Tyre Pressures - Again!

[Wireless]

I can't remember which thread it was now, but I mentioned working tyre pressures out by calculation...

Correct Tyre Pressures for your tyre/vehicle combination?

Ok, I have a nearly standard 5-door Disco V8i;

970kg kerb weight / 1100kg max weight front axle
1055kg kerb weight / 1650kg max weight rear axle

Tyres BFG AT 235/70 - 900kg max weight @ 50psi

So 18kg per psi,

That works out to;

front 26.9 min to 30.6 max psi
rear  29.3 min to 45.8 max psi

So the standard 26psi front is under pressure as no-one is driving or part of the engine has dropped on the road?

Standard 34psi rear assumes a rear axle weight of 1224kg?

Land Rover assumes a continuous load of 169kg or 23st 4lb; one fat owner back seat driving?

Ok, I give up!

[Range Rover Blues]

EEC kerb weights assume a 75kg driver and full tank of gas.  If the front tyres as a little soft it's to induce understeer which is inherantly safer than oversteer.  That said thr RRC tyres presures are 28/34 but with a note that if the car is driven empty for long periods then the back tyres can be run softer for a more comfortable ride.  The LSE are 28/36 with a slightly higher rear axle load than a normal RRC.

Disco is obvioulsy heavier, but the recomended tyre pressures are based on the advice of the OEM supplier's test data and homologation testeing/performance testing of the protortype vehicles.

[andyb]

Gosh that's all a rather scientific approach.......there nothing wrong with that, but....

....I usually start at 30psi all round and go from there. IME 30psi is a good starting point no matter what car it is........transit vans and upwards are a different kettle of worms :)

[python]

i run my disco at 30 psi, no probs.   :)

[Range Rover Blues]

Hmmm :-k, the back end will always be heavier and should thereore be higher pressure :-.

[Wireless]

Ok, did a maximum load calculation for my usage of the Disco (assuming you're right about a 75kg driver included in kerbside weight), including towing, and from calculation I reckon I should be running 30psi front and 39psi rear.

For driving about in it on my own I should run 28psi front and 31psi rear, I have to admit the front tyres do seem soft at 26psi all the time, and the rear a bit hard at 34psi when I'm in the car alone.

Mmm...

Me thinks that for long journeys I should work out a loading vs tyre pressures matrix sheet to keep in the Disco, and possibly keep a record of mpg figures at different loadings/pressures.

Damn, this is turning into some sort of project.

[Jimbo]

I've been running my BFG AT's at 30psi front, 42 rear - the Disco is always well loaded with tools/testers etc. I've just replaced the BFG's with General Grabber AT2's (no BFG's in the country, and no date for their arrival !), got the tyre place to go for 30psi front and 42 rear and the truck is all over the place - either their airline gauge is knacked or perhaps the General's are a bit stiffer in the sidewall ? Going to check the pressures with my own gauge which seems to be fairly accurate.

[Range Rover Blues]

Wirelss, just to make things a bit more complicated, you know how tyres tpressures are suposed to rise when the tyre gets hot? well the rear tyres run in the shaodow of the fronts and on some carts this means that the rear tyres heat up much more than the front.

[Wireless]

I've been running my BFG AT's at 30psi front, 42 rear - the Disco is always well loaded with tools/testers etc. I've just replaced the BFG's with General Grabber AT2's (no BFG's in the country, and no date for their arrival !), got the tyre place to go for 30psi front and 42 rear and the truck is all over the place - either their airline gauge is knacked or perhaps the General's are a bit stiffer in the sidewall ? Going to check the pressures with my own gauge which seems to be fairly accurate.

I bet the max loading and max pressure at cold is lower on the AT2's (when you state stiffer sidewalls), and running the same pressure as your BFG's is causing the new tyres to be over inflated?

Not sure what size BFG AT's you had but to help the 235/70 16's has max 900kg @ 50psi.

[Wireless]

Wirelss, just to make things a bit more complicated, you know how tyres tpressures are suposed to rise when the tyre gets hot? well the rear tyres run in the shaodow of the fronts and on some carts this means that the rear tyres heat up much more than the front.

mmm...could explain a few things, do you have any links for this?

[Jimbo]

I bet the max loading and max pressure at cold is lower on the AT2's (when you state stiffer sidewalls), and running the same pressure as your BFG's is causing the new tyres to be over inflated?

Not sure what size BFG AT's you had but to help the 235/70 16's has max 900kg @ 50psi.

235/70 16's - I checked the pressures (cold tyres) with a digital gauge, 30 front, 42 rear.

I might drop the front to 28 and the rears to 40, and see how it handles.