Tyre Pressures

[davenewellhome]

trooper - 2007-07-31 7:47 PM

 

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)

 

Are you sure your MTPLM is 4000KGs Trooper? I ask because the sum of the maxima of your two axles is only 3750 KGs, a full quarter of a Tonne short of your quoted 4000KGs.

 

Confused it is I am 8-)

 

D.

[Derek Uzzell]

tonyishuk:

 

Michelin's technical handbooks provide a wealth of data about the company's tyres. These include dimensional information about a tyre's section width, overall diameter, static laden radius and rolling circumference, plus the tyre's load/speed index, minimum/maximum rim size, etc.

 

However, if a motorhome owner seeks advice on tyre-pressures based on axle-loadings (VIN-plate or measured), then Michelin's recommendations will be derived solely from a pressure-to-axle-load 'graph' for the particular tyre in question. A simplified version is included in the handbook and comprises a 3-point (sometimes 4-point) scale. For example, for 225/75 R16C Agilis 81 tyres used on a 'single wheel' axle the figures are as follows:

 

Axle-load/Pressure

2640kg/76psi

2410kg/58psi

1950kg/44psi

 

Michelin say that 'joining the dots' permits intermediate pressure-to-axle-load figures to be derived. My experience of doing this and taking advice from Michelin is that 'my' calculations did vary from theirs (as the 'graph' is undoubtedly a curve rather than straight-line), but the variation was acceptably small. It is imperative that owners of motorhomes with 'twin wheel' axles (eg. certain RWD models from Ford, Iveco and Mercedes with paired wheels on each end of their rear axle) reveal this when seeking advice from tyre manufacturers as it affects recommended pressures radically.

 

Things like tyre-footprint, construction, tread pattern, compound, etc. will have been taken into consideration by the tyre manufacturer at the design stage and (if a tyre is to be fitted to a vehicle as original equipment) there will perhaps have been co-operation with the vehicle manufacturer as well. But these factors won't normally have any bearing on Michelin's advice to motorcaravanners about suitable tyre-pressures.

 

Dave:

 

According to his profile "trooper" owns a Burstner Elegance I-685. This is built on a (Fiat) 4000kg chassis that (I'm pretty sure) has VIN-plate axle-load limits of 2100kg (front axle) and 2400kg (rear axle). So trooper's weighbridge-measured axle-load figures of "front 1810 kg, rear 1940 kg" are indeed well within legal limits.

[Dave Newell]

Thanks for that Derek, I must have been having a senior moment and thought he was saying that his MTPLM was 4000KGs but his axle maxima were front 1810 kg, rear 1940 kg rather than the actual weights as measured. :$

 

D.

[Mel E]

Of interest, I have just discovered that Continental have introduced a special Vanco Camping tyre for mobile homes.

 

Mel E

====

[Brian Kirby]

tp002c784tp002c784tp - 2007-07-31 8:06 PM 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

Which, in the nicest possible way, goes to show why Frank should weigh his own van, and get his own recommendation from Michelin (or whoever), based upon the tyres actually fitted to Frank's van. 

Tyre pressures are safety critical, and it is unwise to use anyone else's data as a basis for departing from the handbook recommendation for one's own van.

Last point on this: I used a version of Mel's axle load calculating spreadsheet to "model" different loading conditions in our van.  I also gained a little load/pressure chart from Michelin, which included a recommendation to increase the front tyre pressures by 5% to allow for forward weight transfer on braking.  (Interesting, that bit, innit, Clive?  You must really hammer your van round corners! :-))

What I found was that to satisfy the inevitable "touring" load transfers between front and rear axles - such as, for example, water transfer from the midships fresh tank to the rear mounted (behind the axle on ours) waste tank, plus additions such as bottles etc - while meeting Michelin's recommended pressures for load, the tyres would need to be varied between 54 and 57 psi at the front and between 51 and 54 psi at the rear. 

Inevitably, I adopted the higher result at both ends to be sure of adequate pressures for the worst case, so leaving both ends a bit overinflated relative to load nearly all of the time.  The result was a quieter, smoother ride, improved directional stability, less sensitivity to "tramlines" etc , a quicker "turn in" on approach to bends, and less tail end wallow.  In short, the whole van performed better dynamically than at the handbook 72psi all round. 

However, the main point of this story is that static weighings, as Clive surmises - though not for the reason he cites - do not quite give the whole picture.  One needs to treat all these calculations with a bit of caution, and be prepared to increase the pressures relative to static load by sufficient margin to cater for the way the load changes as one travels.  Simplest rule?  If that sounds like too much complication, stick to the handbook!

[trooper]

hi dave the weights where actual, not as spec, the manufacturers weights are,front 2100, rear 2400, the m/h is a Burstner 684, based on the alko heavy chassis, it has a very good payload.

