Tire Pressure - too much? too little? Photo attached.

Larry N. said:

I just read 571.120, and it has a lot of stuff relating to how warning label values should be calculated, what manufacturers must do, and so on, but nothing that contradicts the typical advice given on this forum about using the tire manufacturer's recommendations about what pressures to use. And I have to believe that the tire maker has a vested interest in maximizing safety with their tires, especially in today's litigious society.

Trucker regulations? The manufacturer has specific recommendations for each tire type AND size, and I doubt the tire knows whether it is on a truck, bus or RV, nor does it consult regulations before blowing or giving an uncontrollable ride (yes, I've had it happen).

You'll need to elaborate a lot to persuade me otherwise.

So you're saying that in order to be within those regs that I have to allow tires to wear way too much in the center or way too much on the edges, and allow them to throw me out of control on certain road surfaces, and to give me a very harsh ride, even though I'm within the structural and design limitations of the tire?

I can't accept your assertion.

Click to expand...

My posts are also laced with established tire industry standards. You will not find anything in tire industry standards that will recommend using less inflation pressures than what has been recommended by the vehicle manufacturer. Tire manufacturer's do not set recommended tire inflation pressures, vehicle manufacturer's do. Tire manufacturers build tires to a set of specifications needed and desired by vehicle manufacturers. They also provide standardized load inflation charts for those tires and they are approved by the tire and rim association (TRA) for use by vehicle manufacturers.

Within 571.120 the vehicle manufacturer is directed to set and display recommended tire inflation pressures for the needs of the vehicle and in compliance with standards set forth in the regulations. If there are deviations such as often used by pick-up owners for towing situations the vehicle owner?s manual will also have recommendations for those loads and inflations.

The tire industry doesn?t give a hoot about inflation pressures vehicle owners use above recommendations and below maximums. If fact, they will slyly hope you practice those procedures, because your actions in those areas will more often than not decrease your tires life expectancy.

IMO RV owners most serious concern should be in the weight and balance of their rig, Truck & trailer or Motor Home & toad. If your unit is not over weight and properly balanced your vehicle?s recommended tire inflation pressures are correct and need to be maintained for every movement and while in storage. 

FMCSA tire regulations have no provision for recommended tire inflation pressures. Their concern is to insure the tire is inflated to a value that will carry the load placed on it.

The bottom line for you seems to be the tire manufacturer?s tire knowledge. They know all about selection of materials and building tires to specifications. Vehicle manufacturer?s select tires from a list of tires specifically designed for the average performance of the vehicle they are to be fitted to. Tire manufacturers test tires on a machine at laboratory conditions. Vehicle manufacturers test tires on a vehicle under actual road conditions after the machine test. Tire manufacturers provide the operating parameters for their tires. The vehicle manufacturer is solely responsible for tire selection and fitment to the vehicle. The vehicle owner is responsible to himself/herself for vehicle safety. The TRA provide lists of acceptable replacement tires to the automotive industry. Vehicle manufacturers approve replacements for RV trailers. When an owner steps outside of those ropes they are on their own - so to speak.

Probably the most visible collaboration between tire builder and vehicle builder can be seen each week on TV when NASCAR is testing, practicing, qualifying and racing their cars. 


I live about 14 miles from the BMW tire testing facility in Greer, SC. Here is a picture of the layout.

http://www.irv2.com/photopost/showfull.php?photo=30525

ralph tait said:

i follow tire side wall inflation.  if it shows max at 125 psi i would set them at 110 to 115 to allow for heat expansion. tire pressure is more about proper tread wear and cooler running not for ride comfort. plus properly inflated will increase fuel economy

Click to expand...

Ralph you are reading impaired

It does nto say "max 125 PSI"

it says Maximum load  xxxx pounds at maximum pressure of 125 PSI

That means if you are running at maximum load, 125 is the proper inflation

But what if you are running 75% of maximum  Then 125 is OVER INFLATED and you will wear the center of the tread before the edges greatly lowering tire life.. You will have a rougher ride, and less control under less than optiomal (or perhaps even optiomal) conditions.... In short.. a bad idea.

