Motorcycle Tires: Pressure – Dial it in.
Tire pressure is a critical “setting” for any bike, on the street or on the track. At the track, motorcycle tires and their pressures can be used like any other adjustment on your bike to change your bike’s handling characteristics. If you need a little more grip, drop the pressure in the rear. If you want longer life, run a little more pressure. So long as you’re within the tire manufacturers’ suggested pressure ranges, “tuning” tires to your liking is just another tool in your riding arsenal.
Over the past 10 years, tire pressure recommendations have strayed away from “cold” pressures (taken before putting tire warmers on) and are nearly exclusively given as “hot” pressures. A common misnomer is that a “hot” pressure is taken after running tire warmers and before you go out on-track.
When a tire manufacturer suggests a certain hot pressure, this should be measured as soon as possible, after an on-track session (at least two laps). Once you get back to your pit, the first thing to do is check your tire pressures and adjust accordingly.
As with any other setting, recording your changes each and every session is highly recommended. If you take multiple pressure readings (cold, after tire warmers, and after your session), you will be able to predict the loss or gain in pressure from cold to warm (before and after tire warmers), and warm to hot while on the track. There is no magic number to set pressures at prior to a given track session, as loss or gains in pressures are dependent on the rider, bike, track surface, ambient temperature, track temperature, etc. Each of these factors can affect the air pressure in your tires and should be recorded so a rider knows what to expect. Here are 2 scenarios that have drastically different impacts on tire pressures.
- It’s the first weekend of the year in April, and temps are in the 50′s.
- It’s the Loudon Classic weekend and temps are touching 90 degrees.
How will your hot pressures be affected? As noted above, there are several more factors that come into play, but generally speaking at low ambient temps (with subsequently low track temps, although bright sunshine can often rise track temps even on a cold day), a rider is likely to experience a LOSS of tire pressure between when the tires are off the warmers (after being warmed for an hour to over 150 degrees), and when the rider comes off the track. It’s quite likely that throughout your session the temperature of the tire carcass will in fact, drop as the ambient temps and track temps combined with heat created by deflection in the tire carcass, are not enough to reach the same temperature achieved with your tire warmers.
Pressure x Volume = (n)(R) Temperature.
Since we’re dealing with air in all cases (although some use nitrogen in their tires which is more stable than air), and the volume of air in your tires remains constant (the tire does not expand or contract like a balloon), the only variable affecting pressure is temperature. A good rule of thumb: for a 10 degree (F) change in temperature = 1 PSI change in pressure within a constant volume (inside your tires).
(If you’re looking for more detailed, technical presentation on pressure and temperature as it relates to tires, click here for a good article to this effect with regard to car racing tires.)
In scenario 2, your tires are at 150+ degrees off the warmers, with an ambient temp in the 90′s the track temperature is likely near 120 degrees. This means your tires are not likely to cool-down on your way from the pits to the track, and once on-track there is more than enough heat created by track temp and tire deflection to raise the temperature (and subsequently the pressure) in your tires well beyond what is achieved by heating the tires with tire warmers.
Why does this all matter?
A tire must be within a certain pressure and temperature range to achieve optimum grip. If you as a rider, over the course of a season or multiple seasons, record things such as ambient temp, cold, warm, and hot tire pressures… you will be able to track and record how many PSI you gain or lose in your tires based on a given set of conditions.
Next time you’re at the track and refer to your notes, you’ll notice a time in the past that was similar and will have a reference to help you expect what will happen to your tires once you’re on-track, and you will be able to set your tires prior to your session (when warm off the tire warmers) based on an anticipated loss or gain in pressure so you ensure your tires are performing their very best while you’re out on track, turning laps, when it matters most.