Date: Fri, 06 Jul 2007 20:32:36 GMT From: Bill Bradley <senator2nospamAMearthlink.net> Subject: Re: 338 hp saab 900 2-door
riserman wrote: > th wrote: >> riserman wrote: >>> Jeremy Brown wrote: >>>> I don't know what to make of this guy: >>>> http://video.google.com/videoplay?docid=7570206935258612439&q=saab >>>> >>>> Jeremy >>>> >>> What a perfect illustration of the limitations of front wheel drive. >>> Under acceleration, the center of mass of the car moves to the rear >>> and there's not enough weight on the front tires to prevent them from >>> just uselessly spinning. >>> >> >> The center of mass does not move under acceleration (except for a >> small shift if the tank is not full). >> > If it's not the center of mass that shifts to the rear under > acceleration, and you may well be right, what resolution of vectors > shifts to the rear? Is it center of gravity or something else? > > Front wheel drive cars lose traction easily under maximum acceleration. > I know this because it wasn't hard to do even in my 1983 900 turbo. > That's why you rarely see front wheel drive race cars. > > riserman It's a torque issue. The tires put a force to the pavement (braking, turning, accelerating). The force is not through the CG, so it creates a torque with the height to the CG as the torque arm (the lower the CG, the less the torque). The only way that the torque can be balanced is by re-distributing the weight force of the car on the tires. Since acceleration creates a clockwise torque that diminishes the force on the front tires [in extreme cases even lifting them off the ground] and increases the force on the rear. Braking does the opposite. Wheel base [or track for turning] and the location of the center of gravity are also factors since they effect the torque arm for the normal force on the tires. Bill
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