Embedded Files
Bicycle and motorcycle geometry is the collection of key measurements (lengths and angles) that define a particular bike configuration. Primary among these are wheelbase, steering axis angle, fork offset, and trail. These parameters have a major influence on how a bike handles.
Wheelbase is the horizontal distance between the centers (or the ground contact points) of the front and rear wheels. Wheelbase is a function of rear frame length, steering axis angle, and fork offset. It is similar to the term wheelbase used for automobiles and trains.
Wheelbase
Wheelbase has a major influence on the longitudinal stability of a bike, along with the height of the center of mass of the combined bike and rider. Short bikes are much more likely to perform wheelies and stoppies.
Steering axis angle
Steering axis angle
The steering axis angle, also called caster angle, is the angle that the steering axis makes with the horizontal or vertical, depending on convention. The steering axis is the axis about which the steering mechanism (fork, handlebars, front wheel, etc.) pivots. The steering axis angle usually matches the angle of the head tube.
In bicycles, the steering axis angle is called the head angle and is measured clock-wise from the horizontal when viewed from the right side. A 90° head angle would be vertical. For example, Lemond[1] offers:
- a 2007 Filmore, designed for the track, with a head angle that varies from 72.5° to 74° depending on frame size
- a 2006 Tete de Course, designed for road racing, with a head angle that varies from 71.25° to 74°, depending on frame size.
In motorcycles, the steering axis angle is called the rake and is measured counter-clock-wise from the vertical when viewed from the right side. A 0° rake would be vertical. For example, Moto Guzzi[2] offers:
- a 2007 Breva V 1100 with a rake of 25°30’ (25.5 degrees)
- a 2007 Nevada Classic 750 with a rake of 27.5° (27.5 degrees)
Telescopic forks on a BMW motorcyclereveal the head angle or rake
Fork offset
Fork offset
The fork offset is the perpendicular distance from the steering axis to the center of the front wheel.
In bicycles, fork offset is also called fork rake. Road racing bicycle forks have an offset of 40-45mm.[3]
Required rake angle arose from early times when lightweight bicycles suffered fork failures from road shock.[citation needed] Most fatigue failures of forks result in a fork blade breaking at the rear edge of the fork crown from repeated vertical road shocks. Before most roads were paved, fork rake had a lower angle so the fork would be loaded axially on rougher surfaces. As most roads became paved, bicycles forks were made steeper, which also gave lighter steering.[citation needed]
In motorcycles with telescopic fork tubes, fork offset can be implemented by either an offset in the triple tree, adding a rake angle (usually measured in degrees from 0) to the fork tubes as they mount into the triple tree, or a combination of the two.[4] Other, less-commonmotorcycle forks, such as trailing link or leading link forks, can implement offset by the length of link arms.
Fork length
Fork length
The length of a fork is measured parallel to the steer tube from the lower fork crown bearing to the axle center.[5]
Trail
Trail
Trail, or caster, is the horizontal distance from where the steering axis intersects the ground to where the front wheel touches the ground. The measurement is considered positive if the front wheel ground contact point is behind (towards the rear of the bike) the steering axis intersection with the ground. Most bikes have positive trail, though a few, such as the two-mass-skate bicycle and the Python Lowracer have negative trail.[6]
Trail is often cited as an important determinant of bicycle handling characteristics, such as here[7] and here,[8] and is sometimes listed in bicycle manufacturers' geometry data, although Wilson and Papodopoulos argue that mechanical trail may be a more important and informative variable.
Example of a chopper with an unusually large rake
where Rw wheel radius, Ah is the head angle measured clock-wise from the horizontal and Of is the fork offset or rake. Trail can be increased by increasing the wheel size, decreasing or slackening the head angle, or decreasing the fork rake or offset. Trail decreases as head angle increases (becomes steeper), as fork offset increases, or as wheel diameter decreases.
Motorcyclists tend to speak of trail in relation to rake angle. The larger the rake angle the larger the trail. Note that, on a bicycle, as rake angle increases, head angle decreases.
