Tyre Pressures

[Guest caraprof]

I share the same motorhome as Terry Peach, who has just been flooded out in Gloucester and who has another thread going about insurance!

He and I exchange tips about our 'vans and he has just told me something that most definitely needs clarification.

Our weights are more or less the same, same 'van, two people etc. and he has emailed Michelin, which has advised him of the correct weights for our tyres, which are the special 'Camping' variety.

My handbook advises 81 psi rear and 67 psi front, which I've been using and I must admit is a bit of a hard ride.

Michelin advises 51 psi front and rear! It's a huge differential and I'd be pleased if any of our resident techies could comment.

Thank you in advance for anyone taking the trouble.

[Mel E]

Frank,

 

I've just sent him an email on the subject.

 

Our Renault Master van conversion has Continental Vanco tyres rather than Michelin. But the recommended pressures in the Renault Handbook are still high, in my view: Front 3.8 bar (55 psi) and Rear 4.4 bar (64 psi).

 

We had our tyres sealed with Ultraseal (see other threads) and I assumed they had inflated the tyres correctly. As Ultraseal maintains pressures without addition, I admit it was the beginning of this year before I checked - to find all four tyres about 0.7 Bar (10 psi) down on the recommendations.

 

So I immediately inflated them to the correct pressures, and, on the next two trips, we noticed how hard the ride had become. So I took 0.5 Bar (7psi) out of each tyre, and it now runs as smoothly as my Citroen Xantia with its famed 'magic carpet' ride.

 

For virtually their whole life (now 18,000 miles), therefore, front and back have been 0.5 to 0.7 Bar (7 to 10 psi) under-inflated. I have checked tread depth and wear right across all four tyres and it is completely even, with no adverse wear on either side or the middle.

 

The fronts are wearing exactly twice as fast as the back, and I calculate that, if I swap front to back at 20,800, they'll all reach 2mm tread depth at the same time - 40,600. I'd much rather fork out for four new tyres at the same time than run two 'getting elderly' tyres on the rear.

 

I have tried to get some info from Continental, but they do not seem to operate the same service as Michelin.

 

There has been a lot on this subject before and Derek Uzzell is probably the resident specialist. Do a search of the archives, and you'll find lots.

 

Mel E

====

[Vernon B]

Frank, there was another long thread on this topic recently which a search should turn up.

 

Suffice to say that I contacted Michelin as a result of that and got much lower recommended pressures than given in the handbook. Michelin are regarded very highly in the business for their technical knowledge so I'd back their advice against any motorhome handbook. Having said that the advised reduction in pressures still seems a lot (between 15-20 psi) so I'm experimenting a little edging my way down; one reason being that although the high pressures stated in the handbook created a harsh ride I was getting very good tyre wear.

 

Vernon

[tonyishuk]

I would second the advice, contact Michelin (via their website viamichelin)

 

Give actual axle loads if possible, if not max permitted axle loads to be on the safe side ( or both). The usually return an eamil reply within 24 hrs.

 

On my Rapido the tyre pressures dropped about 25 to 30 PSI, from the recommended sticker on the door.

 

Rgds

[RonB]

Frank

 

This is a subject that has come up on the forum on lots of occasions. I am sure that there will be loads of info through the Search facility.

 

Suffice to say that I have owned Motorhomes from 3500 to 5000kg loadings which have each had 80psi tyre pressures recommended. Even tag axle tyres which are only sharing a quarter of a 3000kg loading have had the same pressure rating recommended. They cannot all be right.

 

There is a reason, right or wrong, mainly aimed at idiots who vastly overload, why converters recommend these pressures and I suspect that Mel has already outlined them to you.

 

Contact Michelin for a better ride.

 

Ron

[enodreven]

Hi,

 

I would certainly go with Michelin's advice, albeit I would weigh the axles so they can provide the information based upon facts as quite often we hear that the handbook weights don't tally with the weighbridge's.

 

The only other point I would make is get the information in writing/email just in case you have an accident as tyre pressures can be seen as possible contributory factors ??

 

Brian

 

 

[Robinhood]

I've emailed Michelin several times over the years, for different 'vans with different weights.

