A
A-pillar
The roof support on the left and right side of the car’s windshield.
Active Suspension
A cutting-edge suspension system that’s computer-controlled and uses powered actuators to position a car’s wheels correctly to evenly distribute load and handle poor road conditions or disturbances. This system replaces the conventional shock absorbers and spring system.
Aerodynamic Drag
A moving object creates drag when it displaces air in its path. The force exerted by aerodynamic drag is generally measured in pounds and increases in proportion to an object’s square of speed, drag coefficient, and frontal area.
Air Dam
A front spoiler that’s mounted underneath the bumper and designed to limit the flow of air under a car. Air dams can reduce aerodynamic drag, reduce lift, and increase radiator airflow.
Anti-Dive
A characteristic of a tuned-in front suspension that aims to reduce dive under braking by lessening the vertical force and redistributing it.
Anti-Lock Braking System
Commonly referred to as ABS this braking system senses when the wheels are about to lock up and reduces the force of the brake to keep them rotating. ABS can control all 4 wheels of a car, or only 2 (front or back).
Anti-Roll Bar
Sometimes incorrectly called a sway bar, an anti-roll bar is part of the suspension that reduces the severity of the car body rolling or rocking from side to side at speed. It resists the unequal vertical motion of the wheels to improve stability, but it does not affect the suspension when the wheels are moved equally in any direction.
Anti-Squat
This characteristic of the suspension uses acceleration-induced forces in the back suspension to reduce squat. Works in a similar way to anti-dive.
Apex
The clipping point on the inside of a corner that a car passes the closest to.
Aspect Ratio
The ratio between an object’s two dimensions. In the case of tires, the aspect ratio is the proportion between the width of the tread and the sidewall height. A low aspect ratio implies short, wide tires, while a high aspect refers to taller, skinnier tires. Also used to describe the wing of the aerofoil and how its dimension relates to the perpendicular airflow.
Axle Tramp
A type of wheel hop that cars with live axels can perform when the axel slightly rotates repeatedly as the wheels turn, and then springs back into position.
B
B-pillar
The supportive roof pillar between the car’s rear side window and front door window.
Balance Shaft
Balance shafts are not essential to an engine’s operation, but they provide a reduction in vibration by rotating in a way that dampens the shaking movement. They’re becoming increasingly common in cars that have refined engines as they enhance performance and drive comfort. Three-cylinder and V-6 engines use a single balance shaft, while four-cylinder engines use two shafts that rotate in opposite directions on either side of the crankshaft.
Ball Joint
A joint that’s flexible and able to accommodate angular motion with ease. The joint comprises a ball in a socket and is often used in the front suspension of a car due to its wide range of manoeuvrability.
Beam Axle
Also referred to as a dead axle, this axle is rigid and supports the wheels that are non-driven.
Beltline
The bottom edge of a car’s glass panels or glass housing that forms a line.
Bevel Gears
A type of gearset where the gears are shaped like a sliced cone. The cone slice shape allows the axes of the gear to sit nonparallel to one another. These bevelled gears create motion at an angle, allowing two axels to spin at differing speeds.
Boost Pressure
Usually measured in inches of mercury, psi, or bar, boost pressure is the increase in atmospheric pressure produced in the intake manifold by a supercharger.
Brake Bias
The rear or front distribution of a vehicle’s braking power that’s measured as a percentage of total braking force. Also called brake balance it determines how much force the brakes apply on braking, and this can depend on the surface, speed and severity at which the brakes are applied.
Brake Modulation
The ability to control the force applied to the brake to prevent them from locking up. Ideally, brakes should unlock when there’s a slight reduction in pressure, but usually, this isn’t the case. Modulation helps to regulate the pressure and prevent locks.
Brake Torquing
Essentially the power of the braking system and the efficiency it provides. Performance tests often include brake torquing and involve the sudden depressing of the brake followed by accelerating in gear and then releasing the brakes. It’s very effective in cars with turbos as it prevents turbo lag.
Breathing (engine)
The ability of the engine to fill its cylinders with air-fuel mix and discharge any burnt gases that result from this mix. The more air-fuel mixed, the more power a car engine has.
Bushing
A simple type of suspension bearing that allows for limited rotary motion. Typically, a bushing is made of two coaxial tubes made of steel that have a rubber sleeve in between them. The bushing affects handling and harshness due to its compliance.
C
C-pillar
The roof support between a car’s rear window and the rearmost side window. In vehicles that have four side pillars, the rearmost roof support is often referred to as a D-pillar.
