Technical FAQ: Effects of temperature and time on tire pressure

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Dear Lennard,
Forgive me if you’ve dealt with this before, but it’s something that has always puzzled me.

In countless places (in reference to both bikes and cars), I have read that tire pressure is crucial and that altering tire pressure by as little as a couple of pounds can have a significant effect on performance, safety, and durability.

If this is the case, I would think that any tire pressure you started the ride with would be wrong in about ten minutes. Both because the ambient temperature changes wildly from garage to open road, and because tire friction must heat up the tires and thus raise tire pressure.

Also read: Technical FAQ: high-speed shimmy, tubular tires, accuracy in tire pressure

We’ve all done innumerable rides when the air temperature in the garage was 60F degrees, and the air temperature four hours later was 80F with the added influences of the heat of the blacktop, plus the heat of the friction between tire and road.

I once did a century where the temperature was 70F at 7 am, 80F at 8 am, 90F at 9 am, and more than 100F by noon. What’s the correct tire pressure for such a day?

I can’t do the math, but if you put 90 lbs of pressure pre-ride and head out from an 80-degree garage, the tire pressure on a 100-degree day, where I’ve been told the pavement can be 130 degrees, must be astronomical.

So why does no one talk about addressing any of these issues? I’ve never heard anyone suggest that you should release air as the day/road heats up, for instance. I’ve never even heard anyone suggest checking tire pressure after a couple of hours of heating conditions.

Can you explain this?
— Jack

Dear Jack,
This is a great question, and, no, I have never dealt with it before in this column. I asked Oliver Kiesel, tire designer and product manager for Specialized about this, and here is his response:

“We never worked on this extensively, but we did a short study today and came to the conclusion that the air temperature doesn´t change the tire pressure crucially.

There is way more inconsistency and variance in the readings on floor pumps and pressure gauges than in how much the air temperature will affect the tire pressure.

Also, a tubeless or latex tube setup, as you pointed out, loses around 0.5 – 1.0 bar within a day. This is way more influential than the air temperature.

Our Google research and also some tire brand automotive suppliers show that a 10C [~18F] temperature change results just in ~1 psi tire pressure change.

Tires on bicycles don´t heat up too much. They are not heavily loaded and don´t run with high speeds.

There is also cooling air all the time. They mainly run just with the regular air temperature; even if they would run 10°C [18°F] higher, the pressure increase is not crucial to the ride quality or performance.

The study aligns pretty much from my experience with cars.

If you have a car with tire pressure sensors, it´s pretty much identical. The car tires heat up much more, maybe 30-40°C [54-72°F], and the tire pressure increases around 0.3 bar [4.35psi].

— Oliver Kiesel, Specialized Bicycle Components, tire product manager

Also, I want to mention that disc brakes have greatly reduced the heating of tires from braking compared to rim brakes. This was something that, on a hot day under a big guy descending a steep, winding road with rim brakes, especially with carbon rims (which dissipate heat less well than aluminum rims), could increase tire pressure far beyond the kind of changes in ambient temperature we discuss here.
― Lennard

Dear readers,
This question brought up for me a question I have often wondered about, namely how to deal with the pressure loss from a latex inner tube (generally in a tubular tire) over time. Anyone who rides racing tubulars or clinchers with latex tubes knows the routine of having to pump their tires daily, since the latex inner tube bleeds out quite a bit of pressure over a day. In cyclocross, this is not an issue, because the races are short, and the pressure differential between the air inside the tire and the ambient air pressure is quite small, given the low pressures riders run in tubulars in cyclocross (generally, 14-29psi, or 1-2bar). At the high pressures of narrow road tires, however, the pressure differential between outside and inside the tire is large, and the drop in pressure over several hours is noticeable.

I asked this question of Alex Brauns, founder and president of Challenge Tires, and this is his response:

In the old days, when I used to be at the Giro and TDF, I actually found out that mechanics such as Pengo [Enrico, ex Lampre-ISD, now Bahrain-Merida mechanic], Faustino [Muñoz Cambron, ex ONCE/Liberty Seguros, Saxo-Tinkoff mechanic] and Pinza D’oro (Golden Pliers) [ex Saeco mechanic] would actually increase the pressure by about a bar or in that area. During a stage, you have a loss of close to 1bar over a 260km stage, so if the rider wanted 8bars [116psi], he would start with 8.7 to 9bar [126-130psi].

This is still done today to give a little extra pressure at the start. Team head mechanic Jean Marc Vandenberghe at CCC at last year’s team camp was considering 0.5bar minimum.

—Alex Brauns

As Oliver Kiesel mentioned above, this can have a much bigger effect than ambient temperature changes and is worth considering and adjusting for before long rides or races.
― Lennard

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Lennard Zinn, our longtime technical writer, joined VeloNews in 1987. He is also a custom frame builder (www.zinncycles.com) and purveyor of non-custom huge bikes (bikeclydesdale.com), a former U.S. national team rider, co-author of “The Haywire Heart,” and author of many bicycle books including “Zinn and the Art of Road Bike Maintenance,” “DVD, as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.” He holds a bachelor’s in physics from Colorado College.

Follow @lennardzinn on Twitter.