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1 INTRODUCTION TO WHEEL ALIGNMENT A SHORT COURSE ON WHEEL ALIGNMENT, FRONT AND REAR PREPARED FOR THE N.C.R.S. NATIONAL CONVENTION JUNE 29 TO JULY 5, 2012 by: JOE CALCAGNO, RARE CORVETTES rarecorvettes.com, joe@rarecorvettes.com, (831) Pacific Time Zone CAMBER: Camber refers to both the front and rear wheels of a vehicle. Camber is simply the tilting outboard or inboard of the top of the wheel when viewed from the side of the vehicle. If the top of the wheel is tilted out, away from the center line of the car, it is said to have positive camber. If it is tilted in at the top, it has negative camber. Camber is measured in degrees from vertical. If you were to stand out in front of a car which is sitting on level ground, about thirty feet away from the car, looking at the front of the car, you will usually be able to see if the front wheels are tilted inboard or outboard at their tops. All things being equal, a vehicle will pull to the side with the most positive camber. Excessive camber, either positive or negative will cause tire wear. Positive camber in excess will cause wear on the outside shoulder of the tread. Negative will cause the inside of the tread to wear. Buckboards, pulled by horses had positive camber, and it is easy to see. This helped keep the vehicle tracking straight on the roads, just as it does today on most modern vehicles.

2 CASTER: In order to understand the issues involved in front wheel alignment, one must think of the front wheels turning as if pivoting about an imaginary line passing vertically through the steering pivot components. That line went directly through the vertical center of the king pin on early design steering systems. Later, that same line went vertically through the centers of the upper and lower ball joints. Still later, with the introduction of Mac Pherson strut type front suspensions, that same line is used as the measurement line for the caster angle. Caster is the angle of this aforementioned pivot line, in degrees from true vertical, when viewed from the SIDE of the vehicle. Keep in mind that we are viewing the vehicle from the side when considering caster. (Later, we will be looking at that same line, but from the front of the vehicle, and it is a different component of wheel alignment.) Caster is used to stabilize the steering at speed. Wander will result if a vehicle designed for positive caster is set negative on both sides. Tire wear is not an issue as it pertains to caster angles, however, improper caster angles can cause the vehicle to pull to one side. That tendency is used by expert alignment people to help adjust for issues such as the crown of the road. As an example, here in this country we drive on the right side of the road, and the roadway is most often crowned to allow for storm water drainage. Driving on the side of this crown will cause the car to drift off to the right. Caster, and also camber angle adjustments are often used as remedies for issues such as this.

3 TOE: Toe is a very important issue in wheel alignment. If the toe measurements are out of specification, the tire wear will be the most severe of any wheel alignment setting. To visualize toe settings, we must view the wheels from above, as illustrated in the following diagram. Toe is simply a measurement of the difference, usually measured in fractions of an inch, between the forward and the rearward parts of an axle pair of wheels when measured at hub height across the car. In other words, if you look at a wheel from the side, and you visualize the 3 O'clock and 9 O'clock positions, that is what I call "hub height" on a wheel. Simply using a tape measure, measure the distance, at hub height across the axle at the forward most points and then at the rearward most points. If a car has zero toe, the measurement would be exactly the same when you compare the front measurement of the wheel to the rear measurement. Zero toe means that the two wheels are exactly parallel with one another. If the measurement is less at the front of the wheel than at the rear, it has toe in. If the measurement between the wheels is less at the rear of the tires, at hub height, the vehicle has negative toe, or toe out. Tires that have been on a vehicle with excessive toe misalignment will have severe tire wear. That wear can be manifested in several ways: one of which is a scalloped result where you can feel sharpness between tread grooves if you pass your hand over the tread from side to side. As you pass your hand over the tread in one direction, let's say from the outer edge of the tread to the inner edge, you may feel sharpness; because the tread has been lifted on the edge of the raised tread grooves. As you pass your hand in the opposite direction, you will feel only smoothness because, in that direction, the tread has been worn in the opposite way, allowing your hand to glide smoothly across. Also, cupping of the surface of the tread is common on vehicles that have been operated with excessive toe in or toe out. Usually, toe in will not be obvious to the person operating a vehicle because the tires will simply scrape along, all the while scuffing off rubber. On the other hand, toe out will often cause the vehicle to wander; that is, the car will head off to the right, and, as you finally get it back on track, it will then go off to the left. As you drive, you have to constantly correct for the wander.

