Tyre Pressures based on Load Ratings, CP Tyres or Van Tyres.
I have found when I have had the Autotrail Serviced I drive away like the vehicle is driving on Bricks, This is due to the Agent applying the tyre presures as per the door lable or handbook
Taken from an enquiry (in a mag.) about pressures of motorhome tyres, with a supplier:-
Rear axle 1900kg (max 2000)
Front axle 1540kg (max 1850)
Gross 3440 kg (max 3500)
The following is the reply from Craig, the Product Support Engineer at the time from Continental Automobil. He has consented to having his reply published on MHF:
"The pressures you require are as follows:
Front: 3.25bar - 47psi pressure applicable to a maximum permissible load of1590kg. 1850kg Max loading at 4bar - 58psi (1875kg maximum at this pressure)
Rear: 4.25bar - 61psi pressure applicable to a maximum permissible load of1970kg. 2000kg Max loading at 4.5bar - 65psi (2060kg maximum at this pressure)
As a general rule for pressures on this tyre, every 100kg added from the base figure of 1490kg at 3bar (43.5psi) will require an increase of 0.25bar(3.6psi) in pressure up to a maximum of 2060kg at 4.5bar (65psi)
We do get a substantial amount of queries regarding pressures. Unfortunately, there is a distinct lack of understanding on the subject which isn't helped by the apparent reluctance of coach-builders to specify the pressures required. Naturally, all pressures are dependent on load* so providing any queries received contain the information requested, we can advise accordingly. I've helped a few other organisers of motorhome/camping clubs and associations in the past so we are all working to educating the consumers as to what is required. Hopefully this will provide a greater understanding for them."
*Probably the most important point that sceptics about running tyres at maximum pressures should note - and straight from the horses mouth.
"Now those pressures of 47 and 61 psi are FAR removed from what is on MY Fiat door pillar, which says 72.3 and 79.5!! What a difference!! Even Continental's MAXIMUM pressures of 58 and 65 psi are far removed from those quoted by Fiat!!
So, in a nut-shell... don't go by the figures on the label... visit a weighbridge when loaded, (£5 - £10) and note the actual axle weights... then contact the TYRE manufacturer for the relevant pressures! Couldn't be simpler!!
I also asked Craig about tyre ageing... He replied thus:
Age cracking is indeed an issue. In some instances, it is superficial but it is always recommended that the tyres are thoroughly checked before being put back into service. I have examined many tyres that have substantial cracking issues to the tread area and sidewalls. Naturally, continued use when they are affected to such a degree exacerbates the condition. Rubber, being a natural compound, will age over time. There are inhibitors that are added to the tyre compound to help prevent this condition occurring, but the nature of the vehicle application means that they suffer most when left static (particularly under load) for prolonged periods. It does help to cover them to protect them from ultraviolet rays when not in use - providing the vehicles are stored outdoors of course."
http://www.tyresafe.org/tyre-safety/tyre-safety-information
Note: This brochure is now out of date, and does not include all the available current tyre ratings.
Added by myself:-
As stated earlier, by a representative of Continental, "all tyre pressures are dependent on the load" NOT on the maximum rated pressure of the tyre, UNLESS you are running on the maximum permissible load of that tyre. Unfortunately, many manufacturers/converters just quote the maximum tyre pressure for all but their smallest vehicles, which includes Auto Trail, regardless of the actual weight carried, in total contradiction to the varying pressures for specific tyres quoted by Tyre Safety. I know which one I trust...... Continental are much more forthcoming with quoting the correct tyre pressures for a given weight than Michelin, who seen to err on the side of "most are overloaded so we just quote the max pressure" syndrome!
A good guide to the tyre pressure required against the load it is to carry (although not conclusive, and if in doubt, always consult the vehicle manufacturer or tyre manufacturer) is:
We are assuming the following to create a formula:
225/75/R16CP Load rating 116 = Max rating 1250kg per tyre at 5.5bar (80lbs sq.in)
Example:
Maximum Axle Load of Vehicle 2100kg. Two wheels per axle, so each wheel/tyre = 1050kg.
