What does 16v DOHC mean? BikeSocial's guide to technical terms

Phil West
By Phil West
PhilWestNew Former Editor of Bike, ex-Road Test Editor of MCN, ridden more bikes than he can remember. Likes: GTS, Paso, Mantra. Dislikes: own rust bucket LC and 900 T-Bird daily driver.

 

Do you know the differences between bore and stroke? How about rake and trail? What exactly does kerb weight mean and why would a V4 be transversely mounted? 
Read on. Our comprehensive glossary that dissects all of those head-scratching terms from the BikeSocial specification sheet, offers all the answers.

What is capacity?
What are bore and stroke?
What are different engine layouts?
What is inside an engine?
What is power?
What is torque? Why is torque important?
Why quote a top speed?
What is ‘average’ fuel consumption?
Why is tank size important?
What is max range to empty (theoretical)?
What is reserve capacity?
What rider aids are available and what do they do?
What different types of frame are there?
What types of front suspension are available?
What does front suspension adjustment do?
What types of rear suspension are available?
How does rear suspension adjustment affect my bike?
What kind of front brakes are available?
Why are rear brakes smaller than front brakes?
What do the numbers on the tyres mean?
What are rake and trail?
What is wheelbase?
Is seat height really important?
What does kerb weight mean?
What do the dimensions refer to?
What is ground clearance?
What should I look for in a warranty?

 

What is capacity?

The capacity, or ‘displacement’, of an internal combustion engine is the traditional measure of its size and so gives a crude indication of its performance. In very simple terms: the larger the capacity, the more fuel/air mixture is flowed and combusted, the bigger the bang is generated. Specifically, an engine’s capacity is the swept volume of all the pistons inside the cylinders in a single movement from their highest (top dead centre – TDC) to lowest (bottom dead centre – BDC) positions. The usual measure is ‘cc’ (for cubic centimetres) although in the US ‘ci’ (for cubic inches) remains common. In larger engines, ‘litres’ is also referred to, whereby a 1500cc engine is also called a 1.5litre.

 

What are bore and stroke?

These are the key dimensions which determine the swept volume (see Capacity) of a piston in a single cylinder of a reciprocating engine. The bore is the diameter of the cylinder while the stroke is the distance the piston travels in a single movement from its bottom to top position. Therefore, in an engine with a 4cm (40mm) bore and 4cm (40mm) stroke, that cylinder’s capacity is calculated using Pi x radius squared, i.e. 3.1415 x 2 squared (2 being half the diameter and thus the radius), which gives the area of the cylinder = roughly 12.6, which is then multiplied by the bore (4) to give 50.3cc. Of course, if there’s more than one cylinder, that figure is then multiplied by the number of cylinders to give total capacity.

For any given capacity, different bore and stroke dimensions give different characteristics and performance. The longer the stroke the less ability for high rpm and thus lower peak bhp, although this also usually means more flexible, progressive drive. A short-stroke configuration usually means more bhp but is ‘peakier’ requiring higher rpm and more frequent gear changes.

What are different engine layouts?

This refers to the arrangement of the cylinder(s) of a conventional reciprocating internal combustion engine. If has just one cylinder it’s simple: it’s also called a ‘single’ and is usually positioned vertically to the crank and crankcases. If there’s more than one cylinder, however, there are increasing numbers of various arrangements which can affect the look, character and performance of the engine. The following, in ascending numbers of cylinders, are the most common: a two-cylinder engine is also referred to as a ‘twin’. A ‘parallel twin’ such as Triumph’s Bonneville or Yamaha’s MT-07, has the cylinders paired together, side by side usually in a ‘transverse’ arrangement where the crankshaft is perpendicular to the direction of travel. A V-twin is similar but has its cylinders splayed (by varying amounts) to resemble the letter ‘V’. (Ducati, incidentally, insist on calling their version an ‘L-twin’ as its cylinders are arranged at 90 degrees). One famous V-twin exception is Moto Guzzi, who’s characteristic Vee is arranged across the frame with its crank in line with the drive-shaft. While another distinctive twin is BMW’s ‘boxer twin’ where the two cylinders oppose each other horizontally, across the frame with the crank again in line with the shaft drive. Nearly all three-cylinder engines (Triumph Speed Triple, MV Agusta 675 and 800) are, like the parallel twin, ‘in-line, transversely-mounted, across the frame’, as are most fours and sixes – but there are notable exceptions: Triumph’s Rocket3 is in-line but longitudinally-mounted; Honda’s GoldWing is powered by a boxer-style flat six, while Ducati’s new V4 Panigale, like Honda’s VFR and Aprilia’s RSV4, has a transversely-mounted V4.

