4 questions to ask before buying a used truck
Posted: 22-Jan-2020 |

You can't buy a made-to-measure suit at the Warehouse, nor can you spec' a used truck. In both cases, you're limited to what's on the rack or what the dealer has in stock. Most people wouldn't buy a suit that doesn't fit, but apparently a shiny truck can be more appealing than the one that fits the job.

Here are four questions every prospective owner-operator should be asking before they even set foot on the dealer's lot.

What will I do with the truck?
While that question might seem self-evident, it apparently isn't. More than a few first-time buyers seem to have little more than a vague notion of what they want to do. Some seem to be shopping for the next assignment as much as the next truck.

The trouble emerges if they go shopping with the notion that a truck is a truck, and that it will work just fine in a bunch of different applications. That thinking may apply if you're hauling general dry-van freight if you want to switch to flatbedding or hauling containers, but it won't work if you have a notion to take your long-legged highway truck and hitch it to an aggregate trailer or set of Super Bs. Even moving from a 40-ton tandem dry van to a 50-ton tridem dry van can have unpleasant consequences.You can't expect a stallion to do a plow horse's work. At least, not for very long.

How big does the engine need to be?
The answer to this question is absolutely application-dependent. Who wouldn't want a 550-hp/1,850 lb-ft engine to haul 80,000 pounds around NZ? Should you buy the biggest engine you can find? Not unless you want to accelerate your trip to the poorhouse.

Let's face it, if you have the horsepower, you're going to use it, and you certainly do not need that kind of power in a "light-duty" application. Fleets buy 450/1,650 engines for a reason. You should take a lesson from them.

On the other hand, if there's a B-train in your future, then you'll absolutely need big power. You do not want to under- or over-spec' the truck. To make the right decision, you need to be fairly certain what you plan to do with the truck.

Are the gear sets and axle ratios compatible with the application?
The trend in highway trucks over the past decade has been to gear-fast, run-slow — currently known as downspeeding. Consequently, transmissions are typically overdrive ratios of 0.87:1 or 0.73:1, with rear axle ratios of 3.36:1 down to 2.28:1. It's all about lowering engine rpm at highway cruise speed to conserve fuel. With heavier loads, tridems, quads and B-trains, a higher axle ratio is required (3.90, 3.70 or even 4.33) to keep the engine running at higher rpm where the higher horsepower can be found, and to facilitate startability and gradeability.

The bottom line here is that a coveted long-legged large-car may not get a loaded Super-B out of the parking lot, never mind pulling along SH1. And if you're thinking about changing the rear-axle gear sets, you're looking at over $10,000, especially if you're going up to a more rugged axle.

Are the axles, frame and suspension up to the task?
These components are essentially the truck. They are what supports the weight of the load and keep all the wheels pointing in the same direction. Since highway tractors are highly weight-sensitive, they tend to be spec'd with components that have a minimum rating. They are designed and built to work reliably for four to five years, or for the first owner's prospective lifecycle. If the first owner cared about having them last longer, he or she would have spec'd heavier components.

A highway tractor with five axles and an 80,000 lb. GVW will have rear suspension and drive axles rated for 40,000 lb. Some may be only 38,000 lb. in weight sensitive applications. Some highway tractors will have at least 44,000-lb. rear axles and suspension, sometimes 46,000-lb. Meanwhile, trucks spec'd for severe service with multi-axle trailers or trains usually have 46,000-lb.-or-higher axles and suspensions because they are under more stress while driving, and probably face a great deal more stress while navigating job sites, etc.

The axle rating (as well as tire width) determines how much weight can be loaded on an axle or axle grouping. If the equipment isn't rated for a certain weight, it will be fined for overloading.

The same applies to the frame and crossmembers. While frame ratings are not considered when determining vehicle loads at scales, obviously the frame must withstand the vertical and longitudinal loads that it faces. Lightweight, single channel highway truck frames aren't up to the challenge of 56,000 kg (123,000 lb.) multi-axle loads. Sure, they will pull the load, but they won't last long.

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