Aspect ratio is all.
Wider is better. Longer is less good.
The reason is that when you turn a WIDE robot the wheels don't have to move normal to their roll direction (i.e. scrubbing) very much, they just roll. When you turn a LONG robot, the wheels practically do nothing BUT scrub along the carpet. It's this scrubbing along with grippiness that kills you (and your battery).
That the reason that you lower the middle wheels; It effectively halves the Width/Length Ratio because either the front two axles are on the ground or the rear two, but not both. So the distance in the length direction halves while the width stays the same. Less scrubbing so you can keep your grippy wheels.
What do to?
Well, if you're 4 wheels in the corners, you need to either get a third axle and lower that center wheel or you need to get rid of some of that grippiness on at least one set of tires (also you might want to read
Dr. Joe's Seven Stages of Dealing with a 4WD Robot with Grippy Tires)
Dr. Joe J.