Tyre Pressures

[Wirralian]

I have started something which has got me thoroughly confused.

 

I have a Fiat Ace Milano (2004) van with Michelin Camping tyres on the front and Continental Vanco on the rear. On cab door pillar Fiat indicate 79.5 psi tyre pressure and in the owner's handbook quote 70psi for the camping tyre. I have always gone with the handbook as the quoting of 'camping' tyre is more specific.

 

Having recently had the van weighed, I today contacted Michelin for the tyre pressure for the front axle. I was initially quoted 59psi (with 68 rear)and after I sounded a bit suprised a check was made with their technical dept who came up with 50 (front) and 80 (rear)!

 

As for the Continental tyre pressures that is another tale. I contacted their UK head office in Middlesex. The best they could do was refer me to some tyre fitters. They thought that 53psi all round was about right. I then looked up several sites on the net and the only definitive figure was 65psi all round.

 

What a run around!

 

 

 

:-S

[Brian Kirby]

You need to give the tyre manufacturer the exact details of the tyres fitted, that is to say the complete size details from the sidewall including speed rating, as well as the tyre type (for example XC Camping).  You also need to give them the  recorded loads at each axle.  When you get through to either companies technical department, ask for the name and e-mail address of the person you are speaking to, and ask them if they'd mind answering your query by e-mail, so that you have evidence of their advice in case you are ever challenged (for example at a VOSA roadside check) over pressures that differ from those in the vehicle handbook.  Then send them the details of tyre type and size, as above, plus the recorded axle loads.  Both companies have been quite happy to do this in the past.  You should then keep their replies, plus your weighbridge certificate showing the axle loads, in your van with the handbook.

Alternatively, go to www.btmauk.com and download the leaflet entitled "Motorhome Tyres and Your Safety" in which you will find charts giving appropriate tyre pressures for loads for various types and sizes of tyre.  Both Michelin and Continental are members of the British Tyre Manufacturers Association (BTMA), so should be quite happy for you to adopt the pressures recommended in the leaflet.  If in any doubt check, quoting the leaflet: ditto if tyres of the type/size fitted to your van aren't listed.  If taking this route, I suggest you print off a copy of the leaflet, and keep that in the handbook with the weighbridge cert.  Michelin have advised me in the past to add 5% to the actual recorded front axle load when selecting the appropriate tyre pressure, to allow for cornering and braking weight transfers.  Remember that load shifts inside a van as you use it (water and gas in/out etc), so you may need to add a small margin both ends before selecting the pressures.  Underinflation relative to load is dangerous as it leads tyres to overheat and eventually break up.  A little overinflation, on the other hand, has little impact on safety, although it may lead to increased wear to the centre ribs of the tyre and to increased harshness of ride.

Because motorhome manufacturers cannot know how an individual user may load his van, they invariably quote pressures to suit the maximum permissible loads for both axles, a load condition that cannot legally be met in almost all cases, because the plated MAM would be exceeded.  Thus, the handbook pressures are always liable to be too high for load at one axle or the other, and frequently at both - in other words the tyres will almost certainly be technically overinflated at the handbook pressures.  On the other hand, the correct pressures for load give optimum road holding, handling and comfort.  However, the actual laden weight should be checked periodically, since we will add a bit here and there as we go along, so that our vans soon weigh more than when we last visited the weighbridge, with our tyres needing to be run at correspondingly higher pressures.

[RonB]

I think you will find that Michelin have thrown their hand in with the powers to be and now quote 80psi for the rear tyres, no matter what the axle load, in order to allow for overloading. I recently received a letter from them confirming as much and also acknowledging that they were aware that the pressure given was not the 'correct' one technically.

 

As you are only interested in the correct pressure for any given rear loading the way round the problem is to contact them twice, on one such occasion substituting your rear axle weight for your front weight and so getting the correct weight required.

 

How one stands in law is another thing of course. However, my view is that Michelin, or anyone else, cannot prove that your tyres were under inflated when you have proof that says otherwise. Whether they are on the front or back axle has little bearing on the matter and in fact front tyres take a heavier battering than rear ones - for any given weight.

 

[Brock]

Tyre pressures. AAAAAAAAAAAAAAAArgh!

 

Fifteen years of motorhoming and I've never been able to get a definitive answer on what my pressures should be. One suggested that motorhomers were a sensitive bunch always wanting to get the tyre pressures exactly right - and then not changing them when they change the load - whereas van drivers are happy to have a tyre pressure that reflects maximum loading.

