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Dk Vehicle Studio

Sole vehicle platform design engineer · Three-person micromobility startup · Aug 2022 to Aug 2024

A hybrid wishbone and McPherson suspension for a 95 cm platform

Company, brand and product withheld under NDA; figures are simplified or masked.

Plan, front ←LL6tie rod1 tyre2 upright, fixed to3 lower wishbone4 damper mount5 damper, axial force only6 tie rodFd​ · rd​ = Fz​ · rw​about the pivot axisMR = Δsd​ / Δzw​bodyFz​Fy​Fx​Ry​Rz​Rx​Fd​rw​rd​12345bodyFz​Fy​Fx​Ry​Rz​Rx​Fd​rw​rd​12345Plan, front ←LL6tie rod1 tyre2 upright, fixed to3 lower wishbone4 damper mount5 damper, axial force only6 tie rodFd​ · rd​ = Fz​ · rw​about the pivot axisMR = Δsd​ / Δzw​
Fig.
1
Title
The corner as a free-body diagram
Kind
Schematic, principle only, not to scale
Source
Drawn for this site
Note
The damper sits on joints free to rotate, so it carries axial force only, no side load and no bending. The tie rod has the wishbone's length and its inboard joint on the wishbone's pivot axis: bump steer zero by geometry. The moment balance shows the vertical load only.

Problem

The platform had to fit a bicycle lane, stay under 50 kg complete, and ride and handle like a four-wheeler. In a McPherson strut the damper is part of the structure and carries side load and bending, and a damper built for that is heavy; a light bicycle damper is built for axial load only.

Approach

With the upright fixed to the lower wishbone, the side and fore-aft forces at the patch go to the wishbone's pivots, and the damper, free to rotate at both ends, sees only the force along its axis (Fig. 1): the load such a damper is built for. That one decision opened up off-the-shelf dampers and a compact corner. What it fixes in return is the camber path: the wheel's camber follows the wishbone's arc, so I set the arm to use that camber gain rather than fight it.

What I did

  • Sized the corner around off-the-shelf dampers, lighter and cheaper than a bespoke unit.
  • Modelled it in Lotus Suspension Analysis and carried it into Adams Car for full-vehicle studies.
  • Used the camber gain to raise the tyres' cornering force, and set the motion ratio so the damper's stroke covers the wheel travel.
  • Designed the steering knuckle and joints the new geometry needed.
  • Took the mounts and links through stress and fatigue load cases in FEA before release (Fig. 2).
Register excerpt: 5 checks, with load case, loading, material, the source of each check, and masked values
SystemPartLoad caseLoadingForces, momentsStressesMaterialSafety factors (tension, shear, bearing)Source
SuspensionWishbone tubesBraking + cornering, 0.55 g + 0.55 gBending and axial forceAL 6061 T6FEA
SuspensionWishbone tubesMax heave, 2 gBendingAL 6061 T6Bending+Torque
SuspensionDamper mountMax heave, 3 g, stiffest damperTensionAL 6061 T6FEA
SuspensionSteering knuckleMax braking, 0.7 gTensionAL 7075 T6FEA
SuspensionKnuckle bearingBraking + cornering, 0.55 g + 0.55 gBearing reactionBearingBearing Loads
Fig.
2
Title
The corner in the register: 5 of 77 checks
Kind
Workbook extract, values masked
Source
The project's design calculators

Result

  • A corner inside the 95 cm width, with zero bump steer by geometry.
  • Off-the-shelf dampers: lower mass and cost than a bespoke unit.
  • The first prototype is in production with this geometry.