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And my Joker pop-top tyre pressures are 39psi front and 48psi rear as recommended by VW!

This is where anyone running on car tyres could struggle. Some non-commercial tyres have a maximum pressure of 44 PSI, and thats before you even get into speed rating etc etc.

Anyone have any idea what the pressure should be for 205/70 R14 98T XL?

Currently have Semperits on an Atlantic high-top. I'm assuming the pressure noted on the sticker inside the door is for a transporter without high-top and shed loads of wood in it.

cheers,
Rob

I run the VW Carat alloys which normally use 205/70/14 tyres. I fitted std height 205 tyres (205/85/14) to gain a slightly higher gearing (and it does). I don't know what normal Carat tyres should be inflated to, never mind the full height version. I just guessed at 38 front and 48 rear. Feels and looks about right

boiliebob wrote:Anyone have any idea what the pressure should be for 205/70 R14 98T XL?

Rob

If you can get the maximum tyre pressure and the maximum tyre load from the sidewalls of your tyres, and you know your maximum axle loads, then you can use the ETRTO's formula in this thread to get you close. I've used it on various cars and it always gets me close to the values recommended by the manufacturer.

The formula is:

Tyre pressure = Max tyre pressure x (max axle load/(2 x max tyre load))^1.25

Note that the axle loads are in the handbook - they quote 'Normal payload' axle loads of 1100kg front and 1300kg rear, and 'Higher payload' axle loads of 1200kg and 1400kg.

It is confusing, especially if you've changed you van around, or swapped for different size wheels. My Caravelle Syncro now has a pop-top roof and 15in wheels, and tyre pressures are all a bit guess work.

That's a nice little formula thanks.

I've got 17" Borbets on mine (which I'm not a fan of) and never quite knew the correct psi until now

Cheers for your replies Gents.

Working on a higher payload the results of calculations were:

ETRTO's:
Front with 100% optimal grip surface, 38.6psi, 90% optimal grip 44psi
Rear with 100% optimal grip surface, 46.8psi, 90% optimal grip 53.4psi

Using formula above:
Front, 44.8psi
Rear, 47.6psi

So which to use?

Are we going to have 100% optimal grip surface?
If i opt for the 90% figures, front calculations are nearly spot with each other, 44psi.
Rear's, as the max psi of the tyre is 51, i think I should opt for the 47.6 (48) psi option.

Any thoughts?

boiliebob wrote: ETRTO's:
Front with 100% optimal grip surface, 38.6psi, 90% optimal grip 44psi
Rear with 100% optimal grip surface, 46.8psi, 90% optimal grip 53.4psi

Using formula above:
Front, 44.8psi
Rear, 47.6psi

So which to use?

That's odd - when I use the formula I get the same results as the ETRTO spreadsheet (the 100% results). In fact that formula is lifted straight from the spreadsheet. And if you want to check the results for a different contact patch, just divide the max axle load by the contact percentage before calculating the rest of the formula.

Is that the spreadsheet you used for your ETRTO figures? Do you want to share your working?

Hi.
Yes used the spreadsheet in your post. Rechecked 3 times and get same results as above. Also rechecked other results and still have same.
Workings are:
Front axle
750 x2 = 1500 max tyre load
1500/1200= 1.25
51 psi / 1.25 = 40.8

rear axle
750x2= 1500
1500 /1400= 1.07
51/ 1.07= 47.6

Cheers
Rob

OK, starting from:
- Max Tyre Load = 750kg
- Max Tyre Pressure = 51psi

I get, for the front axle with a maximum load of 1200kg:

Tyre pressure = 51 * (1200/(2*750))^1.25
= 51 * (1200/1500)^1.25
= 51 * 0.8^1.25
= 51 * 0.7566
= 38.6psi

And for the rear axle, with a maximum load of 1400kg:

Tyre pressure = 51 * (1400/1500)^1.25
= 46.8psi

umm, seems like I've got my maths completely wrong Thanks for you help, much appreciated!

Rob