What you're describing is "negative camber". Camber is the tilt of the wheel as viewed from front or back of the car. Negative means tipped in on top. I'm not familiar with your suspension system but when both sides are affected about equally, the first thing to look at is ride height. Suspension and alignment experts get pretty picky about that because if it's wrong, there is going to be tire wear even when the numbers appear correct on the alignment computer. That's because the suspension geometry has changed. That changes the motions the tires go through as the car goes up and down over bumps in the road. As a general rule, inexpensive cars that are designed for basic transportation have suspension systems that are more forgiving of sagged springs. Higher-class cars that are designed for improved handling and / or ride quality have a more sophisticated suspension system but all the things designed into it must be as they were designed, and that includes ride height.
Another thing that leads me to this area is that if you look at the rear suspension in terms of cornering, when you make a left-hand turn, the right side of the car drops down. That has the same affect as a sagged spring, and the independent suspension is designed to tip the right wheel inward on top so road forces keep the entire tire tread on the road surface. If high-speed cornering tips the tire in on top, so will sagged springs. That makes them ride on the inner edges.
There is most likely going to be some type of camber adjustment provided by the manufacturer but the range will be limited. Even if the mechanic can get the wheel into specs, the geometry will still be wrong if ride height is wrong. If ride height is correct and there is no camber adjustment, there is usually some type of "problem solver" parts available from the aftermarket suppliers to make it adjustable. I don't think modifications will be necessary because if it was a design issue, we would have heard about this happening to more cars like yours. Ford has always had design problems with their steering and suspension systems and the aftermarket industry has worked overtime trying to correct their flaws. Just look at any older Ranger with the terrible "twin I-beam" suspension. You can see how badly the tires tip out on top when the truck is raised on a hoist. That's what happens going down the road.
Inner tire wear can also occur when "total toe" is not correct. "Toe" is the direction the tires are steering. Basically both tires should be parallel to the frame of the car. If everything else is correct, and toe is off on just one rear wheel, the steering wheel will be off-center. It's possible for toe to be off on both rear wheels. If it is, it is also possible for the steering wheel to be straight. It's important to understand that if toe is not correct on either OR both wheels, making total toe incorrect, that will affect both tires on that axle equally, assuming there's no other variables involved. If either or both wheels are steering away from the center of the car, the "leading edges" will be scrubbed off. At first you'll see a choppy pattern all across the tread, but once those blocks of rubber get worn down enough, the wear will smooth out some but the inner edges will wear much faster. By "leading edges", what I mean is if you exaggerate it for clarity, the wheels are steering away from the center of the car. In effect, they're trying to pull apart. Now imagine the wheels are turned further, ... And further, ... Until they're turned sideways. Now it's easy to see that the inner edge is in front and is the first edge of the tire sliding down the road.
Now imagine if you hold a pencil upright with the eraser on the table. Put a little downward pressure on it and drag it across the table. You'll see the leading edge scrub off and make eraser crumbs, and the trailing edge lifts up off the table and no wear takes place. That's exactly what happens to tires when total toe is not adjusted to specs.
In this case total toe is likely not the cause of the tire wear because you'll never see that by looking at them. You CAN see incorrect camber when it's really bad, and that's what you found. You should also know that camber and toe interact and changing one often changes the other. Experienced alignment mechanics know how to set both of them and both wheels to get the best tire wear and handling.
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Sunday, January 27th, 2013 AT 8:35 AM