It’s probably good that buyers are not usually directly involved in spec’ing trailer axles and suspensions. They make recommendations and requests, sure, but the heavy lifting usually goes on between the trailer manufacturer and supplier of axles and suspensions. Calculating ride height, ensuring frame clearance for all the bits that attach to the axle, and in some cases working out steering geometry is not for the technically challenged.

“As an axle manufacturer, most our specifying is done with the trailer manufacturer,” says vice president – sales and marketing for Eveley International, Geoff Williams. “Design prints are generally exchanged for fit and function to make sure everything is going to work together, taking into consideration the trailer design, trailer frame dimensions, tire size, suspension manufacturer, jounce and rebound of suspension, turning radius, axle capacity, and making sure that local laws are adhered to.”

Many fleets will lean toward certain axles and suspensions because they work in the current application. Chances are, however, that a new trailer will be different than the older trailer it’s replacing. As new business is added and new trailers are required, that old fleet spec’ may no longer be the best option.

The place to start is understanding how the trailer will be used, says Bill Hicks, director of product planning – Americas at SAF-Holland. Spring or air ride? Tire and wheel combinations? Drum or disc brakes? “Many on-highway dry freight vans still have a substantial mix of air versus mechanical springs,” he says. Disc brakes are starting to creep into the picture, too. “You also have to consider frame attachment criteria and suspension ride height, though that can be refined or even predetermined by the trailer builder, depending on the trailer’s intended use.”

When it comes to the axles themselves, customers should consider the application. Such things as axle wall thickness, axle capacity, spindle ends, tapered or parallel, track width, and specific applications, suggests Williams. “Many component manufactures are now directly calling on fleets to promote products such as brake chambers, slacks, seals, tire inflation systems, etc. Most of those requests can be accommodated as the axle is dressed.”

Heavy matters

A variety of products are available when spec’ing for a generic U.S-type operation. Most of it is pretty similar except for -special features to shed weight or ease maintenance, among other things. But when you get into multi-axle trailers for heavy loads, trailers with steerable/-liftable axles, or specialized configurations, there is more to think about.

Consider tire spec’s as an example. “On a steerable axle, a single tire will get you a better wheel cut, something in the range of 25 to 28 degrees, whereas a dual

assembly would be limited to about 20 degrees,” advises Chad Brown, program manager, HT and steerable suspension systems at Hendrickson Trailer Commercial Vehicle Systems. “Depending on the position on the trailer, the more generous wheel cut can save some tire scrubbing and stress on the axle in turns.”

Tire size can affect the choice of axle and suspension as well. A “super-single” tire (as opposed to a fuel-efficient wide-base tire) is several inches taller, so the suspension travel must be matched to it. Switching back to duals at a later date could cause damage.

“A smaller tire might cause the airbags to over extend,” says Paul Brown, marketing manager, Hendrickson Specialty Products – Auxiliary Axles. “Conversely, a larger tire might not give the ground clearance the customer wants when the axle is lifted, say from four inches down to 1.5 inches. Tire choice is often determined by the frame-to-ground distance because we have to match the ride height of the suspension to the tire.”

The height of the tractor’s fifth wheel can also affect suspension travel, especially on the forward-most axles.

So too can tire choices affect the wheel cut on a steerable axle and the placement of components such as tie rods and brake chambers, says Brown. “We can do a preset from 20-30 degrees of steer angle. The farther forward you mount the axle, the steeper wheel cut you need, but you still need the frame and component clearance.”

Trailer suppliers will probably offer a choice of axle and suspension systems, but it might not hurt to consult directly with the manufacturer of such systems.

“Fleets want to maximize their payloads and it’s up to the trailer manufacturer to figure out how to fit a steerable/liftable axles system under the trailer,” says Neil Haslam, head of design engineering at Ingersoll Axles (IMT). “We work with the builder on how to install the axle and design the trailer around it.”

When spec’ing an axle and suspension system for a generic van trailer there are fewer engineering factors to consider than when spec’ing a trailer with a steerable or liftable axle. Haslam says customers need to weigh the pros and cons of single versus dual tires, the desired suspension style, and even the desired turning angle of the axle.

“You have 102 inches to play with under there,” he says. “It’s our job to make everything fit and to work properly. If you get it wrong upfront, it’s not always easy to correct a mistake.”