Thursday, September 22, 2011

Some helpful definitions

The world of automobiles can be a strange and frightening place before you even think of donning fire proof coveralls, helmet, and a Hans device. So to make this a little bit easier on all of us here are some helpful definitions which should come in handy when reading our tech articles. We wont blame you for correcting your friend when he mistakes understeer with roll steer. Go ahead be smug. You are welcome.

  • Roll Center - Both front and rear suspensions have a Roll Center. This is an imaginary point around which the body of the car will rotate in a turn. The attachment points of the suspension components determine the Roll Center.
  • Roll Axis - A line between the front & rear Roll Centers.
  • CG (Center of Gravity) - The center point of the vehicle's mass.
  • Understeer - When the front tires lose traction first.
  • Oversteer - When the rear tires lose traction first.
  • Neutral Steer - The ideal balance when the front & rear tires gradually give up traction at an equal rate.
  • Spring Rate - Expressed in pounds per inch, it is the force necessary to compress the spring, i.e. a 200 lb spring requires 200 lbs to compress it 1 inch, 400 lbs to compress it 2 inches, etc.
  • Motion Ratio - Specifically we usually refer to the relationship between the motion of the wheel and the motion of the spring; i.e. If the spring is half the distance from the control arm pivot as the wheel is, the motion ratio relative to the wheel is .5 to 1.
  • Wheel Rate - The combined effect of spring rate, motion ratio, friction and/or binding of other suspension components measured at the wheel
  • Roll Bind - Any binding of suspension components that occurs as the body of the car leans over in a turn
  • Roll Steer - Generally refers to a steering effect on the rear axle as the car leans over in a corner. Caused by the rear control arms pivoting around their forward mounting point, drawing the axle forward as the arm moves up or down.
  • Bumpsteer - Toe change as the suspension moves up & down
  • Ackerman - Or in other words Toe out in turns. When turning the inside tire must turn more than the outer tire because it is turning on a smaller radius
  • Camber - Expressed in degrees, it is how much the tire leans in or out
  • Caster - The forward inclination of the spindle or strut - like the forks on a bicycle
  • Toe - The difference in the distance between the leading and trailing edge