Tire pressure on hot days?

"Peter Cole" wrote in message news:fFCQa.60384$Ph3.5911@sccrnsc04...
"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?

Something else to consider is the possibility that the temperature of
asphalt and air near the ground is significantly hotter than the air
where most people hold thermometers. I'm not sure how much of this
may transfer to the tire and rim.

On Mon, 14 Jul 2003 18:05:15 +0200, KGB wrote:


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

With tubulars, the rims don't care what the pressure is. The bike would
certainly ride like a rock, though.

--

David L. Johnson

__o | It is a scientifically proven fact that a mid life crisis can
_`\(,_ | only be cured by something racy and Italian. Bianchis and
(_)/ (_) | Colnagos are a lot cheaper than Maserattis and Ferraris. --
Glenn Davies

On Tue, 15 Jul 2003 03:56:22 +0000, Werehatrack wrote:

On 14 Jul 2003 08:56:34 -0700, Sam Huffman may
have said:

I saw a reference to an experiment done on car tires in which an increase
in ambient temperature from 67 to 85 degrees (~ 5%)

Sorry, that's not a 5% change unless you're measuring the temp in
Kelvin...and 85K is mighty cold.

Um, if that were the Kelvin temperatures it would not be a 5% increase,
but a 27% increase. I do believe the guy was converting Fahrenheit to
Kelvin, computing the percentage change there, and noting that.
67F=292.6K, 85F=302.6K (302.6-292.6)/292.6 = 3.4%. Well, OK, he was a
little off.

As far as the rest of the claims go, I*have never experienced it, but
there are those who claim their tires have blown off the rims while the
bike is locked up inside a car. Sun streaming in, no air circulation.

Certainly, though, on a long descent the air inside the tube can be
seriously heated by the hot rims. Melting tubular glue was quite common.

--

David L. Johnson

__o | I don't believe you, you've got the whole damn thing all wrong.
_`\(,_ | He's not the kind you have to wind-up on Sundays. --Ian
(_)/ (_) | Anderson

On Tue, 15 Jul 2003 01:23:41 -0400, "David L. Johnson"
may have said:


As far as the rest of the claims go, I*have never experienced it, but
there are those who claim their tires have blown off the rims while the
bike is locked up inside a car. Sun streaming in, no air circulation.

And temps as high as 180F, possibly higher in the desert Southwest.
Given the rather flexy beads and shallow rims of bike tires relative
to the rim diameters, it would not be surprising to see a tire lift
out of the bead if it was a high-pressure unit to begin with and had a
temp increase of that severity. That's not really a *riding* hazard
situation, though, it's a *storage and transport* issue. Except for
the type of event you describe involving a long descent, I can't think
of a reason to expect this sort of overheating problem *while riding*,
since an environmental temp of that magnitude would have a certain
amount of effect on the rider. Now, if a decent disc brake could be
had for a non-ruinous price, even the braking heat problem could be
ignored.

Certainly, though, on a long descent the air inside the tube can be
seriously heated by the hot rims. Melting tubular glue was quite common.

If it were not for the concerns about weight and aerodynamics, it
would be relatively easy to reduce this problem by making an aluminum
rim with exposed short (on the order of 3 to 5 mm) heat dissipation
ridges on the inner flat surface flanking the spokes. Aluminum
carries heat and sheds it very well, so the effects of prolonged
braking should not be hard to ameliorate in this way. (For the
competition rider, of course, the two conerns mentioned would override
the safety gain...but for the casual or commuter rider, I'd think
there might be a market in areas where the terrain is more vertical
than I'd personally want to try to ride.)

---
My email address is antispammed;
pull WEEDS if replying via e-mail.

Yes, I have a killfile. If I don't respond to something,
it's also possible that I'm busy.

Werehatrack writes:

On 14 Jul 2003 08:56:34 -0700, Sam Huffman
may have said:

I saw a reference to an experiment done on car tires in which an increase in
ambient temperature from 67 to 85 degrees (~ 5%)

Sorry, that's not a 5% change unless you're measuring the temp in
Kelvin...and 85K is mighty cold.

David got it right; I had already converted to Kelvin, though apparently I
should have used a calculator instead of my noodle 3.4% it is.

resulted in tire pressure
increasing from 35 to 36.5 psi (~ 4%), which is probably within experimental
error.

And, quite frankly, that's much less than the variation normally found
from tire to tire on the average real-world non-competition vehicle.
In other words, a difference of no consequence whatever.

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.

