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      01-27-2014, 03:23 PM   #60
bradleyland
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Quote:
Originally Posted by CanAutM3 View Post
This article is full of flaws. It assumes that the path the car travels is linked to the mid point between the front and rear axles; since only the front wheels are steered, this simply isn't true (even on cars that have rear wheel steer, the fronts still do most of the steering). On rear weight biased car, the article concludes that the rear wheels need to cary more lateral load. This is correct, but since they also carry more normal (vertical) load, it becomes a moot point.

A car, as it travels through a bend, also needs to pivot on itself to keep the proper heading. The car does not pivot around its CG nor does it pivot around its "middle". The car pivots around the mid point between the two rear wheels (assuming zero slip angles). Imagine the front wheels steered at 90deg and this becomes very obvious.

To calculate the moment of inertia of an object pivoting around a point, the natural moment of inertia of the object needs to be added to the mass of the object times the square of the distance of its CG relative to the pivot point.

So the closer the CG is to the rear axle, the easier it is to pivot a vehicle. Hence the benefit of a rear weight bias.

The counter point is that when the rear tires start to slip more than the fronts (oversteer), the pivot point progressively moves away from the rear axle towards the front axle. So once you loose the back end on a rear weight biased car, it becomes much more difficult to bring it back.
This is such a fantastic reply. Well said

I'll add that weight distribution is only one factor. You can have a 40/60 (front/rear) weight distribution and still have a horrible handling (and probably dangerous) car. As CanAutM3 mentioned in the last paragraph, the rear-biased car will rotate more easily, but beyond the limit of traction, that rotation can bite you. The trick is in finding the balance between "rotates well" and "wants to swap ends, so watch out".

As has been pointed out, the one thing you don't want is front-heavy weight distribution. You're fighting an up-hill battle at that point. Kudos to BMW for working hard at hitting the 50/50 number, which I would consider a minimum acceptable distribution.
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