Trail can vary as the bike leans or steers. In the case of traditional geometry, trail decreases (and wheelbase increases if measuring distance between ground contact points and not hubs) as the bike leans and steers in the direction of the lean.[10] Trail can also vary as the suspension activates, in response to braking for example. As telescopic forks compress due to load transfer during braking, the trail and the wheelbase both decrease.[11] At least one motorcycle, the MotoCzysz C1, has a fork with adjustable trail, from 89 mm to 101 mm.[12]
The relationship between head angle, rake and trail in a bicycle
Mechanical trail
Mechanical trail is the perpendicular distance between the steering axis and the point of contact between the front wheel and the ground. It may also be referred to as normal trail.[10]
Although the scientific understanding of bicycle steering remains incomplete,[13] mechanical trail is certainly one of the most important variables in determining the handling characteristics of a bicycle. A higher mechanical trail is known to make a bicycle easier to ride "no hands" and thus more subjectively stable, but skilled and alert riders may have more path control if the mechanical trail is lower.[14]
Wheel flop
Wheel flop
Wheel flop refers to steering behavior in which a bicycle or motorcycle tends to turn more than expected due to the front wheel "flopping" over when the handlebars are rotated. Wheel flop is caused by the lowering of the front end of a bicycle or motorcycle as the handlebars are rotated away from the "straight ahead" position. This lowering phenomenon occurs according to the following equation:
f = b sin ∂ cos ∂[15]
where:
f = "wheel flop factor," the distance that the center of the front wheel axle is lowered when the handlebars are rotated from the straight ahead position to a position 90 degrees away from straight ahead
b = trail
∂ = head angle
Because wheel flop involves the lowering of the front end of a bicycle or motorcycle, the force due to gravity will tend to cause handlebar rotation to continue with increasing rotational velocity and without additional rider input on the handlebars. Once the handlebars are turned, the rider needs to apply torque to the handlebars to bring them back to the straight ahead position and bring the front end of the bicycle or motorcycle back up to the original height.[16] The rotational inertia of the front wheel will lessen the severity of the wheel flop effect because it results in opposing torque being required to initiate or accelerate changing the direction of the front wheel.
According to the equation listed above, increasing the trail and/or decreasing the head angle will increase the wheel flop factor on a bicycle or motorcycle, which will increase the torque required to bring the handlebars back to the straight ahead position and increase the vehicle's tendency to veer suddenly off the line of a curve. Also, increasing the weight born by the front wheel of the vehicle, either by increasing the mass of the vehicle, rider and cargo or by changing the weight ratio to shift the center of mass forward, will increase the severity of the wheel flop effect. Increasing the rotational inertia of the front wheel by increasing the speed of the vehicle and the rotational speed of the wheel will tend to counter the wheel flop effect.
A certain amount of wheel flop is generally considered to be desirable. In the magazine Bicycle Quarterly, bicycle dynamics expert Jan Heine wrote, "A bike with too little wheel flop will be sluggish in its reactions to handlebar inputs. A bike with too much wheel flop will tend to veer off its line at low and moderate speeds."[15]
Modifications
Modifications
Forks may be modified or replaced, thereby altering the geometry of the bike.
Changing fork length
Changing fork length
Increasing the length of the fork, for example by switch from rigid to suspension, raises the front of the bike and decreases the head angle. [5]
A rule of thumb is a 10mm change in fork length gives a half degree change in the head angle.
Changing fork offset
Changing fork offset
Increasing the offset of a fork reduces the trail, and if performed on an existing fork without lengthening the blades, shortens the fork. [17]
Legal requirements
Legal requirements
The state of North Dakota (USA) actually has minimum and maximum requirements on rake and trail for "manufacture, sale, and safe operation of a motorcycle upon public highways."[18]
"4. All motorcycles, except three-wheel motorcycles, must meet the following specifications in relationship to front wheel geometry:
MAXIMUM: Rake: 45 degrees - Trail: 14 inches [35.56 centimeters] positive
MINIMUM: Rake: 20 degrees - Trail: 2 inches [5.08 centimeters] positive
Manufacturer's specifications must include the specific rake and trail for each motorcycle or class of motorcycles and the terms "rake" and "trail" must be defined by the director by rules adopted pursuant to chapter 28-32."
Other aspects
Other aspects
For other aspects of geometry, such as ergonomics or intended use, see the Bicycle frame article. For motorcycles the other main geometric parameters are seat height and relative foot peg and handlebar placement.
Motorcycle fork
A motorcycle fork connects a motorcycle's front wheel and axle to its frame, typically via a pair of triple clamps. It typically incorporates the front suspension and front brake, and allows the bike to be steered via handlebars attached to the top clamp.
