Spec’ing tips: Wheels

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TORONTO, Ont. – With three basic designs to choose from, you’re not likely to be overwhelmed by the available options when spec’ing wheels, but there are advantages to each design that should be understood and performance claims that bear investigation.

First let’s take a look at the three main options:

* Disc wheels are one-piece assembly designs with their centre attached to the rim permanently. They are available in either aluminum, a weight-saving, sharper-looking but more expensive spec, and steel. The discs include eight or 10 stud holes and varying numbers of hand holes. By tightening the lug nuts onto the stud bolts the wheel is centred on the axle hub.

On dual wheel positions, the inner wheel is tightened on its studs with the lug nuts, which mount over the studs that protrude from the brake drums. The outer wheel is then mounted and held in place with the outer lug nuts.

* Spoke wheels have three, five or six spokes reaching out from the central hub and are usually offered in steel. A demountable rim is attached to the spokes. With dual wheels, the spokes are made deeper to include the space taken up by the two rims in addition to the necessary spacer placed between them.

* Disc and spoke wheels have been part of the industry since the first trucks at the turn of the century; hub-piloted disc wheels are the latest entrants. Migrating over from the transit bus industry, these wheels are designed to slide over pilot guides included on the hub, which centres them on the hub.

This means flat-faced holes and flanged nuts can be used to hold the wheel in place. Also, because only one set of studs and nuts is employed at the dual-wheel positions, the amount of nut torquing and the need for fasteners is cut in half.

That covers the basic design differences among the three wheel types.

Now let’s consider their performance differences.

Spoke wheel suppliers claim an advantage when it comes to maintenance. This is due to the fact that tires are mounted on the rim and when they need to be changed, only the rim needs to be removed. Spoke wheels also have fewer lug nuts than do disc wheels and if the lugs holding the rim to the spokes work loose, the components will likely escape damage, although the rim will droop on the spokes.

Hub-piloted wheels, of course, also have the advantage of fewer nuts to worry about.

Spoke wheel suppliers also claim an advantage in terms of strength, although disc wheels, in either steel or aluminum can be made to match that strength. For example, Alcoa put its aluminum wheel through a test and reported that it took 157,000 pounds to deflect the forged wheel two inches.

If cost is the most important issue, spoke wheels are likely the best spec. They tend to be the spec of choice on trailers and other applications where price is the overriding factor.

In terms of looks, aluminum wheels can be polished to the brightest finish but there is a premium to pay for that look. Steel wheels are more utilitarian and come in several different hues.

Another issue that bears consideration is the wheel’s ability to conduct heat away from the brake drums. Cooler running brakes add to tire life, particularly to the recapability of inner dualed tires. The different wheel suppliers do conduct tests on their product’s ability to conduct heat away from the brakes so do ask for and compare the results.

No matter which type of wheels you settle on for your operation, proper maintenance will play an important role in getting the most mileage out of the product. Overtightening of nuts is a common industry problem that ends up stretching the studs to the point where they can break.

On disc wheels overtorquing will result in deteriorated chamfers of the holes. On dual wheels the overtightening will pull the two rims too close together, trapping and crushing the separating spacer band.

The only solution is for your maintenance people to check with the wheel manufacturer about the proper torque values for each wheel type and follow them religiously.

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