What’s Caster Got to Do With It?

Experts tell us that caster is the most difficult of the three major alignment angles

to understand and that improper caster settings are less likely to be felt by drivers of today’s front-wheel-drive vehicles.

Of course, then they say irregular tire wear cannot be directly attributable to caster, either.

Sounds like five cups of black coffee and real butter on your toast is really OK. But it isn’t. While caster is an elusive subject, the job is to fix improper caster settings when you find them.

By now you’ve all seen the ubiquitous grocery cart demonstration showing the effect of positive and negative caster. Push the cart forward and it rolls easily, pull it toward you and every egg in the cart begins to crack.

The grocery cart example is a good one. But for an ultimate demonstration of what happens when a massive dose of positive caster is at work, consider the road grader. There is so much positive caster built into these behemoths that the front tires actually tilt over (camber roll) on their sides as they work. Road grader manufacturers designed them this way so they would have enough side force to allow the blade to push dirt.

And, by design intent, vehicle manufacturers have built in 2⊄ to 6⊄ of positive caster for most vehicles – and up to 10⊄ for such vehicles as twin I-beam light trucks and Mercedes-Benz vehicles.

Defining ‘Caster’

This is where a definition of caster would be helpful. Caster is defined as the forward or rearward tilt of the steering axis when compared to an imaginary vertical line (drawn through the pivot points) as viewed from the side. Caster is measured in degrees and plays a role in determining both steering feel and high-speed stability.

Positive caster occurs when the imaginary steering axis line intersects the ground in front of the tire contact patch. Negative caster is when the line intersects the ground behind the contact patch.

Here’s a good example: All bicycles have lots of positive caster. Sight down the frame tube that holds the fork in place. Then extend an imaginary line from that tube to the ground. Notice that it intersects the ground well ahead of the patch.

What does this mean? Simply put, the longitudinal forces acting on the tire’s footprint are located in front of the tire contact patch. Push the bicycle forward without using the handlebars and it’s stable. That’s positive caster. Pull it backward and it becomes unstable and wants to turn. That’s negative caster.

These principles hold just as true for a vehicle. It’s why you will seldom find negative caster built into the front suspensions of modern day vehicles.

Explained in everyday language, caster easier to understand. Think back to the 1973 Monte Carlo, said to be the first radial-tuned suspension on an American-made automobile. Because radial tires did not have the same kind of self-centering feel provided by bias ply tires, more positive caster was designed into the suspension at the factory to improve responsiveness and self-centering or on-center-feel. Factory-added caster also allowed vehicles to handle better.

During the past 30 years car makers have gotten better at it, to the point that 21st century drivers float along with almost no sense that their vehicle may have an alignment problem caused by caster.

That’s why you are more likely to hear about handling complaints from vehicle sensitive purists who enjoy driving, who love their cars and appreciate good handling. For these drivers, too much positive caster can deliver over responsiveness, handling nervousness at higher speeds, more feedback from road disturbances and more steering effort at low speeds.

Positively Positive

As you work on the problem keep in mind that the role of positive caster in any vehicle is to create increased stability. Proper caster angles allow the vehicle to handle properly.

Positive caster causes the front wheels to turn-in equally. If you send a vehicle back out on the road with more caster on one side than the other, the vehicle will tend to pull toward the side with the least amount of caster.

In other words, if the right front is set at 4⊄ positive and the left front at 3⊄ positive the vehicle may pull to the left. Alignment specialists call this caster pull. Your customers call this lousy handling.

Metal Bending Causes

Is there evidence of something the experts call "set back", a metal bending experience? This cannot be detected by eyeballing a vehicle and must also be considered in the total alignment process.

Excessive set back may be caused by hard, metal-bending contact. The lower control arm may have have been banged into a curb and was possibly damaged, or the lower portion of a strut may have been damaged or bent by hard contact. Even the engine cradle may have shifted slightly.

In any case, set back may result in a negative caster condition because a wheel, left or right, was pushed back. Result: The wheelbase is now shorter on one side of the vehicle than the other. Too much negative caster may cause the vehicle to wander and cause the steering experience to become mushy.

The fix in these cases will require some new parts to get the vehicle back to spec. Perhaps the cost will make your customer a little more careful on the road.

Driver Feel Considered

The driver of a Corvette will be far more sensitive to caster pull than the driver of a Buick LeSabre. These are two entirely different types of cars and vastly different driver personalities.

Some drivers aren’t sensitive about the handling of their vehicles. Over time they become accustomed to an out-of-alignment condition. As their vehicle slowly goes out of alignment, they don’t notice it until the situation is so bad they show up on your doorstep with four oddly worn tires.

An obvious fix in these cases is to dial in the caster – and four new tires. But what if the vehicle has a non-adjustable suspension and you can’t adjust caster?

The quick answer is to call one of several aftermarket suppliers and order the part that will allow you to adjust the suspension properly.

Tire Wear Cause?

Ordinarily, caster is not the culprit when tire malwear is involved. However, more positive caster creates more negative camber (camber roll) on the outside wheel and more positive camber on the inside wheel. This can cause excessive shoulder wear. Here’s where and why.

Think back to the road grader example. Unlike a passenger car or light truck, a road grader doesn’t experience body roll because it doesn’t have a spring type suspension. However, because cars and light trucks do experience body roll during cornering, the job at hand is to counteract the effect of camber changes created by body roll.

Keep in mind that while negative camber compensates for body roll, the outside shoulder of the inside tire can be dragged along and may tend to wear faster. While this is not the primary focus in determining the cause of poor tire wear, it’s worth noting.

Getting It Right

No question, alignment is a real balancing act. You and your customer are after the same things: optimum vehicle handling and long tire life. How you get to this point is in the alignment angles.

Caster is related to all of the controlled directions that a vehicle is asked to move at the same time, making it a key driver comfort/tire issue. Caster angle is part of what total alignment is all about. It cannot be left out of the equation any more than a blood pressure check can be left out of your annual physical.

The good news is that the methodology for determining caster angle is readily available from a number of wheel alignment equipment manufacturers.

Determining what angle is best for your customer and their driving needs, well, that’s all up to you.