Tyre Pressures

I first played around with tyre pressures back in the 60's.

One particular vehicle that was well suited to my liking of off road driving was a little 1961 Austin A40 Farina Countryman (mechanically just a Morris Minor 1000 with a station wagon body) identical to the one shown in the adjacent photos.

Back in those days 4WDs were fairly scarce, I can only remember Series I and II SWB Land Rovers in any number also the odd Datsun Patrol and early Landcruiser and surplus LHD jeeps around, none well suited to day-to-day transport as they are today. 

Having a liking for forest trails and beach work particularly around the Coorong in SA (a National Park these days), I soon learnt the importance of tyre pressures.

There was no such thing as all terrain and mud tyres back then, the nearest thing were ones referred to as "winter treads" that were normally only fitted to the rear driving wheels.  The tread pattern as best I recall would have been similar to today's A/T tyres. 

After getting bogged in sand a few times I soon realised that reducing pressures on all four tyres was a huge improvement, so much so that, due to the light weight of the vehicle at around 800 Kg, sand was no longer a problem, it never bogged down, it just sat on top.

Reducing pressure resulted in a wider but more importantly a much longer footprint greatly reducing the psi loading on the ground.  Typically  a 250 to 300% increase in the footprint area is achieved.
 
Not only was the car not sinking into the soft ground but a much larger contact with the ground meant much better traction.

It also give an equivalent to a small reduction of the gear ratio due to reduced rolling tyre diameter, enough to overcome the greater rolling resistance.

This same strategy was just as successful on the Coorong's salt flats where a crust so thin you often couldn't walk on it hid a base the consistency of thick custard.   In both of these environments I could and did literally drive circles around the odd bogged Land Rover, admittedly, in those days they only had narrow (by today's standard) bar tread tyres.   (Couldn't get near them in the mud though.)

Tyres then were cross ply with tubes meaning lowering tyre pressure was a fairly safe affair.   The technique was to lower the pressure watching the tyre bulge to the point just before it creased.    Around 5 psi was a good figure on the rear and one or two more on the front just to lighten the steering a little (no power steering back then).   Slow driving was the order of the day and suited the terrain we drove on very well.

However, I soon learned while touring around the Flinders Ranges north of Hawker that lower pressures were also essential on their sharp stony tracks too but needed to be much more precise.

It soon became apparent that a lower pressure that enabled the tyre to form around the rock and reduce the point loading was imperative (but you have to be a lot more cautious today with radial ply tyres not to exposed the more vulnerable side walls).   For the tyres that I was using at the time, around 18-20 psi worked well.
An additional benefit apart from reducing tyre penetration risk, the tyres climb over rocks much easier. (Imagine a solid wheel with no "give", even a small obstacle will stop it whereas the same size soft tyre will deform and climb over the same obstacle.)   

For mud, with the "winter treads", 20 psi was worthwhile and normal highway pressure was 30-32 psi.
The normal drill was to drop pressure to 20 psi when going off road and then down further if you needed to.

You needed to be careful since it was still possible to sustain minor tread or sidewall penetration on stony ground and because the tube was rarely damaged this could go un-noticed.   This was a potential safety problem if it went un-noticed after the tyre was re-inflated, a blow out was likely on the highway.   It only happened to me once and fortunately I noticed the small split while re-inflating.

Back then, the best, fastest, most reliable tyre pump was one that used one cylinder of your engine as an air pump (you remove one spark plug and screw the hose/air inlet valve fitting into the spark plug hole).
This resulted in tyre inflation that could better any 12V pump today, even at idle speed.   Coupled with the fact that they were tiny (by today's standard) 5.60x13 tyres, re-inflation was very rapid.
In those days with ohv pushrod engines and cast iron heads, frequent use of this type of pump was quick and easy and caused no problems.

This little Austin took us around much of SA, Vic and NSW and to places where only a 4x4 would go today and never once had to call on outside assistance.   I did lose track of how many times that pump was screwed into the engine though.

Every vehicle I have had, from a campervan conversion I did back in the 70s, to vehicles I have used for camping, towing boats and caravans have at some stage benefited from the altering tyre pressures to suit off road conditions.

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Fast forward to the 21st century and we have off road capability over a wider range of terrain that was not possible then for the general public.
But tyres still need to be carefully selected to suit what you hope to do and appropriate tyre pressures are just as important as they ever were.

The construction of cross ply tyres is very different from radial ply, with cross ply design the sidewalls and tread are basically one, resulting in a rigid high aspect ratio tyre.    I am much more particular today with radial tyres than I was back then with cross plies when it comes to off road tyre pressures.

When traveling off road today, I would never ever use any tyre that wasn't rated and stamped LT or Light Truck, never ever.    Non LTs are OK on the highway and in sand but that's about all.
Our Jackaroo, all fueled, loaded up and ready to go is more than three times the weight of the little old Austin and tyres need to be so much more robust.

Passenger rated tyres are just not up to a life off the bitumen today with our present heavy vehicles.   As I have said elsewhere, P rated tyres used off road usually have a life expectancy measured in days not kilometers.

You should however be aware when replacing your P rated OEM tyres for LT ones that the vehicle tyre placard will likely no longer be correct.

This may sound counter intuitive but to carry the same load, the LT tyre will require a higher pressure than was for the P rated tyre.

On the right is a graph generated from the data I took from the Tire and Rim Association tables for tyres that suit our Jackaroo (Trooper).

