On 14 Jul 2003 08:56:34 -0700, Sam Huffman <
[email protected]> 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.
>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. To produce a pressure change of 14%, for instance, the air temp inside would have had to
go up by 74 degrees Farenheit; not impossible, but in normal cycling, extremely unlikely. In road
racing, we expected a tire pressure change from cold to hot of no more than 3 psi; the tires, after
all, are subjected to exposure to a lot of cooling air flow, and while the road interface may get
locally hotter than the air, the core of the tread and the casing shouldn't, nor should the
inflation air inside.
>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.
Beyond that, every automotive reference book in my possession which addresses the topic *except
one*[1] recommends strongly *against* reducing pressure in a hot tire that was at or near
recommended inflation *when cold*, and in point of fact, the vast majority of automotive tires are
explicitly marked with only the cold inflation pressure on the sidewalls. It is expected that the
pressure will rise at the tire's temp goes up. As you noted, 18 degrees (F) of temp change produces
only a 4% variation in inflation pressure; I doubt that this would be enough to cause any problems.
Bear in mind that the tires pretty much have to be designed to allow for that variation, as it is
impractical and unrealistic to expect that people will adjust their tire pressure from morning to
midday in normal riding, and it is fairly common to encounter temp swings of 25 to 30 degrees F in a
day. If you're in a competitive situation, then you will, of course, want to fine-tune the pressure.
Otherwise, my recommendation would be to make sure it's adequate in the morning before setting out,
and recheck it only if there are signs that the pressure is being lost.
[1] that one book I mentioned has a more extensive answer; it states that a passenger car tire which
is more than 15% over the max rated cold inflation pressure when checked hot should be allowed
to cool and then be rechecked, and if it returns to the rated pressure or below, the reason for
the internal overheating should be found and corrected before the vehicle is returned to
service. So, really, even that one doesn't recommend reducing the pressure unless it's
overinflated when cold.
---
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.