 

My current Hymer handbook (not the vehicle handbook) says 5 bar all around for Michelin camping tyres, and 4.1/4.5 (front and rear) for other tyres. Each time my van comes back from servicing, the tyres pressures are less but how much less varies! At least they come back with the same pressure in each tyre.

 

I run at 5 bar and trade ride comfort for the belief that it's probably the best pressure unless I am going to fiddle around so much.

[Guest Tracker]

The Autocruise and the Alko handbooks of my old shape 2006 Boxer Autocruise Starburst on Alko chassis with 3500kg gvw both clearly state 55 psi front and 45 psi rear for the XC Camping tyres and at these pressures it looks right with the van standing with just about the right levels of compression on the tyres and it handles well - or as well as any van can handle - and rides very well too. The tyres never feel more than mildly warm even after a motorway dash at 70 mph - not done very often at that speed - but there are times when the need arises! This is of course well at odds with the door pillar sticker pressures - or it would be if the van had one?

[Derek Uzzell]

Wirralian

 

If you've been running your motorhome's tyres with the handbook's 70psi all-round recommendation and you've been satisfied with the vehicle's performance regarding braking, handling and ride comfort, and tyre-wear has been OK, then (in my view) you might as well continue using 70psi. Even though this pressure may not be optimum, it's at least well on the safe side.

 

Tracker

 

Personally, I'd be very concerned about your 45psi figure - it's startlingly low for a light commercial vehicle, particularly for the rear axle where one might reasonably expect the weight-loading to well exceed that of the front axle.

 

Despite what you say about your motorhome apparently performing OK, I strongly suspect that the Autocruise/Al-Ko 45psi figure is wrong. For motorhomes constructed on the same 3500kg chassis as yours and using 215/70 R15C XC Camping tyres, Hobby advises pressures of 4.5bar(front axle) and 5.5bar (rear axle). While fully acknowledging that these figures are high (probably unduly high for many motorhomes), at least they are rational.

[Brian Kirby]

Rich, I agree with Derek, the quoted pressures for your van do seem oddly low. 

Our last van (Burstner) was on the facelifted (i.e. pre X250) Ducato, albeit without AlKo chassis.  The plated axle maxima were 1,750Kg front and 1,900Kg rear, with an MAM of 3,400Kg.  The handbook tyre pressures (Fiat and Burstner), for 215/70R15 Michelin Camping tyres, were 5.0BAR/72PSI all round. 

Using a formula given me by Michelin, which includes increasing the front axle load by 5%, the pressures for loads at the above axle maxima would be 4.0 BAR/58PSI front and 4.1BAR/60PSI rear.  The difference between the handbook pressures, and the above calculated pressures, being the manufacturer's covert "overload" margin. 

Inverting the Michelin formula, for 45PSI at the rear axle you'd be looking at an actual laden axle load of between 1415 - 1440Kg.  For 55 PSI at the front, using the same formula and allowing for 5% increase in load as above, you'd be looking at between 1645 - 1675Kg.  These look more like unladen, rather than laden, axle loads to me.

Might be worth checking.

[Guest Tracker]

Thanks Derek & Brian and initially I too was concerned about the rear tyre pressures.

However a visit to the weighbridge confirmed that in all up fully laden mode the weight is 1650 kg front and 1510 kg rear - 3220 kg in total and given the weight distribution I was less unhappy than before.

The MAM is uprated from the standard chassis 3400 kg to 3500 kg with the Alko and the user payload is a very generous 739 kg.

The weight distribution is explained by the very long wheelbase that the Alko provides which not only improves ride and handling - and creates a HUGE turning circle too - but tends to take excess weight off the rear axle due to lack of overhang and spread it more evenly between front and rear - the principle of levers etc etc!

The van does sit right with just the right deflection on the tyres as confirmed by a local tyre depot when I asked for an opinion prior to revealing the actual pressures.

We have covered some 14000 miles since buying the van including a winter trip to Morocco and an (allegedly) summer trip to the Outer Hebrides without mishap or overheating and whilst I hear all that you say I am happy with the tyre and van's performance as is and can see no reason to change from the stated pressures - which I can verify if ever called upon - but I am always open to new learnings!

[Brian Kirby]

You're welcome, though my Michelin table does suggest 48PSI for a 1510 Kg rear axle load!  Not a huge difference, though, and I don't know how much reserve your 1510Kg contains.  Nice light van though!

[Derek Uzzell]

Tracker

 

For 215/70 R15 XC Camping tyres my Michelin technical booklets indicate pressures of 51psi and 45psi as being appropriate for, respectively, static axle loads of 1650kg and 1510kg - essentially, the pressures you are using. What perplexes me is that (as I understand it) Autocruise and Al-Ko have somehow been able to predict that all Starburst owners will operate their vehicles with those axle loads.

