Quote:
Originally Posted by DarkSunrise
Yeah just to clarify, I meant that 50/50 would be faster than the Twins' 54/46.
Like you, I also suspect (although haven't seen it quantified) that a moderate rearward balance would be even better for pure performance. Something like the Cayman's 46/54 or even the Carrera's 38/62. You get:
- more even weight distribution under braking
- more rearward weight distribution under acceleration
- less weight upfront for better turn-in
The main downside I see is, if the rear end did let go, it would be a wild ride, as it was with the old 911's. But that might more specifically be a function of the polar moment of inertia rather than weight distribution. If you could centralize the weight sufficiently, you could minimize this downside.
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You're right on two out of three. Less weight up front doesn't necessarily equate to better turn-in. In practice in a street car it usually does, but the question is one of yaw moment generated by the front tires vs polar moment of inertia. A car with a "dumbell" arrangement with the engine in the front, trans in the back (Corvette, Porsche 944) could have more of its weight on the rear wheels, but still not turn-in as well as a more front heavy car with more centralized mass (FT86, Mazda RX7/8).
To Captain Snooze's point, any engineer who says 50/50 is idea is basing their analysis on the most simplified of models. I.e. the bicycle model of cornering dynamics with "large" radius turns and no dynamic cornering (steady state cornering only). Under those conditions, 50/50 is ideal because it minimizes the maximum load on the tires. Since rubber is a non-newtonian material with a non-linear response of coefficient of friction relative to normal force (more load = lower friction) having the load as even as possible results in the highest cornering grip.
In the real world, however, we do more than steady state cornering on large radius turns. We transition from straight to cornering, we do quick left-right transitions, we brake into and accelerate out of turns. All of those favor having more weight on the rear wheels, how much more is dependent on many factors such that there is no "ideal" weight distribution. For a car with a given amount of power at a given racetrack there is an ideal, race engineers in professional motorsport play with the ballast all the time to get it. In practice the twins would be much closer to ideal if we drove them backwards. There's a reason everybody points to the Boxster/Cayman as the exemplar of balance.
Another wildcard is rear-wheel steering. Neglecting tire slip-angles the kinematic point of yaw rotation for a front-steer car is the center of the rear axle (another simplified model, though more sophisticated than the bicycle). With rear wheel steering you can move this rotation point forward and reduce the benefit of rear weight bias to transitional cornering, thus moving the ideal point toward the front wheels. Torque vectoring can have a similar effect. Most street cars have passive rear steering, though all I'm aware of have passive toe-in for stability rather than toe-out for quicker transitions.
Edit: As to tail-swinging with rear weight bias, I doubt a more centralized arrangement than the 911 is possible with 40/60 distribution. The heavy bits are all concentrated close to each other. The older 911s were widowmakers mostly because of the camber behavior of their trailing-arm rear suspensions, the weight distribution didn't help, but the suspension was the real culprit. See E30 BMW (and Z3) for similar problems in a car with 50/50 distribution.