Tyre Pressures are one of the most critical aspects to the performance of a car. Both on the road and the track they make a huge difference to your experience, and for a race team a couple of PSI off and you can be miles off the pace.
For a road car, most of the work is done for you. Manufacturers spend thousands of hours in Research and Development perfecting a tyre and tyre pressure combination to work with every vehicle they release. All you have to do is look up the recommended pressure (which should be either inside the door shut, fuel cap or definitely the owners manual). Manufacturers often specify another pressure if the car is heavily loaded.
Making sure these pressures are correct impacts the handling of the car, but also noticeably on the road, the fuel economy. Low pressures cause the tyre to have higher rolling resistance due to a phenomenon known as hysteresis. The more the tyre deforms as it rolls (which it will do with lower pressure) the more energy is lost as heat. I don’t recommend this, but if you drive for a few hundred metres with a very low or flat tyre, it will get extremely hot. The tyre doesnt have to be flat for the effect to be noticeable though - a couple of PSI low and you will be giving away precious fuel economy. And yes, if you over inflate the tyre, you can actually improve fuel economy - more on why you might not want go too far in that direction next.
In terms of handling the tyre pressure affects two things - the size of the contact patch (the part of the tyre that is touching the road) and the stiffness and overall ability of the tyre to maintain its shape. A low pressure will give you a larger contact patch - its easy to imagine this if you imagine the extreme example of a flat tyre - alot more rubber is touching the road. Larger contact patch means more grip, because there is more tyre on the road. This is why drag cars generally run low tyre pressures - bigger contact patch and more grip off the line. Low pressure also means that the tyre will have less stability and stiffness - this means that during cornering, your tyre may deform so much that the tread of the tyre is no longer in contact with the road, reducing the contact patch and also damaging the sidewall, the rim, and in the worst case scenario causing the tyre to detach from the rim completely. Not good.
Increasing the pressure of the tyre generally reduces the contact patch. As the tyre ‘balloons’ from the higher pressure, the centre of the tyre tread sticks out more and the area around the sidewall can fail to make proper contact with the road. This again is an undesirable effect, as ideally the contact patch needs to be maximised. The higher the pressure the stiffer the tyre becomes - this can have a positive effect on steering feel and responsiveness. It also means the tyre deforms less under load, which can allow for more stable behaviour in the corners.
Somewhere in the middle there is a best compromise - a pressure that provides adequate stiffness for the tyre, whilst also achieving the maximum contact patch. While drivers may subjectively prefer one pressure to another, there will generally be an ‘ultimate’ pressure which provides the best performance in terms of grip. Generally a very good place to start is the manufacturer recommended pressures (even for track work or racing). An exception to this would be if the car is extensively modified (huge weight reduction or slick tyres etc) but generally if that is the case you know what you are doing with pressures, which will be determined through testing!
Track use brings up a few issues with setting the pressures. On track the load going through the tyres (and to a certain extent the brake components) generates a large amount of heat. This heats the air in the tyre and makes it expand, increasing your tyre pressures - this means that if you go to the track, set your pressures at 30PSI and head out for some laps, by the time you come back in the pressures may be at 40-45PSI and the driver will have a noticeable lack of grip. It is for this reason many trackdayers experience horrendous tyre wear and noticeable grip degradation through each session.
The pressure the tyre actually operates out on track is known as the ‘hot pressure’ or ‘operating pressure’. It is relatively easy to set this if you have the luxury on a track or test day - the driver goes out on full attack for several laps until everything is hot, and then comes straight into the pits and the tyre pressures are ‘bled’ down to the correct hot pressure. It is good practice to check and bleed (if necessary) every time the driver comes into the pits.
After the session, if you let the car fully cool you can get an estimate of the ‘cold pressure’ or ‘set pressure’ (the pressure you set the tyre to when cold to achieve the correct hot pressure). Remember that the cold pressure is only right for that particular day, track, setup and conditions. Certain tracks put more load through certain tyres, ambient temperatures, and of course wet vs dry conditions as well as driving style all play a huge role in the cold pressure required. When you have operated a vehicle on the same tyre over a number of tracks for a period of time, you will get a good feel of where your cold pressures have to be to get the hot pressures right out on track. This is very important in racing, as even if you do get to set the pressures in qualifying, conditions can change when it comes to the race. Races are won and lost on tyres, this is as true in club racing as it is in Formula 1, and pressures are a huge part of the equation.
To give some rough examples, on the Civic Cup Car we generally aim for a hot pressure of 30PSI all round (very close to the road pressures recommended by Honda), which equates to roughly these cold pressures:
Front: 21PSI (Dry) 25PSI (Wet)
Rear: 24PSI (Dry) 28PSI (Wet)
As you can see, our car needs lower front cold pressures - this is typical as front tyres tend to work harder in many cars, but front wheel drive also increases the temperatures. These are by no means gospel figures - if we can we will always set the pressures 1-2PSI higher than this (depending on the conditions) and then bleed them down when hot to get the right value.
If you are a regular trackdayer it is well worth your while investing in a high quality pressure gauge with a bleed button. This lets you quickly get around the whole car and bleed the pressures. Take a friend along and get them to help you out, by the time you have got out the car the tyres will have cooled enough to affect the readings. Buy a proper motorsport gauge from a reputable dealer such as Demon Tweeks (we can recommend a few). Even for the road you would be amazed how inaccurate some of the inflators are - we often measure 3-4PSI less on our gauge than our electronic inflator claims.
To wrap it up…
On the Road:
Check your pressures regularly, inflate to manufacturers specifications, increase the pressure if your car is fully loaded and the owners manual suggests you do so.
Use the manufacturers recommended pressures - set them hot. Remember your cold pressures then you will have something to work from next time.
You should know what to do. Test test test!