Tire Inflation Guidelines

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 Common Problems / Posted 2 years ago / 966 views

Proper Tire Inflation

updated 1/13/17

The amount of air (PSI) you should put in your RV’s tires depends on a number of factors; tire size and load range, fully loaded weight of your RV, temperature, tire location (axle). The following will explore these factors and help you arrive at the correct PSI for each tire. If you haven’t weighted your RV yet, consider having your tire inflation values at the max recommended PSI from the tire manufacturer. While this may yield a somewhat harsh ride, it should provide a safe operating margin until you obtain the actual weights.

 

Tire Size:

In the vast number of cases, the tire size and load range specified by the RV manufacturer is the correct size and you should not deviate from it when replacing tires. In a small number of cases, the RV manufacturer may have specified tires that are marginal in supporting the fully loaded weight of the RV. In these cases, you may want to consider increasing the tire size or load range. However, be sure to check the “wheel well” clearance versus the size of the desired tires. Most coaches will have enough clearance on the top of the tire to allow for bumps and coach bouncing due to road bumps. Pay special attention to the space needed to turn the wheels from stop to stop. A good method is to verify clearances of the proposed new tires with the RV at ride height and fully down. Also, be sure to verify that your rims will accept the proposed new tires and verify they will maintain the minimum spacing if going on the rear duals.

 

Tire Load Range:

As mentioned earlier, the RV manufacturer normally has selected the correct load range based on the RV’s maximum expected fully loaded weight. You should never select a tire with a lower load range rating for your RV, as an overloaded tire is subject to catastrophic failure (blow out).

 

Fully Loaded Weight:

Knowing the fully loaded weight of your RV is not sufficient. You need to know the weight at each of the 4 corners of the RV since this is what the tires in each position are going to have to support. The manufacturer may have placed a placard in your RV listing tire size, their estimation for max weights by axle, and suggested PSI values. This is a very rough guide and does not take into account how you load your RV when getting ready for a trip. For example, the placard ratings for the author’s RV are as follows:

  • GVWR:      44600 lbs
  • GAVWR Fnt: 14600 lbs  (115 PSI)
  • GAVWR Rear: 20000 lbs (85 PSI
  • GAVWR Tag: 10000 lbs (80 PSI)
  • GVWR: gross vehicle weight rating (GVWR), or gross vehicle mass (GVM) is the maximum operating weight/mass of a vehicle as specified by the manufacturer including the vehicle’s chassis, body, engine, engine fluids, fuel, accessories, driver, passengers and cargo.
  • GAVWR: gross axle vehicle weight rating (GAVWR), is the maximum operating weight/mass for each axle as specified by the manufacturer including the vehicle’s chassis, body, engine, engine fluids, fuel, accessories, driver, passengers and cargo.

 

So how do you arrive at the needed weight values? First locate a truck stop or repair facility that has a scale. Many truck stops have scales. CAT is among the most common scale. Some repair shops also have scales. Unless the facility has the capability of weighing each corner in tandem, you will need to take multiple readings to arrive at each corner’s weight.

 

Once you’ve identified a facility to weigh your RV, the next step is to load the RV as you would for a trip. This means full fuel and fresh water tanks. Don’t worry too much about the gray and black tanks as they normally carry water transferred from the fresh water tank. Load the kitchen and reefer with the types of groceries you would normally take. Same for the wardrobes. Fill them with the normal amount of clothes you normally take. Don’t forget the basement. Load this with the bottled water, soda, etc., that you normally store here when on a trip and be sure to put the items in the bay where they will be when you travel. As you can see, the easiest way to accomplish this is to weigh the coach at the beginning of a trip. Just be sure to top off the fuel tank before hitting the scales.

 

Weighing Your RV:

If your RV has a tag axle, leave it down for this process. Also, make sure the RV is relatively level when taking measurements. First weigh the front axle. Pull your RV on the scales so that the front tires are on but the rear tires are off the scale. Once you have the front axle weight, pull forward and get the rear axle weight. Be sure the tag axle is not on the scale. Then pull forward so that only the tag axle is on the scale and record that value. Then repeat this procedure with either the driver or passenger side tires on the scale. With these weights you can calculate the weight on each corner and on the tag ends. Alternately you can weigh each corner and tag ends separately.