 

 

What is inside an engine? 

Valves – as in 4v. The inlet and exhaust of fuel mixture and burnt gas from a four-stroke engine’s combustion chamber is controlled by valves. Simpler engines have just one of each, i.e. two valves per cylinder. More modern, high performance engines have two of each, i.e. 4v, which allow the faster flow of gas.

These valves are usually opened, either via rocker arms or directly via a bucket holding shims which allow fine adjustment, via lobes on rotating camshafts in turn driven either by chain or gears from the crank. They’re closed by springs.

Simple, two valve engines have a single camshaft which operates the inlet and exhaust valves. This is referred to as Single Over Head Cam or SOHC.

More sophisticated, high performance engines use twin camshafts – one for the inlet, the other for the exhaust. These are referred to as Double Over Head Cam or DOHC. Although more complex, the advantage of DOHC is four-fold: it allows a four-valve system; there’s more scope for the positioning of the inlet and exhaust valves; there’s more scope for cam tuning and timing and, since the rocker arms are reduced in size or eliminated, there’s less valvetrain inertia.

The main alternatives to overhead camshafts are: an older pushrod system, as used, for example, by Harley-Davidson and Norton, whereby a camshaft in the crankcases actuates two pushrods, one for inlet, one for exhaust, which in turn opens the valves and the desmodromic system, as most famously used by Ducati, which does away with valve springs by having a mechanism which positively opens and closes the valves

‘l/c’ refers to liquid-cooling, whereby the temperature of the engine is regulated via coolant (although some use oil) running around the cylinder and head in channels, driven by a pump, cooled by a radiator and governed by a thermostat. Precise temperature control enables finer machine tolerances and thus increased performance. More basic, traditional (and cheaper) air cooling (a/c) instead relies simply on cooling fins on the cylinder and head.

 

What is power?

Power remains the key measure of a motorcycle engine’s potential performance (although some would argue that an alternative measure, torque, see elsewhere, is more useful) – as is confirmed by the fact that the A1 and A2 learner restrictions both limit power. That’s the simple bit. Then it gets more complicated. In the UK the most common unit of power remains bhp (or brake horsepower) although in Europe kilowatts (Kw) and Ps (Pferdestärke or, literally, 'horse strength') are more common. This power output depends on the size and design of the engine, but also on the speed, or rpm, at which it is running. This latter factor is why maximum or peak power is usually always quoted along with the rpm at which it is generated, for example 50bhp @ 5000rpm. It’s important to realise that this is exactly that, a maximum or peak figure. At other engine speeds the output may be much less depending on the design and character of the engine: a ‘peaky’ engine, for example may have a high power output at high rpm but its output is much less at lower rpm, sometimes making it more difficult to ride without repeated gearchanges. This is what’s sometimes referred to as a ‘peaky engine’

 

What is torque? Why is torque important? 

Ah yes, ‘torque’, the most widely misunderstood and mis-defined term, probably, in all of motoring – which is odd as it’s also probably the most useful.

In the context of engine performance torque is the ‘yin’ to power’s ‘yang’, the flipside, if you like, of the same coin. To be exact: torque is the twisting or turning force which tends to cause rotation around an axis or, in other words, the force most effective at turning a wheel. Which is why torque is fundamental to acceleration and drive rather than the top end punch of power which pushes you to a higher top speed. Got it? It’s traditional measure in motorcycling is in pounds-feet or lb-ft, that being the force in pounds acting on the end of a foot-long lever, but increasingly (and confusingly) also in kg/m and Nm. Like power it varies according to engine speed and design which is why a peak or maximum figure is usually quoted at the rpm speed it’s produced.

