Most everyone I speak to, except for the most rubber-covered caterpillar rats, all forget that there’s an incredibly easy, virtually free way to tune under- and oversteer to car. It’s not suspension, alignment, or even riding technique. It’s as easy as learning about tire pressure.
Tire pressure is so basic, and yet most drivers overlook regular tire maintenance, let alone view it as an adjustment point. For trackdays it is essential, but some basics are forgotten. Tire pressure tuning is not so much about tuning the sidewall flex in my opinion, but tuning the feel and grip, and learning what I want from the tire.
Many people will do a track session or canyon run and check the tire pressure when the tires are warm – the only relevant measure of performance driving. Cold pressure is just a starting point and hot pressure is where all the tuning is done. I’ll temper this by saying there’s something a little, erm, lame on tuning tire pressures for the street.
I’m not going to continue checking my pressure once I have a happy setup. If it makes you feel better, think about this hypothetical on the track. Honestly, there should be two different setups for the road and the track, as the requirements differ significantly between the two. Just don’t be the BMW forum guy who crashes his car into a curb and blames tiny tire pressure differences rather than poor handling. Anyway, we venture into my favorite area of feeling over absolute grip.
Tire pressure is a kind of black art in abstract terms. It’s just as deep and rich as the roll center or sway bars, but much less easy to understand due to the very specific nature of tires. Every tire manufacturer and every model has its own technique with hundreds of parameters. No two tires are ever the same in construction or composition. Tire construction is the part that makes tuning tire pressure less of a math problem and more of a practical one. But with any tire, pressure affects contact patch size, sidewall stiffness, and tire slip angle tolerance.
Slip angle is not a drift angle. Slip angle is simply the effective bending of the tire under load when cornering, more specifically the difference between steering angle and direction of travel on a given tire. Pressure tuning can have a major influence on this. Sidewall stiffness is also an important consideration, as it drastically changes steering feel and limits tire properties. The goal is almost always a more progressive breakout at the limit, and it all starts with tires.
That just means that I don’t have a good base figure for everyone, every car and every tire. All tires behave differently and finding a starting point takes some trial and error. I tend to start with a cold pressure of 30 psi, although your experience may vary. This also applies to a conventional front-engined car. Rear and mid-engine cars will have staggered tire pressures due to their weight balance and tire setups. If all else fails, start with the factory pressure and lower it from there.
Like anti-roll bars, the most important part of tuning balance with tire pressure is the relationship between the tires rather than just having a fixed tire pressure figure that is treated as gospel. In fact, it’s important not to work so much with math and just tinker until everything feels right, especially for people who aren’t trying to set the world on fire with lap times.
There is also a tire pressure issue that is somewhat counter-intuitive. Sometimes a higher tire pressure provides more grip and sometimes the opposite. It’s all about finding a suitable window where the tires want to work. Fortunately, tires are incredibly sensitive to pressure changes. A single psi makes a big difference, so tire tuning can be quick and easy. All you need to do is release the pressure in precise steps.
I like to start by letting some air out of the front tires to feel how they react. When on a track day I release a pound at a time until I feel there is less return or negative results like understeer or imprecise steering. There is a threshold, and part of the purpose of pressure tuning is to have as little hot tire pressure as possible while maintaining good grip, feel and wear. But only mess with one axis at a time. If there isn’t much difference, I reset the pressure and start fiddling with the rear.
According to this technical magazineLower rear tire pressure is beneficial for handling as it allows for a progressive breakout and a softer sidewall, allowing for some rotation and a sense of rotation. This theory is rock hard in my book and has worked well for me. I sometimes run my bum a pound or two less than my fronts, but I also use a staggered joint for extra frantic handling. Anyway I start fiddling with the back the same way I did the front until I notice if it likes the settings or not.
By doing it this way, I can see how the car handles pressure shifting forward and back, as well as even pressure all around. The car felt different on all three setups and I found a trail I liked.
You can ask questions about increasing the pressure above the factory recommended pressure. In my experience, this has never worked out well. I tried, and my car just felt like garbage. I’d tell just about anyone to start with the factory settings and work your way down, although I’ve found that race alignment shops often have helpful specs to start from factory. That’s how I started.
As far as I understand, tuning tire pressure can kill understeer on mid-engined and rear-engined cars more effectively than front-engined cars. I have no practical experience with such wealth, but my friends testify to it. Also keep in mind that automakers do everything they can to make a car safer for the consumer, even performance cars. Tire pressure is one of the tools used by engineers to make cars inert. There is always a pressure setting that makes the car more neutral. Taken to the limit, the car can become an oversteer monster. Autocross folks sometimes pump rear tires to 40 psi for maximum oversteer balance.
In the end it all depends on the car, the day, the driver and the road or track. I say it often and it never gets boring: go out and tinker. This is free, easy and can have a shockingly good effect on any car.