"How To....."

Tyre Pressures - How to calculate what pressures to use

(June 05, 2017)

For non-standard vehicles, there's no 'book value' for the tyre pressures a Split Bus should be inflated too. So here's a calculation to follow to give you guidance to the pressures.

 

Listed weight of the vehicle, lets say, a late model Splitscreen Pickup = 1,120kg (with a tilt & bows fitted)

 

The maximum axle weights for Splitscreens are front 950kg / rear 1150kg

That equates to a balance ratio of front 45% / rear 55%

 

With just the cover fitted, full of fuel & driver 1,230kg, giving front weight of 554kg and rear weight of 677kg.

 

The maximum payload of a Pickup is 765kg

So combined weight is 1,230kg + 765kg = 1,995kg, giving front weight of 898kg and rear weight of 1,097kg.

 

Now turn to your tyres that are fitted. All tyres will have the markings that either give the exact ratings, or a series of code numbers that can be looked up on the industry standards tables.

 

1. For the example of the Pickup;

    Front tyres are 165/70/14 with a maximum inflation pressure of 50psi, and maximum load of 515kg

    Rear tyres are 185/75/14 with a maximum inflation pressure of 70psi, and maximum load of 850kg

 

Now the calculation for just the cover fitted, full of fuel & driver;

    Front Ratio = 2x max load of the tyres  =  2x 515  =  1,030  =  Front 1.86

                              Weight of the axle             554           554

 

    Maximum pressure  =    50    =  27psi front tyre pressure when not loaded

         Front Ratio               1.86

 

 

    Rear Ratio = 2x max load of the tyres  =  2x 850  =  1,700  =  Rear 2.55

                              Weight of the axle             677           667

 

     Maximum pressure  =    70    =  27psi rear tyre pressure when not loaded

           Rear Ratio              2.55

 

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Now the calculation when carrying a full payload;

    Front Ratio = 2x max load of the tyres  =  2x 515  =  1,030  =  Front 1.15

                              Weight of the axle             898           898

 

    Maximum pressure  =     50    =  43psi front tyre pressure when FULLY loaded

         Front Ratio               1.15

 

 

    Rear Ratio = 2x max load of the tyres   =   2x 850   =   1,700  =  Rear 1.55

                              Weight of the axle             1,097          1,097

 

     Maximum pressure  =     70     =  45psi rear tyre pressure when FULLY loaded

           Rear Ratio              1.55

 

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2. For the example of an empty Panel Van;

 

The vehicle was weighed on an MOT brake tester. Front weight was 520kg and the rear was 575kg.

 

    Front tyres fitted are have a maximum inflation pressure of 44psi, and maximum load of 550kg

    Rear tyres fitted are have a maximum inflation pressure of 44psi, and maximum load of 550kg

 

Now the calculation for just half full of fuel & driver;

    Front Ratio = 2x max load of the tyres  =  2x 550  =  1,100  =  Front 2.12

                              Weight of the axle             520           520

 

    Maximum pressure  =    44    =  21 psi front tyre pressure when not loaded

         Front Ratio               2.12

 

 

    Rear Ratio = 2x max load of the tyres  =  2x 550  =  1,100  =  Rear 1.91

                              Weight of the axle             575           575

 

     Maximum pressure  =    44    =  23 psi rear tyre pressure when not loaded

           Rear Ratio              1.91

 

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Assuming the same 765kg payload, split 45% / 55% across the axles. 

 

Now the calculation when carrying a full payload;

    Front Ratio = 2x max load of the tyres  =      2x 550     =   1,100  =  Front 1.27

                              Weight of the axle            (520+344)          864

 

    Maximum pressure  =     44   =  35 psi front tyre pressure when FULLY loaded

         Front Ratio               1.27

 

 

    Rear Ratio = 2x max load of the tyres   =     2x 550       =   1,100  =  Rear 1.1

                              Weight of the axle            (575 +420)          996

 

     Maximum pressure  =     44     =  40 psi rear tyre pressure when FULLY loaded

           Rear Ratio                1.1

 

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