Dynamic 4-link SuspensionET 493Cole PrejeanAdvisor: Dr. Ho-Hoon Lee

What is a 4-link?• A 4-link suspension is a four bar linkage system used in drag

racing applications.• It is used to connect the rear axel to the chassis, directing the

force of acceleration.

4-link directs forces to provide ideal traction

Typical 4-link suspension

4-link suspending rear axel

How it works

Forces Acting on Rear Tire

All About Friction• The weight of the rear tires on the track is used to compute

the frictional force.• The coefficient of friction µ of rubber on concrete is .60-.80.• If the weight of the car is 2450 lb. and only rear tires are on

track we can assume max Ff.• Ff=µN. Ff=2450(.8)

Instant Center

Instant Center (IC)• Imaginary point at which bars will intersect.• Many different intersect points can be used with existing systems.• Different adjustment holes locate linkages.

Analysis of Launch• Before the car accelerates, the front to rear weight is about

equal (depending on static weight bias) which means the force of the surface is acting almost equally on all tires.

• Once acceleration is applied to the rear axel, pitch rotation of the car occurs, thus applying more weight to the rear of the car, since the system is suspended by front and rear shocks.

Data Aquisition

High IC• A high intersect point directs forces upward, trying to push the

car skyward.• This reaction forces down the rear axle, providing a higher

reaction force against the tire. Extends rear shock absorbers.• Less force applied in horizontal direction.

High IC

Low IC• A low IC will push the chassis towards the ground. • Traction is reduced.• Compresses rear suspension.

Low IC

Force of Acceleration• Acts from rear axle.• From data acquisition, acceleration reaches a max of 3 times

force of gravity.•Using yields 7350 lb.

•This is max force acting on system.

Difference Example• High IC creates 300lb. More downward force on rear axle than

the low IC.• This will create more traction.

Objective• Create method to change angle of bottom bar during race,

resulting in repositioned IC.• Changing IC will redirect forces acting on chassis providing

ideal traction at different positions on race track.

The plan• To move the bottom bar, instead of adjustment holes, a pivot

system will be used.• The angle will be controlled by an actuator on each side of the

system.

Pivot Groove

TimelineTask Result

3/12 Approval of project

Complete

3/20 Find max acceleration, research of chassis dynamics Complete

3/28 Calculation of all forces acting on system

Complete

4/3 Determine how changing force angles act on rigid body

Complete

4/10 Apply forces to 4-link configurations

Complete

4/18 Show that difference of IC locations changes force on tires

Complete

5/1 Construct draft of pivot bracket

Complete