Is irregular wear not a problem with retreads?

In some ways, the statement is true, but not really that surprising.

Why not? 

The conditions that cause irregular wear generally do not discriminate between new tires and retreads. Most irregular wear problems are the result of application and maintenance issues. And application is a very large factor. For example, the majority of irregular wear occurs on steer axles.

Why is that?

The steer position is the most demanding one. For one thing, these tires are often carrying loads very near their maximum capacity.

As an example, imagine a typical, 18-wheeler, with a maximum weight of 80,000 pounds. The weight distribution looks something like this:

A 295/75R22.5 G-load rated tire will have a capacity of 6,175 lb at 110 psi. So, that means it could easily be at (6,000/6,175) about 97 percent of its maximum rated load.

And that’s pretty much all the time, because steer axle load doesn’t vary that much, regardless of whether the rest of the vehicle is fully loaded or not.

Steer tires are under a lot of stress.

What about drive and trailer tires? 

If we use a typical inflation of 95 psi and the same size and load rating tire for drive and trailer, we find the maximum load capacity for those tires is 5,070 lb.

Since the maximum legal load for these positions is 4,250 per tire, we see that drive and trailer tires are generally loaded to no more than 83 percent (4,250/5,070) of their capacity.

And unlike steer tires, payload has a huge effect. If anything, drive and trailer tires are often overinflated. And irregular wear is much more likely on an underinflated tire than on an overinflated one.

Tire Load as Percent of Rated Capacity

Steer Tires: 6,000/6,175 = 97% (at 110 psi)

Drive & Trailer Tires: 4,250/5,070 = 83% (at 95 psi)

Are there other factors?

Steer tires control the direction of the whole vehicle. As much as 80,000 pounds of vehicle is being pushed left, right, or held in the center of the road by just two tires. That’s additional stress.

How does this relate to retreads?

First of all, only two out of the typical 18 positions on a tractor-trailer combination vehicle are steer positions. That means only 11 percent of the tires under an “18-wheeler” are steer tires, whether they are retreads or not.

In addition, vocation also affects the situation. P&D, LTL and waste haulers tend to use retreads extensively, including on steer positions, while long distance line haul fleets tend mostly to use retreads on drive and trailer positions.

Are there other application effects?

Waste, P&D and some LTL fleets, which tend to use retreads extensively, also usually make shorter, more localized hauls, where there’s more stopping, starting, turning, backing and maneuvering than in coast-to-coast truckload work.

That means more scrub, and more scrub means some irregular wear gets “erased” almost as quickly as it appears. Really short-haul or P&D fleets, for example, rarely complain of irregular wear, whether they use retreads or not.

What about “casing growth”? 

You sometimes hear about “casing growth” as a factor in irregular wear, and that by the time tires get to the stage where they’re retreaded, casing growth is pretty much over.

The analogy is a bit like buying a new pair of shoes. They might be a bit tight and uncomfortable at first, but after a while, they stretch and conform to your foot. And if you take them in to be resoled, that process is not going to change how they fit or wear.

Is that how it works with tires?

Casing growth tends to be greatly exaggerated. Modern, all-steel tire casings don’t grow all that much, and tire design engineers have become very skilled at estimating and compensating for casing growth effects.

Casing growth can affect the tire footprint as it rolls down the road. So do load, speed and tread wear. All of these things are precisely considered when a tire is designed.

Bridgestone designs each tire so that once casing growth has stabilized (by about 20,000 miles or so) the tread footprint is nearly ideal, and remains that way for the remainder of original tread life.

So the casing inside a retread is fully “broken in”?

To some extent. Plus, to make a quality retread, you mount and inflate the old casing onto what amounts to a lathe, buff it to a very precise size and true up its roundness.

So, any residual effects of casing growth are essentially “machined away.”

Are there other differences? 

Besides proper inflation and vehicle maintenance, one factor that affects irregular wear is proper mounting. If a tire isn’t mounted concentrically, it’s not going to wear optimally.

When tires are brand new, beads are stiff and tight and it may be hard to get the tire onto the wheel, then to get the tire concentric with the wheel.

After a tire has been in use for some time, as it will be by the time it hits the retread stage, getting it onto and off the wheel is a bit easier, as is the process of getting the tire concentrically mounted. And that too can reduce irregular wear.

So, is irregular wear really less of a problem with retreads?

It might appear that way, largely because the most troublesome irregular wear problems occur on steer tires. Steer tires are outnumbered by other positions, and in waste, P&D and LTL fleets, where retreads are extensively used, there often is a lot of scrub that tends to erase irregular wear effects.

As we said in the beginning, irregular wear is largely the result of application and maintenance effects. Your best defenses are careful selection of the tires you use, coupled with conscientious maintenance best practices, including regular inflation pressure checks and tire inspections.