Our Experts on: Tyre pressure, part 2

March 3, 2008 at 8:11 am 4 comments

We explored Tyre Pressures on the CarbonBlack blog in the last post. Now, let’s move on to a specific example, using our newfound tyre knowledge for some detective work.

First, consider the tyre pressure for a VX/VT series Commodore. 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 2.6 tonne. That’s a pretty overloaded Commodore. With two passengers and luggage, you’re weighing in at about 70% of that. According to the Tyre Standards, that means you only need about 23 psi in the tyres of your average Commodore.

But if you look at the actual tyre placard on a Commodore, it says to inflate the tyres to 26-29 psi. Why the difference? First off, because engineers generally don’t specify pressures that are barely adequate – that way lies trouble. Secondly, a Commodore with tyres inflated to 23 psi would handle like mush. As we’ve noted before, the higher the pressure, the better the handling.

Here’s where our detective work comes in. What if we look at the tyre placard for a Calais? For those not acquainted with it, the Calais used to be thought of as half of a limousine ride – a luxury car. However, if you look at the tyre tables, the tyre pressure specified is 36 psi, significantly higher than the Commodore. That gives you some quite twitchy steering, and means you feel a lot more bumps – not exactly what’s expected when you buy a luxury vehicle.

So what caused the engineers who designed the VE Series Holden Calais to specify such high pressures? GMH is very coy about the weight of the car- it cannot be found in the owner’s manual, only the maximum weight permitted over each axle. The answer might 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. Reportedly, the engineers were chasing a figure 0.1 litres per 100 km lower. Their solution apparently 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 the fillings in your teeth can stand the ride – increase the tyre pressures, and your fuel consumption will reduce. Even maintaining the pressure at that intended 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.

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: Our Experts, Tyre safety & maintenance, Tyre Technology. Tags: , , , .

Our Experts on: Tyre pressure, part 1 Our Experts on: Hot Tyres and the Grand Prix

4 Comments Add your own

  • 1. Our Experts on: Tyre pressure, part 1 « The Tyre Blog  |  March 3, 2008 at 8:13 am

    […] Next on the CarbonBlack blog, we use our newfound knowledge of tyre pressures in some detective work. […]

    Reply
  • 2. David Matthews  |  March 9, 2008 at 4:39 am

    I weighed an 18 month old VE Calais recently. It weighed 900 kg over each axle, total weight 1780 kg (experimental error?), fully fuelled, no driver or passengers. Add in 73 kg for driver and passenger on front axle, 63 kg on rear axle, and 27.2 kg for luggage, total vehicle weight adds up to 1943.2 kg in ‘normal trim i.e. 2 up, with luggage.
    This makes it around 98 kg heavier than the model it superseded.

    Assuming a 50/50 weight split front to rear, then the weight on each tyre is 486 kg. To carry this load, as if that were the only criterion, which it is not, the pressure required at normal speeds is 22 p.s.i. (150 kPa) in the 225/55R17 97V tyres fitted.

    Since new, the car has done 30,000 km of country miles, the computer shows 9.7 litres per 100 km, running the 36 p.s.i. specified. Not real bad for such a big car, loaded with extras! The new owner has set his tyre pressures now at 28 p.s.i. to get a better ride, and re-set his computer. I’ll report in another 20000 km or so on fuel consumption. He’s much happier with the ride, after all, a 70 year old is entitled to a bit of comfort!

    Reply
  • 3. Terry Benson  |  October 4, 2009 at 9:42 pm

    Dear Sir,
    I have a 1996 VS Commodore Acclaim sedan could you please recomend tyre pressure for a couple and two suit cases in boot for a long trip.
    Regards Terry Benson

    Reply
  • 4. David Matthews  |  October 29, 2009 at 6:30 am

    Terry, You’ll find the recommended tyre pressures for your 1996 Holden on the tyre placard on the drivers side door. The pressures you would choose would be the “Normal Load” pressures, unless you intend to drive very quickly (above legal speeds) in which case you would choose the “High Speed” pressures.
    2 passengers and their luggage is “Normal” But if you want to save fuel as well, put an extra 4 p.s.i. ( 30 kPa) in them for your trip. Have a good one!.

    Reply

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