Maximum Load stated on tyre 1250kg., at Maximum Tyre Pressure of say 80lbs/sqin. (5.5bar)
(Maximum loadings and pressures are always printed on the tyre)
1050kg (axle load per tyre) is 84% of 1250kg max tyre load, so 84% of 80lbs sq.in is 67lb sq.in., assuming the vehicle is running fully loaded. If you ran the tyre at maximum stated pressure, it would be some 20% over-inflated.!!
You will find however, most stated axle loads will add up to more than the maximum load capacity stated for the vehicle, to take some account of unequal loading, so you will need to weight the vehicle when you have it fully laden, including driver and passenger/s, to determine the pressures even more accurately.
However, using the maximum axle load is a good guide.
As an example, I used my previous vehicle (an Autocruise), and worked out the tyre pressures, comparing them with the converters recommended pressures:-
Rear axle max load is 2050 /2 (two wheels) so 1025 per wheel.
Tyre load for the tyres on this vehicle are rated at 1120kg at 65lbs/sq.in.
1025 is 91% of tyre load (1120) at 65psi - so 91% of 65 is 59.1
Motorhome converters stated tyre pressure is 57lbs sq.in, and taking in the fact that the vehicle maximum loading is 100kg (25kg per wheel) less than the two axle maximums added together, is spot on with my calculations, and it works exactly the same on the front axle too.
As the user payload is often an issue, most vehicles will be running at or near maximum payload (some over probably), so using the maximum permissible weights is a very good guide to your tyre pressures.
NOTE: Contrary to the belief of some, it is NOT necessary or compulsory to fit CP (camper) tyres, and as long as you fit van/commercial tyres that have at least the same weight and speed ratings (or higher is fine), then these are totally adequate for normal use.
So - what is the difference, well CP tyres generally have a couple more plies in the wall, making it firmer, and thus allowing higher pressures to be used, usually up to 80 lbs-psi.
Why? because some motorhomes don't move from one month to the next, because many have too little payload and are overloaded, so manufacturers Michelin & Continental produce a range of CP tyres to combat the transgressors if you like. If you use your unit fairly regularly, and don't overload/exceed the maximum user payload, then normal van tyres are fine! If you do overload, then you need to look at either having the GVW increased (if possible), or take less stuff!
In my personal opinion (but after doing considerable research into the subject, so not pie-in-the-sky comments):-
Disadvantages of using CP tyres: A harder ride, less deflection of tyre on the road so less tread footprint on the road (especially when vastly over-inflated, which many are if you believe the converter), which could possibly lead to longer stopping distances (depending on tyre and pressures), possible greater chance of wheel spin (less grip), quicker wear on front wheel drive vehicles (depending on tyre), expensive, ....
If you regularly and can easily spin the front wheels when you pull away, there are only two reasons, too much weight to the rear, and/or too high tyre pressures.
Disadvantages of using van tyres: *Sidewall cracks may occur a little earlier as the tyre flexes a bit more especially if left stood in one spot for very long periods (but see below), possibly slightly worse MPG due to more tread on the road (rolling resistance greater)
* Most motorhome tyres become due for replacement due to their age rather than wear, so sidewall deterioration is not an issue, unless you tend to try and extend their life beyond the recommended replacement time of 5 - 7 years old. All tyres will deteriorate with age, whatever type they are, so always check them regularly. I used to run Sprinter service vans, fully loaded 100% of the time, and got 100,000 miles out of tyres in 3 - 4 years. Most motorhomes don't do that much in 20 years!
TIPS:
If you store the vehicle over winter rather than use it, increase the tyre pressures to maximum pressure or up to 5%* above max shown on tyre while in store (remember to adjust before use). That way it will reduce the risk of flat-spotting where the tyre is sitting. Also try to move it occasionally, so the tyres are not on the same spot for months on end. *The pressures increase by up to 10% when the tyre is hot, so can take the extra without risk, as long as you take them back down again before use.
If you use commercial van tyres instead of CP tyres, those with 10 ply sidewalls will be almost identical.
As long as the speed rating and load rating are at least the same as the quoted CP, they are fine. Many you will find have a higher load and speed rating - even better! 8 ply van tyres may flex a little more on cornering, but are still ok, especially on the smaller lighter units.
Remember, many m/h suppliers converters just tell you the max. tyre pressure to cover themselves for the overloaders, which is not always very helpful!
