Tyre Pressures

Added info.

Continental's chart, as posted earlier, states that for the tyres fitted to my vehicle, the pressure for the rear tyres, with a 2130kg axle load, is 65.3 psi

For the front, the chart only goes down to a front axle loading of 1730kg, and for that the tyre pressures are 43.5 psi, which. A tad far removed from Fiat's 80 psi.

As I said earlier, I am now running at 55 psi at the front, which from Continental's chart is for an axle axle loading of 2070kg.

"If all of this 390kg, was added the rear axle loading it would of course now take it to 2115kg, so over the 2000kg limit you refer to.

However, if we assume assume that only 70% of this 390kg is loaded onto the rear axle, and the remaining 30% applied to the front axle, then 273kg would be added to the rear axle loading, and 117kg added to the front axle."

Dave, firstly, my comment re 70/30 of passenger weight distribution was based on their seating position in relation to the two axles.

however, the fulcrum effect of any load placed behind a rear axle depends on the length of the overhang and the wheelbase.

a simple example....a van has a 4m wheelbase and a 2m overhang (not unusual with some of the latest AS and swift and AT and etc  coachbuilts...) 100kg placed at the rear of the locker will exert 150kg loading onto the rear axle (and remove same from front axle).

it was your earlier comment suggesting that any remaining payload would be placed 'over or behind' the rear axle which led me to believe you had a garage or larger rear underbed locker...

...and, thus, your above comment re the effect of that hypothetical extra 390kg is not true.

unless it is placed between the axles (this may be what your comment was describing) it cannot have the effect you describe.

if it were all over the rear axle the spread would be 0/100 front/rear, but the real effect of putting (say) 390kg behind the rear axle would be as in my earlier post, approx 450kg rear and a reduction on the (already light) front axle...

looking at it another way, what it means is that any nominal spare capacity on the rear axle of (say) 200kg is not, in fact 200kg...

as illustrated above (albeit an extreme example) it might only take around 150kg placed in a rear locker to cause a force of 200kg on that axle...... 

now, from your figures, I don't think you will be overloaded, however, unless you know the overhang ratio etc, it's difficult to precisely predict how load position truly affects axle loadings.

i think you are doing the right thing, to get totally loaded up and then re-weigh...

despite the fact that I'm sure youll be fine, the numbers might still surprise you.

good luck.

EDIT: having just noticed it's a Bessacarr 412 (6m and no rear storage) it looks like all the available storage is under the lounge seating which will spread the weight fairly evenly.

i don't see a real problem, but weighing is still good practice as youve discovered.

Certainly an interesting discussion 

For now, I am comfortable with the reduced pressures that I have set (55psi front, and 65psi rear), but I will, when the occasion arises, and are fully loaded, re-visit the weighbridge, and get it weighed with us onboard.

On driving the vehicle, I quickly knew that that things weren't right, as the ride was extremely hard, and the steering at speed, like driving on marbles. This is why my attention turned to tyre pressures.

It would appear that mho manufacturers, and Fiat, apply a 'blanket' 80psi all round, to allow for a maximum loading on the tyres, irrespective of axle loading, but in many cases this would equate to grossly over inflated tyres for the actual load carried. Especialy the fronts.

I have to wonder how many mho owners are driving around  with 80psi all round, just because the 'book' says so, even though though the tyre manufactures say different.

Having sent my weighbridge results to Continental, I received another reply this morning which gave this info:

" ContiVanco Camper 225/75 R16 (CP*) 116 R tyres.

Front Axle: 1280 Kg - 3.0 bar/ 43.5 psi (maximum weight for pressure given = 1730 kg)

Rear Axle: (single fitment) 1680 Kg - 3.5 bar/ 50.8 psi (maximum weight for pressure given = 1740 kg)"

Of course, these weights were without passengers, clothing, and food, but even so, still far removed from 80psi.

80psi is for a real axle loading of 2500kg, and 70psi for a front axle loading of 2500kg. Of course these figures are for the tyres fitted to my vehicle.