 

Why quote a top speed?

Historically the measure, in miles per hour (mph), which more than any other separates the ultimate performance machines from the pursuing mob, today, however, it’s not quite so simple. Since the early 1990s most litre-class bikes have been capable of 175mph or more reaching its peak with Suzuki’s 1999 Hayabusa which was famously measured at nearly 200mph thanks to its combination of 175bhp and class-leading aerodynamics. However, fearing a speed war and a legislative backlash the leading manufacturers soon after entered into a ‘gentlemen’s agreement’ to thereafter limit their machines’ speeds to 300kph or about 185mph. This is still supposedly in place today, although Ducati’s Panigale V4 S has been recorded at 191mph…

 

What is ‘average’ fuel consumption?

Average fuel consumption is the measured average rate at which a given motorcycle consumes fuel over distance. In the UK the usual unit to display this is mpg – or ‘miles per gallon’. Generally, smaller capacity engines produce higher mpg figures than larger ones as, by definition, they burn less fuel, although factors like combustion efficiency, tolerances, state of tune etc can affect this. Importantly, how a bike is ridden has a significant effect, too. Predominantly urban riding in low gears consumes more fuel as does track or performance riding at high rpm. Generally, the best mpg figures are produced with the machine in top gear but at as low rpm (and thus speed) as practical. For all of the above, ‘average mpg’ figures, calculated over as long a distance and variety of riding conditions as possible are the best as they are the most comparable and realistic.

Some manufacturers publish their own ‘claimed’ figures although these should be taken with a pinch of salt as they are often measured in favourable, not necessarily realistic conditions. That’s why BikeSocial endeavour to, wherever possible, publish our own ‘tested’ figures.

 

Why is tank size important?

Simply this is the capacity by volume, as usually presented in litres, of a motorcycle’s fuel tank and, as such, a useful guide to its range. Typically, motorcycles have fuel tanks of between 15 and 18 litres. Those of lightweight or learner machines are often smaller, so that a full tank doesn’t make the bike too heavy, while those of extreme ‘adventure’ bikes, whose whole remit is long-distance travel, can be up to 30 litres.

 

What is max range to empty (theoretical)?

Range is the theoretical maximum distance any given motorcycle is calculated to be able to travel as a function of its average fuel consumption multiplied by its fuel capacity. In the UK this calculation is slightly complicated by the fact that we use imperial ‘miles per gallon’ to measure consumption and decimal litres to measure fuel tank capacity. A further complication is that, in reality, prudence usually ensures we refuel long before the range is covered. Even so, it’s a useful guide to how far a bike can travel before needing to refuel and is particularly useful for touring machines.

 

What is Reserve capacity?

This refers to a machine’s extra or ‘reserve’ fuel capacity beyond its normal volume. Although today rarely the case, traditionally motorcycle fuel tanks were operated by a fuel tap which had three positions: on, off and reserve. When you ran out of fuel in the ‘on’ position you were able to ‘switch to reserve’ which commonly released enough fuel for a further 10-15 miles or so. However, since the onset of modern fuel injection systems, fuel taps have almost universally become a thing of the past.

 

What rider aids are available and what do they do?

Ever since the onset of ‘ride-by-wire’ (debuting with the 2007 Yamaha R6), fuel injection governed by electronic control units (ECUs) and inertial measurement units (IMUs) which sense a bike’s attitude, there has been a seemingly endless stream of ever more sophisticated electronic systems available which can control engine performance and braking automatically and can also be tailored to personal taste and conditions. These take the form of switchable fuelling maps (or riding modes), launch control, traction control, cornering ABS and more, all at the flick of a switch.

 

 

What different types of frame are there? 

