ASC Suspension Basics

A summary of points relevant to our car

Onboarding Resource

Static Analysis -Front Suspension

• inside wheel of solar car vs FSAE usually outside wheel analysis because N force >

Force analysis and designing parts for loads

• 2g bump, 1g lateral force turn, 1g braking input forces referenced to the contact patch

  • keep in mind the changes in N force during all ^ behaviors

  • MINIMUM

• statics problem w 3 torques 3 forces

  • if pass forces on one component - pass to the next component - to the frame - determine if the frame mounting point will fail at these conditions

• combined loading conditions AND separated loading conditions

  • one loading condition may be HELPING the other one - so important to do combine + separate because it may pass in combined but not separate

  • ex) braking + cornering - cornering may be helping alleviate the > N force on front axles during braking and < load from pitching forward - vice versa

brake perform analysis

• VDR Requirements - will car survive brake system, will brake system survive, will car perform and pass dynamics brake performance (4.7 m/s^2)

• pedal force required? - panic stop - when our car is decelerating at this rate ^ (to pass) then how much is the pounds of force on pedal to achieve that?

  • is this a reasonable number? too much or too little force to achieve is not good

  • hard to modulate brakes that react too easily - preventing from locking up

  • convergent behavior

• what point do front lock up vs required force to gen 4.72 m/s^2 gradual braking

  • 50 lbs F to braking performance vs 60 lbs to lock up - driver needs to be good at working in the spot between - decelerating but not locking up

• at point F wheels lock up - did rear wheels lock up yet

  • wheels that lock up are not preventing slip - no lateral acceleration or cornering force

  • locked up wheels have now lost their ability to stop themselves from sliding

  • front with braking force and imbalances / if one wheel is more brake force or CG not centered or sidewind - back wheels will slide to one side and create a yaw and now unstable = spinout

• sim - 200lbs F on the brake pedal and box - does the mount and hydraulics survive?

sample brake calculations

Camber gain from bump - good because roll out and camber neg counteracts roll (counteracting the wheel from lean out or pos camber induced by roll) and in theory the wheel will be up and down and leaning out less

• shorter upper A arm and longer bottom for negative camber gain

• angled up upper A arm / tilt it down to inboard

• change the lengths of control arms, angle of them → optimizing for camber gain

Open

yellow neutral, orange bump

Scrub caused by bump - reduce > bc > energy loss

• caused by suspension motion → lateral component - lateral movement of tire side to side

• bump - suspension compresses - angled arms become horizontal and push the wheel out - vice versa, creating a scrub radius from bump

• aim to keep contact patch at same point - the tire can move ( camber change?) - optimizing for negative camber gain in bump doesnt solve this though

• neutral bump start with control arms horizontal - the displacement when the control arms change angles during dump will be less

• longer control arms CAN decrease scrub but also decrease camber gain - compromise

Leading arm suspension - MIN scrub bc constrain to vertical movements but V LITTLE camber gain

• control arms and inboard and outboard same

Nat frequency to determine springs - easy method for solar cars - take calc weight from axle and derive springs from that natural frequency - consider when motion ratios are factored in

• front > natural frequency than rear - can do rear x 1.15 = front

  • effective spring rate at the wheel finding

• > frequency = > stiffness = < grip - soft suspension / soft and high temp tires > grip

• → front understeer bc of the grip - obviously more grip means more potential to oversteer

• passenger cats 1.0 - 1.3, sports sedans 1.2 - 1.5, sport cars 1.5 - 1.8

  • more damper dependent ^ when softer

• race cars w aero > 1.8 frequency - > stiffness so vehicle handling < performance

Caster - increases steering effort and feedback (good) - increases negative camber gain with steering angle (x,z plane outboard) - having wheels lean into turns and induce camber

• pos - lean back - DB, preferred

• neg - forward

• built-in -

• more caster = more steering torque

• helps self center steering

Kingpin inclination - caster but front of the car - angle of steering axis from vertical

• camber gain while steering

• > KPI = positive camber gain - not good - we want negative camber gain on the outside wheel

scrub radius

Camber and jacking force