The fork and its attachment points on the frame establish the critical geometric parameters of rake and trail, which play a major role in defining how a motorcycle handles and dives during braking.[1]
Variations
Variations
Over more than one hundred years of motorcycle development a variety of front fork arrangements have been tried, several of which remain available today.
A telescopic fork uses fork tubes which contain the suspension components (coil springs and damper) internally. This is the most common form of fork commercially available. It may or may not include gaiters for protection against abrasive elements on the suspension cylinders. The main advantages of the telescopic fork are that (i) it is simple in design and relatively cheap to manufacture and assemble; (ii) it is lighter than older designs using external components and linkage systems; and (iii) it has a clean and simple appearance that bikers find attractive.
Conventionally, the fork stanchions are at the top, clamped to a triple tree, (aka "yoke"), and the sliders are at the bottom, attached to the front wheel spindle.. On some modern sport bikes, this system is inverted, with "sliders" (complete with the spring/damper unit) at the top, clamped to the yoke, while the stanchions are at the bottom . This is done (i) to reduce unsprung weight by having the lighter components suspended, and (ii) to improve the strength and rigidity of the assembly by having the strong large-diameter "sliders" clamped in the yokes.[2] The inverted system is referred to as an upside-down fork, or "USD" for short. A disadvantage of this USD design is that the entire reservoir of damping oil is above the slider seal so that, if the slider seal were to leak, the oil could drain out, rendering any damping ineffective.
The first production motorcycle with hydraulically damped telescopic forks was the German BMW in 1935.[citation needed] However, undamped telescopic forks were used on bikes made by The Scott Motorcycle Company from the beginning of production in 1908,[3] and the Danish Nimbus used them from 1934 on.[citation needed]
Trailing link
Trailing link
A trailing link fork, which may or may not be telescopic, suspends the wheel on a link (or links) with a pivot point forward of the wheel axle. Most famously used by Indian Motocycle; it was also used by BMW for its early bikes.
A leading link fork, which may or may not be telescopic, suspends the wheel on a link (or links) with a pivot point aft of the wheel axle. Russian Uralmotorcycles still use leading link forks on sidecarequipped motorcycles, and aftermarket leading link forks are often installed today on motorcycles when they are outfitted with sidecars, they are also very popular with trikes, improving the handling while steering or braking. The most common example of a leading link fork is that found on the Honda Super Cub.
Springer
The springer fork is an early type of leading link fork. A springer fork does not have the suspension built into the fork tubes, but instead has it mounted externally, where it may be integrated into the triple tree. This style of fork may be found on antique motorcycles or choppers, and is available today on Harley-Davidson's Softail Springer.
While it may have an exposed spring near the triple clamp, a springer fork is distinguishable from a girder fork by its two parallel sets of legs. The rear is firmly fixed to the bottom triple clamp (usually brazed or welded). A short leading link holds the wheel and the forward leg which actuates the springs (usually mounted on the triple clamp).
BMW's version of oil-damped telescopic fork, on a 1939 R12
Ural's variant of the leading link fork
Telescopic
Telescopic
Leading link
Leading link
Unusual "trailing bottom link" on a Honda Rune
Trailing link fork on a 1928 Indian Big Chief
Early Harley-Davidson with springer fork
The telescopic Earles fork was a variety of leading link fork where the pivot point was aft of the rear of the front wheel ─ this was the basis of the Earles' patent.[4] Designed by Englishman Ernest Earles, this triangulated fork actually caused the front end of a motorcycle to rise when braking hard — the reverse of the action of a telescopic fork. It was designed to accommodate sidecars, and from 1955 to 1969, BMW used the fork even though most of its motorcycles were sold as solo bikes.
Earles
Girder
Girder
Earles fork on a 1968 BMW R60/2
One of the earliest types of motorcycle front suspension, the girder fork consists of a pair of uprights attached to the triple clamp by linkages with a spring usually between the top and bottom triple clamps. The design reached its peak in the "Girdraulics" used on "The Vincent" motorcycle. Girdraulic forks featured forged alloy blades for extra strength and hydraulic damping.[5]
While it may have an exposed spring near the triple clamp, a girder fork is distinguishable from aspringer fork by the wheel being fixed firmly to the (usually a long diamond shape) upright. The pivot points are short links mounted to the top and bottom triple clamps. The spring is (usually) mounted to the girder and compressed against the upper triple clamp.
