Tire pressure on hot days?

[SirSnodgrass]

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?

[SirSnodgrass]

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.

[MrWiZo]

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???

[DrummerGirl]

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.

[MrWiZo]

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.

[SirSnodgrass]

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.

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.

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,

[SirSnodgrass]

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.

[britneybabie]

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

[MrWiZo]

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?

[H3g3m0n]

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