[Guest caraprof]

Brian, Thank you for what I know is good advice but here is the reason why I've not yet weighed my van.

I took it to my local council weighbridge and asked the bloke if he'd give me a front and rear axle weight and a whole-vehicle weight while he was at it. He told me that it would be £10 per weigh and asked for £30. I told him politely to bugger off!

A couple of days later I phoned his boss who said that the lad is a fool and he should have charged me just £10 for the certificate for the whole vehicle and simply jotted down for me the front and rear axle weights.

In the meantime I've decided to get a bike rack fitted and buy two bikes, so I thought it politic to wait for my weigh-in until that job was done.

In the meantime, Terry Peach emailed me the response that he'd had from Michelin, which prompted my thread. However, the gist of my thread wasn't really about the exact pressures for my 'van, but was more about the huge differential between the manufacturer and Michelin.

I shall have mine weighed fairly soon when the rack is fitted and the second bike has arrived and of course, the weights will be different then from Terry's (we have absolutely identical models by the way).

But I know what will happen! Michelin will, no doubt, quote me much lower pressures than those in my handbook, but when that happens I am now much more confident about following its advice, especially after Dave Newell's comments.

[tp002c784tp002c784tp]

Hi Caraprof

One thing that has not been mentioned in this thread about pressures, By law you must have the correct pressure in all your tyres, so how can you drop your pressures from 67 and 81 down to 51 all round and still be within the law.

 

Who would the law believe tyre manufacturers or the vehicle hand book.

 

Terry

[davenewellhome]

tp002c784tp002c784tp - 2007-08-07 7:29 PM

 

Hi Caraprof

One thing that has not been mentioned in this thread about pressures, By law you must have the correct pressure in all your tyres, so how can you drop your pressures from 67 and 81 down to 51 all round and still be within the law.

 

Who would the law believe tyre manufacturers or the vehicle hand book.

 

Terry

 

If you can demonstrate that you have at least attempted to establish true axle weights by means of a weighbridge and produce a printout of an e-mail from the tyre manufacturers stating the required pressures for the tyre size, type and loading as per the weighbridge certificate then any authorities would accept that you'd acted responsibly and not apportion blame to you.

 

D.

[Derek Uzzell]

Terry:

 

You say "By law you must have the correct pressure in all your tyres". That's a very authoritative statement, but would you like to quote how that law is worded please, so we know where we stand? If tyre pressures are to be judged legally "correct", there must be a legal definition as to what "correct" means.

 

Tyre-pressure recommendations given in a light commercial vehicle handbook are normally 'worst-case single-figure' purely because the vehicle manufacturer can have no clue how a buyer of that vehicle will be loading it. This is why the data in most instances equate to the pressure requirements necessary to support the vehicle's maximum authorised individual axle-loadings. That the vehicle would be seriously illegal overall-weight-wise if its axles were all loaded to their maxima is ignored. Motorhomes are a different kettle of fish as their owners are perfectly capable of measuring their vehicles' 'normal use' axle-loadings that will often be well below the axles' permitted maxima.

 

My Hobby's maximum permitted axle-loads are 1665kg (front) and 2250kg (rear). Its real-world axle-loadings - that's with the vehicle in 'holiday trim' with full tanks and allowing for anything extra I can imagine carrying (like 100 or so bottles of wine!), plus an allowance for rear-to-front weight transfer under braking - come to 1520kg (front) and 1800kg (rear). For these axle-loadings Continental recommended pressures of 3.1bar (front) and 3.65bar (rear). I favour a firmish ride, so I use 3.2bar (front) and 3.8bar (rear).

 

For static axle-loadings of 1665kg or 2250kg Continental advised 3.24bar and 4.76bar respectively, but surely you wouldn't suggest that I alter the pressures to these figures to cope with hypothetical weight increases of 125kg on my Hobby's front axle and 450kg on the rear axle when there's absolutely no possibility of such loadings being realised?

 

These single-figure tyre pressures seem to engender a 'one size fits all' philosophy in many motorcaravanners. With cars, the manufacturer often provides tyre-pressure recommendations relating to more than one driving scenario. For our Golf Estate car VW advise 2.0bar (front and rear) for a scenario involving 3 people + luggage on board, while 2.2bar (front) and 2.8bar (rear) are specified when 5 people + luggage are to be transported. It's very rare for our car to carry more than 2 people (a lightweight me + a lightweight wife)), so I stick to 2.0bar pressures all round, resulting in a firm but comfortable ride. I'm confident that nobody on this forum would suggest that I really should always use 2.2bar(F)/2.8bar® in our Golf's tyres just in case I suddenly encountered 3 massive hitch-hikers with haversacks one day and wanted to give them a lift. But that's exactly the mind-set some motorcaravanners appear to have.

[Mel E]

Derek,

 

You don't mention your MAM/GVW, but if it's the standard 3500 Kgs, then on those axle weights, you have just 50Kgs left for your good self after loading those 100 bottles of wine!