 

My current Rapido is rated at 3850kg MAM, and the Fiat handbook pressures for my tyres are 79.5psi all round.

 

Previous requests to Michelin have had pressures quoted much below Fiat or converter recommended values, and sure enough, as copied below, they quoted lower values for this vehicle:

 

==

Thank you for your recent e-mail.

 

The pressures that we would recommend based on the loads quoted are as

follows;

-

 

Front axle 1850kg - 60psi

Rear axle 2120kg - 64psi

==

 

This is on Michelin 215/75 R16C *Camping* tyres, and is quoted at the maximum front and rear axle loads for the particular vehicle.

 

(And to be honest, the vehicle was shipped to me at pressures closer to these than the Fiat quoted values).

 

I would drop them an email - from previous experience the ride at the lower pressures is generally much better.

 

BTW, I keep a copy of the eMail in the vehicle in case I have to satisfy our friend in blue!

 

[Guest caraprof]

Thank you for all the advice. One or two of you have missed something though, Michelin has been emailed (read my post thoroughly) and are recommending 51 psi front and rear as opposed to the manufacturer's recommended 67 psi front and 81 psi rear.

I was querying why there may be such a huge differential but the consensus seems to be that Michelin are the ones to trust. Like some other contributors I've decided to experiment by using pressures about halfway between Michelin's and the manufacturer's and will take it from there.

Thanks again for all the advice.

[Robinhood]

I did realise that Michelin had been emailed ;-).

 

The point is that they will give pressures based on the axle loadings quoted to them. (or possibly on the basis of a particular base vehicle, which they will look up the respective axle loadings for - I must admit I've never tried that).

 

It isn't clear from your note what basis Michelin were asked to quote on.

 

What you need to be sure is that the pressures you use are relevant to the loadings *you* actually run at. If you've been quoted 51 all round, it implies to me relatively equal loading between front and rear axle. Whilst not impossible, most 'vans have a larger legal limit on the rear axle, and many will tend to run heavier on the rear. (this coincides with the different manufacturers quoted pressures).

 

I would have thought it more normal to be quoted higher pressures at the back, and would suggest you try Michelin yourself with specific axle-loadings.

 

If you can't get to a weighbridge, and unless you are already running illegally overweight, a good starting point is to use the maximum front and rear axle loads for your vehicle, as the quote for these gives you the maximum pressures you need if you remain legal, and also gives a bit of leeway on maximum overall weight.

 

 

[Guest caraprof]

Robinhood - 2007-07-30 10:14 PM I did realise that Michelin had been emailed ;-). The point is that they will give pressures based on the axle loadings quoted to them. (or possibly on the basis of a particular base vehicle, which they will look up the respective axle loadings for - I must admit I've never tried that). It isn't clear from your note what basis Michelin were asked to quote on. What you need to be sure is that the pressures you use are relevant to the loadings *you* actually run at. If you've been quoted 51 all round, it implies to me relatively equal loading between front and rear axle. Whilst not impossible, most 'vans have a larger legal limit on the rear axle, and many will tend to run heavier on the rear. (this coincides with the different manufacturers quoted pressures). I would have thought it more normal to be quoted higher pressures at the back, and would suggest you try Michelin yourself with specific axle-loadings. If you can't get to a weighbridge, and unless you are already running illegally overweight, a good starting point is to use the maximum front and rear axle loads for your vehicle, as the quote for these gives you the maximum pressures you need if you remain legal, and also gives a bit of leeway on maximum overall weight.

I wasn't think of you actually when I said the bit about reading my post thoroughly. One or two suggested emailing Michelin, when it's already been done.

Michelin were emailed with front and rear axle weights and came up with this (you will see of course that front and rear weighs are almost identical):

The tyre size quoted above is able to run at the following axle loads/pressures, when fitted to a 'single fitment' axle (one tyre only each end of axle):

          225/65 R 16CP 112Q    ‘XC Camping'

         Axle weight             Pressure

          Kgs                          psi (bar)

Quoted wts: 1610           51 (3.5)        Front

Quoted wts: 1620           51 (3.5)        Rear

 +10% front axle:

Normally, we would suggest an increase of the front tyre pressure by 10% above the final calculated 'static' value, to help the front axle tyres cope more easily with the weight transfer forward under braking (providing the maximum pressure is not exceeded). The static axle weight provided is so low (as is the rear axle), however, that the minimum pressure level should be considered as having this 10% already included.