Cam Profile
The shape of the lobes on a camshaft. The shape or profile determines the time that a valve opens and its potential lift.
Camber
Used in steering and suspension design the camber refers to the angle of a car’s wheels when viewed from the rear or front. If the wheels tilt outwards, the camber is positive and if they tilt outwards, it’s negative.
Camshaft
A rotating shaft that’s fitted with several cams that have lobes that push valve lifters. When this happens, it converts rotary motion into linear. Camshafts are integral to piston-driven engines as they regulate the opening and closing of all the piston valves when in motion.
Carbon Fibre
A carbon composite that’s lightweight yet very strong and flexible. Carbon fibre is made up of strands of fibre that are bound by resin, heat, pressure or vacuum to create a composite that’s on the higher end of the price spectrum.
Caster
An angle measured on minutes and degrees that measures a car’s steering axis from the side. It identifies the backwards or forwards slope of the lower steering pivot points.
Catalytic Converter
Often simply called a “catalyst”, this is the exhaust emission control device that is responsible for converting pollutants and toxic gases created by the engine. The converter creates a catalysing redox reaction that changes the chemical nature of emissions and makes them less harmful.
Centre Differential
A differential in four-wheel-drive car engine systems that’s used to distribute power to both the front and rear differentials.
Chassis
The supporting frame of the car onto which all mechanical and other parts are attached. If a car has unitized construction the chassis is everything but the body of the vehicle.
Coil Spring
A flexible resilient metal spring that’s wound into a spiral and can be compressed and “bounce back” without damage. Usually used in suspension, coil springs have multiple applications in cars.
Combustion Chamber
The space in the cylinder above the top of the piston. This chamber is formed by the cavity in the cylinder head and the top of the piston, and the majority of air-fuel combustion occurs here. The shape and design of this chamber greatly affect a car’s performance as it influences engine emissions, power and fuel efficiency.
Compliance
The give or resilience that the suspension bushings incorporate to assist in absorbing the impact of a bump or jolt. If a car has good compliance it allows the wheels to adjust when hitting a bump but doesn’t allow for lateral movement when cornering. This adds to the car’s stability.
Composite
Any type of material that is made up of two or more components. Carbon Fibre and Fibreglass are both examples of composite materials.
Compression Ratio
A ratio calculated between the combined volume of a combustion chamber and cylinder when the piston is at its bottom stroke and the volume when it’s at the top. A higher compression ratio indicates that more mechanical energy can be drawn from the air-fuel mix, however, a higher ratio also makes detonation a more likely occurrence.
Connecting Rod
A metal rod that connects the piston to a crankshaft throw.
Constant-Velocity Joint
A universal joint that is designed not to allow any cyclic fluctuation between the speed of the input and output shafts.
Control Arm
Also called an A-arm or wishbone, this suspension element has two joints on one end (usually on the side of the chassis) and a single joint on the other.
Cornering Limit
The maximum speed that a car can negotiate a corner or curve.
Coupe
A car with a sloped roof and two side doors. According to SAE standard J1100, a coupe has less than 33 cubic feet of rear interior volume. This means that not all two-door vehicles are automatically coupes.
Crankshaft
A shaft driven by cranks that are coupled by connecting rods to pistons in the engine. By working concurrently, the crankshaft and con rods are able to turn the pistons’ reciprocating motion into a rotary one.
Cylinder
The straight-sided round cavity where pistons pump upwards and downwards. A cylinder is usually made of cast iron and forms part of the engine block.
Cylinder Head
An iron or aluminium casting that houses intake and exhaust ports, combustion chambers, and some or all of these valvetrains. The head or heads are always located above the cylinders.
Cylinder Liner
Also called a sleeve, the cylinder liner is the circular housing that the piston moves in when the cylinder doesn’t form an integral part of the block.
D
dBA
The measurement unit for decibels. Named after Alexander Graham Bell it measures the intensity of sound in a logarithmic way. Every 3dB increase in sound represents a doubling of the sound pressure. The “A” represents the fact that the measurement is taken with an A-weighted scale. This scale approximates a human’s ear sensitivity to varying frequencies.
de Dion Suspension
A suspension system where the rear wheels get bolted to a lightweight, transverse, rigid structure. The wheel’s rotation is powered by universal-jointed half-shafts that get attached to a body-mounted differential.
Dead Pedal
A footrest on the left of the furthest left pedal. This resting pedal gives drivers a place to brace when cornering hard.