The frame is framework which forms the skeleton of a motorcycle, onto which the engine, suspension fuel tank, seat etc is attached. It can come in various forms. Traditionally, having evolved from bicycles, many most motorcycles employed tubular steel ‘cradles’, whereby a ‘loop’ of metal held the engine. Sometimes two loops were used, meeting at the steering head. These are ‘double cradle’ frames. Sometimes the loop split into two just for the bottom part of the frame. These are ‘semi-double’ cradles. In the early 1980s such frames began to be made of box-section aluminium rather than tubular steel with, for lightness. Then, from the late ‘80s, sports bikes in particular began to use ‘twin spar’ designs whereby to large beams of aluminium (usually), either cast or extruded instead of cradling the engine wrapped around each side of it in a straight line between the swinging arm pivot and steering head, a design which was both stronger and lighter still. This design still largely prevails today. There are a handful of exceptions. Ducati traditionally use a tubular steel trellis which also wraps around the sides of the engine; some employ large diameter tubular steel or cast aluminium ‘spine’ frames, where the engine hangs from a single beam, while BMW’s boxers do without a conventional frame instead having smaller sub-frames at front and rear bolted directly to the engine.

 

 

What types of front suspension are available?

Most motorcycles still use telescopic forks with most of those, except for lightweight or budget bikes, now using the more rigid, inverted or upside-down (usd) design first introduced in the early 1990s, whereby the broader and thus more rigid outer ‘slider’ forms the longer, upper rather than lower part of the leg. This also has the advantage of reducing the machine’s unsprung weight. The diameter of the more slender, internal part of the leg is often quoted as an indicator of its strength. For example, 45-50mm is today commonplace whereas less rigid telescopics in the 90s were more usually 35-40mm. The only common alternative to telescopics in mainstream motorcycling is BMW’s Telelever system which is a development of the Saxon-Motodd system developed in the 1980s. Instead of having springs in the forks this system has an additional swingarm that mounts to the frame and supports a separate shock absorber unit. This separates the front forks steering and suspension functions and eliminates fork dive under braking by causing the bike’s rake and trail to increase during braking instead of decreasing as with traditional telescopic forks.

 

What does front suspension adjustment do?

On conventional suspension units three main types of adjustment are sometimes available.

 What is preload

‘Preload’ is simply the amount the springs are compressed while the suspension is fully extended. The fork preload adjuster bears down on the fork spring and shortens or extends it accordingly. Many think that changing preload affects spring stiffness, and while you can compensate to a certain extent for a too-soft or too-stiff spring by using preload, the right move in that situation is to change the spring itself. The adjuster is usually a large nut on top of each leg.

 What is compression damping?

‘Compression damping’ determines how fast the suspension can compress when you hit a bump and as such is what gives a bike its feeling of plushness or stiffness. With too much compression damping the forks cannot compress quickly enough after hitting a bump causing a jarring sensation. With too little, the weight transfer of the bike as you accelerate or brake will give a wallowy sensation. The compression damping adjuster is sometimes a screw at the bottom of the fork leg, sometimes a screw on the top of one of the legs.

 What is rebound damping?

‘Rebound damping’. Once your suspension has compressed over a bump, rebound damping determines how fast the suspension can ‘rebound’, i.e. extend to keep the wheel in contact with the ground. Too much rebound damping will keep the suspension compressed when it should be extending to follow the road on the downside of a bump and the wheel will lose contact with the ground. Too little and the suspension will extend fast enough to push the bike up forcibly, giving a loose feeling.

Increasingly, modern machines also have electronic suspension systems whereby settings vary according to selected modes and can even be ‘active’ where the settings are altered in real time according to prevailing conditions and type of riding.

 

What types of rear suspension are available?

Most modern motorcycles’ rear suspension use a swinging fork which holds the rear wheel and pivots on the rear of the frame whose movement is controlled by a ‘single shock’ or monoshock ­– a single combined spring and damper unit. Commonly this is done via a pivoting linkage or link which allows less shock shaft travel in the initial movement of the rear wheel. This is also known as ‘rising-rate’ rear suspension. Some more traditional machines such as retros, however, instead of a rising-rate monoshock, employ a more old fashioned ‘twin shock’ rear suspension system with a shock unit on each side of the swinging fork.

 

How does rear suspension adjustment affect my bike?

