Technical Topics No. 5.

TYRES AND TYRE PRESSURE:

In light of the number of tyre punctures encountered on a recent joint club trip I have put together some information about tyres and tyre pressures. In all there were 7 punctures experienced, 4 of which occurred to Goodyear Wrangler tyres. As far as I know the NSW Jackaroo Club was the only club at this tri-state gathering that experienced punctures.

First of all I need to make it clear that I am not a tyre expert and the suggestions contained in the following article are from what I have learned from experience and from other four wheel drivers. It seems that selection of the appropriate tyre pressure for travelling on non sealed roads is a matter of judgement backed by experience rather than a science.

Tyre Placard.
The recommended cold tyre inflation pressures on the tyre placard of the Jackaroo is as follows:

Tyre 245/70R16

FRONT.

REAR.

210 kpa. (30.5psi)

240 kpa. (35psi) Up to max load.

Tyre 225/75R16 or 235/70R16

FRONT.

REAR.

210 kpa. (30.5psi)

260 kpa. (38psi) Up to max load.

Travelling on sealed roads.
The tyre pressures to use on these roads are those listed on the vehicle's tyre placard, remembering these are cold inflation pressures. If you are inflating your tyres at the local garage, when the air in the tyre is hot you should add an additional 28 kpa (4 psi). It is strongly recommended that you acquire your own tyre pressure gauge as many of those used at service stations are inaccurate. On the Jackaroo the tyre placard lists the rear tyre pressure as 240 or 260 kpa depending on the tyre fitted. If the vehicle is being used unladen, say going to and from work etc, this rear tyre pressure could be adjusted downwards to say 20 to 30 kpa (3 to 4 psi) above your front tyre pressure to attain a more comfortable ride and reduce centre tread wear. However, when a load is added to the vehicle, rear tyre pressure should be increased to that recommended on the placard. Some vehicles have drawer systems fitted and carry tools/recovery gear at all times and if this is the case with your vehicle you will need to experiment with rear tyre pressures to get the best results.

The above pressures apply to Jackaroos and for other members who drive different vehicles the rear tyre pressure could be adjusted to say 20 - 30kpa (3-4 psi) above the front tyre pressure that you use when the vehicle is unladen.

To check if you are running the correct tyre pressures measure them cold and then again when they are hot. If you are running with the correct pressures the hot pressures will be 28kpa (4psi) higher compared to the cold reading. You will need to drive the vehicle at least 2 to 3 kms at moderate speed to get the hot tyre pressures. If the measurement between hot and cold is greater than 4 psi your cold pressure is too low. Similarly if the difference is less than 4 psi the tyre pressures are too high. You will need to use the same tyre gauge to get an accurate comparison.

Correct tyre pressure provides proper sidewall deflection and safe tyre operating temperatures. Under inflation results in excessive heat , reduces tyre life , lower load carrying capacity and leads to possible tyre failure. Under inflation also increases side wall deflection increasing the risk of a sidewall puncture and increases rolling resistance. Over inflation makes the tyre more puncture prone in the tread area.

Stony or Gravel Roads.
There are a number of factors that can cause tyre puncture on stony or gravel roads the main ones being as follows;

• Under inflation. Rocks or other sharp objects such as tree roots can more easily pierce the tyre side wall. Driving over sharp objects lying in mud holes can result in punctured side walls.
• Over inflated tyres. Running over inflated tyres can result in punctures in the tread area as the tyre looses it's ability to mould to the shape of the stone.
• Using highway oriented tyres, such as Goodyear Wranglers off-road or on stony roads. I consider high speed rated highway tyres puncture prone.

For other mid size makes of 4WD's the same approach as outlined above could be used as a guide. Reduce the rear tyre pressure to around 225/230 kpa (32/24 psi). For the larger 4WD (Nissan and Toyotas) reduce the rear tyre pressure to 250/255 kpa (36/37 psi) as a starting point. Again, your travelling speed will have to be reduced accordingly, especially on the larger 4WD's with their large carrying capacity.

In any exercise in tyre pressure reduction it is critical that you observe sidewall deflection (bulge) and if it appears excessive then increase your rear tyre pressures to reduce the deflection - this would be the guiding rule.

Once you come back on to a sealed surface or a good dirt road where you will travel at higher speeds you should increase your tyre pressures to minimise heat build up and possible tyre failure.

To facilitate tyre pressure changes it will be necessary to have access to a 12 volt electric or the old fashioned human operated air pump, the latter being a museum piece. There are many electric operated units on the market. Some though, like Big Red, are expensive. Generally you get what you pay for with the more expensive pumps allowing tyre inflation at a much quicker rate. In my experience I have noticed that some of the cheaper pumps have much difficulty attaining pressures over 240 Kpa (35 psi).

Please remember that when travelling with reduced tyre pressures you must reduce your speed to minimise heat build up in the tyres and to allow for change in steering response etc.

It is recommended that if you are going on a trip and you have tubeless tyres buy yourself a plug repair kit. Please remember that any such field repairs must be considered a temporary fix and the tyre should be presented to a tyre repairer or garage for a proper examination and the fitment of a permanent repair. It may transpire that the tyre cannot be fixed properly once an internal examination have been carried out by a qualified person.

There is one more factor that can result in a puncture. It is called bad luck and it can happen to any one anywhere. You can take all the right precautions and still end up with a puncture.

Finally, one practice I make is that whenever I stop, morning tea, lunch etc, I walk around the vehicle checking not only the tyres but look for other possible problems like leaking fluids, sticks caught up underneath, loose radio aerial etc. A practice that has paid dividends over the years. Another practice I use is to keep an eye on the vehicle in front where ever possible as I might observe that vehicle is getting a flat tyre before the driver does - sometimes you can travel on a dirt road for some distance before you realise that you have a flat tyre.

Tyres are the vehicles contact with the ground and it is important that we look after them. Tyres should be inspected for physical damage (don't forget BOTH sides of the sidewall) and the pressure checked regularly. Tyre wear should be checked every 5,000 Km or less. Irregular wear can be due to incorrect wheel alignment, or incorrect tyre pressures.

Please remember that the above quoted pressures are a guide only and common sense should be used in relation to tyre care and driving technique when travelling on rough stony roads. All the best in the bush.