Detonation
Also known as “knock” detonation occurs when an unburned air-fuel mixture in the combustion chamber explodes spontaneously after a spark plug has fired. The explosion is triggered by the pressure and heat of the air-fuel mix that’s already alight. Detonation isn’t desirable and adds stress to the engine.
Differential
Also called a “diff” this gearbox is specially designed for the torque feed to be split and sent to two outputs that can rotate at varying speeds. Differentials in axles split torque evenly but if they’re in a 4-wheel drive system this changes to an uneven distribution.
Disc Brakes
The full term for these is calliper disc brakes and they consist of a disc rotating at the same speed as the wheel. Callipers straddle the discs and can squeeze from the outsides to clamp down and cause a slowing effect. Disc brakes are more linear in their response and safer than drum brakes in wet conditions and higher temperatures
Dive
The dip that occurs in a car’s nose when the brakes are applied hard. Dive is caused by the dip in the load transfer from the rear to the front suspension and it occurs due to the car’s centre of gravity being affected.
DOHC
Stands for Double Overhead Camshaft. In a DOHC engine, there are two camshafts in each cylinder head. There is one camshaft that operates the intake valves, while the second actuates an exhaust’s valves.
Downforce
Vertical force that directs downward when an object such as a car body creates airflow.
Drag Coefficient
The measure of an object’s aerodynamic sleekness. Signified by Cx, a sleek car will have a drag efficiency or Cd of about 0.30.
Drivability
The overalloperating qualities of the powertrain. Drivability includes cold and hot starts, the smoothness of idling, throttle response, altitude change tolerance and power delivery
Driveline
All the components of a drivetrain barring the transmission and engine.
Driveshaft
The shaft responsible for transmitting power from the transmission to the differential.
Drivetrain
The components of a car that work together to create the power that keeps the wheels turning. Literally the train of parts that creates momentum.
Drum Brakes
A brake type with an iron casing that’s shaped like a drum. This drum rotates when the wheels do and has curved brake shoes that get forced into contact with the drum to create the braking functionality.
E
Exhaust-Gas Recirculation
EGR is the method used to reduce oxides of nitrogen or NOx exhaust emissions. It does this by recirculating the engine’s exhaust gas and directing it into the intake manifold. The exhaust gas absorbs heat during combustion and reduces the temperatures reached.
Engine Control System
May be referred to as the ECU, which stands for Engine Control Unit, this computerised brain regulates the operation of the engine by regulating various factors. It regulates the RPM, intake airflow, and coolant temperature by using sensors and then controls variables according to a pre-programmed schedule.
EPA Fuel Economy
Environmental Protection Agency laboratory fuel-economy tests that use simulated weight and drag to facsimile real driving conditions. These tests were updated for 2008 model year cars to better reflect current cars and performance.
Exhaust Manifold
The pipes and passageways that collect the gases created by various parts of the exhaust ports and push them towards the mufflers and catalysts in the system. Free flowing manifold passages improve breathing and performance.
Exhaust Port
The passageway that leads from the exhaust valve to the manifold in the cylinder head.
F
Feedback Fuel-Air-Ratio Control
One of the features of a computerised and controlled fuel system. A sensor measures the oxygen content of the exhaust to keep the fuel to air ratio at the perfect level for ideal combustion levels.
Fibreglass
A type of strong composite material that’s made up of interwoven glass fibres. Often used in racing or sports car bodies due to its lightweight properties.
Final-Drive Ratio
The last set of gears that connect the engine to the axel.
Floorpan
The most important and the largest of all stamped metal parts in the body, a floorpan is made from smaller stampings that are combined to create the floor. This pan determines most of a car’s dimensions for its internal and external panels and is often the foundation that the mechanical parts are built upon.
Fluid Coupling
Can be any type of device that transfers power through a fluid. Fluid couplings have two fans sealed in a housing filled with oil or other viscous liquid. The fans churn the liquid up and create movement that powers the output fan.
Flywheel
A heavy disc-shaped wheel found on the end of the crankshaft to give its rotary inertia a boost. The flywheel smooths the engine’s power pulses and stores energy to crate consistently smooth rotation.
Four Valves Per Cylinder
A valvetrain that has 4 valves in its combustion chamber. Typically, there are 2 intakes and 2 outtakes or exhausts. The increased number of valves provides improved breathing for engines and allows spark plugs to be positioned closer to the combustion chamber.
