How does this sound for tyre pressure

[prelude]

I was scouring the internet regarding tyre pressures since I changed tyres on my P38. Generally 28psi (1.9 bar) front and 38 psi rear (2.6 bar) is what I come up with. I am assuming though that this is related to the tyres the car came fitted with. Changing them to a different size and/or load capacity (stiffness in other words) to me suggest that those pressures may need to be recalculated.

I came across this link on another land rover site which makes sense to me but I wonder what other people make of it:

Optimum Tyre Pressure - On or Off Road 4x4 Driving Land Rovers (http://www.landyonline.co.za/techtalk/tyre_pressures.htm)

As an example: I have fitted goodyear wrangler MT/R kevlars recently in LT235/85R16. Which have a maximum load of 3042lbs (or 1380kg's) and a maximum inflation pressure of 80 psi (or 5.5 bar). When I take my range fully loaded and the tyres spec through the calculation on the above link I get:

fully loaded rangy (3000kg) / maximum load of 4 tyres (5520kg) X maximum pressure (5.5 bar) = 3 bar (44 psi) pressure per tyre.

Looking at the load distribution of the P38, derived from the original tyre pressures it looks to be about 42% front and 58% rear. If I apply these figures to the 3 bar (44 psi) average presure I get 2.5 (36 psi) front and 3.5 (51 psi) rear.

that's a helluva difference so I am somewhat cautious to simply beleive one source. Btw, I took my p38 as an example but this will apply to any laro of course.

What do you guys think?

-P

[simonmelb]

You're over complicating it.

This is a very good guide for typical Australian high speed dirt road travel:

http://www.l8ter.com/wp-content/uploads/2015/08/tyre-pressure-pdf.jpg

Those tyres you have are very tough and can cope with lower pressures. As stated in the lower Rh corner of the guide you can adjust pressures by eye to equalise the contact path length.

[Tombie]

Nope. I wouldn't do it..

Having said that - I wouldn't run a P38 at 28/38 either.

[jcamp]

How does it feel!

With a FFR 109 years ago after shifting weight forward (toolbox on top of the winch etc) finished up with 15psi front and 60 psi rear

[Disco-tastic]

Are talking for bitumen or dirt roads?

When i had that size tyres (maxxis bighorns) on a disco 1 i ran 40psi front and 36-45 rear depending on loads (i generally left ot at 38 unless it was a long, laden trip).

With any tyre i pump them to a pressure that looks good to the eye and then monitor them for wear and adjust accordingly. I have always ended up between 36-45 for a 4wd.

Cant help for dirt roads as i dont do much of that.

Cheers

Dan

[prelude]

Thanks for the drawing :) For now I was talking bitumen though just to get started.

In the mean time I found another formula that simplified reads

front axle weight / ( 2x max load tyre) x max pressure and if I take my numbers once again I end up with:

front: 1260 / (2 x 1380) x 5.5 = 1.9 or 28psi again
rear: 1260 / (2 x 1380) x 5.5 = 2.6 or 38psi again

Funny enough, back to what land rover suggested. I am confused :)

I'll air them up whatever "looks and feels" right for me for now but I am curious the the science behind all this.

@tombie, why would you not air a p38 up to those pressures and what WOULD you do?

Thanks!

-P

[ramblingboy42]

How does it feel!

With a FFR 109 years ago after shifting weight forward (toolbox on top of the winch etc) finished up with 15psi front and 60 psi rear

reading that makes me cringe.

I'm glad you are driving it.

It must understeer like a pig in ****......

[alien]

I set things up with a little less maths.

For asphalt I air up until the side wall stops standing up and add 5-10 psi.
After 1 hour stop and fairly quickly check pressures.
From the first reading with a cold tyre I look for a 4psi rise( some stiffer LT tyres look for 6psi maximum).

On high speed dirt deflate till you see the side wall "bag out"/softening, drop a further 5 psi.
As above it should be checked but it's fairly hard to actually do this.

For rock crawling harder tracks drop another 5psi and this is my start point in sand also.

For the record I run BFG KM2 mud tyres. LT 235/85R16's.
The Defender and van sit with @50kg to spare on each axle, close to 4ton GCVW.
Front and van 44-46psi.
Rear 60-64psi, 52psi with no trailer.
Any less and I see scrub marks on the side walls and it feels "loose" to drive.
I also cheat and use a tyre moniture rather than stop after the 1 hour mark.

[Tombie]

Unladen - remembering my vehicles tend to be a little heavier (bits [emoji41] )

Lara was 38 front and 40 rear (road trim)
Fully loaded 38 front and 46 rear

D2 36/42 road and 38/46 loaded

D4 36/40 road and 38/44 loaded

Offroad (sand) 18 front and 24 rear
Offroad (rock crawling) 15 front and 18 rear