Tyres and Pressures

[johnlc]

I've read a lot of threads about this and the gist seems to be that tyre pressures should be set based on the axle weights and got from the specific tyre company, not the motorhome manufacturer, or base vehicle manufacturer.

I have a SWB Ducato and the axle weights when fully loaded in trip condition with two people in the cab are

Front - 1510kgs

Rear 1400 kgs

This is marginally less than the total weight of 2930 when on the weighbridge.

 

I contacted Pirelli about the recommended tyre pressures and they said

"We are unable to advise what the correct pressure is for your particular tyres as it is the vehicle manufacturer that devises such pressures. This is due to the axle weight split of the vehicle and certain pressures required for laden and un laden weights".

I reminded them of the quoted laden weights which is all I am interested in and they finally suggested 3.1 bar front and rear.

The Fiat manual recommends 4 bar +/- 0.05, but does not relate this to a specific weight.

 

Given the ride in my van is generally fine, but very crashy/bangy on uneven roads (as if the bump stops are being hit?) does the 3.1bar suggested pressure seem about right? Would it soften the ride a bit?

It seems quite a big difference from Fiat's recommendation.

The gross vehicle weight for the van in 3000kg, but I have had this uprated to 3200kg by SV Tech, in order to give more flexibility on payload should it be needed.

The tyres are Pirelli Chrono 215/70 R15C

 

Thanks

 

 

 

[Keithl]

Hi John,

 

My first port of call is normally the TyreSafe brochure Link but unfortunately it doesn't list your size :-(

 

So my next resource is the Continental tyres brochure Link. This lists your tyre size with a 109 load rating on pages 22/23 (across the double spread) and for your axle weights I would read off tyre pressures of Front 3.2 bar and rear 3.1 bar (without delving into maths to extrapolate exact figures).

 

This is VERY close to the 3.1 bar Front & Rear you have been told by Pirelli so I would think good to go with.

 

If you Google for long enough you may be able to find tables for Pirelli but my experience is that most manufacturers have similar figures for the same load rating.

 

If you are into maths you can plot axle load capacity against pressure to get a range of load/pressure figures.

 

Keith.

[Derek Uzzell]

John

 

As I read the Fiat handbook data for a 3000kg SWB van, your Fifer’s maximum axle loadings are likely to be 1630kg (front axle) and 1650kg (rear axle). For those axle loadings and 215/70 R15C tyres, static inflation pressures would be around 3.5bar.

 

(The SVTech uprating exercise was discussed at some length in this earlier forum thread.)

 

http://www.outandaboutlive.co.uk/forums/Motorhomes/Motorhome-Matters/Air-Assist-Suspension-on-SWB-PVC-any-benefit-/35116/

 

Pressure recommendations in vehicle handbooks will always err on the high side as it’s safer to somewhat over-pressure a tyre than under-pressure it. And, as the vehicle manufacturer cannot know how an owner will load the vehicle, the advised pressures will always relate to (at least) the maximum axle loadings.

 

There are also potential benefits in running tyres a little above the minimum pressure able to cope with a particular static load - steering will be lighter, handling will be sharper, braking and cornering should be improved. (Michelin used to suggest adding 10% to the static-loading pressure for front tyres to cope better with cornering/braking loads.)

 

As far as I’m concerned the cardinal rule should be to use the manufacturer’s recommended pressures initially (in your case 4.0bar front and rear) unless one is certain they are wrong (as was the case with my Hobby motorhome). If the vehicle’s ride then proves to be very harsh, and weighing the vehicle in fully-loaded state indicates that its axle loadings are well under their maxima, reducing the tyre pressures should be expected to improve the ride quality to some degree.

 

I suggest you lower the pressures to 3.5bar/51psi (front and rear) and experiment. I’m doubtful that reducing your 4.0bar tyre pressures will produce a significant improvement in the ride quality unless you lower them to the absolute minimun for the measured axle loadings, when there’s a very real chance that the motorhome’s handling will be compromised.

 

As you say that your Fifer’s ride quality is generally fine, except on uneven roads, you might find that going to 3.5bar will be sufficiently high to retain taut handling but help with the crashy/bangy behaviour.

 

Find a road that causes crashing/banging at the 4.0bar pressures, drop the pressures to 3.5bar and see what effect that has. If things are no better, drop the pressures to 3.1bar and see what that does.

