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Tire pressure


aunut

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I have been inflating my tires to the psi of 110 cold as shown on the sidewall. However, I just looked at the placard by the driver seat and it says 90 psi cold. These are the original factory tires. Why the difference? Am I over inflating?

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The PSI on the tires is the MAXIMUM to which they should be inflated, not necessarily the ideal PSI. Tires should be inflated based on the weight they're carrying.

 

The first step, then, would be to get your rig weighed...by each tire position ideally, but at least get each axle weighed. Then look up the tire manufacturer's tire inflation chart to see where the PSI should be set.

 

I suspect you will find that the PSIs shown on the placard are enough to handle the axles' GAWR.

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Follow Linda lead. My tires and rims (G114) on the trailer are rated at 4800 @ 125 (or so). 6500# on each axle (- some), so the 'ideal' pressure / weight according to Goodyear - 85?? - so I run about 95 in them. With independent suspension, I figure that 1 or 2 is always going to be in a divot someplace. (Oh, and it's a tri-axle)

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The PSI on the tires is the MAXIMUM to which they should be inflated, not necessarily the ideal PSI. Tires should be inflated based on the weight they're carrying.

 

The first step, then, would be to get your rig weighed...by each tire position ideally, but at least get each axle weighed. Then look up the tire manufacturer's tire inflation chart to see where the PSI should be set.

 

I suspect you will find that the PSIs shown on the placard are enough to handle the axles' GAWR.

So, at 110 I should be safe at all loads. I don't carry much cargo weight and am usually way under max. I would think that I would get better gas mileage at the higher psi, maybe just a rougher ride?

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So, at 110 I should be safe at all loads. I don't carry much cargo weight and am usually way under max. I would think that I would get better gas mileage at the higher psi, maybe just a rougher ride?

 

I was adamantly told otherwise by a tire dealer in Wyoming who had installed my new Michelins. He said that over inflating them relative to the load is bad for the tires as well as producing an unnecessarily harsh ride. With car tires I knew that over inflation results in the tread being "bowed" instead of being flat on the ground and he said that the same is true for truck tires despite their size and weight. There is no question but that our ride is quite a bit more comfortable with the tires set at pressures that are appropriate for the load.

 

However, don't go by the placard in your MH since that isn't based on your actual weight. At least have your axles weighed at a CAT Scale so you'll know your actual load. Better still have each wheel weighed.

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I was adamantly told otherwise by a tire dealer in Wyoming who had installed my new Michelins. He said that over inflating them relative to the load is bad for the tires as well as producing an unnecessarily harsh ride. With car tires I knew that over inflation results in the tread being "bowed" instead of being flat on the ground and he said that the same is true for truck tires despite their size and weight. There is no question but that our ride is quite a bit more comfortable with the tires set at pressures that are appropriate for the load.

 

However, don't go by the placard in your MH since that isn't based on your actual weight. At least have your axles weighed at a CAT Scale so you'll know your actual load. Better still have each wheel weighed.

Thanks. Will do.

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The answer given by Linda is the best answer for the tires described in the original message BUT the answer may be different for different tires in RV use.

RVs vary widely in design and in how they are used and the tires used on the RVs vary widely as well. Motor home tires differ from smaller to larger. Trailer tires differ widely from commercial quality tires to Light Truck Tires to Special Trailer tires and all in varying sizes. Tow vehicle tires vary from passenger car tires on SUV and 1/2 ton pickups to commercial Truck and Bus tires. There are some general rules but because of the wide variety of tires used and the way they are used we must understand that there are different mitigating factors to consider in each situation. Tire inflation issues for a travel trailer with 14" ST tires are not the same as a Motor home with 22.5" tires. Motor home owners may need to understand more issues if they pull a cargo trailer with ST tires.