Four-Wheel Drift
Drift occurs when the wheels slide sideways uncontrollably, and this term refers to when a car corners ad all 4 wheels operate at large slip angles.
Four-Wheel Steering
A steering system that allows for the steering of the front and rear wheels. This allows for improved handling and better manoeuvrability at speed.
Fuel Injection
A mechanical, electronic, or computerised system that controls the flow of fuel to an engine by measuring its needs. Fuel injectors or pumps regulate the flow of fuel and maintain it while the car is in motion.
G
The universally accepted unit for road holding or lateral acceleration. A single g is equal to 32.2 feet per second. This is the rate an object accelerates at when it’s dropped at sea level. The g force is often used to describe the push back force that a driver feels when cornering.
Gearset
Two or more gears grouped together that are used to transmit power.
Greenhouse
The portion of a car’s body that’s above its beltline.
Ground Effect
A phenomenon that takes place when airflow between a fast-moving object (such as a car) and the ground creates a downforce.
H
Half-Shaft
Used in independent suspensions systems, a half shaft is a rotating shaft that’s articulated. It’s used to transmit power from a differential to one of the car’s wheels.
Handling
A term that refers to a car’s behaviour and its directional control. A car with good handling performs well and offers a smooth driving experience.
Heel-and-Toe
A driving technique that combines downshifting while braking. It requires a driver to use all three pedals in a manual car at the same time, depressing each one slightly to create the desired action. To heel-toe brake, you need to brake with the toe of your right foot while using the heel or side of the foot to push the accelerator and increase the RPM. The left foot is in use on the clutch pedal as normal. The sequence that needs to be followed requires you to brake using your right toe, depress the clutch with the left foot, shift the gears to neutral while braking, press the accelerator and shift to a low gear, release the clutch and the brakes. All this should be done in a second.
Heim Joint
Also known as a “spherical rod-end” this very rigid articulating join is found in precision linkage. Because they can locate wheels incredibly precisely they are often found in racing cars.
Helical Gear
A gear type that features teeth cut at slanting angles to the gear’s circumference. Helical designed gears deliver an even, constant tooth loading in a gearset which helps to reduce noise.
Hemi
Any engine with hemispherical combustion chambers in the cylinder head. A four-valve design may be more efficient, however, a Hemi head creates extra room for large valves pairs and has great breathing characteristics.
Horsepower
The recognised unit used to measure the power of an engine. The higher the horsepower, the more powerful the engine. One horsepower is equal to 550 foot-pounds per second and this is the power needed to lift 550 one foot from the ground in a single second – or a single pound 550 up for the same duration.
Hotchkiss Suspension
A shaft-driven form of power transmission, this type of suspension characteristic has a live-axle rear suspension with leaf springs. These springs handle the axels’ location and bounce.
Hydraulic Lifter
A device that maintains zero clearance in the valvetrain. It does this by using valving pressure to adjust the oil pressure and amend its length accordingly to create an environment of zero clearance. Hydraulic lifters are noise and maintenance-free.
I
Independent Suspension
Suspension where the camber of the wheel is not affected directly by the movement of an opposite wheel and instead, acts independently.
Intake Charge
The fuel and air mixture that flows into the engine.
Intake Manifold
The passage network that directs the air or air-fuel mixture away from the throttle body to the intake ports that are found inside the cylinder head. The flow tends to run from the throttle body into the plenum (a chamber). The plenum feeds individual runner tubes to each intake port. The breathing of an engine enhances if the intake manifold has been configured to optimize the intake system’s pressure pulses.
Intake Port
A cylinder head passageway that leads from a manifold to the intake valve or valves.
Intercooler
An air to air or air to liquid cooling device that exchanges heat generated by compression in any supercharger. An intercooler is similar in many ways to a radiator as it has large passages for intake flow, and uses water, or air from outside to bring down the temperature of the intake flow inside it.
J
Jounce
The wheel motion that leads to compression of the car’s suspension.
Jounce Bumper
A type of elasticated cushion that gradually stiffens the suspension again at the end of every jounce travel.
K
Kickdown
An automatic downshift that’s caused by depressing the accelerator.
Knock Sensor
A sensor designed to improve engine power and efficiency. The knock sensor is mounted on the engine and detects high-frequency vibrations triggered by detonation. Once detected the system allows the engine to operate as close to its detonation point as possible without causing any damage.
L
Lateral Link
A wheel suspension link aligned to restrict or limit any sideways motion.
Leading Link
A wheel suspension link designed to resist longitudinal motions. This link is mounted to the chassis behind a wheel.
