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Tyre pressures - Leap of faith?
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usercostaexpress
Posted: 24 June 2020 8:24 AM
Subject: Tyre pressures - Leap of faith?
 
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Location: Swift Autocruise 164 Select 6.4m PVC


This is my problem. Continental 16" CP tyres, The continental technical guide tells me ideal pressures for my van are 48 front and 55 rear. The fiat handbook for my tyres says 79.5 psi front and back, Swift have stamped on the door 79.5 psi front and back. Continental make the tyres and should know, the van is very comfortable at the lower levels, however, and here is the problem, the pressures are so far away from Fiat and Swift that I cannot bring myself to drive that low and I am currently sat at 65 psi all round for no other reason than that's what my last van was set at. I like the drive with Continental's recommendation, just wondering if any one has actually made the leap of faith or compromised like I am currently doing
userHarveyHeaven
Posted: 24 June 2020 8:38 AM
Subject: RE: Tyre pressures - Leap of faith?
 


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we go with the information supplied in the van which taakes into account the weight of the vehicle. Any lower and the tyres get hot more swiftly. We have a tyre pressure monitoring system which my husband is very happy with even though to set it all up is a bit of a faff.
useraandy
Posted: 24 June 2020 8:43 AM
Subject: RE: Tyre pressures - Leap of faith?
 
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costaexpress - 2020-06-24 8:24 AM

This is my problem. Continental 16" CP tyres, The continental technical guide tells me ideal pressures for my van are 48 front and 55 rear. The fiat handbook for my tyres says 79.5 psi front and back, Swift have stamped on the door 79.5 psi front and back. Continental make the tyres and should know, the van is very comfortable at the lower levels, however, and here is the problem, the pressures are so far away from Fiat and Swift that I cannot bring myself to drive that low and I am currently sat at 65 psi all round for no other reason than that's what my last van was set at. I like the drive with Continental's recommendation, just wondering if any one has actually made the leap of faith or compromised like I am currently doing


I did a few years ago. The Fiat sticker recommended 79.5 rear and 72 front. I can't remember exactly what the Continental figures were but they were similar to yours. After about 5,000 miles there were clear indications of under inflation in the wear pattern so I reverted to the original pressures. Shortly after that, at just under five years old, I changed them because the side walls were deteriorating.

I now have ordinary van tyres (Michelins) and set the pressures according to the calculator here https://www.tyresafe.org/check-your-pressures/motorhomes/. In 12,000 miles I've experienced no problem with ride, handling, or uneven wear.

Edited by aandy 2020-06-24 8:44 AM
userDon636
Posted: 24 June 2020 8:46 AM
Subject: RE: Tyre pressures - Leap of faith?
 
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My van is 6.4m on the maxi chassis and has run at 51psi front and 48psi rear for the last six years, based on weighbridge weights and Continental charts. The van may say 79psi but on the maxi chassis the weight can be up to 4250kg so maybe it accounts for the maximum weight possible. My view is that Continental must know what they are doing so I run with their figures. The ride at 79psi must surely be punishing.

Edited by Don636 2020-06-24 8:48 AM
userSteve928
Posted: 24 June 2020 9:17 AM
Subject: RE: Tyre pressures - Leap of faith?
 


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I use Conti's advised pressures at max axle load for my Michelin Agilis CPs, even though we're quite a bit under max on the front axle.
Van is stickered at 79.5 all round but at that pressure the harshness and vibration was ridiculous; I simply wouldn't want to use the van if it was like that..
It now drives well, looks right and the tyre temperatures are normal.

From Continental:
"Based on the 225/75 R16 CP 116R and the maximum axle weight you have provided (1850kg front/2000kg rear) the recommended inflation pressure is:
Front: 3.5 bar (51 psi) / Rear: 4.25 bar (62 psi)"
userDerek Uzzell
Posted: 24 June 2020 9:26 AM
Subject: RE: Tyre pressures - Leap of faith?
 


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Although tyre manufactuers undoubtedly "know what they are doing”, I’d question whether motorists do...

