Wednesday, August 15, 2018 Detailed Auto Topics
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A new set of premium tires wears out in just 20,000 miles. The wheel alignment shop says everything is okay, but is it? Why would tires continue to wear though the alignment is set?

Dynamic and static wheel alignment

A new tire destoyed by improper wheel alignment

What many understand about wheel alignment is only a part of the story. The alignment-angles they check and adjust, and the specifications they use, are for static alignment. That is, the way the wheels are set with the vehicle setting still. Dynamic-alignment is the angle the wheels are at when the vehicle is in motion. They engineer this into the chassis and suspension and if static-alignment is okay; dynamic should be as well.

Problems begin when anything alters the original design. Many things can change dynamic-alignment and go undetected in a static check. Typical examples include, a change in vehicle height, worn or bent components and any modification to the wheels or suspension. To find these type problems, technicians need a thorough knowledge and some simple diagnostic measures.

The effect of ride height on tire wear

All wheel alignments depend on a level vehicle, sitting at the proper height. If the vehicle leans to either side, it drastically affects wheel alignment. The same is true with ride-height. Static alignment-specifications are for a vehicle at stock height. When the position of the vehicle changes up or down, the specifications also change, to an unknown. Calculating the dynamic requirements without extensive tests is impossible. Because dynamic-alignment has changed, setting the alignment on a raised or lowered vehicle to perfect static specifications results in worn tires.

Air flow pushes the vehicle down at speed

To illustrate, consider a typical vehicle setting at stock height. Lower control-arms angle downward at the wheel end. The arm pivots at the attachment to the crossmember and travels in an arc. As speed increases, air flow pushes the vehicle down, reducing the ride height. The distance between this pivot and the lower ball joint gets longer as the angle decreases. This pushes the bottom of the tire out and produces negative camber.  Camber changes, because steering axis inclination or the angle of the pivot points change.

Lowering a vehicle will change the wheel camber

Static alignment allows for the normal change in rolling camber, by presetting the wheel more positive. This works well if the arc of the lower control arm remains as designed. Artificially raising or lowering a vehicle changes the angle. The lower pivot no longer travels through the same range. Static alignment that is set perfectly will not produce the desired dynamic-alignment, on a vehicle with altered height.

Attempting to correct for camber

Modifications in the vehicle stance also change static camber. When a vehicle is lowered, camber moves toward negative. Steering axis inclination is not adjustable on many vehicles. Some aftermarket kits that attempt to correct camber, changes the included angle. The included-angle and wheel-offset control the scrub-radius.

Scrub radius and toe change 

modifying the included angle will change the scrub radius and the toe characteristics

The term scrub-radius, refers to the distance between the pivot point of the suspension and the actual center of the wheel. This distance is critical. It controls whether the wheel will dynamically toe in or out. Scrub-radius will also determine how much toe will occur. Toe in and out are extreme tire wear angles. Setting the static toe perfectly, will not provide the proper dynamic toe, if the scrub radius is wrong.

An included angle will also change if the strut or knuckle is bent. This results from impact with curbs, potholes or a collision. Detecting this type of damage requires training and tools designed to measure these angles. Often, they do not equip alignment shops to handle this. At least, they should measure included angle and compare from one side to the other. A difference of one degree or more suggests something wrong.

Wheel offset and its effect 

Wheel offset must be correct for scrub radius to be right

Wheels with a different offset will change the scrub radius. Offset is the difference between the center of the tire and the mounting face of the wheel. Manufacturers carefully engineer this into the original wheel and it must remain. Replacing wheels with an improper set can create a tire wear problem, and suspension abnormalities.

The next article in this series covers the toe on turns and other dynamic alignment aspects.





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