Quote:
Originally Posted by CanAutM3
I agree, the term "bias" is not the most scientifically accurate here, but it is the one often used in the automotive jargon.
The reason why a greater rear weight distribution is favourable to create nimble handling has to do with polar moment of inertia. I am not sure how familiar you are with physics and engineering mechanics, but the polar moment is the "rotational mass" of an object; the greater the polar moment of inertia, the greater the force (torque) needed to change the rotational speed of the object. The polar moment of inertia of a mass element is defined by the product of the mass element with the square of the distance of the mass element to the centre of rotation (I = m * r^2). So the further the mass element is located from the centre of rotation, the greater its polar moment of inertia. That is why mid-engines cars are considered more nimble, because they have a lower natural polar moment of inertia since their mass is concentrated closer around the centre of gravity.
Further, when an object rotates around a point that is not its centre of gravity, the objects natural polar moment of inertia needs to be added to the polar moment of inertia created by the distance between the centre of gravity and centre of rotation. - Itotal = Iobject + mobject * r^2
- where r is the distance between the centre of gravity of the object and the centre of rotation
And this later point is key as to why rear weight distribution is preferable for nimble handling in vehicle dynamics.
When a vehicle is going through a turn, it does not pivot around its centre of gravity, it pivots around the mid point between the rear wheels (on a front wheel steered vehicle). Imagine the front wheel steered at 90 degrees, at this becomes quite clear. So considering all of the above, it becomes obvious that the further the centre of gravity resides from the rear axle, the greater the force needed to get the car to change direction. And this is to the tune of the square of that distance (i.e. twice the distance requires 4 times the force). So the greater the weight distribution towards the front, the more the front tires need to work to get the car to change direction.
Now the above only remains 100% true when the front and rear wheel slip angles are the same. The rotation point will vary with varying degrees of slip angle difference between the front and rear axles. When the front slip angle is greater than the rear (understeer), the pivot point shifts further to the rear and the the rear slip angle is greater than the front (oversteer) the pivot point shift towards the front. In the extreme case of a spin, the car pivots almost around the centre point of the front axle. And that's where it becomes interesting from a vehicle dynamics standpoint. A car with a rear weight distribution will require much more effort (force) to recover from a spin because the centre of gravity is located further from the front axle. Remember that unrecoverable snap oversteer reputation of early mid engine Ferraris, Toyota MR2 and 911?
I don't recall exactly where I specifically read all this, but I got to learn this while working on the design of our Formula SAE back in my engineering studies. If ever you want to dig deeper on the topic, two books that were useful to me at the time were "Fundamentals of Vehicle Dynamics" by T.D. Gillespie and "Race Car Vehicle Dynamics" by W.F. & D.L. Milliken, both published by the SAE. |
I really appreciate the time you take to explain this. This makes sense now. I wonder what's the weight distribution of my 335i convertible. Also, from a marketing point, BMW are not totally wrong when they say "50/50 is the key, 'whispering' (for a front engine car)". I guess ATTESA (omitting the other reasons for the sake of this argument) like systems is to mitigate this problem.