Tyre manufacturers’ testing is covered here

https://www.tirereview.com/tire-testing-process/

Continental’s Technical Databook lists data obtained during testing. For a particular tyre specification and a specific load placed on that tyre an inflation-pressure datum will be listed, but it would be well to treat this as an absolute minimum and it’s unlikely to include allowances for (say) weight transference fowards under braking or side loads placed on front tyres during hard cornering, and certainly not what the vehicle will feel like to drive when the listed pressure is used.

Lowering pressures from an advised 80psi Front & Rear to 48psi (F) and 55si (R) is a very large reduction and - although a big improvement in ride would be a totally predictable result - I’d expect handling to become ’sloppier’ and tyre wear to rise.

The advised inflation-pressures for my Rapido’s Michelin 215/70 R15CP tyres are 5.0bar/72.5psi (F) and 5.5bar/80psi (R) when the motorhome IS AT MAXIMUM LOADED WEIGHT. I found the ride very harsh at those pressures and the steering lighter than I liked. I reduced the pressures by 0.5bar F & R to 4.5bar/65psi (F) and 5.0bar/72psi (R) and that’s made a worthwhile improvement on ride quality and steering ‘feel’.

I’m perfectly happy to use Continental’s inflation data as a guideline, but I’ll always treat the figures as minima and rely on my own judgement as to how much higher pressures I’ll choose to use.
usercolin
Posted: 24 June 2020 9:33 AM
Subject: RE: Tyre pressures - Leap of faith?
 


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I was very sceptical of the online recommended tyres, but at Fiat recommended pressures the ride was abysmal, my solution was to take note of tyre temperature then reduce the pressure 5psi for each trip, taking note each time we stopped to ensure tyres were not overheating. When I got down closer to Continentals recommended pressure I was not happy with handling so settled on a pressure slightly higher, but which seemed hardly any harsher and allowed some flexibility with loading.
p.s. the Fiat pressures for my van equate to 2.5t per axle.

Edited by colin 2020-06-24 9:34 AM
userRobinhood
Posted: 24 June 2020 9:46 AM
Subject: RE: Tyre pressures - Leap of faith?
 


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Steve928 - 2020-06-24 9:17 AM

I use Conti's advised pressures at max axle load for my Michelin Agilis CPs, even though we're quite a bit under max on the front axle.


Since 2002 I have run every van on CP tyres at lower than the (generally maximum) inflation figures on the door opening.

Back then, even Michelin would readily quote much-reduced pressures for such tyres, if they were quoted axle loadings. Nowadays, you're only likely to find Continental doing that (though I understand Michelin may still recommend reduced *front* pressures for a given axle loading).

I have never replaced a tyre, nor experienced undue wear or cracking in that period, and the ride and handling has been (subjectively) much improved, albeit the extent varying by vehicle.

My own practice is to weighbridge-check my vehicle fully loaded, check the individual axle weights and add a margin (generally 10%, but if this leaves it well away from the maximum axle loading, I'll increase slightly - my vehicles have generally been a good bit under maximum axle loadings). I then use the pressures from the Conti tables relevant to those (with margin) weights.

Since 2002, I have had one coachbuilt equipped from the factory with non-CP tyre (a Hobby on a Ford Transit). The recommended pressures in the door opening were those for the maximum axle-weights on a non-CP tyre, considerably lower than those that would have been quoted for a CP-tyre, and absolutely coincident with the Conti tables for a non-CP tyre. I used those pressures.

Patently, the decision on tyre-pressures is a personal one, but if you wish to vary them starting point *must* be to understand your running weight (per axle) and any margin you desire to add for different loading, etc.

Edited by Robinhood 2020-06-24 9:47 AM
usercostaexpress
Posted: 24 June 2020 10:29 AM
Subject: RE: Tyre pressures - Leap of faith?
 
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Location: Swift Autocruise 164 Select 6.4m PVC


Thanks everyone some great answers, The current ride at 65 psi all round isn't too bad, however, I think I'm going to move down in steps, just a little on the back to 62 and start the front at 60, next trip try 55 and then 52 in line with @Epic Contributor as I have the same axle weights and tyres. I know I'm being over cautious, however, with no spare wheel (nor on my last van, nor two of our cars) tyre management has moved back onto the radar. Not going anywhere for a couple of weeks so it will be a while before I can report back.
userBrian Kirby
Posted: 24 June 2020 1:18 PM
Subject: RE: Tyre pressures - Leap of faith?
 


