Huffman" wrote in message

The bike techs were pumping tires up to full pressure, but a friend
advised
her to drop the pressure to about 125 psi from her usual 175 psi due
to
the
anticipated heat. (PV=nRT, I suppose).

I'll be doing some light touring later this summer in similar
conditions,
and
am wondering if this is something I should watch out for? I.e. let air
out
of
my tires mid-day? I generally run close to the maximum listed tire
pressure. Or is this yet another tire pressure myth?

If you're smart enough to write PV=nRT to the group then use it.
Calculate
the P-delta for your T-delta.

I can do the calculation; it looks like a roughly 10% pressure increase as
a
result of temperature increasing from 60 - 95 degrees. However the
question is
whether other factors mitigate or aggravate this.

I saw a reference to an experiment done on car tires in which an increase
in
ambient temperature from 67 to 85 degrees (~ 5%) resulted in tire pressure
increasing from 35 to 36.5 psi (~ 4%), which is probably within
experimental
error.

However the same experiment found that the tire in direct sunlight
increased
from 35 to 40 psi (~ 14%), which assuming the same result applies to
bicycle
tires, might become problematic if someone rides near or at maximum
pressure
normally.

I was wondering if anyone had any experimental or anecdotal evidence that
might apply to cycling.

Thanks,
Sam

Anecdotal? OK; ask Joseba Beloki about hot days, long descents and tubular
pressures.

175psi is pretty much pointless anywhere outside of *maybe* a velodrome
anyway, so why push it?

SB

"Sam Huffman" wrote in message
news

My sister was in a race in last month during which the temperature went from
about 60 degrees in the morning to 95 degrees in the mid-afternoon.

The bike techs were pumping tires up to full pressure, but a friend advised
her to drop the pressure to about 125 psi from her usual 175 psi due to the
anticipated heat. (PV=nRT, I suppose).

I'll be doing some light touring later this summer in similar conditions,
and
am wondering if this is something I should watch out for? I.e. let air out
of
my tires mid-day? I generally run close to the maximum listed tire
pressure. Or is this yet another tire pressure myth?

The real danger is tube heating from the rims during prolonged braking. This
is usually only a concern in winding, mountainous, descents where excess speed
has to constantly be shed. In these circumstances, rims can sizzle water and
it's a good idea to lower the pressure well below blowoff maximums before
descending. Where the actual blowoff is relative to the sidewall/rim specs is
hard to know, but some circumstances deserve a conservative approach.

Sam Huffman: wrote:

My sister was in a race in last month during which the temperature went from
about 60 degrees in the morning to 95 degrees in the mid-afternoon.

The bike techs were pumping tires up to full pressure, but a friend advised
her to drop the pressure to about 125 psi from her usual 175 psi due to the
anticipated heat. (PV=nRT, I suppose).

She normally runs 175 psi?!!! Wow! That must make for a hard ride.

I normally wouldn't worry about a 30 degree change in air temperature,
but at 175 psi I might. What can really cause problems is rims heating
up from prolonged braking on long mountain descents.

Art Harris

(John Forrest Tomlinson) writes:

Sam Huffman wrote in message ...
a friend advised
her to drop the pressure
to about 125 psi from her usual 175 psi

What sort of race was this that riders would use 175psi in normal
weather? That's remarkably high for road racing. Was it a track race?
What kind of tires does she use?

It was a triathlon; she uses tubulars, not sure of the brand. I doubt she
would notice if the tire pressure were 175 psi or 125 psi, but that is the
pressure the LBS told her to use.

She's been told by other shops to keep them pumped high because she's heavy
compared to other racers (150 pounds) and lower pressure will cause rubber
deformation which, in conjunction with other factors, can cause the glue to
fail.

She'd be the first to admit that her sport is caught up in over-hyped
equipment.. At all of her races I've been to there's always some old guy on
a Schwinn who finishes relatively high in the standings, and well before a
lot of 20-somethings with fancy equipment.

Sam

On 14 Jul 2003 07:15:47 -0700, Sam Huffman
wrote:

I'll be doing some light touring later this summer in similar conditions, and
am wondering if this is something I should watch out for? I.e. let air out of
my tires mid-day? I generally run close to the maximum listed tire
pressure. Or is this yet another tire pressure myth?

Thanks,
Sam

How accurate is your gauge? Most people have no idea.


Barry

When I do a packed tour, I normally increase my tire pressure by 10%
(100 110 psi) dou to the simple fact that I'm carying (over)ten
percent more weight.

Even the heat of the California (or Carolina!) summer sun, I have never
had a tire blow out.

I weigh 175, usually carry about 27-30 lbs in the summer, I have
Weinmann touring rims and Hutchinson clincher tires rated at 100 psi. I
normaly ride them at 100 in the rear, 90 front (unladen bike).

May you have the wind at your back.
And a really low gear for the hills!
Chris

Chris'Z Corner
"The Website for the Common Bicyclist":
http://www.geocities.com/czcorner