Trailer Tyre pressures

[RogerB]

Having just purchased an Ifor Williams CT 115 single axle trailer for my 7 I'm surprised to see the recommended pressure is 95 PSI. As this is the max stated pressure on the Cargomax tyres I would have expected the running pressure would be somewhat less.

I'm interested to know others views. 

[ECR]

This from the caravan Club: (dont know why the font colour changes ....)

 

Trailer Tyre Pressure

One of the most important safety aspects of towing is having the correct tyres fitted on your caravan and just as important having the tyres inflated to the correct pressure. The simple fact is caravan tyres often get overlooked and are not checked regularly. On your tow vehicle, there is usually a sticker conveniently located in the door frame or fuel filler cap reminding you of the correct tyre pressure. Unfortunately this is not always the case with all caravans.

Your Caravan Handbook usually quotes the tyre pressure for the original tyre size fitted and the caravan loaded to the MTPLM (Maximum Technical Permitted Laden Mass). As the load in your caravan can change or on older caravans, the tyre size might have changed it’s worth knowing how to calculate the correct pressure from the markings on the tyres fitted to your caravan.

You first need to be able to decipher the code on the tyre wall… this is a typical code:

205 / 65 R 15 94 H

The first part of the number is the tread width, in this case 205 mm wide. The second number is the Aspect Ratio which means in this case the tyre wall height is 65% of the width of the tread. The next letter “R” is the construction, in this case Radial, the following number “15” is the rim diameter (in inches). The next number “94” is the load rating – more of this in a moment and finally the last letter “H” is the speed rating for the tyre.

Load Rating

The load rating for the tyre can range from 60 to 130 which equates to a maximum load per tyre of between 250 Kgs to 1900 Kgs. The first thing to check is the tyres are rated for the MTPLM of your caravan, so we need to know the load index ratings.

 

From the table above, you will be able to cross reference the load rating code number to the actual maximum permissible load the tyre is rated for. So in our tyre data example from above, we can see that a load index of “94” means the maximum load for the tyre is 670 Kg’s. Remember that is the load for ONE TYRE… so on a single axle those two tyres could carry a maximum load of 1340 Kg’s. The recommendation from the tyre industry though is you should never exceed 90% of the tyre’s load index, so in this case the maximum axle weight would be 1251 Kg’s

The other bit of information we need to know from the tyre is the Maximum Tyre Inflation Pressure and Load. This is usually located on the side wall below the tyre data in smaller text. It will say something like “MAXIMUM LOAD 720 Kg’s MAXIMUM PRESSURE 52 PSI” it might have the pressure given in Bar.

Once you have the MTPLM of your caravan and the maximum pressure for your tyre we can now work out the correct tyre pressure.

Take the figure for the maximum pressure for the tyre and divide it by the maximum weight from the table above. Now multiply this figure by the actual load you are going to put on the tyre – so half the MTPLM for a single axle caravan or a quarter of the MTPLM for a twin axle caravan. The resulting figure should be the correct inflation pressure for each tyre.

Lets look at an example for a single axle caravan:

Tyre Details:  205 / 65 R 15 94 H – Maximum Inflation Pressure 60 PSI

Weight of caravan: 1200 Kg’s

60 / 670 = 0.08955 (Max Pressure for tyre divided by load rating)

0.08955 x (1200 / 2) = 53.7 PSI (inflation factor times half the weight of the caravan)

So the correct tyre pressure is 54 PSI

Working out for a twin axle caravan:

Tyre Details:  205 / 65 R 15 94 H – Maximum Inflation Pressure 60 PSI

Weight of caravan: 1800 Kg’s

60 / 670 = 0.08955 (Max Pressure for tyre divided by load rating)

0.08955 x (1800 / 4) = 40.29 PSI (inflation factor times one-quarter of the weight of the caravan)

So the correct tyre pressure is 41 PSI

Top Tip: As a rule of thumb, the inflation pressure of a hot tyre is usually about 4 PSI higher than when cold. If you check your tyre pressures when hot and it is more than 4 PSI higher, then the cold inflation pressure was too low. If the hot inflation pressure is less than 4 PSI difference than the cold pressure, then the cold pressure was too high.

This is derived from: “For every 10 Deg F temp change the tyre pressure changes by 1 PSI”

[RogerB]

Well thanks ECR that makes more sense. For my single axle trailer running on 155/70R12 104/101 N tyres.

Each tyre is rated at 900kg

Max pressure 95 PSI

Anticipated Gross weight of Trailer + Car = 1062 kg

My calculation:  95 divide by 900 x 531 = 56 PSI.

[Jonathan Kay]

"This from the caravan Club: (dont know why the font colour changes ....)"

I think that's because from From the table above, you will be able to cross reference... onwards it's a hyperlink. The browser changes the colour to show this.

Jonathan

[7 wonders of the world]

Really neat guide there, could someone add this to the tech guides...?