4 That's because the car is following one wheel and dragging the other one sideways. After you correct it, it then follows the other wheel. Wander, no matter what the cause, takes all the fun out of driving, and it kills tires. Also, it is dangerous because it is difficult to control the car and keep it in the lane. STEERING AXIS INCLINATION: As I mentioned earlier, in order to understand the issues involved in front wheel alignment, one must think of the front wheels turning as if pivoting about an imaginary line passing vertically through the steering pivot components. Here again we are looking at that same vertical line, but with STEERING AXIS INCLINATION, we are looking at it from the FRONT of the car instead of from the side as we did when considering caster. Steering axis inclination (SAI) also called king pin inclination (KPI) and also called ball joint inclination, is a measurement, in degrees, of the steering pivot line we talked about earlier, through the upper and lower pivot points and compared to true vertical. This feature is important for several reasons, but it is most noted for its result of slightly lifting the front of the vehicle during a turn, which, because of gravity will cause the steering wheel to return to the straight ahead position when released after the turn is complete. Note that the spindle is driven down into the road surface during a turn. Since it cannot go down due to the road, the car lifts slightly. This same thing happens with bicycles. That is the reason that one can ride with "no hands". The bicycle will stay in the straight ahead position because of the rider's weight. SAI is not adjustable. It is measured only to check the condition of components. If it is out of specification, there is most likely a bent or badly worn part.

5 INCLUDED ANGLE: The included angle is the angle measured when we take the steering axis inclination and add the camber angle. Remember, we are looking from the front of the vehicle to measure SAI, and also when considering included angle. If the camber is positive, the wheel out at the top, the included angle will be greater than the SAI by itself. If the camber is negative, it will cause the included angle to be lessened as degrees will be subtracted from the SAI. This angle is not measurable. It will be affected negatively through the use of incorrect, or reversed wheels, or through the use of the wrong size tires. SCRUB RADIUS: The scrub radius is simply the measurement, on the surface of the road, between the intersection of the SAI and a line measured vertically through the center plane of the wheel. If the tire center is outboard of the intersection of the SAI on the surface of the road, it is said to have a positive scrub radius. The opposite, or the center point of the tire occurring inboard of the SAI point would indicate a negative scrub radius. This measurement, if not equal from one side to the other, will cause the vehicle to pull to one side. Rear wheel drive vehicles usually have a positive scrub radius while front wheel drive vehicles will usually have a negative scrub radius. This is not an adjustable measurement. The scrub radius is used to help make turning easier by walking the tire

6 around a small circle instead of making it twist in its own foot print during a turn while standing still. TOE OUT ON TURNS: When turning a corner, the outside front wheel describes a larger circle than does the inside wheel. For this reason, the steering is designed to open the toe settings during turns. The steering arms are designed to do this automatically during every turn. The measurements are given in alignment specification books, and are checked, using the degree markings on the turn plates which are part of the alignment equipment. The toe out on turn angles are not adjustable. If they are out of specification, there is probably a bent steering part that needs to be replaced prior to completing the alignment.

7 RIDE HEIGHT: The riding height is a very important issue as it is a test of the condition of the vehicle suspension, especially the springs. If you think about the design of the front suspension, you will note that the control arms are different lengths. That is, the upper control arm is shorter than the lower one. For this reason, you can visualize that the wheel tilts in and out at the top as it moves up and down while going over bumps in the road. Remember, we adjust camber, the inward and outward tilt of the top if the wheel, as one of the primary adjustments of wheel alignment. The ride height is the place in the arc of motion where the wheel comes to rest. Therefore, it is directly affected by the condition of the spring. The spring dictates where the wheel ends up in the arc, therefore, it dictates the camber of the wheel when it is at rest. For this reason, the spring is extremely important in the overall geometry of the suspension and steering systems. The ride height is usually measured in inches from the road surface to a given point on the body or chassis. It is not adjustable except on vehicles with torsion bar suspension, as those springs can be adjusted. On all vehicles with coil or leaf springs, the springs, and their bushings and mounting points will have to be brought into specifications in order to achieve the correct ride height before the alignment is done. It is the ride height that establishes the basis for the angles in the alignment. It is not possible to get the vehicle into proper specification with springs that cause the suspension to rest at a lower or at a higher point than that which was intended. THRUST ANGLE: The thrust angle is the angle that the rear wheels are pointing relative to the center of the car. If the vehicle has adjustable rear wheel toe, it is important to first adjust that before going to the front wheels. While adjusting rear toe settings, the thrust angle will be brought into line with the centerline of the vehicle. The adjustment of the thrust angle will enable the front wheels and the steering wheel will be centered. The rear wheels on Hotchkiss design rear axles, or solid tube rear axles, are not adjustable, so the thrust angle needs to be checked to see if it is acceptable before going forward. Sometimes, bent or broken rear springs, damaged spring bushings or mounting perches, or a bent axle housing can cause the rear axle to track to one side. If that is found, repairs must be made or the vehicle will "dog track" down the road. Modern alignment equipment is designed to measure the thrust angle because it is so important to the outcome of a

8 front wheel alignment.