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Old 07-03-2007, 02:10 PM   #3
Bill Harris
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Join Date: Mar 2002
Location: Ooltewah, TN
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Exclamation Re: rear shock setting

Here is a long winded answer to a seemingly simple question.

The answer is yes, and here is why.

When you launch the rear axle housing tries to rotate such that the pinion yoke tries to move toward the floor of the car. The leaf springs (and traction bars) transfer that force to the front spring mount which is attached to the chassis/unibody. At the front spring mount the rotational force is redirected such that is is essentially vertical, which tries to lift the chassis with respect to the axle housing, what is usually called "separation".

So why control the separation? The lifting of the chassis (and often the entire front end of the car in a wheelstand) takes energy. Most of the energy is stored as potential energy in that the chassis lifts in relationship to the rear axle, as well lifting the front end as the chassis rotates around the axle. The rest is stored as elastic deformation of the chassis and rear springs (bending). Since the motion takes place over a period of time, power is used to create the motion (power is energy applied over time). The only source of energy is the torque that is applied to the rear axle through the drivetrain.

Recall that: "For every action there is an equal and opposite reaction", Newton's third law of motion. In order for the power (torque applied over time) to be applied to the tires and move the car forward, the axle housing has to push against something. If that something is extremely rigid, then all of the torque is applied to the tires as it is supplied by the drivetrain (think hard-tail dragster). However, if the axle housing is pushing against something that is compliant, or soft, then some of the drivetrain energy is used to move the housing against it's resistance and less torque is applied to the tires. The nature of the resistance that is seen by the axle housing has a direct affect on the amount and rate that torque is applied to the tires. In other words, by controlling the "softness" of what the housing pushes against, you control the power that is applied to the tires for a period of time (until all the softness or compliance is taken up).

Without shock absorbers, the only thing that resists the separation would be the springs themselves. When you lift the chassis away from the axle you are UN-loading the springs, and they don't provide much resistance to that force. If you add the shock absorbers between the axle and chassis, they provide additional resistance to the lifting force depending on how hard it is to extend them. The spring itself is very important of course, and may times multileaf springs are clamped together to increase their stiffness which absorbs power through frictional losses.

Since it takes time and energy to lift the chassis and front end, that power is not available to the tires. If you had perfect traction then all of that power would be wasted and the car will not be as quick as it could be. However, since traction is usually a major limitation on a car equipped with leaf springs (like a stocker with 9" tires) the control of how much power is applied to the tires is very important. Rear shock absorbers that have adjustable extension rates are a tuning method to control the resistance to the axle housing rotation. A shock that is easily extended will present a softer resistance to the axle housing than a shock that is hard to extend. The body will separate more with the soft setting and more energy will be stored in the chassis than a stiff extension setting. The more energy stored, and the longer it takes to store it, the less power is applied to the tires. This is often referred to as the amount of "hit" on the tires. A very rigid rear suspension with a stiff shock extension will result in a harder "hit", since more of the torque is getting to the tires and less is being used to power the chassis movement.

The "separation" is a visual indication of how much energy is being stored in the chassis. The amount of front end lift is another indication. If you watch a pro-stock car you will see very little movement in either the front end or the rear end. The sophisticated suspensions and chassis on these cars allow nearly all the available power to be applied to the tires to move the car forward. Power application is controlled more by clutch slippage than by trying to store excess power in the chassis. Stockers with automatic trans, lots of torque and limited tire size can maximize performance by redirecting excess power to the chassis according to how good the track is hooking.
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