Leaf Spring
A type of spring that’s long, thin, and flat and bends in deflection when force is applied to it. Leaf springs are generally found in car suspension systems.
Lift
The vertical force that directs upward when airflow is pushed around a moving car
Lift-Throttle Oversteer
Related to handling. Occurs when the rear tires lose some of their cornering grip when the pressure on the accelerator is lifted slightly on a tight corner.
Limited-Slip Differential
A differential that’s been fitted with a mechanism to limit torque and speed differences of its two outputs. Limited slip ensures that some torque gets distributed to both wheels even if one is on a very slippery surface.
Line
The pathway through a corner that has the best late braking point, and entry and exit speeds.
Link
A suspension member that features a joint on either of its ends.
Live Axle
A rigid axle that incorporates axle shafts and a differential to provide power to two wheels that it can support.
Lockup
The point when tires start to skid when braking at speed. On the verge of lockup, a tire’s maximum force is exerted, this means that when a lockup is imminent a car’s shortest stopping distances are produced. Front wheel lockup is more stable than back wheel lockup, which can cause a slide.
Locking Differential
A differential with two outputs that can get locked together. This locking action eliminates any differential action and maximizes traction when in slippery conditions.
Locking Torque Converter
A torque converter that’s been fitted with a locking clutch. This clutch can be engaged to eliminate slip between a torque converter’s input and output. This can improve fuel efficiency and performance.
Loose
A colloquialism for oversteering.
M
Main Bearings
Engine block bearings that provide support to the crankshaft.
Mid-Engine
A type of chassis layout where the engine sits behind the passenger compartment, ahead of the axel.
Monocoque
Also called a called “unit” or unitized construction, this type of body structure differs from a framework of thick members and instead draws its strength from slimmer, carefully joined and shaped panels that create rigidity.
Multileaf Spring
A leaf spring that has several leaves all bound together using steel bands.
Multilink Suspension
Rear suspension that has four links and an absence of struts. A multilink suspension assigns itself to each wheel individually it greatly improves handling and ride.
N
Neutral Steer
When the car turns at exactly the same rate as the steering wheel is turned. Technically an ideal state of balance, it doesn’t provide as much stability as a minor understeer.
O
On-Centre Feel
The feel and response rate of the steering wheel when it’s centred. In a car with good on-centre feel, the steering wheel returns to the centre even when deflected slightly.
Opposite Lock
A driving technique where the steering wheel turns in the direction away from the turning point. Opposite lock controls a car when it’s oversteering and the tail is swinging out.
Overdrive
Any gearset where the output shaft rotates at a faster speed than the input shaft. Overdrive gears reduce engine RPM and improve fuel economy, so they’re found in most cars today.
Overhead Cam
A valvetrain arrangement with the camshaft/s in the engine’s cylinder head/s. A single-overhead-cam (SOHC) has one camshaft that works with all the valves in the cylinder head, while a double-overhead-camshaft (DOHC) works separately to actuate the intake valves and operate the exhaust valves.
Oversquare
An engine with a bore larger than its stroke.
Oversteer
A handling term that refers to when the rear tires’ slip angles are greater than the front tires. Oversteering cars are not desirable and are considered “loose” as the tail has a tendency to swing out.
P
Panhard Rod
A long lateral link sitting parallel to the axel and attached on one side. The other side is attached to the body to provide the axel with lateral location.
Pent-Roof
A combustion chamber usually used with four valves per cylinder. It has an upper surface that resembles a slightly peaked roof.
Pitch
Diving and squatting are pitching motions that are caused when a car’s nose or tail bounces up and down on its horizontal axis.
Planetary Gears
A gearset with all the gears on a single plane that’s usually found in automatic transmissions. The way the gears are grouped around one another are like the planets that surround the sun. The middle gear is the sun gear, and the planet gears are mounted around its circumference.
Plenum Chamber
A chamber between the runners of an intake manifold and the throttle that’s used to evenly distribute the intake charge. This enhances engine breathing.
Polar Moment of Inertia
The resistance an object has to rotational acceleration. In a car, this refers to any resistance felt when turning the steering wheel.
Port Fuel Injection
A fuel injector that has at least one mounted injector in the intake port per cylinder. The injector tends to be mounted close to the port on the air intake manifold ad is designed to improve fuel distribution. It also allows for greater intake manifold design flexibility and this improves engine breathing.
Pound-Feet
The unit that measures torque. One pound-foot equals the twisting force that’s produced when a one-pound force gets applied to a one-foot-long lever.
