Motorcycle frame

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Triton: A Triumph engine in a tubular steel Norton Featherbed frame Triton Norton-Triumph motorcycle with polished frame and tank.jpg
Triton: A Triumph engine in a tubular steel Norton Featherbed frame

A motorcycle frame is a motorcycle's core structure. It supports the engine, provides a location for the steering and rear suspension, and supports the rider and any passenger or luggage. Also attached to the frame are the fuel tank and battery. At the front of the frame is found the steering head tube that holds the pivoting front fork, while at the rear there is a pivot point for the swingarm suspension motion. Some motorcycles [1] include the engine as a load-bearing stressed member; while some other bikes do not use a single frame, but instead have a front and a rear subframe attached to the engine. [2]

Contents

Materials

In the early days, motorcycles were little more than motorised bicycles, and consequently frames were tubular steel. While the use of steel tubing is still common, in modern times other materials, such as titanium, aluminium, magnesium, and carbon-fibre, along with composites of these materials, are now used. As different motorcycles have varying design parameters (such as cost, complexity, weight distribution, stiffness, power output and speed), there is no single ideal frame design, and designers must make an informed decision of the optimum choice. [3]

Steel

In Europe and the USA, steel tubing was the default material until recent times. All the major manufacturers (AJS, Ariel, BSA, Matchless, Norton, Sunbeam, Triumph, Velocette, BMW, DKW, Ducati, Moto-Guzzi, Harley-Davidson and Indian) used steel tubing.

Examples

Aluminium

Unlike the development seen in bicycle frames where tubular steel frames were succeeded by frames welded out of round or ovalized aluminium tubing aluminium in motorcycle frames is almost exclusively welded in more angular forms, and often forming a monocoque frame.

Examples

Carbon fibre

In 1983, Armstrong motorcycles produced a 250cc Grand Prix motorcycle using a revolutionary carbon fiber frame. [4] Following the technology being used by the Formula One industry, Armstrong designers Mike Eatough and Barry Hart created the first motorcycle using a carbon fiber frame to compete in Grand Prix racing. [4]

Examples

Magnesium

Magnesium framed Elf 5 Elf5.jpg
Magnesium framed Elf 5
Examples

Titanium

Examples

Composite

Greeves 250DCX Sportsman 1962 Greeves 250DCX Sportsman 1962.JPG
Greeves 250DCX Sportsman 1962
Examples

Types

Spine or backbone

The motorcycle engine is suspended from a single spine. Spine could be a solid structure.

Examples

Single cradle

The motorcycle engine is held in a single cradle with a single spine.

Examples

Half-duplex cradle or double cradle

The motorcycle engine is held in a double cradle with a single spine and single downtube.

Examples
Double cradle frame on Honda CB750 Honda CB750a.JPG
Double cradle frame on Honda CB750

Full duplex cradle

The motorcycle engine is held in place within a pair of separate cradles. The Norton Featherbed frame was the classic example, but many "duplex" frames actually have a single spine beneath the tank.

Examples
2011 Kawasaki ZX10-R below airbox.jpg
Twin-beam frame on Kawasaki ZX-10R
Buell Lightning XB9SX CityX.jpg
Aluminum beam frame on Buell Lightning

Perimeter

Also called beam or twin spar, two beams wrap around the engine to join the steering head and swing arm in the shortest distance possible for better rigidity. Beams are usually made of pressed metal (steel/aluminium). The trellis frame employs the same concept but uses welded members to form a trellis instead of pressed metal.

Antonio Cobas is credited with pioneering the modern, aluminum perimeter frame chassis now used on many modern racing and production motorcycles. [7] In 1982, Cobas developed a stronger and lighter aluminum twin-beam chassis to replace the steel backbone frames. [7] The technology was copied by major motorcycle manufacturers and by the 1990s, all the major racing teams in Grand Prix competition used the aluminum frame design pioneered by Cobas. [8]

Examples

Pressed

Pressed steel frame on Ariel Arrow Ariel Golden Arrow.jpg
Pressed steel frame on Ariel Arrow

The frame is pressed or stamped from sheet metal to form a car-type semi-monocoque. One of the earliest examples was the 965cc motorcycle produced by Louis Janoir in 1920, which used pressed steel for the frame, rear swinging arm, and the front forks. The frame may be entirely pressed (Ariel Arrow), or may have just a pressed aft section connected to the steering head by a conventional steel tubular spine (Honda Super Cub). Both the Super Cub and the Arrow also have pressed steel forks, instead of conventional telescopic forks.

