March 15, 2010 at 1:53 am Leave a comment

Our readership survey shows that the greatest interest in articles on our website relates to articles on tyre pressure. Since basically, this is the one parameter that the user has under their direct control, this is probably not surprising. So I thought that I would expand a little further on the subject.

The modern steel belted tyre has made a myth of many strongly held beliefs that go back in history to the days of bias tyres, sometimes called CROSS PLIES. Some of these were that under-inflated tyres wore on the shoulders, while over-inflated tyres wore out in the centre of the tread first. This was before those pesky Frenchmen invented radial tyres and front wheel drive (traction avant) motor cars. Today, front wheel drive tyres wear more rapidly on the front, with rapid wear both in the crown and the shoulders of the tyre, because it is steering and driving both at once; while on a rear wheel drive car, the rear tyres wear in the crown, and the front tyres wear on the shoulders, because that is where they are doing their main work- developing grip.

The steel belts of a radial tyre are laid criss-cross under the tread running at a low angle to the direction of rotation. They brace the tread. The tread rubber, and the tread pattern particularly, are then held stable, and the pattern’s major function of providing drainage channels for water trapped under the tyre to escape, is assured.

So what is the role of tyre pressure?

First, and most important, is that it is the air that carries the load. The air pressure inside the tyre and rim tensions the tyre cords in the casing, which allows the tyre to support the weight of the car. The quickest way of proving this is the extreme method of letting the air out, and seeing how far you get- like nowhere.

So assuming that there is air in the tyre, how much is enough?

According to the Tyre and Rim Association load tables, 4 tyres under a Holden Commodore inflated to 250 kPa (36 p.s.i.) can carry a load of 650 X 4 kilograms= 2.6 tonne.

Now you would admit that that’s a pretty overloaded Commodore.

Since a V6 VT/VX series Commodore with two passengers in it, with luggage for two in the boot weighs:-

Car tare weight 1665kg (V6), 1682kg (V8), when fully fuelled

2 passengers weigh 73 kg over the front axle, 63 kg over the rear,

Luggage for two………….27.2 kg

Total weight (mass) is therefore around 1845kg, give or take a few, according to the Tyre and Rim Association Standards. Optional extras also have to be added in as well.

This represents only 71% of the load carrying capacity of the tyres when they are inflated to 36 p.s.i., which is their maximum. And according to the Tyre Standards, only 23 p.s.i. is required to carry the load of the car when operated at normal highway speeds. With that load on it, and with that pressure in it, the tyre is the shape that the engineers designed for it to operate satisfactorily.

Yet the car engineers do not as a matter of course, specify pressures that are barely adequate to carry the load of the car and its occupants. So there has to be more involved than just load carrying. A placard for the VX/VT series Commodore appears below. Note particularly the pressures specified for the 225/60R15 tyres on this model- 29 p.s.i., and the 225/55R16 tyres at 26 p.s.i. under load conditions similar to those quoted above.

Note also that further pressure increases are specified for (a) extra load (all seats occupied, and their luggage too; and (b) extremely high speed (above 160 km/k) in this case.

Tyre Placard
Motor car engineers, and the driving public, expect that their cars will “handle”. This is not necessarily for

use at high speeds in today’s highly regulated road systems ( read radar traps) but is something else called steering response. A popular catch phrase for this a decade ago was “radial tuned suspension”. Nowadays, phrases like “turn in” get bandied around.

From the load tables, around 24 p.s.i. is enough to satisfy the tyre engineers that their tyre is inflated to its design shape that will be durable, and hold together under the loads the tyre is carrying. With such a pressure, the modern car would handle like mush, and drivers would not be happy. Car engineers specify pressures in the region of 28 to 34 p.s.i, with appropriate corrections for high speed. A tyre placard which specifies the pressures for a Holden Calais appears below. You will note that these are up in the 35 p.s.i. range for the same sizes as were fitted to the VT/VX series above.

Note also that NO pressures increases are specified for a full load, only for speeds above 160 km/h. In other words, the standard inflation pressure can cope wit high load conditions.

The Calais used to be thought of as half way to a limousine ride, but with 35 p.s.i. in the tyres, the limousine buyer will be disappointed. The steering response (ability to change direction quickly, as in negotiating a course of witches hats) is extremely good. Drop the pressures to 29 p.s.i., and it’s a different motor car. Softer ride, not nearly as twitchy for the type of buyers who buy a Calais. After all, these are the type of buyers who want a bit of luxury, and all the mod cons added in.

So what does extra pressure do? One- it shortens the footprint, 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. How much, and whether it entirely compensates for less drainage time, is a matter entirely for the design of the tread pattern, the speed, depth of water, 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.

The second thing that air pressure does, once the load and speed requirements have been met, is stiffen the sidewalls. Experienced tyre men caught without a pressure gauge, compress their thumb into the sidewall in the shoulder area, which has no bead stiffeners, and no steel belts in that part of the tyre. They get very good at estimating pressure in the tyre. Incidentally, a tyre has to be below about 17 p.s.i. for it to “look flat”.

So a turning input from the steering, down through the steel rim, and the flexible rubber sidewall, will get to the road surface where it has to do its work, with more sensitivity- 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”.

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, which can give a bone shaking ride, and transmit a lot of thumps and bumps from concrete road joins and the like to the cabin. There are more components in the tyre to accomplish this, which is why they cost more.

So what caused the engineers who designed the VE Series Holden Calais to specify such high pressures. The answer will be found in the delays that accompanied the release of the new VE model Holden. Loaded with extras, the VE Calais did not meet the intended design parameters for fuel consumption. It ended up with the engineers chasing a figure 0.1 litres per 100 km lower. The solution was to increase the tyre pressures. This lowers the rolling resistance of the tyres, and improves fuel consumption. There is a lesson here for all of us, if your fillings in your teeth can stand the ride. Increase the tyre pressures, and your fuel consumption will reduce. Even maintaining the pressure at the intended design pressure will pay off in lower fuel consumption.

So have a look for your tyre pressure placard somewhere on the car, or in your car handbook. Mostly, it’s on the driver’s door or door pillar. It specifies the pressures that the car engineers are happy with to make the car ‘handle’ the way they want it to. If the fillings in your teeth can’t cope with it, accept that there is a penalty involved, paid for at the fuel bowser.

Or else just call it a “Sports Calais”!

DISCLAIMER.

The writer drives a VX SS Commodore, and would just love to get his hands on a VE 6 litre “SS”, because “it handles”.

Entry filed under: Tyre Industry, Tyre safety & maintenance, Tyre Technology. Tags: , , , , , , , , .

Correcting Drive.com.au article on tyre sizes from Jan 22 Golf GTi teams up with Pirelli

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