Open Shoulder Tires vs Closed Shoulder Tires

Normally loaded, tires have a flat, even footprint. When empty at highway speed, the only center of the tread makes contact with the pavement.

There's some debate over the impact running empty has on tire wear, but fleets that run a high ratio of light or unladen miles seem to report higher instances of irregular wear such as cupping, scalloping, and flat-spotting. It's not clear whether that irregular wear is a direct result of light tire loading or collateral damage resulting from some other condition, but evidence suggests tires tend to perform much better under pressure than when lightly loaded.

"By far, the largest contributing factor to tire wear is improper tire pressure for the anticipated axle load," this "With tires typically pressurized for a loaded condition, trailers operating with a high percentage of empty or lightly loaded miles can see a decrease in tire life as a result of the tires being over-inflated for the reduced axle load."

Tread rubber is constantly being scuffed away from a tire at predictable rates under normal loads. But when running light, it wears at a slower rate. If some other factor is causing irregular wear - such as bad alignment or loose wheel bearings - the wear caused by the problem would probably keep pace with normal tire wear under normal loads. But compared to slower "normal" wear rates under light loads, the irregular wear might appear to be more aggressive or more pronounced. This could lead to the assumption that running light or empty is the root cause of the accelerated wear.

I have over the years found a higher-than-normal incidence of loose wheel bearings, improperly matched tires in dual assemblies, and improper inflation.

"Was it the high empty miles? No, it was a product of poor maintenance."

Changing the footprint:
"The unloaded tires just skip down the road without any weight to bear down on the tire," .

The tread surface is generally pretty close to flat, or square with the road, at operating pressure. Under load, contact with the pavement is pretty even across the tread face. But when empty at normal operating pressure, the center of the tread tends to be slightly higher than the shoulder. This is more acute at high speed, where centrifugal force acts on the tread, causing it to extend outward at the center.

What you get in this situation is an egg-shaped footprint, with the center of the tread having a slightly larger circumference than the edges. This increases the scuffing action, because the edges are not revolving at the same rate as the center of the tread.

"In addition, an unloaded tire that bounces along the road actually slows while it is airborne, causing a minor bit of scuffing each time it regains contact with the road. In this situation the tread surface is equal to a dual tread application therein where the low part of the tread or the parts of the tread that are damaged have a lower height. Because of this on the same tread base the damaged portions of the tread will delete at an accelerated rate for lack of traction just like running one tire with lower tread depth next to a tire with good tread depth on a dual wheel drive type application.

Tire pressure has a major role to play here, as does the damping effect of the suspension.

"Think of a fully inflated tire as a Super-Ball," he says. "With relatively little weight to keep them on the ground, they'll bounce like crazy at normal inflation pressure. The suspension can mitigate this, but only to a certain extent."

Correct tire pressure for the load would have a large influence on tire wear, in theory, but, adjusting tire pressure downward for one half of the trip is a task few drivers would undertake.

Suspension solutions:
Low-spring-rate suspensions could also help to reduce irregular wear by maintaining better contact between the tire and the road.

"Air suspensions typically have a lower spring rate than leaf spring suspensions, and they maintain a relatively constant low spring rate whether the tractor is loaded or empty".
Air suspensions also have shock absorbers that dampen the suspension movement, further improving the suspension's ability to maintain tire contact with the road. This application seems to be solely viable for interstate freight commerce and seems to be a further liability and adds to the premature wear of steer tires under heavier tear weights or uneven terrain.

Leaf springs provide a stiffer ride on an empty steer axle. The tires themselves can easily become the only dampening component resulting in a varying load on the tire as it encounters irregular road surfaces, other than a correctly functioning shock absorber. On a axle with only vertical or "perpendicular" movement only a oil type shock should be used as it only adds resistance and not downwardforce which changes spring rate "Leaf spring suspensions that do not have shock absorbers, but the leaf springs themselves provide some damping characteristics."

In many high-empty-mile applications, tires and suspensions run either fully loaded or empty, so "tuning" the suspension and tire pressure to work equally well at opposite ends of the spectrum is realistically out of the question. The main examples of this are on every major tire manufacturers tire wear charts under corrective measures, claiming there are none.

"For air suspensions, the air pressure is used to maintain the tractor's height, so changing air pressure would have little effect on the spring rate," In contrast "Leaf springs can be designed to provide varying spring rates at different loads, but the amount the spring rate can be varied is limited."

Start with tire matching:
Since there are no spec'ing options to help manage this unique situation, the duties seem to fall on the maintenance department's shoulders. This issue is more and more cropping up constantly in his alignment shops. Suggest the jump-in point is careful tire matching.

"Mixing brands and models of casings in a dual assembly encourages irregular wear. They don't all have the same sidewall flex, or even with the identical sizing the same ride height "The more focus on matched brands, models, circumference, and inflation pressure, the better the tires will wear - even under poor terrain conditions."

And the shock absorbers on spring suspensions need particular attention. "That's where all the damping takes place. If they're failing, your tires will fail, too."

Thinking outside the box:
Since it's practically impossible to spec or tune a tire. The suspension can somewhat be tuned to an application operating at opposing ends of the spectrum, other measures might do the trick. What follows are suggestions, not tried-and-true solutions born of million-mile field trials. They have some merit on paper, but the effectiveness will be up to you to prove. The adding to or the corrective actions or placement thereof components that change spring weight tolerances or spring rate can effectively change the footprint and, by doing so minimize the wear from the overloading of a suspension.

The consensus seems to be that alignment plays a role in certain types of irregular tire wear, and wear is exacerbated when tires are run lightly loaded. Keeping the vehicle properly aligned may minimize your irregular tire wear.

Joshua Reeves 4/16/24

 

So, here's my experience with truck drive tires:

Closed-shoulder tires ride nicer, make less noise, and give you better fuel economy. They also wear more even, and if you are in the 'mountain west' a lot during the winter, they are much easier to hang iron onto than a lugged tire is. On the downside, they kinda stink in the snow, and sometimes REALLY stink in the snow. My preference would be these if I drove just in the south, or along the west coast.

Open-shoulder (or "lugged" tires) can be noisy, and can wear in 'choppy' patterns and throwing chains on them can be une batard of a job, but if you drive in snowbelt areas or enter a lot of muddy drop lots they really are the way to go. Despite all the snow we get near the Great Lakes, absolutely NOBODY EVER uses chains up here. Even with 4" of fresh snow the traffic still goes at least 50 mph, which is about 20 mph over the absolute top speed for truck chains. So, in the northeast, they are just about required. If you drive in the area of the 'front range' of the Rockies, you may or may not wish to use them, so that those times you don't REALLY need chains (and there is no DOT monkey telling you what to do..) you'll be better off going without the iron. You'll just have to deal with the lugs when you absolutely have to chain-up......