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P R O D U C T   F O C U S

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Why do spread axles eat tires?

In the straightaway, they don’t, really. But they make up for it in the turns. Next time you have a chance, observe a spread axle trailer executing a tight turn. You’ll probably hear the tires squealing and growling as they’re dragged across the pavement. 
     And, if you watch closely, sometimes you’ll even see the tires rotate first one way then stop then rotate the opposite direction. All in the course of a single continuous turn. No wonder spread axle tire life is so short.
     Side forces, including scuffing, scrubbing, squirming and dragging of the tires on spread axles can result in rapid wear, irregular wear, shoulder rib tear or occasionally even belt separations. And that cuts both mileage and retreadability.

What can we do about these side forces?

Not much. They’re inherent in the geometry and dynamics of spread axle trailers. With every vehicle maneuver, tires are subjected to side forces. That’s one reason tires used in long distance hauling tend to last much longer than those used in pickup and delivery work.
     But the farther the axle gets from the center of the turn, the greater these side forces and the wear they cause can get.

Whoa! This is getting too complicated!

It’s not as bad as it sounds. Take a look at the pictures, and you’ll see. When the midpoint of an axle coincides with the center of a turn, wear and side forces are at their minimum but still there.
     With a tandem, the center of the turn may not coincide with either axle center. Even if the center of the turn matches the center of the tandem, there’s still a lot more scrub and side forces on the tires in a tandem than on those on a single axle.

What happens as the axles
get farther apart?

We already know that side forces increase the farther the center of the turn is from the center of the axle.  With a spread axle, this distance is even greater than with a conventional tandem. 
     And you can see that no matter where the center of the turn is, one or both of the axles is a long way from it. The farther the axles are apart the greater the spread the greater the side forces.
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indent.gif (821 bytes)indent.gif (821 bytes) What does this do to the tires?

It can cause rapid wear. And the larger these side forces, the more rapid overall tire wear can be. (In our surveys, the tires on the front axle of the spread tandem are the most troublesome).
     Sometimes, after just a few thousand miles, shoulder edge wear can be so severe that it looks like the tire’s shoulder has been “carved” away.
     And, these incredible side forces are also very hard on the edges of the steel belts. Because the tires may be dragged sideways, under very heavy loads, while hardly turning at all, these side forces could also compromise casing integrity.

How can the new R196 help?

First off, the R196 is deeper than most trailer tires. At 16/32”, the R196 carries fully a third more tread than the 12/32” tires used on many trailer positions.
     And, the R196 also incorporates a special, high scrub resistance tread compound that’s designed to wear slowly but run cool, even under stress.
     Finally, the tread of the R196 is a bit wider than normal. That means side forces are distributed over a larger area, reducing the harm they can cause. 
     So, if rapid wear is the problem, the R196 gives you more tread depth, more tread area and a slower-wearing compound to fight it.


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What about the shoulder wear?

During the development of the R196, much of which took place in Canada (where spread and tri-axles have always been more common than in the U.S.)  Bridgestone engineers evaluated a variety of shoulder designs.
     Not surprisingly, what worked best was a design similar to that of the R250F, a radial Bridgestone had developed for fleets in short-haul, high-scrub operations.
     Bridgestone engineers took the basic R250F shoulder shape, then added hundreds of tiny stress-relief sipes along each shoulder edge. The squarish shoulder of the R196 stands up well to side scrubbing, and the sipes help relieve stress concentrations that could initiate irregular shoulder wear.

Would the R196 be suitable for tri-axles?

Since increased side forces are typical with tri-axles,
they often exhibit many of the same wear conditions seen on spread axles. The R196 should be an excellent performer with all sorts of multi-axle configurations including tri-axles.

How can I try the R196 on my trailers?

See your dealer. The four most popular sizes of the R196, the 11R22.5, 11R24.5, 295/75R22.5 and 285/75R24.5 are in production and on their way into the market right now.

So, are all the problems
of spread axle tire wear solved?

Sorry. It’s not that simple. Spread axles will probably always be tire-eaters. But the R196 slows down the process a lot in many cases. For fleets that have been plagued by rapid wear, irregular wear and belt separations on their spread axle tires —or that have had to resort to more expensive, deeper tread steer radials, the R196 can be a real, cost-effective answer. And these days, that’s what we all need.


What good are spread axles anyway?

As you know by now, even in the best of circumstances and even with tires like the new R196, spread axles are tough on tires. So why use them?
The only good reason we know of to use spread axles is in situations where you need to increase the load on the trailer axles. Federal bridge formula weight laws allow spread axles to carry more weight than conventional tandems. This gives you increased flexibility in load positioning on the trailer.

This ability to increase the load over the one axle set can be very useful if you’re hauling something with a very concentrated mass, like a big casting, or a coil of steel or a piece of heavy machinery.
     Even though you can’t exceed the 80,000 lb. total for the whole vehicle, more of the weight can be placed on the trailer axles. This can allow more flexibility in loading, and can also reduce stress on the center of the trailer frame.

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Why don’t all trailers have spread axles?
In a sense, all of them do. It’s just that the spreads are usually less than 96 inches. And there may be good reasons for not using spread axles: If your loads are relatively uniformly distributed, spread axles may offer no advantage. 


Important Note:

The diagrams in this article are based on federal guidelines.
Individual state load limits may vary.
Please be sure to check local regulations before changing loads
or switching to spread axle trailers.


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