Introduction
When you visit a motorcycle manufacturer’s website, you will notice the specification page for each motorcycle size. The specifications will give you a complete picture of the bike, including its size, power, features, physical dimensions, weight, and more. You may understand some of the specifications and may not understand many. This article briefly explains the major specifications of a motorcycle.
However, do not trust the specification sheet and buy a motorcycle. Sitting on a motorcycle and a trial ride are the best ways to determine whether a bike suits you and your physique.
Terms used in motorcycle specifications
Engine
Your motorcycle is run by an internal combustion (IC) engine. An IC engine can be 2-stroke or 4-stroke, but all present-day motorcycles have 4-stroke engines. 2-stroke engines are obsolete since they do not conform to state emission norms.
You may read the article ‘Why two-stroke engines were killed‘ to know the reasons for the obsolescence of two-stroke engines.
The major parts of a 4-stroke engine are the engine cylinder, piston, connecting rod, crankshaft, crankcase, spark plug, inlet, and exhaust valves. The piston reciprocates up and down the engine cylinder, and a connecting rod connects the piston and crankshaft. The crankshaft converts the piston’s reciprocating motion to rotary motion.
When the piston moves up and down, the bottommost point of the piston is BDC (bottom dead center) and the topmost point of the piston is TDC (top dead center). The movement of the piston from BDC to TDC or TDC to BDC is called a stroke. The crankshaft makes one revolution per two strokes.
The four strokes of a 4-stroke engine are:
- Suction
- Compression
- Power
- Exhaust
Suction stroke: The piston moves down from TDC to BDC. The valve at the inlet is open, and the valve at the exhaust is closed. The vacuum in the cylinder allows the air-fuel mix to enter.
Compression stroke: The valves at the inlet and exhaust are closed. The piston moves up from BDC to TDC and compresses the air-fuel mix. When the piston reaches the TDC, the sparkplug sparks to ignite the air-fuel mix.
Power stroke: The high power created by the combustion of air-fuel mix pushes the piston down to BDC with force.
Exhaust stroke: The piston moves BDC to TDC; the valve at the exhaust opens and the burnt gasses rush out.
The image below explains the four strokes of a 4-stroke motorcycle engine. The supply of air-fuel mix can be through a carburetor or fuel injection system. Most of today’s bikes work with a fuel injection (FI) system.
A DTS-i (Digital Twin Spark ignition) engine uses two spark plugs (in place of one) to enhance fuel efficiency, and power, and reduce pollution.
You may view this YouTube video showing four-stroke engine animation.
Engine cooling system
A reciprocating internal combustion (IC) engine generates enormous heat and this heat must be dissipated to maintain an optimum engine operating temperature and efficiency. There are three types of engine cooling systems:
- Air-cooled engines
- Air-oil-cooled engines (also called oil-cooled engines)
- Liquid-cooled engines
Air-cooled engines: The outer of the cylinder head and block has typical fins to dissipate the engine heat to the surrounding air. This is generally found on lower rpm engines and low cc (100 to 125 cubic centimeters) engines.
Oil-cooled or air-oil-cooled engines: This is essentially an air-cooled engine with an additional facility for cooling the hot engine lubricating oil and recirculating it. The oil is cooled with a radiator. This cooling system is normally found in mid-size, 150 to 250 cc bikes.
Liquid-cooled engines: The design of these engines has water-cooling jackets for passing the liquid coolant. When the engine becomes hot, a liquid coolant is passed through the engine jackets, and the hot coolant (coming out of the engine) is cooled in a radiator before recirculating it. This cooling system is normally found on higher size and higher rpm engines (more than 250 cc high powered bikes).
Number of valves
A motorcycle engine must have a minimum of two valves (1 inlet and 1 exhaust) per cylinder. But it can have multi-valves per cylinder also viz. 3 valves (2 inlet and 1 exhaust), 4 valves (2 inlet and 2 exhaust), or 5 valves (3 inlet and 2 exhaust).
SOHC and DOHC engine
SOHC (single overhead cam) and DOHC (double overhead cam) are the systems used for opening and closing valves.
Number of cylinders
A motorcycle engine can have one to six cylinders. Normally, single-cylinder engines are limited to 350 or 400 cc engines. Engine capacity higher than 400 cc will have 2 or more cylinders. The arrangement of cylinders can be V-type engines, horizontal inline cylinder engines, etc.
