The problem
Small electric cars have heavy floors and tight packaging around the battery, motor and drive shafts. The front suspension geometry decides camber control, steering feel and tyre wear. Double wishbone suspension offers good control but its hard points interact in ways that are hard to see without a kinematic model. This project builds that model and uses it to make design decisions.
Objectives
- Define hard points for upper and lower wishbones, upright, tie rod and spring-damper.
- Use a Motion study to plot camber, toe, caster and track change through bump and rebound.
- Find roll centre height and scrub radius, and tune hard points to meet targets.
- Check control arm and upright strength under loads exported from the motion study.
What you need
Tools and toolboxes
- SOLIDWORKS 3D Sketch and Layout — Hard points as 3D sketch points with dimensions driven by global variables, for quick geometry changes.
- SOLIDWORKS Assembly with mates — Concentric and coincident mates at bushings and ball joints, with a linear motor at the wheel centre.
- SOLIDWORKS Motion Study (Motion Analysis) — Bump and rebound travel, angle and displacement plots, spring force and joint reactions; Motion Analysis needs SOLIDWORKS Premium or a Simulation Professional or Premium licence.
- SOLIDWORKS Simulation (Static) with Import Motion Loads — Strength of lower wishbone and upright using joint forces from the motion study; manual load application is the alternative.
- Excel or MATLAB — Camber, toe and roll centre calculations from exported coordinates and comparison with targets.
Software
- SOLIDWORKS 2023 or later (Premium for Motion Analysis)
- SOLIDWORKS Simulation
- Excel or MATLAB
Extra parts and data
- Tyre and wheel data — Wheel offset, tyre size and contact patch location set scrub radius and the wheel envelope.
Key concepts
- Camber gain
- How camber changes as the wheel moves in bump. Negative camber gain in bump helps keep the outer tyre upright in a corner when the body rolls.
- Bump steer
- Unwanted toe change as the wheel moves vertically, caused by a tie rod that does not follow the wishbone arc. It should be small and predictable.
- Roll centre
- The point about which the body rolls, found by the intersection of lines through each wheel's instant centre and contact patch. Its height affects load transfer and jacking.
- Scrub radius and kingpin inclination
- The distance at ground level between the steering axis and the tyre centre line. It affects steering feel, braking stability and steering effort.
Step-by-step guide
- 1
Set targets and packaging
Define an example small electric car: front track 1,350 mm, wheelbase 2,400 mm, front axle mass and a wheel travel of +80 mm bump and −70 mm rebound. Set example targets: camber gain in bump around −0.5 to −1 degree per 25 mm, bump steer under 0.1 degree over travel, roll centre height in a stated range and a small positive or slightly negative scrub radius. Mark packaging zones for drive shafts, steering rack and battery.
Checkpoint: A target table and packaging zones, all labelled as example values.
- 2
Place hard points
In a 3D sketch in the assembly, place points for lower and upper wishbone inner pivots, outer ball joints, tie rod inner and outer points, spring-damper mounts and wheel centre. Drive their coordinates from global variables in a table. Draw lines between them to see the arms and steering axis. Compute kingpin inclination, caster and scrub radius from these points.
Checkpoint: A 3D layout where changing a global variable moves the hard point, and steering axis angles are reported.
- 3
Model parts
Model the wishbones as simple A-arms between their hard points using sweeps of tube or pressed profiles, the upright with ball joint and tie rod bosses, the tie rod, and a spring-damper as two parts. Use steel or a forged aluminium alloy from the material library. Keep geometry linked to the hard-point sketch so the model updates when points move.
Checkpoint: Parts update correctly when a hard point is moved and rebuild without errors.
- 4
Assemble the kinematic model
Fix the chassis. Add concentric mates at inner pivots, treating bushings as rigid; add ball-joint behaviour at outer joints with coincident point mates. Fix the rack in the straight-ahead position so steering is locked. Check that the wheel moves with one degree of freedom. Add a linear motor at the wheel centre to drive vertical travel from full rebound to full bump.
Checkpoint: The assembly moves through full travel with one degree of freedom and no over-defined mates.