Examples

Monocoque

More common in the car world, a monocoque frame comprises a structure where loads are supported through its external skin. On motorcycles they are used almost exclusively on racing motorcycles.

French industrialist and engineer Georges Roy attempted in the 1920s to improve on the bicycle-inspired motorcycle frames of the day, which lacked rigidity. This limited their handling and therefore performance. He applied for a patent in 1926, and at the 1929 Paris Automotive Show unveiled his new motorcycle, the Art-Deco styled 1930 Majestic. Its new type of monocoque body solved the problems he had addressed, and along with better rigidity it did double-duty, as frame and bodywork provided some protection from the elements. Strictly considered, it was more of a semi-monocoque, as it used a box-section, pressed-steel frame with twin side rails riveted together via crossmembers, along with floor pans and rear and front bulkheads. [12]

1968 Ossa 250 cc Grand Prix racer OSSA 250cc Monohull 1968 Santi Herrero c.JPG
1968 Ossa 250 cc Grand Prix racer

A Piatti light scooter was produced in the 1950s using a monocoque hollow shell of sheet-steel pressings welded together, into which the engine and transmission were installed from underneath. The machine could be tipped onto its side, resting on the bolt-on footboards for mechanical access. [13]

A monocoque-framed motorcycle was developed by Spanish manufacturer Ossa for the 1967 Grand Prix motorcycle racing season. [14] Although the single-cylinder Ossa had 20 horsepower (15 kW) less than its rivals, it was 45 pounds (20 kg) lighter and its monocoque frame was much stiffer than conventional motorcycle frames, giving it superior agility on the racetrack. [14] Ossa won four Grands Prix races with the monocoque bike before their rider died after a crash during the 250 cc event at the 1970 Isle of Man TT, causing the Ossa factory to withdraw from Grand Prix competition. [14]

Notable designers such as Eric Offenstadt and Dan Hanebrink created unique monocoque designs for racing in the early 1970s. [15] The F750 event at the 1973 Isle of Man TT races was won by Peter Williams on the monocoque-framed John Player Special that he helped to design based on Norton Commando. [16] [17] Honda also experimented with the NR500, a monocoque Grand Prix racing motorcycle in 1979. [18] The bike had other innovative features, including an engine with oval shaped cylinders, and eventually succumbed to the problems associated with attempting to develop too many new technologies at once. In 1987 John Britten developed the Aero-D One, featuring a composite monocoque chassis that weighed only 12 kg (26 lb). [19]

An aluminium monocoque frame was used for the first time on a mass-produced motorcycle from 2000 on Kawasaki's ZX-12R, [20] their flagship production sportbike aimed at being the fastest production motorcycle. It was described by Cycle World in 2000 as a "monocoque backbone...a single large diameter beam" and "Fabricated from a combination of castings and sheet-metal stampings". [21]

Single-piece carbon fiber bicycle frames are sometimes described as monocoques; however as most use components to form a frame structure (even if molded in a single piece), [22] these are frames not monocoques, and the pedal-cycle industry continues to refer to them as framesets.

2006 ZX-14 cutaway showing its aluminium monocoque frame Kawasaki ZZR1400 2006 KGTW2.jpg
2006 ZX-14 cutaway showing its aluminium monocoque frame
Examples

Semi-monocoque

If a "monocoque" frame uses additional support, such as tubes or longerons, the frame is more properly called a "semi-monocoque". An example is Peter Williams' semi-monocoque Norton racer.

JPS Norton with semi-monocoque frame 1973 Norton Monocoque at 1999 TT cropped.JPG
JPS Norton with semi-monocoque frame

Trellis

Steel trellis frame (red) on a Ducati Monster 1000. The engine is a stressed member. S2R1000-101 v2 1024 web.jpg
Steel trellis frame (red) on a Ducati Monster 1000. The engine is a stressed member.