 

Just so everyone knows, a case of 6 glass bottles of wine weighs in at 7.6 Kgs!

 

Mel E

====

[Basil]

I find all of this very strange. At the end of the day if you use the tyre pressures as stated in your handbook, that is surely what you can stuff under the nose of anyone who has a legal reason to require you to show it and if your tyres corespond to that and you are observing your plated wieghts, what is the problem?

 

Bas

[Derek Uzzell]

Basil:

 

Personally, I've no problem whatsoever with what you are suggesting. If a motorhome's tyre-pressures conform to 'handbook' recommendations and the motorhome owner is a) confident that those recommendations are trustworthy and b) happy with the driving experience that results (ride is comfortable, handling/road-holding is fine, braking is secure, no evidence of tyre overheating/unusual wear, etc.), then there's no good reason to alter them and, as you rightly say, there's no potential for 'those in authority' to argue that one is using wrong pressures.

 

In my particular case, not only did I have no faith in the pressure recommendations in the handbooks for my motorhome, I knew from experience that they were wrong or unsuitable. That's what caused me to contact Continental initially in order to obtain some credible stop-gap pressure figures to use until I could weigh the vehicle. Once I had weighed the motorhome I could then revise those pressures based on the weigh-bridge measurements.

 

Genuinely wrong handbook pressures are probably (hopefully!) very rare. As I said earlier, it's far more common for handbook recommendations to relate directly to the motorhome base-vehicle's maximum permitted axle-loadings. If a motorhome's normal loaded state produces a situation where its axle-loadings are well below the permitted maxima (and this has got to be pretty prevalent), then the result is likely to be a noticeably hard ride that soon becomes irritating to the motorhome owner. In such cases it makes sense to weigh the motorhome and consult the tyre manufacturer. On receiving information from the tyre maker it's obviously up to motorcaravanners whether or not they continue to use handbook pressure recommendations and endure the hard ride or alter them in line with the tyre manufacturer's advice. That's what free will is all about.

 

(If anyone wants to know how one can possibly know (or suspect) that tyre pressure recommendations are "wrong or unsuitable", then I'm afraid that, unless it's blatantly obvious (like 35bar instead of 3.5bar), you'll need to rely on experience, gut-feeling or educated guesswork.

 

For example, the very high 80psi specified on my Hobby's cab-door plate for the motorhome's rear tyre-pressures (see my first posting in this thread) possibly harks back to early variants of this model that had Michelin XC Camping tyres as standard, whereas my own vehicle has 'ordinary' tyres with a stated design-pressure maximum of 70psi. But where the somewhat low 44psi front-tyre pressure recommendation came from is anybody's guess.

 

I wrote to Ford(UK) about this as the cab-door plate is a proper Ford-marked item rather than having being added by Hobby during the conversion process. The telephone response that resulted was, I suppose, predictably unproductive, with the Ford spokesman essentially saying it's a German motorhome so all bets are off, and me replying that it's got a Ford plate with wrong data on it and, now that you know about it, you can please yourself whether or not you take any follow-up action.)

 

 

General comments:

 

It seems from motorhome forum postings (and I'm not just referring to this forum) that many motorcaravanners demand black-or-white answers to what are really grey questions.

 

I weighed my motorhome on a DIY French weigh-bridge and, to obtain the readings, someone had to press a button in the little hut alongside. I drove the vehicle on to the weigh-bridge platform and my wife pressed the button.

 

Prior to consulting Continental (the manufacturer of my motorhome's tyres) I adjusted the values of the measured axle-loadings to include allowances for a) my wife being on board, b) several more cartons of wine (I already had 60 or so bottles squirreled away in the motorhome), and c) water transferring from the Hobby's 100 litres rearwards-mounted fresh-water tank to its frontwards-mounted 100 litres waste-water tank. Then, once I had received Continental's advice based on my modified figures, I added some extra poundage to the front-tyre recommendation to allow for front-to-rear weight transfer under braking and a couple of psi to their rear-tyre recommendation 'just for luck'.

 

I'm not advocating that other motorcaravanners carry out similarly elaborate calculations - that's how I did it and I'm satisfied with the result. As a precautionary measure, in the unlikely case I ever need immediate access to the information, I keep copies of my correspondence with Continental with my motorhome documentation as 'provenance' for the tyre-pressures I've chosen to employ.

 

It can never be emphasised too strongly or too often that tyre-pressures should NEVER be too low for the job in hand. According to Michelin under-inflating a tyre by 20% reduces its life by around 26%. (Overloading has a similar effect - 20% overload decreasing a tyre's life-span by approx. 26%). Even more important than their effect on tyre longevity, of course, is that under-inflation and overloading are bloody dangerous practices.

 

 

Mel E:

 

My Hobby's MAM is indeed 3500kg.