1934 Cotton with girder fork
Saxon-Motodd (Telelever)
Saxon-Motodd (Telelever)
The Saxon-Motodd (marketed as Telelever by BMW) has an additional swingarm that mounts to the frame and supports the spring. This causes the trail and castor angle (rake) to increase during braking instead of decreasing as with traditional telescopic forks.[6] In the 21st century, BMW's boxer twins are equipped with Telelever forks.
Hossack/Fior (Duolever)
Hossack/Fior (Duolever)
The Hossack/Fior (marketed as Duolever by BMW) separates completely the suspension from steering forces. It was developed by Norman Hossack though used by Claude Fior and John Britten on racebikes. Hossack himself described the system as a 'steered upright'. In 2004 BMW announced the K1200S with a new front suspension that appears to be based upon the design. As of 2006, the Duolever is on the K1200S, K1200R, and K1200GT.
BMW Telelever fork on an R1200GS
Coaxial steering front suspension
Coaxial steering front suspension
Developed by MotoCzysz for their C1 and awarded United States Patent 7111700 on September 26, 2006. It is a fork with "coaxial steering and suspension components, and having telescopic forks. Swing weight of the forks is dramatically reduced by removing their suspension components to the central location, coaxially within the steering tube. Ride height can be adjusted without loosening the forks in the triple clamps. A shock tube disposed substantially coaxially within the steering tube wherein the shock tube includes a passage therethrough substantially coaxial with the steering axis; an upper triple clamp and a lower triple clamp coupled to the shock tube; a pair of sliding-tube forks each having an upper fork tube coupled to the upper triple clamp and to the lower triple clamp, and a lower fork tube; a coil-over shock disposed within the shock tube; a front wheel rotatably coupled to the lower fork tubes; a pair of bearings rotatably coupling the shock tube to the steering tube; and a top bolt coupling the shock tube to the upper triple clamp and having a passage therethrough substantially coaxial with the steering axis; wherein the coil-over shock includes a setting adjustment mechanism which is accessible via the passages through the top bolt and the shock tube."[7]
This particular fork, as implemented on the MotoCzysz C1, also has adjustable trail, from 89 mm to 101 mm.[8]
Non-forks
Non-forks
Main article: Hub-center steering
There have been several attempts to implement front steering and suspension without using anything that could be described as a "fork". Examples include hub-center steering, used as early as 1920 on the Ner-a-Car,[9] and implemented in the 1990s on the Bimota Tesi[10] and the Yamaha GTS1000[11]
A single-sided girder "fork" was used by the German firm Imme[12][13] between 1949 and 1951.[13]
Fork tube
Fork tube
Duolever front fork
Generally employed in pairs, fork tubes (or "stanchions") link a motorcycle's front wheel to its frame. They typically house the front suspension and on telescopic fork systems compress and extend to adjust for inconsistencies in the road.
Inside most tubes are springs, fork oil, and air, creating a shock absorber. Some forks allow pressurized air to be added through a valve in the top of the fork to stiffen the suspension. Another method employs a screw to compress fork spring to increase or decrease spring pre-load.
Some forks also allow damping through variably sized orifices controlling the flow of fork oil. The larger the orifice, the more free the flow and the less damped the fork. A selector atop the fork engages the desired hole size and corresponding damping rate.
Cartridge forks provide regressive damping. Self-contained cartridges within the forks contain spring-covered orificies regulating fork oil flow. The springs resist low forces and thus provide high damping rates. Higher forces compress the springs, allowing more oil flow and less damping. Thus the fork is stiffer when responding to small bumps but will soften as larger ones are encountered.
Triple tree
Triple tree
A triple tree ("triple clamp" (US) or "yoke" (UK)) attaches the fork tubes to the frame. Most bikes have upper and lower triple trees, providing two solid points for connecting the forks to the frame.
Telescopic upside down (USD) fork with stanchions at the bottom. The right fork tube can be seen held by the upper and lower triple clamp of the triple tree.
Hub-center steering
Hub-center steering
The upper triple clamp joins the top of the fork tubes and the handlebar to the frame of a Honda 919
Hub-center steering (HCS) is one of several different types of front end suspension/steering mechanisms used in motorcycles. Hub-center steering is characterized by a swingarm that extends from the bottom of the engine/frame to the centre of the front wheel instead of twoforks.