A trellis frame connects the steering head to the swingarm pivot as directly as possible using metal tube arranged in triangulated reinforcement. Using lattice girder principles, a trellis frame is typically constructed of round or oval section metal tubular segments that are welded or brazed together. A well-designed trellis frame should provide a strong, lightweight structure that simplifies placement of engine and components, and gives good access for maintenance. Although construction of a trellis frame needs a more complicated process than, say, an alloy beam frame, it requires only a simple jig and a competent welder. No heavy capital outlay is required, so a trellis frame is ideal for a model that may be made in relatively small numbers. For this reason, the trellis frame has found favour with European manufacturers, and Ducati in particular.

Some motorcycles, such as the Yamaha TRX850, have hybrid frames that employ alloy castings at the swingarm pivot area. Another variation is to suspend the engine from a trellis frame, but have the swingarm pivot cast into the rear of the engine.

Examples

Engine as a stressed member

Harley-Davidson Model W with structural tubes bolted directly to engine case to complete the frame triangle Harley-Davidson Sport 558 cc 1921.jpg
Harley-Davidson Model W with structural tubes bolted directly to engine case to complete the frame triangle

For rider comfort, a motorcycle's engine can be mounted on rubber bushings to isolate vibration from the rest of the machine. This strategy means the engine contributes little to frame stiffness, and absorbing rather than dissipating vibration can lead to stress damage to the frame, exhaust pipes, and other parts. [3]

Instead, if the engine is rigidly mounted to the frame, vibrations pass to and are dissipated via the whole frame, and the rider. Rigid mounting allows the engine to contribute to the overall stiffness of the frame. It also becomes possible to mount the swingarm directly to the engine rather than the frame, avoiding the need for frame members extending downward to the swingarm pivot. By increasing the number of mounting points between the engine and frame, vibrations and stress can be better dissipated in the frame, typically creating a triangle between the swingarm in the rear, the cylinder head at the top and the lower crankcase area at the front. If a rigidly mounted engine not only contributes to, but is critical to, the stiffness of the frame, and is an integral part of closing the triangle or trellis structure that transfers force from the headstock to the swingarm, to the point that without the engine the frame would be deformed, the engine is called a stressed member, or a lifted engine. Sharing the load between the engine and frame reduces the overall weight of the motorcycle. [3]

Stressed member engines were pioneered at least as early as the 1916 Harley-Davidson 8-valve racer, and incorporated in the production Harley-Davidson Model W by 1919. [23] This was called a keystone, or diamond, frame. [24] [25] The 1946 Vincent Series B Rapide was designed with an advanced chassis, termed a "tour de force for its day," [26] that included a stressed member engine. During early testing of the 1983 Kawasaki GPZ900R, twin downtubes were included, creating a full cradle, but the downtubes were found to carry little load, so they were removed, relying entirely on the combination of the steel backbone and engine for chassis rigidity. [27] BMW's R1100 series twins of 1994 relieved the frame of stress entirely, with the engine carrying the total load from the front Telelever fork to the rear Monolever. [28] [29]

Stiffness

Frame stiffness is a problem for motorcycle designers, as it was for the bicycle frames that motorcycles are descendants of. [30] [31]

Modifying the stiffness of a factory-produced frame can be undertaken to improve handling characteristics. This is often done by triangulating the factory frame. [30] [32] Triangulation is a technique used in many engineering applications to stiffen structures. [33] However doing so can also have undesirable effects if it overloads other parts of the frame, as a flexible frame acts as a spring to absorb some loads. [34]

Lateral stiffness

In the 21st century, advances like high power engines driving high traction tires, [35] and better-performing suspension components, especially forks, [36] led to a situation where designs with increased overall frame stiffness were made available to consumers. Analysts differ on whether infinite lateral stiffness is desirable, [36] or whether a finite degree of built-in flex is preferable. [37] [38]

See also

Related Research Articles

V-twin engine

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Monocoque Structural design that supports loads through an objects external skin

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Kawasaki Ninja ZX-7R

The Kawasaki Ninja ZX-7R was a motorcycle in the Ninja sport bike series from the Japanese manufacturer Kawasaki produced from 1989 until 2003. It remained largely unchanged through its production. Kawasaki used inverted forks starting in 1991, added ram air using a single tube, and in 1996, twin tube ram air and Tokico six piston brakes and fully adjustable suspension. From 1989 through 1995 in the US market, Kawasaki called the ZXR-750 and ZXR-750R the ZX-7 and ZX-7R respectively. Starting from 1996 Kawasaki dropped the ZXR name worldwide and the former ZXR-750 was now ZX-7R and the limited edition homologation special ZXR-750R/ZX-7R started in 1991 was now ZX-7RR.

