Solid beam axles rely largely on the pneumatic properties of the tyres at their ends. This form of suspension has rarely been used in the last fifty years except for the occasional van.
Double wishbone suspension is also unsuitable for everyday road use although the precise of movement it allows makes it for race circuit use - e.g. Formula One.
The MacPherson Strut advances wishbone technology and with the incorporation of a pushrod system can allow a 'stub axle' to be steered as well. This makes it perfect for front as well as rear use.
Engine position and transmission layout greatly affect the handling characteristics of a car. The engine block, one of the heaviest components, within the vehicle should not move its centre of gravity too far from the middle of the vehicle - otherwise its balance is affected.
Similarly rear wheel drive has a tendancy to promote oversteering characteristics, whilst a front wheel drive system, tends toward understeer.
For example, the mid-engine roadster, MR, exploits both its tendancy to oversteer and its optimum centre of gravity creating the highly reactive and nimble handling required of a sports car.
Most family cars are configured as FF - front wheel drive. Its tendancy to understeer, in a predicable manner, in an emergency situations is widely perceived as a 'safety feature'.
Four wheel drive transmission - 4WD is now regularly exploited by both rally and circuit based cars. Normally neutral steering characteristics can be varied dynamically with the aid of computer assisted power distribution. Eg. Mitsubishi Lancer, Subaru Impreza and Nissan Skyline.
Oversteer This is the condition where a car turns through a greater angle than the steering angle of its front wheels. Also known by racing drivers as a 'tail slide'.
Understeer In this situation the front wheels have insufficient grip to affect a turn - the car tends to drive in a straighter line than its steering angle - ploughing stright on.
In order to propel a car along the road you need to connect the power developed in its engine to its driven wheels. This is done by connecting the crankshaft to a gearbox, with an intervening clutch mechanism, to a final drive differential - which attaches to the axle of the driven wheels. This whole arrangement is collecticely referred to a transmission.
The clutch is a mechanism that allows the connection of the crankshaft to be temporarily disengaged from the gearbox whilst the driver selects a different gear.
For a given engine output (rpm) the eventual vehicle speed is determined by the 'gear ratio' of the 'speed gear' (i.e. 1st, 2nd, 3rd etc.) that is currently selected.
The ratio of a specific 'speed gear' is determined by comparing the number of teeth on this gear compared with the number on the secondary driven pinion. This connects the gear at the end of the crankshaft to the 'speed gear'.
Reverse gear is implemented by disengaging the secondary pinion and effectively connecting the driven pinion directly to the 'speed gear' - this reverses the rotation of the driven axle.
The Four Stroke Power Cycle
Most sports cars use a petrol engine that operates on a four stroke cycle. To produce one pulse of power - the piston must travel up and down the cylinder four times.
1. Induction Cycle
The process begins with the inlet valve open. Rotation of the crank shaft moves the piston down the cylinder sucking the fuel air mixture
2. Compression Cycle
Both the inlet and exhaust valves are now shut. Raised by the rotating crankshaft the piston compresses the mixture into the combustion area of the cylinder.
from the carburettor past the open valve.
4. Exhaust Cycle
3. Power Cycle
With the valves shut, a spark jumps across
Exhaust gas from the power cycle leave the combustion chamber through the open exhaust valve. Pressure created by the rising piston aids this process. When
the electrodes of the spark plug igniting the mixture. Energy is released as it burns and quickly expands. This pushes the piston toward the bottom of the cylinder driving the crankshaft round half a turn.
the piston reaches the top of the cylinder at the end of this stroke, the exhaust valve closes, the inlet valve opens and the cycle begins again with induction.
During the four strokes, the crankshaft rotates twice, but since the valves need to operate only once during each cycle, the camshaft that opens them is driven at half crank speed - rotating once every four strokes.
Valves, Cams and Camshafts
As we have already seen; two valves are needed to control the the four stroke combustion cycle. An inlet valve controls the fuel air mixture and an exhaust valve governs the escape of the spent gases. During the compression and combusion cycles both valves are closed.
DOHC - Dual Over Head Cam
The exhaust valve is heated by the escaping hot gases. Typically operating at temperatures around 800°C, it is made from a heat resistant steel alloy.
Because the inlet valve is cooled by the incoming mixture it runs much cooler.
The arrangement of valves and cams is varied, but the major designs are shown here. As the camshaft rotates the lobes of the cams depress the poppet valves, opening them and compressing their valve springs. As the cam profiles smooth out the springs expand re-closing the valves.
The most efficient, and hence powerful engines use multiple inlet and exhaust valves. This arrangement allows the central location of the spark plug. Combustion spreads evenly throughout the combustion chamber (at the top of cylinder). This allows designers to increase the compression ratio - which means more horsepower.
By altering the profile of the cam, usually flattening the tip, the duration of the valve's open state can be increased - allowing more gas to escape or to be introduced into the cylinder.
