Tires at different pressures on same axle??

[Gordon Dewald]

Originally Posted by FastEagle

Unfortunately many Motor Home owners wrongly apply that standard to their Motor Homes instead of following the Federal Motor Vehicle Safety Standards to which their vehicles were built to. FMVSS regulations require recommended tire inflation pressures. The tire industry never, under normal circumstances, recommends inflation pressures below the vehicle manufacturer’s recommendations found on the vehicle certification label, tire placard or in the vehicle owner’s manual.

I found that the recommended tire pressure on my certification label corresponds with the max GAWR rating on the axles also on the label using the tire manufacturers inflation charts for the tires installed on our unit.

Since I am well below the GAWR axle rating on all axles I feel confident in reducing the pressure to the tire manufacturers inflation chart settings. I add 5 lbs pressure to address unequal loading when driving for things like rutting in the road surface.

[FastEagle]

Originally Posted by Gordon Dewald

I found that the recommended tire pressure on my certification label corresponds with the max GAWR rating on the axles also on the label using the tire manufacturers inflation charts for the tires installed on our unit.

Since I am well below the GAWR axle rating on all axles I feel confident in reducing the pressure to the tire manufacturers inflation chart settings. I add 5 lbs pressure to address unequal loading when driving for things like rutting in the road surface.

Whenever a statement such as yours above involves vehicle safety it needs an answer that is best descriptive for the situation. I could quote Michelin, Goodyear or Toyo....etc...etc... but each may differ in content. So here is a reference to a link that can answer with authority.

National Highway Traffic Safety Administration (NHTSA)

[Chinewalker]

Originally Posted by FastEagle

Whenever a statement such as yours above involves vehicle safety it needs an answer that is best descriptive for the situation. I could quote Michelin, Goodyear or Toyo....etc...etc... but each may differ in content. So here is a reference to a link that can answer with authority.

National Highway Traffic Safety Administration (NHTSA)

Nice. Thanks for the link.

[Gordon Dewald]

Originally Posted by FastEagle

Whenever a statement such as yours above involves vehicle safety it needs an answer that is best descriptive for the situation. I could quote Michelin, Goodyear or Toyo....etc...etc... but each may differ in content. So here is a reference to a link that can answer with authority.

National Highway Traffic Safety Administration (NHTSA)

I think I found the brochure you are referring to regarding the tire inflation on the manufacturers placard.

It does say "Manufacturers of passenger vehicles and light trucks determine this number based on the vehicles design load limit, and the vehicles tire size."

I wonder why they did not simply say inflate to the max tire pressure on the side of the tire? It appears they agree to let the manufacturer use the same tire inflation chart to determine a reduced pressure for the max design load.

The brochure does not address the topic of maximum tire pressure when the vehicle weight is below GVWR. Is this deliberate on behalf of the authors? Is it an oversight or a topic that is too complex to deal with in a brochure? There are a couple things that make me wonder?

Skid resistance and braking.

If each axle is loaded to the GVWR and the tires inflated to the recommended pressure there would be a resultant tire ‘patch’ contact area. The patch helps determine handling and braking characteristics that is part of the tire design. With reduced loading the tire patch size is decreased. Will this affect the tire handling and braking characteristics? Another interesting topic.

A tire that is inflated to carry a load greater than is being applied provides a harsher ride and will cause extra shock to the chassis and house components.

I believe the statement in the brochure as printed. We can agree to disagree on the application of reduced tire pressure per inflation charts vs maintaining max pressure at any weight.

[FastEagle]

Originally Posted by Gordon Dewald

I think I found the brochure you are referring to regarding the tire inflation on the manufacturers placard.

It does say "Manufacturers of passenger vehicles and light trucks determine this number based on the vehicles design load limit, and the vehicles tire size."

I wonder why they did not simply say inflate to the max tire pressure on the side of the tire? It appears they agree to let the manufacturer use the same tire inflation chart to determine a reduced pressure for the max design load.

The brochure does not address the topic of maximum tire pressure when the vehicle weight is below GVWR. Is this deliberate on behalf of the authors? Is it an oversight or a topic that is too complex to deal with in a brochure? There are a couple things that make me wonder?

Skid resistance and braking.

If each axle is loaded to the GVWR and the tires inflated to the recommended pressure there would be a resultant tire ‘patch’ contact area. The patch helps determine handling and braking characteristics that is part of the tire design. With reduced loading the tire patch size is decreased. Will this affect the tire handling and braking characteristics? Another interesting topic.

A tire that is inflated to carry a load greater than is being applied provides a harsher ride and will cause extra shock to the chassis and house components.

I believe the statement in the brochure as printed. We can agree to disagree on the application of reduced tire pressure per inflation charts vs maintaining max pressure at any weight.

Tire inflation charts are approved by the TRA. For the most part, equal sized tires will have the same chart for all manufacturer’s of a specific size and design. For instance, the ST235/85R16E will have a maximum load capacity of 3640# at 80 PSI. The LT235/85R16E will have a maximum load capacity of 3042# at 80 PSI. Different design, different materials, different results.

Treads don’t provide tire strength. They provide durability for an assortment of tire functions including wear.

Load inflation charts are developed in a collaborative effort by tire and vehicle manufacturers. Vehicle manufacturers are responsible for selecting all OE tires. They are also responsible for establishing and setting recommended (correct) tire inflation pressures for OE tires - see FMVSS 571.120 paragraph S5.3.1. It takes the ambiguity out of it.