We will try to answer frequently asked questions about motorcycle tire pressure. How does it look from the point of view of physics and… power loss of the motorcycle?
A motorcycle riding has an engine running, which produces power. Even riding at a constant speed, this power is consumed for something. The vast majority of it is air resistance, but also tire rolling resistance. The latter depends largely on tire pressure. The lower the pressure, the more power the motorcycle has to lose to move. On the other hand, the higher the pressure, the smaller the surface area of the wheel in contact with the ground and the greater the stiffness – that is, less comfort and even less grip.
In sports, on the other hand, tires must be warm, because only then does the racing compound work properly. So that they don’t cool down, the pressure must be low enough so that the energy lost to rolling resistance is high enough to keep the tire cool all the time. We need to reach an equilibrium when the amount of energy produced by deforming the tire at high speeds is the same as the energy lost through cooling.
This is where physics helps a bit. But about that in a moment. Gas transformations at constant volume are described by the equation of state of a perfect gas pV=nRT.
R is the universal gas constant
n is the number of moles of gas
T is the temperature in Kelvin
P is the pressure in Pascals
However, there is no need to be afraid of this equation, because we immediately throw out of it things that do not change. And neither the amount of gas changes (we don’t deflate the air, after all), nor the volume of the tire (because it doesn’t swell like a balloon, but is rather the same shape). The gas constant is constant even in name.
So what we are left with is p/T=const.
Meaning that the pressure/temperature ratio is always the same. Just remember! Temperature is given in Kelvins – so 0 degrees Celsius is 273 Kelvins. So there you go!
Let’s assume that we pump the cold slicks (rear) at 1.8, and their temperature is 20 C. That is, 293K (1C = 1K only the scale starts in a different place). We heat these tires to 100 C = 393K. Our equation then looks like this:
1,8/293=x/393
(1,8*393)/293=x
X=2,41
There you have it! A warm rear slick (inflated to 1.8 in the cold) has a pressure of 2.4!
Today, many sport racing tires work optimally at 1.4- 1.5 bar. Then in the cold this tire should have a pressure of 1.1 bar. The formula below:
1,5/393=x/293
X=1,1
Back to the fact that physics helps. Well, the higher the pressure, the less energy is used for heating. Which means that a cold tire with low pressure heats up quickly, and the more the temperature rises, the higher the pressure becomes and the less the tire heats up. And after some time we have a state of equilibrium. That’s why choosing the right pressure is so important, because it is what determines whether the tires are at the right temperature. That is – whether this state of equilibrium will settle at the right temperature or not.
If we exaggerate the pressure, the tires will never warm up. If we give too little, despite the fact that they will be hot, they will still deform too much (we will be driving on the so-called flak).
Important : what temperature the tires reach on the track will also depend on how we drive. A beginner driver, driving slowly, will not maintain the same tire temperature as a competitor.
That was about racing, and now about the street!
On the street, where few people use racing tires (racing tires are impossible to warm up in the city) we no longer have to deal with warming tires in blankets. Rubbers in this type of conditions should actually work at any temperature – of course, they hold better when they reach a minimum temperature. And here we should follow the manufacturer’s recommendations. This one, however, always gives the pressure “from – to”, and the maximum pressure for the tire. What effect does this have on power? So see a comparison of tires with higher pressure and with lower.
Both graphs are for the size of racing wheels – that is, the classic R17 180. The black line shows the measurement of the tire inflated to 2.7 bar, and the red line for the tire inflated to 2.1 bar. The difference, as you can see, is huge. In my experience, above 2.7 bar there is almost no gain.
In other words: if you can, inflate the wheels more strongly – the motorcycle will become a little more rippable. Of course, you should not overdo it, but it is worth knowing. Keeping in mind that we need about 7 kW in total to move at 100 km/h, and about 55 kW to move a motorcycle at 200 km/h, you can see that pressure matters a lot at low speeds, and less at higher speeds. Although, of course, at these higher speeds, we may always be missing just those last 10 kW.
Summary
At the races, set the recommended temperature hot, and measure after the session – if it dropped it means that you need to go out on a slightly higher one.
On the road, pump as much as is comfortable, but above 2.7 bar there is no more power gain.
Second opinion
If you do not have high-end blankets for racing tires ( indicating the temperature) , then on the track you will find it difficult to control the exact temperature of the tires, and you will not be sure whether you really have the correct pressure (whether they are really heated or not) when you leave on reheated rubbers. On the Poznań track, this is compounded by frequent problems with voltage drops. Tires heated for an hour in the morning do not necessarily reach the same temperature as those heated for an hour in the afternoon. That’s why, personally, at practice I always go down after two laps, check the pressure of the reheated rubber and confirm its compliance with the World Championship teams’ recommendations for the correct pressure for reheated tires in the given conditions. Sometimes the information the teams have is slightly different from the manufacturer’s recommended standards. However, this won’t matter to beginner and intermediate drivers – just make sure you stick to the official recommendations for the tire.
