Tyre pressures

[Whitehorse]

Hi guys,
What tyre pressures are people running on dirt/gravel. I've looked on the forum but it seems most of the responses are about towing and sand.
On my last trip I didn't air down and I was fine but it was pretty firm and dry. I'm expecting softer and wetter conditions this time around.
Cheers, Whitehorse

[damos75]

For me, I generally go with the following.

Around town - 40psi
Dirt/Gravel - 30psi
Really rocky stuff etc - 25psi
Sand - 14psi

[Merts]

It really depends on your specific setup.

How heavily loaded is your rig?
What size tyres are you running?
What construction and type are they? LT or P?

For example, for my setup.
Front axle load is about 1260kg, and rear 1460.
Based on those loads, my 265/65r17 LT tyres are ideally at around 36psi front, and 42 rear (cold pressures) for on-road driving. These figures are calculated using ratios between the tyres recommended maximum loads and pressures, and the actual loads.

Off road, for general rocky and/or muddy tracks, I drop to low 20s front and mid 20s back. Say about 23 and 27.

For lots of soft mud, or crawling over rock steps, I might go down a bit further, 20 and 24.

For sand, also 20 and 24, but if it's very soft I might drop to 15 and 18 (warm pressure) to get up a particular dune or through a dry and soft area.

[JohnFromPerth]

I have been using the suggested pressures in the handbook of 29 front and 32 rear.
May go up 5 pounds all round if carrying a big load.
Suggested pressures seem a good compromise. Plenty of grip on wet bitumen and good on dirt roads.
Using as supplied 265/60R18 Bridgestones.
In general I think most 4x4 drivers over inflate their tyres. This results in less grip, centre wear on tyres (hard to see) and harsh ride. Also makes the tyre prone to cuts in stoney conditions.

[Jamoer]

Do you fuys feels jittery when running over 33psi on both front and rear? Mine felt jittery when we drive over uneven asphalt patches.

[dragonjon]

ive had my sport since january 2016
always 36psi allround still on original tyres 46000 klms
no handling problems at all,
tow a caravan sometimes never have changed pressure

[ron2503]

Do you fuys feels jittery when running over 33psi on both front and rear? Mine felt jittery when we drive over uneven asphalt patches.

Mine was very jittery especially over 80k's. Upgrade suspension and no problem since.

[Jamoer]

Mine was very jittery especially over 80k's. Upgrade suspension and no problem since.

May I know what suspension upgrade did u do? I am currently running 24s and quite jittery. Changed the shocks using Silver Bilstein but still jittery.

[JohnFromPerth]

I run 33psi front and 36psi rear with a moderate load.
Thought handbook pressures looked like just for a soft and comfy ride.
No jitters or any other problems at those pressures on tarmac and hard gravel.
Would go back to book pressures for rocky tracks or mud and as low as 15 psi for soft sand.

[ron2503]

I have Outback ArmourTrail front and back. The difference from original is like chalk and cheese.

[denisp]

Put BFG ATs (LT) tyres on our PS from new.


Inflated to 36ps all round as best first guess, 20% higher than labelled for highway (P) tyres.


Recent recommendation from BFG is 45psi all round which is 50% higher than PS labelled tyre pressure.



Thought that as a bit high so only inflated to 42psi. Fuel use is lower with the higher pressures.


Twice today on damp roads rear wheels spun, luckily straight line accelerating from stop.


Will drop pressure to 40psi tomorrow and check again.


Denis

[old Jack]

Hi Denis,

According to the Toyo Load Inflation Tables, a LT 265/65R18 tyre inflated at 35psi will carry a weight of 832kgs so that is 1664kgs per axle. Since the Pajero Sport has a maximum front axle load of 1260kg and a maximum rear axle load of 1600kg, your tyre's are well and truly inflated enough at 36psi. I would say they were over inflated unless you were fully loaded at GVM and GCM. Sure you gain some extra economy by over inflating your tyre's but it is at the expense of ride comfort, extra load on shocker bushes and a decrease in the tyre's ability to flex like it is designed to do for optimal road handling and increases the chance of tyre damage if you hit a pot hole and spoon drain at speed. On an unladen PS 32 psi in the front tyre's and 30 psi in the rear tyre's when running LT construction tyre's is more than adequate.

OJ.

[JohnFromPerth]

Old Jack is right. Most people over inflate their tyres.

I run 33psi front and 36psi rear for moderate loads.

I have a tyre pressure monitoring system, the screw on type that replace the valve caps, with a receiver that plugs into the lighter socket. Gives tyre pressure, tyre temperature and battery voltage.

It is interesting to watch the tyre pressures rise as you drive and the tyres warm up.
After half an hour or so of highway running my tyre pressures stabilise at 37 psi front and 40 psi rear.

So if inflating your tyres cold, realise that they are going to run at least 4 psi higher when warm, and even higher if it is a hot day.

[nj swb]

I have a tyre pressure monitoring system, the screw on type that replace the valve caps, with a receiver that plugs into the lighter socket. Gives ... tyre temperature...

No, it doesn't. The overwhelming temperature influence in these sensors is ambient temperature, and provides little useful information about what's really going on inside the tyre. With internal sensors I can see temperatures more than 20 degrees above ambient where travelling companions in similar vehicles with external sensors are lucky to show 5 degrees above ambient.

So if inflating your tyres cold, realise that they are going to run at least 4 psi higher when warm, and even higher if it is a hot day.

Yes, tyre pressures are generally always specified as cold inflation pressures, not hot, which raises the issue of cold inflation at what ambient temperature? As you note, ambient temperature will change the tyre pressure without turning a wheel. High school physics tells us that going from cold overnight to a hot day can produce a 10% change in tyre pressure, before you bring in effects of the sun bearing directly on a tyre - an hour after dawn I've seen a 2psi difference from one side of the car to the other.

Factors like these can change tyre pressures before the car starts to roll, which is why I believe that being too hung up on pressure and pressure rise can be a waste of time. A real time tyre pressure monitor that enables the driver to keep an eye on pressures throughout the day is invaluable - I wouldn't like to be without one again.

[JohnFromPerth]

Quite right nj swb, pressure monitors on the valve stem do give mainly ambient temperature.

However, the one time I have had a puncture with this system, the temperature did rise as the pressure dropped, so some heat must be transmitted through the valve stem. And yes I do see temperature variations depending which side of the vehicle is in the shade, but that is probably just sun on the sensor.

As you say, a better system is the in-tyre monitor if you are interested in true temperature rise, but they are much more expensive and need the removal of the tyre from the rim to fit.

I believe most manufacturers regard ambient (cold) temperature to be 25 degrees C.

Anyway, the main thing to monitor is tyre pressure, especially if it is going down.