Tyre Pressure Calculator

[Fletton]

Thanks for all the advice. The bit I don't quite get is that all the rear axle figures appear to be above the 69psi written in the tyre, and if it is safe to exceed that

@Mr Chrysalis ... My guess is a big no no...
another thought - if I understand what you are saying - is that your tyres may be underrated for said vehicle and axle weights...

 

[peterc10]

I am sure I have read somewhere that that pressure can be exceeded. Ater all Fiat or the converter put the tyres on and usually recommend 5.5bar all round! They would not do that unless they were sure it was OK. Think it is something only relevant to the USA.

But it is another reason to phone Continental's technical advice line just to make sure.

 

[Mr Chrysalis]

I am sure I have read somewhere that that pressure can be exceeded. Ater all Fiat or the converter put the tyres on and usually recommend 5.5bar all round! They would not do that unless they were sure it was OK. Think it is something only relevant to the USA.

But it is another reason to phone Continental's technical advice line just to make sure.

I got a very quick response from Continental:

Thank you for your email and I completely understand your concern.

It is extremely likely that the relevant information supplied with the chassis is documented for use as a commercial vehicle, therefore proposed before the conversion to a motor home. For this reason, Continental tyres would always recommend having the motor home weighed on a weigh bridge at your heaviest travelling weight, including liquids to achieve specific axle weights. In some cases the coach builder will also put their recommended pressures on the vehicle plate. However, the majority put this to the maximum inflation pressure of the tyre to cover all loads being applied and not specific loads which often, are not near the plated weight you actually travel at.

The volume and pressure of the inflation medium is what carries the load and if it is not sufficient then the tyre can suffer damage, hence why it is extremely important for the correct loads to be obtained, in order to inflate the tyre correctly.

Experience shows that due to the weight distribution of vehicles in the leisure industry such as motor homes the likelihood of overloading or uneven weight distribution tends to put a greater load on the rear axle rather than on the front axles, where the weight tends to remain more consistent. It is true that when braking occurs the weight transference is to the front, but this only tends to be for short periods, whereas overloading on the rear axle can be present over long periods, when the tyres are rotating at high speed.

This is the main reason that with CP Camper type tyres the inflation pressures at the rear is deliberately set higher. Although this also has a bearing on the contact area when negotiating grassy parking areas and also to the sidewall deflection, which influences the body roll.

The 69 psi written on the side of the tyre is for the Northern American market only and the inflation pressure of the tyre can exceed this value within its safety operating range.

Based on the following:

ContiVanco Camper 215/70 R15 (CP*) 109 R tyres.

Front Axle: 1850 Kg - 4.25 bar/ 61.6 psi (maximum weight for pressure given = 1885 kg)

Rear Axle: (single fitment) 2000 Kg - 5.5 bar/ 79.8 psi (maximum weight for pressure given = 2060 kg)

Rear Axle: (twinned fitment) 2000 Kg - 3.0 bar/ 43.5 psi (maximum weight for pressure given = 2640 kg)


(*CP- Camper) (Single fitment (S) = two wheels per axle, Twinned fitment (T) = four (or more) wheels per axle)

I have included the chart for these specific tyres to help with pressure vs. load for when you have had the vehicle weighed accordingly.



I hope this helps and any further enquiries (no matter how small) please do not hesitate to ask, we are always happy to help!


Best Regards

Alex Wilson
Product Support Engineer


I am visiting the weighbridge tomorrow, so now I know the 69psi has no relevance, I can set the tyres correctly

 

[Mr Chrysalis]

I got a very quick response from Continental:

Thank you for your email and I completely understand your concern.

It is extremely likely that the relevant information supplied with the chassis is documented for use as a commercial vehicle, therefore proposed before the conversion to a motor home. For this reason, Continental tyres would always recommend having the motor home weighed on a weigh bridge at your heaviest travelling weight, including liquids to achieve specific axle weights. In some cases the coach builder will also put their recommended pressures on the vehicle plate. However, the majority put this to the maximum inflation pressure of the tyre to cover all loads being applied and not specific loads which often, are not near the plated weight you actually travel at.

The volume and pressure of the inflation medium is what carries the load and if it is not sufficient then the tyre can suffer damage, hence why it is extremely important for the correct loads to be obtained, in order to inflate the tyre correctly.

Experience shows that due to the weight distribution of vehicles in the leisure industry such as motor homes the likelihood of overloading or uneven weight distribution tends to put a greater load on the rear axle rather than on the front axles, where the weight tends to remain more consistent. It is true that when braking occurs the weight transference is to the front, but this only tends to be for short periods, whereas overloading on the rear axle can be present over long periods, when the tyres are rotating at high speed.

This is the main reason that with CP Camper type tyres the inflation pressures at the rear is deliberately set higher. Although this also has a bearing on the contact area when negotiating grassy parking areas and also to the sidewall deflection, which influences the body roll.

The 69 psi written on the side of the tyre is for the Northern American market only and the inflation pressure of the tyre can exceed this value within its safety operating range.

Based on the following:

ContiVanco Camper 215/70 R15 (CP*) 109 R tyres.

Front Axle: 1850 Kg - 4.25 bar/ 61.6 psi (maximum weight for pressure given = 1885 kg)

Rear Axle: (single fitment) 2000 Kg - 5.5 bar/ 79.8 psi (maximum weight for pressure given = 2060 kg)

Rear Axle: (twinned fitment) 2000 Kg - 3.0 bar/ 43.5 psi (maximum weight for pressure given = 2640 kg)


(*CP- Camper) (Single fitment (S) = two wheels per axle, Twinned fitment (T) = four (or more) wheels per axle)

I have included the chart for these specific tyres to help with pressure vs. load for when you have had the vehicle weighed accordingly.



I hope this helps and any further enquiries (no matter how small) please do not hesitate to ask, we are always happy to help!


Best Regards

Alex Wilson
Product Support Engineer


I am visiting the weighbridge tomorrow, so now I know the 69psi has no relevance, I can set the tyres correctly

Weighbridge says we are just about at the limit, 3650kg. I could not find one to weigh individual axles so will assume 1750 front and 1900 rear and inflate according to the table provided by Continental

 

[peterc10]

Weighbridge says we are just about at the limit, 3650kg. I could not find one to weigh individual axles so will assume 1750 front and 1900 rear and inflate according to the table provided by Continental

Most weighbridges should be able to weigh the axles as long as the approach to it is level. Just drive on so that your front wheels on and obtain a reading. That will give you the front axle's load. Then drive further on so that all wheels are on and obtain another reading. That will give you the total load. The difference between the 2 will be the rear axle load.

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[Mr Chrysalis]

Most weighbridges should be able to weigh the axles as long as the approach to it is level. Just drive on so that your front wheels on and obtain a reading. That will give you the front axle's load. Then drive further on so that all wheels are on and obtain another reading. That will give you the total load. The difference between the 2 will be the rear axle load.

Yes, that is what I said to the operator, but he was adamant that it would not do it, and would not try it. I'll try another one later - this one was handy as it was local. As a chartered mechanical engineer, I am confident that provided the vehicle is reasonably horizontal and you can get the front wheels somewhere near the middle of the weighbridge the readings should be correct. To be fair to the operator, his weighbridge was about 20m long so my wheels would have been close to one end if I had only rolled the first axle in