By Roger Marble
Andy recently wrote to me about 4-corner weights, as follows:
Good afternoon, Roger. I wanted to double-check myself regarding tire position weights. In reading your blogs, you said for double-axle trailers the tire pressure should be the max cold pressure amount as stamped on the tire because trailer sway and turning forces on those tires are different than on a motorhome. Is it important to get individual tire position weights for trailers?
As I just got a new 5th wheel trailer and I am getting ready to install a Tire Tracker tire pressure monitoring system, I have a few questions:
1. Do I need to get 4-corner weights (or in this case, 8 tire position weights)?
2. Is load on each tire position important for trailers vs. motorhomes, or is axle weight sufficient? (I can get overall and axle weights easily enough. And, I just got an appointment for tire position weights with a SmartWeigh club in FL, if you think it is important.)
3. When I go to get the weights, should ALL holding tanks be full or only Fresh Water? (The SmartWeigh group literature states that they only want the fresh water tank full.)
The 4 tires on my 5th wheel trailer (and the spare) are:
Carslie CSL16 ST235/85R16
Load index 132/127
Speed Rating: M (81 MPH)
Max Load Single 4400 lbs. at 110 psi. cold
Max Load Dual 3860 lbs. at 110 psi. cold
DOT JETB 1821
The 6 tires on my Ford F450 DRW are:
Max. Load Single 3970 lbs. 110 psi. cold
Max Load dual 3750 lbs. 110 psi. cold
Then they have LRG in an oval followed by 128/126N in another oval and then DOT B6 YB NFL X 0621 in various ovals
I don’t have weights on the trailer yet, but the GAWR is 7,000 lbs.; GVWR is 17,000 lbs.
Thank you very much. —Andy
The “4-corner weight” is usually talking about Class A motorhomes and the data shows that some number of those units can be significantly (1,000# or more) unbalanced side to side on an axle.
Smaller RVs can probably get away with just learning the weight on each axle with the RV and TV loaded to its heaviest. Then calculate 51% or 52% for each axle, and then use that weight number to confirm you are not exceeding the load capacity of the tires.
For towables, including 5ers, your calculated heavy weight should be no more than 90% of the tire capacity when inflated per the Load & Inflation tables. I covered the reason for this 90% limit in my blog post on Interply Shear. The RVIA (see that Gold sticker near the door on the trailer) has a requirement that tire capacity should be = 110% of GAWR.
The max load capacity for a tire is load in pounds shown on the tire.
Your Dry Weight is not important when we are talking about tire loading. The GAWR is just a number for all the tires on any one axle. The tire on the passenger side has no idea what the load on the driver side is, so you can’t average the axle weight to learn the max weight on the heavier loaded tire.
“Dual” loading is when two tires are mounted side by side on one end of an axle, as we see on the rear of most Class C motorhomes and you have on “dually” pickup trucks like your F-450. Dual does not apply to RV trailers. LT tires do have different load capacities if mounted on the front (single) or mounted on the rear (dual).
You’re correct that you do not need both freshwater and holding tanks full. Think of your loading when you start a trip. Fresh water is normally full, so is propane, gas or diesel, and food pantry, but holding tanks are empty. Don’t forget your tool box. That would be what I would call “heaviest expected weight.”
This is when you get on a truck scale and get readings with just one axle on a scale pad. You may need a couple of readings depending on pad spacing and your axle locations. Get both trailer and truck weights, as you should also confirm you are not overloading your TV tires either.
Example: Suppose you have tires that say 3,900# @ 80 psi. Your GAWR on the certification label is 7,000#. RVIA requires the tires have a capacity of 110% of the 7,000 or 7,700 total, or 3,850# each. So you might think you are OK. But if your RV axle is “out of balance” side to side by 1%, or 3,885# on the heavy end, you would have a tire in overload. Not a great deal, but in overload assuming the scale is accurate +/- 1% and you have a 100% accurate pressure gauge. What if your axle is unbalanced by 200# or 300#? TVs are normally more balanced, so just take the axle load and divide by the number of tires on that axle.
BUT the RVIA 110% requirement went into effect in November 2018, so there are many RVs out there that only require that tires be capable of supporting 100% of GAWR, so that 10% “cushion” is gone.
I strongly support the RVIA’s 110% load capacity. In fact, my Interply Shear data suggests that something closer to 125% would be desirable. But the RV companies simply have not designed their vehicles with large enough wheel wells to allow that large of a tire, even if the cost penalty was less than $200 an RV.
Back to your original question: You do not need to learn the individual loads on each tire position. BUT I do suggest you assume at least a 1% out of balance and apply that to the weight reading you can get at truck scales. You just need to pay attention to where each axle is on which scale pad.