Pull Rod Suspension (A Brief Overview)

In this brief article, we will discuss the Pull Rod suspension design and its advantages.

What is a Pull Rod Suspension?

The origin of the Pull rod suspension can be traced back to Brabham Automotive’s late-1970’s BT49 Race Car in Formula 1. 

A Pull rod suspension is a type of multi-link suspension mechanism used primarily on F1 Racecars. The Pull rod suspension setup is different from a multi link suspension setup seen on road cars in that the Springs and Dampers are located inboard of the chassis, instead of outboard.

How does a Pull Rod Suspension Work?

• The Pull rod suspension uses a bell-crank lever mechanism to convert the vertical motion of the wheel into the horizontal motion of the shock absorber. 
• This is done by using a bell-crank lever that has 3 connecting points or pivots. 
• The multi-link suspension has one control extra rod, also called a ‘PullRod’, linking the Upper end of the knuckle to the outer pivot point of the bell-crank. 
• The bell-crank’s lower pivot is mounted on the chassis at the bottom and the third inner pivot point connects to the horizontally mounted coilover outer mount. 
• The Coilover’s inner mount is connected to the chassis.
• When the wheel moves upward over a bump or in a corner, the PullRod causes the bell-crank lever to rotate and ‘Expand’ the Coilover Spring-Damper. In a similar fashion, the coilover Compresses, when the wheel moves downwards.
• The difference between the pull rod and push rod suspension is that in the case of the push rod suspension, the spring compresses with upward movement of the wheel. And in the case of the Pull Rod suspension, the spring expands with upward movement of the wheel. 

What are the advantages of the Pullrod suspension?

The benefits of a Pull rod suspension setup are as follows:

Reduced Unsprung Weight

• The weight of the shocks, springs, and reservoirs is moved over to the chassis. 
• As a result, the lower control arm can be made much lighter.
• The Pull rod suspension design approach, therefore, yields a drastic reduction in unsprung weight.

Easy Adjustment of Suspension Wheel Rate

• In a Pull rod Suspension, one of the biggest advantages is that the suspension Wheel rate can be increased or decreased without changing the suspension spring.
• By using different bellcrank motion-to-spring compression ratio i.e., changing one of the bellcrank’s arm lengths, the wheel rate can significantly change. 
• The relationship between Wheel Rate and spring rate is,

Wheel Rate = Spring Rate * (Motion-to-spring compression Ratio ^ 2) * Spring Angle Correction

• Since the wheel rate is directly proportional to the square of the motion-to-spring compression ratio, small changes in the ratio lead to a significant change in Wheel Rate.

Easy Ride height Adjustment

• The Pullrod suspension Ride height can be adjusted conveniently just by adjusting the length of the pull rod.
• Unlike in a conventional Multi link suspension, the need for adjusting the pre-load on the springs is eliminated.
• Corner suspension fine-tuning can also be easily done just by adjusting the pull rods.

Improved Aerodynamics

• Professional open-wheel race cars are lightweight. They need favorable aerodynamics in order to ensure perfect grip at all speeds at all 4 corners. 
• The pullrod suspension helps aerodynamics since the Coilover shock absorbers can be neatly tucked away underneath the bodywork and minimize drag due to obstruction by coilovers.

Conclusion

In this brief article, we have discussed the Pull Rod suspension design, and its advantages.

In case of any questions or queries, please feel free to get in touch.