 

Starburst has a 3500kg MAM chassis and it's invariably the case that the sum of individual axle load-maxima will at least equal the MAM. I would have guessed that a Starburst's maximum axle-loads (ie. the loads shown on its VIN-plate) would be roughly in line with Brian's Burstner at 1750kg(Front) and 2000kg(Rear) and, consequently, all the companies involved with your motorhome's manufacture (Peugeot, Al-Ko and Autocruise) would recommend in their handbooks tyre pressures appropriate to those axle-load data.

 

I can well understand a Starburst owner weighing his/her motorhome, establishing that the vehicle's real-world axle-loads are significantly below the VIN-plate limits and adjusting the pressures to match. What I do find strange is that the Autocruise and Al-Ko handbooks for Starburst apparently ignore the likelihood that an owner will operate the motorhome loaded to (or very close to) its VIN-plate MAM and/or axle weight limits. For instance, you've got 280kg of MAM left to play with. If you chose to exploit that 'allowance' and - for the sake of argument - positioned that load directly above the rear wheels, then you'd need to increase the rear tyre pressures to around 56psi to deal with the rise from 1510kg to 1790kg.

 

I've seen a good many motorhome-related handbooks and all the tyre-pressure recommendations in them have erred on the side of caution and assumed the vehicle may be operated at VIN-plate maximum axle loads. I just find it peculiar that Autocruise and Al-Ko handbooks choose to do otherwise.

[Guest Tracker]

First of all my apologies to Wirralian for high jacking his thread - it was never my intention when I posted some examples of tyre pressures from the Autocruise / Alko handbook and how they seem to be far more realistic than most handbooks that quote ridiculously over high pressures.

 

Derek,

 

The axle limits for the standard Peugeot chassis are shown as 1750kg F and 1900kg R with a MAM of 3400kg.

 

The Alko uprates that to 1750lg F and 2000kg R with a revised MAM of 3500.

 

In addition the Alko must be lighter than the steel Sevel offering which is partly where the very high payload figure of 739 comes from.

 

Most Autocruise vans of similar age and similar size have a payload of around 560 to 580(ish)kg so I can only assume that not only does the Alko uprate by 100kg it also adds in the weight saved by it's own presence - possibly around 60 to 80kg - a hidden bonus of Alko over steel maybe?

 

I note your comments re extra payload and this is why I have visited the local weighbridge twice in the last year to check that all the extra gubbins carried since getting the van has not distorted the weight balance.

 

All we need to do now to make it our ideal van is get the batteries to last more than two days in winter - but that's another story!

 

Thanks for your input Derek - very knowledgeable as ever and much appreciated!

 

 

 

 

 

 

 

 

[Wirralian]

Many thanks, Brian and Derek especially for such helpful responses.

 

John

[Derek Uzzell]

Tracker

 

Al-Ko chassis are also made from steel, though their perforated, bolted-together construction and revised rear axle/suspension may well save weight compared to original welded-steel chassis. However, I don't really see why that should have an impact on recommended tyre pressures in this instance.

 

I'm reluctant to harp on about this, but I still can't fathom why Autocruise and Al-Ko, for a motorhome with a MAM of 3500kg and maximum axle-loads of 1750kg(F) and 2000kg®, should, in their handbooks recommend tyre pressures appropriate for a motorhome with axle-loads of 1650kg(F) and 1510kg® and, consequently, a MAM of 3160kg. I'd expect at least 54psi(F) and 63psi® to cope with the 1750kg/2000kg VIN-plate axle-loadings. In fact, I'm surprised that Al-Ko (who will have supplied just the bare chassis-cab to Autocruise for conversion into a Starburst) would be in a position to recommend specific tyre-pressures at all.

[Guest Tracker]

After many years of driving all manner of vehicles one gets a feel for handling and ride control to the extent that when tyres are under inflated the pronounced body roll on bendy roads and reluctance of the vehicle to resume an upright stance before entering the next bend can be very dangerous and can easily induce enough understeer to frighten one witless. Similarly over inflated tyres give such a hard ride and so much bounce that on a bumpy and twisty road oversteer can easily be induced if driving with some gusto. Similarly the experienced driver can generally feel by the way the vehicle responds to steering input when he has a puncture, and if he is good at it, on which wheel.

 

Whilst driving a motorhome with gusto on twisty and bumpy roads is not to be advised - nor is it for the faint hearted - the above effects are much magnified due to the height and weight of this type of vehicle.

