When buying an RV, owners are exposed to a number of new concerns they probably never faced with their regular passenger car. These include the actual weight of the vehicle, the weight distribution, and its effect on tire durability.
Passenger cars are reasonably balanced side to side. If there is a moderate front/rear imbalance it is taken into consideration when the car manufacturer sets the recommended inflation for the tires, as shown on the tire placard. Also, car manufacturers will normally design with a margin of at least 10% to 20% or greater on tire load capacity.
RVs are different. They have heavy components such as slides, battery banks, generators and holding tanks that can quickly add lots of weight off-center.
RV weight is seldom what you think and can result in significant side-to-side unbalance. Even multi-axle trailers can be unbalanced axle to axle, and that can affect tire life. RV manufacturers sometimes have tire load margins as low as 5% or less for an RV when normally loaded. So it is easy to end up with a significantly unbalanced load and even overload on the tires.
More than half of RVs have an overload condition
Real-life measurements by engineers show that more than half of the RVs on the road today have one or more tires and/or axles in an overload condition. This is an undesirable and unsafe condition as it can cause uneven tire wear and poor fuel economy. Even worse, it can lead to tire failure if the unbalance is significant.
The best way to learn the actual loads on each tire position is with individual position scales.
These are not easy to find but you can check the Recreation Vehicle Safety Education Foundation (RVSEF)Â Weight & Tire Safety Program, that is a service offered at many RV rallies and events. Check their website to see if you are traveling to a location where they offer the service. Escapees also offers a similar “4-corner-weight” service at some locations.
Can use regular truck scales
If you can’t find a location to do individual tire position weights, you can use some regular truck scales such as CAT Scales, or Certified Public Scales Locator from Penske. Some farm co-ops or feed mills and some sand and gravel yards can also provide the needed service. You will need a scale with enough side clearance to weigh one side at a time (see the worksheets). So check before you travel a long distance for a weighing. If single-position scales are not available, you will need to do some simple math to calculate your actual weights. You can find worksheets from Bridgestone/Firestone at Fifthwheelst.com.
SPECIAL WARNING: CAT Scales and probably some other certified truck scales do not want you to do one-side weighing as they feel it can affect their scale accuracy. Many CAT Scales have been fitted with guardrails to prevent vehicles from being off-center.
If you can’t get the weight for each end of each axle you will need to apply a “fudge” factor to the axle readings to get reasonably close to the real weight numbers. So, until you can get real “4-corner weights,” I suggest you assume your heavy end of each axle has 51% of the axle reading. NOTE: Class A RVs should use 52% to 53% if they have slides or residential refrigerators or granite countertops.
The worksheet above can still be used, but you will need to do the simple calculation as we need to select the tire inflation required to support the heavier end of each axle.
When you need to weigh your RV
You will not need to get corner weights or do the calculations every trip or even every year, unless you make a significant modification to your RV. Just be sure you have loaded your RV with everything you would normally take on a long trip and be sure to include water and fuel when you are getting your RV weighed.
Once you know the real tire loads, you should consult the tire Load/Inflation tables to learn the MINIMUM inflation for all the tires on that axle. While most tables have similar or identical numbers, there are a small number of tires that are different. So it is important to use the tables published by your tire manufacturer if you can find that information. With all this information and the math done, you select the minimum cold inflation based on the heaviest loaded tire on each axle.
Motorhomes should generally set the cold inflation at least 5 psi or 10% higher than the minimum without exceeding the max rating for your wheels.
Multi-axle trailers should set the cold inflation to the pressure molded on the tire sidewall, but need to adjust the RV load so they have at least a 10% margin if they cannot increase the inflation. This is because RV trailers and 5th wheels place significantly higher structural loads on their tires.
Roger Marble
Check out my Blog www.RVTireSafety.Net
Have a tire question? Ask Roger on his RV Tires Forum here. It’s hosted by RVtravel.com and moderated by Roger. He’ll be happy to help you.
Read more from Roger Marble on RVtravel.com.
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