  • Note: The Tag axle down pressure setting can be adjusted to vary the weight on the front axle. Increasing the tag down-pressure will shift more weight to the front axle. Reducing the down-pressure will reduce the weight on the front axle. The only time one should adjust this is to compensate for an overweight front axle. In all cases, be sure to record your tag axle pressure reading and leave it set at that value. Changing the down-pressure will require a re-weigh of the RV.

 

Determining Correct Tire PSI:

Each tire manufacturer has a table of inflation PSI vs weight for each RV and Truck tire size they offer. It will be located on their web site. For example purposes here we will use the author’s weights and tires. Keep in mind your values will differ.

  • Tires: Goodyear Tires: G670 295/80R 22.5  Load Range: “H”
  • Front Driver corner weight: 6800 lbs
  • Front Passenger corner weight: 6900 lbs
  • Rear Driver corner weight: 10280 lbs   (5140 lbs / tire)
  • Rear Passenger corner weight: 12500 lbs (6250 lbs / tire)
  • Tag Driver corner weight: 4620 lbs
  • Tag Passenger corner weight: 3000 lbs
    • Note that the rear axle exceeds the mfgr’s spec but this is OK since there is not 10000 lbs loaded on the tag axle. This also assumes the tires are capable of supporting the rear axle corner weights.

 

This link will display the inflation tables for various popular RV tire brands. The Goodyear tables are of interest for this discussion.

http://www.rvsafety.com/tire-inflation-tables

The inflation table indicates we must have 110 PSI in each front tire. This will support a max of 7070 lbs for a single tire, so it will be sufficient for the 6900 lbs in the right front of the RV. Note we always want to fill all tires on the same axle with the same PSI. The table indicates the rear tires also require 110 PSI each to handle the 6250 lbs / tire on the right rear of the RV. The table also indicates 80 PSI is required for the 4620 lbs on the driver side tag. However, the author puts 90 PSI in each tag axle tire as these tires are subject to severe lateral stresses on turns and low air pressure can cause sidewall failure due to this stress.

 

Filling The Tires:

Now that we know the tire manufacturer’s recommended PSI values we can adjust the tire pressure. First, ALWAYS fill tires when they are cold. PSI will differ with temperature, increasing as it gets warmer. The author normally leaves them sit for at least 4 hours and preferably overnight. Also consider the effect of the sun on a summer day. Preferably both sides of the RV will be in the shade. Tire manufacturers specify cold inflation values at an ambient temperature of 65°F. The following table will help you determine the PSI offset depending on the ambient temperature. (Use of a laser temperature probe is an easy way to determine ambient temperature at the tires).

Recommended Cold Inflation

Note that suggested PSI values are based on a 65 degree ambient. The table provides the offset for given temperatures. For our example, we’ll assume the ambient is 90 degrees. Thus to obtain 110, 95, and 90 PSI, we would fill the tires to 116, 100, and 95 PSI respectively. It is not recommended to drop below the manufacturer’s Cold Inflation PSI value at temperatures below 65 degrees as bead separation can occur at low PSI values.

A few final notes;

  • Use the tire inflation guide from your tire manufacturer.
  • Avoid adjusting tire PSI right after you’ve been driving. Tires will easily gain 20 PSI or more from the heat generated by rolling contact with the road surface.
  • Moisture in the tire can cause deterioration and failure of the steel bands that make up the sidewall. If you carry a tire inflation tool, consider adding a moisture filter to it.
  • Be sure to check the maximum PSI that your tire rims are rated for and don’t exceed this value at cold pressure. Normally this isn’t an issue but check to be sure.
  • Consider investing in a tire monitor system. It will show you tire PSI and temperature (including your tow car) while driving. They are especially handy for spotting slow leaks. The cost of a system can be considerably less than the cost of a blowout.

 

Hopefully this helps answer questions about proper inflation of your RV’s tires. If you have any questions, post them on the BAC Forum and we’ll get you the answer.

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