The proper pressure the tread lays flat on the road.> The load is evenly distriputed across the tire,, Control is thus maximized as the full tire width is in contact with the road (most surface)

Too low a pressure and only the edges of the tire are gripping the road, plus side wall flex is increased and control is thus lost more easily once again.

Scale it and use the manufacturer's chart.. The only way

(used to have a car with a slow leak and a curve that was a killer (Seriously it killed people)  When the tire was about 10 PSI low.. I felt it on that curve).

My posts are also laced with established tire industry standards. You will not find anything in tire industry standards that will recommend using less inflation pressures than what has been recommended by the vehicle manufacturer. Tire manufacturer's do not set recommended tire inflation pressures, vehicle manufacturer's do. Tire manufacturers build tires to a set of specifications needed and desired by vehicle manufacturers. They also provide standardized load inflation charts for those tires and they are approved by the tire and rim association (TRA) for use by vehicle manufacturers.

Click to expand...

Perhaps you misunderstood some of that.

The bottom line for you seems to be the tire manufacturer?s tire knowledge.

Click to expand...

The bottom line for me is safety! I mentioned losing control due to pressure too high. Fortunately those times were only momentary. And that was on an Interstate highway more than 10 mph below the speed limit. Squirrelly steering and sidewind sensitivity are other safety issues (nothing to do with regulations) that are affected by tire pressure being too high.

If you are strictly by the book, as you read it, then I hope you're not near me when you lose control.

That's my last word for this thread.

Gary RV Roamer said:

If by "even tire pressure" you mean the same psi on every tire, that is definitely NOT a requirement or even desirable. All the tires on the same axle must be inflated the same psi. but different axles commonly have different psi requirements. Often the rear axle will use less pressure than the front, and a tag axle yet again different.  The tire placard provided by the coach manufacturer will typically show different psi for front and rear axles.

But you are right that great precision is not required. It just has to be "enough". Greatly exceeding the "enough" can lead to harsh riding, poor handling, and uneven tire wear, but the psi has to be way high to do that. 10 psi over is not excessive at all.  However, I have seen coaches where the difference between the required minimum for the load and the recommended psi was substantial, e.g. 30-40 psi. RV dealers and tire shops tend to recommend high pressures, sometimes extreme, to avoid ever being accused of under-inflation. That came about after the under-inflated tire fiasco with the Ford Explorer many years ago.

Click to expand...

Thank you for clarifying my comment...I should have been more precise regarding "even"

I would think that on an '05 RV, the tires have already been replaced once because of age and possible dry rot.  If not, I would be more concerned about that than fine turning the air pressure.

If they have been replaced, were they replaced with the same size and ply?  If so, most seem to agree going with the OEM sticker.  If not sure or the tire load rating different, I would take the weight listing from the MFG and compare that to the tire mfg chart and adjust appropriately.  That should get you really close unless you want to go to the trouble of weighing each axle which I would not do unless you have handling issues.

Running your tires at a higher pressure than what is needed to carry the load of your rig, will unnecessarily increase the harshness of the ride and contribute to wagging when semis are passing you.  Been there and done that.

Not trying to hyjack the thread, but I have a similar question.  I certainly know the basics--inflate to the vehicle manufacturer's specs, not the tire max pressure on the sidewall.  However, my Adventurer sticker is 70# all around.  On 6 tires.  That sounds so low that I got recommendations from my tire place "experts" to increase it a bit.  I had them bump it to 90# across the board, but now am second guessing myself.  A little overinflated to cover "stuff" we tend to store in the coach sounds OK, maybe get a little better gas mileage.  Certainly not to the max on the sidewall (which I believe is 110).  (More) opinions?  It should not be necessary to weigh the coach to get decent inflation numbers; I have never weighed my truck.  We do have a toad, but now flat tow, so no additional weight on the rear of the coach.

Sailorkane said:

It should not be necessary to weigh the coach to get decent inflation numbers

Click to expand...

That is a curious attitude. Weighing is the ONLY way to get accurate numbers for proper pressure in your tires. Randomly adding air is exactly that, guessing.  And a four corner weight is recommended. Since most RV's tend to be way off balance on weight side to side, a four corner weight is the only way to properly set air pressure.

SargeW said:

Weighing is the ONLY way to get accurate numbers for proper pressure in your tires.