The advantages of using a hub-center steering system instead of a more conventionalmotorcycle fork are that hub-center steering separates the steering, braking, and suspension functions.
With a fork the braking forces are put through the suspension, a situation that leads to the suspension being compressed, using up a large amount of suspension travel which makes dealing with bumps and other road irregularities extremely difficult. As the forks dive the steering geometry of the bike also changes making the bike more nervous, and inversely on acceleration becomes more lazy. Also, having the steering working through the forks causes problems with stiction, decreasing the effectiveness of the suspension. The length of the typical motorcycle fork means that they act as large levers about the headstock requiring the forks, the headstock, and the frame to be very robust adding to the bike's weight.
Hub-center steering systems use an arm, or arms, on bearings to allow upward wheel deflection, meaning that there is no stiction, even under braking. Braking forces can be redirected horizontally along these arms (or tie rods) away from the vertical suspension forces, and can even be put to good use to counteract weight shift. Finally, the arms typically form some form of parallelogram which maintains steering geometry over the full range of wheel travel, allowing agility and consistency of steering that forks currently cannot get close to attaining. The hub center steering's achilles heel, however, has been steering feel. Complex linkages tend to be involved in the steering process, and this can lead to slack, vague, or inconsistent handlebar movement across its range.
Hub-center steering systems have only appeared on a very few production motorcycles, and not with any great success. Evolution, rather than revolution, tends to drive advancements in new models, and dictate sales. After so many years of telescopic forks, people are used to riding a bike that handles in a specific way, and almost expect the limitations, and compensation is part of the experience. Also there is a depth of knowledge known about fork based chassis design that attempts each year to get around the limitations through technological advances on the current system. Thicker and thicker fork tubes are used to reduce flex, special coatings are used to aid stiction, and greater and greater steering angles are used to counteract dive.
The hub-center steer concept is a very old one used as early as 1920 by Ner-a-Car, and enjoyed an aftermarket vogue in the 1970s through the work of Jack Difazio in the UK. The late Mike Tomkinson (of Mead & Tomkinson), aided by sons Chris and Patrick, pioneered the use of hub-centre steering in 24-hour motorcycle endurance racing. Their first machine, "Nessie" (qv), was powered by a Laverda 1000cc triple; but they later designed a Kawasaki-engined bike that became known as Nessie II. The Tomkinson's efforts encouraged Elf in the 1980s to create a succession of GP race bikes. In the 90's there was a flurry of action, first was the Bimota Tesi 1D in 1991 (designed by a young Massimo Tamburini of 916 fame) however this was expensive and was only ever produced in small numbers. Then in 1993 Yamaha launched theGTS1000 based on James Parker's RADD design. It raced at the Isle of Man TT but was always blighted with a reputation for being a bit heavy and clumsy in use. In 1995 Michael Tryphonos built a prototype based on the Defazio system that did race at the Isle of Man with some success reaching 11th in the Senior TT.
Royce Creasey, designer of feet forwards motorcycles, is an ardent advocate of HCS.
Currently, Bimota's Tesi 3D and the Vyrus 984C3 2V and the 985C3 4V are the only production motorcycles using hub-center steering systems, however Italjet also use hub-center steering on their top of the range scooters with much success. Sidecar manufacturers occasionally employ hub-center steering in their designs. A notable example being the GG Duetto.
The Vyrus 985 C3 features hub-center steering.
Hub-center steering in Bimota Tesi
Suspension
Suspension
The typical motorcycle has a pair of fork tubes for the front suspension, and a swingarm with one or two shock absorbers for the rear suspension.[1]
A motorcycle's suspension serves a dual purpose: contributing to the vehicle's handling and braking, and providing safety and comfort by keeping the vehicle's passengers comfortably isolated from road noise, bumps and vibrations.
Main article: Motorcycle fork
Front suspension
Front suspension
The most common form of front suspension for a motorcycle is the telescopic fork. Early front suspension designs used frames with springs.
Some British manufacturers (e.g. Greeves) used a version of the swinging arm for front suspension on their motocross designs. A single-sided version of the idea is also used in motor scooters such as the Vespa.
The Hub-center steering as developed by Ascanio Rodorigo, on a concept associated to Massimo Tamburini is a complex front swingarm alternative system that entails suspension and steering, as seen in projects such as Bimota Tesi and Vyrus motorcycles.