Power
The rate that work gets performed in the car. Power is measured in horsepower and proportional to RPM and torque.
Power Band
The RPM range that an engine delivers its power over. The power band will extend from below the torque peak to the pinnacle point of performance.
Powertrain
A combination of a transmission and an engine.
Profile
A tire’s aspect ratio.
Progressive-Rate Spring
A spring that has an increasing spring constant. If a spring requires 100 pounds of force for the first inch, the second inch would require 200 pounds of force and so on. The more pressure the spring is under, the stiffer it becomes.
Psi
The measurement for pounds per square inch. This is the common unit used to measure pressure. At sea level, normal atmospheric pressure is 14.7 psi.
Push
A colloquialism for understeer.
Pushrod
The term for any rod used to transfer force in compression.
R
Rack-and-Pinion
A steering mechanism with a gear in mesh that has a toothed bar. This bar is called the “rack.” The ends of the rack link with tie rods to the wheels. When the steering shaft causes the gear to rotate it moves the rack side to side to turn the wheels.
Rebound
The opposite of jounce, refers to the motion of a wheel extending the suspension.
Recirculating-Ball
Part of the steering mechanism that has a worm gear. The worm gear causes a metal block with teeth to move forwards and backwards. When this happens, ball bearings placed in a recirculating track are used to reduce friction that forms between the block and the worm gear. When the block moves to and fro the teeth rotate a gear that is connected to the steering arm and this moves the steering linkage
Redline
The line on the RPM counter that indicates the maximum RPMs for the engine per minute. A tachometer is the official name for an RPM counter and some have a single red line, while others have two lines, with one indicating the maximum sustainable RPM, and the other the absolute maximum.
Ride Height
The measurement between the ground and a fixed reference point on a car’s body. Different manufacturers use different points but this dimension can be used to This dimension can be used to measure the height of the car’s body from the ground and the suspension deflection amount.
Ride Steer
Also known as “bump steer” this undesirable condition causes the steering wheel to move slightly when the suspension is compressed or expands.
Rigid Axle
A simple suspension that is non-independent. It consists of a rigid transverse member with wheel hubs that have been bolted solidly on it. The axle is attached to the car’s body by a combination of suspension arms and links, or it uses leaf springs.
Ring-and-Pinion Gear
Any gearset with a small gear that rotates a large-diameter annular.
Roadholding
The ability of a car to grip the surface of the road and remain upright. Measured in gs, road holding is also described as “lateral acceleration” as cornering deviates from the straight line.
Road-Load Horsepower
The amount of power the wheels need to move a car forward at a steady speed. This power varies and it depends on a car’s speed, mechanical friction, and aerodynamic drag. The tires’ rolling resistance also have an impact. Road-load horsepower differs from engine power because mechanical losses can sap the engine’s output at the driving wheels and flywheel.
Roll
Often inaccurately referred to as”sway” or “lean,” roll refers to how the body of a car rotates on the longitudinal axis. Roll happens when cornering as the car’s centre of gravity is higher than the axis that it rotates upon.
Rubber-Isolated Crossmember
A structural member that’s laterally aligned and attached to the frame or body of a car with rubber isolators that absorb vibration. If suspension or drivetrain components use these isolating cross members the noise of a vehicle is greatly reduced even under extreme conditions or high speeds.
S
SAE: Society of Automotive Engineers
A professional association of transportation-industry engineers that sets automotive-industry standards. The SAE regulates measuring, testing and designing cars and their components.
Scrub Radius
Also known as the “steering offset.” the scrub radius is the distance from the point of the steering axis intersecting the ground to the longitudinal line running through the middle of where the tire contacts the road.
Sedan
A car with a fixed roof with at least 4 doors. According to the SAE standard J1100, a sedan is also applicable to a fixed roof car with two doors and a rear interior volume of 33 cubic feet
Semi-Elliptic Leaf Spring
More advanced than a standard leaf spring, the elliptical leaf spring is attached to a car’s body on each end and to the suspension or a component thereof in the middle. A shackle allows for changes in length as it moves downwards and upwards.
Semi-Trailing-Arm Suspension
An independent rear-suspension system. In this system, each wheel hub is only located by a large, triangular-shaped arm that has two points of pivoting. From the top, the line from the two pivots will fall perpendicular and parallel to the longitudinal axis of the car.
Series (Tire)
A tire’s aspect ratio numerical representation of. An 80-series tire has an aspect ratio of 0.80.