Those tires must have been getting awfully hot. or must have been in
the freezer when they were aired up.

The tires in the experiment were 35 psi at 67 degrees (no direct sunlight),
36.5 psi at 85 degrees in the shade, and 40 psi at 85 degrees in the
sunlight. So based on that, 4% of the increase was from ambient temperature
and 10% was from sunlight.

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

The easy test you can do yourself is this; try changing the tire
pressure on your bike by the 4% amount you mentioned, and see if you
can detect the difference in a ride. I think you'll discover that you
can't.

My concern isn't the 4%, but rather the 14%. My guess would be the additional
10% from direct sunlight is a constant adder on top of the pressure change, so
on a 60F - 95F day, one might expect a 17% increase in tire pressure. From
that and the other input I've gotten it sounds reasonable to adjust tire
pressure mid-day, though probably not crucial, given that a tire rated at 110
psi is more likely to blow if it's at 130 psi than at 110 psi.

Thanks for the input,
Sam

On 14 Jul 2003 08:56:34 -0700, Sam Huffman
wrote:
I saw a reference to an experiment done on car tires in which an ambient
temperature from 67 to 85 degrees (~ 5%) resulted in tire 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 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.

Additional factors to keep in mind:
1. Direct sunlight on a black tubless tire with a wide auto tread and high
profile would definately heat the tire significantly. The same light on a
narrow bicycle tread, combined with the possible lighter color of the
sidewall (or even the tread) and the fact that the tire has layers of nylon
or kevlar between the tread and the tube (vs. just steel braid, covered in
rubber, with no tube in an automotive tire), would have a different result.

2. While riding, the tire is subject to wind, and maybe shade, which cools
it down, but is also subject to heating factors including normal (friction,
rolling resistance) and climate-related (pavement in hot sun on a hot day
get HOT; try cooking a frozen pizza on it and see what I mean).

3. Uhh, I forgot. Stupid phone calls distract me. What do people think
they're doing, calling me at the office during standard business hours to
discuss, of all things, business?

--
Rick Onanian

On 15 Jul 2003 08:49:20 -0700, Sam Huffman
wrote:
The tires in the experiment were 35 psi at 67 degrees (no direct 36.5 psi
at 85 degrees in the shade, and 40 psi at 85 degrees in the

Maybe more importantly, size (width, profile, and rim) and position (front
wheels turned away from car into sunlight? Wheel removed from the car and
left standing in sun?).

The factors relating to pressure change in an automotive tire that I can
think of work this way:
- Width: If the tread sees the sun, then it is more surface
area getting direct heat.
- Wider tires whose tread are sheltered by the car have more
air in them to heat up for the same surface area exposed,
so they will not heat up as quickly, and probably not as much.

- Profile: This is the sidewall measured from the rim to the tread.
This will get the most sunlight, and is generally thinner than
the tread, probably resulting in transmitting the heat sooner
and more strongly.

- Rim size: Other two measurements being the same (if measured in
absolute units, instead of the percentages used to sell the
tires), changing the rim size of the tire results in, of course,
more air volume, as well as more profile surface area (though
less change in tread surface exposed to the sun).

Bicycle tires have small profiles, depending on how you're comparing.
Consider that a bicycle rim is bigger than an automotive rim, and a bicycle
tire's outside diameter is yet smaller. Then calculate in the percentage
difference in width, and you can figure out how a bicycle tire's profile
relates to an automtive tire's profile, and also air volume.

Now that I've said all that, I must say...I'm not nuts enough to actually
DO all those calculations. Personally, I'd just try my luck at my preferred
pressure.

Where can I get 175psi tires, and what tubes will keep that pressure
without leaking? My two road bikes have 125psi tires (Hutchinson Carbon
Comp on my Giant TCR2, and Serfas Seca on my recently rescued, still un-
identified Peugeot).

Who am I kidding...125psi is more than stiff enough for me!

Oh, one more thing: At 175psi, I doubt that rolling resistance has a chance
to heat the tire.
--
Rick Onanian

On 14 Jul 2003 09:07:55 -0700, Sam Huffman
wrote:
not. My cheapo floor pump goes to 160psi I think, so presumably some

Ah, there's the other problem. If you come up with tires, tubes, and rims
that do 175psi...How the hell do you pump them up that high?

I'm in the roofing business, and although my compressors can provide
120psi, I can't get anywhere near it by the time it gets through a presta
valve...I had to buy a floor pump, and it's tough pumping 125psi. How the
hell do you get 175 in there???

--
Rick Onanian