Vincent Black Lightning with Girdraulic front suspension
Telescopic forks
Telescopic forks
In 1934 Nimbus was the first manufacturer to produce a motorcycle with hydraulically damped telescopic forks. Most motorcycles today use telescopic forks for the front suspension. The forks can be most easily understood as simply large hydraulic shock absorbers with internal coil springs. They allow the front wheel to react to imperfections in the road while isolating the rest of the motorcycle from that motion.
The top of the forks are connected to the motorcycle's frame in a triple tree clamp (otherwise known to British riders as the top yoke and bottom yoke), which allows the forks to be turned in order to steer the motorcycle.
The bottom of the forks are connected to the front axle around which the front wheel spins.
On typical forks, the upper portion, known as the fork tubes, slide inside the fork bodies, which are the lower part of the forks. As the tubes slide in and out of the body they are telescoping, thus the term telescopic forks. The fork tubes must be smooth to seal the fork oil inside the fork, and typically have a mirrored finish, though some fork tubes, especially those on off-road motorcycles, are enclosed in plastic protective sleeves, known as gaiters.
"Upside-down" (USD) forks, also known as inverted forks, are installed inverted compared to typical forks, with the tubes at the bottom and the bodies at the top. This decreases the unsprung weight of the motorcycle and improves its handling. USD forks are usually found onsportbikes, though Honda's large power-cruiser, the Valkyrie, sported USD forks.
Pre-load adjustment
Motorcycle suspensions are designed so that the springs are always under compression, even when fully extended. Pre-load is used to adjust the initial position of the suspension with the weight of the motorcycle and rider acting on it.
The difference between the fully extended length of the suspension and the length compressed by the weight of the motorcycle and rider is called total sag. Total sag is set to optimize the initial position of the suspension to avoid bottoming out or topping out under normal riding conditions. Bottoming out" occurs when the suspension is compressed to the point where it mechanically cannot compress any more. Topping out occurs when the suspension extends fully and cannot mechanically extend any more. Increasing pre-load increases the initial force on the spring thereby reducing total sag. Decreasing pre-load decreases the initial force in the spring thereby increasing total sag.
A few motorcycles allow adjustment of pre-load by changing the air pressure inside the forks. Valves at the top of the forks allow air to be added or released from the fork.[2] More air pressure gives more preload, and vice versa.
Damping adjustment
Some stock telescopic forks have external adjustments for damping. The adjuster turns a rod inside the fork which brings different sized orifices into alignment with the damping fluid flow path inside the fork. Smaller orifices restrict the flow of the fork oil more and give greater damping.
Fork oil
Since forks act as hydraulic shocks, changing the weight of the fork oil will change the damping.
Cartridge forks
Cartridge forks use internal cartridges with various leaf springs covering orifices to control the damping of the fork.
Some of the leaf springs lift with little force allow fluid to flow through the orifice. Other springs require greater force to lift and allow flow. This gives the fork digressive damping, allowing it to be stiff over small bumps, but get relatively softer over larger bumps.
Also, the springs only allow flow in one direction, so one set of springs controls compression damping, and another rebound damping. This allows the dampings to be set separately.
Cartridge emulators are aftermarket parts that make non-cartrdige forks behave like cartridge forks.
Gas-charged cartridge forks
In 2007 the gas-charged bolt-in cartridge set for modern sportbike forks became available. This kit is legal for supersport styled classes of racing, which regulations do not allow a complete fork replacement, and force competitors to use the stock fork casings.
Brake dive
Applying the brakes of a moving motorcycle increases the load borne by the front wheel and decrease the load borne by the rear wheel due to a phenomenon called load transfer. For a detailed explanation and a sample calculation, see the braking section of the Bicycle and motorcycle dynamics article.
If the motorcycle is equipped with telescopic forks, the added load on the front wheel is transmitted through the forks, which compress. This shortening of the forks causes the front end of the bike to move lower, and this is called brake dive.
Brake dive can be disconcerting to the rider, who may feel like he or she is about to be thrown over the front of the motorcycle. If the bike dives so far as to bottom out the front forks, it can also cause handling and braking problems. One of the purposes of a suspension is to help maintain contact between the tire and road. If the suspension has bottomed out, it is no longer moving as it should, and is no longer helping to maintain contact.