 

I have long experimented with motorhome tyre pressure - since the 70's long before it became politically incorrect to do so - and always with safety in mind. I long ago came to the conclusion, with the aid of a good friend who was a very wise and experienced tyre supplier and ran his own tyre supply and fitting business, that motorhomes often sat on the road without any tyre deflection at all suggesting a significant over inflation because tyres are designed to deflect where they contact the road surface. At the other extreme under inflated tyres have visibly too much deflection at the point of contact.

 

In years past when I experimented widely with tyre pressures I found that it was always wiser to have the rears between 5 and 10 psi more than the fronts as that helps prevent understeer. I also found that a change of pressures of at least 10 psi was needed before any real difference could be felt on the road and even then it often needed brisk driving on a bumpy twisty road to exagerate and highlight any issues. The usual 'window' of pressures that worked well depended on the van but most were carried out with either Bedford CFs or early A/S Talisman which did not have the huge overhangs of newer vans and were found to be from about 47 to 57 psi front and from about 52 to 62 rears with the happy point for handling and ride being around 50F and 57R.

 

I cannot stress enough that this is simply my own experience and I do not recommend that anyone else emulates it - unless you are competent and experienced enough to carry out your own experimentation with tyre pressures. And if you do - be aware that Mr Plod may take a dim view if you get it wrong as will your insurers in the event of an accident. You have been warned!

 

So with all the various theories flying about regarding 'correct' tyre pressures I still feel inclined to do it my way - especially when the makers handbook supports my own feeling about using the sort of tyre pressures that I have found to work well and which have been safe for over 35 years and probably close to 100,000 miles of motorhome use.

 

I've never had a blow out or an over heated tyre on a long fast motorway trip or a handling issue on twisty roads - plus my van always sit nicely on the road with just a small amount of deflection at the point of contact.

 

I am inclined to agree that 55/45 psi certainly sounds on the low side of safe and yet the van has behaved with perfect road manners on all surfaces over some 14000 miles. Maybe this is because the longer wheelbase and lack of 'pendulum' effect of the long overhang and highly loaded rear axle normally associated with motorhomes reduces the tendency for the back end to step out of line when changing direction rapidly with all the stress that it places on the tyres - and the driver?

 

I am not a vehicle designer nor am I a tyre expert but maybe a longer wheelbase allows a lower tyre pressure more in keeping with the actual weight of the vehicle and less as a compensation for the common but surely bad design practise of too much rear overhang in the name of cost cutting rather than using an appropriately long chassis for the conversion?

 

I dunno! But I am happy with what I am doing and I have two handbooks to support it!

 

I am also happy to hear the views of others in the process of making an informed decision - any other thoughts anyone?

 

 

[Derek Uzzell]

Tracker

 

I'm not quibbling over the tyre pressures you are using as they conform to Michelin's data for the your Starburst's tyres.

 

Starburst has a MAM of 3500kg and a payload of 739kg. I don't know if the latter figure includes allowances for freshwater, gas, etc. but, even if it didn't, it's still a very healthy payload for a fairly compact motorhome. 'Empty', your Starburst will weigh 2761kg and your weighbridge-confirmed axle-weights indicate that, in fully loaded trim, it weighs 3160kg. This means that, fully loaded, your motorhome is carrying an extra 399kg of 'stuff' - so you are hardly choosing to travel super-light.

 

The tyre pressures advised in your Autocruise and Al-Ko handbooks evidently match (pretty much exactly) your particular motorhome's axle-weights when the vehicle is in fully loaded state, but what I continue to ask myself is "How?" How can Autocruise and Al-Ko have known in advance that you would be carrying 399kg, distributed throughout your motorhome so that it would produce your 1650kg/1510kg axle loads? What if you chose to carry, say, 699kg of load within the vehicle rather than 399kg, or decided to hang 150kg of tow-bar/scooter on your Starburst's rear? It seems to suggest that Autocruise and Al-Ko have psychic powers.

 

I dread to say this, but 'official' tyre-pressure advice is not always correct. The pressure recommendations provided by Ford on my Hobby's door-pillar data-plate are definitely wrong and I've spotted obvious errors in Michelin's own handbooks. Even though the pressures you are using are 'right' for your motorhome's Michelin tyres and match the Autocruise/Al-Ko handbook recommendations, I still think there's something 'wrong' with the handbook advice - perhaps as a result of typographical errors or a confusion between bars and psi. While there's no good reason for you to alter the pressures you've been using to date, it might be interesting to know what pressures Al-Ko would currently suggest if you contacted them and provided tyre and chassis VIN-plate details. As motorhome manufacturers are rarely pro-active when it comes to providing tyre-pressure advice, if the Al-Ko handbook's advice were iffy there's every chance the Autocruise handbook's advice would be too.

