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How to Weigh Your RV
In the previous document we discussed what the various weight ratings are. Now all we have to do is figure out how to weigh our RV. I guess it might be a bit heavy to drive onto the bathroom scale so we'll have to find something bigger. Truckers weigh their vehicles regularly so see that they are legal. So, the best bet is to find a truck scale someplace. Typically if you go into a Flying J or other truck stop you will be able to find a drive-on scale. Most of these are CAT scales. No they're not related to Caterpillar engines - they'll work just fine on a Cummins powered coach too. A CAT scale is a Certified Automotive Truck scale. Truckers rely on these scales because their license are tied to how much weight they can legally carry. If they need to haul more, they need to pay more for a license plate with a higher weight rating. If they exceed their license rating there are some very hefty fines levied. CAT scales will certify that the weight is accurate and if there is a problem, they'll pay the fine so you'll find these all over at various truck stops. For a listing of scale locations throughout the US select the CAT Scale Locations link to find out where they are. In addition to truck stop scales you may find a local gravel and concrete products company or trucking terminal that has a scale. Many times they'll let you use their scale if you ask nice. Other places to get them weighed are at the large FMCA conventions where a third party, such as RV Safety Education Foundation, will come in and weigh your coach as well as put on helpful seminars on RV weights. Some states will leave their scales on when they are closed. Washington and Oregon welcome RVers to come in and use the scales when they are closed.
The minimum is to be sure that you get each axle weighed separately. Knowing the overall weight of the RV isn't as important as the per axle weight. Each axle is designed to carry a certain amount of weight. Part of this capacity is determined by the tires and the pressure within each tire. As the weight goes up, so must the air pressure. If you have excessive weight on the rear axle and it's light on the front you will undoubtedly have handling issues with a front end that doesn't like to stay put. You may also have too much weight on the rear axle for those tires. In that case you'll have to slide some of the weight (cargo) forward. It's time to put the toolbox in the front of the RV and the badminton birdies in the back. Once you have the axle weights within the GAWR for each axle you then take these weights and go to your tire pressure chart for the particular tire that you are using. Take this weight and compare it to the chart to see how much minimum air pressure you need to have in those tires. Note that this is the minimum air pressure designed to carry that amount of load. You need to keep the air pressure somewhere above the minimum (chart) pressure and the maximum air pressure stamped on the tire's sidewall. If you can't do that, you either need to shed some pounds or find tires with a higher rating. There's a great document in this library explaining tire pressures better at This Link in the Tires and wheels category. You may or may not be using the same tire pressure on the front axle as the rear. If you want to run equal tire pressure you can, as long as you raise the lower pressure to equal the higher pressure. The important thing is to be sure that you stay above the minimum and below the maximum for each axle. If you do run higher pressures than required you will have a bit of extra load carrying capacity, which isn't bad in case you add a little more "stuff" some day after you've had your RV weighed. The only drawback is that it "might" ride a bit rougher with higher air pressures because the sidewalls don't give as much.
Now that we've discussed the importance of weighing front to rear let's also consider that many RVs are not equally loaded from left to right. This is going to depend on a ton of variables such as where the water heater tank is located, where you load your cargo, which side the refrigerator is on, and a host of other floor plan considerations. Ideally you would run your RV through a 4 point weighing operation. In this version you would weigh each wheel position individually. You would then know how your load is balanced left to right. If you can't shift any load from side to side then you will have to air each tire on the axle to equal pressure. But, instead of taking the average weight (axle weight divided by 2) for each tire you would now air each tire up to the minimum required air pressure of the tire with the most load on it. Never run different air pressures on the same axle so air up the lighter tire to the same amount. It'll have more air than it needs but it will be balanced and you won't have any pulling problems.
Not every place has 4 point scales. They are generally available at the large rallies but not at truck stops. There is a work-around though. If you can drive your RV onto the scale so that one side is on the scale and the other is off, you can get the readings for that side. Then drive around the second time and weigh the other side. Add them together to get the totals. Unfortunately this generally doesn't work at most truck stops because they have guide poles along the side so you can't get out of line. You probably can do that at a state scale that is closed though as well as many of the private business scales in your area. Ideally, a 4 point weigh is the best way to go. If you absolutely can't find a 4 point weigh location be sure to add a bit extra air pressure to the tires to allow for any left-right weight disparities. It's better to run a bit extra air than to be short and have the tire overheat and fail.
For an example of weighing your RV, let's look at the following scenario. Remember that these are example weights only so you will have to substitute your numbers when checking your RV.
We'll assume that the RV being weighed has a GVW of 29,000 lbs, a front axle weight rating of 12,000 and a rear axle rating of 20,000. Running across a set of scales we get the following 4 point readings.
If we add the 2 front wheel weights together we have a total axle weight of 10,820 lbs (5,200 + 5.620 = 10,820). This is well under the front axle weight rating of 12,000 lbs, so we're fine here. When we add the two rear wheel weights together we have a total rear axle weight of 17,250 lbs (8.870 + 8,380 = 17,250). That's 2.750 lbs under the rear axle rating of 20,000 lbs so we're fine here too.
Adding the front and rear axle weights together (10,820 + 17,250) gives us 28,070. That's 930 lbs under the 29,000 lb GVWR so we're good here too. Note that the weight ratings of both the axles add up to more than the GVW rating. This is typical and gives you the ability to not have to be quite so picky about evenly distributing your weight from front to rear. In any case, DO NOT exceed the GVWR of the coach, even if each axle's weight is within the GAWR for that axle. The axles may be able to hold up under that load but the brakes, suspension, and other components are not designed to exceed the GVWR.
So far we see that we are within specs but now we need to check the tires to ensure that we have enough air in them. We'll assume that the following chart represents our particular tire inflation pressures.
When we check the above load inflation table we find that the left-front corner weighs 5,200 lbs. According to the chart, we need a minimum air pressure of between 100 and 105 PSI to safely carry that much load. However, the right-front corner weighs 5,620 lbs, which requires close to 110 PSI of air. Always match the air pressure in any tires that are on the same axle and always match them to the higher weight rating so we'll run both of the front tires at 110 PSI.
On the rear axle we have duals so well be using the lower row of numbers from the above chart. At 8,380 lbs our right-rear corner requires 95 PSI. The left-rear weighs 8,870 so we'll need 100 PSI. Therefore we'll be running 100 PSI in both sets of duals in order to keep them equal. While you should never mix pressures across an axle, it is acceptable to have different tire pressures on one axle than another, so we don't need to run the rear axle at the full 110 PSI that the front axle is using.
For a sample chart that you can print and fill in your own weights, please select the following link. Be sure to refer to the proper load inflation chart for your specific tires when determining what your minimum tire pressures need to be.
Submitted by Mark Quasius - 3/29/06
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