FSAE Rear Suspension (A Brief Overview)

In this brief article, we will discuss the types of rear suspension setups used in the FSAE design competition.

What types of Rear Suspension setups are used in the FSAE competition?

The types of suspension used in the FSAE competition are:

  • Pushrod Suspension
  • Pullrod Suspension

The Formula SAE is a student design competition organized by SAE International at various member Universities that teach courses related to the design and manufacture of Automobiles. It is aimed at giving a practical hands-on experience of developing an automobile within the context of a competition where students form racing teams and manage the design, manufacture and racing of their own unique race car.

Pushrod Suspension

  • A Pushrod suspension is a type of multi-link suspension mechanism used primarily on F1 Racecars. The Pushrod 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.
  • Observing from published literature, the Pushrod type of suspension seems to be the most favored choice of rear suspension architecture since this is the setup used in Formula race cars as well.
  • The Pushrod 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. 
  • The multi-link suspension has one control extra rod, also called a ‘PushRod’, linking the lower end of the knuckle to the outer pivot point of the bell-crank. 
  • The bellcrank’s lower pivot is mounted on the chassis at an elevated point 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 PushRod causes the bell-crank lever to rotate and compress the Coilover Spring-Damper. In a similar fashion, the coilover expands, when the wheel moves downwards.?

Pullrod Suspension

  • The Pullrod suspension, although not as popular as the Pushrod setup, still finds moderate application in the FSAE competition teams
  • The pullrod 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 bellcrank’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 pullrod and pushrod suspension is that in the case of the pushrod suspension, the spring compresses with upward movement of the wheel. And in the case of the pullrod suspension, the spring expands with upward movement of the wheel. 

What are the advantages of the Pushrod and Pullrod suspension

The benefits of using a Pushrod or Pullrod 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 Pushrod suspension design approach, therefore, yields a drastic reduction in unsprung weight.

Easy Adjustment of Suspension Wheel Rate

  • In a Pushrod 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 arms 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 Pushrod suspension Ride height can be adjusted conveniently just by adjusting the length of the pushrod.
  • 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 pushrods.

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 pushrod 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 types of rear suspension setups used in the FSAE design competition.

In case of any questions or queries, please feel free to ask.

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