[johnlc]

Keith and Derek

Thanks for the information and your helpful suggestions. I will try, as soon as I can, reducing the pressures to 3.5 bar initially, and then 3.1, and gauge what the impact is.

I was concerned about the impact on handling when Pirelli suggested 3.1 bar, but we'll see.

If it seems desirable to reduce the pressures I wonder that the impact will be when the vehicle is left stationary for quite a while, as it tends to be over winter? My simple logic seems to suggest higher pressures will keep the sidewalls more taut and help when the vehicle is static for prolonged periods. What do you think?

[Keithl]

johnlc - 2015-03-26 7:06 PM

... I wonder that the impact will be when the vehicle is left stationary for quite a while, as it tends to be over winter? My simple logic seems to suggest higher pressures will keep the sidewalls more taut and help when the vehicle is static for prolonged periods. What do you think?

 

Simple answer John, don't leave it static for any length of time!

 

We try and use ours at least once every month to keep the batteries charged and everything running smoothly and nicely aired.

 

Keith.

[johnlc]

Keith

I agree. We use ours for a day trip about once every 3 weeks over winter, but it is still a lot of standing around! Perhaps it would be better to keep the tyre pressures higher i.e. at the 4 bar during winter?

[colin]

johnlc - 2015-03-26 7:06 PM

 

Keith and Derek

Thanks for the information and your helpful suggestions. I will try, as soon as I can, reducing the pressures to 3.5 bar initially, and then 3.1, and gauge what the impact is.

I was concerned about the impact on handling when Pirelli suggested 3.1 bar, but we'll see.

If it seems desirable to reduce the pressures I wonder that the impact will be when the vehicle is left stationary for quite a while, as it tends to be over winter? My simple logic seems to suggest higher pressures will keep the sidewalls more taut and help when the vehicle is static for prolonged periods. What do you think?

We did a similar thing to Derek's advice, gradually lowering pressure till we got to recommend pressure for axle load, we then compromised by increasing pressure to around halfway between tyresafe pressure and Fiat pressure. Be interesting to see if you decide to make same compromise.

[Derek Uzzell]

johnlc - 2015-03-26 7:36 PM

 

Keith

I agree. We use ours for a day trip about once every 3 weeks over winter, but it is still a lot of standing around! Perhaps it would be better to keep the tyre pressures higher i.e. at the 4 bar during winter?

 

My motorhomes used to languish at my home from November to March, though I used to move them every now and again to alter where the tyres were in contact with the ground.

 

Received wisdom is that inflating the tyres to the pressure shown on the the sidewall (I’m guessing around 65psi/4.5bar for your Pirelli tyres) should help to combat tyre deformation when a vehicle is long-term static and I used to do this.

 

In your case, as you plan to use your motorhome every 3 weeks, ‘over inflating’ the tyres during the winter is unlikely to be beneficial. If your motorhome were to be unused for 3 months it would be another matter.

[Tracker]

Derek Uzzell - 2015-03-27 8:15 AM

My motorhomes used to languish at my home from November to March, though I used to move them every now and again to alter where the tyres were in contact with the ground.

 

Goodness me Derek - how many have you got - one at a time is enough for most of us - a bit like wives!

[Derek Uzzell]

I bought a Herald motorhome in 1998 and sold it in 2004. I then bought a Hobby motorhome in 2005 and sold it in 2014. At present I do not own a motorhome.

 

Both the Herald and the Hobby used to hibernate - hence "My motorhomes used to languish at my home from November to March...”

 

Happy now?

 

[johnlc]

Just to report back...

I lowered the pressure from 4 bar to 3.1 bar front and back, as recommended by Pirelli for my current tyres. It wasn't a fully accurate comparison as the van wasn't fully loaded for a trip away, but we do keep a lot of stuff in it anyway. The ride was definitely less jarring on bumps, but of course they are not eliminated given the state of our roads! Handling didn't seem negatively affected even in the strong cross-winds occurring when I was on the motorway. All in all, if I stick with the same tyres, I would prefer these lower tyre pressures.

I also contacted other tyre companies to see what the recommendations would be for their tyres given my axle weights

Michelin Agilis Camping - 3.5 bar front and 5.5 bar rear (this rear figure irrespective of axle weight!)

Continental Vanco Camper - 3.25 front and 3.5 bar rear

I haven't managed to get any figures yet for the Hankook Vantra LT tyres that have been well reviewed.

 

The recommended tyre pressures are influencing me regarding choice of tyre if I replace some, or all of my tyres. But I'll post separately about that!