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costaexpress - 2020-06-24 10:29 AM
Thanks everyone some great answers, The current ride at 65 psi all round isn't too bad, however, I think I'm going to move down in steps, just a little on the back to 62 and start the front at 60, next trip try 55 and then 52 in line with @Epic Contributor as I have the same axle weights and tyres. I know I'm being over cautious, however, with no spare wheel (nor on my last van, nor two of our cars) tyre management has moved back onto the radar. Not going anywhere for a couple of weeks so it will be a while before I can report back.

But, beware that important information is missing. You say your van has 16" Continental CP tyres. Just to be certain, are these 225/75 R 16 CP?

You say the recommended pressures in the Continental Technical Databook are 48psi (3.3 bar) front, and 55psi (3.8 bar)rear.

From the Continental Technical Databook, those pressures imply axle loadings in the region of 1845kg front and 1875kg rear, and an actual laden weight of 3,720kg. However, according to Swift, your maximum permissible MAM is 3,500kg. Have you weighed the van at a weighbridge, fully laden, as the tyre pressures you are taking form the Conti Databook, seem to indicate a possible overload?

Based on actual weighbridge data, with our van fully laden, I have been running it since we first used it "in anger", in spring 2018, at 51psi , (3.5 bar) front, and 58psi (4.0 bar) rear, and it is fine.

So, if you are confident that your axle loads were obtained at full load condition (and that the van is not consequently overloaded), I can't see why using the pressures recommended by Continental should not be adopted as the minimum safe working pressures. It would not be safe to go below these pressures, but there would be nothing against increasing them if you don't like the way the van rides or handles at those pressures, subject to not exceeding the pressures recommended by Swift.
userEJB
Posted: 24 June 2020 2:12 PM
Subject: RE: Tyre pressures - Leap of faith?
 
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My experience is near to 65/55 are normally the correct and sensible pressures...….3500 +/-Kg MHs.
80PSI is the normal commercial industry recommendation 'catch all' number for the rear tyres...a bit lower for the fronts.
userBrian Kirby
Posted: 24 June 2020 2:59 PM
Subject: RE: Tyre pressures - Leap of faith?
 


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Brian Kirby - 2020-06-24 1:18 PM

costaexpress - 2020-06-24 10:29 AM
Thanks everyone some great answers, The current ride at 65 psi all round isn't too bad, however, I think I'm going to move down in steps, just a little on the back to 62 and start the front at 60, next trip try 55 and then 52 in line with @Epic Contributor as I have the same axle weights and tyres. I know I'm being over cautious, however, with no spare wheel (nor on my last van, nor two of our cars) tyre management has moved back onto the radar. Not going anywhere for a couple of weeks so it will be a while before I can report back.

But, beware that important information is missing. You say your van has 16" Continental CP tyres. Just to be certain, are these 225/75 R 16 CP?

You say the recommended pressures in the Continental Technical Databook are 48psi (3.3 bar) front, and 55psi (3.8 bar)rear.

From the Continental Technical Databook, those pressures imply axle loadings in the region of 1845kg front and 1875kg rear, and an actual laden weight of 3,720kg. However, according to Swift, your maximum permissible MAM is 3,500kg. Have you weighed the van at a weighbridge, fully laden, as the tyre pressures you are taking form the Conti Databook, seem to indicate a possible overload?

Based on actual weighbridge data, with our van fully laden, I have been running it since we first used it "in anger", in spring 2018, at 51psi , (3.5 bar) front, and 58psi (4.0 bar) rear, and it is fine.

So, if you are confident that your axle loads were obtained at full load condition (and that the van is not consequently overloaded), I can't see why using the pressures recommended by Continental should not be adopted as the minimum safe working pressures. It would not be safe to go below these pressures, but there would be nothing against increasing them if you don't like the way the van rides or handles at those pressures, subject to not exceeding the pressures recommended by Swift.

Apologies for re-posting my own post, but I was being rushed to finish, and omitted to add that my above comments regarding tyre pressures are only valid if the tyres fitted to costaexpresse's Autocruise are 227/75 R 16 CP (as seems most probable), and if the pressures cited were the result of taking the van to a weighbridge to obtain actual, fully laden, axle loads.

