When we speak of two-wheeler technologies these days, more often than not, discussions are focused around engine development, electronics and suspension. Chassis – that bunch of steel tubes and/or aluminium plates – tends to get ignored. These days, when new bikes are launched, OEMs’ press releases are often filled with details of how engineers extracted every last bit of horsepower from the engine, details of every conceivable electronic rider aid and details of adjustable/active suspension. The chassis? Not so much. And yet, a bike’s chassis, even though it remains hidden under bodywork and lets the engine be the centrepiece in terms of style, remains one of the most important components of a two-wheeler. Chassis design and construction defines the way in which a bike rides and handles, and over the last many decades, OEMs have continued to experiment with various types of construction, all of which have their own positives. Here, we take a look at some of the more recent developments in two-wheeler chassis design.
Chassis design for powered two-wheelers (motorcycles and scooters) evolved from bicycle chassis about a century ago. Early efforts were pretty basic, with engines being bolted on to beefed-up bicycles frames. Of course, as motorcycle engines became bigger and more powerful, manufacturers felt the need to develop proper chassis that could keep pace with power output. From there, development branched off in various directions, with OEMs experimenting with multiple permutations and combinations of materials, arrangement and construction. The basic purpose remained the same – the chassis would act as the base for mounting a bike’s engine, fuel tank, handlebars and suspension (including the swingarm) – but the ways in which this could get done invited experimentation, which continues to this day.
Over the years, types of two-wheeler chassis used by OEMs includes the steel single or twin downtube cradle-type frame, steel tube perimeter chassis, aluminium twin-spar frame, steel tube trellis frame, hybrid (steel tube trellis section combined with aluminium side plates) frame and aluminium/carbonfibre monocoque chassis. Surprisingly enough, almost every single type of frame mentioned in the list above is still in use today, on modern motorcycles, depending on price, positioning and intended usage. While more basic, commuter-spec two-wheelers are often fitted with single or double downtube cradle-type frames, more expensive, higher-end bikes usually get steel tube trellis or aluminium twin-spar chassis. Some manufacturers are also using carbon/aluminium monocoques, which do away with steel tubes and/or aluminium spars, thereby reducing weight and complexity.
CHALLENGES IN DESIGN
A motorcycle’s chassis must provide the ideal balance between stiffness and controlled flex, in order to provide the optimum ride and handling. The chassis must facilitate the placement of the engine in a way that allows it to adhere to the basic principles of mass centralisation (where most of the two-wheeler’s mass is placed as centrally as possible between the front and the rear wheels, and is also placed as low as possible, without compromising ground clearance), and must facilitate the mounting of suspension components in a way that allows those components to work in an efficient manner.
Engineers who design chassis systems for four-wheelers have a relatively simpler task – largely, they only have to ensure rigidity and stiffness, letting suspension deal with all the movement. On the other hand, motorcycle chassis designers also have to account for ‘controlled flex,’ which allows the frame to work in harmony with the suspension. A very stiff chassis, with zero flex, can potentially lead to loss of grip at either the front or the rear end, with the motorcycle simply bouncing off road irregularities. Sometimes, especially when cornering, two-wheeler chassis must also take on some of the functionality of the suspension, flexing in a controlled manner to ensure that the wheels do not lose contact with the tarmac.
It is also true that better, higher-quality suspension components, with minimal unwanted suspension movement, can be used with stiffer, more rigid chassis. But due to reasons of cost and complexity, suspension components used on most mainstream two-wheelers are far from being perfect, which is why it’s essential for two-wheeler chassis engineers to build controlled flex into the system. Different kinds of chassis deal with this issue in different ways. We’ll come back to this point later, as we take a closer look at some chassis types, and see what pros and cons they have to offer.
STEEL TUBE SYSTEMS
An established school of thought in motorcycle chassis design is that nothing withstands flex better than a stiff, heavy tube of steel. This thought was the underlying principle behind the single / twin steel downtube chassis used on motorcycles a century ago, and remains true to this day for bikes where simplicity and low costs are the most important factors. The setup is simple – the chassis is made of steel tubes – one main tube of larger diameter (above the engine in the ‘backbone’ type of chassis, and in an upright position, parallel to the engine, in the single/twin downtube cradle-type frame), and with other smaller-diameter tubes that provide mounting points for the swingarm and suspension.
On slightly higher-end machines, the backbone/downtube kind of steel tube chassis was replaced with the ‘perimeter’ type of frame, where a steel tube structure completely envelops the engine, with the aim being to connect the steering head to the swingarm pivot in a straight line, in as short a distance as possible. In the 1960s-1970s, as Japanese two-wheeler OEMs took the lead with engine development, power output from bigger engines often outstripped tyre and chassis capabilities, resulting in bikes that were difficult to control under hard acceleration and/or at higher speeds. At that time, companies like Bimota (a small-scale, very high-end two-wheeler OEM based in Italy) started the trend of using Japanese motorcycle engines, alongside carefully crafted high-spec chassis that was engineered to handle very high levels of performance. Chassis specialists in the UK, companies like Harris Performance and Spondon, also started creating bespoke chassis for production motorcycles, bettering OEMs’ efforts by a large margin.
While specialist companies did take the lead with motorcycle chassis development in the 1970s and eary-1980s, OEMs did figure things out eventually and by the late-1980s, Japanese are European motorcycle manufacturers were building highly capable two-wheeler chassis on their own, finally catching up with their engine development departments. This also led to the development and use of aluminium twin-spar chassis, which is currently used by most Japanese and European OEMs for all their high-end machines.
