Tyre pressures Autotrail Mohican

[breakaleg]

Took my van to a weighbridge, fully laden for a two week break (even the waste water tank was full, forgotten to empty it.

 

Weighed the front and rear axle separately then the whole van. total cost of £13.00.

Emailed Michelin with the results.

 

they replied wit , front 50psi rear 55psi.

Printed the lot and keep in the glove box (just in case)

Pete

[jadatis]

@ breakaleg

Can you give me the exact data of the weighing bridge and the specifications of the tyres. Tyres: load-index or maximum load in kg or LBS, and the pressure on the sidewall "at xxx psi or kPa"

and from the car te axle-weigts front and rear.

Then I can give an example here. I expect that those pressure advices michelin gave were excactly for the weights you gave them, and that if you calculate with some extra for savety and weight shifting during the vacation, It has to be higher.

 

@ Brian Kirby

It is since the Ford Explorer/Firestone afaire that the pressures are adviced so high and the manufacturer can be held responcible for to low tyre-pressure advice. That is why they advice much to high mostly. Read already story's of campers with front wheele drive that did not have enaugh grip coming out of difficult situations, to my opinion because of the to high pressure.

 

[spartan3956]

Thats interesting after using a weighbridge so i didnt do to bad getting 55psi all round by trial and error.

 

I have had the tyres Derek above 60 psi and the ride was so rough it was uncomfortable, Steering was very light and the unit wandered especially on the motorway near the legal limit. I started to reduce the pressures gradually until i was happy with the handling.

 

A good guide would be to check the tyre depths for wear across the width of the tyres .

A reduction in wear in the middle to high a pressure if on the sides to low a pressure.

 

I would say that tyre pressures are critical with such a heavy vehicle, most of the handbooks quote tyre pressures for base vehicles even loaded the MH is much taller and the handling is effected by cross winds.

 

I did check the fuel consumption when lowered to 50psi and it did suffer

 

 

[Derek Uzzell]

spartan3956 - 2009-12-02 5:48 PM

 

A good guide would be to check the tyre depths for wear across the width of the tyres .

A reduction in wear in the middle to high a pressure if on the sides to low a pressure.

 

This used to be a reasonable rule of thumb years ago when tyres were cross-ply construction, but is of little practical value nowadays as the circumferential belt of a radial tyre inhibits deformation of the tread. You might get some indication of the type you suggest if a tyre were monumentally under- or over-inflated (say a tyre being run at 30psi when it should be run at 70, or vice versa), but otherwise not. In both of the examples I've suggested, inspection for tyre wear will be academic as massive under-inflation will rapidly destroy the tyre, while serious over-inflation will result in a bone-shaking ride.

 

As your Mohican is built on the 'old model' Ducato, with different size/pattern tyres to Carol's, the pressures you've chosen for your motorhome will not directly apply to her vehicle.

 

 

[spartan3956]

Derek

I have only given my oppinion

The handbook states 80psi from the road testing this again in my oppinion is far to high.

Knaus state a similar psi with regard to the Maxi chasis based Fiat vehicles but the owner forum have recommended 60psi.

 

Tyre depth checking on any vehicle is a good guide to any problems especially on the front.

Working for a major car dealer tyre depths are always checked every sevice and always checked at MOT.

 

Can you still get cross ply tyres i dont know of any modern manufacturer fitting them?.

 

 

 

 

 

[Derek Uzzell]

spartan3956 - 2009-12-03 8:28 PM

 

...The handbook states 80psi...

 

I understood from your earlier posting in this thread that your Mohican's handbook advised 60psi. If you were initially using 80psi pressure for your motorhome's front tyres I'm not the least bit surprised that the steering proved to be ultra-light and that the vehicle wandered about.

 

Cross-ply tyres are still manufactured for specialised applications (eg. for use on old cars or agricultural machinery).

[Brian Kirby]

spartan3956 - 2009-12-02 5:48 PM Thats interesting after using a weighbridge so i didnt do to bad getting 55psi all round by trial and error. I have had the tyres Derek above 60 psi and the ride was so rough it was uncomfortable, Steering was very light and the unit wandered especially on the motorway near the legal limit. I started to reduce the pressures gradually until i was happy with the handling. A good guide would be to check the tyre depths for wear across the width of the tyres . A reduction in wear in the middle to high a pressure if on the sides to low a pressure. I would say that tyre pressures are critical with such a heavy vehicle, most of the handbooks quote tyre pressures for base vehicles even loaded the MH is much taller and the handling is effected by cross winds. I did check the fuel consumption when lowered to 50psi and it did suffer

The problem, as I see it from what you have said so far, is that you still don't know what is the safe starting point for your tyre pressures.  If I've missed it, I apologise, but I haven't noticed you say you have actually weighed each axle at your maximum touring trim, obtained the recommended pressures for those loads, and noted these as the pressures that you must not stray below.

Trial and error is all very well, but the error, in the absence of the above information, is liable to be that of under-inflation which, as I'm sure you are only too aware, causes tyres to overheat and, ultimately, self-destruct.  Some trial, some error, to coin a phrase!

Experimenting within the range of the lowest recommended pressure for load, up to and including the blanket fail-safe pressures recommended by vehicle producers, is fine to establish where comfort and handling reach the best compromise, but one must always be aware of both the upper, and more crucially the lower, limit.

This, above all else, is the reason why handing out general advice, on a public (or any other for that matter) forum, that this, or that, tyre pressure works on any particular vehicle is unwise.  Not everyone who reads will pause to think (because they will not all understand) about the pressure differences necessary to accommodate different actual loads, tyre sizes, and tyre ratings, possibly leading them to blindly assume it is safe to follow what has been said, with potentially disastrous consequences.

If there is a recommended pressure in a handbook, follow it: it will at least be the fail-safe option.  If the ride, or handling, of the vehicle seems unsatisfactory at those pressures, go to a weighbridge with the vehicle in full touring trim, including all passengers, and get it weighed to show the actual loads on the individual axles.  Get, and keep, a weighbridge ticket.  Relay the axle load information, together with vehicle type, and tyre size and type - including aspect ratio - to the actual manufacturer of the tyres, and get their recommended pressures for your actual axle loads, and ensure it is confirmed in writing.  Regard these as the minimum safe pressures to use with the laden vehicle.

Whatever then happens as you drive around, whatever checks your vehicle is subjected to, you can produce evidence to demonstrate why you are not using the handbook pressures which, for legal (and insurance) purposes, will be the presumed default pressures.  Anything less, without the evidence, will be liable to be construed as under-inflation, and may cause you difficulties.

[spartan3956]

I take your comments on board not every one has the back ground and has to guided by the handbook quotes.

 

I did this initially , but found that the pressures quoted did not suit the touring trim of my vehicle.

 

I have travelled extensively in the uk and Europe for years and have never had any problems with tyres.

 

However i do change the tyres every 5 years way before the tread is at its legal limit.

ive seen enough old caravans turn over due to tyre wall/steel core failure. You dont find that advice in the handbook. Would you be uninsured if you had not replaced them.

Its a good job we have MOT tests.

 

 

 

 

[Derek Uzzell]

The UK's MOT test (which, of course, does not apply to caravans) takes no account of the age of a vehicle's tyres. Legally, there's no reason why one should not use a vehicle with 10-, 20-, 30- or 50-year old tyres as long as the tyres comply with UK regulations regarding visual condition and tread depth.

 

I don't know what an insurance company's reaction would be if a vehicle were involved in a 'tyre-related' accident and it was discovered that its tyres were really, really old. I believe the UK tyre-industry recommends replacement no later than 10 years, but organisations like the Caravan Club advise a shorter duration.