Of the two most common wear patterns, one Is most often attributed to under inflation, which It could be, provided the tire has been driven that way for a very long period.  Sometimes seriously low pressure or overloading can make the wear show sooner, as will hard cornering, of course. The second wear pattern Is usually blamed on incorrect camber. The interesting point here Is - how much is incorrect?  On some vehicles, the preferred spec Is +l/2-degree, ±l/2-degree. Although many people, companies and text books have failed to acknowledge this fact for years,, It Is now being widely accepted that a more common cause of this wear pattern is weak shocks and/or springs due to resulting excessive side-scuff.  Here's why.  When the  tire Is pulled Inward, due to excess control arm arcing, the tire's Inner tread ribs wear more because they are. In a sense, dragged under due  to distortion as the tire Is pulled inward.  This scuffing simply takes more rubber off the Inner tread ribs.  Now, when the control arm comes back to the normal horizontal position. It pushes the wheel back outward.  Now the outer edge or tread ribs drag under and therefore scuff more. You may wonder why this action doesn't cause a feathered Inner edge due to the distortion.  It does In fact, but the "feather" Is promptly removed as the tire scuffs back In the opposite direction. Another common cause of this same "under Inflation" wear pattern Is dynamic, or load-toe change. Here's why. Suppose the car drops due to a dip In the road, or the wheels rise due to a hump in the road.  In either cause, the control arm arcs up and the spring compresses.  If there Is a mechanical problem that causes the wheels to toe out at this time, the tire tread tries to go out but can only go a small amount, of course.  This distorts the side wall and sets up a scuff action similar to side-scuff with the same result - Inner tread wear.

Now, on the rebound, as the control arm straightens out and the car level Is out, the wheel toe comes back to zero.   But as rebound continues (especially with weak shocks) where the car rises and control arms are downward, the same mechanical conditions may cause the wheels to toe In.  This will cause the tire tread to track inward but Is restrained by the side walls and so scuffing of the outer tread takes place. Next, consider this.  If a toe change problem exists, along with weak springs and shocks, you can see how this wear pattern would develop within a few thousand km. Does this mean that 1 1/2-2° will be excess, and will therefore cause wear? On the other hand, many vehicles today have +1 1/2° preferred with ±l° allowed.  Does this mean that a tire on this vehicle will not develop camber wear until the camber exceeds +2 1/2°? And will It also develop camber wear if/the angle Is less than +1/2°?   It Is very confusing to say the least!

Now let's look at this fact from another angle, so to speak. How many cars with Independent rear suspension (or front, nowadays for that matter) have you seen with several degrees camber (2° or more and negative or positive) and yet found either normal tread wear after thousands of kilometers, or (If It's got weak shocks and sagged springs) "under Inflation" type wear pattern?  Since most of the time you will not see a "camber" wear pattern on this vehicle, we have to ask that when we do see a "camber" wear down do we know that camber is the real cause? The fact Is we don't. What you are most likely looking at when you see tire "camber" wear on independent suspension systems (either front or rear) Is in fact caused by toe change with up and down movement of the vehicle, or load-toe change as described earlier.   The vehicle may have good shocks but most likely has weak parts. For some strange reason most vehicle owners and many automotive industry personnel seem to feel
that If the tires are worn, all the car need is an alignment and that this alignment will compensate for worn parts and sagged springs.  In fact that's why most people believe the angles were, or are adjustable - to compensate for wear, etc.!

Nothing could be farther from the truth! Angles are in fact adjustable to allow the manufacturer to establish the designed-in angles during assembly, while assembling the vehicles from various parts that need not be, or are not, made to exacting specifications.  In other words, adjustments were used to compensate for manufacturing tolerance, but were never Intended to compensate for wear. They also happen to be   very useful to the mechanic when repairing the vehicle for the same reason; that is, to enable him/her to re-establish to designed-in operating angles. In recent years the manufacturers have Improved production methods to the point where they can assemble the vehicle with more precise parts to a more precise body or frame, and establish the designed-in angles without the need for final adjustments other than toe. And let's hope they don't take that adjustment away from us!  That would be worse than hiding the mixture screws on the carburetor, or having no Ignition timing adjustment.

The result of all this to some service shops is that an "alignment" now takes  a  little  longer.    Since  there  are  no  camber  and  caster adjustments on some cars, they feel they have to create them by bending or shimming components that should not be tampered with. And of course, there Is always the so called "specialist" out there that feels he has to set the angles to his "time proven" settings,  even though they may be somewhat different from those recommended by the manufacturer. The point here Is that if we replace the proper worn parts and restore the vehicle to the right height, the angles will usually be within spec.
If not, there's likely a bent frame or component that was not detected.