Shazaam's Tech Library - What Tyre Pressure ?

A useful technical article from guest contributor Larry Kelly of San Diego CA (aka Shazaam!).

The answer is that for a tire to perform at its best it has to reach, but not exceed, the temperature at which the rubber compound gives the maximum grip.

The difficulty is, predicting the temperature that the tires will actually reach for each combination of bike model, suspension set-up, rider weight, cornering and throttle technique, air temperature, road temperature, road surface and curvature ... just to name a few variables. This is why tire testing is so important, and also why so much time is devoted to it by race teams.

For street bikes, Ducati plays the averages. When they do development work on each bike model, Ducati test riders and engineers work with their OEM suppliers, Michelin and Pirelli, to come up with a tire that gives a good overall performance (wet, dry, good milage, etc.) Along the way, the tire pressure that works best for these OEM tires for AVERAGE ROAD USE is determined, and that’s the number that ends up in the Owners Manual. As power increased and tire construction and rubber formulations changed over the years, so have Ducat’s recommendation for tire pressures. Early 916’s for example, had a recommended pressures of 32F/35R, but 30.5F/32R is now recommended for 99X’s.

So when you move to a tire that isn’t OEM, then you have to do your own tire testing to determine the best pressures FOR YOU, and you have to hope that the tire manufacturer has done the necessary development work to give you a good starting point.

The non-Ducati OEM tire manufacturers play the average-game as well, so when they recommend tire pressure starting points, they do it for an average bike, whatever that means. The trouble is the tire manufacturers don’t ever tell you what’s the optimum tire temperature for the rubber formulation for each kind of tire. Some reps will tell you the optimal increase (%F/%R) in pressure from cold to running condition, which is more useful information than cold tire pressures.

So it’s really all up to you. You'll get a lot of opinions on what tire pressure to run, but the correct tire pressure for you is not a matter of polling other rider's opinion or asking the manufacturer’s rep at the track.

Here are the basics you'll need to decide for yourself.

For the street, start with the bike manufacturer's recommendation in the owners manual or under-seat sticker for OEM tires and similar models from other manufacturers. This is the number that they (Ducati and the OEM tire makers) consider to be the best balance between handling, grip and tire wear. Further, if you're running alloy wheels on poor pavement, consider adding 2 psi to the recommended tire pressure just to reduce the likelihood of pothole damage. Just as you would for a car, increase the pressure 2 psi or so for sustained high speed operation (or two-up riding) to reduce rolling friction and casing flexing. Check your tire pressure regularly as they say.

In order to get optimum handling a tire has to get to its optimum temperature which is different for each brand of tire. Most of us don't have the equipment needed to measure tire temperature directly so we measure it indirectly by checking tire pressure since tire pressure increases with tire temperature. Tire temperature is important to know because too much flexing of the casing of an under-inflated tire for a given riding style and road will result in overheating resulting in less than optimum grip. Over-pressurizing a tire will reduce casing flexing and prevent the tire from getting up to the optimum operating temperature and performance again suffers. Sliding and spinning the tires also increase tire temperatures from friction heating.

A technique for those wanting to get the most out of their tires on the street is to use the 10/20% rule.

First check the tire pressure when the tire is cold. Then take a ride on your favorite twisty piece of road. Then, measure the tire pressure immediately after stopping. If the pressure has risen less than 10% on the front or 20% on the rear, you should remove air from the tire. So for example, starting at a front tire pressure of 32.5 psi should bring you up to 36 psi hot. Once you obtain this pressure increase for a given rider, bike, tire, road and road temperature combination, check the tire pressure again while cold and record it for future reference.

Each manufacturer is different. Each tire model is different. A tire design that runs cooler needs to run a lower pressure (2-3 psi front) to get up to optimum temperature. The rear tire runs hotter than the front tire, road and track. So the rear tire cold-to-hot increase is greater. Dropping air pressure has the additional side effect of scrubbing more rubber area.

As an example, when I used the tire pressures recommended by Ducati (32F/35R) for my 916 on my favorite road, I got exactly 10/20% on a set of Bridgestone BT-012SS. So I guess I'm an average rider, and the BT-012SS runs at an average operating temperature compared to other brands.

For the track you'll have to drop the cold tire pressures an additional 10/20%. Track operation will get tires hotter (increasing the cold-to-hot pressure range) so starting at say 32/30 psi now should bring you up to the same temperature (and pressure) that 35/39 psi gave you for the street. Don't even think about running these low track cold pressures on the street.

Finally, dropping tire pressures on street tires for track use has its limitations, so street compound tires on the track often get too hot and go beyond sticky to greasy. That's why you have race tires. Race tire compounds are designed for severe operation at these higher temperatures for a limited number of thermal cycles. On the other hand, a race tire on the street usually won't get up to the appropriate temperature for good performance. At street speeds, the race compound often won't perform as well as a street tire.

Please note that cannot accept any liability for the accuracy or content of this section. Visitors who rely on this information do so at their own risk. If you are unsure it's worth contacting your local Ducati dealer who will be able to help. Do not attempt a repair or modification if you do not have the correct tools or knowledge to do so.