Suspension Kinematics by sanmelody


									Suspension Kinematics

  Reasoning and Approach
•   Design assumptions & goals
•   Wheelbase
•   Track width F/R
•   Predictability
•   Roll Center
    – Migration, height, roll axis
• Camber variation
    – Front and rear
• Front & rear design criteria
    – Adjustability
• Unequal length, non-parallel A arms
• Allows most design flexibility IE:
  maintaining desired roll center and camber

• Most FSAE teams use this design
• ~700lbs with driver, weight biased towards
• “Engine limited” in low rev range (primarily
  corner exit’s, unless down shifting mid
• Tires can tolerate some camber change

• Car will probably tend towards oversteer
         AutoX Environment
• Hardly ever in steady state (if ever)
• Response > Power
• Predictability is important to maintain limit
• Make car easy to handle at the limit
  – Predictable
• Make sure camber curves stay reasonable
  – Roll, pitch, steering, and combos of these

• Find a balance between two above traits
  – They are not mutually exclusive!
• Car should be stable in high speed
• Car should rotate easily in tight corners,
  without over-steer
• Mike Doyle: “Cut hard and then hold”
• Larger than 60”, but most schools stay
  below 70”
• Smaller = more responsive
  – Less steering angle needed
  – Creates packaging issues

• 65 inches chosen
     Trackwidth / Wheelbase
• Important ratio relating width to length
• 2005 top cars examined from R&T article
  – All cars fell between 67-78%

• We opted for a midrange 70% ratio
             Trackwidth F&R
• Front track > Rear track
  – Increased front roll stiffness
     • Helps decrease oversteer
  – Less chance of clipping cones
     • Rear follows slightly larger radius

• Front track: 48”
• Rear track: 46”
• Limit is easily found and maintained
  – Car is easy to drive at limit
  – Driver isn’t left guessing
  – Maximize “available contact patch”
• What makes a car predicable?
• WT takes time T, allows driver to feel car
  through transients (driver feedback)
• Elastic and Geometric WT takes place at
  different rates!
• A predictable car should maintain a
  constant ratio of Elastic to Geometric WT
• Make sure tires stay relatively flat with
  – Grip is proportional to contact patch size!

• Final Summation: Tires make up for small
  changes in camber through flex, so
  predictability due to WT more important to
         Roll Center Migration
• Lateral Migration
   – More lateral movement means less roll on that side of
   – In our case: Inside extends more than Outside
• Effect = stiffer spring on outside

• Less spring compression = less Elastic WT,
  more Geometric WT!
   – Can cause increased under/oversteer
       Limiting RC Migration
• Helps solve predictability issues
• Keeps “rate of roll generation” constant
  – Dampers, springs, ARB’s move more

• Final Summation: Lateral RC movement
  acts as a form of “dynamic anti-roll” that
  changes with roll angle
                  RC Height
• Height of RC is a form of static “anti-roll”
• Above ground = smaller roll moment
  – Larger Geometric WT component
     • Better response
  – Less reliance on ARB’s
• Keep ratio of Geometric & Elastic WT
  const F&R by keeping the same roll
  moment F&R
                  Our car
• RC height
  – Front: 2.125” above ground
  – Rear: 2.423” above ground

• RC Migration at 1.5deg of body roll
  – Front: 0.795” toward inboard side
  – Rear: 1.067” toward inboard side
                  Roll Axis
• Line drawn between F & R RC’s
• Relative motions of RC’s important
• Skewing roll axis causes pitch/yaw
  – Hurts predictability

• To keep roll moments constant F&R =
  Slope of roll axis must equal slope of mass
  centroid axis of car
          Camber Variation
• “Ideal” for braking = no camber gain
• “Ideal” for cornering = single pivot
• Goal is to keep tire at optimum angle to
  ground in all situations

• For AutoX, camber variation in roll is most
• Often times keeping RC’s in check gives
  you lousy camber variation, or vice versa
• Finding a balance is an iterative process

• Finding “acceptable” values of camber
  gain F&R in roll and heave vs “acceptable”
  amounts of RC migration
    Clues to Camber variation
• The body should not be rolling, without the
  front wheels being steered
• Camber variation in roll is less vital in the
  front because of camber gain due to
    Clues to Camber Variation
• In the rear, camber on outer tire should be
  examined during squat and roll, to allow
  early throttle application out of a corner

• In general, camber variation in roll is more
  important in the rear because caster
  cannot be adjusted to make up for lack of
                  Our car
• Camber variation in heave
  – Front: -1.067deg per inch
  – Rear: -1.322deg per inch

• Camber variation in roll (laden tire only)
  – Front: 0.572 deg per deg
  – Rear: 0.488 deg per deg
                 Front end
• Ratio of mechanical to pneumatic trail
  – Allows driver to feel tires before washing out
• Enough (or adjustable) caster
  – Camber gain while turning
                Front End
• Scrub radius affects feedback under
  braking and increases caster effects
  – diagonal weight jacking, unloads inner rear
  – “tram-lining”
• Keep fairly small
  – B/C we have manual steering, fast ratio
• One solution: KPI, but side effects include
  – Decreased camber gain due to caster
  – Increased self-centering force
                Front End
• Ackermann
  – Increases response
  – Used to cause yaw moment while turning to
    rotate car
  – Heats tires
  – Reduces need for F toe-out
  – Less understeer
• Adjust using replaceable brackets
                   Rear End
• Minimize “scrub radius”
  – Less deflection under power
     • Make upper brackets higher profile
• Maximize toe stiffness
  – Links as far apart as possible
• Emphasize camber variation in roll
  – Caster cannot be adjusted
           A Arm size/shape
• Front
  – Steering linkage packaging
    • Toe stiffness, interference
  – Highest loading
  – Right triangle weaker than isosceles design
    • Find balance, maximize FOS
• Rear
  – Pushrod paths past axles
  – Maximize toe stiffness
• Camber
  – Shims between upright and upper a arm
    • Add or remove to change camber
• Caster
  – Offset upper and lower A arms
    • Keeps A arm design simple: one fixture
    • Adjust by rotating wheel package & using new
      mounting hole on upright bracket
• Toe
  – F&R uses toe links with threaded rod ends
  – Rear toe links mount to chassis, not A arms
     • Better load paths, no bending moment induced
• Anti dive/squat
  – Small area of adjustment on inboard brackets
  – Also adjusts pitch center location
      Factors that make our car:
• Understeer                   • Oversteer
  –   LSD                        – Weight bias towards
  –   Wider front track            rear
  –   Higher F natural freq.     – Same tire size F&R
  –   Not enough ackermann       – Higher rear RC
                                 – More RC migration

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