Shock reaction on 4-link launch

[mike fabish]

We race a 4-link rear engined dragster and we do not have any data collection capability for the action of the rear shock absorbers. I was wondering what the shocks "do" at the release of the trans-brake, do they compress or extend,or maybe the correct question is; What are they supposed to do? Will additional power from No2 at launch or shortly thereafter mandate a change to the shock settings.Any comments would be welcome. Thanks Mike Fabish

[BillyShope]

Whether the shocks extend or compress, on launch, depends on your 4link settings. If the instant center falls on a line passing through the rear tire patch and having a tangent (slope) less than the ratio of center of gravity height to wheelbase, the rear will squat and the shocks will compress. If the IC falls on a line with a tangent greater than that described, the rear will rise and the shocks will extend.

Personally, I would attempt to adjust to a condition in which the rear would neither squat nor rise for then I wouldn't have to worry about rear springs and shocks. They'd just be going along for the ride.

See pages 4, 5, and 6 for a simple test to determine the wheel loadings on launch without even starting the engine:

[Mike Peters]

Is not the very first movement of any set-up an extension of the shocks due to the reaction of the pinion pushing down on the ring gear?

[BillyShope]

Is not the very first movement of any set-up an extension of the shocks due to the reaction of the pinion pushing down on the ring gear?

The rear axle assembly is connected to "the world" at the links and at the tire patch. It is the forces at these points which determine what moves where. Specifically, these forces can cause the shock to either extend or contract.

The force to which you refer is called an "internal" force, meaning that other internal forces cancel it out. In this case, the separation force is reacted through the case.