Shift Gate
A transmission linkage mechanism designed to control the gearshift lever’s motion. The shift gate is usually internal. However, in some cases such as Mercedes-Benz automatics and Ferrari five-speeds, it’s designed to be an exposed guide around the shift lever.
Shock Absorber
A device designed to convert motion into heat by forcing oil through a small internal passage in a tubular shaped housing. Sock absorbers are used mostly to dampen the oscillations in suspension and they only respond to motion.
Single-Rate Spring
A spring that has a constant spring rate. The same amount of force will deflect the spring by the same amount. For example, if 100 pounds of force can deflect the spring one inch, another 100 pounds will deflect it another inch. Force can be applied until the spring bottoms or fails completely.
Skidpad
A testing area that’s smooth and flat designed for use in roadholding tests under various conditions.
Slip Angle
The difference in angle between the direction a tire is moving and the plane of the wheel it’s on. Slip angle is caused by deflections in the sidewall when cornering. If the slip angle and cornering force have a linear relationship then the car is easier to control.
Slushbox
The colloquial term for a car with an automatic transmission.
SOHC
The acronym for a single overhead camshaft. SOHC engines use a single camshaft in each cylinder head. This shaft operates the intake and exhaust valves simultaneously.
Space Frame
A tube frame is made up exclusively of small-diameter short tubes. The tubes get welded together in a way that loads them primarily in compression and tension.
Spoiler
An aerodynamic body part or add on that changes the airflow direction to create better slip and reduce lift or aerodynamic drag, or to improve engine cooling.
Squat
The opposite of dive, squat is when the car’s rear end dips during hard acceleration. Load transfer from the front to the rear of the suspension creates squat.
Steering Axis
The line intersecting the upper and lower steering pivots on the wheel. In a vehicle that has strut suspension, the axis is the line through the ball joint on the bottom and the strut mount on top.
Steering Feel
The relationship between a car’s handling and the steering wheel. Steering effort should be minimal and effortless and there should be limited friction.
Steering Gain
The relationship between the steering wheel’s position and effort and the yaw. All three need to be proportional and have a gradual build up.
Steering Geometry
Design variables that fall outside of the mechanism that affects the steering behaviour of a car. They include ride steer, scrub radius, toe-in, trail, linkage arrangement and camber and caster.
Steering Response
The sensitivity of a car’s tires to the steering movement.
Straight-Line Tracking
When a car can resist any irregularities in the road and remain on a straight course without any corrections to the steering.
Stroke
The full travel distance between a piston’s travel in a cylinder.
Strut
A support element in the car’s suspension that uses a reinforced shock absorber as one of the locators for the wheels.
Sump
A small space under the crankshaft in the engine block where the oil drains into.
Supercharger
Usually only applicable for mechanically driven compressors, it can be used to describe all types of compressors including turbochargers. A supercharger is an air compressor that forces air into an engine so that it can inhale independently.
T
Targa
Popularised by Porsche, this cat body style has a removable roof. It is similar to a convertible but the car has a fixed rollbar that runs behind the front seats.
Throttle-Body
A housing that has a valve that regulates the airflow through an intake manifold. Usually located between the intake plenum and air cleaner for maximum efficiency.
Throttle-Body Fuel Injection
A fuel injection type where the injectors are in the throttle body of the engine allowing them to feed fuel to more than a single cylinder at a time. This type of fuel injection is more economical as it uses fewer injectors and pushes air and fuel through the intake manifold.
Toe-Control Link
Also known as active toe control this multilink suspension lateral link is designed to control the wheel direction of the car when the suspension moves around
Toe-In
The intentional nonparallel orientation is of two wheels opposite to one another. To measure toe-in you can subtract the distance between the front of a pair of tires from the distance between the rear edges. The toe-in dimension is positive when the front tires are turned towards the car’s centre line.
Toe Steer
The minor changes in a wheel’s direction occur without any steering input. Toe steer may get caused by suspension deflections, obstacles in the road, or ride steer.
Torque
The force that the drive shaft is subjected to. Torque is measured in pound-feet.
Torque Converter
A type of fluid coupling that transfers power created by the engine to the drive load. In an automatic car, the torque converter connects the power source to the load that needs moving.
Torque Steer
The tendency of a car to turn towards a specific direction when in motion. Front-wheel drive cars commonly have torque steer because the reaction forces in the half-shafts can generate uneven steering forces in the car’s front tires.
