Vehicle Pull, Alignment, and Uneven Wear

We talk to people all the time that think that because their car doesn't pull to one side the other, that it therefore must be in alignment. Sadly, this is not a reliable test. A vehicle can have a pull and wear the tires just fine, or it can drive perfectly straight and wear the tires out in a hurry. In this article, we will discuss the common causes of vehicle pull - alignment or otherwise. To do this, we'll divide this article into three sections:

• Alignment And Vehicle Pull
• Uneven Wear And Vehicle Pull
• Other Causes Of Vehicle Pull

Alignment And Vehicle Pull

Vehicle pull can be caused by misalignment, but it can also be caused by uneven wear on the tires, a low tire on one side, or a tire separation. Throughout this article, we will be referring back to alignment and the specific kinds of uneven wear it causes, but in this section we are dealing with vehicle pull, and we'll seek to explain our opening statement.

Alignment has three main parameters: Camber, Caster and Toe-in.

Camber is the tilt of the tire from top to bottom. If the tire tilts in at the top (towards the vehicle), it has "negative" camber. If it tilts out at the top (away from the vehicle), it has "positive" camber. Typical camber on modern vehicles is from 0 to 2 degrees negative (in at the top), while many classic cars (50's to 70's) used 0 to 1 degree positive camber (out at the top). Camber can cause a pull, but it doesn't do it by being extremely negative or positive, but by being different from side to side. If camber is the cause of your pull, it will always pull to the side with more camber (from negative to positive). If your car has .5 degrees negative camber on one side and 1 degree negative camber on the other side it will pull to the .5 degree side (since -.5 is more than -1). Or if it has 1 degree positive camber on one side and .5 degrees positive camber on the other, it will pull to the 1 degree side (since 1 is more than .5). However, a difference in camber side to side will not result in tire wear, only extreme amounts of camber, negative or positive, will cause tire wear, but this wear will be relatively slow.

Caster is a little harder to explain, but it has to do with the angle between the ball joints, or on a strut car, from the top of the strut to the lower ball joint. You can think of this angle like the forks of a bicycle. Bicycle forks angle out towards the front at the bottom, (or in towards the vehicle at the top) which is "positive" caster. Positive caster is what allows you to ride your bike with no hands. It is no different on four wheel vehicles. If you don't have enough positive caster (forks too little angle toward the front) the car will wander and feel unstable. If you have too much positive caster, the vehicle will be harder to turn. However, just like with camber, vehicle pull is not caused by too much or too little caster, but by side to side imbalance. As far as we know, all four wheel vehicles are designed with positive caster (forks out towards the front at the bottom). So we are talking about more or less positive caster. Camber may pull toward positive (more tilt in at the top) but caster will pull toward negative, or less caster. So if you think of the two front wheels like a pair of cycle forks spread out at a distance, more positive caster on one side would mean that one of the forks of the "bike" was further forward than the other, (though this is really just an angle measured in 10ths of degrees). So the vehicle would pull to the side with the "fork" further back. Again, caster, like camber can cause pulls, but caster will rarely cause much wear. (Note: caster is only significant on wheels that turn, so unless your vehicle has rear wheel steering, caster will only be adjustable in the front.)

Toe in is just like it sounds. If you look down at your feet and your toes are farther out than your heels, this simulates "toe out", if your toes are farther in than your heels (pigeon toed) that simulates "toe in". Obviously, you can have toes and heels even (zero toe) or you can have one pointing out and one in. However, even if one is in and one is out, you will simply turn the steering wheel to go straight down the road. Therefore, toe in (or out, or both) can definitely make your steering wheel crooked when driving straight, but it almost never causes a pull. On the other hand, though it may not cause a pull, it is the single greatest cause of premature tire wear. With enough of an error in toe your tires can wear out in a real hurry, the worst we've seen is 3 days on brand new tires! And 5,000 miles on brand new tires is not at all unusual, even though the vehicle may track perfectly straight down the road!

So: Camber and caster will cause your vehicle to pull if they're uneven, but will cause little tire wear, while toe in won't cause your vehicle to pull, but can cause extremely fast tire wear. This is why your vehicle can pull and not wear out tires, or it can drive perfectly straight and destroy them in a hurry!

Uneven Wear and Vehicle Pull

We have to preface this by saying that most uneven wear is caused by misalignment. However, if you align the car and the tires still have uneven wear, the vehicle will often still pull. This is one of the reasons that there has to be a distinction made between a pull caused by wear and one caused by alignment. Granted, the alignment may have caused the wear, but then the wear can cause the pull.

There are two main types of uneven wear that can cause your vehicle to pull: Cupping, and wear on one side. "Cupping" is a term for a wear pattern where the tread blocks along the circumference of the tire have high and low spots. One type of cupping can be caused by rear wheel misalignment of front wheel drive cars. Another, more common type of cupping is the norm for the front on four wheel drives and some commercial vehicles. The second type is also seen on some passenger cars but is less common. In either case this type of wear can definitely cause the vehicle to pull. In most cases, the only thing that can be done for this type of wear is to keep the tires rotated. Granted, this can cause a shake also, but that's a different article.

The other common type of wear that can cause a pull would be wear on one side. This is caused by alignment or inherent in the vehicle design. What happens here is similar to what happens when you roll a styrofoam cup on the ground, it rolls in a circle. In this case, unless it's an inherent design issue, alignment is necessary to correct the wear problem. However, aligning the vehicle will only keep the tires from getting worse, if you don't replace the tires or rotate them to the back, the vehicle can still pull after it's been aligned.

A separated tire will also cause the vehicle to pull if it's on the front. Though technically this is not uneven wear, it is effectively the same thing. A separated tire is when the layers of the tire begin to de-laminate or "separate". There are two type of separations, sidewall and tread. A sidewall separation is when there is a bulge on the side, usually running from the rim to the tread face. This is almost always caused by some sort of damage, and not usually a defect. This can cause a pull but won't always. The tread separation is usually only seen after the tires get old or worn and is usually a defect, but can also be caused by damage. The most common type will be a bulge along the side of the tread face in one spot. When this happens on the front, it will push the vehicle away from the separated side of the tire. (So even if the bad tire is on the left side, it can still pull to the right.) As a side note, the warning sign for a separated tire will be vehicle wobble (front or rear) at 15 to 20 MPH.

Tire Cupping

Rear Wheel Misalignment Wear

Wear On One Side

Tire Seperation

Other Causes Of Vehicle Pull

Possibly the most common other cause is having one low tire. Obviously, a leaky tire should be fixed and that should correct the problem. Most of the other causes of vehicle pull are fairly obscure. The only one we seen more than a few times is a sticky brake caliper. We've even seen the brake calipers act up only when they get real hot. However, the main point here is that vehicle pull can be caused by various mechanical issues, so tires and alignment aren't the only causes of pulls, just the most common.