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Wednesday, February 15, 2017

Tow vehicle tire question

Here is a question from an RV Forum:

"Last week I purchased new E rated tires for my F150. Went with the BF Goodrich KO2 (275/65/18) and so far I'm very happy with them. These are larger than the tires that came from the factory (265/60/18). I'm pulling my trailer for the first time this weekend so had a quick question.

"The tire shop originally put the PSI at 35 based on the door sticker, but after reaching out to BF Goodrich they suggested to bump it up to 45 for everyday driving. They were not able to give any recommendations for tire pressure while pulling a trailer since there are so many variables like trailer size, weight and cargo. The tire shop suggested going to 50 pounds and then deflating back down after the trip. I'm just curious if others actually bump up their PSI while towing, and have you noticed a difference in doing this? Just trying to figure out if there is a benefit to doing this. Thanks in advance for your feedback."

Some important information is missing, so let's identify our assumptions:

1. We are talking about the tow vehicle tires - the F150.

2. The original poster didn't identify the OE (original equipment) tires as being P type or LT type. This makes a major difference and providing that information up front will eliminate guesswork for those trying to help with the answer. With a 35 psi door jam sticker number, I suspect the OE tires are Passenger type and the replacement tires are LT type.

3. Since it is the air pressure and not the tire construction that supports the load on a tire, buying LR-E (80 psi) tires when you never run higher than 65 (LR-D) would be a waste of money. Also if you are changing from P type tires with a max inflation of 36 to 41 you need to confirm the wheels are rated for higher load and inflation of LT tires.

4. The door jam pressures are based on the car company making some estimates on how much and how often you have the vehicle empty or fully loaded. You might check the owners manual and see if they give an empty and loaded tire inflation suggestion. The inflation in the OE tires is what is needed to support the GAWR but few people run that heavy all the time with their pickup.

5. I suggest you get the F150 on some truck scales when empty and again when fully loaded with the trailer also fully loaded, and learn the real facts of the various axle loads under both conditions.

6. Knowing the real loads, you can use the tire Load & Inflation tables to learn the MINIMUM cold inflation pressure for the F150 for the two situations. I suggest you use a margin of +10% of the table inflation to establish your CIP (cold inflation pressure). See other posts on weight and inflation if you need more details.

7. Trailer tires are a completely different situation and should always be inflated to the tire sidewall inflation. You should also run no more than 85% of the trailer tire max load rating when on a scale. Cornering, sway and side wind loads have been shown to shift loads side to side by 10% or more. Also the Interply Shear on tires in trailer application needs to be considered as this is a major contributor to trailer tire belt separations.

Hope this info helps clarify.

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Monday, January 23, 2017

Quick post on RV weight

Read a question on an RV forum about "4-Corner weights vs. just getting axle weights." Here is my reply.

Getting 4 corner weights (actual load on each tire for trailers or each position on motorhomes) is definitely worth doing, once. Once you figure your numbers you are typically good to go for the life of the tires or unless you change something big.

Do it fully loaded,  i.e., fuel, water, canned goods, fishing gear, clothes, books, people, etc.

Based on actual data from RVSEF, very few RVs have side-to-side loading at 50/50%. Some owners have discovered one axle end as much as 1,000 lbs. heavier than the other!

Once you know for a fact that you have at least a few hundred pounds "cushion" (being under your RV GAWR [gross axle weight rating] and GVWR [gross vehicle weight rating]), you don't need to do corner weight again unless you make a major change or remodel of the RV (e.g., adding a residential fridge or granite counter-top) and have a reasonable balance to start with.

I do suggest at least once a year a quick check of axle loading by going through a truck stop scale. You can compare the truck scale numbers with your corner weight totals to confirm your RV hasn't suffered from weight creep, as some of the drivers may have noticed over the years.

Weight terminology tidbits:

SCWR (sleeping capacity weight rating)

The manufacturer’s designated number of sleeping positions multiplied by 154 pounds (70 kilograms) which is the official weight of people in cars and RVs.

CCC (cargo carrying capacity)

The GVWR minus each of the following: unloaded vehicle weight, full fresh (potable) water weight (including water heater), full LP-gas weight, and SCWR.
This new label permits the buyer/owner to determine the carrying capacity (CCC) based on a personal calculation of actual passengers carried, the amount of fresh water onboard, and the amount of LP-gas carried.

More info on terms and abbreviations can be found HERE

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