 

Thanks for your comments and information

 

 

[Derek Uzzell]

All 215/70 R15C tyres with a 109 Load Index will have a similar pressure-to-load profile. So contacting Hankook about Vantra LT 215/70 R15C inflation pressures is unlikely to be profitable as Vantra LT tyres’ pressure-to-load profile will be much the same as that of equivalent Pirelli, Michelin, Continental, Avon, Dunlop (etc. etc.) products.

 

“Camping-car’ tyres that carry the “CP” marking (eg. 215/70 R15CP 109) are a mite different as the E.T.R.T.O (European Tyre and Rim Technical Organisation) recommends that tyres with this classification be inflated to a high pressure when they are fitted to a motorhome’s rear axle. This recommendation disregards the vehicle’s actual rear axle loading. (And, in my view, deliberately choosing CP-marked tyres for a SWB panel-van conversion would be an odd thing to do.)

 

Michelin follows the E.T.R.T.O’s recommendation rigidly when providing advice on suitable pressures for the Agilis Camping pattern, while Continental’s stance is more flexible regarding Vanco Camper. This explains why Michelin advised a 5.5bar rear axle pressure, but Continental advised 3.5bar.

 

What you are attempting to do is improve your vehicle’s ride quality by playing about with the tyre pressures, whereas these are just one element of a multi-part equation. It’s likely that, whatever 215/70 R15C (or CP) tyre you fit, 3.25bar (front) and 3.5bar(rear) will be reasonable compromise pressures given the 1510kg/1400kg axle loadings you quoted. (Me, I like to have more pressure at the back than the front even if the static rear-axle loading may be lower than the front one.)

 

But the rear springs, dampers and ‘spring assisters’ will also contribute to the vehicle’s ride quality. You said earlier that the ride was “...very crashy/bangy on uneven roads (as if the bump stops are being hit...” and it may well be the case that the ‘bump-stops’ are currently having more of an impact on the ride harshness than the 4.0bar tyre pressures you’ve been using. This theory was explored last June when you asked about air-assistance and I suggested that, if you wanted to confirm the effect of the bump-stops’ on the harsh ride, you could temporarily remove them and see if that signifcantly improved matter and, if it did, replacing them with an air-bellows system could be worthwhile.

 

 

[George Collings]

Derek last posting answers the question and gives the solution.

 

Road imperfection are filtered out by both the tyres and suspension. The tyres deal with the small bumps and the springs the larger. If the tyres are inflated harder than the load demands its the small bump harshness that is felt.

 

The basic problem is that these vans are commercial vehicles primarily designed to carry goods not passengers. Goods don't complain about the ride but passengers do. Running near their allowable load there is virtually no reserve suspension travel to absorb large road imperfections..This is particularly true when cornering.

 

By fitting air assistance the vehicle rides higher giving more reserve travel and a much more progressive stiffening improving ride and handling.

 

The van shown was a simple Ducato PVC running close to but not over its designed limit.

 

See Pic:

 

[johnlc]

Derek and George

Thanks for your comments.

I haven't tried removing the bump stops as Derek has previously suggested as I am not technically competent enough to do so, and spending money to experiment when other things are essential isn't an option. Derek suggests it is not worth bothering with CP tyres but doesn't say why. I did ask Michelin about this and they said the only difference was that the CP tyre could take higher pressures. Given I don't need this and they are more expensive I won't bother with them. Although I have only had my van for a year, it is 6.5 years old now, and done only 24,000 miles. The front tyres need replacing soon because of tread wear but the rear ones have loads left on them. Given there are no visible signs of tyre wall defects I am inclined to keep the rear tyres and put some non CP tyres on the front. Also, given that France requires tyres of the same make on the same axle ( I think) , and my spare is unused this would make sense.

Overall, for 95% of the time the ride in our van is fine. Improving it a little with altered tyre pressures doesn't cost anything. As we don't need the extra payload of 100kg that air assistance would give, we'll probably leave it at that.

Thanks for all your advice.

[colin]

I would suggest you swap rears to front and put new tyres on rear.

[Brian Kirby]

johnlc - 2015-03-29 7:08 PM................... Derek suggests it is not worth bothering with CP tyres but doesn't say why.

The implication, as you have realised, is that your van is relatively light, so the extra benefits of the CP type tyres would not be realised. So, little point in spending the extra if you won't benefit from it.