As with the front suspension, three main types of adjustment are sometimes available on the rear shocks:

What is preload?

‘Preload’ is simply the amount the springs are compressed while the suspension is fully extended. The fork preload adjuster bears down on the fork spring and shortens or extends it accordingly. Many think that changing preload affects spring stiffness, and while you can compensate to a certain extent for a too-soft or too-stiff spring by using preload, the right move in that situation is to change the spring itself. The adjuster is usually a large either above or below the spring on the unit itself although sometimes remote preload adjusters are mounted on the side of the machine.

What is compressions damping?

‘Compression damping’ determines how fast the suspension can compress when you hit a bump and as such is what gives a bike its feeling of plushness or stiffness. With too much compression damping the rear shock(s) cannot compress quickly enough after hitting a bump causing a jarring sensation.

What is rebound damping?

‘Rebound damping’. Once your suspension has compressed over a bump, rebound damping determines how fast the suspension can ‘rebound’, i.e. extend to keep the wheel in contact with the ground. Too much rebound damping will keep the suspension compressed when it should be extending to follow the road on the downside of a bump and the wheel will lose contact with the ground. Too little and the suspension will extend fast enough to push the bike up forcibly, giving a loose feeling.

Increasingly, modern machines also have electronic suspension systems whereby settings vary according to selected modes and can even be ‘active’ where the settings are altered in real time according to prevailing conditions and type of riding.

 

 

What kind of front brakes are available?

Modern motorcycles, almost without exception, use hydraulically-operated disc brakes front and rear, although historically drum brakes were common and are still used occasionally at the rear. In simple terms, a disc brake’s power is increased by the diameter of the disc, number of pistons in the brake caliper and rigidity of the caliper itself. The maximum size of most discs today, due to available space, is 320-330mm; after a brief flirtation with six-pistons, four-piston (with twin opposed pistons) calipers are now the norm (although cheaper, lighter machines often have only twin or single-piston calipers) while radially-mounted, one-piece (or ‘monobloc’) caliper designs are the current most optimum, effective types. Larger bikes invariably also use twin disc systems with one on each side of the front wheel. Some machines, notably Moto Guzzis and the Honda Gold Wing, also use ‘linked’ braking systems the front brake lever also operates to some degree the rear brake, and vice-versa. ABS, meanwhile (or Anti-Lock Braking Systems) have been an EU legal requirement on bikes over 125cc since 2016.

 

Why are rear brakes smaller than front brakes?

As the great majority of a motorcycle’s braking is undertaken by its front brakes, due to a combination of weight transfer and geometry, the rear brake on many machines, although a legal requirement, is of little significance, which is why it’s often merely a small disc with a simple one or two-piston caliper. Indeed, some older bikes still have even more basic drum rear units. The one slight exception to this, however, is cruisers which, due to more weight at the rear and sometimes a narrower front tyre contact patch, need to use their back brakes more. ABS is a today a legal requirement on new machines.

 

What do the numbers on the tyres mean?

Virtually all modern road tyres are tubeless radials although tubed cross-plies are still used in off road competition. They’re dimensions are usually in the following convention – 120/70 V 17 – where 120, in mm, is the tyre’s nominal width, measured from sidewall to sidewall (NB: the actual physical width of the tyre on the rim can differ from the nominal width), ‘70’ is its ‘aspect ratio’, or the height of the sidewall expressed as a percentage of the width, ‘V’ is its maximum speed rating where R = 170kph, V = 240kph and Z = 240kph+, and 17 is its diameter in inches.

 

 

What are rake and trail?

 These are the two most critical measures of a motorcycle’s steering geometry and thus the best indicators, all else being equal, of its handling characteristics

 What is rake?

Rake (also sometimes called caster) is measured in degrees and is the angle from vertical of the motorcycle frame’s steering head. Customs or cruisers usually have a lot of rake – their forks stick way out in front. Whereas sports bikes have much less, and their forks are closer to vertical. A typical sports bike might have a rake as steep as 25 degrees, while a chopper may be closer to 35 degrees. A tourer might typically be around 29 degrees.

