Tire Overload

We left the Flying C Ranch around 10:30am and followed I-40 westbound to US285 north (map). The wind had shifted and we had a tailwind on I-40. Of course this meant a crosswind as soon as we started north. We were going up and down grades with the net result being another gain in elevation.

Our destination was only about 75 miles away. We wanted to spend some time in Santa Fe, New Mexico. A week should allow us to sightsee and get a feel for the place. On the way, Donna looked at different RV parks and campgrounds on her laptop. We debated going to a casino about 15 miles north of town for the weekend. Parking at the casino is free and they offer a shuttle to downtown Santa Fe.

Eventually we decided against it. We wanted to be in Santa Fe where we could walk to restaurants and shopping or take a short transit bus ride to the city center. Our first choice, Trailer Ranch RV Park was full, so we booked a week across the street at Los Suenos de Santa Fe RV Resort. This is a convenient location with everything from taquerias to natural food stores to large department stores within easy walking distance.

We have a long pull-through site. The first site they gave us didn’t work out. It was also a long pull-through, but it had a tree on the left rear corner and a stump on the right rear. It was impossible to turn a rig a long as ours without hitting either the stump or the tree. If I didn’t have the trailer I probably could have jockeyed back and forth and got in. But if I didn’t have the trailer, I wouldn’t need a long pull-through site.

One of the workers told me to move down to another site that was more accessible and we set up without any issues in site 88. There are empty sites on both sides of us.

After we set up, I saw a motorhome trying to get into a back-in site. He had a tow dolly on the rear and couldn’t make it work. He eventually gave up and upgraded to a pull-through – site 86, two sites down from us.

These sites slope from left to right slightly. I saw him back his coach over boards he placed behind the wheels to raise the right side. Then I noticed he had improperly supported the rear of the coach. Here’s a picture – I’ll explain the problem with this.

Coach raised by driving onto boards

Coach raised by driving onto boards

The rear wheels are duals. He placed boards under the outer wheel only. The load at the right rear of his coach is intended to be supported by two tires. Having boards under one tire means the entire load is supported by that tire – the inner wheel is just hanging in space.

I’ll use my rig as an example and put some numbers to this. My coach is fitted with 295/75 X 22.5 tires. The tire pressure placard placed by the manufacturer (Western Recreational Vehicles) calls for 110 psi in the front tires and 95 psi in the rear.

I’ve weighed my coach on a certified scale and have the axle weights. My front axle is rated for 12,000 lbs. My actual front axle weight is under that by several hundred pounds. My rear axle is rated for 19,000 lbs – my actual rear axle weight is just over 18,800 lbs.

Looking at the load and inflation tables for my tires (Toyo 295/75 X 22.5 load range H), I can see how much load they can safely carry when inflated to the pressures listed on the placard. The load and inflation table has two columns for each pressure figure – one for tires in single wheel applications (front) and one for tires in dual wheel applications (rear). The dual application always has a lower weight rating due to inconsistencies in how duals carry the load. Things like pressure differential between the two tires or tire diameter variation or even road crown can affect how much load each tire carries in dual configurations.

Back to my example. Toyo says with 110 psi in single wheel application, my tires can support 6,175 lbs. Multiply this times two front tires and I can safely carry 12,350 lbs – a safe margin over the gross axle weight. Since I don’t know my side to side variation, having 110 psi gives me a margin of safety. My 11,000+ lbs front axle weight might be closer to 6,000 on one side rather than evenly distributed. The only way to know that for sure is to have each wheel load weighed separately.

On the rear, Toyo says 95 psi in dual wheel applications can support 5,070 lbs. Multiply this by 4 tires and I have 20,280 pounds of load capacity for the rear axle – again, a safe margin over the 19,000 gross axle rating and my actual weight of 18,800 lbs.

Now, what happens when I place a board under oneĀ rear wheel? That makes that wheel equate to a single tire application. The load and inflation tables tell me that a single tire inflated to 95 psi (remember – that’s what my rear tires call for) can support 5,510 lbs. Let’s assume that my rear axle weight is evenly distributed side to side. If we divide my actual weight of 18,800 by two we see that each side of the rear axle is carrying 9,400 lbs.

That single rear tire with board underneath is severely overloaded. Please don’t do this. The tire carcass may suffer damage from being overloaded in this manner. In my neighbor’s case, the problem is made worse because the board is smaller that the width of the tire, so the entire contact patch of the tire isn’t supported.

Today we have beautiful weather. The high is supposed to reach 80 degrees under clear skies with very low humidity. Last night, the temperature dropped to the low 50s and we slept with the windows open. We’re at an elevation of 6,650 feet above sea level now. We’ll take a bus downtown and go exploring today.

5 thoughts on “Tire Overload

  1. Ray

    I see this tire issue often, or front wheels off the ground completely or even rear wheels off the ground makes me nervous but some folks don’t seem to care. Great blog, thanks.

  2. Maryann

    Hi, Hope you enjoy your time in Santa Fe. I’m going to give you a couple of recommendations that you might check out: Canyon Road (very artsy but fun to look at) and the Shidoni Foundry and Galleries. We like eating at the Blue Corn restaurant which is near the square in Santa Fe, but there are many places to eat there in Santa Fe.
    Maryann

  3. Pingback: Free Parking and Good Food - Flying The Koop

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