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      12-12-2013, 05:59 PM   #246
Second Lieutenant

Drives: M3
Join Date: Feb 2010
Location: USA

iTrader: (0)

Originally Posted by Craigy View Post
Don't be so surrious.

He's just picking on you since you're coming in, telling someone they're wrong, and not providing any actual input as to a correct answer.

Frankly I buy the "it understeers because there's a couple hundred less pounds in the back" answer. Makes sense to me. Unless you want to tell us the correct answer...
I wouldn't say I didn't provide ANY actual input to a correct answer, but you're right I didn't completely spell it out. Of course we could have elaborated on that and had some discussion, but the conversation took a different turn...

So we have the car going around corner. The centrifugal force acts on the car, but it's the tires that provide traction for the car and determine where it's going (understeer/neutral/oversteer). Each tire has a limited amount of grip that can go to braking, accelerating, cornering, or a combination of those forces. These traction forces are developed by the friction between the tire and the pavement, and the magnitude of a friction force is dependent on the mass of the object pushing down on the friction surface (among other variables). For example, if you slid a small wooden block over a sheet of sandpaper by just pushing it from behind, then it will slide somewhat easily. If you put a 5 lb weight on the block, then it will take more force to slide it over the surface because the frictional force has increased due to the increase of the mass of the wooden block pushing down on the friction surface.

So getting back to the car, when mass is removed from the rear, it takes away from the magnitude of the traction force that the tire can provide through friction. With the rear tires having less traction available to them because of the loss of mass pushing down on them, they will lose traction earlier. This situation is made worse when entering a corner because the rear tire also has the acceleration force to deal with to go along with the cornering force (since we should be on the throttle through a corner). So it will exceed its available traction sooner than before, meaning the tail of the car has less grip and will swing out more easily, aka oversteer.

Aside from the technical explanation, I'm experiencing this exact problem in my track car that has had extensive lightweight mods in the back seat/trunk, but limited lightweight mods towards the front of the car.