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Tyre pressures - Leap of faith?
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userRobinhood
Posted: 25 June 2020 2:57 PM
Subject: RE: Tyre pressures - Leap of faith?
 


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costaexpress - 2020-06-25 2:20 PM

Now I just need to get over the feeling that I'd rather stick my head in the sand and remain in ignorance, after all 'what f I don't like the answer!'.


...you know it makes sense!

TBH, whilst I think you might be surprised at the overall weight, given your posting on accessories and loading I doubt you'll find yourself over the MAM of 3500kg, or over the individual max axle weights unless the design is particularly poorly balanced.

You'll certainly also be well placed to close the debate here on your tyre pressures and have an easy mind.

Given load and distribution have quite an effect, however, I would weigh in full "going away" order, with you (and any other usual incumbents) in the 'van, fuel, water and gas as full as possible.

My 10% allowance on the measured axle loads is there simply to allow for varied loading under different circumstances (and, as I say, still results in pressures (well) below the 5/5.5 bar recommendations.
userDeneb
Posted: 25 June 2020 3:10 PM
Subject: RE: Tyre pressures - Leap of faith?
 
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Brian Kirby - 2020-06-25 2:45 PM



I believe most, if not all, of the old mechanical weighbridges are now out of use for trading standards purposes, so the sorts of inaccuracies you refer to should not arise on weighbridges that come under trading standards control. But, perhaps folk would be wise to check whether the weighbridge they use is regularly checked by trading standards.


Well I can't argue with what the operator told you. He's quite correct that the individual axle weights are likely to be less accurate if you can't place the axles near the centre of the plate, but to be honest that just makes weighing both axles more useful, not less, as by using subtraction you are just deducting one possibly inaccurate weight from the gross to derive another. At least if you have two less than accurate weights which are ikely to show a similar degree of error, by comparing the sum of both to the gross you have a much better idea of the likely true figures for both.

Two of our local bridges certified for enforcement are traditional plate bridges. One belongs to the county council and operates in 20kg increments. I know because I have both used it for enforcement and training for my weighbridge operator's licence. The second belongs to our local council and uses 30kg increments according to their published information.


Edited by Deneb 2020-06-25 3:11 PM
userSteve928
Posted: 25 June 2020 3:10 PM
Subject: RE: Tyre pressures - Leap of faith?
 


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I often divert to the DVSA weighbridge nr. Stirling as it's a drive-over type that weighs single axles rather than plate type; you can only get a total weight by addition of the individual axles. It's self service when un-manned although I admit to doing an about turn on one occasion when I found it to be manned, just in case One quiet day I did drive around and around and repeat the weighings 3 times and they were consistent, which was nice.

Recently I bought a Reich weight scale because they were on offer at Obelink.
I can't say I'm impressed as you get wildly different readings depending on how smoothly you can manage to drive over it - and it presents quite a resistance to you so doing.
I've now constructed a plywood ramp with a plate that sits on the Reich to allow a smooth and consistent passage across it but have yet to give it a thorough test to see if gives an improvement. It does now give repeatable readings using my loaded wheelbarrow though!
It will be nice to have individual wheel weights, if I can manage to get them consistent and in agreement with the DVSA's weighbridge totals for the axle.
userDeneb
Posted: 25 June 2020 3:25 PM
Subject: RE: Tyre pressures - Leap of faith?
 
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Steve928 - 2020-06-25 3:10 PM

I often divert to the DVSA weighbridge nr. Stirling as it's a drive-over type that weighs single axles rather than plate type; you can only get a total weight by addition of the individual axles. It's self service when un-manned although I admit to doing an about turn on one occasion when I found it to be manned, just in case One quiet day I did drive around and around and repeat the weighings 3 times and they were consistent, which was nice.



Dynamic axle weighbridges typically have an accuracy of plus or minus 150kgs per axle in "drive over" operation. From that you can see that you could drive over one all day as many times as you liked, and unless you had made substantial changes to the weight of your van between each pass, it would be pretty surprising if you hadn't obtained consistent readings with a relatively light vehicle!
userEJB
Posted: 25 June 2020 3:26 PM
Subject: RE: Tyre pressures - Leap of faith?
 
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Steve...…."It will be nice to have individual wheel weights, if I can manage to get them consistent and in agreement with the DVSA's weighbridge totals for the axle"

Happy days does that mean you will be able to have different pressures in all four of the wheels instead of boring old axle
pressures?


