You want to match the tire pressure with the weight on the tires. Doing so will give you a more comfortable ride and protection against blowouts. To do this requires you to weigh all four corners. Then go to the manufacturer's charts and use the pressure specified for the weight. Use the highest corner weight on the front and the rear so that both rears have the same pressure and dito for the fronts. Your option would be to set all tires to the highest weight.
I had my rig weighed by a local who used a portable scale to weigh each corner.
How many of you have done the math - compared the load carrying capacity of the tires as listed on the vehicle tire placard with the GAWR's? I'll bet you find that recently produced vehicles (say 2008 and newer) have more tire capacity than the minimum to carry the GAWR. That's the vehicle manufacturer's take on the use of the tire load tables.
Doesn't that mean you have to adjust what you get when you weigh the vehicle by the same factor? In other words, you should not use the published tire load tables as is.
Well that certainly stirred up the hornet nest didn't it.
YES, YES,YES, you can run the tyres at the 110psi maximum pressure stamped on the sidewall. They won't mind a bit.
However, as you probably realise, that may not be the most ideal pressure for you, your passengers and the motorhome - for several reasons.
1 - tyre wear is certainly going to be only on the centre 1/3 of the tread.
This has bad results -
The tyres are going to be worn out far, far earlier than they would be if the wear was shared equally right across the tread. So there is a direct cost involved.
Your footprint will also be 1/3 what it should be and this WILL have effects on how well they grip the road and that MUST reduce the safety margin under severe braking and evasion manoeuvres. What price do you put on safety.
2 - the tyres are more likely to tramline because they are just too tight and the sidewalls will have no squirm-factor (no, not a scientific term)and will be acting more like crossply tyres. This can cause stability problems and require more attention to driving.
3 - You are losing a big part of your suspension travel and shock absorbing power of your suspension. Tyres at the right pressure for the load can deform to soak up quite large bumps so if they are too tight, the other parts of the suspension have to do all the work and the shock absorbers will also be under more stress.
You will end up with fast-wearing tyres, a harsh ride, your vehicle will fall to pieces that much quicker, and in an emergency situation your steering and stopping will be noticeably less-responsive.
Only you can decide whether the slight inconvenience of weighing the coach and getting the pressures right outweighs the verifiable disadvantages of running with grossly-overinflated tyres.
One thing to consider is that especially in our situation, if a MH really did need to run at maximum sidewall pressure because they were that heavily loaded, it would probably be smart to fit tyres with a higher load rating.
The other thing is that the "standard" pressure to run truck tyres (yes, I realise yours are smaller) at seems to be 100psi - perhaps only because it is a nice round figure - so unless your loads are up there with trucks, maybe you don't need quite so much
CapriRacer wrote: How many of you have done the math - compared the load carrying capacity of the tires as listed on the vehicle tire placard with the GAWR's?
CapriRacer nailed it. I did the math. My tires at max air have a greater load capacity then my axles. Adding it all up I have an 18K GVWR chassis rolling on recommaned size and load range tires with a capacity of 26.6K lbs. I did inflate up to max once and the ride was pretty harsh and the coach would wandering pretty badly.
Inflating to tire max without knowing your weight will give you a very wide safety margin incase you are over loaded. Take the ten minutes to drive thru a CAT scale. If not for tire pressure reasons at least to ensure you are not overloading the chassis.
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I think there is some practical reality that one needs to consider. First of all, the statements that over inflated tires are more prone to blow outs is just plain false. As long as the tire is within the maximum rated pressure, the tire is fully designed to handle that pressure. Consider that if the tire is running at maximum load and maximum air, reducing the load, without reducing the air will reduce stress on the tire compared to maximum load. Less load = less stress period. Its pretty simple physics. You can not invent more stress with less weight. DO not get confused with stress that is transferred into the coach. Yes, it will transfer more stress to the coach, but that is the transfer of existing stress. A tire that is matched to the load, actually absorbs the stress, rather than transferring it to the coach. YOU CAN NOT INCREASE STRESS, you can only choose where the stress is absorbed.
As far as wear, I think we imagine things incorrectly. Tires have belts wrapped around that tire that prevent the tire from expanding by any large amount, the belts tend to keep the tread area fairly flat. The reduction in contact patch from excessive air is mostly for to aft, not so much side to side. This can be very easily seen if you look at contact patches of tires going around corners. The contact patch changes to V-shaped, rather than lifting the whole side of the tire. As it turns out, I run my tires over inflated do to the improved handling characteristics. Since I drive my coach rather than letting it sit, I wear out my tires. Tires are overinflated by about ~20%. My rear set is down to the tread indicators. You have to study the tire carefully to see that the inside has ever so slightly less tread than the outside. In otherwords, the tire wear difference is negligible over the full life of the tire. If you have really wide car tires, you will see it, but for narrower high pressure RV tires, not so much. AS far as traction, I defy any logical explanation that can prove it is significant. Under inflated tire will absolutely give less traction, but not so much for overinflated tires. The reason is that anytime you are in a situation where traction is critical, there is significant weight transfer to the tires most critical to the traction component. (braking results in weight transfer to the front wheels, swerving side to side will transfer weight to the outside tire) Since the weight on the critical tire is increased, that will make a matched inflated tire become effectively under inflated for the load at that time. In some extreme cases you maybe able to measure some minor difference for an over inflated tire, but the practical reality is its pretty insignificant, particularly because of the weight transfer.
