Six-bar Suspension

For Long Travel

A design algorithm was created that synthesizes suspension linkages that feature the Watt I six-bar mechanism. Watt I mechanisms offer motion capabilities beyond four-bar double wishbone designs, however their design in not intuitive so we depend on the mathematics to find linkage designs for us. The algorithm was applied to the design of a long travel suspension for use on an SAE Baja vehicle. The resulting linkage has the following specifications:

  • 16 inches of travel
  • Less than 1 inch of track change
  • 62 inch track
  • 12 inch ride height
  • Roll center height about 1.68 inches below ground
  • 1.13 deg/in camber gain for upper 8 inches of motion
  • Minimal camber gain for bottom 8 inches of motion
  • Neutral camber when the wheels are fully drooped

The roll characteristics of the vehicles have been designed such that the outside tire is perpendicular to the ground during cornering loads.  Shocks can be mounted to the small inboard links eliminating the need for additional pushrod and rocker links to move the shocks inboard and manipulate mechanical advantage for whatever spring size and travel. With these links eliminated, the suspension showcased here has the same number of links as a traditional double wishbone and rocker setup.

A fifth-scale prototype of this suspension was built.  It’s geometry is exhibited in the video below.

At the heart of the design algorithm is a synthesis method for four position motion generation of Watt I linkages. This is a simplification of general motion synthesis for Watt I’s that allows eight positions. The simplification comes in the form of prescribing the positions of the ground pivots and end effector pivots. This has the particular advantage in suspension linkage design of allowing us to locate the chassis mounting points and upright mounting points. Since many of the algorithm’s results suffer from packaging problems or linkage defects, the algorithm includes the ability to search for design candidates within zones of chassis pivots and upright pivots.

A common solution for achieving a long travel suspension is to design a long parallelogram four-bar linkage. However, these linkage suffer from track change and packaging issues.

For Racing

The same synthesis algorithm was used to design a novel racing suspension. The goal is to maintain near vertical alignment of the wheels to the road during cornering. The complete suspension is analyzed as a symmetric planar 12-bar linkage with ground pivots located at the contact patches. The design procedure specifies the vehicle chassis orientation and the tire camber angles of the vehicle when cornering.  As well, two task positions of the wheels with respect to the chassis are specified for suspension movement in straightaways.  The result is 18 design equations with 18 unknowns that have a total degree of 2,097,152, though only 336 roots.

Fifth-scale prototype of racing suspension

M. Plecnik and J. M. McCarthy, 2014. “Vehicle suspension design based on a six-bar linkage,” Proceedings of the ASME 2014 IDETC/CIE Conference, Paper No. DETC2014-35374, August 17-20, 2014, Buffalo, New York, USA.

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