ALUMINIUM TWIN-SPAR CHASSIS
From the mid- to late-1980s onwards, two-wheeler manufacturers like Bimota, followed by the Japanese big four (Honda, Suzuki, Yamaha and Kawasaki), moved away from steel tube perimeter frames and started working with twin-spar aluminium beam frames, especially for high-end, performance-oriented motorcycles. Aluminium, which is lighter than steel, was formed into large cross-section ‘beams’ and the same were used to build stiff, lightweight chassis that brought major advances to the way bikes handled. Along with this, swingarm design also changed – OEMs moved on from oval- or box-section steel tube swingarms, to much larger cross-section aluminium swingarms that provided improved rigidity and flex-resistance at high speeds. In the late-1980s and early-1990s, manufacturers like MV Agusta, Ducati and Honda also started using single-sided aluminium swingarms with great success, and continue to do so today.
While the basic underlying principles for designing light and stiff aluminium frames for bikes have not changed much over the last three decades, OEMs still continue to innovate, especially now that chassis systems work alongside electronically-controlled suspension. The newest aluminium beam frames, especially on high-end bikes, usually feature gravity cast components (for lightness and strength) that are welded together to form a single, compact unit. The chassis also uses the engine as a partially or fully stressed member, mounted rigidly to the frame at the cylinder head and at the crankcase. These types of chassis also feature aluminium swingarms (usually upward truss type) made of gravity cast and forged components, providing maximum strength at the lowest possible weight. Exotic materials like magnesium and/or carbonfibre are sometimes used for the subframe, which provides mounting points for the seat unit, rear footpegs and, in some cases, even the exhaust.
STEEL TUBE TRELLIS FRAME
While most Japanese and European OEMs have been using aluminium twin-spar beam frames for their high-end two-wheelers for the last two decades, some – most notably Ducati and MV Agusta – have stuck with the steel tube trellis frame. With this, instead of using lightweight aluminium, OEMs use triangulated steel tubes for creating stiff, compact chassis units that are sometimes relatively less expensive to build as compared to aluminium beam frames. Steel tubes are inherently highly resistant to twisting forces, hence motorcycle frames made of triangulated (for maximum strength) steel tubes work very well even in the most demanding of applications. Furthermore, the tooling costs required to make an all-new steel tube trellis frame are lower than those for an aluminium beam chassis, and changes to the chassis, as and when required, are relatively easier to make.
Ducati still uses steel tube trellis frames for all its bikes except the highest-end Panigale machines, while MV Agusta uses ‘hybrid’ chassis, which utilise a mix of steel tube trellis sections along with aluminium side plates. In either case, the ride and handling characteristics provided by the chassis are of the highest standards, matching the standards set by aluminium beam frames. These days, OEMs use large-diameter, thin-walled steel tubing for reduced weight, without compromising rigidity or resistance to flex. It’s also relatively common for OEMs to use die-cast aluminium swingarms (twin-sided or single-sided units), which work just as well with steel tube trellis frames as they do with aluminium beam frames.
While some Italian OEMs have traditionally preferred this kind of chassis setup, even some Japanese OEMs have moved from using aluminium beam frames to steel tube trellis frames. Kawasaki is one example, with the company having engineered an all-new steel tube trellis frame for its top-of-the-line H2 sportsbike, following it up with another trellis frame for the new Z900. The reasons for this move include lower tooling costs, more flexibility (in terms of the ability to make modifications when required) and high structural/torsional strength offered by steel tube trellis frames. Another important reason, also, is that steel tube trellis frames bring the engine out in the open, which helps with heat dissipation – an important factory when designing chassis for very high performance bikes.
While the monocoque chassis is commonly used on all modern cars, its use in the world of two-wheelers has only been occasional at best. This kind of chassis doesn’t use any steel tubes or aluminium beams etc. Instead, an all-encompassing single-piece outer ‘shell,’ which can be made of aluminium or even carbonfibre, which takes the load of all components, often using the engine as a fully stressed member. Monocoque chassis are currently used on a handful of very high-end bikes, including the street-oriented Kawasaki ZX-14R and the track-focused Ducati Panigale 1299.
Monocoque chassis can be extremely rigid and not extremely expensive to produce, but at times, there can be issues with excessive rigidity. Unlike steel tubes or aluminium beams, it’s difficult to engineer ‘controlled flex’ characteristics into monocoque shells, which is why their use is more common in the world of cars rather than two-wheelers. That said, monocoque chassis can be quite compact, help reduce the sheer number of components on a bike and can offer certain benefits in terms of packaging, which might make this form of chassis attractive for electric bike manufacturers, who need space to pack in a lot of batteries. Should be interesting to see what kind of usage this type of chassis sees in the world of bikes in the years to come.
Unlike cars, most of which use a monocoque chassis, there is still a lot of diversity in the world of two-wheelers in terms of chassis types currently being used. As listed above, bikes can still have very basic steel single or twin downtube chassis, or a steel tube trellis frame, or an aluminium beam frame, or an aluminium/carbonfibre monocoque frame. Each has its merits and demerits, and intended usage of the machine dictates the kind of chassis it gets.
Large diameter, thin-walled steel tubes remain the most popular material for creating trellis frames, while large cross-section thin-walled aluminium beams are used for twin-spar frames that are usually lighter that steel tube frames. Both can be equally strong and rigid though, a fact that is also applicable to the far less common monocoque type chassis. The latter, by using the engine as a fully stressed member, can also help reduce the total number of parts on a bike, and might, in the future, be a useful option for electric bike manufacturers who need extra space to package batteries and electric motors. However, it’s expected that development of all chassis types will continue in the future, with application depending on intended use as well as cost and complexity concerns.
TEXT: Sameer Kumar