Edited by EJB 2020-06-25 3:26 PM
userTeamRienza
Posted: 25 June 2020 3:37 PM
Subject: RE: Tyre pressures - Leap of faith?
 
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Bear in mind that tyre pressures can increase by as much as 10psi when up to temperature, so adding extra air before a long journey may simply increase wear on the tyres.

Davy
userBrian Kirby
Posted: 25 June 2020 7:20 PM
Subject: RE: Tyre pressures - Leap of faith?
 


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EJB - 2020-06-25 12:01 PM
I wonder why tyre manufacturers have advised lower pressures for many years.......when MH manufacturers are allegedly more expert in the subject. Really?
Derek Uzzell - 2020-06-25 8:57 AM
DickB - 2020-06-24 10:19 PM
...The figures given by the chassis manufacturer are based on the maximum loading of it as a commercial vehicle....

That’s not the case for motorhomes where ‘camping-car’ tyres are factory-fitted as standard equipment.
Ever since Noah put wheels on the Ark and drove it off Mount Ararat, there have been just two sets of advised tyre inflation-pressures for Fiat Ducato-based motorhomes
- for 15” wheels 5.0bar (front wheels) and 5.5bar (rear wheels).
- for 16” wheels 5.5bar (front wheels) and 5.5bar (rear wheels).
and the motorhome’s design and how many axles it has are immaterial.
It’s completely predictable that this will result in a harsh ride, but few motorhome manufacturers are prepared to diverge from the above figures.
I remember talking to a Continental tyre technician years ago and him telling me that their CP-marked camping-car tyres were designed to cope with the high pressures, adding that the motorhome might shake to bits or the axles become damaged, but the tyres ‘can take it’.

Is there a misunderstanding, I wonder?

The question about who knows best which pressures to use is that each knows which pressures to use, but only the owner of the motorhome, having weighed their van in full operating trim, knows what actual load each axle is being asked to carry.

The motorhome manufacturer, and the base vehicle manufacturer, only know what the safe maximum load for each axle is, but cannot know whether a subsequent owner will load the axle to that degree. So, they give the "fail-safe, worst case, pressure that is appropriate to the maximum load condition. If a tyre that fails in use it will then be because of a defect in the tyre (whether from manufacturing or damage) or because the owner has overloaded their van, and so overloaded the tyres. All they are doing is protecting themselves from liability for accidents caused by owners overloading their vehicles, which, in their shoes, is the sensible thing to do.

OTOH, once the owner knows their actual, laden, axle loads, and can advise the tyre manufacturer what they are, the tyre manufacturer can then advise the use of lower pressures appropriate to the actual axle loads. Continental very conveniently publish their excellent Databook, that allows owners to select the optimum pressures for their actual axle loads. Michelin will only advise reduced pressures for front tyres - apparently based on their experience of French motorhomers suffering from rear blow-outs that Michelin became convinced were due to overload, which the motorhomers vehemently denied. So, to protect their reputation, they decided all motorhomers overloaded their rear axles and henceforth refused to advise any pressure reduction for the rear axle on CP (and I believe, if "motorhome" is mentioned, any other!) tyres.

That is how the differences arise, and both are, in fact, "right".
userDon636
Posted: 25 June 2020 8:23 PM
Subject: RE: Tyre pressures - Leap of faith?
 
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Brian, that’s pretty much what I said in that I thought the pressures stated on the motorhome might be related to the maximum possible axle weights. For my 6.36m maxi van the maximum axle weights are Front - 2100kg and Rear - 2400kg. Based on the Tyresafe charts (which I have found to be very close to the Continental charts) these maximum weights would give pressures of F - 62psi and R - 69 psi. There is a plate on the door jamb which gives pressures of F - 65psi and R - 72.3psi which are 3psi above the Tyresafe pressures but clearly in the same ballpark. For my loaded van the pressures are F - 1800kg and 51psi, R - 1700kg and 48psi. Quite a difference once weight is taken into account.
userChrisD
Posted: 25 June 2020 9:00 PM
Subject: RE: Tyre pressures - Leap of faith?
 
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Evening all

I did post on this issue in 2017 when I bought new my Roller Team 746.
Amongst the questions that need to be answered is this one:
What is the legal position in the event of an accident if the tyre pressures are not as the vehicle manufacturer/converter specifies and have been varied. Sadly I had to attend a road accident inquest some time ago and the evidence of the traffic investigator was very detailed including the tyre pressures of each wheel against those that the tyre pressure 'should have been'

Just a thought
userDon636
Posted: 25 June 2020 9:23 PM
Subject: RE: Tyre pressures - Leap of faith?
 