My advice would be to consult the tyre manufacturer, and not assume that 80psi all round is correct. After all, it is the tyre that is carrying the weight, and who knows the tyre's capability better than the manufacturer of the tyre.

In the case of my tyres, 80psi is the absolute maximum pressure, and I would not be happy running them at the limit.

As I said BB, an interesting discussion, and one which I hope may have been of interest to others.

Dave, payloads (and associated axle weights) are always 'interesting' discussions.

opions vary, even within the industry...

eg....Continental will respond directly to a customer and will recommend appropriate pressures based on axle weights...

Michelin, on the other hand, seem only to do this for the front axle, recommending 80psi for the rears, irrespective of weights....the reason given is that there is a 'tyre body' (can't remember their name) that stipulates this and they won't go against this.....despite doing as Contindid for many years...

Thirdly, we have the converters, who seem to think that 80 all round will do the job....

despite some of their customers rejecting this and inflating to (say) Conti's recommendation, some converters won't help with getting TPMS  updated in line with the change, merely saying 'it isn't possible' so customers either suffer a hard ride or a continual being in the cab...

re: payloads specifically, some are totally oblivious of even the MTPLM let alone individual axle weights, just loading a van, because it's a van and has plenty of space....

some recognise these limits, yet 'sail close to the wind' or bury their head in the sand....

some manufacturers (certainly, swift, AS, AT)  are marketing vans at 8m  on a 3500 chassis.....with huge overhangs, storage under rear beds etc.

some of these have 'payload' in MIRO of just. over 200kg....the rear axle must be close to busting the limit before anyone gets in it....

fortunately, there are also plenty of vans with decent payload at 3500 and plenty of owners who are aware of their vans technical properties and act accordingly.

happy vanning.

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DD, yes, I thought Michelin used to respond as you said, too.

however, I picked up my comments from MMM possibly 6 mths ago...

i can't remember the name of the tyre 'body' whose "thoughts' Michelin now side with...

perhaps Google MMM Michelin pressures will,throw it up..

have to go, cinema awaits.... 

DD, I think it's Tyresafe that only give a 'calculated' pressure for the front and 80 for the rear...

ive just put my figures into their website calculator and got the fugures below...

firstly, here is Contis reply for my weights...

ContiVanco Camper 225/75 R16 (CP*) 116 R tyres.

Front Axle: 1600 Kg - 3.0 bar/ 43.5 psi (max weight for pressure given = 1730 kg)

Rear Axle: 1820 Kg - 3.75 bar/ 54.3 psi (max weight for pressure given = 1840 kg)

here is the Tyresafe web response....along with their rear axle loading 'advice/rider'

Suggested cold tyre inflation pressure:

Front Axle : 44 psi / 3.06 bar

Rear Axle : 80 psi / 5.5 bar

Do not exceed the maximum cold inflation pressure marked on the tyre sidewalll

Advice :

Tyre valves must be suitable for the inflation pressures and for the wheels, and must be in good condition. Metallic or special high pressure valves are normally required with CP-type tyres. Seek the advice of a tyre specialist.

CP-type tyre construction enables the use of higher inflation pressures to provide resistance to the difficult conditions of use encountered on motorhomes. Therefore, when CP-type tyres are fitted on the rear axle in a single formation set the inflation pressures to 5.5 bar (80 psi) for all loads.

im sure it's this last para (rear axle only) that Michelin are now noting when giving out to customer requests....whereas they never used to...

its interesting that Coninental still give figures based on both axle weights.

what could Tyresafe know that Continental don't?

im more than happy with my figures from Continental.

PS, re the noise thread ...I won't mention the ear plugs to L

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I dont know about anyone else ? But to me, having a Tyre pressure monitoring system that isnt settable by the driver is an anathema. So all vehicles running around with tyre pressures ALWAYS set for maximum loading and everyone onboard getting a very uncomfortable ride ?

Not me, IF i purchase such a vehicle, I would get the system replaced by a customer settable commercially available system, as they are a useful addition, and evidently can fail an MOT if its not working ?

But compulsory over inflated tyres ???  A step too far.