 Final recommended pressures:

     Front: 51 psi

    Rear: 51 psi

.

A huge difference from the manufacturer's pressures!

[Clive]

I believe that one important criteria is being overlooked here, especially for those with coachbuilts. Tyre manufacturers will hapilly give a pressure for a given axle loading which will be perfectly OK for the tyre and provide a comfortable ride, BUT these pressures do not necessarilly take account of the higher than normal centre of gravity of a motorhome compared with a conventional car. Even van conversions have a higher C o G than a plain white van with a load in the back.

To get a soft ride requires increased tyre deflections and radial tyres are very good at permitting this while at the same time keeping the tread flat on the road. This was after all their biggest advantage compared with older cross ply tyres.

But if a tall motorhome is pushed a tad hard round a bend or a roundabout the weight transfer will cause increased deflection in the outer tyres (still well within their tyre ratings) and an increase in body roll. At this time you will be wishing you were travelling a tad slower or had not lowered your tyre pressures. The motorhome manufacturers have a reason for specifying these high pressures.

So when you lay your pristine "comfortable" motorhome on its side at the roundabout and the police check your tyre pressures and find that they are well under the pressures specified in the motorhome drivers handbook how will you anwser them? Will Comfort pay out if you admit you had purposely lowered the pressures below the specified levels to get a softer ride on somebody elses say so?

 

Lastly a small detail, the maximum pressure for Micheling camping tyres was I thought 80 PSI and not 81??? Typo perhaps?

 

I cringe when I hear owners say "I blew the tyres up to the pressure in the handbook then lowered them a bit at a time until the ride was nice". Its wearing blinkers to the dynamics of the situation.

 

Think on!

 

C.

 

 

[Guest caraprof]

So Clive, what should I do? Lunar states 81 and 67 and Michelin 51 each.

I'm happy to take your advice.

I'm really not bothered about a hard or soft ride, I just want to do the right thing but I get conflicting advice.

[Bazza454]

Clive - 2007-07-30 10:35 PM I believe that one important criteria is being overlooked here, especially for those with coachbuilts. Tyre manufacturers will hapilly give a pressure for a given axle loading which will be perfectly OK for the tyre and provide a comfortable ride, BUT these pressures do not necessarilly take account of the higher than normal centre of gravity of a motorhome compared with a conventional car. Even van conversions have a higher C o G than a plain white van with a load in the back. To get a soft ride requires increased tyre deflections and radial tyres are very good at permitting this while at the same time keeping the tread flat on the road. This was after all their biggest advantage compared with older cross ply tyres. But if a tall motorhome is pushed a tad hard round a bend or a roundabout the weight transfer will cause increased deflection in the outer tyres (still well within their tyre ratings) and an increase in body roll. At this time you will be wishing you were travelling a tad slower or had not lowered your tyre pressures. The motorhome manufacturers have a reason for specifying these high pressures. So when you lay your pristine "comfortable" motorhome on its side at the roundabout and the police check your tyre pressures and find that they are well under the pressures specified in the motorhome drivers handbook how will you anwser them? Will Comfort pay out if you admit you had purposely lowered the pressures below the specified levels to get a softer ride on somebody elses say so? Lastly a small detail, the maximum pressure for Micheling camping tyres was I thought 80 PSI and not 81??? Typo perhaps? I cringe when I hear owners say "I blew the tyres up to the pressure in the handbook then lowered them a bit at a time until the ride was nice". Its wearing blinkers to the dynamics of the situation. Think on! C.

Clive,

You're spot on, many thanks.

This is more or less the same response that I got from Knaus when I questioned the tyre pressures. 