 

RVs by design have a much greater likelihood of uneven loads across the axle, therefore, wheel position weighing is the gold standard for determining how these loads are distributed on the chassis of the vehicle. This uneven loading can result in under inflation if the axle weight is assumed to be equal (axle weigh/2). If only axle weights can be done then a significant reserve capacity or safety margin should be considered in tire pressures to cover the possibility of uneven weight distribution, which is very common in RVs. Then when the opportunity is available get wheel position weights to learn the intricacies of the specific RV and how it is loaded. It could be that this RV happens to be well balanced side to side. RVs are designed to be comfortable living quarters not simply carry heavy loads from point A to point B like a box truck or semitrailer. RVs do carry heavy loads but these loads are distributed in such a way to make for a comfortable living space and therefore often subject to load balance issues. The other factor is that different people load stuff differently, which can help balance the loads or unintentionally cause greater balance differences. Wheel position weighing gives each RV owner important detailed information about how each RV carries the loads down the road, allowing the owner to make the best possible adjustments to safely enjoy RVing. (one such adjustment is proper tire pressure to carry the load)

 

Too high a tire pressure will be detrimental to tire life. A significantly higher pressure than is needed for the load increases the likelihood for impact damage. Too high also decreases the contact patch of the tire to the ground, which reduces braking capability and maneuverability. It is safer to be higher than lower but neither is good for the life of the tire or the vehicle. Under inflation will cause damage to the tire much more quickly than over inflation.

 

Understand that the number on the sidewall of the tire is NOT telling you what you should run the tire. This number is to identify the load carrying capacity of the tire and the pressure needed to carry that load.

 

The recommended tire pressure placard is what Linda said. It is a best guess and is required to provide enough carrying capacity to cover the gross axle weight rating (GAWR). This assumes that the weight is evenly distributed across the axle and as stated above, this is often not the case with RVs.

 

We could confirm this if you look up the GAWRs (front and rear) on the federal certification sticker (somewhere around the drivers compartment) and give the tire size,make/model, and max load single and dual, so that we could look up the inflation table for those tires. Also check to make sure the load carrying capacity on the front tires is the same for the rear tires. It is common that the same tire (make/model/size designtation) will be used everywhere but the front tires will have a higher load carrying capacity than the rear tires. This can be identified by comparing the max load single/dual and or by comparing the Load Range ID for front a rear tires. (a letter designation).

 

Also, is the tire pressure recommendation the same for front and rear axle? On this vehicle, the GAWR should be different for each axle and that will be shown on the federal certification sticker. It is possible and likely that the tire pressure recommendation is also different.

 

110 psi is 20 psi over the recommended pressure which is high for this kind of tire. However, since we do not know the actual measured loads that is only an assumption that the loads are under the GAWR and evenly balanced.

 

In this case, if the recommended pressure is the same for front and rear then running 5-10 psi higher than recommended (without exceeding max pressure for the tire) would help to cover possible unbalanced issues until actual wheel position weights can be determined. It is quite possible that once actual weights are taken that even 90 psi is significantly more than needed.

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RVs by design have a much greater likelihood of uneven loads across the axle, therefore, wheel position weighing is the gold standard for determining how these loads are distributed on the chassis of the vehicle. This uneven loading can result in under inflation if the axle weight is assumed to be equal (axle weigh/2).

 

 

Absolutely!

 

As an (admittedly extreme) example: Say you have a 6,000# axle. You weigh the loaded axle and come up with 5,000#. Great, you think...I'm 1,000# under the axle's weight rating! But then you weigh each tire position and find one tire is carrying 3,000# and the other is carrying 2,000#. While you'll probably never get each side of an axle to weigh exactly the same, 1,000# difference between one side and another is way too much difference and, depending on the tire, the 3,000# one may be overloaded.

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I put a question much like this a long time ago, and got much of the same answers are appear here now, and I STILL do not understand ---- my guess is that many of you, too ----- do not really know the answers to these questions. Go by what the truck manuel tells you to do, or go by the max as stated on the side of the tires. I give up. Perhaps I should just "split the difference" and inflate somewhere in the middle of the two ranges.

