By: Aman Mishra

Every automobile that we come across daily was once a design on a piece of paper. Not one design but many with some on the table and some in the trash bin. It is a process that involves series of attempts from scratch to production. A motorcycle/bike is designed and developed with several stages of testing. Each and every part is first sketched on papers as well as on computers. Just like other parts, there is one very crucial part that does an elderly job of firmly holding other parts together in place and is known as a frame. 

🚨I’m not a bio student so please pardon me if I haven't used the exact biology terms in my explanation. I actually took an example of human body as it is the closest thing to me that resembles a motorcycle frame🚨

 

 Think of a human body for a min which has so many(206) bones that together form a skeleton. Skeleton saves all the organs from getting damaged by any external force. Similarly in a bike, frame is the skeleton and saves other mechanical parts from getting damaged in any crash. People often get confused between a chassis and a frame. Some even treat both as the same part with different names. Let me take you back to human body in which skeleton supports the organ. In a skeleton, all bones hang on a chief centre support. This support is a chain of small interlocked bones which form a column known as spine or backbone. In a bike, spine or backbone is the chassis. Chassis has different mountings on which all parts fit along with the frame. Chassis has components like tires, axle assembly, steering, brakes, engine and transmission assembled on it whereas a frame is the main structure fixed to a chassis.

 Generally, it has a wishbone type of design. Frame absorbs most forces that are transferred from rotating wheels to the body. Handling and comfort of a bike could be improved by using nice pair of tires, improved suspension system and also a better frame.

The main parts of a frame are:-

• 🚲Top tube is where the tank and seat is assembled.
• 🚲Steering headtube is where the front fork is fitted.
• 🚲Downtubetakes up whole weight of the engine.
• 🚲Swingarm pivot is where the swing arm with a wheel is bolted.

Additional pivots in the frame are provided to which either mono or twin coil spring suspension is installed. To be précised, coil spring is assembled in between frame and swingarm.   

HIStory or HERtory?

Motorcycle is nothing but a cycle with an engine. Earlier these motorized cycles ran at a speed of 20km/h and had small light weight engines that didn’t transfer much force to the parts. Slowly when the evolution of bikes started, it increased the weight with power production and ultimately increased the average as well as top speed. This induced different types of transverse forces caused by cross winds, steering and road conditions to the parts(mainly engine). Also, the location of engine was high due to the absence of any support like a frame. These two factors caused tremendous shaking that either loose bolted the parts or even in some cases made the engine to fall out of the bike while riding. Installing a frame gave a huge opportunity for the developers who could then advance to assemble engines close to the ground. This design reduced vibration and shielded parts to some extent. Still wondering how? Imagine a 50 storey building that is suffering from an earthquake. Greater vibrations would be felt by the person on 50^th floor than the one on the ground floor.

 Frames were first introduced in a motorcycle prior to WW1. This made heavy and powerful engine installation possible while diminishing the cons of a frameless bike. Earlier motorcycle had that common cycle frame i.e. diamond design but in the beginning of 20^th century after numerous R&Ds, bikes started getting those distinguished frame. However, Antonio Cobas led the foundation brick of modern aluminium perimeter motorcycle frame. Different types of frame got developed with development in automobile design. Five basic types of frames installed in a motorcycle are:-

🚴Spine frame

🚴Single cradle frame

🚴Duplex or double cradle frame

🚴Perimeter or twin spar frame

🚴Trellis frame

🚴Stamped/Pressed frame

🚴Monocoque frame      

  

Spine frame – A spine frame has only the spine section to mount the parts. In this type, downtube is absent which makes the engine cantilever. Less amount of steel is used to build a spine frame which makes it light and cheap. This makes it least efficient at high speed and on bumpy terrain. Due to the absence of downtube, it is less capable of withstanding transverse forces. However, toptube has greater diameter than the remaining tubes and acts like the main load bearer. Spine frame is mostly used to mount smaller capacity engines in low budget bikes, e.g. Honda CS90, Hero CD100 etc.    

Single cradle frame– Cradle is a type of bed hanging on rocker arms. The downtube which was absent in a spine frame is present in this type. With the introduction of downtube, a single cradle frame looks complete or kind of a looped or closed structure. The most basic one is a bicycle frame. The engine sits firmly inside the cradle section just like a baby sleeping inside a cradle bed. Engine is provided with an additional support by the downtube resulting in a better performance than a spine frame. But it becomes unstable and shaky when rider applies hard brake or when going through high speed corners. This happens when overall forces become unbalanced at the moment of high lean angle cornering and hard braking. The material chosen to build this frame is pure steel. Obviously, it is heavy but cheap at the same time and finds its place in low budget bikes. Cradle frame allowed the developers to reposition and assemble the engine much closer to the ground and increase stability. Modern low budget bikes along with small capacity dual sports come with a single cradle frame as it offers better stability than a spine frame. It is the most widely used frame due to its simple design, e.g. Royal Enfield 350, Jawa Yezdi 350 etc.

Duplex or Double Cradle frame – Two is better than one. By this line I mean a single cradle has a single downtube that supports the engine but fails to give results when it goes through hard braking from a speed of 130km/h. Hence, an evolved version of a single cradle frame is a double cradle frame having two downtubes instead of one that provides extra support to the engine and transmission. Extra tube increases the weight but still the overall frame is comparatively light. Duplex frame is made of steel and is a good overall package of strength, rigidity with light weight. The difference could be easily felt when you ride a Jawa Yezdi(single) and a Royal Enfield Continental 650GT(duplex) and this is the reason why this frame is used in performance bikes, e.g. Kawasaki W800, Yamaha Bolt etc.