Engine displacement
Displacement of a reciprocating engine is the amount of the cylinder volume displaced by its piston when it moves up from BDC (Bottom Dead Center) to TDC (top Dead Center) and it is expressed in cc (cubic centimeters) or cu in (cubic inches).
Displacement gives the volume of the air-fuel mix drawn into the engine cylinder. So, the higher the engine displacement, the higher its power.
Bore and stroke
Bore is the internal diameter of the engine cylinder and stroke is the distance between BDC and TDC.
If you know the bore and stroke of a motorcycle engine you can calculate its displacement.
The Bore × Stroke of a single-cylinder engine is 72 mm × 85.8 mm or 7.2 cm × 8.58 cm.
So, displacement = π (3.14) × (7.2/2)2 × 8.58 = 349.15 cc
Fuel system
A fuel system defines how the fuel is supplied to the engine and it can be a carburetted system or a fuel injection system. In a carburetted engine the vacuum created in the cylinder sucks a mix of air and fuel from the carburetor. In a fuel injection system, the vacuum in the cylinder sucks the air and the fuel is injected separately.
Compression ratio
The compression ratio explains the extent of compression of the air-fuel mix before it is ignited by the spark. If the compression ratio is 9.5:1 and the engine displacement is 349 cc, it means 349 cc of air-fuel mix enters the cylinder and is compressed to 36.74 cc before a spark ignites it.
Maximum torque and maximum power bhp
Maximum torque: You can explain torque as the twisting force. A motorcycle with higher torque can carry more load at a consistent speed, and climb up a hill with load and similar things. A motorcycle with low torque can give you speed, but high torque gives you the ability to ride on steep and hilly roads.
Torque is measured as force× distance and its unit indicates this.
Torque is expressed in Newton-meter (Nm) in metric. 1 Nm implies one newton of force applied with one meter of leverage.
Torque is expressed in pound-foot (Lbf-ft) in the imperial system and it implies one pound of force applied with one foot of leverage.
The maximum torque of a motorcycle is always given at an engine rpm (revolutions per minute).
Maximum power: The power of a motorcycle engine is the power output of its engine. The engine power can be expressed in horsepower (HP), brake horsepower (BHP), PferdeStarke or PS (French horsepower), or kilo Watts (kW).
One horsepower indicates the energy required to lift a weight of 250 kg by one foot each second. Horsepower helps measure the relative power of engines, electric motors, etc.
BHP or brake horsepower is slightly less than the HP of an engine or a motor. One BHP is less than one HP because BHP considers frictional loss.
Watts are one of the fundamental units of power and it can be used for measuring electric, thermal, and mechanical power. 1 kilowatt is 1000 watts.
The BHP of a motorcycle indicates its ability to accelerate fast and move fast. But speed depends on other factors like the weight of the motorcycle and its aerodynamic construction. This is the reason why racing bikes have a high bhp, low weight, and aerodynamic body. Racing bikes can accelerate faster and move at high speeds.
The maximum power of a motorcycle is always given at an engine rpm (revolutions per minute).
You will get a conversion between the different units of torque and power on the internet.
Start system
The start system can be electric start and kick start or only electric start. A combination of electric start and kick start is preferred since you can easily kick start the bike if the electric start fails due to a discharged battery. If your motorcycle is stuck on the road with a discharged battery and there is no kick start, you are forced to take the help of a good Samaritan rider to jump-start your bike.
Transmission or gearbox
This gives the number of speeds and type of transmission. A 6-speed manual transmission or gearbox means it has 6 gear positions and gear shifting is manual.
Final drive
The final drive connects the engine transmission to the rear wheel. The final drive can be chain, belt, or shaft drive.
Brakes
The front brake of a motorcycle is operated by the brake lever at the right handlebar and the rear brake is operated by pressing the brake pedal on the right leg. The brake can be of the disc type or drum type and the normal combinations are both front and rear drum brakes, front disc and rear drum, or disc brake at both front and rear wheels.
Braking system
The braking system of a motorcycle can be:
- Normal braking system
- Combined braking system (CBS)
- Anti-lock braking system (ABS)
Normal braking system: This is the simplest system where the rider activates the front and rear brakes by pressing the corresponding brake lever/pedal. It is left to the discretion of the rider when to use the rear or front brake or a combination of both brakes.
Combined braking system (CBS): In this system, when the rider activates the front or rear brake, say rear, the motorcycle system applies the brake simultaneously on both front and rear brakes. This type of brake system helps safe braking.
