Thread: my tyre pressure experiences

Interesting calculations.
What vehicle?
Our Defender on 235/85/16LT Hankook ATs runs 46 rear and 36 front on bitumen.
I lower them by about 30% on easy off road, but go down to low 20s on sand.
Strong sidewalls are important if pressures are being significantly lowered.

Originally Posted by DiscoMick

.................
Strong sidewalls are important if pressures are being significantly lowered.

As is consideration for ambient temps speed and terrain.

If you run them too low on corrugations on a hot......tulip killing.........40C+ day in Oz you'll run the risk of catastrophic sidewall failure methinks.

DL

Heat is the killer of sidewalls if you are running lower pressures at more than 70-80 KM/H then you need to consider speed. Mightn't be an issue if you are driving back to camp after a trip in Tassie but not so good if you decide that 100KM/H on the Oodnadatta track is appropriate when you have dropped your pressure to low 20s.
On the 130 with 265/70/16 I generally drop to 24 front/26 rear on most off-road tracks and adjust from there if I need more traction. On the Disco 3 28psi was my go to pressure off-road running 265/60-18 tyres. It's pretty easy to check your sidewall temperature, if it's too hot to touch slow down or add more air.

Regards,
Tote

Tyre pressure is always a compromise, between ride, rolling resistance, ground pressure, handling, and tyre life. So there is no ideal pressure.

Of course, you could always go by the driver's manual. But even there, there are problems knowing what to do. I have in front of me, owner's manuals for Series 2a 1963 and 1970 and Series 3 1981. (these are essentially identical vehicles)

Tyre pressures for the same radial tyre, same size for a 109. Pressures in psi, in the order 1963, 1970, 1981

Front, normal use, load under 250kg - 20,25,25 Load over 250kg - 20, 25, 25 Emergency soft - 15, 15, 15

Rear, normal use, load under 250kg - 20, 25,25 Load over 250kg - 35, 35, 42 Emergency soft - 26, 35, 35

For my 1986 110, similar vehicle, same radial tyres, they have a more complicated schedule. (pressures psi front/rear)

Load up to 500kg up to 105kph 35/35 Up to 120kph 45/45 Over 120kph 50/50

Load 500-1000kg " 35/45 " 47/55 " 55/62

Load over 1,000kg " 40/50 " 50/60 " 60/65

And tyres may be reduced from these pressures for cross country operation by 15psi.

So for the same tyres on similar vehicles we have a recommended pressure range from 15 - 65psi depending on load, speed, conditions - and when the recommendation was made!

On my 2a, mostly used on unsealed road, but short on bitumen trips, I use 30/30psi or 30/40 loaded - but 35/35 for long highway trips, and as low as 12/12 when the paddock is very soft.

On the 110, I routinely use 35/35 unloaded, but 35/45 when fully loaded, increasing to 40/55 on long trips with temperatures over 40. And I have used as low as 15/15 when the road is very bad, especially deep dry sand, although it also helps a lot on soft mud with no bottom.

Low tyre pressures should be avoided where the ground is hard and there are sharp stones or sticks, to avoid sidewall damage, although low pressures do help adhesion on steep rocky surfaces.

Originally Posted by DiscoMick

Interesting calculations.
What vehicle?
Our Defender on 235/85/16LT Hankook ATs runs 46 rear and 36 front on bitumen.
I lower them by about 30% on easy off road, but go down to low 20s on sand.
Strong sidewalls are important if pressures are being significantly lowered.

It's a 1998 P38 range rover.

I agree on sidewall strength but that also determines tyre pressure in a significant way since very stiff sidewalls tend to bulge less and to get the same footprint length you need to air down more. In reducing tyre pressure I tend to do two things: look at the sidewall bulge visually, I have a MT with sidewall biters that offer good protection but I never run them so soft that they already make significant contact with the surface when stationary. I also stop after driving a while and feel the tyre temperature by hand, both sidewall and tread. If I can't touch it properly, it's too damn hot (and thus soft).

Regarding the different tyre pressures per year, that is an interesting little nugget I guess that is because tyres have evolved and thus the manual had to keep up? I have also noticed a trend like that with oil type and change interval on the rover V8 engine. That was more of an environmental and marketing though I guess since afaik there was never any change in the internal tolerances that it warranted thinner oil and a longer change interval.

Anyway, I digress... All in all I found the experiment to be worth while since it gave me a bit of a basis to go on when I am on my trip. I guess I ended up a bit too soft in the end and I should run a tad more. With temps in northern sweden already down to freezing at night and around 8c during a sunny day, my tyres stayed very cool.

Thanks for sharing!
Cheers,
-P

Originally Posted by prelude

It's a 1998 P38 range rover.

I agree on sidewall strength but that also determines tyre pressure in a significant way since very stiff sidewalls tend to bulge less and to get the same footprint length you need to air down more. In reducing tyre pressure I tend to do two things: look at the sidewall bulge visually, I have a MT with sidewall biters that offer good protection but I never run them so soft that they already make significant contact with the surface when stationary. I also stop after driving a while and feel the tyre temperature by hand, both sidewall and tread. If I can't touch it properly, it's too damn hot (and thus soft)................
Cheers,
-P

Have you considered fitting a TPMS Tyre Press. Monitoring System as they can show temperature as well? They do work as I was on a dirt road one hot day, nearly 40 degrees and was looking at a map deciding where to go next with the engine idling, and a breeze blowing in the direction of one of the rear tyres and the TPMS alarmed at the 50 degree setting.

It's on my todo list It is a bit of work to figure out what system to use. I think the type that sits on top of the valves is a bit of a risk but the same goes for the internal type, when changing tyres you could well have them damaged... I will have to look into this.

-P

Originally Posted by prelude

It's on my todo list It is a bit of work to figure out what system to use. I think the type that sits on top of the valves is a bit of a risk but the same goes for the internal type, when changing tyres you could well have them damaged... I will have to look into this.

-P

Like any number of others I have the external screw in type (Safety Dave version). Never had a problem towing or off road - but I do add a little piece of garden hose on each stem for added protection. Externals of course is easier to use if changing wheels.