When you alter a vehicle´s ride height, you automatically will change the wheel angles, including camber, caster and toe.
Depending on vehicle suspension design, lowering the ride height will cause the wheels to either move in a positive or a negative camber direction. In some cases, the factory camber adjustment range may be adequate to obtain the desired wheel angle, but in other cases, you may easily run out of adjustment (this depends on the specific vehicle and the extent of ride height change). As a result, an aftermarket camber adjustment system may be required.
The toe angle also will change whenever ride height is altered. Depending on the suspension and steering design, lowering the ride height will cause the wheel toe angle to move in or out. Since the vast majority of steering system tie rods offer ample adjustment range, regaining proper toe angle should not present a problem. Bear in mind however, that toe must be checked and adjusted after ride height has been determined.
While the caster angle of the front wheel locations will change slightly -- lowering the vehicle may create a more positive angle, while raising may lessen the positive angle -- caster may or may not be easily adjustable, depending on the vehicle´s suspension design. For example, if the vehicle features twin-arm front suspension with adjustable upper or lower control arms, you may be able to easily compensate for any caster angle change. However, if the front suspension features a MacPherson strut design with a "fixed" caster angle, you may be stuck with the resulting change. In most cases though, the caster angle probably won´t change enough to create a drivability problem.
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ADJUSTING CAMBER
As we´ve already stated, if you alter ride height, you´ll change camber, caster and toe. Front-axle toe is easy to readjust and will not require specialty parts. However, if the vehicle features a strut-type front suspension, it´s very likely that aftermarket adjustment parts will be needed.
Generally speaking, there are two common methods of providing camber angle adjustability on a strut-equipped vehicle.
1. A common approach involves the use of offset, or eccentric, washers and indexable bolts at the lower strut-to-steering knuckle attachment on those vehicles that feature a two-bolt attachment between the strut and the knuckle. While the upper bolt/washer system may provide clamping force only, the lower bolt/washer system may feature eccentric washers.
The offset washers ride against a small flange adjacent to the strut´s lower mounting hole. The bolt will feature a flat-sided shank that keys itself to a flat-sided hole in the washers. As the bolt head is rotated, this moves the eccentric washer against the strut flange, forcing the bottom of the strut to move the spindle inboard or outboard, changing the camber angle as a result.
While most vehicles do not feature an eccentric system, the dealer may offer an eccentric fastener package as a "crash" repair part. Such eccentric bolt/washer packages are easily obtained through several aftermarket sources and are readily available. This approach may provide in the neighborhood of +/- 2 degrees of camber range, depending on the specific vehicle and the specific eccentric bolt/washer kit.
2. An alternate method involves the installation of an upper camber plate system. While more expensive to purchase and potentially more labor intensive to install, a plate system will offer easier adjustment and a potentially greater adjustment range. In addition, depending on the plate system selected, this also may provide a limited caster angle adjustment.
This type of adjuster installs at the top of the strut tower and will feature a multi-piece sliding plate system that allows inboard/outboard positioning of the top of the strut for camber changes (and may offer fore/aft positioning for caster changes). A series of bolts secure the plates to the strut tower. When loose, the plate system may be adjusted.
Once the desired angle(s) is achieved, the bolts are tightened to lock the adjusted setting. For serious performance alignment tuners, this approach offers distinct advantages, since camber adjustment is easily accessed underhood as opposed to undercar.
If the vehicle at hand features eccentric bushings at the upper or lower control arm pivot locations, this may allow enough camber adjustment range. If not, an aftermarket eccentric bushing with greater range may be the ticket.
For a street-only application, your goal should be to adjust camber, caster and toe as close to factory specifications as possible, regardless of your ride height situation. For competition use, the goal will be to obtain a full tire-to-road contact patch that matches the overall characteristics of the vehicle in critical turns on a specific track in order to turn the quickest possible lap times.
A word of caution: If the vehicle is equipped with upper and lower A-type control arms and performance aftermarket coilovers, you´ll need adequate room between the upper control arm and the coilover to be able to adjust the upper control arm fore/aft for caster adjustment. On some street rod applications (depending on brand of frame and suspension pieces), the upper control arm may be too narrow to allow enough caster adjustment at the upper arm.
Talk to the supplier to make sure that caster will be adjustable. On the other hand, the frame maker already may have designed the pickup points for the control arms to obtain a useable caster angle without the need for extensive adjustment. Naturally, when dealing with 100% custom components on a from-scratch build, many variables enter the picture that can affect suspension geometry.
Adjusting camber (and potentially caster) will differ greatly depending on the suspension design used -- coil-sprung A-arm setups, torsion bar systems, strut systems, etc. Check with performance-oriented aftermarket suspension suppliers to determine what systems are available for the specific vehicle. In most cases, suspension modifications to street production vehicles will involve relatively easy bolt-on procedures.
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CAMBER ADJUSTMENT PROS & CONS
Many street vehicles originally were designed to use just a bit of positive camber, with good reason. As the vehicle is loaded with driver and passengers, the suspension is compressed. As the suspension compresses, the wheel´s camber angle will change to a less-positive range. That´s automatic, simply due to the pivoting action of the suspension parts.
