Hi,

I was talking to a Porsche guy last week and he said something that made me think but couldn't come up with a clear explanation. Porsches all have a heavy bias toward the rear with regards to weight distribution, the way those Balco-enhanced beetles sit, you just know the weight is mostly over the rear tires. Now I've been told many times that a sports car aspires to have 50/50 distribution, Porsches are more like 40/60 if not more. How is it that they handle pretty good? The guy told me that when accelerating and braking, what matters is not STATIC balance (which most sports car have, with the 49/51, 48/52, or even 50/50 weight distribution), but rather DYNAMIC balance. He said that when a Porsche brakes, the rear weight is transferred to the front, getting very close to 50/50, and during cornering, all that rear weight is controlled by shocks/springs, anti-roll bars, and whatever Porsche does with their suspension tuning. I've read that since Ferdinand demands that the engine sits in the rear, Porsche's engineers HAVE to compensate for that "disadvantage" and engineered out the rear bias, essentially saying that they start out with an imperfect design and over the years have pretty much "perfected" it. So this implies that for all other sports cars, the DYNAMIC balance is more like 70/30 or 60/40 towards the front. I don't know all the engineering principles, but all I know is that my car handles great (2002 Z06), and the S2000 too. This forum is great! Very open-minded about other cars. Any thoughts about what the Porsche guy said?

 

The Porsche guy was right that it's dynamic weight distribution that matters, not static distribution. There's no way to precisely calculate dynamic distribution from static distribution -- the 50/50 stat is largely a marketing construct. It also doesn't take moment of inertia into account.

It's worth noting that there's also no magic "correct" balance number. As with all things, what distribution you aim for depends on your priorities. Shifting weight back improves traction under straight line braking and acceleration, but also increases moment of inertia and potentially makes the car more of a handful in transitions.

Last but not least, someone should verify this, but I believe Formula 1 cars also have a very rearward-biased static weight distribution.

Steve

 

I was also thinking that most passing in racing (road race, not NASCAR roundy-round) is in the braking zone so a car that brakes better has an edge that even a mild HP difference won't overcome.

 

I've thought about this "perfect 50/50" weight distribution also and have also concluded this turned out to be more or less a marketing term.

The static weight distribution is just the starting bias for the dynamic weight distribution, that is just one factor in determining overall traction. And ultimately it's getting the most, maximizing the use, and optimizing the balance of traction is what it's all about. As long as the weight distribution is not so far off as to overwhelm one or more tires, tire and suspension design/setup can be changed to suit whatever application the car is designed for.

One extreme example is a top fuel dragster. Huge rear tires, RWD, with most of the weight at the rear and that huge rear wing to maximize traction for the holeshot and straight line acceleration. Another example is oval dirt racers which shift weight to the left for all that left turning.

Another thing is that weight distribution is more than just front/back but also whether the weight is near the center of the car and near the bottom of the car.

The more weight in the center of the car the smaller the polar moment of inertia, so easier for the car to rotate. Race cars in race series with weight limits still reduce weight to below the minimum and then add it back on as ballast near the bottom of the car. I'm guessing this is to move more weight below the roll axis to reduce car lean, i.e. outside wheel weight transfer, i.e. increase in total tire efficiency, i.e. more traction ... i.e must stop and go to sleep.

 

to throw some heat into the debate i'll argue that just because the current 911s handle well doesn't mean 50/50 distribution is marketing hype

http://autozine.kyul.net/911/911_5.htm

so it would seem that the current 911s require a lot of tweaking to make up for the rear bias.

 

regarding the formula 1 cars

http://www.indiacar.com/index2.asp?pagenam.../wtdistinf1.htm

seems like teams are using ballast to get better weight distribution.