TYRE PRESSURE AND YOUR VEHICLE
Tyres have a significant effect on a vehicle’s ride, comfort, handling, steering effort, traction, noise and fuel efficiency. There are many things to consider when selecting tyres for your vehicle and its intended application and there are also many things to know about setting up tyres for correct operation.
At Vista Hire, our staff are experienced 4WDers who know how tyre pressure can impact your vehicle's safety. If you're an existing client and want some advice on tyre pressure, just give us a call. Or if you're looking for a hire car company that provides sound advice and expertise, call Vista Hire on 1300 993 767.
Review the tyre placard
The tyre placard gives information about the maximum load carrying capacity per axle, the recommended tyre and rim size and recommended tyre pressures. All of Vista Hire’s rental 4WD vehicles are fitted with a tyre placard placed on the b pillar of the driver side of the car.
We like to use the recommended pressures as a guide or starting point, but it’s important to understand how your vehicle is setup, what accessories have been added, and the intended use of the vehicle.
You can see on the placard of this 2014 Toyota Hilux 4WD, that the unladen recommended pressures are the same for the front and rear axles. In this case they are 240kPa (kilopascals) which equates to 34.8 psi (pounds per square inch). You can also see that Toyota recommend that with a payload over the rear axle, rear tyre pressure should be increased to 300 kPa or 43.5 psi. Let’s look at the reason why this recommendation has been made.
What happens when tyre pressure is lowered
Where a tyre makes contact with the road, there is some amount of deformation from round.
We call this the contact area or contact patch.
The part of the tyre involved in this contact is the tread.
When pressure is lowered in a tyre, the sidewall bulges out, but it doesn’t affect the width of the patch that contacts the ground.
So it’s only the tread that gets longer, but not wider.
If you’re not convinced of this, see the prints that follow.
The following prints give an idea of the contact area of a BF Goodrich all terrain tyre (Vista Hire’s favourite all round tyre) on a Toyota Hilux at 40 and 20 psi.
Looking at the patches made by the tyres, we can see the width stayed consistent across the two pressures tested, but the length changed. We can work out the area each tyre has in contact with the ground, and from this, the approximate weight per square centimetre the 4WD vehicle exhibits on the road.
If we assume a GVM of 2,780 kg and to simplify the calculations we assume the vehicle is loaded for even weight distribution between all four wheels, we get (2,780 / 4 = 695) that each wheel exhibits 695 kg of downward pressure.
At 40 psi our contact area is approximately 17.8 cm x 22.2 cm = 395 cm2
This means 695 kg / 395 cm2 = 1.76 kg/cm2 of pressure is under each wheel.
At 20 psi we have approximately 17.8 cm x 26.0 cm = 462 cm2 of contact area.
695 kg / 462 cm2 = 1.50 kg/cm2 is under each wheel at 20 psi.
By reducing tyre pressure from 40 to 20 psi we have reduced pressure on the ground by 15%, spreading the load to effectively make the 4WD lighter in any one spot. The relationship between pressure and contact area isn’t linear, so making incremental changes below 20 psi will have a dramatic impact on lowering the downward force of your 4WD and this is why it is advisable to reduce pressure considerably when driving on surfaces like sand. (Please note, Vista Hire 4WDs cannot be hired for beach driving.)
Measure the Static Load Radius
Now because it’s impractical to measure the contact patch of each wheel, and we know there will be some difference in the weight distribution between the front and rear of the vehicle, particularly when a 4WD’s tray is loaded, it’s useful and easy to measure the static load radius.
The static load radius is the distance between the centre of the wheel hub to the ground.
Once a suitable tyre pressure has been chosen on a reference wheel, the static load radius can be measured and pressures adjusted on all remaining wheels (including trailers if running the same tyre and wheel) using the radius measurement. This is not an exact science, and further adjustment may be necessary, but it’s a good starting point and gives us confidence we have the correct pressure differential front and back for how our car is loaded.
Consider the vehicle accessories
Looking at the tyre placard on the Vista Hire Hilux we see a recommendation for 8.7 psi difference between front and rear tyres for a loaded vehicle. This is a significant pressure difference and highlights the importance of setting pressures depending on how your vehicle is setup. Bear in mind that if your vehicle has accessories installed such as a bull bar, winch, second spare tyre, tool box, internal or external ROPS, dual battery or anything that adds substantial weight to specific areas of the car, you will need to consider these add-ons when deciding on the pressures to use.
talk to us about tyre pressure on your rental vehicle
Our experience from assessing the wear and damage to tyres after covering millions of kilometres in all sorts of terrain, gives us confidence in the choice of tyres we fit to our hire fleet.
If you have any questions about adjusting the tyre pressures on your rental 4WD, the staff at Vista Hire will be more than happy to share their experience.
Need to hire a 4WD or want more information? Call us on 1300 993 767