[Robinhood]

Clive - 2007-07-30 10:35 PM

 

I believe that one important criteria is being overlooked here, especially for those with coachbuilts. Tyre manufacturers will hapilly give a pressure for a given axle loading which will be perfectly OK for the tyre and provide a comfortable ride, BUT these pressures do not necessarilly take account of the higher than normal centre of gravity of a motorhome compared with a conventional car. Even van conversions have a higher C o G than a plain white van with a load in the back.

 

*snip*

 

So when you lay your pristine "comfortable" motorhome on its side at the roundabout and the police check your tyre pressures and find that they are well under the pressures specified in the motorhome drivers handbook how will you anwser them? Will Comfort pay out if you admit you had purposely lowered the pressures below the specified levels to get a softer ride on somebody elses say so?

 

*snip*

 

 

Certainly a point worth considering *but* lets be clear here, the pressures requested (both by myself and the OP - or at least his friend) were explicitly for XC Camping tyres, normally fitted to motorcaravans, and generally to coachbuilts and A-class conversions, rather than van conversions.

 

In addition, my particular request was specific about the vehicle type involved, and this was acknowledged twice in the reply.

 

If push came to shove, I think it would be easy to argue that the manufacturer of the tyre was more qualified to give advice than the manufacturer of the base vehicle (assuming the appropriate facts were under consideration).

 

BTW, I'm not entirely convinced about the argument on COG, there's a hell of a lot of air in my van at higher levels that would be occupied by goods in a fully loaded delivery van.

 

 

[Guest caraprof]

Now I'm even more confused! Help!

[davenewellhome]

Frank, go with Michelin's advice. They make the tyres and have been doing so for a long time. I respect Clive's wisdom and input on may things but I'm afraid I disagree with his overall conclusion on this one. Printing out the e-mail and keeping it with you in the 'van is a good idea as you then have something more than just your word that a wiser person than us gave you the tyre pressures to use. If you don't want to park your motorhome on its side at a roundabout don't go so quickly.

 

D.

[Guest Syd]

davenewell@home - 2007-07-31 7:23 AM

 

Frank, go with Michelin's advice. They make the tyres and have been doing so for a long time. I respect Clive's wisdom and input on may things but I'm afraid I disagree with his overall conclusion on this one. Printing out the e-mail and keeping it with you in the 'van is a good idea as you then have something more than just your word that a wiser person than us gave you the tyre pressures to use. If you don't want to park your motorhome on its side at a roundabout don't go so quickly.

 

D.

 

A point worth remembering here is that Michelin made formula one tyres for years but still got it very badly wrong in America, so wrong that they almost killed Ralph Schumacher and were forced to admit that their tyres simply were not up to the task. They just could be wrong here too

[Dave Newell]

With the greatest of respect to all involved you cannot seriously be advocating taking the advice of an amateur over a professional, seriously? 8-)

 

D.

[Derek Uzzell]

I shall try to avoid going over old ground. As far as I'm concerned pretty much everything worth saying about 'camping' tyres for motorhomes and peculiar tyre-pressures being advised in handbooks has already been covered (repeatedly) on this forum.

 

My own experience differs somewhat from Mel E's regarding Continental. My Hobby is also fitted with Vanco-8 tyres and, in July 2005, when I acquired the vehicle and decided that I couldn't trust Ford's or Hobby's pressure recommendations, I e-mailed Continental for ball-park advice based on axle-loadings given on the Hobby's VIN-plate and in its Certificate of Conformity. Continental evidently don't operate like Michelin as the response to my request came from a Support Engineer based in Rugby having been re-routed there from Continental(Germany).

 

About a year later, having finally got round to weighing the Hobby in loaded state, I re-contacted the same engineer and received pressure advice based on the measured loadings. (I also asked him what temperature Continental considered 'cold' with regard to the usual recommendation that tyre-pressures be measured cold. He replied "the benchmark within Europe is 20 degrees C, however please take that with a 'pinch of salt' and use whatever the ambient temperature at the time is".)

