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 Â Load range EÂ (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.
My reply
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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Please correct if I’m wrong. A tire engineer once told me to inflate the tires to the specs printed on the sidewalls. It the tire is a ten ply and the sidewall says max pressure 80 psi I usually set them to spec. Not the door panel that recommends 55 psi. Many vehicles come from the factory with under rated tires or minimum rating. If you upgrade to a better load range and maintain the same psi rating you are allowing more sidewall flex. Wasn’t this the issue with Ford Explorers and their tire failure? Under inflation. Thank you for the service you provide to the industry. Education is priceless. Steve
The MAX tire pressure on the side wall is a don’t exceed number not an inflation guide. Don’t inflate your tires to that number!
Calculate the tire pressure from the tire spec’s (load range speed rating etc) the weight on the tire in normal use on your vehicle and a tire pressure guide.
If the number is even close or over the MAX number it is the wrong tire for you application.
Not quite. Each tire has a stated Max Load capacity. The inflation is the MINIMUM inflation required to support that Max Load. Inflating above that inflation number WILL NOT increase the load capacity. Some applications do specify a Cold Inflation level with the same PSI as the number molded on the tire sidewall so we can’t say “Never exceed the number on the tire sidewall.” Some people incorrectly think that if the inflation exceeds that number when hot they need to bleed pressure off. This is absolutely the wrong thing to do. You can learn more by reviewing the Inflation posts on my Blog http://www.RVTireSafety.Net which is sponsored by RVTravel.com
Wondering which tire company your “Tire Engineer” designed tires for. He/She should know that the tire inflation on the Certification Label is the responsibility of the vehicle MFG. He/She should also know that it is a Federal Regulation that the tires, when inflated to label inflation, be capable of supporting the GAWR so while there might be better tires available the OE tires can’t be “underrated” or a recall and fines are possible. While sometimes the Certification Lable may specify the same inflation as molded on the tire sidewall that is not an “always”. Yes, changing tire size or Load Range will require some investigation to learn the correct minimum, again you can’t make broad generalities.