Michelin tyre pressure

[macca59]

Hi, we have a Rapido 7076DF, 31/2 ton motorhome. I have tried a number of times to find out what tyre pressures should be for Michelin 215/70R15 CP. The plate on the cab doors says Front should be 72.3 and the Rear should be 79.5. When we've spoken to other motorhomers they say these figures are far too high. We've been told figures from Front 64, 65 or 66 to Rear 69, 70 or 72. We've spoken to Fiat and they say the figures are only for reference. We've found it almost impossible to contact Michelin regarding this problem. Would anyone be able to help us with this problem, we would be most grateful.

 

[Caddies104]

Hi

We have the same size tyres on our van and also weighs 3 1/2 ton, the book says, 5 bar on the front ant 5.5 on the back(cold). A bar is 14.7 lbs so roughly 73 on the front and 80 on the back... Just a guide but should be ok set at this, as your cab door says.....

 

[Derek Uzzell]

This inquiry also on "Hints and Tips" forum.

 

Not sure why it should be on both, but I've copied my Hints and Tips response below:

 

"Macca

 

This earlier thread may help to explain why it’s difficult nowadays to obtain tyre-pressure advice from Michelin regarding their ‘camping-car’ (“CP” marked) tyres.

 

http://www.outandaboutlive.co.uk/forums/Motorhomes/Motorhome-Matters/Michelin-tyres-Pressures-yes-I-know-I-know-/28892/#M332680

 

A Rapido 7076DF is about 7 metres long, quite heavy empty, and the tyre size is not excessive. Assuming that you’ve been operating the vehicle at the 72.3psi (front axle) and 79.5psi (rear axle) pressures you’ve quoted, unless you’ve found the ride-quality unduly harsh, I suggest you stick to those pressures.

 

If you have found the ride-quality to be very harsh, it may be possible to reduce the tyre pressures to improve matters, but, to be sure of doing this SAFELY, you’d need (as has been advised so many times on this forum) to have your motorhome weighed to establish its axle-weights when normally loaded.

 

As was explained in the thread I gave a link to above, Michelin has become reluctant to provide axle-weight-specific advice for the rear axle of a motorhome fitted with Michelin ‘camping-car’ tyres, advising 80psi irrespective of a vehicle’s rear axle measured weight. It should be possible to side-step this, but – if you have in mind to alter the 72.3/79.5 pressures – you are going to have to weigh your motorhome first, irrespective of where you get advice from afterwards.

 

If you say to Michelin “We have a Rapido 7076DF, 31/2 ton motorhome with Michelin 215/70R15 CP tyres – what tyre pressures should be used?” you are unlikely to get a useful answer. If you want to play about with the tyre pressures, you’ll need to provide Michelin with measured axle-weights. Even if Michelin only provides a specific pressure recommendation for the front axle (and a ‘one-size-fits-all’ 80psi figure for the rear axle) that will be a start.

 

There is absolutely no way of knowing whether the 72.3/79.5 pressures are “far too high” for your motorhome without knowing what its measured axle-weights are, and the only way to find out the axle-weights is to measure them. The “...Front 64, 65 or 66 to Rear 69, 70 or 72” suggestions are finger-in-the-wind guesstimates and any fool can do that.

 

It needs saying that, if your Rapido’s ride-quality is harsh, to alter matters significantly by lowering the tyre pressures will require a significant pressure reduction. Dropping a pressure from, say, 80psi to 65psi should make a noticeable difference, but dropping it from 80psi to 75psi almost certainly won’t.

 

It also needs emphasizing that, although it may well be practicable to reduce the 72.3/79.5 pressures safely and improve your Rapido's ride-quality as a result, it’s much safer to have tyre pressures that are a bit too high than to have tyre pressures that are a lot too low."

[macca59]

Hi Derek,

thanks for all the information provided. I will probably take your advice and leave the pressures as 73 and 80. Will probably try to find a weigh-bridge somewhere and get the motorhome weighed., Thanks once again for all your help.