Dave
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Well that certainly stirred up the hornet nest didn't it.
h
and finally led to some decent discussion and showed the varied viewpoints instead of just bashing the OP because he didn't bother to correct a misspelling.
Many/most of us have the same tires that are used on trucks. Many vocational trucks(dump ect.) run empty half of the time. They may go from an empty weight of 20-24k to 60k loaded. Their tires will be inflated to the max. They seem to get reasonable mileage from their tires. More than the average RVer will do before the tires age out.
The theory that over inflated radial tires will loose 2/3 0f their contact patch is carried over from bias tires and even then is overblown. We are talking truck tires. I agree with Dave on this. Does an empty dump truck ride rough? Watch one go across an uneven RR crossing.
jolooote wrote: 2006 National RV Class'A'. New, Goodyear, G647, 245/70R-19.5,
Load Range 'G' tires on Ford F53 20500# chassis.
The MAX preasure on side of tires is 110 psi. Is it ok to just run em at that preasure? No, I'm not goin 2 get it weighed. No, I'm not going to mess with manufacturers Tire preasure tables.
I'll be traveling across the US to Colorado & there will probably be temps into the 100s in Kansas, etc. The preasures listed inside the coach by the builder (85) seem very low. I don't want the tires to be overheated if preasure is too low.
I won't comment on your attitude against doing it right, so to best answer your question......
Don't exceed the wheel PSI, no matter what the tire max is. If a wheel should ever explode while airing, however rare, you're dead.
85psi sounds about right for max GVW, but I like a 10psi cushion, so I'm guessing 95 for best results unless the wheel is only rated for 90.
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Daveinet wrote: I think there is some practical reality that one needs to consider. First of all, the statements that over inflated tires are more prone to blow outs is just plain false. As long as the tire is within the maximum rated pressure, the tire is fully designed to handle that pressure. Consider that if the tire is running at maximum load and maximum air, reducing the load, without reducing the air will reduce stress on the tire compared to maximum load. Less load = less stress period. Its pretty simple physics. You can not invent more stress with less weight. DO not get confused with stress that is transferred into the coach. Yes, it will transfer more stress to the coach, but that is the transfer of existing stress. A tire that is matched to the load, actually absorbs the stress, rather than transferring it to the coach. YOU CAN NOT INCREASE STRESS, you can only choose where the stress is absorbed.
As far as wear, I think we imagine things incorrectly. Tires have belts wrapped around that tire that prevent the tire from expanding by any large amount, the belts tend to keep the tread area fairly flat. The reduction in contact patch from excessive air is mostly for to aft, not so much side to side. This can be very easily seen if you look at contact patches of tires going around corners. The contact patch changes to V-shaped, rather than lifting the whole side of the tire. As it turns out, I run my tires over inflated do to the improved handling characteristics. Since I drive my coach rather than letting it sit, I wear out my tires. Tires are overinflated by about ~20%. My rear set is down to the tread indicators. You have to study the tire carefully to see that the inside has ever so slightly less tread than the outside. In otherwords, the tire wear difference is negligible over the full life of the tire. If you have really wide car tires, you will see it, but for narrower high pressure RV tires, not so much. AS far as traction, I defy any logical explanation that can prove it is significant. Under inflated tire will absolutely give less traction, but not so much for overinflated tires. The reason is that anytime you are in a situation where traction is critical, there is significant weight transfer to the tires most critical to the traction component. (braking results in weight transfer to the front wheels, swerving side to side will transfer weight to the outside tire) Since the weight on the critical tire is increased, that will make a matched inflated tire become effectively under inflated for the load at that time. In some extreme cases you maybe able to measure some minor difference for an over inflated tire, but the practical reality is its pretty insignificant, particularly because of the weight transfer.
You really need to conduct a thorough investigation into tire technology and design before you make any more replies regarding tires. I worked for one of the big 3 tire companies for many years and not one thing you said is accurate. However, you are free to inflate your tires at whatever pressure you chose. I just hope no one reading your post follows your advice.
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No, I'm not goin 2 get it weighed.
No, I'm not going to mess with manufacturers Tire preasure tables.