 

Recommended figures for the Hobby were:

 

Ford cab-door plate - 44psi (Front axle) and 80psi (Rear axle)

Hobby handbook - 61psi (F) and 70psi ®

Ford handbook - 47psi (F) and 67psi ®

 

I now use 47psi (F) and 55psi ® which provide a firmish but fairly supple ride - or as "supple" as one can reasonably expect given the design and original purpose of the vehicle - with precise-enough handling. The 'hand on tyre-wall' test (hammer the vehicle down the motorway for a while, then stop and immediately check how hot the tyres feel) indicates absolutely no sign of excessive heating at these pressures, which is hardly surprising as they are a bit up on Continental's recommendations. Anyway, it should be apparent from the above that there can be wide variations in tyre-pressure recommendations depending on where the data comes from.

 

There's no doubt that (assuming your motorhome is fitted with tyres that are suitable for its design weight limits and you aren't compromising this suitability by overloading the vehicle) for basic safety reasons you should definitely err towards using higher rather than lower tyre-pressures. Having said that, if your motorhome is shaking the fillings out of your teeth on less than billiard-table-smooth roads, then it makes sense to confirm the suitability of the tyre-pressures you are employing. As commercial-vehicle tyres are designed to run at significantly higher pressures than car tyres, you will probably need to alter pressures by a good chunk to notice a real difference in ride quality. For instance, I'm aware of a difference in ride and handling if I alter the pressures in my VW Golf's tyres by 4 psi, but I'm sure I'd need to double (perhaps triple) that variation to notice any real change on the Hobby.

 

I may well have said this before, but, if you check your own tyre-pressures, make sure your gauge is providing accurate readings. Dial-gauges particularly don't like rough treatment and can be unpredictably inaccurate. I've got two gauges: one is accurate across its whole pressure scale, but the other over-reads by about 20% below 35psi, then progressively corrects itself and is accurate again by 60psi. My mechanic pal tells me this is commonplace and there's no guarantee where in the scale such inaccuracies may occur. There's little point in seeking advice from a tyre manufacturer regarding optimum pressures and being meticulous in following that advice if you've got a wonky gauge.

 

Syd:

 

While Michelin will openly admit that pressure-recommendation information in their handbooks has occasionally been incorrect (usually type-setting problems) it's unlikely their e-mail/phone recommendations will be wrong (provided, of course, they are given the right information to work from!!). Anyway, as like-for-like tyres from different manufacturers will share similar pressure-to-load characteristics, it is possible to perform a rough cross-check if one has access to the different manufacturers' data. Personally, I've always found advice from Michelin credible and when I have cross-checked (as I'm a disbelieving beggar) I've never discovered any contradiction.

[Mel E]

Clive,

 

If anyone drives a motorhome round corners/roundabouts in a manner likely to put it on its side, then they'll manage it whatever the tyre pressures.

 

All I can say is that:

 

1. being a van conversion, ours is not top heavy compared to the average loaded van. Remember that very few vans carry goods dense enough to fully use the load space, and we have all the heavy items on or near the floor: gas bottle, fresh water (waste is even lower: underneath the van), leisure batteries, fridge, cooker, loo cassette, water boiler, water pump and heater. Only a few light ply cupboards reach to the top and they only ever have very light things in them.

 

2. the degree of roll on corners is not detectably different with the lower tyre pressures - it's very small unless you try and take 'the racing line'.

 

3. as has been made clear in this post, 'recommended' tyre pressure can vary very widely for the same vehcile between tyre and vehiccle manufacturers - not to mention converters.

 

Mel E

====

[Brian Kirby]

caraprof - 2007-07-30 11:06 PM Now I'm even more confused! Help!

Frank

When I read your original post, way up top there, you seemed to me to be saying your friend had e-mailed Michelin with his axle loads, but not that you had done so yourself.  You seemed to be extrapolating from his experience and advice to you van.

If you were, may I suggest you don't: but get your own van to a weighbridge, fully laden, and get a proper certificate recording the overall laden weight of the van, plus the loads on each axle.  (This can usually be done by asking the operator to set up the weighbridge for a delivery: first weigh the whole van [weight 1], then drive the front wheels just off the platform and take the second weight [weight 2, rear axle only], the weighbridge should then calculate the third weight [weight 3], which is 1 - 2 or, in the case of a motorhome, the front axle load).

