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Commercial Tires

Two of a Kind: Mixing Sizes, Construction Leads to Trailer, Dually Disasters

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Why do people seem to lose their ability to reason when selecting tires for trailers and dually trucks? Since tire manufacturers build a specific tire designated as ST – Special Trailer – it is reasonable to assume that they did so for a purpose. Maybe to keep the scene depicted below from happening?

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Take a good look. This is a rural, two-lane highway with little shoulder area and less room to maneuver. From what I can tell, the driver-side tires blew first, causing the trailer to swerve across the centerline and into oncoming traffic. Fortunately, there was no one passing at that moment. The trailer and whatever was on it landed in this field just off of the road. There was no license plate, VIN or anything else to identify the owner.

Just in the last few months, I have noticed a marked increase in passenger-rated tires being used on trailers. To further complicate the issue, many aren’t matched sets. The owner used whatever tires he could find at the $20 used-tire super outlet or something.

I personally had to deal with this back in November. I met a local home remodeler one morning to help load materials needed to repair a hurricane-damaged home. The minute I walked over to the trailer, I noticed mismatched, passenger-rated tires. As I climbed into the truck, I told my friend that I would buy him a new trailer tire as soon as we got back.

Well, as it turns out, we made it all of 1.4 miles before being flagged down by another vehicle. As we pulled over and inspected the tire, my friend asked if we could “pump it up.” You can imagine how I felt at this moment, so I calmly suggested replacing the tire. The picture below is a close representation of what my friend had on his trailer. I see this all the time.

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A P225/75R15 was matched with a P205/65R15 on the passenger side. The driver-side tires were destroyed, so I couldn’t tell what they were.

Another point is to make sure that all of the tires are of the same construction. As you know, ST tires are built in both bias-ply and radial construction. Each has its own handling characteristics and rolling resistance, so don’t mix construction types, either.

If you have seen this in your shop, here’s a suggestion: Take a picture of a tangled mess like the one on this page and put it at the counter. You are likely to get new business based on the image alone, before you even start to tell the story.

Trailer tires generally are improperly stored and sit for prolonged periods of time, exposed to the sun’s UV rays. Then, when needed, they suddenly go from sitting for a year or two to running down the road at 70 mph, building up heat and stress. If an ST tire shows signs of cracking and oxidation, it warrants a consultation with the owner to discuss possible internal damage and the need for replacement, even if the tire has plenty of tread.

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This principle of maintaining the same size of tires applies to dual rear-wheel trucks, as well.

And, the amount of wear has to be considered, too. Just looking at the sidewall and seeing the same size isn’t going to prevent anything if one tire is worn more than the other. New guidelines from RMA state that there should be no more than 1/4-inch difference in overall diameter between dual tires, both on a trailer or a dually application.

Determine the difference by checking the tread depth – 4/32 of tread depth equates to 1/4-inch in overall diameter. Anything outside of this range poses a potential liability issue for the owner and the installer – you.

When the front tires on a dually truck show irregular wear, RMA guidelines suggest using the diagrams on the next page to rotate the tires.

Note that Diagram 1 is in the process of being updated to reflect the new guidelines that we mentioned earlier regarding the 4/32 tread depth difference. In a scenario where the front tires are worn significantly, the only option is to correct the condition that caused the irregular wear and replace the tires on the front. A difference greater than 4/32 will cause an overload condition on the taller tire, which could lead to failure.

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The same conditions must be met when performing a standard six-tire rotation, as illustrated in Diagram 2.

I have also seen some LT applications on dually trucks in which the owner thought that a wider tire was going to provide more traction. However, the driver bought wider tires to fit on the OE wheels. Most dually wheels are only 6 inches wide. The widest tire that will fit properly is a LT235/85R16. Note that a LT245/75R16 is rated for 6.5 inches up to 7.5 inches.

Other considerations for replacing tires on a dual-wheel truck are minimum dual spacing and load capacity. Minimum dual spacing is determined by measuring the distance between the centerline of each dual wheel.

The Tire and Rim Association can provide the specifications for these minimums. For instance, the LT245/75R16 that we just mentioned has to have a minimum dual spacing of 11.34 inches.

And, don’t forget air pressure. Some trailer tire manufacturers are suggesting inflating to maximum psi ratings. They cite the variable use of trailers as the reason. For example, the trailer may be hauling a tractor one day and an empty wooden crate the next. Best to be safe than sorry, they say.

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As for dually trucks, pay close attention to the load and inflation table. There are two separate lines and scales for single-wheel and dual-wheel applications. An important point to make here is that wide tires on narrow wheels (on any vehicle) will decrease the load capacity by as much as 10% if the beads are drawn in too close relative to the section width.

Last, but not least, look closely at load ranges. ST and LT tires are built in several load ranges, so never assume they are the same. I have seen mislabeled tires before; if you don’t look at the sidewall, you may not notice the difference.

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