From other posts above, I sense a certain laxity in using these pressure for load tables, with some posters apparently relying on a deal of guesswork in arriving at tyre pressures, or even relying on posts from others as to the pressures they use. Don't do this!

There is a deal of engineering that goes into the construction of tyres, and into establishing the optimum pressures for given axle loads.

Over-inflation relative to actual load, providing the pressure used is within the stated limits for the tyre, may impact on ride comfort, handling, braking performance, and wear around the centre band of the tyre, but it will not be dangerous - which is why van converters invariably recommend using the maximum permissible pressures. After all, they cannot know, or reasonably forecast, how an individual owner may load their van, or even whether they will actually check its laden weight. So, in advising the tyre pressures to use, they err on the side of caution. Those maximum pressures may knock your fillings out, but they won't kill you!

The danger arises when tyres are under inflated relative to load. So, if one intends to lower the tyre pressures for whatever reason (most frequently comfort! ) it is essential to do this in the full knowledge of the loads that will fall on the van's axles when the van is at its maximum actual load as it would be when in use - which means with everyone and everything on board, including all camping bits and bobs, clothing, food, water, fuel, gas, booze, books, toys, occupants, pets etc. etc., so that it is never likely to be run heavier. Then visit the weighbridge in that condition to get the actual axle loads, and only then adopt the next tyre pressure above those actual loads, and periodically check that they do not fall lower in use.

Under inflated tyres will run hot, and the hotter they get, the more prone to catastrophic failure (i.e. blow-out) they get, and that, and the probable resulting loss of control, in a vehicle weighing 3.5 tonnes or more, is liable to prove fatal for someone.
userwhiskers
Posted: 24 June 2020 9:41 PM
Subject: RE: Tyre pressures - Leap of faith?
 
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In line with weighbridge weights and Continental data, I am running at 58psi all round. As has been said by others, at 80psi the camper was shaken to bits. Mrs C and I do not carry a lot of baggage.
userDickB
Posted: 24 June 2020 10:19 PM
Subject: RE: Tyre pressures - Leap of faith?
 
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This discussion is endless. The figures given by the chassis manufacturer are based on the maximum loading of it as a commercial vehicle. The converter may put in a different figure, but again based on his opinion of maximum load. The only way to determine the correct pressures is to do a proper axle weight check on a weighbridge and then use the tyre manufacturer's settings accordingly. Just like a car, if you are loaded up with more than 2 passengers, you increase the pressure then you would do the same with a motorhome. However, I check my motorhome weight every year when fully loaded and set the appropriate pressures. If I run more lightly loaded, less fuel, water or stores, then I could drop the pressures, but in practice the variation is minimal.

Dick %
userDerek Uzzell
Posted: 25 June 2020 8:57 AM
Subject: RE: Tyre pressures - Leap of faith?
 


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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’.
userDeneb
Posted: 25 June 2020 9:11 AM
Subject: RE: Tyre pressures - Leap of faith?
 
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Fully agree with Dick. The important points to remember are that when you decrease a tyres running pressure, you also decrease its load capability and its ability to dissipate heat (and the tyre will generate more heat if it is underinflated for the load that it is bearing, compounding the problem). This may not manifest itself in any obvious way for quite some time, unless you were to remove the tyre from the wheel, examine it internally and know the signs of stress or damage to look for, but it can eventually result in catastrophic tyre failure such as sudden deflation (a blowout) caused by failure of the sidewall structure, which can in turn cause a sudden loss of vehicle stability that may sometimes be difficult to control.

Altering the relationship between the front and rear tyre pressures as recommended by the vehicle manufacturer can also adversely affect the handling capabilities of the vehicle, at worst by inducing oversteer that can be a sudden and extreme event with little or no advance warning felt by the driver.

In other words, think very carefully when choosing running pressures and preferably utilise the tyre manufacturer's technical guidance or seek their advice. I appreciate that many are often wary of making recommendations though, due to the current climate of liability and litigation issues.
userDeneb
Posted: 25 June 2020 9:20 AM
Subject: RE: Tyre pressures - Leap of faith?
 