Since the auto maker´s objective is to let the tires see a full contact patch of tread to the road as the car travels down the highway, the car maker initially compensates with just a touch of positive camber angle; they assume that once the car is actually being driven in a straight line, the dynamic camber angle will be close to the perfect "zero."
In a performance situation, we use the same approach. Since our objective is still to see a full contact patch of tire tread to the road, we adjust the angle into the negative range initially.
When the suspension is under load in a severe turn, the tires that were at first mostly touching the ground on the inside shoulder, are now fully contacting the road surface during a hard turn. And since our real objective is to enter and exit a turn at a higher speed, we´re more concerned with making use of the tread in the turns as opposed to the straight line.
The downside of modifying the camber angle should be obvious. You increase the chance of unevenly wearing the inside tread areas during normal daily driving, and you increase the chance that the car will exhibit some excess road "wander" when driven in a straight line.
If you understand the trade-offs, you´re then able to make an informed decision. Ideally, the vehicle should be aligned to OE specs during the week and readjusted with more negative camber on the weekends for any "competition" use.
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TWO WHEELS VS. FOUR WHEELS
When you "align" the wheels on a vehicle, you are adjusting the wheel direction based on a point of reference. That point of reference is critical, since it provides the entire basis of your alignment work. If you only refer to the geometric centerline (the center of the rear axle), you´re not considering the thrust direction of the rear wheels at all, and that´s a huge mistake.
A thrust condition always causes the front wheels to steer into the direction of the thrust line in order to retain vehicle direction. If the front wheels were not adjusted parallel to the thrust line, constant steering input is needed by the driver, premature tire wear (on both front and rear) will result, and poor directional control is a constant.
When you only consider the geometric centerline of a vehicle as the point of reference for a front wheel alignment, you´re assuming that the rear axle is located where it´s supposed to be, and that´s a big assumption. The rear axle may be offset from center, or it may be "crooked," causing the rear wheels to point right or left.
In summary, performing two-wheel alignment is like trying to plot the shortest route from a specific address in Cleveland to "Europe" instead of to a specific city location. In order to create an accurate point of reference, you´d need a specific address in a specific country. In order to perform an accurate alignment, a geometric centerline alignment is not the answer.
THRUST-LINE ALIGNMENT
"Thrust line" alignment considers the actual location and direction of the rear wheels. Never assume that the thrust line is parallel to the geometric centerline. When adjusting the direction of the front wheels, the front wheel alignment should be set parallel to the direction of the rear wheels, in reference to the actual thrust line.
When aligning the front wheels on a vehicle that offers no rear wheel adjustment, setting the front wheels according to the thrust line is the only accurate method of front wheel alignment. The thrust line may not be parallel to the vehicle body, but at least the alignment technician can make the front wheels parallel to the rear wheels, and that´s the most important job. It´s called doing the best you can, given the limitations set by either poor OE vehicle assembly or previous vehicle damage.
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TOTAL FOUR-WHEEL ALIGNMENT
Total four-wheel alignment is possible on a vehicle that features both front and rear wheel adjustment. Once you know the thrust line of the rear axle, you can determine the "thrust angle." Thrust angle is found by comparing the geometric centerline to the thrust line. If the rear wheels are pointed to the right, the thrust angle is referred to as positive. If the rear wheels are pointed to the left, the thrust angle is negative. On a vehicle that offers rear-wheel adjustment for toe and camber, you can use the thrust angle information to adjust the thrust angle to zero (or at least near-zero).
A non-zero thrust angle can create a number of problems, including "dog tracking" (where the vehicle looks crooked as it travels straight ahead). A thrust angle that´s out-of-zero also can affect the turning angles of the front wheels, causing toe, camber and caster-related handling and tire wear (by the way, caster, indeed, can be a tire-wearing angle, contrary to the opinion of some).
As a driver continually applies steering input to compensate for the bad thrust line, the front wheels are not in the same position as they were during the alignment job. As the wheels are turned, camber and caster angles of each wheel begin to change slightly as the suspension and steering experiences its normal dynamic state that occurs during turns. This continual attempt to drive straight ahead with uneven (side-to-side) toe, camber and caster will create premature wear on tires and steering parts. The bad thrust angle causes the front wheels to be steered in-parallel to the rear, resulting in a false centerline steer. The steering input causes uneven front wheel angles during straight-line driving and the non-level steering wheel position. Thrust angle misalignment also can create handling problems, causing the vehicle to turn-in quicker in one direction.
In a perfect world, the alignment technician can adjust the rear thrust angle to zero (now parallel to the front axle centerline), and then adjust the front wheels parallel to the rear wheels. The result is a vehicle that travels in a straight line with minimal driver input, and level centerline steer.
Obviously, the front or rear toe will not always be set to zero, since OE specs must be followed in a static position (toe is set in or out to compensate for expected dynamic travel as the vehicle rolls down the road). The point is that all four wheels should travel in-parallel when the vehicle is in motion on a straight and level road.