Featherbed frame was a type of duplex frame developed by the famous UK brand Norton. It was the best frame in terms of handling around the twisting course of Isle of Man Tourist trophy in 1950 era. Isle of Man TT is the most dangerous race on the planet and Norton’s featherbed frame performed so well that riders felt like riding on a cosy featherbed and that is how the frame got its name.

Perimeter or twin spar frame – All the above mentioned frames were reliable but the standard was limited to small engine commuter bikes with low top speeds. Most energy produced by the engine got wasted in form of vibrations and restricted the bike to reach its limits. This called for a totally new type of frame that was least flexible. Perimeter frame was derived from high speed racing bikes so it was already a successful frame. Racing motorcycle are no doubt based on performance and years of research work brought developers to the conclusion that rigidity(less flexibility) of a bike increased if the distance between headtube and swingarm was decreased to the lowest value. Perimeter frame has two rectangular/box section main beams or spars moved outwards to the perimeter of the bike. They are merged with headtube on one end and to the swingarm pivot on the other. Engine gets boxed in between the main spares and is assembled on mostly five mounting points. Such a design helps to share overall stress on the frame by the engine making it a stress member. The engine seems to hang from the bottom area due to the absence of downtubes. Perimeter frame looks bulky but never go on its looks as it is made of light weight aluminium. Whole weight of the rider, engine and other parts gets concentrated on to the frame. This is when the two main beams that form a closed structure take up these loads with other forces to provide rigidity and stiffness. Steel is replaced by pressed metal aluminium which is strong and light at the same time and basically used to construct a perimeter frame for a better high speed(above 200km/h) performance. This is why MotoGP bikes have perimeter frames making it a crucial part for the riders to achieve high speed and corner without slowing down much. Mostly the bikes having such a frame are costly, e.g. Bajaj Dominar 400, (my fav) BMW S1000 RR etc.         

Trellis frame – A trellis frame is the younger member of perimeter frame. The motive behind building one remains the same which is to connect headtube to swingarm pivot with the minimum distance in between them. Instead of two rectangular section main beams, here there are two sets of primary tubes running from headtube to swingarm pivot. Further, the space in between one pair of tube is occupied by short aluminium set of tubes welded in criss-cross or triangular manner forming a trellis. This provides reinforcement to this pair of tube. The adjacent pair of tube has a similar structure. Due to the presence of trellis, extra weight of solid structured main beams of a perimeter frame is much reduced and gives a better core strength or rigidity or stiffness to the bike. Cornering tries to induce lateral flex to the frame otherwise but in a trellis frame, flexibility is properly dealt. Manufacturing a trellis frame doesn’t require costly and massive machines. It could be hand-built and is simple in construction. Also, it allows easy access to the engine and transmission, e.g. Ducati Monster 1200, MV Agusta Brutale Dragster 800RR etc.

Stamped/pressed frame– Bikes having stamped frame have freaky design and styling. Thin and light sheets of steel are hard pressed by a RAM which is called as stamping. These sheets are stamped into dies and then welded together to form sections. This forms a continuous series of sections starting from the headstock to the rear fender forming the spine. There is no support from the beneath and engine along with transmission is firmly bolted from the top. Often bikes with pressed frames have stamped steel front forks in place of regular tubular forks. Stamped frame offers enough rigidity to the bike but due to the absence of downtube and access weight of spine, it doesn’t handle corners well and even shakes when brakes are applied hard. Some bikes of 60s and 70s era had this frame installed in them, e.g. Ariel Arrow, Honda CB92 Benly etc.

Monocoque frame – Monocoque is a French word that means “single shell”. In this type, frame along with body parts take up the external forces. Engine and transmission become the stress member and absorb the forces from reaching the rider. Engine, transmission and frame together form a single or mono unit to reduce maximum flex. Main beams are chopped and made smaller in length to be called as semi-beams. This reduces the weight of frame. A typical perimeter frame is used to build a monocoque frame. The two semi-beams are fixed to the headstock. Triple clamp has handle tightly bolted on it. Headstock has triple clamp fixed on one side and front fork on the other side which completes from steering system of a motorcycle. Semi-beams have mounts on the other side for engine assembly. Engine has several mounts located on front and rear heads, transmission case and oil sump. Front head and transmission case mounts are bolted to the semi-beam while oil sump mount is pivoted to swingarm. The rear engine head mounts are bolted to the rear split perimeter frame portion. This type of frame is built together with the engine making them an integral part of each other. In a monocoque frame, engine is not surrounded by the frame but perfectly fits the engine due to the size which is equal to the space in between two semi-beams. Generally, a 4-cylinder engine is selected to fit in the size. In this way the engine as well as the semi-perimeter frame both become a single stress member unit and take up loads equally and reduce flexes. Also, it allows the engine to be placed in mid-section of the bike at a lower height improving the centre of gravity resulting in better rigidity and handling while braking and high speed cornering. Monocoque frames are build using expensive materials like magnesium and carbon fibre along with method of production, making it an overall expensive frame. Fabrication and production focus mainly on precision. These reasons make it a rare frame in a bike even after impressive performance output, e.g. Kawasaki ZX-14R, Ducati Panigale V4 etc.

          Spark out of the box!!!

The famous U.S. based company Harley Davidson founded a sub brand Buell. It produced some sensational bikes but out of them, there was one which was really astonishing. That is the Buell XB9R Firebolt because it has a perimeter frame and a fuel tank. Well, many bikes have already!! Actually it is the X factor of the bike which uses the perimeter frame as a fuel tank. Yes, it stores the fuel in the main beams of the perimeter frame and lubricating oil in the rear swingarm in order to make the bike agile and nimble.

This information would definitely prove to be handy in choosing the type of frame you wish to install in your custom bike. Do not forget to scroll down and have a look at one of the most complex shaped bikes on this planet. Use the above details and your understanding and let me know in the comment box about the type of frame used in the bike. Thank you!!!