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Derek Uzzell - 2020-06-25 8:57 AM

That’s not the case for motorhomes where ‘camping-car’ tyres are factory-fitted as standard equipment.



The point about Camping Car tyres though is that their primary design consideration is to better resist the "pressures" of spending long periods of time in static positions whilst coping with what are often relatively heavily laden vehicles. The tyre's construction and its operating pressure are important features of that capabability, which assume as much, if not more importance than its ride comfort.
usercostaexpress
Posted: 25 June 2020 10:08 AM
Subject: RE: Tyre pressures - Leap of faith?
 
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Location: Swift Autocruise 164 Select 6.4m PVC


Thanks once again for all your help. A couple of points have arisen, I do have 225/75 R 16 CP tyres, I have not however, been to a weighbridge, merely guessed at my likely maximum axle loadings to be on the safe side. I travel solo and do not particularly load the van up with all the goodies associated with a couple on holiday. The point about lower tyre pressures generating more heat is interesting as I often drive longish distances in the van (maybe 600 miles in a 24 hour period) and I guess the tyres must get quite hot under those conditions, so would the recommendation be to add a few psi when covering longer distances? The van is 1850/2000 max 3500. The vans handbook book states 795kg payload although a few upgrades will have reduced that to closer to the 500kg mark (Awnings, twin batteries etc).
userDeneb
Posted: 25 June 2020 10:36 AM
Subject: RE: Tyre pressures - Leap of faith?
 
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costaexpress - 2020-06-25 10:08 AM

The point about lower tyre pressures generating more heat is interesting as I often drive longish distances in the van (maybe 600 miles in a 24 hour period) and I guess the tyres must get quite hot under those conditions, so would the recommendation be to add a few psi when covering longer distances?


As long as the tyre pressures are sufficient for the load being borne, the distance being travelled is immaterial. Tyres will always generate heat when in use, it is the amount of heat and their ability to dissipate it that matters.

You will see in most if not all car handbooks a recommendation to increase tyre pressures for high speed running, and usually for increased loading, the latter based on the assumption that most cars are usually driven with only one or two occupants and light luggage, so recommendations are made for occasions when the car is more heavily loaded.

With commercial vehicles, the assumption is generally that they are bought to carry a load, that they will both normally be laden, and the vehicle will have been purchased by the operator to suit the gross weight under which they normally wish to operate it (e.g. taken to extremes a company that requires a small car-derived van to deliver sandwiches would not consider buying a 38 tonne artic for that purpose).

So operating weight and speed are the main considerations. You shouldn't need to change tyre pressures based on distance, and high speed considerations can be pretty much ignored for motorhomes.
userRobinhood
Posted: 25 June 2020 11:07 AM
Subject: RE: Tyre pressures - Leap of faith?
 


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costaexpress - 2020-06-25 10:08 AM

The vans handbook book states 795kg payload although a few upgrades will have reduced that to closer to the 500kg mark (Awnings, twin batteries etc).


...that 795kg figure looked (very) high given my experience of the XLWB van (and assuming the model in your profile).

The current Swift brochure shows the basic payload for the 164 as between 490 and 523kg, depending on engine and devoid of other options. (and very light on gas, and absolutely no water!).

My guess is that, even with your professed light loading, you will be running close to the MAM of 3500kg.

If you want to reduce the tyre pressures from the specified maxima (and I certainly would), I'd strongly suggest you remove the guesswork, and visit a weighbridge "fully loaded" to determine the overall weight and axle loadings. (My local weighbridge charges just £6 for overall and both axle weights) Then take the pressure calculations from there (and I would suggest with at least a 10% uplift to the axle weights, even this is likely to leave you with pressures (well) under 79.5psi).

Until such weighing, I would be wary of reducing the pressures, (preferring to have a real baseline) but personally, in the interim without such weighing I certainly wouldn't countenance going below those relevant to maximum axle weights (which on your tyre, from the Conti tables, maps to the region of 47psi F 61psi R (The quoted profiles vary for front and rear use on CP tyres)
usercostaexpress
Posted: 25 June 2020 11:52 AM
Subject: RE: Tyre pressures - Leap of faith?
 
