Tech help!
TECH HELP EDITOR
Nick Fisker
Base vehicle expert
Q What’s the correct tyre pressure?
I recently bought a Marquis Majestic 120 and have been researching the correct tyre pressure for it. The current tyres are 215/70R15 CP 109 R, which have a maximum psi of 69. I’ve been on the TyreSafe website calculator and, for the maximum loading for the ’van (1,750kg front and 1,900kg back), it gave a pressure of 63 psi front (no problem), but 80 psi for the rear, albeit it was red flagged.
I will get the ’van weighed to get the correct allowance for the van/tyres as soon as I can, but are the tyres that have been put on not the best ones to get the best payload? Also, is there a way of calculating this backwards so using the right tyres for getting the best permissible payload?
Janet Ingham
A If your tyres have a load rating of 109, each tyre can support a weight of 1,030kg. So, if every tyre is 109 rated, each axle can support a maximum weight of 2,060kg and the total weight they can support is 4,120kg. This sounds fine for a typical 3,500kg motorhome. Yes, you could get tyres of a higher load rating if they’re available in your size, but the higher the load rating, the stiffer the tyre sidewall will be and so the harsher the ride.
As for the correct tyre pressure, weigh the ’van on a weighbridge – this is essential for not only determining optimum tyre pressures but also for checking that your ’van is not overweight. Weigh each axle separately (or one axle, then the whole ’van and do the subtraction).
Armed with the axle weights, contact the tyre manufacturer and get its recommendation. It’s wise not to go over the maximum sidewall pressure.
Many tyre organisations recommend that you simply run at the maximum pressure stamped on the sidewall if you have CP tyres as they suspect that many motorhomes tend to travel at or above their design weight (and because most tyre centres don’t weigh vehicles).
However, this one-size-fits-all approach doesn’t allow for different motorhome designs that have different front and rear axle loadings.
The other point to make is that tyre pressures aren’t set in stone – you can experiment with the pressure between the tyre maker’s recommendation and the maximum sidewall
