When a truck is aligned, it means that all the tyres are rolling in the same direction that the vehicle is travelling in. While that is not always as easy to achieve, as it sounds, it does pay to go straight.

Ideally, when a truck is travelling in a straight line, all of the axles are parallel – and perpendicular to the vehicle centre line – and all the tyres are rolling in a straight line, too.

What are the benefits of alignment?

Probably the one we think of first is reduced operating costs through longer tyre life. But that’s just the beginning. Not only do tyres on a properly aligned vehicle last longer, but some manufacturers suggest that there are significant improvements in fuel economy, component wear, even less driver fatigue.

Does the whole vehicle have to be aligned?

Ideally, yes. Many of us tend to think of alignment on trucks the way we think of it on our personal motor cars. And there, generally, we think only of aligning the “front end.” On a passenger car, that may be enough. But on a vehicle that has tandem axles, like a truck, aligning only the front end is only doing a small part of the job.

Why is that?

Because, while a misaligned front end on a truck can cause similar problems to one on a motor car, the other axles can have a huge effect, too. In fact, we’re going to look at alignment in order of which axle set tends to cause the most problems.

Where do we start?

Let’s begin by talking for a moment about vehicle “tracking.” In a perfect world, trucks would travel only in perfect straight lines from one location to another, never having to turn. And tyres, by their nature, would last longest it they only rolled in perfectly straight lines. 

If this were the case, as the truck went down the road, all axles would be absolutely parallel, and all would be perfectly perpendicular to the vehicle centreline. And this, incidentally, is one of the reasons long distance line haulers tend to have such long tyre life – because they travel in what amounts to mostly straight lines. There’s very little in the way of side forces acting to scrub rubber off their tyres.

But trucks have to turn.

Of course. And whenever a tyre runs, side forces act on it that cause the tread to wear. The trouble is, sometimes a tyre is turned, and it’s so slight that we barely notice. And that’s what alignment is all about, trying to remove even the tiniest turning side forces.

So which axle is most critical?

The drive tandem. Alignment experts tell us that this is the most important set of axles to have properly aligned, and the one that can cause the most trouble if it is not. That’s because even if all the other axles are correct, misaligned drives can force the vehicle to track improperly. And there are two basic ways that drive axles can be misaligned.

In one case, if both axles are more or less parallel but not perpendicular to the vehicle centreline, we have what is called a “thrust” angle problem. As the diagram shows, the drive axles are trying to push the vehicle away from the centreline.

If the axles are not parallel, the problem is described as a “scrub” angle problem. In this case, the drive axles are trying to turn the vehicle. Either way, to bring the truck’s travel back into a straight line, the driver has to turn the steering wheel.

Because the “thrust” or “scrub” forces from the drive axles are more or less constant, the driver’s steering input has to be constant.

How does this affect wear?

Because the steering tyres are constantly turned, slightly, this can cause them to wear more quickly. The drive tyres, which are constantly trying to go the wrong direction, may wear faster, too. And, because it takes more energy to turn than to go in a straight line, fuel economy can suffer.

Drivers may complain that the truck “pulls” to one side or the other. In fact, “pull” is an important clue to drive axle misalignment. In extreme cases, the pulling may even contribute to driver fatigue complaints because they’re “fighting” the wheel.

Tandem Misalignment Conditions

If drive axles are not parallel, the result can be“scrub” forces, causing the vehicle to turn out of straight-line travel 
Drive axles that are parallel, but not perpendicular to the vehicle centreline, produce “thrust” forces, pushing the vehicle out of straight-line travel.
Trailer axles may be misaligned in the same way as drive axles, producing rapid or irregular wear on all tyres, wasting fuel and requiring drivers to apply continual steering corrections to fight “wander.”

What about the trailer tandem?

Trailer axles can be misaligned in the same way that drive axles can. In fact, we use the same terms for trailer axle misalignment, “thrust” and “scrub”. The difference is that while misaligned drive axles tend to push the vehicle out of straight-line travel, misaligned trailer axles tend to drag the vehicle.

What effect does that have?

While misaligned drive axles often produce a steering “pull”, misaligned trailer axles often produce “wander”. The vehicle seems to track OK for a while after a steering correction, then drifts, first one way then the other.

How much of an effect can tandem misalignment have?

Let’s do the maths. Suppose that there’s just a 58mm misalignment between steer and drive tyres, and that the truck-tractor has a wheelbase of 460 centimetres.

If you locked the wheel and drove for 1 kilometre across a huge flat surface, like a salt flat or something, you’d end up one mile away and 11 metres to one side of you starting point. 

That’s equivalent to scrubbing the tyres across the road surface 11 metres per single kilometre. Over a year’s driving, that would amount to dragging the tyres sideways for about 1,100 kilometres! That’s going to produce some severe tyre wear, and it all comes from just 58 millimetres of misalignment. 

As we’ve seen so many times before, irregular wear comes from irregular abrasion. And with today’s long-wearing tyres, tiny irregular wear forces that persist over thousands and thousands of kilometres can add up to severe wear conditions. Good maintenance and proper alignment are critical to minimising these wear forces.