- 5
Run kinematic sweeps
In the Motion study, run the bump-rebound sweep and plot wheel centre vertical position, camber angle, toe angle and track change. Export to CSV and process in Excel or MATLAB. Compute camber gain, bump steer and track change rate. Compare with targets. If bump steer is large, check the tie rod inner point height and length relative to the wishbone arcs.
Checkpoint: Camber, toe and track curves with computed gains compared against targets.
- 6
Find the roll centre
At each travel position, compute the instant centre of the upright from the intersection of upper and lower wishbone lines in front view, then the line from the contact patch through the instant centre. For a symmetric car at ride height, the roll centre is where this line crosses the centre plane. Track how roll centre height moves through travel and in roll, using opposite travel on each side.
Checkpoint: Roll centre height at ride height and its migration through travel and roll.
- 7
Tune hard points
Change upper arm length and inner pivot height to adjust camber gain and roll centre, and tie rod points to reduce bump steer. Use a Design Table or a script in MATLAB to try a set of variants. Keep inside packaging zones and keep ball joint angles within the parts' allowed range. Record the best hard-point set and the trade-offs it makes.
Checkpoint: A tuned hard-point set meeting all kinematic targets within packaging, or a stated compromise.
- 8
Add steering and check
Free the rack and add a rack-and-pinion mate. Sweep steering from lock to lock at ride height and plot wheel angles, checking Ackermann and steering axis behaviour. Then combine steering and bump at the extremes and check ball joint angles and wheel envelope clearance to the arch and drive shaft with Interference Detection. Note the minimum clearance at each extreme and the ball joint angle used compared with the maker's limit, and record the turning circle at full lock.
Checkpoint: Steering and combined bump-steer checks show no interference and ball joint angles within limits.
- 9
Export loads and check strength
Define example load cases: vertical bump at a stated multiple of static load, cornering lateral force and braking longitudinal force at the contact patch. Apply them in a Motion Analysis study and plot reaction forces at each joint. Use Import Motion Loads into a static study of the lower wishbone and upright, or apply them manually. Refine mesh at bosses and check factor of safety.
Checkpoint: Factor of safety above an example 1.5 against yield for each load case at a converged mesh.
- 10
Drawings and report
Make a hard-point table drawing and part drawings for the wishbones and upright, with GD&T on ball joint tapers and bushing bores. Write up targets, hard points, kinematic results, roll centre, tuning, steering checks and strength. State limits: rigid bushings, no compliance, quasi-static loads. Recommend a kinematics and compliance test or a measured wheel alignment on a prototype. List the global variable values used so the model can be rebuilt exactly.
Checkpoint: A hard-point table, drawings and a report with kinematic curves and strength results.
Testing & validation
- Kinematic curves are smooth and symmetric about ride height where expected.
- Roll centre from coordinates matches a graphical construction at ride height.
- Bump steer and camber gain meet targets.
- Strength checks meet factor-of-safety targets with mesh convergence.
Expected results
A parametric double-wishbone suspension with tuned hard points, camber, toe, track and roll centre curves, steering and clearance checks, and strength-checked control arms and upright.
Research angle for your M.Tech thesis
Small EVs place heavy batteries low in the floor and often use compact motors and drive shafts, which constrains hard points. A study can define the kinematic targets for such cars, perform a sensitivity analysis of each hard point on camber, bump steer and roll centre, and use optimisation to find robust hard points within packaging. Adding compliance and validating against a measured alignment rig would raise its research value.
Take it further
- Add bushing compliance and compare kinematics and compliance results.
- Integrate a drive shaft for an in-board motor and check plunge.
- Run a fatigue study of the lower wishbone with a load spectrum.
- Compare with a MacPherson strut on the same car.
Viva and review questions
Why do you want negative camber gain in bump?
When the body rolls in a corner, the outer wheel moves into bump. Negative camber gain then keeps the outer tyre more upright and improves grip.
What causes bump steer?
A tie rod whose arc does not match the arc of the wishbones, so the upright is pushed or pulled as the wheel moves vertically, changing toe.
How did you find the roll centre?
From the front-view instant centre of each upright, then a line from the contact patch through it, intersecting the vehicle centre plane.
How did you get loads for the strength check?
From joint reactions in a Motion Analysis study with example load cases, imported into a static Simulation study of the arms.
Learn it at DIYguru
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