 

................................. The front tyres need replacing soon because of tread wear but the rear ones have loads left on them. Given there are no visible signs of tyre wall defects I am inclined to keep the rear tyres and put some non CP tyres on the front. Also, given that France requires tyres of the same make on the same axle ( I think) , and my spare is unused this would make sense...................

I'm unconvinced tha France requires this for foreign registered vehicles, or that anyone checks. The only time, AFAIK, it would become an issue would be if you had damaged a tyre that could not be repaired and needed a replacement. Then, unless a French fitter has a replacement that matches the remaining tyre on the same axle, he will be likely to refuse to fit just one, odd, tyre. But, that is the fitter responding to official reccomendation (or possibly regulation, that I think has been criticised in France for being inexplicit) on the basis that if he doesn't do so, and you subsequently have an accident, he might become liable. I don't think it is any more an issue than that. As your vehicle is UK registered, it has only to comply with UK law, which is less demanding in that respect than French. But, the French fitter must treat your vehicle as though it were French if he works on it. I wouldn't worry.

[Derek Uzzell]

This is the 2013 forum thread that originally discussed the requirement in France that same-make tyres musyt be fitted to the same axle

 

http://www.outandaboutlive.co.uk/forums/Motorhomes/Motorhome-Matters/urgent-advice-re-tyres/31371/

 

I don’t believe there is any question of a UK-registered vehicle being somehow illegal in France if it has a mix of makes of tyre on the same axle. It’s just that (as Brian says) a French tyre-fitter may refuse to fit a single tyre if doing so produces a conflict of makes. This is one good reason for carrying a spare wheel (or even just a spare tyre) if practicable when travelling in France.

 

Regarding CP-marked ‘camping-car’ tyres, this Continental webpage advertises their merits

 

http://tinyurl.com/o9yt6wh

 

The CP-marking is a designator that the E.T.R.T.O has agreed can be shown on a tyre’s sidewall to indicate that the tyre manufacturer is marketing the tyre with motorhomes particularly in mind - but no independent testing is carried out to prove that a CP-marked tyre meets the tyre manufacturer’s claims. Continental, Michelin and Pirelli are currently the only manufacturers marketing CP-marked tyres.

 

The reasons I said above " (And, in my view, deliberately choosing CP-marked tyres for a SWB panel-van conversion would be an odd thing to do)" are a) a vehicle like John’s that started its life with non-CP-marked tyres, that is driven relatively lightly loaded and is being criticised for a harsh ride, is most unlikely to benefit from a tyre intended for heavily-loaded motorhomes and designed to be run at high inflation pressures, b) only three manufacturers make CP-marked tyres and availability has, in the past, sometimes proved to be very difficult, c) ‘camping-car’ tyres are specialised products and their price usually reflects this - you won’t find a cheap CP-marked tyre.

 

Received wisdom (and the tyre manufacturers’ advice) is that a vehicle should have its ‘best’ tyres on the rear axle

 

http://www.michelin.co.uk/tyres/learn-share/care-guide/ten-tyre-care-tips

 

In John’s case I’d be considering fitting new Hankook Vantra LT RA18 tyres all round. This assumes that he plans to keep his Fifer for a significant period, in which case he may as well fit brand-new tyres now and getting several years use out of them when they are at their best, rather than fit two new tyres now and another two, say, 3 years down the line, not long before he sells the motorhome.

 

[George Collings]

On modern cars tyres are an integral part of the suspension and on some very expensive models specific to one model. Clearly for these its essential that pairs on an axle or even the whole car are matched but the legislation appears to be blanket. For LCVs where far less sophistication is involved as long as tyres are matched to their loading the rules are being over zealously applied.

 

In a similar vein I have heard of owners trying to register their RHD motorcaravans in France but being refused because the habitation door was on the wrong side. Clearly for a similar vehicle being used as a minibus an exit to the centre of the road was dangerous but if its ok for ordinary cars then its Ok for a motorcaravan.

 

Clearly the officials concerned had enjoyed the how to wear blinker training day.

[johnlc]

Thank you again for your thoughts and information. I had thought of doing what Derek suggested but my wife keeps mentioning about changing the van for one with a shower in it. As we are planning a longish trip to Scotland in mid-April I was hoping she would make up her mind before then but that seems unlikely given we will try wildcamping for the first time on that trip, and the lack of a shower may make her mind up then! Pity it will be too cold to have showers outdoors!

Thanks again

(Derek - I have PMd you)