On a vehicle with a non-adjustable rear, a four-wheel thrust line alignment approach always should be used. On a vehicle that provides rear wheel adjustment, a "total" four-wheel alignment always should be performed. A front-wheel-sensor-only alignment never should be performed on any vehicle, because it´s impossible to know if the front wheels are parallel to the rears.
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TIRE WEAR
Chassis misalignment is directly responsible for a host of tire wear problems.
Camber. On either front or rear axle positions, an off-camber problem will cause premature shoulder wear, as the incorrect vertical stance of the tire concentrates the scrub area away from the centered location of the tread. Excess negative camber exists when the top of the tire leans too far inboard. The result is an over-concentrated amount of scrub on the inside shoulder/tread area. When you inspect a tire with a smoothly worn inside tread area, suspect a negative camber situation.
By the same token, when excess positive camber is the case, the top of the tire leans outboard, and the excess wear will be concentrated on the outside tread/shoulder area.
Bear in mind that a camber-only misadjustment will result in a fairly uniform wear along the concentrated wear path. If a choppiness, roughness or scalloping is also evident, one or more additional factors such as worn shocks, loose control arm joints or bushings, loose strut to spindle bolts, etc., are at play. Excess camber, in either positive or negative direction, coupled with a low tire inflation will result in a more pronounced roughness along the wear path, especially at the edges of tread blocks that lie along the concentrated scrub area.
Toe. Equal but incorrect toe settings, causing both steer tires to roll ahead at an angle that´s non-parallel to the vehicle thrust line, will cause a "feathered" wear across the tread face. This results in angled wear across the tread blocks that create a slightly raised tread block edge in the direction of the excess toe. In the case of toe-in, the raised feathered edge will exist toward the inboard edge of the tread blocks; with toe-out, the raised edges will be created on the outboard side of the tread blocks.
In the case of unequal toe, it´s automatically assumed that the tire featuring the greatest out-of-toe setting will wear irregularly. For the sake of example, let´s say one wheel is set at zero toe and the other at one-quarter-inch toe-out. Even though one wheel is at zero, both tires will wear badly.
Since one wheel is out beyond spec, the driver will compensate by steering against the directional pull in order to steer down a straight road. As a result, the tire that was set at a static zero is being dragged off-line to follow. In addition to individual wheel toe, the "total" toe is the more important condition to consider.
The positioning of the rear axle will directly affect the wear on both front and rear tires. If a rear axle is crooked, a new and incorrect thrust line is created.
Caster. Although not commonly targeted as a tire-wearing angle, improper steering axle caster can affect wear in combination with other factors. If the caster is too positive, there may be more of a tendency to wear the tires´ outside shoulders during severe-angle turns. Also, if caster is excessively unequal, this condition will create a vehicle directional pull that will force the driver to compensate by countersteering.
For example, if the right steer wheel position is more positive than the left, the vehicle will tend to pull/wander to the left. As a result, the driver will be forced to steer slightly to the right in order to maintain vehicle course. This continual compensation-steering can promote a feathered-edge wear across the tread face, similar to that of toe-induced wear. In other words, if you notice a feathered-type wear across the tread face, don´t automatically assume that an incorrect toe setting alone is the culprit.
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PRE-ALIGNMENT CHECKLIST*
1. Test-drive the vehicle.
2. Check vehicle ride height and correct as needed.
3. Check and correct tire inflation.
4. Inspect tire condition. Replace/rotate as needed. If tires or wheels are unidirectional, follow proper rotation procedure!
5. Check tire sizes. Brand, model, must be same at all four wheels. Sizes must be same on each axle.
6. Inspect steering components for looseness, wear and damage, and for improper installation.
7. Inspect suspension parts for damage and wear.
8. Inspect the brake system for potential causes of vehicle pull as well as an overall system check. Check calipers, hoses, hard lines, master reservoir, etc.
9. Check driveline items such as driveshafts, U-joints, CV joints, etc.
10. Be aware that if you´ve recently had engine or transmission work performed on a front-wheel drive vehicle, the engine mounting cradle may have been reinstalled incorrectly, which will cause a pull.
* If the vehicle is equipped with an electronic leveling system (which lowers the vehicle at high speed and raises it during low speed, etc.), it´s necessary to read the shop manual for the correct pre-alignment procedure.
ALIGNMENT ANGLE TIPS
1. If the rear wheels are not adjustable, at least establish the rear axle thrust line, and align the front wheels according to this direction of rear wheel travel.
2. If the rear wheels are adjustable, adjust individual rear toe to create a zero thrust angle (in line and parallel to the geometric centerline).
3. Left and right camber should not differ by more than one-half degree. This applies to both front and rear axles.
4. Left and right caster should not vary by more than one-half degree.
5. Do not assume that the front toe on a front-wheel-drive vehicle should always be set out (negative), or that front toe on a rear-drive vehicle should always be set inboard (positive). OE designs in recent years have created suspension systems that require just the opposite in many cases. Don´t assume that a "generic" spec will work on all vehicles. Always check the OE specs!
6. Both thrust line and thrust angle is easily measured using a four-wheel alignment system. By reading individual rear toe and total rear toe, the alignment machine performs these calculations for the technician.
