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It’s All About The Weight

With New Year’s Resolutions just a short time away, it’s time to talk about weight – not just “total weight,” but where that weight is located and how it moves around…wait…moves around?

No, we’re not talking about the extra pounds that may have come from Christmas cookies and pies, but the weight – and how it moves – on your car.

The 944 and its cousins are well known for its 50-50 weight distribution, or to be more exact, the 25-25-25-25 weight distribution.  The goal is to have equal weight on all four wheels with the driver, all while “static” – standing still.  While the girth of the driver and also throwing in the weight and number of passengers, the goal is the same – equal on all four.  So let’s look at a little math.

Let’s say that your car weighs 3,000 pounds with you – the driver – in it.  The goal would then be to have 1,500 pounds on the front and 1,500 pounds on the rear, with that split to 750 pounds on each tire.  Again, this is a “perfect world” goal, and there are many variables that can easily change these numbers, such as fuel load.  (A gallon of gas weighs about six pounds, so a full tank – say 17 gallons, weighs 102 pounds, while a half-tank weights 51 pounds – significant difference when figuring weight distribution.)

So we also know that while static weight is nice to know, we tend to drive our cars, and that is when “movement of weight” becomes important.  How much weight is applied to what end and eventually to what tire makes a difference in steering, braking and acceleration.  This “dynamic weight distribution” is not only important on the track, but also in everyday driving.

For our purposes of discussion here, let’s agree that the total weight of the car does not change when we start talking about dynamic weight distribution.  It just makes things easier to understand at this point.  Since the total weight – 3,000 pounds – will not change, we need to then talk about how that weight is distributed once we are moving and driving.

So let’s say that the traffic light changed and you need to stop quickly.  You jump on the brakes and the nose dives as you slow.  What you have done is “moved weight” from the nice 1,500 pounds front and rear to put more weight on the front tires, hence the nose diving down.  For out discussion, let’s say that the act of braking in a straight line moved an extra 1,000 pounds to the front axle – you hit the brakes pretty hard.  (Remember being told that the front brakes do most of the work?  This is why…)  So where did that thousand pounds come from?  The rear axle.

As you are slowing, the front axle now has 2,500 pounds on it, which helps with braking.  But the rear axle now weighs only 500 pounds.  The rear of the car lifts up, transferring the weight to the front.  But the front brakes are up to the job, so no worries.  This dynamic weight distribution occurs every time you hit the brakes to some degree.  This is why track training classes always harp on braking technique – you are changing the weight distribution of the car, which has an effect on handling.

When you stop at the light, the car sets back down on all fours – static weight distribution.  The light turns green and you accelerate, moving front end weight back to the rear wheels.  That helps with traction, although in most road cars the transfer of weight in acceleration is not very dramatic.  On the drag strip, high powered cars will actually transfer enough weight to lift the front wheels completely off the ground, taking all the front axle weight and putting it on the rear.  We don’t really need to worry about that though.

Turning also shifts weight.  You take a curve at speed, and the body leans towards the outside of the turn, shifting weight from the inside wheels to the outside wheels.  Again, on the street this is not a big shift and therefore is nothing to worry about.  On the track, it can have an effect on steering, braking and traction as you are shifting weight from the inside front wheel, effectively making it lighter and losing some ability to steer and/or brake.

Note the inside front wheel off the ground and the outside rear compressed almost to the ground

Here is an example.  While in practice at Daytona International Speedway, my son and I both noticed that the car had a noticeable understeer on exit to the east horseshoe in the infield.  We made some adjustments to the sway bar and shocks, but were never able to dial it out.  A few days later I received a photo from a friend of the car coming out of the turn and saw that the inside (right) front wheel was completely off the ground by a couple of inches.  The act of  turning had shifted the weight completely off the right front wheel and moved it out to the left front wheel, added to the act of accelerating which moved the rest of the weight to the left rear wheel.  The car was trying to negotiate the turn with only one front (steering) wheel on the ground, so it lost some of its ability to effectively turn the car.  Our mistake in setup was concentrating only on the front end while ignoring the rear.

The lesson here is that you have to understand where the weight is going as you turn, brake and accelerate, which changes how the car turns, brakes and accelerates.  Drivers on track are coached to lift momentarily before entering a mild chicane in order to put a little more weight on the front wheels to help them steer more effectively.  Careful application of throttle is necessary to keep from “unloading” the front wheels and making them too light to negotiate turns.  “Threshold braking” provides improved braking control by controlling the movement of weight front to rear.

Finally, think about what happens when you brake hard and turn hard at the same time.  Braking hard puts a lot of weight on the front wheels, then turning hard shifts that weight to the outside front tire.  In our example, let’s say you brake and turn hard at the same time.  Braking puts an extra thousand pounds on the front axle, then turning hard shifts shifts 750 pounds to the outside front tire.  Do the math – that puts 2,500 pounds on the front axle, then distributes that total unequally to the front tires – the outside front tire gets a whopping 2,000 pounds on the outside front tire and only 500 pounds on the inside front tire.   Go look at the sidewall of your tire and see the “maximum load” shown in it.  My bet is something around (or under) 1,500 pounds…so you have dynamically overloaded the outside front tire and lightened the inside front tire, taking away a lot of its ability to help brake and steer.  The turn also shifts some of the 500 pounds in the rear to the outside, taking away traction from the rear – can you say, “Spin out?”

Your car likes its static 25-25-25-25 weight distribution.  Then you get in and upset that formula, and the car does not like being upset.  So remember that you don’t want to upset your car.  You have heard it before – “Smooth is Fast.”  “Smooth” helps keep from upsetting the car.

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Kevin Duffy, Author and Chief Geek View All

Kevin Duffy is an Associate Professor of Criminal Justice at Daytona State College in Florida and a dedicated car guy. He divides his time between teaching criminal justice topics in the online environment and working on/driving cars, particularly Porsches. Kevin is one of the principals in InspiringLifeOver50.com.

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