The downside could be twitchy handling if you miss the mark and set it with "toe out." Most manufacturers spec the alignment so that it winds up at zero with the car at road speed under power with the flexible suspension components compressed or stretched. If you alter this to zero while the vehicle is stationary, who knows what it will be when you are driving?
Also, you could have accelerated tire wear, additional drag on the car, and reduced fuel economy.
That's one risky alignment! When we'd align it at the shop most things would be set for 0 camber and 0 toe and whatev caster. Camber will actually reduce contact patch (which is why extreme camber makes traction hard to attain) but will improve cornering. Toe is just toe, it should be 0 regardless, even though it's usually speced to something very nearly but not always 0.
Basically you zero everything. What would be the negative drawbacks of this?
Depends on the vehicle.
Some vehicles have alignment adjusted for optimum handling or traction, as opposed to fuel efficiency. Some vehicles have it already adjusted for efficiency; those are the vehicles you don't want to change.
A VW Karmen Ghia, for instance, benefits heavily from a zeroed alignment in regard to efficiency. Its alignment was originally speced for cornering capability, as opposed to efficiency.
camber or caster wont effect tire wear or fuel efficiency unless it really extreme.
the toe angle is what will eat your tires and cause more drag. ideally if your car isnt a race car, set the toe to 0 all around.
i anticipate some opposition as far as the facts of camber and yes excessive camber does a distinctive type of tire wear, but the acceleration of this wear is dictated by the associated toe. usually when you change your camber(or anything in your suspension for that matter) the toe is changed just from the adjustment in suspension geometry. i hope i dont get flamed for this, but if you think about it, it makes much more sense.
the bottom line: dont worry so much about your camber unless its really bad, just get the toe to 0
don't waste your time or time will waste you
Should I get out flame's? I don't think so. In my experience, the toe is by far the most critial, in terms of tire wear and mileage. What I do is set up the camber for very nearly vertical and then check to see if I'm getting any pull to one side or the other. On my van I had to set it so that I have a small amount of positive on one side and a small amount of negative on the other, to get it to not pull to one side. Once I got that neutral, I adjusted the toe-in and it drives and handles real well. As far as tire wear, with the small amount of camber I had to put in, it doesn't seem to increase tire wear, as nearly as I can tell. At least it's not enough to make it worth trying to get the rear alignment done, so it doesn't have the slight pull.
I have a tool I built for doing the toe-in alignement that work's pretty well and it's relatively easy to use. If ya'll promise to be nice to thisisn'tjared, I could share it. I've thought about getting it copyrighted, or something, but I don't have the money and I don't know how much of a market their might even be. I just get galled every time I take a car to get it aligned and I get some technician who "alway's" finds something wrong, so I pay for a realignment and then I end up having to go back and fix what he adjusted, so it will drive straight and not scrub the tire's into the belt's.