Torsion Bar
A spring is made up of tubular or long and solid rod that’s fixed on one end to the chassis and the suspension. To attach to the suspension the bar is twisted.
Traction Control
A wheelspin prevention electronic control system that detects when a wheel is about to break traction. When detected, it reduces the engine power and may apply the brakes to prevent this from happening.
Trail-Braking
A type of technique whereby the driver starts to brake before entering a turn and then continues to do so while easing into a corner. When the cornering forces build up the drive reduces braking pressure, and the cornering grip improves concurrently. This increases vertical loading and tire traction, improving the handling while cornering.
Trailing Arm
A suspension element that’s made up of longitudinal member with a wheel hub attached rigidly to the trailing end and that pivots from the body at its forward end.
Trailing Link
A wheel suspension link is aligned to resist longitudinal motions. This link is mounted to the chassis in front of the wheel.
Transaxle
When a differential and transmission get combined in a single integrated assembly.
Transmission
The engine’s gearbox has different selectable ratios. Each gear is designed to match the engine’s RPM or torque.
Tread Squirm
The a little bit of flexibility between a tire’s tread and the carcass. Often felt in new tires, tread squirm is not dangerous unless the tread is not suitable for the terrain. Now tires with small deep tread have a lot of tread squirm while slick tires used for racing have very little as they don’t have much tread at all.
Tube Frame
A car frame comprising of welded rigid tubing. These tubes are easier to manufacture than unitized frames if a small quantity is required.
Tumblehome
The convex curve of a car’s side body.
Tuned Intake and Exhaust Systems
Types of intakes and exhaust systems that harness resonances and pressure pulses in passages and chambers of the exhaust and intake manifolds. By doing so they increase the flow of intake charge from the combustion chambers.
Turbocharger
A supercharger that’s powered by exhaust-driven turbines. A turbocharger uses a centrifugal-flow compressor that can efficiently operate at the high rotational speeds the exhaust turbine produces.
Turbo Lag
The lag is the delay between a car’s accelerator getting depressed and the time that it takes for the supercharged engine to adjust to the power curve and pick up speed.
Turn-In
The transition point between driving in a straight line and starting to corner.
U
Understeer
The sensitivity of steering where the front tires’ slip angle is greater than the rear tires. Understeering can lead to a car resisting turns and pushing to stay straight.
Unitized Construction
Also referred to as”unitary construction”, “unibody” and “unit-body” this type of car body construction doesn’t need a separate frame to offer support for mechanical components. It may utilise monocoque construction or have strong structural elements that make up its construction.
Universal Joint
Also called U joints, these joints facilitate the motion of drive shafts as they move up and down so that the power can be transferred. The simplest type of universal joint is called a “Hooke joint” and this causes the output shaft to speed up and slow down with every revolution, depending on the angle difference between the shafts.
V
Valve Float
An issue in high RPM engines where valve lifters lose contact with cam lobes due to weak springs that cannot overcome the momentum of the valvetrain components. Valve float stops the higher RPM operation, and it can cause damage to the valvetrain over longer periods.
Valve Lifter
Sometimes referred to as a “valve follower” this cylindrical shaped component presses against the lobe of a camshaft. It moves up and down when the cam lobe rotates and usually has an oil-lubricated surface that glides over the cam lobe. If a car has “roller lifters,” these small rollers are in contact with the cam lobe and can reduce the friction between a cam lobe and lifter.
Valvetrain
The valvetrain is all the parts that make the valves work together. It includes the camshaft and its related components as well as the parts that convert the camshafts rotary motion, and all other valves in the chain of operation.
Viscous Coupling
A mechanical coupling that transfers torque and rotation by using viscous fluid to create balance. This type of coupling is used to limit speed differences between components and to stabilise the output of axles or differentials.
W
Waste Gate
The valve that controls the flow of gases from the exhaust system in a turbocharged engine. The wastegate allows some of the engine’s exhaust flow to be diverted from the turbo to regulate the speed of the turbine.
Wheel Hop
A suspension characteristic that causes a wheel or wheels to move up and down, creating a hopping action. Wheel hop is undesirable and can be caused by multiple factors including insufficient shock damping, a lack of or poor torsional axle control and an excess of unsprung weight.
Y
Yaw
The movement of an object on an axis as it rotates. In the case of a car, it relates to how far off its axis a car shifts when turning.
Z
Zero-Offset Steering
A steering system that minimises the steering effects when accelerating or braking on different types of surfaces. The system’s geometry has a scrub radius of zero.
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