Kwik Fit getting cross about cross climates

[Derek Uzzell]

This early-2014 forum discussion may be of interest

 

https://forums.outandaboutlive.co.uk/forums/Motorhomes/Motorhome-Matters/New-tyre-blowout/33821/

 

Michelin used to happily offer (over the phone or by email) inflation-pressure recommendations for their ‘camping-car’ tyres (front and rear axles) based on axle weghts provided by the person seeking advice. The requisite data (as is now the case for Continental tyres) were in fact available in a Michelin booklet aimed at tyre fitters (I scrounged a 2004 copy) and - besides giving simple pressure-to-weight recommendations - Michelin used to suggest that the front-tyre pressure be raised by 10% to better cope with braking/cornering loads.

 

The 2004 Michelin booklet included pressure-to-weight data for XC Camping tyres with (as I said above) a footnote that 80psi be used on the rear wheels of motorhomes operating in Continental Europe and that ‘metal’ tyre valves should be fitted.

 

For what it’s worth, I have a selection of tyre gauges - some analogue, some digital: some psi only, some with multiple pressure scales - and some are more accurate than others. Obviously (as motorhome tyres tend to be inflated to pressures much higher than used on cars) the gauge must be suitable, and - so I understand - a gauge tends to be most accurate within its mid-range. I use a digital gauge (free from the TyrePal stand at an NEC show) if I not too fussed and an expensive USA-made psi-only analogue gauge if I’m in a picky mood. Both gauges provide near-enough the same readouts, though the USA gauge is easier to use. What I don’t use is the gauge on any of my several tyre pumps, as those gauges are just for entertainment.

 

[Steve928]

I wonder what accuracy one can expect from TPMS sensors?

My Michelin-branded digital gauge and my 0-100psi pen-type gauge give the same readings (within the limitations of reading the graduations on the analogue scale) but both sets of TPMS sensors in my summer and winter wheels read consistently 2.5psi lower.

 

I'd have thought that the technology within a (good quality, Schrader) TPMS sensor might be the most accurate but then I've had no reason to doubt my gauges before. The question of course is how can one ever determine which is the correct reading?

 

 

[colin]

rayc - 2021-03-12 7:18 PM

 

The recommended tyre pressure of 79psi is only for the specified tyres i.e. CP ones of the size stipulated on the plate. If the tyres were changed to a C type tyre for example the pressure would need to be as per the replacement tyre load table.

As this, the 118 load index of cross climates is way beyond what a light chassis can take, so you put in the appropriate pressure for that tyre.

[rajohno]

Steve928 - 2021-03-14 10:03 AM

 

I'd have thought that the technology within a (good quality, Schrader) TPMS sensor might be the most accurate but then I've had no reason to doubt my gauges before. The question of course is how can one ever determine which is the correct reading?

I was always told as long as you use the same gauge all the time then that is the one to believe, using different gauges especially between wheels, may result in uneven pressures or unnecessary worry thinking tyre's are under or over inflated when checked.

With TPMS I would use the reading as guidance, I would check pressures and adjust pressures with a quality gauge, once the pressures are set, then look at the TPMS make a note of the reading and then use that as a base reading when on the road etc.

[EMS]

We have a Dethleffs and the handbook gives the recommended tyre pressures against the axle loadings.

So easy trip to the weighbridge and then adjust the tyre pressures accordingly, they are significantly lower than the Michelin values.

[ron.]

Michelin still quote an optimum pressure for their front tyres based on the actual axle loading, as against a standard 80psi quote for the rear tyres to allow for presumed likely overloading. My method over the years is to ask Michelin the pressures to use giving the correct weights and from this I will know that the front figure is correct. I then get a friend to ask Michelin for pressures but using my rear loading for his front which gives me the optimum figures all round. As already mentioned, you should ignore any labels giving tyre pressures if you want the optimum figure as a) nobody knows the weight you will be carrying, and b) only the actual tyre manufacturer knows it’s optimum pressure.

 

 

[Derek Uzzell]

I once tried a similar ploy (purely out of curiosity) with Michelin, saying that I had acquired a set of near-new “Agilis Camping” CP-marked tyres and fitted them to a ‘white van’. I asked for front and rear tyre inflation-pressure advice based on weighbridge-measured axle-loadings, but was told that CP-marked tyres were intended for motorhomr usage and the rear tyres should then be inflated to 80psi.

 

All light-commercial vehicle tyres with a similar specification (eg. 215/70 R15C) and a similar Load Index will have similar inflation-pressure-to-load values, so there’s little gain trying to ‘spoof’ Michelin into providing other-than-80psi advice for their CP-marked tyres. Pressure-to-load information for Continental tyres is readily available online and the data for (say) a Continental 215/70 R15CP tyre can be applied to a Michelin 215/70 R15CP equivalent.

 

I’ve gained the impression that motorhome owners seeking pressure-to-load advice by contacting the tyre manufacturer believe this will somehow ‘legitimise’ them ignoring the tyre pressure recommendations published in their vehicle handbook(s). However the responsibility for not adhering to the handbook pressure recommendations will always remain the motorhome owner’s.

[Deneb]

ron. - 2021-03-23 9:38 PM

 

As already mentioned, you should ignore any labels giving tyre pressures if you want the optimum figure as a) nobody knows the weight you will be carrying, and b) only the actual tyre manufacturer knows it’s optimum pressure.

Your sentence should end with "for it's load bearing capability".

 

But you have forgotten to also add, "Only the vehicle manufacturer knows the pressures required and optimum front to rear ratios for the safe handling of their vehicle."

 

I have personal knowledge of the fatality of a test driver involved in exactly that type of testing, as well as numerous on the road collisions ranging from damage only to fatal, where the circumstances were directly attributable to unsuitable tyre pressures, so the effect of unwittingly tinkering with tyre pressures should not be underestimated.

 

Derek Uzzell - 2021-03-24 7:04 AM

I’ve gained the impression that motorhome owners seeking pressure-to-load advice by contacting the tyre manufacturer believe this will somehow ‘legitimise’ them ignoring the tyre pressure recommendations published in their vehicle handbook(s). However the responsibility for not adhering to the handbook pressure recommendations will always remain the motorhome owner’s.

And the driver's as well, if different.

[ron.]

Deneb - 2021-03-24 9:34 AM

 

But you have forgotten to also add, "Only the vehicle manufacturer knows the pressures required and optimum front to rear ratios for the safe handling of their vehicle."

This is the very suggestion which I believe to be wrong. The vehicle manufacturer cannot know what weight you will be carrying (which could vary from a driver only plus his camping and personal needs to a family of four or more and their needs in the same type van, etc, etc) and so in most cases will quote tyre pressures based on the top end of the loading figures allowed. However, these are not the optimum running pressures and more often than not result in a safe but bone hard ride. On the other hand, and I can only repeat, if you give the likes of Michelin the type of vehicle, the tyres fitted and the real life axle loadings on them (but bearing in mind my previous post) they will provide you with the optimum pressures which are both safer and more comfortable.

 

I think it would be interesting to know how many experienced motorhomers stick to the vehicle manufacturers recommended pressures as apposed to the tyre manufacturers recommendations - not many I would wager.