For the mathematical minded, the calculation is as follows:
(Using the same assumption as before - ie. 225/75/R16 116-Q rated (Max load 1250kg per tyre) and an axle maximum load of 2100kg run at max load. (adjust your figures to suit)
Max Tyre Pressure 80 / (tyre load rating x 2) x Axle Load 2100
2100x(80/(1250x2)) = 67.2lbs sq.in
In simple terms.. 80psi / (2xmax load) 2500 = 0.032 x 2100* rated axle load = 67.2
*You can adjust this figure as necessary after a weighbridge visit fully laden.
Of course, if when fully loaded the actual axle weight is proportionately less, then you can use this calculation with the known weights given on a weighbridge, to give the correct pressures.
Hope this helps.
NB: There is a new 225/75/R16 Michelin Agilis CP with a load rating of 118-R - so make sure you check your load rating first, before calculating.
Load Rating
Speed rating
Although Tyre Safe do not include the latest tyre size of CP (Camping) tyres, using the exact same calculation % formula that they use for all other available (or previously available) CP tyres, here are the pressures against load for:-
The Michelin Agilis 225/75R16 CP only. Speed rating of Q = 99mph
Maximum load rating (116) of 1250kg per tyre at 80psi. (For 118 ratings see chart below)
(This is based on single rear axle vehicles - not twin wheel or tag axles)
Tyre Pressures (cold)
Lbs sq.in Bar Max. Axle load (2 tyres) % of Max Load
58 4.00 1938kg 77.50
60 4.16 2001kg 80.05
62 4.25 2035kg 81.385
65 4.50 2130kg 85.18
69 4.75 2223kg 88.93
70 4.82 2250kg 90.00
73 5.00 2317kg 92.68
76 5.25 2408kg 96.34
80 5.50 2500kg 100.00
This calculation is based on exactly the same as Tyre Safe calculations of tyre pressures for:-
195/75R14 CP, 195/75R16 CP, 195/65R16 CP, 215/75R16 CP, 215/70R15 CP, 225/70R15 CP, 225/65R16 CP
Over-inflated tyres put less tread between you and the road, the only contact you have between being safe, or not.
http://www.tyresafe.org/images/tyre-safety-guide/motorhome-leaflet.pdf
Taken from an enquiry (in a mag.) about pressures of motorhome tyres, with a supplier:-
Rear axle 1900kg (max 2000)
Front axle 1540kg (max 1850)
Gross 3440 kg (max 3500)
The following is the reply from Craig, the Product Support Engineer at the time from Continental Automobil. He has consented to having his reply published on MHF:
"The pressures you require are as follows:
Front: 3.25bar - 47psi pressure applicable to a maximum permissible load of1590kg. 1850kg Max loading at 4bar - 58psi (1875kg maximum at this pressure)
Rear: 4.25bar - 61psi pressure applicable to a maximum permissible load of1970kg. 2000kg Max loading at 4.5bar - 65psi (2060kg maximum at this pressure)
As a general rule for pressures on this tyre, every 100kg added from the base figure of 1490kg at 3bar (43.5psi) will require an increase of 0.25bar(3.6psi) in pressure up to a maximum of 2060kg at 4.5bar (65psi)
We do get a substantial amount of queries regarding pressures. Unfortunately, there is a distinct lack of understanding on the subject which isn't helped by the apparent reluctance of coach-builders to specify the pressures required. Naturally, all pressures are dependent on load* so providing any queries received contain the information requested, we can advise accordingly. I've helped a few other organisers of motorhome/camping clubs and associations in the past so we are all working to educating the consumers as to what is required. Hopefully this will provide a greater understanding for them."
*Probably the most important point that sceptics about running tyres at maximum pressures should note - and straight from the horses mouth.
"Now those pressures of 47 and 61 psi are FAR removed from what is on MY Fiat door pillar, which says 72.3 and 79.5!! What a difference!! Even Continental's MAXIMUM pressures of 58 and 65 psi are far removed from those quoted by Fiat!!
So, in a nut-shell... don't go by the figures on the label... visit a weighbridge when loaded, (£5 - £10) and note the actual axle weights... then contact the TYRE manufacturer for the relevant pressures! Couldn't be simpler!!
