Why a 50/50 Weight Distribution Doesn’t Work

Posted on January 14, 2015


Why a 50/50 Weight Distribution Doesn’t Work

By: Rob Oakman

2009 Corvette Z06 (Courtesy of autoblog.com)

2009 Corvette Z06
(Courtesy of autoblog.com)

Alot of car companies tell you their sports car has a “Perfect” 50/50 weight distribution. The only thing perfect about a 50/50 split is how nice it looks on paper. If you want to go fast on track, you want something closer to a 40/60 bias to the rear, and here is why.

To get through a corner fast you require the ability to change direction quickly. This is determined greatly by where you put the weight on your chassis. I’m going to explain this using the analogies of a figure skater, a lever, a transport truck and a whip. Stick with me here, this is going to be fun.

Angular Momentum Described by the great source OUCHMATH (courtesy of ouchmath.wordpress.com)

Angular Momentum Described by the great source OUCHMATH
(courtesy of ouchmath.wordpress.com)

First the figure skater. If an object is spinning, like a figure skater, given no other influences they will turn directly around their centre of gravity. When the skater brings their arms in towards their centre, the spin accelerates. This is because of a thing called Conservation of Angular Momentum (Don’t worry. There isn’t a test). When you enter the corner at Turn-In you need to induce a rotation. This is literally just a controlled spin that allows you to point your chassis into the corner and the direction you want to go. Just like the skater, the more centralized the weight on your chassis the faster you can rotate (I covered this in Why a Porsche spins, a Corvette pushes and a Lamborghini wants to kill you).

Your rear is the load. The front is the Effort. The fulcrum is your center of gravity between your tires. (courtesy of NCSM)

Your rear is the load.
The front is the Effort.
The fulcrum is your center of gravity between your tires.
(courtesy of NCSM)

A 50/50 split means you rotate in place from your centre but you obviously don’t want to spin. You want to get into the corner. If the figure skater sticks one arm out and they don’t compensate for the imbalance they begin to wobble. What is interesting about this is that their body still wants to turn around its centre of gravity so its acts like a lever with its fulcrum off-centre. Remember science class? The area closest to the fulcrum travels less distance and can exert more force than the part furthest from it. That is where the advantage of a rear bias comes in. When you move the centre of gravity back its like moving that fulcrum back. Your front tires are doing the steering so they are the point of force or effort. The rear tires follow along so they are the point of load (see the diagram at left). As you rotate, the front end is leveraged into the corner. A 50/50 weight distribution won’t do this, and a front bias will see the back end want to slide out towards the outside.

Momentum is an issue too. When something is moving it wants to keep moving until enough force is applied to change the direction. Here is where the truck and whip come in.

When a truck is carrying a load — whether on a trailer or just on the chassis — the driver needs to slow all the way down before they reach a corner. The weight of the load shifts forward under braking and the front tires can’t handle the turn. So if they don’t want the trailer to push them wide into a pole, they need to pull the weight of the load through the corner instead. At turn-in they get on the throttle to maintain speed. This is called powering through the curve.

A similar layout to a porsche. (courtesy of The Gilmer Mirror)

A similar layout to a porsche.
(courtesy of The Gilmer Mirror)

A car is exactly the same… seriously. As you decelerate the weight on the rear wants to shift forward just like a load on a truck. Too much weight bias on the front end and the extra weight shifting forward will cause you to push on entry like so many a sports cars. And the further from the centre of mass the weight is, the more pronounced a problem you are likely to have. Powering through the curve works just fine for a truck, but it is not the fastest way through a corner because you loose so much time under braking.

As I said, when you decelerate weight shifts forward, so a 50/50 chassis will bias to the front on entry while you are decelerating, putting extra pressure on the front tires causing them to slide. At some point you are going to have to accelerate. When you do, the weight shifts to the rear again. The problem is that as the weight shifts back the momentum at the rear increases and the front decreases. When the front end finally grips the track it snaps the rear around the corner like a whip at an s&m party. This is often called snap-loose or push in/loose out. Anyone who has driven a Porsche in anger knows what I am talking about.

Devo knows what I'm talking about too. (Courtesy of clubdevo.com)

Devo knows what I’m talking about too.
(Courtesy of clubdevo.com)

If you feel these symptoms on track, don’t give up. To fight the push on entry try trail-braking and additional rear brake-bias (if you have that option on track). To handle the whip effect on exit, you need to be easy on the throttle, unwind the wheel carefully, and most importantly, be ready.

Too much rear bias feels almost exactly the same except that you won’t get loose on exit.

To summarize:

By shifting weight back your chassis will want to move away from the imbalance, you’ll reduce the momentum pushing on your front tires while decelerating, and you’ll reduce the tendency of the rear end to “whip” out under acceleration. Obviously you can go too far and shift too much momentum onto the rear tires, but that’s what set-up days are for. Learning.

Remember to have fun out there and thanks for reading.
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