If you prefer to quote only the permissible individual axle loads to Michelin, which may be what your friend quoted, I'd still suggest you repeat the exercise for yourself, even if you know the answer will be the same.  Why?

If it is your own van's actual (or maximum permissible) axle loads that you quote to Michelin, preferably by e-mail, you'll get a reply, identifiably from Michelin, that will demonstrate, in conjunction with your weighbridge certificate/vehicle plate, to anyone who has a right to know, that you have taken good and sound advice before varying your tyre pressures from those in your handbook. 

Not essential, but it should help to avoid any awkwardness, should you get pulled into a roadside check.  However, even if you choose to rely upon the plated limits and not the weighbridge figure, I'd still get the van weighed as above, just so that you can prove, if ever you have to, that you are aware of the maxima, and the normal running axle loads, and that you have sound grounds for believing your van is not overloaded.

It may be a bit of a fag, but it's tidier that way!

[Clive]

Seem to have woken a few up ey?

Dave I will sort you out next time we get together (Malvern with luck) and the same applies to Brian if he gets there??

 

Centre of Gravity varies greatly in the Panel Van range I accept. However when the firm I work for (now part time) test a new van for use as a service van they put a few pallet loads of cast iron fork lift truck wheels inside to bring it up to max GVW. Low Cof G but heavy!

 

 

I still believe that the tyre manufacturers are not able to provide a definitive pressure based on axle loads alone. C of G MUST have an effect on the handling and weight transfer when cornering and is the reason higher pressures are given in the motorhome manufacturers handbooks.

The Camping tyre arguement is something else. From my viewpoint it is a way of attempting to make over weight vehicles handle a bit taughter by increasing the pressures from a typical 65 up to 80 PSI! But if a vehicle is that loose it NEEDS 80 PSI to make it handle better then what better acceptance of the need to consider C of G?.

 

You must make up your own minds on this but my advice is to fall in line with the owners handbook. The pressures and tyre types tabled would not have been plucked out of thin air.

 

Over

(I will now duck)

 

 

[trooper]

I weighed my m/h today fully loaded plus a 56 pound weight in the back to allow for waste, wine etc. I then rang michelin to get the pressures as I did on my last m/h.The tyres are 225 75r 16 cp camping. It appears these are the heaviest of this type of tyre. The max weight is 4000kg, axle weights, front 1810 kg, rear 1940 kg, they recommended 65 psi all round as that was as low as their charts etc went, he actually said the tyres were over specified for the weight of the vehicle, and that i had a good safety margin, i obviously was quite pleased, I must admit I learned

more by ringing than I would if I had E mailed (lol)

[tonyishuk]

If I recall from a previous thread,

Someone spoke to the man at michelin who said that it was not just a matter of weight / tyre pressure.

 

The information that Michelin gave on all their tyres was also based upon the footprint of the tyre on the road. This depended on a number of factors: weight, side wall design, the way the tyre deforms at speed. ( Tyre tread moves outwards due to centifugal force,) and a whole lotta other stuff.

 

Point being :

1) The make em, so should know best.

2) Keep the email for reference, they all have reference numbers that can be traced back to Michelin.

3) NEVER buy a vehicle with out a spare wheel ( Nothing to do with the topic, but sound advice)

 

Rgds

[tp002c784tp002c784tp]

Hi All and Frank

As I forwarded this information about Tyre pressures to Frank I must apoligise to Frank, As I have been busy with my insurance company for the last two days I have only just read this thread and I did not realise Frank was starting it.

When I read the weights Frank has quoted by Michelin off the letter sent by them to me I acually re-read my origanall email to Michelin because the weights of the vehicle quoted by them for my recomended pressures did not seem right, I looked at my first email I had sent to them and the weights I quoted was.

Total Weight 3,050kg

Front Axle 1,460kg

Rear Axle 1,590kg

I do not know where Michelin aquired the weights of 1,610kg and 1,620kg because it certainly was not in the information I sent to them.

I have just emailed Michelin with the above correct weights and see what they come up with this time.

 

Once more I must appogise to Frank for the duff information I sent him , but I did send this data to Michelin before I went on holidays but did not get the reply until I returned.

 

Terry