Performance Glossary of Terms
Everything you thought you should already know, but were afraid to ask.
A-arm: a wishbone (see wishbone) shaped member. Usually, the a-arm has
a pivot axis intersecting the end of each leg and a multiple axis pivot such
as a ball joint at its apex.
Acceleration: the change in velocity over time. Acceleration can be
lineal (such as pulling away from a stop light or braking?the only difference
is one is positive acceleration and one is negative) or rotational. Rotational
acceleration is when the velocity changes direction in a circular manner. For
example, even if you maintain your same speed while going around a corner, your
velocity is changing direction (remember that velocity is described as motion/speed
in a specific direction). Therefore, when you turn a corner, you are experiencing
rotational acceleration (you can feel it by your rear end wanting to slide to
the outside of the corner).
Anti-Dive: suspension geometry that resists pitch movements under braking.
Anti-Roll Bar (Sway-Bar): a torsion spring that resists independent motion of
the wheels on opposite sides of the car. It increases weight transfer at the
end of the car at which it is mounted. Read?a bar/spring that reduces cornering
body roll/lean buy tying the inside and outside suspension together.
Balanced: neither oversteer nor understeer. It is when the front and
rear tires/wheels track the same around a corner?the slip angles are the
same. When the vehicle is balanced you are able to drift
Body Roll: tendency of a car to lean toward the outside of a turn.
Bump Steer: when the front wheels are turned or change position due
to suspension movement (like when you hit a bump mid corner).
Bump Stop: elastic cushion that softens the impact when suspension travel
reaches the end of its travel.
Camber: the vertical angle of the wheel with respect to the body of
the car. Positive camber is where the top of the wheel sticks out farther than
the bottom of the wheel (think of the bull legged cowboy). Negative camber is
where the bottom of the wheel sticks out farther that the top (think of the
someone who is "knee nocky"). Lowered cars typically show negative
camber. Typically, small amounts negative camber provides the most grip. Running
too much negative camber will prematurely where out the inside of your tires.
Caster: defined as the forward and backward angle of the wheel in relation
to the steering axis
Center of Gravity/Mass: the point/position where you could theoretically
balance a car on something like a pencil (obviously a pencil wouldn't support
the weight?but you get the idea!).
Contact Patch: The contact patch is where the rubber meets the road?literally.
Four tires means four contact patches.
Control Arm: a swinging suspension member to which the axle stub/hub
carrier is rigidly attached.
Damper/Shock Absorber: controls body movement by converting suspension
movement into heat. For example, when you hit a bump, the spring is compressed
and now has "potential" energy and wants to expand back to its normal
position. The damper/shock absorber controls the rate that the spring expands
by converting that motion to heat.
Double Wishbone/Double A-arm: the suspension configuration that most
"Formula" type racecars employ. It consists of an upper and a lower
A-arm that control the hub carrier.
Heel Toe Downshifting:
Independent Suspension: suspension setup where a disturbance on one
wheel does not affect the opposite side wheel. It is the opposite of a solid
axle that connect both wheels together. In a solid axle configuration a bump
on one side affect the opposite side. Most cars now employ independent suspension
on at least one axle.
Initial Understeer: reluctance of vehicle to begin rotating about its
vertical axis on entrance to a turn.
Kinetic Energy: energy of an object due to its motion.
Lateral Acceleration: rate of change in velocity (rotational acceleration)
in the horizontal plane. Read?cornering acceleration.
Lowering: reducing the height of a vehicle's center of gravity.
MacPherson Strut: a telescopic member incorporating the damper (shock
absorber) with the wheel rigidly attached at its lower end by linkages that
pick up lateral and longitudinal (up and down) forces. The upper end is connected
to the body.
Multi-Link: similar to a double wishbone setup but uses multiple links
to transfer lateral and longitudinal (up and down) forces.
Off-Set: the distance between the plane where the wheel connect to the
hub and the center line of the wheel.
Oversteer: When going around a corner the rear end of the vehicle becomes
loose and slides to the outside of the corner. In other words the rear end doesn't
track the front wheels. In effect, what happens is your "line" through
the corner is "tightened"?sometimes to the point where you find
yourself off the road with the front end pointing in the wrong direction. Oversteer
occurs because the cornering forces on the rear exceed the tires' ability before
the cornering forces overwhelm the front end. In other words, you have too much
grip in the front of the car and not enough in the rear. Sometimes, mild oversteer
is desirous on the track. However, a balanced vehicle or even one with mild
understeer is much easier to drive at speed (there are a few racing drivers
that prefer a loose rear end). To overcome the grip imbalance, more of the cornering
burden needs to be transferred from the rear end to the front. You could also
increase the rear end's gripping ability (i.e. larger tires?but different
tires sizes front and rear does not work well with all-wheel drive). The balance
can be changed in a couple of ways. For example, you could change the tire pressure
front to rear, stiffening the front end with a larger anti-sway bar so it has
more grip, etc.
Plow: Severe understeer.
Potential Energy: energy of an object due to its position or stored
energy. A compressed spring has potential energy as well as a vehicle sitting
atop a hill.
Semi Trailing Arm: control arm configuration where the pivot axis is
angle with relation to the vehicle centerline.
Skid Pad: flat area (usually pavement) where a vehicle's lateral or
rotational acceleration is measured?usually 200 or 300 ft in diameter.
Vehicle is driven around at maximum speed in a continuous circle to determine
maximum grip. Read Skid Pad as a test ground for a car's static grip/handling.
Slip Angle: the difference between the orientation of the wheel plane
and the direction of wheel travel.
Snap Oversteer: oversteer that begins suddenly and/or without much notice?usually
faster than the driver can correct. Old Porsche 911's used to have a reputation
of snap oversteer when the throttle was lifted mid corner (driver induced error).
Sprung Weight/Mass: portion of a vehicle's mass that is supported by
the springs. Movement of mass is independent of road surface.
Suspension Travel: the difference between the lowest and highest extremes
of suspension movement.
Terminal Oversteer: tendency of vehicle to continue rotating about its
vertical axis when exiting a turn.
Trail Braking: since a tire can only provide a maximum grip of 100%
you cannot brake and get 100% cornering ability. Therefore, trail braking is
gently trailing off the brakes as you increase steering into a corner. The advantage
of trail braking is that by braking (not at 100%) you increase the load (temporarily
change the weight distribution) on the front tires, which gives it greater cornering
Toe: the difference in distance between the front edge of the tires
from side to side and the distance of the rear of the tires (same axle) measured
from side to side. It's easiest understood in terms of people. Just think of
someone who is pigeon toed has "toe in" and some who is duck footed
as "toe out".
Turn In: the initial response of a vehicle when changing from a straight
line to turning.
Understeer: Understeer is the opposite of oversteer. It occurs when
the front end is over burdened before the rear end (the rear can still say "bring
it on baby!"). The front-end wheels are turned at a greater angle than
the vehicle's direction. Understeer gives you that "plowing" feeling.
In the extreme case, it is like driving on ice where you try to turn but the
vehicle keeps going straight. Most manufacturers dial understeer into all of
their cars because an understeering vehicle is much safer for the majority of
the sue happy public who don't know how to drive.
Unsprung Mass: mass acted directly upon by the road surface?i.e.
the wheel, tire, hub, spindle brakes, and a portion of the suspension including
the control arms, spring, and damper.