Motorcycle manufacturers call this braking system by other names viz. Integrated braking system, Synchro, or SBT brake, but they all mean the same.
Anti-lock brake system (ABS): ABS is a system created to detect wheels lock up when braking and immediately reinstate the rotation of the wheel to ensure the stability of the bike. ABS is a safe braking system to avoid locking up of wheels when the brake is applied suddenly.
The ABS system has sensors on the front and rear wheels to detect their rotational speed, ECU (electronic control unit), and disc brakes. The sensors on the wheels continuously monitor the speed and send signals to ECU when it detects a wheel lock-up.
The ECU regulates the brakes to avoid locking up and continuously applies and releases the brake to allow smooth and controlled braking. ABS helps prevent bike accidents.
The ABS can be single-channel or dual-channel. A single-channel ABS activates the front wheel brakes only since the front wheel has more chance of skidding. A dual-channel ABS is activated for both wheels.
Suspension system
The front and rear wheels are connected to the chassis through a suspension system. The ideal purpose of suspension is to take all the shocks due to the ups and downs of the road and not transfer them to the rider. But, even in high-end bikes, a part of the shock gets transferred to the rider. There is suspension at the front wheel called front suspension, and the rear wheel called rear suspension.
In a suspension system springs are used for absorbing the shock and dampeners to control the motion of the spring.
Front suspension: Most of the motorcycles use telescopic front suspension containing two fork tubes with springs and a dampener. Another popular type is an inverted fork and here the fork tubes are turned upside down. In both designs, the slider tube moves up and down into the bigger tube with spring and the movement is within the allowable limit. The dampener dampens the up-down spring movement.
Rear suspension: Most of the present motorcycles use swingarm-type rear suspension. The swingarm fastens the motorcycle chassis and the wheel and moves in an arc when the bike encounters the up and down of the road. The rear shock absorber can be mono-shock or dual-shock absorbers.
Dimensions of a motorcycle
The specifications of a motorcycle give the following dimensions.
Seat height: Seat height is the height measured between the top of the seat and the ground. This is an important spec for a rider because when the rider sits on the bike, the feet should firmly touch the ground to ensure safe riding.
Rake and trail: Please see the image to understand these dimensions.
Wheelbase: This is the distance measured between the centers of the front and rear wheels.
Length and height: These are the overall length and height of the motorcycle.
Width: This is the maximum width of the motorcycle with all parts assembled.
Ground clearance: This is the dimension between the bottommost part of the motorcycle and the ground.
Fuel tank capacity in Liters: This is the fuel holding capacity of the fuel tank and it includes reserve capacity wherever applicable.
Kerb weight: Kerb weight is the weight of the motorcycle ready for ride, meaning it is filled with fuel and all the oils. Kerb weight is also known as wet weight. Some manufacturers mention dry kerb weight or weight without fuel and oil.
Motorcycle Tyres
Your motorcycle tires are in constant contact with the ground and give traction and movement to the bike. Fixing the recommended tire on your motorcycle makes your ride safe. The letters and numbers you see on a tire have a meaning that can be understood by referring to a tire chart. Let us understand this briefly.
The marking on the tire is 140/70 17 66 P
The first three digits 140 is the tyre width in mm. The second set of two digits 70 is the aspect ratio. 17 is the wheel diameter in inches. 66 indicates the corresponding load-carrying capacity of the tire as given in the tire chart. As per the tire chart, the number 66 corresponds to the 300 kg carrying capacity for the tire. The alphabet P gives the maximum speed for the tire, as the tire chart P indicates the maximum speed is 94 MPH or 150 KMPH (the maximum speed, however, depends on the recommendation of the motorcycle manufacturer).
Please visit https://www.tvsmotor.com/media/blog/how-to-read-two-wheeler-tyre-markings for more information.
Safety
The safety of the rider and the passenger should be of utmost importance when riding a motorcycle. Please wear an ISI-approved full-face helmet and other safety gear when riding a motorcycle and this applies to both the rider and the passenger.
The design of the anti-lock braking system (ABS), is not meant for the shortening of braking distance and ABS cannot prevent wheel skidding during cornering. You must not feel overconfident about ABS.
Please ride carefully, and safely, and wear all your safety gear to make riding a pleasure.
You may read the article ‘Popular Motorcycles in India.’
You may read the article ‘15 Things to Consider Before Buying a Motorcycle.’