Click to expand...

Yup!  It's not even difficult, not sure why people find it so challenging.

It should not be necessary to weigh the coach to get decent inflation numbers; I have never weighed my truck.

Click to expand...

You don't "have to weigh". Like on your truck, you can use a generic recommended pressure from the vehicle manufacturer that will be adequate for general use. It will almost surely be higher than necessary, but it will be safe enough. There should be a federal tire placard on the wall next to the driver seat, with max axle loads (GAWR) and factory tire size & pressure.  Weighing lets you optimize for max tire life and ride quality, though, and also shows if an axle is overloaded. RV Safety Foundation statistics show that overloading is VERY common, over 50% of RVs has one or more axles over its max weight spec!

If there is no placard tire pressure recommendation, use the axle max weight (GAWR) as the estimated weight, divide by the number of tires on the axle, and get the psi values from the tire maker's inflation tables. Add 10 psi to allow for an unbalanced load (one side heavier than another) and temperature variations during the day. That should get you close enough for safety as long as the RV isn't actually overloaded.

No one replied to butch50s comment. Put a chalk mark across the tile and drive for a few tire rotations to check tire wear. It seems like a reasonable way to see if tire pressure is close to good. Any comments?

I have been using the weigh and do the math system for many years,,, I am also a tight wad with money,,,I have recently replaced all my tires (with in two years), the fronts first then the rears..These puppies cost a fortune!! and I will not throw my money away by guessing about the pressures (at $709.00 per tire!)  Across the four years now and 22 thousand miles they are wearing perfectly,, no cupping on the steers nor uneven treads anywhere ,,good enough for me

The chalk method would probably be ok on a car or truck tire, but on a RV a little weight difference from side to side can  make a big difference. That is why tire manufactures advise weighing the vehicle, then using the side with the highest weight, set the pressure from the tire  chart and carry it across to the other side of the axle. Chalk marks are going to be really spit balling. 

RVs by their very nature are notoriously over loaded or have unbalanced loads or both conditions. For most users with such conditions their first clue is badly worn or failed tires. Then they seek solutions from others with not much more knowledge or experience than themselves.

So let?s go searching for more knowledge and experience with the subject at hand.

The reference provided below is going to tell us to get our RVs down to GVWR or less. It?s also going to tell us to balance our loads. It?s all in chapter four.

However, the entire PDF is worth reading. It?s loaded with all the good stuff.

http://www.mcgeecompany.com/wp-content/uploads/2010/12/complete-manual.pdf 

There was no photo.. The best way to tell if your tires are over/under/properly inflated is a scale.

Specifically you need to weigh tall 4 courners. There are two ways to to this

First. and lowest cost is to find a FLAT segmented scale, not a Bridge scale (most CAT scales are Bridge scales)

Prepare the RV as though starting out on a trip. full fresh water, empty waste tanks, Fully loaded

Park on the scale so each axle is on a segment and get a weight.

Now drive around so ONLY ONE SIDE of the RV is on the scale and get a weight
The first reading gives you axle weights
The second the weight on that side

Axle = Side = Other side

Now you can go to the tire company and download the inflation chart for your tires.

Option 2. a bit more expensive, but easier.

RVSAFETY.COM  had a link on their page for Aweigh We Go as I recall (or weigh yhour rig)

They bring portable scales out and do the job for you.  They do a pre-appointment interview so they come with the proper tire inflation chart and they test your tire pressure gauge as well.. This is the DELUX method.

Option 3: Buy a couple beers for the Motor Carrier Enforcement Officer.... Same scales RV-Safety uses.

FastEagle said:

The reference provided below is going to tell us to get our RVs down to GVWR or less. It?s also going to tell us to balance our loads. It?s all in chapter four.

However, the entire PDF is worth reading. It?s loaded with all the good stuff.

http://www.mcgeecompany.com/wp-content/uploads/2010/12/complete-manual.pdf

Click to expand...

Excellent resource FastEagle - thanks for sharing!  I wish all RV's would read it.

Follow the sticker. Large trucks (which is basically what a motorhome is) are constantly changing weight due to loading and unloading cargo. They certainly do not needed to have their tire pressure adjusted everytime they are loaded/unloaded.