OHV - Over Head Valve
SOHC - Single Over Head Cam
Carburettors, Fuel and Air
As part of the induction cycle a mixture of fuel and air is introduced into the combustion chamber. As the piston descends in the cylinder, creating a vacuum, air is drawn (via an air filter) from the atmosphere through the carburettor where fuel is added, in liquid form, via a
tube known as the main jet. Fast flowing air across this nozzle atomises it onto tiny droplets which are further vaporised prior to ignition by heat present in the engine block and cylinder head.
'fuel is initially pumped from the petrol tank into the float chamber adjacent to the throttle body (a floating valve prevents any overflow)'
Fuel is drawn from the float chamber by the 'venturi effect'*. A restriction in the throttle body (the venturi) accelerates the air which creates a partial vacuum - atmospheric pressure in the float chamber pushes fuel through the main jet.
'a pivoted disc known as the throttle valve controls airflow'
For efficient combustion, a constant air fuel mixture - between 12:1 and 16:1 is needed. If the throttle is fully opened, more air it drawn through which increases the pressure differential - more fuel is 'sucked' in ... the mixture becomes over-rich (the opposite is also true). To start an engine from cold, mixtures as rich as 1:1 are sometimes required. A 'choke' can enhance the overall richness of the mixture.
The Venturi Effect Discovered by observation by Daniel Bernoulli, a Swiss mathematician and physian in the 18th century. Fluid flowing through a passageway accelerates as it passes through a constriction which also causes a drop in pressure at that point. The Bernoulli Effect, as it is more correctly known, is responsible for aerodynamic lift. The profile of an aerofoil accelerates air flowing over its upper surface compared with that of the lower producing a lower pressure on the upper surface. Taken together, and acting against gravity, the effect is known as lift.
The power that an engine develops depends on how much energy is released during each power cycle. This is directly related to how much petrol is mixed into the fuel air mixture created by the carburettor. Theoretically the more this is compressed the greater the amount of energy will be released. The compress-
ion ratio is the amount by which the original volume is reduced by the piston during the compression cycle. Very high ratios (greater than 8:1) usually result in uneven detonation, with loss of power, due to the distance of the mixture from the spark. This is
referred to as 'pinking' or 'knocking'.
When the bore size of a cyclinder approaches that of the
piston stroke length - this is referred to as 'squareness' - square configurations such as that found in the engine of the legendary Subaru Impreza 22b deliver maximum torque very early within their rev range; usually at around 3000rpm.
Turbos and Turbocharging
As we have already seen. The more fuel you can ignite within a given period of time the more power an engine will develop. Several strategies exist for achieving this ! The first is obvious - add more cylinders, bigger ones ... or both. This approach is typified by the 'American Muscle Car'. Skimming the engine block or using thinner gasket material increases the compression ratio of an engine - with consequentially more power ... the 'head job'.
The process of turbocharging uses an entirely different technology to achieve both increased compression and greater engine throughput. Turbocharging is a type of forced induction system. Power from the engine is used to squeeze
more air into the cylinders; then just add more fuel and you've got more power - simple !. Turbochargers are constructed from two linked sections; a turbine that drives a centrifugal pump via a linked shaft. Exhaust gases, straight from the manifold, are directed across the blades of a turbine which causes the impellor to draw and compress ambient air - driving it toward the inlets of the engine. Provided that the fuel injection system or carburettor can continue to deliver increasing amounts of fuel to match the increasing airflow, an increasing amount of power will be developed.
Before the compressed air can be mixed with fuel it needs to be cooled.
The Laws Of Thermodynamics dictate that 'energy cannot be created or destroyed'.
So when a gas is compressed the molecules collide more often - raising its overall temperature. This side effect results in poorer combustion, however, it can easily be addressed by passing the air through an 'intercooler' to radiate the waste energy. Because of the increased
Camouflaged, pre-marked body of the Wild Commando is both lightweight and tough.
The 144mm diameter large tires with aggressive block pattern are fit for a true R/C Commando and afford superior grip both on and off road.
Resin frame combined with duraluminum alloy deck realize superb, Commando level durability. Double wishbone suspension features two dampers per wheel to absorb shock when tackling the toughest of terrain.
18 class, 3cc engine employs large heat sink and cooling fan. Idler needle screw enables smooth transition from low to high rpm.
Comes equipped standard with special lightweight, heat resistant resin chamber with manifold.
Large double-disc brakes realize effective cooling and feature triple layer brake pads to give the Wild Commando incredible stopping power.
150cc high-capacity fuel tank comes with lightweight, easy-to-use choke pump.
Big, "Commando" size tires require a big steering servo. The Wild Commando comes equipped with 9kg/cm high-torque servo to realize sharp steering.
R/C mechanics sealed in a compartment that effectively shuts out debris, protecting the Commando's guts from dirt and water.
Wheel and trigger-type transmitter is perfect for controlling the Wild Commando. Wheel realized top steering control. Throttle trigger controls forward and reverse movement with ease.