The Watts link is mostly used...
The Watts link is mostly used in road-racing cars. The advantage of this system is that as the chassis moves vertically, the rear solid axle does not move laterally. With some rear suspension systems, lateral movement of the rearend would cause rear steer. The Watts system eliminates the lateral movement that creates the rear steer. The rear moment center height also stays very consistent with right- and lefthand turns, such as are encountered with road racing.
There are additional effects created by the Panhard/J-bar angle that tend to leverage and exert influential forces on the suspension that promote load transfer and those effects will be covered in future articles on chassis dynamics. Suffice it to say that excessive Panhard bar angles can have a negative influence on your setup.
There are eight primary components that affect the magnitude of the rear roll angle. These are: 1.The weight of the sprung mass of the portion of the car supported by the rear suspension (scale weights at the LR and RR tires minus unsprung components).
2.The height of the center of gravity of the sprung mass.
3. The height of the rear moment center.
4. The magnitude of the lateral force.
5. The overall spring stiffness as well as side to side spring split (i.e., right rear stiffer than the left rear spring, and vice versa).
6. The width of the spring base, which is the distance between the centers of the top of the springs.
7. The rear sway bar (if used) has a large effect of antiroll and must be taken into account. The larger the diameter of the bar in this system, the more resistance there is to roll.
8. The track banking angle.
Note: The car "feels" the spring base at the top of the springs. In a coilover sprung car, the distance between the tops of the mounting bolts is the rear spring base. For a big spring car, the distance between the centers of the top of the springs is the spring base. In a leaf-spring car, the distance between the centers of the leaf springs is the spring base.
Now that we know something about the cause and effect of roll angle for each suspension system, the key to creating a balanced setup is to change spring rates and moment center locations so that each end of the car will want to roll to the same angle, or what we call "having the same desires."
A typical rear sway bar installation...
A typical rear sway bar installation uses a high mount and arms with several holes to use to adjust the length of the arm, which changes the rate of the assembly. Note that the link between the sway bar arm and the rearend is at 90 degrees off the arm and directly above and inline with the motion of the axletube.
In order to set up our cars, we would need to be able to change component values in order to bring balance to the desires of the front and rear of the car. These changeable components include the spring rates, the moment center locations front and rear, and the static weight distribution.
In Part 3 of this series, we will examine how we can apply this technology to set up some typical types of stock cars. We will show different setups that might apply to your type of racing and why those setups will be better
A sway bar is a device that produces a resistance to roll when used in either the front double A-arm or straight-axle suspension system. Its influence in a AA-arm suspension is much less than when used in the solid-axle suspension.
The sway bar has a stabilizing effect on the AA-arm system and helps control oscillations common with that type of suspension. The sway bar is more effective in a straight-axle suspension when, for reasons of design limitations, the rear roll angle is hard to control, such is the case with big spring/truck arm suspensions.
The tendency in recent years has been to use softer front spring setups and larger diameter sway bars. Racers have found limitations to the usefulness of this trend, but have found that softer springs and larger diameter sway bars help to lower the car and promote a more efficient aerodynamic configuration when the car is at racing speeds. This does not preclude the need to find a balance in the desires of the front and rear.