Page 5 - Travelor - Summer 2016 - Issue 34

DRVC Travelore - Page 5
scales at each of the
four corners. If you
cannot find a properly
equipped RV repair
facility to do a four-
corner weight measure-
ment, a measurement
of at least the front and rear axle is better than noth- ing. You can find certified public or commercial scales at moving or storage lots, farm suppliers with grain eleva- tors, gravel pits, recycling companies, and large, commer- cial truck stops for this type of weighing. Allow adequate time, since the entire weighing process can take up to 30 minutes.
Next, take those weights and consult your tire manufac- turer’s load/inflation tables to find the air pressure needed to safely carry the load your coach is carrying. The tables will give you a range of weights for each 5 psi increment
of air pressure. These tables will give you a minimum air pressure for that tire regardless if you are under the lowest weight for that tire. All tire air pressure ratings are to be set at 65 degrees F temperature so only adjust the air pres- sure in the morning before you start driving.
All tires on the same axle must carry the same air pressure but the front or rear axles may carry different air pres- sures based on the recorded weights. For instance, both front tires must be the same and all four rear tires must be at the same pressure. Tag axle tires should also carry the same pressure.
I will give a good example of why knowing your 4-corner weights are so important. Using my own 40-foot Damon on a Freightliner chassis, I know that the weights are as follows:
Left front Left rear
5,250 lbs 9,200 lbs
Right front 5,600 lbs Right rear 10,150 lbs
If you use the chart from Michelin (see image inset)
that represents my Michelin XZE tires, I know that the 275/80R22.5 load range G front tires can handle my weights at 100 PSI plus 5 PSI for what-if’s (what if I stop and add something else of significant weight) to be able to properly carry the 5600 lbs that are on the heaviest front tire (right front). If I just weighed the front axle, I would come up with 10,850 lbs divided by 2 = 5,425 or 95 PSI plus 5 PSI for what-ifs and I would not have enough for the right front corner.
The rear weights have a marked difference. The right rear weighs 10,150 so I would need 100 PSI + 5 PSI for what-ifs for a total of 105 PSI. On a single axle scale, the readings would be 9200 + 10150 = 19350 divided by 2 = 9675. By using the chart below I would use only 90-PSI +5 PSI for what-ifs and I would not have enough air in each right side tires to properly carry my weights. (NOTE: Michelin often uses weights X 2 to list in their charts so be careful how you read the chart.)
Rear tire weights (dual) are always rated lower than the same tire on the front. This is because the rear tires dis- perse heat slower due to their proximity to one another. Make sure to note the maximum weight at the maximum air pressure written on the sidewall of your tires, when consulting the charts. Some newer tires have slightly dif- ferent air pressure ratings for the same size and load range of tires.
ABOUT THE AUTHOR - Dick Lorntson is the president and founder of Pre- cision Frame & Alignment in Elk River, MN. He has been actively involved in the alignment, suspension and tire industry since 1977. For questions or comments about this article, Dick can be reached at 1-866-473-2632 or by email at rlorntson@precisionframe.com
From the chart (see image), I see that my tires must carry a minimum of 70 PSI even if my coach is lightweight for my tires.
Tire charts are available on the internet or at your local tire dealer