Our Experts on: Tyre pressure, part 1
Given that tyre pressure is one of the few things about a tyre that you, the average driver, have direct control over, it’s no wonder that a lot of people take an interest in it. Tyre pressure can have a surprising influence on your car’s performance and fuel efficiency, so it’s worth understanding.
Originally, back in the days of bias tyres, your tyre pressure was highly important to the health of your tyre. If you over-inflated a tyre it wore heavily on the centre of the tread, and if you under-inflated it wore on the shoulders (edges) of the tyre. This was before those clever Frenchmen invented radial tyres and front wheel drive (traction-avant) motor-cars. The steel belts of a radial tyre are laid criss-cross under the tread, bracing it. The tread rubber, and the tread pattern particularly, are then held stable, and the pattern is able to perform its major function of providing drainage channels for water trapped under the tyre to escape
So what is the role of tyre pressure? First, and most obviously, it is the air that carries the load. The air pressure inside the tyre and rim creates tension, which allows the tyre to support the weight of the car. The quickest way of proving this is the rather extreme method of letting all the air out and seeing how far you get. Incidentally, a tyre has to be below about 17 p.s.i. for it to look “flat”.
We’ve established what not enough air can do, what about too much air? First off, it shortens the footprint (area of tyre in contact with the road), which lessens the time available for the tyre to drain water out from under it. In compensation, it increases the pressure under the contact patch, increasing grip. Whether it entirely compensates for the drop in drainage time is a matter entirely for the design of the tread pattern, the speed, depth of water, texture of road surface, and how much tread pattern depth remains on the tyre. Naturally, the less pattern depth, the more drainage is impaired- worn tyres don’t perform as well as new tyres in the wet.
With extra pressure in the tyre, a turning input from the steering will get to the road surface with more sensitivity as well. Things happen quicker and more predictably if the tyre is pumped up “hard”. The tyre distorts less in a sideways direction under extreme cornering forces too – sporting drivers say that the car “handles”. That’s something to notice – the steering response (ability to change direction quickly, as in negotiating a course of witches hats) gets better as tyre pressure increases.
High performance and premium quality tyres have complex bead and lower sidewall structures and low profiles, so that the message can get to the road quicker WITHOUT the necessity to run high pressures. A tyre with extra pressure can give a bone shaking ride, and transmit a lot of thumps and bumps from concrete road joins and the like to the cabin – not exactly ideal for the driver. There are more components in premium tyres to accomplish this feat, which is why they cost more.