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I can’t see how you could possibly be at fault for using pressures recommended by the tyre manufacturer - assuming they were in line with verified axle leadings.

Edited by Don636 2020-06-25 9:24 PM
userDeneb
Posted: 26 June 2020 9:11 AM
Subject: RE: Tyre pressures - Leap of faith?
 
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ChrisD - 2020-06-25 9:00 PM

Evening all

I did post on this issue in 2017 when I bought new my Roller Team 746.
Amongst the questions that need to be answered is this one:
What is the legal position in the event of an accident if the tyre pressures are not as the vehicle manufacturer/converter specifies and have been varied. Sadly I had to attend a road accident inquest some time ago and the evidence of the traffic investigator was very detailed including the tyre pressures of each wheel against those that the tyre pressure 'should have been'

Just a thought


Having been involved in those type of investigations, it is a good question. The collision investigator would normally use the tyre pressures advised on the vehicle data plate and/or in the driver's handbook as a baseline for any single stage production vehicle. For a motorhome, the same would apply if there was an updated pressure label or data supplied by the final stage converter.

A deviation of a few psi from those figures would not normally be an issue, provided that both or all tyres across an axle were compatibly inflated. The default position of tyre and vehicle manufacturers when legal queries arise is that any deviation from recommended pressures can be assumed to have a detrimental affect on the handling of the vehicle in the absence of definitive proof to the contrary.

Now I know that some tyre manufacturers advise different MINIMUM pressures for the load capabilities of their tyres, and you may have received advice from a tyre manufacturer about those pressures in response to a specific query. In the event of an incident however, depending on the circumstances, it may not be possible for an investigator to obtain the actual axle weights of the vehicle concerned after the fact.

The other point to bear in mind related to advised tyre pressures, is that the minimum load bearing pressures are just that, and simply relate to the ability of the tyre to safely bear the stated load under normal operating conditions. Variations in tyre pressures can also have a substantial affect on the steering and handling characteristics of a tyre, and altering the ratios between the pressures of front and rear tyres can also affect these characteristics. Vehicle manufacturers go to great lengths through physical testing of their vehicles in safe environments to determine the handling characteristics of their vehicles and suitable safe tyre pressures, which are not only related to load, but also how they affect the handling of the vehicle under various and sometimes extreme conditions. Deviations from those pressures may appear to be perfectly fine under normal driving conditions, but it is often in emergency situations, such as having to make a sudden and sharp steering correction, or brake suddenly whilst the vehicle is unsettled by negotiating a bend on traversing a rough surface, on greasy, loose or slippery road surfaces etc, that adverse effects may appear, sometimes leading to sudden and severe loss of control with little or no advanced warning.

When tyre manufacturers give customer advice, they can only do so based on their knowledge of the load bearing characteristics of their tyres, not on the handling characteristics which may vary by make and model of vehicle according to other aspects of the vehicle manufacturer's design considerations.

So the bottom line is that, depending on how far the pressures vary from the vehicle specific recommendations, you could be in a difficult position say in the event of a serious or fatal collision, even if clutching a piece of paper from a tyre manufacturer. To counter that, it would be necessary to employ an independent expert who may of course agree or disagree with the prosecution expert's conclusions. In such an event, the two experts may be invited to attempt to reach a jointly agreed conclusion. If that proved impossible, the matter could be in the hands of a jury according to the evidence presented.

If the incident leading to the investigation can be attributed to an unsuitable tyre pressure or pressures, in the absence of any other possible contributory factors, it may be very difficult to refute.

This could well be why certain tyre manufacturers are reluctant to advise on varying tyre pressures, particularly for rear axles where significant reductions in pressure are likely to contribute to more serious handling anomalies in extremis.

usercolin
Posted: 26 June 2020 10:02 AM
Subject: RE: Tyre pressures - Leap of faith?
 


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Or to look at it in another way, what are the dangers of over inflated tyres? The recommended inflation figures for my van allow for a total weight of 5tonne, yet the last two times I've driven it was at less than 3tonne.
userDeneb
Posted: 26 June 2020 10:40 AM
Subject: RE: Tyre pressures - Leap of faith?
 
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colin - 2020-06-26 10:02 AM

Or to look at it in another way, what are the dangers of over inflated tyres? The recommended inflation figures for my van allow for a total weight of 5tonne, yet the last two times I've driven it was at less than 3tonne.