I also asked Craig about tyre ageing... He replied thus:
Age cracking is indeed an issue. In some instances, it is superficial but it is always recommended that the tyres are thoroughly checked before being put back into service. I have examined many tyres that have substantial cracking issues to the tread area and sidewalls. Naturally, continued use when they are affected to such a degree exacerbates the condition. Rubber, being a natural compound, will age over time. There are inhibitors that are added to the tyre compound to help prevent this condition occurring, but the nature of the vehicle application means that they suffer most when left static (particularly under load) for prolonged periods. It does help to cover them to protect them from ultraviolet rays when not in use - providing the vehicles are stored outdoors of course."
http://www.tyresafe.org/tyre-safety/tyre-safety-information
Note: This brochure is now out of date, and does not include all the available current tyre ratings.
Added by myself:-
As stated earlier, by a representative of Continental, "all tyre pressures are dependent on the load" NOT on the maximum rated pressure of the tyre, UNLESS you are running on the maximum permissible load of that tyre. Unfortunately, many manufacturers/converters just quote the maximum tyre pressure for all but their smallest vehicles, which includes Auto Trail, regardless of the actual weight carried, in total contradiction to the varying pressures for specific tyres quoted by Tyre Safety. I know which one I trust...... Continental are much more forthcoming with quoting the correct tyre pressures for a given weight than Michelin, who seen to err on the side of "most are overloaded so we just quote the max pressure" syndrome!
A good guide to the tyre pressure required against the load it is to carry (although not conclusive, and if in doubt, always consult the vehicle manufacturer or tyre manufacturer) is:
We are assuming the following to create a formula:
225/75/R16CP Load rating 116 = Max rating 1250kg per tyre at 5.5bar (80lbs sq.in)
Example:
Maximum Axle Load of Vehicle 2100kg. Two wheels per axle, so each wheel/tyre = 1050kg.
Maximum Load stated on tyre 1250kg., at Maximum Tyre Pressure of say 80lbs/sqin. (5.5bar)
(Maximum loadings and pressures are always printed on the tyre)
1050kg (axle load per tyre) is 84% of 1250kg max tyre load, so 84% of 80lbs sq.in is 67lb sq.in., assuming the vehicle is running fully loaded. If you ran the tyre at maximum stated pressure, it would be some 20% over-inflated.!!
You will find however, most stated axle loads will add up to more than the maximum load capacity stated for the vehicle, to take some account of unequal loading, so you will need to weight the vehicle when you have it fully laden, including driver and passenger/s, to determine the pressures even more accurately.
However, using the maximum axle load is a good guide.
As an example, I used my previous vehicle (an Autocruise), and worked out the tyre pressures, comparing them with the converters recommended pressures:-
Rear axle max load is 2050 /2 (two wheels) so 1025 per wheel.
Tyre load for the tyres on this vehicle are rated at 1120kg at 65lbs/sq.in.
1025 is 91% of tyre load (1120) at 65psi - so 91% of 65 is 59.1
Motorhome converters stated tyre pressure is 57lbs sq.in, and taking in the fact that the vehicle maximum loading is 100kg (25kg per wheel) less than the two axle maximums added together, is spot on with my calculations, and it works exactly the same on the front axle too.
As the user payload is often an issue, most vehicles will be running at or near maximum payload (some over probably), so using the maximum permissible weights is a very good guide to your tyre pressures.
NOTE: Contrary to the belief of some, it is NOT necessary or compulsory to fit CP (camper) tyres, and as long as you fit van/commercial tyres that have at least the same weight and speed ratings (or higher is fine), then these are totally adequate for normal use.
So - what is the difference, well CP tyres generally have a couple more plies in the wall, making it firmer, and thus allowing higher pressures to be used, usually up to 80 lbs-psi.
Why? because some motorhomes don't move from one month to the next, because many have too little payload and are overloaded, so manufacturers Michelin & Continental produce a range of CP tyres to combat the transgressors if you like. If you use your unit fairly regularly, and don't overload/exceed the maximum user payload, then normal van tyres are fine! If you do overload, then you need to look at either having the GVW increased (if possible), or take less stuff!
