By Roger Marble
I recently received this request and supporting information regarding tire inflation:
Can you help me find the proper tire pressure for the following?
Winnebago Class C motorhome on 2018 MB Sprinter chassis
Dual rear wheels, Tires are LT215/85R16 115/112Q Load range E (Continentals)
GVWR = 11030
GAWRF = 4410
GAWRR = 7721
Door placard says 61psi, front and rear.
Actual weight as loaded, including driver & passenger
Front = 4200
Rear = 6600
Total = 10800
According to the Michelin load chart for this size tire, the front about matches my placard at 61psi
However, if my rear with duals is 6600, should I set the rear pressure closer to 50?
At 61psi the rear impact harshness seems high over road cracks and expansion joints.
Would love to hear your comments as I’m thinking about lowering my rear pressure some. Trying to improve ride, but not hurt tire performance.
Looking at your GAWR and scale readings tells me you are not overloaded. You are relatively close on the front and only have a 5% margin at 210#. The rears are better off with a 14% margin. Given that I have seen some Class C RVs have 3% to 4% side-to-side unbalance, I would be more comfortable if you could reduce the actual load on the front tires until you can confirm actual “4-corner” weights at a building supply scale or gravel pit, or some other location where you can get the individual tire loading.
Your weights in the rear are a bit lighter than I normally see such as on my 24′ Class C, which has scale tire loading of 1,900# and 2,100# on F (4,000 axle) and 3,550# and 3,850# on R (7,400 axle).
But back to your situation
I confirmed that the weight chart from Michelin is the same as the general industry standard. Sometimes Michelin has different numbers than the rest of the industry. So unless we are running Michelin tires, I tend to not even look at the Michelin charts. I also prefer to deal with individual tire loading, and some Michelin charts are axle loads.
4,200 x 51% = 2142#. Consulting the chart, I find 60 psi as the minimum. I also always suggest +10% inflation margin to learn the cold set pressure which gives 66psi.
6,600 x 50% gives 3,300#. 51% gives 1683, and the chart suggests 45 psi as the minimum. Adding my 10% yields 50 psi.
I add this inflation margin to avoid the need to adjust my tire pressure whenever the ambient temperature changes. I don’t adjust tire pressure till the 10% drops to 5% inflation margin.
BUT the 51%/49% side-to-side split is conservative, and I am not comfortable suggesting lower than Certification Label inflation without knowing actual tire loading. So I suggest:
66 psi on the Front and no less than 61 psi on the Rears.
Ride and harshness are really a function of chassis design, springs and shocks. Using the tires to “improve ride” by lowering pressure without all the numbers may result in shorter tire life.
I trust that this is clear enough such that others can do the calculations on their own.
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