Tyre Pressure Calculation

I came across this calculation recently and thought it might be good to share with the club. For caravan s particularly it is problematic as to what tyre pressure to use. So here goes.

1 You have to know max tyre pressure and load rating for your tyres. This can be easily obtained by reading the side wall of your tyres. e.g.
For load rating some tyres have this tyre code written on them :
185 R14C 102/100 Q

Where 185 is tread width, R is radial tyre, 14  is diameter in inches, C denotes Commercial, 102/100 is Load index number and Q is speed rating.

We are interested in the load value, which in this case is 102/100.  This is only an index number and you will have to refer to the table below to determine the maximum load your tyres can carry. But which one is it ? This is quite simple. The 102 refers to a load rating if used on a single axle. The 100 is the load rating if used on a twin axle ‘van. The following table shows the load rating

The Max pressure for this size on my van is 65psi. Once again this is on the side wall of the tyre.

2. The max on road weight of the van. For Example my van weighs 2740kg unhitched. The drawbar weight, which is generally 10% is 280kg.
Subtract 280 from 2740 = 2460kg (This is the rolling weight of my van)
As my van is a tandem axle (4 Wheels) divide 2460 by 4 = 615kg per tyre.

3. Pressure Calculation:

Divide the maximum tyre pressure by the load rating of your tyre and then multiply this answer by the actual load each tyre is carrying. This answer is the optimal pressure for that tyre.

Tandem Axle:

65 / 800 = 0.08125 (Max Pressure for tyre divided by load rating)
0.08125 x (2460/4) = 49.96PSI (inflation factor times quarter the weight of the caravan)
So the correct tyre pressure is 50PSI

Single Axle:

Assume that the van weight is 1500kg
65 / 850 = 0.07647 (Max Pressure for tyre divided by load rating)
0.07647 x (1500 / 2) = 57.35 PSI (inflation factor times half the weight of the caravan)
So the correct tyre pressure is 58 PSI

4. Motorhomes:

I read somewhere that the tyre pressures shown on the vehicle are the recommended pressures by the manufacturer for the maximum axle loading of that vehicle. If your vehicle gets a COF you will know the axle loadings from your COF. You can then use the above calculation to determine a more load dependant pressure.

If your motorhome is on a WOF you will have to weigh your van to find out your axle loadings to be able to use this formula.

Disclaimer:

This information I have cleaned from the following websites and if you wish further information please look at these sites.
The Trudgians - tyre pressure calculator
Caravan Chronicles  (a good blog to look at, this is where I got most of my information. Please have look.)
Tyre Pressure Calculator
Motorhome Tyre Pressures Use the “Check Tyre Pressures” tab to select your vehicle type.

E&OE This is a guide only with cold tyres.