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Hi, although mine is a 2019reg it is a 2017 van. I have just checked the brochure for my model and it is 765kg, the 795kg is for the 115 engine, so I guess the newer models come with more kit and slightly heavier. Regardless I agree it needs to go to a weighbridge at some stage and in the meanwhile I will run with continentals recommendations on the assumption that I am at the full 3500kg.
usercostaexpress
Posted: 25 June 2020 11:54 AM
Subject: RE: Tyre pressures - Leap of faith?
 
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Thanks, understand distance driven not the factor and as you say with a 130 euro6 engine, speed is the last thing I need to worry about!
usercostaexpress
Posted: 25 June 2020 11:55 AM
Subject: RE: Tyre pressures - Leap of faith?
 
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Apologies see i should press quote first when making a specific response rather than a general comment
userEJB
Posted: 25 June 2020 12:01 PM
Subject: RE: Tyre pressures - Leap of faith?
 
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Sorry to 'Quote'...hate it!

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’.
userRobinhood
Posted: 25 June 2020 12:34 PM
Subject: RE: Tyre pressures - Leap of faith?
 


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costaexpress - 2020-06-25 11:52 AM

Hi, although mine is a 2019reg it is a 2017 van. I have just checked the brochure for my model and it is 765kg, the 795kg is for the 115 engine, so I guess the newer models come with more kit and slightly heavier. Regardless I agree it needs to go to a weighbridge at some stage and in the meanwhile I will run with continentals recommendations on the assumption that I am at the full 3500kg.


I've just checked that, and it certainly is the figure quoted in 2017. From experience it still looks very high to me, and I note that the 2018 models (also quoted on a 115 engine, but now badged Swift rather than Autocruise, so maybe some differences) show a payload 230kg less for the equivalent van. (which is more in line with my expectation).

I also note the weasel words "All weights estimated" in the headings for 2017 version, but not the 2018, which rather reinforces my opinion that the payload figure you're working with may be unreliable.
userBrian Kirby
Posted: 25 June 2020 1:25 PM
Subject: RE: Tyre pressures - Leap of faith?
 


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costaexpress - 2020-06-25 10:08 AM
Thanks once again for all your help. A couple of points have arisen, I do have 225/75 R 16 CP tyres, I have not however, been to a weighbridge, merely guessed at my likely maximum axle loadings to be on the safe side. I travel solo and do not particularly load the van up with all the goodies associated with a couple on holiday. The point about lower tyre pressures generating more heat is interesting as I often drive longish distances in the van (maybe 600 miles in a 24 hour period) and I guess the tyres must get quite hot under those conditions, so would the recommendation be to add a few psi when covering longer distances? The van is 1850/2000 max 3500. The vans handbook book states 795kg payload although a few upgrades will have reduced that to closer to the 500kg mark (Awnings, twin batteries etc).

With apologies to others who may have responded to this query, but I think the answer to the above question - "so would the recommendation be to add a few psi when covering longer distances"? - has to be a resounding no.

The answer to that question is to do as already advised, and fully load the van as it would be when in use, and then take it to a weighbridge and weight the van (which is already guessed to be overloaded) to get its actual laden weight, and the actual load on each axle - remembering that the weighing should take place with the driver on board and sitting in the driver's seat.

Make sure you get, and keep, the weighbridge certificate for your visit, which will show the date, the vehicle registration number, and (usually) the actual laden weight of the vehicle and the load on one axle. The difference between these is the load on the other axle. To obtain the single axle load it is usual to drive the van forward or backwards until either front or rear tyres are just off the platform. The weighbridge operator will tell you which.

You will then, and only then, have the loading data against which the Continental Tyres Technical Data Book is designed to be used. Then adopt the tyre pressures recommended for those loads, noting that the pressures for front axle and rear axle will vary even if both axles bear the same load, and adopting the nearest pressure for load above the actual weighbridge values. Then note these pressures as the minimum to be used. Failing that, just stick to the pressures recommended by Fiat/Swift.

If, when driving at the Continental recommended pressures, you do not like the ride or handling of the van, increase the pressures as you suggest, in say 5psi increments, front and/or rear, until satisfied, and then note the enhanced pressures for future reference.

