Suspension Physics: Weight Transfer and Shocks

In High School, first period started at 7 am sharp.  One year, that first period class was PHYSICS.  You know, the science kind of physics, not the climb-the-rope-in-the-gym kind.  Theory of Everything.  Steven Hawking.  At 7 am.  Retention was questionable at best.  So today there is a bit of a review in order.

We all accept that in a car, there is a thing called “weight transfer.”  Weight moves forward when we hit the brakes, backwards when we hit the gas, and to the side when we steer.  Braking and steering at the same time transfers weight out and forward to the outside front tire.  The car dips and leans, telling us that we are transferring weight.  But does the weight actually move?  Well…it doesn’t move like when you hit the brakes hard and that thing in the back seat is now in the front passenger floor, but it does move in relation to the suspension and wheels.

We have two things in the suspension that help us with weight transfer – springs and shocks.  (We call it “springs,” but it can be torsion bars or some other form of springy thing that is between the wheel and the body.)  The springs allow the body to move on the wheels and tires, “suspending” the body and insulting it from the harsh realities of the road.  But springs also allow the body to transfer weight, which is part of the performance of the suspension.

Uncontrolled, the springs would be an enemy.  Think of the leaf springs in the old horse-drawn buckboards in the movies – bouncing around and making for a less-than-comfortable ride along with questionable cornering ability.  So we add shock absorbers to control the movement of the springs.  The shocks keep the spring movement slow, and adjustable shocks let us further dial in the compression and rebound rates of the shock, helping to keep the wheels on the ground and control the amount and speed of movement in the spring.  

So when we hit the brakes hard, the front end goes down because the weight transfers forward.  Hit the gas and the rear end squats as weight moves to the back.  So let’s look at an example of a car that is 3,000 pounds.  (We chose 3,000 to make the math easier.). Also assume that the weight is equally distributed on the wheels, with 1,500 on each axle and 750 pounds on each wheel.

When you hit the brakes, the front end dives.  The weight moves from rear to front, putting more weight on the front wheels by taking weight from the rear and placing it forward.  For our example, let’s say you hit the brakes hard so an additional 1,000 pounds is transferred forward – 500 pounds is left on the rear wheels while 2500 pounds is on the front wheels.  The rear lifts up and the front dives down.

For our example, let’s also say that your 944 has 250 pound front springs.  That means that it takes 250 pounds of pressure to compress the spring one inch.  We are putting an extra 1,000 pounds on the front axle (500 pounds on each wheel) which means that the springs can compress two inches.  (Understand please that we are over-simplifying this for our example.  The actual distance that the nose dives will be a little different depending on a lot of other factors.)

The shocks, though, want to try to control this compression, or more exactly the speed of the compression – slow it down a bit by resisting the movement.  How the shock is valved will determine the speed of the shock’s action.  On some shocks, the speed (valving) is adjustable so that you can dial in the action of the shock.  It is also important to know if the shock is adjustable in both directions – compression and rebound – and if they are independently adjustable in either direction.  Obviously, this can get a little complicated.

Many of us run Koni Sport adjustable shocks and struts – a typical upgrade on a 944.  You should know that the adjustment on them is only rebound adjustable.  That means that your adjustment has nothing to do with the adjustment for compression.  It only adjusts the speed (resistance) of the extension of the shock.

Therefore, to control the speed of the weight transfer under braking you need to adjust the REAR Koni’s.  To speed up the weight transfer for more bite on the front tires, adjust the rear shocks to be softer.  To slow down the weight transfer to the front tires, adjust the rear shocks to be firmer.  (I hope this makes sense.)  On your Koni’s adjusting the rear shocks is much more difficult and time consuming when compared to adjusting the front struts.

The same is true for weight transfer to the rear – the front struts are rebound adjustable only, so to slow the transfer to the rear, make the front struts more firm.

However, also understand that adjustments to the shocks/struts will also have an effect on body roll in a turn.  Just as the car will transfer weight front and rear, it will transfer weight left and right.  Remember that to slow the roll, adjust more firm on the side opposite the roll – when the car is rolling to the right, adjust the left side.  Remember, too, that the sway bars also help to control body roll, so adjustments to one will have an effect on the other.

With that all said, we normally adjust the struts to the same adjustment and the rears to the same adjustment.  When shaking out a car for a new track, we will normally set both front and rear shocks to the medium (center) adjustment and go from there.  We monitor tire pressures to see where the work is happening.  If the front tires gain 8 psi in a session and the rears only gain 3 psi, the front tires are working harder.  We can assume that the car is understeering.  Understeer condition can be alleviated with sway bar and shock adjustments.  If the opposite happens, oversteer can also be reduced with sway bar and shock adjustments.

Be sure to only make one change at a time – otherwise you don’t know what is working and what is not.

Next time we will look at shock and sway bar adjustment.

 

Comment with your experiences with your shock adjustments!