How to guesstimate correct tire pressure in the winter?

Question

I ride a rather generic Merida hardtail mountainbike throughout the year. I don't own a pump with a gauge so I usually pump my tires at gas stations (that have automatic pumps with gauges).

The problem is the following: I pump the tubes outdoors, but the bike is stored indoors most of the time. Usually I just fill the tires so that they're a little bit softer than usual as to avoid having too high pressure after the air warms up indoors.

Is there a good method to guesstimate how much lower (in bar) the pressure should be related to the norm to accommodate for this change? Is this worth worrying about to begin with?

If it matters, the temperature difference between out- and indoors can easily be 35°C (~95°F; -20°C outside, +15°C inside).

Answer 1

Ideal Gas equation: pV=nRT

With +15oC and -20oC the pressure ratio between the two is: p1/p2 = (273+15)/(273-20). Or 13% higher pressure when put in the garage.

However that is assuming you are pumping it to maximum pressure, that the tyre can no longer (or very slightly) expand, making V (volume) constant.

That is the basics for your assumption. However, the tyre would flex and expand ever slightly. So, 5-10% would be good approximation to me, depending on how much you put into the tyre, and how big is your tyre.

Saving you all the calculation, I would suggest for

• Road tyre : 700x18-24c, 85-90 psi at the gas station (less than 10% of the maximum pressure, but you need more traction if temperature is less than 0oC!!!)
• MTB tyre: 26x1.2-1.5", 55-85 psi at gas station MTB tyre 26x1.5"+, 20-42 psi
• Hybrid/CX: 700x24 to 700x40, 45-85 psi.

Just a rough guide, but please make sure you check maximum tyre pressure, then subtract 10% ( just don't do bicycle stunt indoor...) Edit 2: This is for your maximum Cold-Tyre Pressure only. Your target should be within recommended (i.e. Cold-Tyre pressure at recommended pressure and make sure +10% and it should not exceed Maximum pressure).

So I believe the answer is 10% more of the recommended pressure would not do any harm if put in the garage (checking a few popular tyre in the shop)

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EDIT: I have one tyre blown up when I put close to recommended maximum pressure, and let it exposed to the summer sun locking to the pole. It was the loudest bang I have ever heard.. and I would be extra careful from now and then when pumping an old tyre pass recommended pressure (leave alone the maximum recommended)

Answer 2

The right answer to this (as with all tire pressure questions) is try it out as the sidewall ratings (and anyone else who gives you an answer with a number in it) is likely giving you nonsense. In fact, you should not be near the max sidewall rating in winter (or in summer) in most cases and thus this is not something you should be worrying about if your pressures are set correctly.

The right pressure depends on what kind of conditions you encounter and the amount of weight on the bike -- the pressure should be high enough to avoid damage due to road hazards / pinch flats and not give excess rolling resistance, but low enough not to give a harsh ride or promote damage when hitting a road hazard or reduce control (overinflated tires are bouncy).

In winter, this is especially important depending on your riding conditions. Big tires run at low pressure are good for snow, studded tires for ice also need to be run at low enough pressure for the studs to catch on the ice if they're commuter studded tires, etc. The pressure will almost surely not be the same as the one you're running indoors or in the summer, so its not worth calculating -- you need to go outside and do some experiments to set the tire pressure (it will most likely be lower than your summer pressure). An example from Peter White says that he runs his Schwalbe Ice Spiker Pro at 25 psi, for example and recommends no more than 60 psi on the thinnest tire he sells (Nokian A10). This is purely an example though, and you need to validate the tire pressure which works for you in winter on your own; A winter in South Beach (Miami, FL, USA) is well different from a winter in Winnepeg (Manitoba, CA). In the case of South Beach, your winter and summer setups are probably the same. In Winnepeg, well, they'll be very very different (probably fatter lower pressure tires in the winter). Even if the temperatures are the same, you still have the rest of the conditions (cold+dry is very different from cold+wet) affecting your setup.

Also, invest in a bike floor pump -- gas station pumps often have poor control of how much air is pumped in relative to the volume of a bicycle tire and can't always reach adequate pressures. Your tires/rims may not like the gas station pump one day.