More rake does two things: one, it makes the wheelbase longer, and two, it generally increases a motorcycle’s trail.

What is trail?

Trail, usually measured mm (although inches remains common the in the US), is the distance between the point of the front wheel’s contact with the ground and a line drawn through the axis of the steering head.

Normally, this contact point is some distance behind the projected line – it ‘trails’ behind the axis, hence the name. A great deal of trail (110mm+) makes a bike difficult to steer, too little (under 90mm) makes it steer quickly but be increasingly unstable.

 

What is wheelbase?

Wheelbase is one of the key measurements which define the character and handling of a motorcycle. In being the distance, usually in mm, between a bike’s front and rear axles, it gives an indication, everything else being equal, of a machine’s stability (generally, the longer the wheelbase the more stable it is in a straight line). By contrast, the shorter the wheelbase (everything else being equal, the more nimble and quicker steering a bike can be. Of course, many other factors can come into play (steering geometry, weight, wheel diameter), but in general terms sports machines tend to be shorter (with wheelbase’s rarely over 1400mm) and cruiser or touring bikes longer (sometimes over 1500mm).

 

Is seat height really important?

The height, usually in mms (although again it’s still sometimes quoted in imperial inches) from the ground to the lowest point of a motorcycle’s saddle when positioned upright and is thus useful when gauging the manageability of any given machine – especially for shorter riders. Most larger bikes are typically between 720 and 770mm. Retro roadsters and learner bikes can be under 700mm, big off-road or adventure machines, due to their larger wheel diameters and longer travel suspension to gain the required extended ground clearance, can be well over 800mm. It’s worth remembering, though, that many modern machines have either seats which are height adjustable (usually by around +/- 20mm, or have lower or taller accessory saddles available as extra cost options. Finally, it’s also important to realise that seat height isn’t the only factor in machine manageability. A bike’s slimness makes it easier to manage, as does its weight, whether that weight is carried high or low, and also its overall bulk.

 

What does kerb weight mean?

The weight of a motorcycle, usually quoted in kgs, fundamentally affects its performance and manageability which is why their figures are so commonly quoted, especially in comparison road tests. A common problem, however, is that those figures are usually the manufacturer’s own, so should be taken with a pinch of salt, while different types of weight measurement are used. Kerb or ‘wet’ weight (spelt ‘curb’ in the US), is the most common and is the total weight of a bike with all operating consumables such as oil, coolant and a full tank of fuel. However, ‘dry weight’ is also sometimes quoted which is the weight of a machine WITHOUT all necessary consumables and is therefore significantly less.

 

What do the Dimensions refer to?

The most commonly quoted dimensions of a motorcycle, apart from those for Seat Height, Wheelbase etc which are dealt with separately elsewhere, are its length (L), width (W) and height (H). These are usually quoted in mm (although the US still defers to imperial inches) and are invariably the manufacturer’s own quoted figures. In each case it’s important to remember this is a MAXIMUM, so height, for example, is commonly that to the top of any screen, width is that between the ends of the mirrors and so on which, often, is not particularly useful to a rider and is why these dimensions figures are considered less important than things like kerb weight and seat height.

 

What is Ground clearance?

Sometimes referred to as Ride Height, ground clearance, usually measured in mms, is the amount of space between the contact patch of a motorcycle tyre and the underside of the bike’s chassis. In other words: it’s the shortest distance between the ground and the underside of the engine, frame or fairing and useful to know especially over rough terrain. While only relevant on road bikes if having to traverse a kerb or similar, off-roaders generally benefit from as much ground clearance as possible.

 

What should I look for in a warranty?

The warranty is effectively the guarantee offered to the purchaser by the motorcycle’s manufacturer. The convention in motorcycling is for it to cover the cost of any parts and the workshop labour required for any repair if any part of the motorcycle becomes defective over a finite period. This means that the warranty can usually pass on to a new owner if it’s sold on within that time period. Most commonly in motorcycling that finite period is two years irrespective of mileage, which is what’s referred to as a two year/unlimited mileage warranty. It is, however, becoming increasingly commonplace for three-year warranties to be offered.

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