Brake dive with telescopic forks can be reduced by either increasing the spring rate of the fork springs, or increasing the compression damping of the forks. However, all of these changes make the motorcycle less pleasant to ride on rough roads, since the front end will feel stiffer, in the 1980s various manufacturers attempted to get round this by methods of anti-dive such as:
- ACT: Developed by Marzocchi and fitted to Buell motorcycles such as the Buell RR 1200 (1988).
- ANDF (Anti Nose Dive Forks): This was fitted to a number of Suzuki GSX models and the RG250.
- AVDS (Automatic Variable Damping System): This was fitted to a number of Kawasaki motorcycles.
- NEAS (New Electrically Activated Suspension): As fitted to the Suzuki GSX-R 1100 and GSX-R 750 Limited Edition.
- PDF (Posi Damp Fork): This was fitted to the Suzuki RG500 and GSX-R 750 and worked by brake fluid pressure closing a valve in the mechanism when the brakes are applied, restricting the flow of damping oil and slowing fork compression. The valves are spring loaded so if the wheel hits a bump when the brakes are on, they bounce off their seats and restore the flow of oil for a moment to allow the suspension to absorb the shock.
- TCS (Travel Control System): Anti-dive system with variable damping. TCS was introduced on the FZ 400 R (1984, only for the Japanese market).
- TRAC (Torque Reactive Anti-dive Control): This was fitted to a number of Honda motorcycles such as the CB1100F, CB1000C, andVFR750F and worked by utilizing a pivoting caliper that activated a valve in the fork leg.
Another method to reduce or eliminate brake dive in telescopic forks is to use a reactive link or torque arm to connect the braking components to the motorcycle frame via the triple clamp.
Some fork designs mitigate dive, eliminate it, or even reverse it without affecting the front suspension adversely. The Earles fork is among the latter; when braking the front brake hard, the front end of the motorcycle actually rises. BMW's Telelever fork is designed to nearly eliminate dive, but could have been designed to eliminate it completely if the manufacturer chose to do so. Leading link front forks, such as used on some Ural motorcycles, can also be designed either to reduce or eliminate dive.
Saxon-Motodd (Telelever) fork
The Saxon-Motodd (marketed as Telelever by BMW) has an additional swingarm that mounts to the frame and supports the spring. This causes the rake and trail to increase during braking instead of decreasing as with traditional telescopic forks.
Hossack/Fior (Duolever) fork
The Hossack/Fior (marketed as Duolever by BMW) separates completely the suspension from steering forces. It was developed by Norman Hossack though used by Claude Fior and John Britten on racebikes. Hossack himself described the system as a 'steered upright'. In 2004 BMW announced the K1200S with a new front suspension that is based upon this design.
Single-sided
Single-sided
A single-sided front swingarm suspension was used on the Yamaha GTS1000, introduced in 1993. The GTS used the RADD front suspension designed by James Parker. A single sided girder fork was use by the German firm Imme between 1949 and 1951, and the Vespa scooter has a single-sided trailing-link fork. More recently[when?], the ItalJet "Dragster" scooter also uses a single-sided swingarm suspension, though unlike the GTS1000 there is no upper control arm; the upper part of the suspension on the Dragster serves only to transmit steering input.
Rear suspension
Rear suspension
BMW's Telelever
While front suspensions were almost universally adopted before World War I, several manufacturers did not use rear suspension on their bikes until after World War II. However, motorcycles with rear suspension were offered to the public before World War I. Notable among these are the 1913 Indian Single with a swingarm suspended from a leaf spring and the 1913 Pope with wheels supported on a pair of plungers which were each suspended by a coil spring.[3][4]
Plunger rear suspension on a BMW R51/3
Early rear suspensions
Early rear suspensions
Plunger suspension
Several motorcycles before and immediately after World War II used plunger suspension in which the vertical movement of the rear axle was controlled by plungers suspended by springs.[5]
Notable manufacturers of bikes with plunger suspension include Adler, Ariel, BMW, BSA, Indian,MZ, Norton, and Zündapp
Swingarms
Swingarms
Main article: Swingarm
The basic motorcycle swingarm is a rectangle, with one short side connected to the motorcycle's frame with bearings so that it can pivot.[6]The other short side is the rear axle around which the rear wheel turns. The long sides are connected to the motorcycle's frame or rear sub-frame with one or two shocks with coil-over springs.
In production motorcycles, swingarms are not exactly rectangular, but their function can be more easily understood by thinking of them as such.
