Tyre pressures - Leap of faith?

[costaexpress]

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

[HarveyHeaven]

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.

[aandy]

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.

[Don636]

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.

[Steve928]

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)"

[Derek Uzzell]

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 ® 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 ® 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 ® 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.

[colin]

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.

[Robinhood]

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.

[costaexpress]

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.

[Brian Kirby]

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.

[EJB]

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.

 

[Brian Kirby]

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.

[whiskers]

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.

[DickB]

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 %

[Derek Uzzell]

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’.

[Deneb]

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.

[Deneb]

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.

[costaexpress]

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).

[Deneb]

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.

[Robinhood]

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)

[costaexpress]

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.

[costaexpress]

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!

[costaexpress]

Apologies see i should press quote first when making a specific response rather than a general comment

 

[EJB]

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’.

[Robinhood]

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.