Over-inflated above the manufacturer's recommended figures, or according to the actual axle-weights of your vehicle?

In the second case, a harsher ride and possibly accentuated tread wear. In the first case, possible adverse effects on the handling of the vehicle. But in any case, there will be less risk of seriously adverse effects on the handling of the vehicle than with under-inflated tyres unless you go to extremes.

A lot of car manufacturers now publish higher than normal inflation pressures for economy, with little if any compromise to handling albeit a noticeably harsher ride.
userplop
Posted: 26 June 2020 11:53 AM
Subject: RE: Tyre pressures - Leap of faith?
 
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As part of getting ready for going and being concerned about my VIN plate (https://forums.outandaboutlive.co.uk/forums/Motorhomes/Motorhome-Matters/VIN-Plates-Chausson-650/55518/), tyre pressure checks were next.

Fiat Ducato 2020 3500kg rated and all tyres are set at 5.5 bar - 80 lbs from the dealer.

Michelin 225/75R16CO 116 tyres with heavy duty valves.

The sticker inside the passenger door says this is correct but the ride is pretty harsh.

Looking on https://www.tyresafe.org/check-your-pressures/motorhomes/ with 1850 front and 2000 rear axle it suggests:
Front: 53 psi / 3.67 bar
Rear: 80 psi / 5.5 bar

Amusingly different values.....

I've called Michelin who said their standard advise pressures for those tyres are:
Front: 65 psi / 4.5 bar
Rear 80 psi / 5.5 bar

And again.

Michelin guy said he will email me local approved suppliers and to take it to them and get it weighed there and see what they suggest.

This would appear to be a surprisingly dark art....

We've done 494 miles from new now on day trips and we're seeing friends tonight about 75 minutes away for early bbq so I've lowered the front to 4.5 bar to see what difference it makes to the ride - we won't be at high speed or for very long.

My daily run about is a Skoda Octavia Greenline Estate - this runs quite high tyre pressures as part of it's eco credentials and does 63 mpg at 80 mph up and down the motorway and about 70mpg when going slower more locally, which is mental for a decent sized estate.

The van has improved from the initially indicated 22mpg to just under 25mpg so far as it loosens up a bit, so it will be interesting to see what this does to that as well. I'll track it on the next fill up as well and see what comes of it.

Who knew it could be so baffling?
userBrian Kirby
Posted: 26 June 2020 1:08 PM
Subject: RE: Tyre pressures - Leap of faith?
 


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ChrisD - 2020-06-25 9:00 PM
Evening all
I did post on this issue in 2017 when I bought new my Roller Team 746.
Amongst the questions that need to be answered is this one:
What is the legal position in the event of an accident if the tyre pressures are not as the vehicle manufacturer/converter specifies and have been varied. Sadly I had to attend a road accident inquest some time ago and the evidence of the traffic investigator was very detailed including the tyre pressures of each wheel against those that the tyre pressure 'should have been'
Just a thought

This is exactly why I said above to get the dated weighbridge certificate bearing the vehicle registration number, and to make sure that either the tyre pressures used are as published in the Continental Tyres data book (or for that matter the Tyresafe publication) or are as advised in writing by Michelin. You then have evidence that you have taken a logical and responsible approach to varying away from the base vehicle/conversion manufacturer's recommendations. This principle remains the same whether the van is on CP tyres or plain vanilla van tyres. One is catering for an absolute worst case, so that on any given day the actual loads will be lower.

The tyre pressures must be suitable for the loads they are carrying, which means the van must be weighed at its absolute maximum weight in use, when using the axle loads to determine tyre pressures. One thing that has worried me when people have reported visiting a weighbridge to get axle loads is those who have reported that the fuel tank, or the fresh water tank, were "about" (whatever percentage) full. Given that the approximation will have been gained from a notoriously inaccurate water tank gauge, and an almost equally inaccurate fuel gauge, they know nothing! Gas cylinders, and water and fuel reservoirs should all be full, and so should toilet flush tanks for vehicle so equipped. If a post accident investigation revealed that the vans actual weight, or the actual axle loads, exceeded the loads stated on the weighbridge certificate, one would be in a very difficult position.

Deneb will know, but I assume that these kinds of very detailed investigations would most probably follow an accident that involved loss of life or serious injury and, unless there is no obvious cause for the accident, will be triggered by a suspicion that defective, or wrongly inflated, tyres were the cause, or were a contributory factor.