You will then have the weighbridge certificate, plus the Continental Databook information, for reference to demonstrate, if ever challenged, why you are running your van at lower pressures than those stated by Fiat/Swift, and indicated on your van.
userDeneb
Posted: 25 June 2020 1:56 PM
Subject: RE: Tyre pressures - Leap of faith?
 
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Brian Kirby - 2020-06-25 1:25 PM

Make sure you get, and keep, the weighbridge certificate for your visit, which will show the date, the vehicle registration number, and (usually) the actual laden weight of the vehicle and the load on one axle. The difference between these is the load on the other axle.


To be strictly accurate Brian, the difference is the load on the other axle plus or minus the compounded errors in the actual weights obtained, resulting from the weighbridge tolerance. Static weighbridges normally operate in increments of between 20 and 50kgs, so calculating a weight be subtraction could in the worst case give an error of plus or minus 100kgs on the indicated weights, without allowing for any discrepancy in the accuracy of the bridge itself, although that would be rare. If the bridge is regularly tested and certified (not all private bridges are) then the operator should know the accuracy as well as the tolerance level at which the bridge operates.

Calculating weights by subtraction from physical weighing of gross weight and other axles used to be frowned upon for enforcement purposes unless the dimensions of the vehicle, plate and surroundings made it physically impossible to calculate an individual axle weight in any other way. Whilst I'm not suggesting that you should equate weighing for our purposes with enforcement procedures necessary to ensure acceptance in a court, if a MH owner is going to weigh their vehicle it is surely in their interests to follow best practice so that they can be as confident as possible in the outcome. If nothing else, physically weighing both axles individually does provide a very useful mechanism for cross checking and comparing the other readings obtained.

A 50kg or even 100kg tolerance is next to nothing when weighing a 30 plus tonne vehicle, even for enforcement purposes with the tolerances allowed before prosecution or prohibition, but it can make a significant difference to the calculated weight borne by say a 1700kg axle
usercostaexpress
Posted: 25 June 2020 2:20 PM
Subject: RE: Tyre pressures - Leap of faith?
 
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One thing I am certain of now, thanks to all your replies, there is one thing I clearly need to do, and that's to head to the weighbridge and know what my starting point is before I even ask the question. 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!'. No, seriously I'll book an appointment at least I will know then.
userBrian Kirby
Posted: 25 June 2020 2:45 PM
Subject: RE: Tyre pressures - Leap of faith?
 


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Location: East Sussex. Motorhome: Knaus Boxstar 600 Street


Interesting. It may depend on the weighbridge and its sensitivity. I asked the operator of the one I use (at a builder's yard, for weighing bulk goods) and his advice was to weigh the whole and deduct the weight of the heaviest axle to get the front axle weight, in preference to weighing both axles individually and adding them to get actual laden weight.

His reason was that the platform was long enough to take an articulated truck, and that for best results the load should align as nearly as possible to the centre of the platform. So, the recorded load with the van placed centrally would be the most accurate, and is the load value that is most likely to be challenged/checked.

In weighing either axle individually, because the axle load would of necessity be well off the platform centre with a 6.0 metre van, those weights would a bit less accurate, so he preferred the whole weight less one axle, to the sum of the weights of both axles, to arrive at actual laden. This is an "electronic" weighbridge that employs load cells under the platform, and is regularly checked by trading standards. It seems reasonably accurate as, when I walk on it, it tells me my weight to within a few kg!

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.
userRobinhood
Posted: 25 June 2020 2:47 PM
Subject: RE: Tyre pressures - Leap of faith?
 


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Deneb - 2020-06-25 1:56 PM

To be strictly accurate Brian, the difference is the load on the other axle plus or minus the compounded errors in the actual weights obtained, resulting from the weighbridge tolerance.


TBH, I think the "overall plus one axle" approach is often taken because:

i) the weighbridge operator can't be a*sed and would rather just take two readings rather than 3

ii) the weighbridge charges (an extortionate amount) for each individual weighing.

My local authority combines both of these

My nearest weighbridge, however, is a coal merchant, and is quite happy to do as many readings as you want (in one visit) for the princely sum of £6, and issues a ticket. (mine shows a <10kg variance between the overall weight and the sum of the axle weights).
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