 

A personal-experience comment of your first paragraph...

 

As I've mentioned before on this forum, my Hobby fell victim to the dreaded Transit Mk-6 tyre-valve failure disease, where one rear-wheel valve failed in transit (Ho Ho!) and the other when I had stopped to change the wheel. I shall never know exactly when the first valve gave way but, with the benefit of hindsight and being on-the-spot when the second valve packed up, it's likely that I drove for a couple of hours with 25-or-so-psi in the right rear tyre rather than its usual 55psi. At the start of the journey I had felt the vehicle 'jink' slightly as I negotiated a roundabout, but, as I was leaning the vehicle fairly hard on its outer tyres, I just thought "That's a bit odd - must have been a stone under the tyre". For the rest of the trip, on a mixture of French main roads, twisty secondary roads and a final stretch of autoroute, I didn't notice anything in the vehicle's behaviour to cause me alarm. It was only when I had slowed right down on leaving the autoroute that I could hear excessive tyre noise and guessed I'd got a seriously deflated tyre. I've been driving since 1960; I'm well versed in the delights of over- and under-steer: I can detect the effect of relatively small changes in tyre pressures on cars and I'm happy to adjust my vehicles' tyre pressures to suit my personal preferences. But, even though one of my Hobby's rear tyres was hugely under-inflated, I failed to notice any adverse effect during normal driving. I'm sure I'd have noticed immediately if the valve-failure had been on a front wheel, and I'm fairly confident, if I'd attempted some serious high G-force roundabout rotating, that I'd have spotted something was wrong. But, in normal driving, nothing untoward was evident. I suppose there are two morals to this tale: one - that a motorhome with a well under-inflated rear tyre may handle OK (at least for a while!), and two - that it's a good idea to fit clamp-in tyre valves to Mk 6 Transits.

[johngee]

Oh no! Reading this thread has given me something else to worry about!

 

Checked my pressures - 60 front, 65 rear.

Looked at the sticker on the door pillar - 70 front, 79 rear.

OMG!!

Looked in the motorhome blurb - 60 front, 65 rear.

 

Phew, panic over, aaaaannnnnnndddddd relaxxxxxxxxxxxxxxxxx.................

[brom]

I've found this thread very interesting, as my own Trendscout (Ford Transit Mk7, LWB, FWD, 2.2 litre High-top, 3500kg chassis, maximum permitted on front axle 1750kg, and back axle 2250kg - total 4000kg(?)) has always felt as if the tyres were over-inflated. The van seems bouncy, and doesn't handle quite as sportily as some Fiats I've driven. The steering is more twitchy, too.

 

So, as we've often intended, but never before 'had time', it was off to the weighbridge, with 90% water (90litres), 90% fuel, full gas and all our normal clutter on board - and driver and passenger, too. That also includes the towbar and spare wheel. Total weight came out at 2,940kg, leaving a spare payload of 560kg. Front axle load = 1520kg, rear axle= 1400kg, with 20kg lost somewhere in between.

 

Armed with that, I checked the tyres - Turkish-made Bridgestone R623, tyre size being 215/75/R16C. I rang Bridgestone UK and left the details of the tyres, and they emailed to request full details of the Transit, as they said the calculations might vary according to the van spec.

 

I spoke to the (very helpful) chap there, and gave him the details. I wanted to know what variation on pressure would be needed if load rose by,say, 50kg. He gave me the tyre pressures recommended for the pressures I had supplied - and they were an eye-opener!

 

He emailed the results to me, and I shall, as suggested on the thread, put the print-out in the van's papers, for production if necessary.

 

The sticker on the B-pillar of the van says pressures should be 50.8psi front and 68.9psi rear, at max load. And that's what I had them on, on a 'better safe than sorry' basis. The 'correct' figures from Bridgestone are 41psi (front) and 39-41psi (rear)! The chap thought (as I did) that that sounded a bit low, so he suggested 43 (front) and 47 (rear) as a compromise, giving lots of leeway for extra weight, and he put that in writing, which was very helpful of him.

 

The moral of which is, of course, that you should always take the trouble to check weights at the weighbridge with your normal full load on board, and then check with the tyre manufacturer what the pressure should be. It seems from what has been said previously on the thread, that not all are as helpful as Bridgestone.

 

I have just let twenty pounds of pressure out of each back tyre, and seven out of the fronts. It will be very interesting to see how that affects handling and fuel consumption....

 

Brom