The method of determining the new pressure is as follows (I am using the Jackaroo as an example from the vehicle tyre placard) . . . . .
Front tyre pressure - 210 kPa,
from the graph, 210 kPa supports 890 Kg (this therefore is the vehicle manufacturers requirement, the tyre must be inflated to support 890 Kg), 
again from the graph, to support 890 Kg, the 245/70 LT tyre must have 320 kPa, and a 225/75 LT must have 355 kPa.

Therefore, whereas our original P tyres were 210 kPa (30 psi) front and 240 kPa (35 psi) rear, the LT tyres need to be 320 kPa (46 psi) and 345 kPa (50 psi) respectively to support the same load.

A warning in the same document . . .

And a quote from Toyo on replacing P rated with LT rated tyres "LT-metric tires have to be inflated to a higher pressure than that described on the vehicle's Tire Information Placard."

But be warned, nearly all tyre outlets I spoke with were extremely ignorant of the need to increase tyre pressures (let alone by how much or how to calculate the new figure) when changing from P to LT tyres as they were for the 9% de-rating factor if your vehicle specifies LT but you want to substitute a P tyre.   See In Search of a Tyre


This will always get an argument but I would only consider using what I consider to be the best all terrain tyres on the market, B F Goodrich All Terrain A/T, Bridgestone D694 or Toyo M55s, all LT all terrain tyres of course.
And no, I don't get a zack from anyone for saying that.    

With our Jackaroo and using the BFGs, pressure was 44 psi front and 48 psi rear, on the rare occasions on sand I deflated to 20 psi and on stony trails and tracks (that is in most forests) I'll drop to 30 psi.
I'll use this as a starting point with our new Bridgestones.
(We got 87,000 Km from our previous BFG A/Ts with a uniform 4.5 mm tread depth remaining with more than 50% of that mileage towing our caravan.)

With our caravan that has 195/R14 LT 8 ply tyres with a load rating of 950 Kg @ 60 psi, I run 58 psi on sealed roads, 30 psi in sand and 45 psi off road.

Following this strategy, I haven't lost a single tyre since using LTs, not even a puncture except on one occasion when I picked up a nail around town.

Off road tyre pressures could be a science if you could factor in all variables but since this is not possible, it will remain an art form based on the terrain, weight of vehicle, type of tyres and what works for you (driving style) and what doesn't over a period of time.

It is imperative however that tyres be returned to correct pressures as soon as possible and at all times on sealed roads, it will greatly reduce tyre temperature and therefore wear not to mention safety and better fuel economy.
Goodyear estimated (Publication No. NHTSA-2000-8572-26) that for every 10 kPa (1.5 psi) drop in inflation below optimum pressure, tread wear would increase by 2.6% over the life of the tyre.


Another method to determine optimum on-road pressure since this will depend on the load carried by the tyre is from the NRMA web site (a lot of other good info there too).

A method often seen to estimate pressure is one derived from a simple ratio formula involving maximum tyre design load, actual tyre load and maximum design tyre pressure.
While this method, which gives a linear relationship, gives a rough approximation when the tyre is in the upper limits of its' load, it is less accurate for a lightly loaded tyre. (Compared to TRA pressure/load tyre tables.)
It does however correctly determine that a LT tyre needs higher pressure than otherwise identical P tyre to support the same load.

A quote from Bridgestone USA  http://www.tiresafety.com/maintenance.asp

"Inflation Pressure 
Proper inflation pressure is essential for achieving maximum performance and mileage. Improper tire inflation pressure can cause severe internal tire damage, which can lead to sudden tire failure and resulting in serious personal injury or death. Improper inflation pressure may result in rapid or irregular wear. Pressures should always be checked when the tires are cold and at least monthly. Under normal tire operation, approximately 1psi of tire pressure will escape every month. Also, for every 10 degrees F change in ambient temperature, tire pressure will change by approximately 1psi.
Vehicle manufacturers list recommended tire pressures for original vehicle tires in the owner's manual or on a placard on the end of the driver's side door or in the glove box. 
For continuous high speed driving, tire pressures should be increased by 3 to 5psi above the normal cold inflation recommended."
However, for passenger tires, never exceed the maximum inflation pressure molded on the sidewall. The inflation pressure for light truck tires may exceed that molded on the tire by 10psi. Any recommended front to rear pressure differential should be maintained."

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In conclusion I must add that I would never go anywhere without a decent 12V pump, air pressure gauge, a full repair kit and on holidays when we will be going off road, also a larger 72 lit/min air pump such as the one shown on the right as well as my home made bead breaker. 
And an inner-tube, just in case I run into a tyre that is difficult to re-seat on the rim, (I don't subscribe to the LPG or petrol and match bead sealing technique).
If you don't have them, you know what Murphy's Law says.



Finally, one of the cheapest, handiest gadgets you could have is a tread depth gauge such as the one I use shown here.   With this compact little gadget you can detect tyre wear problems due to incorrect alignment and pressure very early.   
A simple moving plunger type with a calibrated scale, it can resolve 0.5 mm variations and costs around $7.   Very quick and simple to use.  
I check the inside, centre and outside tread depth of all four tyres usually every month or so at home or every week while away, takes less than two minutes.
With 4x4 tyre rubber costing between $20 and $30+ per mm per tyre (2009), it's worth keeping an eye on and not just relying on a 10,000 Km rotation to pick up any problems. 
(Due to its small size, about 75mm long, it's much more convenient to use than a vernier caliper.)


I also have a page concerned with the selection of my most recent tyres.

  
Below is a pressure conversion table
 and
below that a speed and load rating table,

but as I have said elsewhere, they may contain errors.