Saturday, December 16, 2017 Detailed Auto Topics
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We check and correct three main angles when doing a wheel alignment. These angles are toe, caster and camber. The toe angle will wear tires quickly if it is not correct. Setting the camber and caster angles helps the vehicle drive straight and helps with tire wear. Adjusting each angle to specifications is not enough. To provide a proper wheel alignment, we must also understand how these angles interact with each other.

Point one 

Wheel alignment machines do NOT align vehicles. A properly trained technician is necessary

 

When engineers design a vehicle, they provide specifications for caster, camber and toe. Setting the wheels to these specifications is important, but alone will rarely result in a properly driving vehicle. Unless we understand the relationship among these settings, simply adjusting caster, camber and toe may not stop wear or make the vehicle drive properly. 

What is wheel alignment camber?

Positive and negative camber, front and rear

To help understand camber, imagine standing in front of the vehicle and looking straight at the tire. If the tire stands straight up and down we say it has zero degrees of camber. Zero camber means the wheel does not lean in or out at the top.

Camber is always expressed in degrees. If the wheel leans in at the top (toward the center of the vehicle) we say that it has a negative camber. A tire that leans out at the top has a positive camber. We use the same term whether the tire is on the front or rear of the vehicle.

They provide specifications for static camber in a range of degrees. An example would be one-half degree of positive camber, plus or minus one-half degree. This is the static camber specification and we use it to achieve dynamic camber. We set wheel alignment to static settings. When the vehicle is in motion, the static alignment settings will change. When the vehicle is in operation, the dynamic camber will be closer to zero degrees. This is due to dynamic factors built into the suspension.

A camber of zero degrees is best for tire wear.  With a zero degree camber, we spread the load evenly across the tire tread. The load is placed on the inside the tire tread with a negative camber. Using a positive camber shifts the load to the outside. The specification engineers give us, allow for this. With a slight positive or negative static camber, they attempt to achieve near-zero rolling camber.

Camber and vehicle pull 

 

Camber causes a pull toward the positive wheel

Improper camber will wear our tires, but it will also affect handling. With other factors equal, a vehicle will pull in the direction that has the most positive camber. For instance, with a camber of zero degrees on the left wheel and one degree positive camber on the right wheel, produces a pull to the right.

zero camber is more positive than negative camber

Even if the camber is not positive on either side, it will still pull toward the side that is more positive. An example is, if the right wheel were negative one degrees and the left wheel was zero degrees. Zero degrees is more positive than negative-one, so the vehicle veers to the left.

camber forms a cone and thus pulls toward the most positive side

Camber causes our vehicle to pull because of conicity. This means we form a cone if we consider the intersection of the centerline of the wheel and the surface of the road. If we roll a cone, it moves in a circle. The same is true with the cone we form with positive camber. Our vehicle attempts to roll is a circle around the intersection of the wheel centerline and the road.

As an example, suppose the specification given is one-half degrees positive camber plus or minus 0.5 degrees. If one wheel is set to zero, and the other wheel is set to one degree, technically the vehicle is "in specifications." Such an alignment will not drive straight. The relationship of camber side to side must be correct.

Slight side to side variation, within specification, is useful to compensate for road crown, torque steer and passenger loading. This is where a skilled technician custom-tunes the wheel alignment to achieve a straight drive. Because camber wears tires, we use great discretion between directional adjustment and tire wear.

Dozens of factors determine the preferred camber setting. The ideal camber setting is within the specification range, but may vary from one vehicle to another. Observing the wear on the old tires is very valuable, before we set the wheel alignment with new tires.

Point two 

Buying tires and having the alignment set elsewhere is a bad idea.  Seeing the wear that exist, before deciding on the best possible settings is one of a technicians most powerful tools

If we do not trust a tire vendor to align the vehicle properly, we cannot trust them to properly install tires or to balance them properly. 

Tire wear is not corrected by wheel alignment

once wear is establish on a tire, alignment will not correct it 

Setting the wheel alignment on a vehicle with worn tires is of very little use. Radial tires conform to the road. If the shoulder of a tire is worn, it will continue to wear though the camber is corrected. We must align a vehicle properly the first time. Once an improper alignment causes tire wear, it will continue.

Point three 

Camber and toe wear that is established will continue, though camber or toe are corrected

Wheel alignment caster

The caster is a bit harder to visualize than camber and toe. The caster is the angle formed by a line through the pivot points of the wheel, compared to vertical. We think of the caster angle, viewing the wheel from the side. If the upper pivot point is directly above the lower pivot, the wheel has zero degrees of caster.

With a positive caster the upper pivot point is closer to the rear of the vehicle than the lower pivot. A negative caster is the upper pivot closer to the front of the vehicle than the lower pivot point. We set nearly all modern vehicles with a positive caster. In the past, they use a negative caster on a few vehicles, to lower steering effort.

wheel alignment caster is similar to a caster on a cart

A positive vehicle caster angle works similarly to a caster wheel on a cart. The pivot line of the wheel contacts the floor, in front of the wheel’s centerline. With the positive caster angle, the wheel follows the direction in which we push it. This causes the wheels of our car to track straight and return to center when turned.

Why a negative caster pulls

Tilting the axis of pivots on the spindle, causes the end of the spindle to rise or fall when we rotate it. When we turn the wheel out from the center, we push the weight of the vehicle up. By turning in, we allow the weigh of the vehicle to fall. This force causes the wheel to naturally toe in.

Caster causes tires to turn inward when weight is applied 

Because the weight of the vehicle causes the wheels to turn to the inside, the side with the least caster exerts the least force. This means that the wheels naturally turn toward the side with the least caster angle. We say a vehicle pulls toward the more negative caster angle.

The caster by itself does not cause tire wear, but the caster angle can cause the toe to change. With a more positive caster angle, the wheels will toe in. An insufficient or excessive caster angle can cause toe wear, even with the toe set to specifications.

The caster also affects the camber. Because of the caster angle, the wheels lean when rotated off the center. This is why we adjust the caster angle to specifications, before we adjust camber or the toe.

Because the caster is not a severe wear angle, we use it to make a vehicle drive straight. By varying the caster, within the specifications, we can make a vehicle pull to one side or the other. This is handy when allowing for road conditions.

Road crown and a custom wheel alignment

roads lean toward the drainage so that water will run off

For example, a vehicle set equally side to side may veer right. Roads are not flat and must lean slightly toward the drainage system. A crown of about ½ inches per foot of road width, helps prevents water from accumulating on the surface. Since most drainage in the US is to the right shoulder, most vehicles will drift right, without and alignment compensation.

A skilled technician drives the vehicle on a known route, before attempting wheel alignment. From this they judge the effect of the road crown on a particular vehicle. The influence varies from negligible to quite noticeable, depending on many factors.

The technician may apply a slight caster and/or camber compensation to the alignment to achieve a straight drive without tire wear. Properly done this allows the vehicle to track straight on most roads.

Properly setting caster, camber and toe will help prevent tire wear, achieve straight tracking and a centered steering wheel. Custom setting alignment, within specifications is where a professional alignment technician comes in. This is where AGCO has built their reputation since 1974. AGCO will set you straight.





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