C. SD,

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Tire inflation pressures for Motor Homes (MH) differ somewhat from tire inflation pressures for RV trailers. In this installment I’m only going to address MH tire inflation pressures.

 

Because there is more than one FMVSS regulation at work in this post I’m not going to keep referring to them. If you want verification on a point in question just ask and I’ll post the part of the regulation (s) in question.

 

It is undisputable. Vehicle manufacturers have the sole responsibility for selecting and fitting the Original Equipment (OE) tires and rims to your MH. The regulations they must follow to successfully completion tells them they must provide some load capacity reserves with their tire selections. Because they also determine the vehicle’s GVWR and GAWR they will normally exceed the GVWR with the total GAWR. Tire selection is determined by each individual GAWR. The wording for OE tire fitment goes something like this; “Tire size designation and the recommended cold inflation pressure for those tires such that the sum of the load ratings of the tires on each axle is appropriate for the GAWR.” So, the recommended cold tire inflation pressures depicted on a MH certification label, tire placard and in it’s owner’s manual is not an arbitrary figure, it’s the correct cold tire inflation pressure. It is not, under normal circumstances, going to be disputed by the tire industry. In other words; For OE tires, NEVER use less tire inflation pressures than those found on the labels.

 

The confusion factor; Cargo! In a perfect world the MH will be balanced at all times and the recommended tire inflation pressures would always be correct. But, they are big, cumbersome vehicles with numerous compartments for cargo, cargo that will unbalance the loads across (maybe) all of the axles. The best way to counter the unbalanced conditions is simple. Weigh it and rebalance it. Wont work? In pops the tire industry load inflation charts and the misuse starts. Rule 1, don’t use less load inflation pressure than what has been recommended. Rule 2 don’t exceed the tire’s maximum load inflation value. Rule 3 don’t exceed an axle’s GAWR value. Best rule, balance your cargo loads.

 

Replacement tires will often require new recommended cold inflation pressures. A good tire retailer will insure the new tires have the capacity to equal or exceed the load capacity of the OE tires. They will do that by manipulation of the tire load inflation pressures via a load inflation chart to a value that will accomplish the task. Very good ones will insure the vehicle owner knows the new recommendations and an auxiliary tire placard will be placed adjacent to the original tire placard. The auxiliary placard is authorized by the DOT.

 

Best reference for verification listed below.

 

http://www.trucktires.com/bridgestone/us_eng/press/zip/WeighForm.pdf

 

 

FastEagle

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I put a question much like this a long time ago, and got much of the same answers are appear here now, and I STILL do not understand ---- my guess is that many of you, too ----- do not really know the answers to these questions.

To got into great detail you need some understanding of physics and tire design, but the basics are really fairly simple.

 

Consider a tire that has gone flat and the shape that it takes. As you inflate that tire the amount of tire touching the road decreases. The reason is that more air pressure carries the weight of the axle with less total area of tire surface. Since each tire is designed to operate with a particular amount of tire surface touching, more air pressure means less surface touching so at some point the tire will have lifted too far and have too little surface touching. You can also easily see that the tire becomes harder and harder as you add air pressure to the inside. Since tires are designed to flex as they roll down the road, too high an air pressure means that they don't flex enough and too low means they flex too much.

 

The weight of the RV riding on the axles is what determines how hard that tire must be in order to flex the proper amount when in use. If the tire is too hard or over inflated, the tire becomes like a rock and will not flex to absorb bumps as it should and too little tire surface will contact the road so vehicle handling is poor. If the tire has too little air pressure it becomes too soft and winds can move the vehicle side to side as you travel as well as the tread scrubbing around on the road making the vehicle difficult to control.