Ducati 916

The Ducati 916 is a fully faired sport bike made by Ducati from 1994 to 1998. Featuring a 916 cc (56 cu in) fuel injected, 4-valve, desmo, liquid-cooled, 90° V-twin engine in a trellis frame with a single-sided swingarm and USD forks, the 916 is frequently cited as one of the most beautiful motorcycles ever.

Suzuki GSX-R series

The Suzuki GSX-R is a series of sport bikes made by Japanese manufacturer Suzuki. Current models are the GSX-R125 and GSX-R150 since 2017; GSX-R600 which was manufactured from 1992 to 1993, and then since 1997; the GSX-R750 since 1985; and the GSX-R1000 since 2001.

Swingarm

A swingarm, or "swinging arm" (UK), originally known as a swing fork or pivoted fork, is a single or double sided mechanical device which attaches the rear wheel of a motorcycle to its body, allowing it to pivot vertically. The main component of the rear suspension of most modern motorbikes and ATVs, it holds the rear axle firmly, while pivoting to absorb bumps and suspension loads induced by the rider, acceleration, and braking.

Softail

A softail is a motorcycle with its rear suspension springs or shock absorbers located out of direct view, so as to look like a hard-tail motorcycle.

Honda RC51

The Honda RC51, also known as the RVT1000R or VTR1000 SP1, is a 90° V-twin motorcycle produced by Honda from 2000 to 2006. Built by Honda to prove a point that Honda could outsmart Ducati on the track using a V-Twin.

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.

Ducati L-twin engine

The next new Ducati engine to appear after the Ducati Apollo was the 90° V-twin, initial Grand Prix racing versions being 500 cc, and the production bikes were 750 cc. There was also the Ducati 750 Imola Desmo that won at Imola in 1972. These engines had bevel gear shaft drive to the overhead camshaft, and were produced in round, square, and Mille crankcases. In the 1980s these gave way to the belt drive camshaft engines that have continued to this day, in air-cooled and liquid-cooled form. The Mille used a plain bearing crank, like the belt models.

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Ducati SuperSport

The Ducati SuperSport and SS are a series of air-cooled four stroke desmodromic 2-valve 90° V-twin motorcycles made by Ducati since 1988. A limited edition SuperSport called the SuperLight was sold in 1992. The name harked back to the round case 1973 Ducati 750 Super Sport, and the 1975 square case 750 and 900 Super Sport. The appellation 'SS' was applied only to the later belt drive (Pantah) based models.

Kawasaki Ninja ZX-9R

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Kawasaki KR500

The Kawasaki KR500 was a racing motorcycle manufactured by Kawasaki from 1980 to 1982 for competition in the Grand Prix motorcycle racing series. The motorcycle was powered by a 494 cc two stroke engine, and used an aluminium monocoque frame, similar to the 1979 Honda NR500 racer, aimed at improving aerodynamics with a small frontal area, improving chassis stiffness and reducing weight.

Ducati ST series

The Ducati ST series is a set of Italian sport touring motorcycles manufactured by Ducati from 1997 through 2007. In order of release, the series comprised five distinct models: the ST2, ST4, ST4S, ST3, and ST3S. Intended to compete with other sport-tourers such as the Honda VFR, the ST Ducatis had a full fairing, a large dual seat and a relaxed riding position for both rider and pillion. The ST bikes had a centre-stand, and could be fitted with optional matching luggage.

Ariel Ace

The Ariel Ace is a 2014 sports motorcycle that is manufactured by the British Ariel Motor Company in Crewkerne, Somerset, England. It uses the 1,200 cc (73 cu in) V4 engine from the Honda VFR1200.

Norman Hugh Hossack is a Scottish inventor and engineer, who invented the Hossack motorcycle front suspension system, used on some BMW Motorrad K series motorcycles.

Ducati Panigale V4

The Ducati Panigale V4 is a sport bike with a 1,103 cc (67.3 cu in) desmodromic 90° V4 engine introduced by Ducati in 2018 as the successor to the V-twin engined 1299. A smaller engine displacement version complies with the Superbike category competition regulations which state "Over 750 cc up to 1000 cc" for three and four cylinder 4-stroke engines.

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Sources