When a swingarm is present on only one side of the motorcycle, this is known as a single-sidedswingarm. Notable examples include the Honda VFR800 and the BMW R- and K-series. Single-sided swingarms make rear-wheel removal easier, though they generally increase the unsprung weight of the rear suspension. This is due to the additional material required to give identical torsional rigidity to a conventional (two-sided) swingarm setup. For this reason sports bikes are rarely seen using the setup. Notable exclusions are the Ducati 916 which was intended to be taken endurance racing, the MV Agusta f4 which has a hollow interior for reducing weight (a magnesium version is also available), and the Ducati 1098, which was given a single sided swingarm purely for styling reasons.
On many shaft-drive motorcycles the drive shaft is contained in one of the long sides of the swingarm. Notable examples include all post-1955 BMW models prior to BMW's use of the single-sided swingarms, Urals, many Moto Guzzi twins, the Honda Goldwing, the Yamaha XS Eleven, and theYamaha FJR1300.
The BMW R- and K-series combine a shaft-drive contained in the swing arm with a single-sided swingarm, and the combination is marketed as the Paralever. Newer Moto Guzzi motorcycles use a similar arrangement marketed as the CA.R.C. ("CArdano Reattivo Compatto" - Compact Reactive Shaft Drive).
For motorcycles with chain drives, the rear axle can be adjusted forward and back in relation to the swingarm, to adjust chain tension.
Shock absorbers
Shock absorbers
Main article: Shock absorber
The hydraulic shock absorbers used on the rear suspensions of motorcycles are essentially the same as those used in other vehicle applications.
Motorcycle shocks do differ slightly in that they nearly always use a coil-over spring. In other words, the spring for the rear suspension is a coil spring that is installed over, or around, the shock.
In terms of adjustment, rear shocks span the range from pre-load adjustments only to racing shocks with adjustments for pre-load, and four different kinds of damping. Most shocks have internal oil reservoirs, but some have external ones, and some offer air-assisted damping.
A number of companies offer custom-built rear shocks for motorcycles. These shocks are assembled for a specific motorcycle and rider combination, taking in to account the characteristics of the motorcycle, the weight of the rider, and the rider's preferred riding style/aggressiveness.
Twin shock absorbers
Twinshock refers to motorcycles that have two shock absorbers. Generally, this term is used to denote a particular era of motorcycles, and is most frequently used when describing off-road motorcycles.
During the late 1970s and 1980s, motorcycle rear suspension design and performance underwent tremendous advances. The primary goal and result of these advances were increased rear wheel travel, as measured in the how far the rear wheel could move up and down. Before this period of intense focus on rear suspension performance, most off-road motorcycles had rear wheel travel of about 3.5–4 inch (9–10 cm). At the end of this period, most of these motorcycles had rear wheel travel of approximately 12 inch (30 cm). At the beginning of this period, various rear suspension designs were used to reach this degree of performance. However, by the end of this period, a design consisting of using only one shock absorber (instead of two) was universally accepted and used. Motorcycles with only one shock absorber are calledmonoshock motorcycles. The performance of monoshock motorcycles was vastly superior to twin shock motorcycles. Accordingly, this design distinction is readily used to categorize motorcycles. Since monoshock motorcycles have been the norm since the 1980s, the term "twinshock" is now used to categorize vintage motorcycles. This distinction is important in that it provides classes used for vintage motorcycle competition. For example, vintage motocross races are held for older motocross motorcycles. To prevent the better-performing monoshock motorcycles from dominating the competition, there are separate competition classes for monoshock and twinshock motorcycles, which prevents them from competing directly against each other.
Mono-shock
On a motorcycle with a mono-shock rear suspension, there is only one shock that connects the rear swingarm to the motorcycle's frame. Typically this lone shock absorber is in front of the rear wheel, and uses a linkage to connect to the swingarm.
Mono-shocks eliminate torque to the swingarm and provide more consistent handling and braking. They are also easier to adjust, since there's only one shock absorber to adjust, and there is no worry about matching two shocks. Also, the linkages used to connect the shock to the swing-arm are frequently designed to give a rising rate of damping for the rear.[7]
Honda refers to its mono-shock designs as Pro-link suspensions, Kawasaki as Uni-Track, Suzuki as Full-Floater and Yamaha as Monocross.
BMW's Paralever rear suspension on aR1200GS
Moto Guzzi's CA.R.C.
Google Sites
Report abuse