 

You really do not need to understand the principles of tire design but just realize that the design of each tire is intended to be soft enough to absorb some impacts, but hard enough to not flex too much. Achieving that means that you must adjust the inflation pressure inside of the tire based upon the weight the tire is carrying. Since the engineers design tires to operate at specific pressures for different amounts of weight carried, base your inflation pressure on the charts that they supply. Since the RV manufacturer didn't know what tires you would replace the ones it came with and he can only guess how much weight you will add to the RV, he can't know accurately what pressure is best for the tires that you have installed at this time.

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Our RV weights change frequently. If we are going to the back country to boondock for a couple of weeks we load up on food, water and propane. I vary the tire pressure some but I don't weigh each time so I try to always run just a little extra pressure. I prefer the tires to be slightly over inflated than under inflated. The weight on our HDT can vary by thousands of pounds, depending on if we carry the car and RZR or not. We also have a couple of 100 gallon water tanks, extra fuel and storage. I would guess we run the tires over inflated now and again.

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Those of you pulling trailers actually need to adjust the rear tire pressure on your trucks when pulling the trailer and not pulling. I did this on my dually fairly faithfully. The front tires I would leave alone but the rear tires running around without the fiver I would run at 35 psi and when pulling the fiver with a 2500 lb pin weight I would raise the tire pressure in the duals to 60 psi. This was based on the weight on each tire according to Michelin's pressure chart. If I left the rears tires at 60 psi with no load the truck rode rough and traction was diminished.

 

I do this on any truck where the load changes.

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I didn't want to adjust the rear pressures on my F250 because it was a pain to get them back to the maximum rated pressure, what they needed to carry my load. I discovered just how small the tire contact patch is when your tires are way over-inflated for your load, thankfully with no damage aside from my nerves. If you don't adjust pressures as needed you will be way low on rear traction empty or badly under-inflated loaded.

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Some comments from an actual tire engineer.

 

FastEagle is correct that the vehicle MFG has the responsibility to provide the owner the information for tire inflation, tire size & load range and for the max axle and vehicle load ratings. The issue is that the tire inflation number must be high enough to support the maximum load on the axle i.e. GAWR.

 

In reality we would hope that you are not running at the max capability of the tire or axle. It is always a good idea to have a margin.

Unlike aircraft in the Navy we don't have a trained tire specialist checking our tires before every trip so we need to take appropriate steps to take variations in operating conditions, potholes and punctures into considerations. We also do not have systems to measure weight and balance as aircraft do so we need to build in some additional margin.

 

Get the RV weighed using the "gold standard" of learning individual corner or tire loading. It is not unusual for side to side variation on an axle to be 1,000# so simply assuming a 50/50 split on a CAT scale axle weight can be very misleading.

 

Knowing the actual load on the heaviest end of each axle after making an effort to get closer to 50/50 we can then consult Tire Company Load Inflation tables. HERE is a blog post with many/most of the appropriate links for various brands of tires along with info on RV tires in general.

 

Having consulted the table and using the heavier end of an axle we know the MINUMUM inflation needed for all the tires on that axle based on our personal unit. Remember the RV assembler was guessing how many bowling balls you cart around in your collection all the time. :lol:

 

To avoid chasing a change in inflation of a couple psi due to time or temperature change, and to provide a margine I recommend adding 10% to that minimum inflation to learn your Cold Inflation Pressure or CIP should be.

 

Set your TPMS warning pressure to the Minimum from the tables as you want to minimize the time you ever run underinflated. Hopefully the brand TPMS you have or purchase has an early warning system that will warn you of a leak from the HOT running pressure and not have you wait till your tire has lost 20 psi or more and is running underinflated.

 

NOTE the above is for Motorhomes. Trailers are a little different due to the axle palcement. Trailers still need to balance load and to not exceed axle or tire capacity but trailers should use a CIP that equals the inflation on the tire sidewall. This is to lower the Interply Shear forces that try to tear the tire belts off the carcass. You can Google "Interply shear" if you want to learn more on that topic, but I can tell you you want this as low as possible.

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