 

You'll see from the above that I had factored the wine into my weight calculations. I thought my wording had covered that, but there you go!

[Basil]

Hi Derek,

 

I believe I understand now from your point of view that having changed from a completely different type (not just make) of tyre you wanted a revised psi figure for your new ones. Thanks for your explanation.

In our case I have the same type of tyres, though more modern tread pattern, as origionally fitted so use the figures as stated on the door pillar and in the handbook, i.e. 59 front and 65 rear.

 

Bas

[Derek Uzzell]

Basil:

 

I'm not sure how you've reached that conclusion.

 

There is no doubt that the tyre-pressure recommendations on my Hobby's Ford-provided door-pillar plate are wrong. The recommendations in the Hobby handbook supplied with the motorhome are 'generic' and, in fact, clearly relate to Fiat-chassis Hobby models rather than Ford-based ones like mine. The recommendations in the Ford Transit Owner's Handbook relate to panel-vans or chassis-cab vehicles, not to the specialised platform-cab chassis on which my motorhome is constructed.

 

Until I weighed my motorhome I used the pressures recommended in the Transit Owner's Handbook for chassis sharing the same wheelbase, weight characteristics and tyre size as my motorhome. However, I was always aware that these were based on maximum permitted axle-loadings and that my motorhome's actual loadings were likely to be significantly different. But no tyre changes - make or type - were ever involved.

 

Having said that, there may well be a need to revise inflation pressures when tyres are changed.

 

My previous motorhome was a 1996 Transit-based Herald Templar and the handbooks (Ford and Herald) gave the same tyre-pressure recommendations. Although it was evident that these related to the vehicle's maximum permitted axle-loadings I never found the ride to be in overly harsh. So I never diverged from the handbook recommendations even after weighing the Herald (and consulting Michelin) had revealed that the recommended rear-tyre pressures could be safely reduced. However, it may be worth highlighting that the Ford/Herald handbook recommendations were 41psi (Front) and 53psi (Rear), nothing like the ultra-high pressures that are sometimes specified for Fiat/Peugeot-based motorhomes.

 

When the Herald's tyres were replaced, a different Michelin tyre (Agilis 81) was fitted with a higher load-bearing capability than the originals that were no longer available. Based on Michelin's advice the replacement tyres were run at slightly higher pressures than before to reflect their different technical characteristics.

[Basil]

Derek Uzzell - 2007-08-10 9:45 AM

 

Basil:

 

I'm not sure how you've reached that conclusion.

 

.

 

From your statement

 

""For example, the very high 80psi specified on my Hobby's cab-door plate for the motorhome's rear tyre-pressures (see my first posting in this thread) possibly harks back to early variants of this model that had Michelin XC Camping tyres as standard, whereas my own vehicle has 'ordinary' tyres with a stated design-pressure maximum of 70psi. But where the somewhat low 44psi front-tyre pressure recommendation came from is anybody's guess. ""

 

So presumably that, combined with your feeling that the pressures were not right made you investigate further.

 

Sorry am I missing something else?

 

Bas

[Guest caraprof]

As the instigator of this post (who's just returned from a few days away) I've found all of this very helpful, as well as originally a little confusing, as some of our very knowledgeable people seem to contradict each other! Like medicine, some aspects of motorhoming seem to be an inexact science!

However, can I make this assumption?

Many motorhome manufacturers base their tyre pressures on the base vehicle's recommended figures. These may well assume that the base vehicle will be carrying anything from hundreds of parcels to builder's supplies.

Very few motorhome manufacturers seem to go to the trouble of calculating the typical and likely axle weights of their creations and simply rely on the vehicle manufacturer's figures.

When the vehicle maker decides on a recommended pressure it does not take into account the rather different 'Camping' tyres that will eventually be used.

If I am reasonably correct in these assumptions then surely the wisest thing to do is to consult the real experts (the tyre manufacturer) and use its recommendations?

I wait to be shot down in flames!

[robin]

This may be interesting ....

 

M'home -

 

Weinsberg Imperiale 670 on Renault Master 3.5t plate (2006)

(for those that don't know Weinsberg are part of Knauss / Tabbert group and my 'van is a more or less re-badged Knauss Sun Ti)

 

Front Axle 1870k (max)

Rear Axle 2050k (max)

 

XC Camping 225 / 65 R 16

 

Weinsberg Handbook Pressures

Front 3.8 bar (55 psi)

Rear 4.4 bar (63.8 psi)

 

Michelin advice

Front 4.14 bar (60 psi)

Rear 4.48 bar (65 psi)

 

So Weinsberg and Michelin agree on the rears, I don't know why not on the front. It may be that I am quoting max axle weights which are only likely to be reached if the 'van was plated up to 3.9t

 

I am settled to 4.1 front and 4.5 rears.

 

Cheers