In my personal opinion (but after doing considerable research into the subject, so not pie-in-the-sky comments):-
Disadvantages of using CP tyres: A harder ride, less deflection of tyre on the road so less tread footprint on the road (especially when vastly over-inflated, which many are if you believe the converter), which could possibly lead to longer stopping distances (depending on tyre and pressures), possible greater chance of wheel spin (less grip), quicker wear on front wheel drive vehicles (depending on tyre), expensive, ....
If you regularly and can easily spin the front wheels when you pull away, there are only two reasons, too much weight to the rear, and/or too high tyre pressures.
Disadvantages of using van tyres: *Sidewall cracks may occur a little earlier as the tyre flexes a bit more especially if left stood in one spot for very long periods (but see below), possibly slightly worse MPG due to more tread on the road (rolling resistance greater)
* Most motorhome tyres become due for replacement due to their age rather than wear, so sidewall deterioration is not an issue, unless you tend to try and extend their life beyond the recommended replacement time of 5 - 7 years old. All tyres will deteriorate with age, whatever type they are, so always check them regularly. I used to run Sprinter service vans, fully loaded 100% of the time, and got 100,000 miles out of tyres in 3 - 4 years. Most motorhomes don't do that much in 20 years!
TIPS:
If you store the vehicle over winter rather than use it, increase the tyre pressures to maximum pressure or up to 5%* above max shown on tyre while in store (remember to adjust before use). That way it will reduce the risk of flat-spotting where the tyre is sitting. Also try to move it occasionally, so the tyres are not on the same spot for months on end. *The pressures increase by up to 10% when the tyre is hot, so can take the extra without risk, as long as you take them back down again before use.
If you use commercial van tyres instead of CP tyres, those with 10 ply sidewalls will be almost identical.
As long as the speed rating and load rating are at least the same as the quoted CP, they are fine. Many you will find have a higher load and speed rating - even better! 8 ply van tyres may flex a little more on cornering, but are still ok, especially on the smaller lighter units.
Remember, many m/h suppliers converters just tell you the max. tyre pressure to cover themselves for the overloaders, which is not always very helpful!
For the mathematical minded, the calculation is as follows:
(Using the same assumption as before - ie. 225/75/R16 116-Q rated (Max load 1250kg per tyre) and an axle maximum load of 2100kg run at max load. (adjust your figures to suit)
Max Tyre Pressure 80 / (tyre load rating x 2) x Axle Load 2100
2100x(80/(1250x2)) = 67.2lbs sq.in
In simple terms.. 80psi / (2xmax load) 2500 = 0.032 x 2100* rated axle load = 67.2
*You can adjust this figure as necessary after a weighbridge visit fully laden.
Of course, if when fully loaded the actual axle weight is proportionately less, then you can use this calculation with the known weights given on a weighbridge, to give the correct pressures.
Hope this helps.
NB: There is a new 225/75/R16 Michelin Agilis CP with a load rating of 118-R - so make sure you check your load rating first, before calculating.
Load Rating
Speed rating
Although Tyre Safe do not include the latest tyre size of CP (Camping) tyres, using the exact same calculation % formula that they use for all other available (or previously available) CP tyres, here are the pressures against load for:-
The Michelin Agilis 225/75R16 CP only. Speed rating of Q = 99mph
Maximum load rating (116) of 1250kg per tyre at 80psi. (For 118 ratings see chart below)
(This is based on single rear axle vehicles - not twin wheel or tag axles)
Tyre Pressures (cold)
Lbs sq.in Bar Max. Axle load (2 tyres) % of Max Load
58 4.00 1938kg 77.50
60 4.16 2001kg 80.05
62 4.25 2035kg 81.385
65 4.50 2130kg 85.18
69 4.75 2223kg 88.93
70 4.82 2250kg 90.00
73 5.00 2317kg 92.68
76 5.25 2408kg 96.34
80 5.50 2500kg 100.00
This calculation is based on exactly the same as Tyre Safe calculations of tyre pressures for:-
195/75R14 CP, 195/75R16 CP, 195/65R16 CP, 215/75R16 CP, 215/70R15 CP, 225/70R15 CP, 225/65R16 CP
Over-inflated tyres put less tread between you and the road, the only contact you have between being safe, or not.
http://www.tyresafe.org/images/tyre-safety-guide/motorhome-leaflet.pdf
.jpg)
.JPG)
.jpg)
.jpg)
.jpg)
.jpg)