Having said that, there is no guarantee that a correctly inflated tyre will remain so in use. Slow loss of pressure due to a defective valve, as experienced by Derek Uzzell, for example, could result in a sudden tyre failure due to underinflation, but not because the vehicle owner had chosen to underinflate the tyres. Ditto the consequences of striking an object on the road that, although it had damaged the tyre, had not immediately caused it to deflate.
userDerek Uzzell
Posted: 26 June 2020 2:34 PM
Subject: RE: Tyre pressures - Leap of faith?
 


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For what it’s worth, this issue came up here years ago...

If I remember correctly, following an accident involving a motorhome it was found that the vehicle’s owner had inflated its tyres to the high pressures that were recommended for ‘camping-car’ tyres when the tyres fitted to the motorhome were actually ‘white van’ type.

The insurance provider quibbled over the claim, saying that the owner had not taken due care and that the ‘over inflation’ might have significantly affected the motorhome’s on-road behaviour and this might have contributed to the accident. This was all very hypothetical and, when advice was sought from the tyres’ manufacturer, the latter said that the high pressures were still within the tyres’ ‘safety window’. The claim was then settled...

(I’ve always said that, if a motorhome owner wants to know the ‘correct’ inflation-pressures to use for his vehicle, it’s the pressures stated in the motorhome’s documentation. And if those pressures shake the owner’s fillings loose, tough luck - those pressures are still the only ‘correct’ ones.)
userDeneb
Posted: 27 June 2020 11:13 AM
Subject: RE: Tyre pressures - Leap of faith?
 
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Brian Kirby - 2020-06-26 1:08 PM

Deneb will know, but I assume that these kinds of very detailed investigations would most probably follow an accident that involved loss of life or serious injury and, unless there is no obvious cause for the accident, will be triggered by a suspicion that defective, or wrongly inflated, tyres were the cause, or were a contributory factor.



National policy when I retired, and I can't believe that it will have since been relaxed, was to conduct a thorough examination of all vehicles involved in collisions involving fatalities, injuries likely to prove fatal or result in life changing conditions. We would also examine vehicles in other scenarios where investigating officers felt it necessary to cover the possibility of a driver later alleging a defect as a defence during prosecution, where a driver had alleged a defect to be the cause of a collision, or simply where no cause for a collision was apparent and a prosecution was being considered. Also to provide expert evidence to officers dealing with dangerous defects found during roadside checks, and criminal acts involving motor vehicles etc.

A lot of time was spent investigating allegations from drivers who tend to invent defects after the crash, thinking it will absolve them of responsibility for their appalling driving, and full examinations have to be carried out in most such cases, since a driver's allegation that "something went wrong with my brakes" might involve an issue with the braking system, or tyres, suspension, steering, etc.

As far as tyres are concerned, it was very rare to encounter a sudden failure with no prior warning unless resulting from an impact or penetration by an object laying on the road surface. Many sudden catastrophic failures result from tyres being used over a period of time with damage or low inflation pressures which basic regular pressure checks and maintenance should have picked up long beforehand. For instance, lowering tyre pressures too far below the manufacturer's recommendations, whilst appearing to be adequate during a static check, can result in a situation where the inner sidewall folds excessively under cornering, generating excessive localised heat which progressively affects the structural integrity of the tyre over time. The evidence is plainly visible on the inner carcass of the tyre when removed from the wheel, but would often not be apparent externally until it had catastrophically failed after repeated use, e.g. a "blowout". The only prior indication available to the driver or owner would be that the pressure was substantially lower than specified by the vehicle manufacturer.

That is one reason why I would caution against lowering tyre pressures excessively - more than a single figure psi decrease on a MH for example. It also means that if you lower pressures to the absolute minimum recommended for the load rating applicable to your axle weight, you have very little leeway in the event of a slow pressure loss caused by a leaky valve, breakdown of the seal between the tyre bead and rim (not uncommon on alloy wheels or steel wheels with minor impact damage to the edge of the rim, for instance) or an object such as a screw or nail penetrating the tread. The only way to counter that is to be absolutely fastidious in regular checks, both of tyre pressure and condition (checking for the presence of objects etc.) and removing any wheel/tyre which does exhibit a slow leak for further examination to resolve the problem. Relying on pressure monitoring systems alone, which sometimes do not alert until a substantial reduction in pressure has been detected (20% over a period of at least 10 minutes for ECE approved systems) is not advisable.
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