Where in areas like engines, braking systems and suspension technology, two-wheeler manufacturers have made great progress over the last 4-5 decades, transmission is one area where things have, apparently, not changed too much. For more than half a century, the basic format has remained the same – a hand-operated clutch and foot-operated five- or six-speed transmission. Of course, part of the reason why things haven't changed too much is, one, this format pretty much works very well for most riders, and two, unlike car drivers, two-wheeler riders haven't exactly been demanding automatic or semi-automatic transmission. Most riders are happy to trade ease of use for the sheer 'control' that a fully manual transmission provides. And yet, with the inevitable march of technology, changes have started creeping in gradually. Here, we take a look at some of the more recent developments in two-wheeler transmission technologies.
CONTINUOUSLY VARIABLE TRANSMISSION
Honda is the one company that stands out in having always taken the lead with trying to develop alternative two-wheeler transmission. Their efforts started in the late-1950s, with the C100 Super Cub, which was equipped with an automatic centrifugal clutch that allowed riders to start the engine and change gears without needing to use a clutch. This was followed by the Juno 125 in the early-1960s, which was fitted with a continuously variable hydraulic transmission (a forerunner of the present day CVT), and in the late-1970s, the Honda CB750A, which was fitted with a semi-automatic torque-converter type transmission called 'Hondamatic.' While the torque-converter type transmission never really took off for two-wheelers, in the 1980s, Honda also took the lead with developing continuously variable transmission (CVT) for two-wheelers, which did away with conventional gears altogether and went on to became a huge success worldwide. In fact, dozens of OEMs currently use some iteration of the CVT, especially for small-capacity scooters and general-purpose motorcycles.
With most two-wheeler CVT applications, the system consists of two pulleys (one for the engine, the other for the rear wheel) positioned perpendicular to their axes of rotation, with a V-belt running between them. There are no fixed gear ratios in this system, but during actual road use, 'gear ratios' are changed based on the engine load, by altering the effective diameter of both pulleys. The distance between the two pulleys and the length of the V-belt remains fixed, maintaining constant tension in the belt. However, as the effective diameter of the front and rear pulleys changes, effective gear ratio is altered, which leads to increased and decreased road speeds, while engine rpm remains constant.
With CVTs, even on small-capacity two-wheelers with relatively low power output, the V-belt itself must remain stiff and cannot be allowed to stretch in the pulleys' axial direction. Instead, the belt must only make small radial movements while sliding in and out of the two pulleys, thereby 'changing gear.' Depending on application, belts of different materials can be used in CVTs, including steel-reinforced belts that suffice for most two-wheeler applications.
While most OEMs use CVT only for scooters, Honda and Aprilia are two manufacturers that have also used this kind of transmission for relatively bigger bikes, with varying levels of success. The CVT is considered to be extremely user-friendly, but can have some disadvantages, including higher fuel consumption and an unnatural 'feel' during hard acceleration, where the engine keeps spinning at one specific rpm (the point of optimum power and torque delivery) while the vehicle picks up speed gradually. It must be noted that CVT is more suited to general purpose two-wheelers meant largely for city use. Due to the inherent limitations of the two-pulley system, CVTs do not work with powerful, high-performance machines.
SEMI & FULLY-AUTOMATIC TRANSMISSION
Even as the CVT became ubiquitous on smaller two-wheelers, Honda was already preparing for next steps in the ongoing evolution of two-wheeler transmission. Pioneered by European car manufacturers in the early-1980s, dual-clutch transmission (DCT) was being seen as the next big thing in cars, and there was no stopping Honda from bringing this innovation to the world of two-wheelers as well.
In 2009, Honda announced the world's first dual clutch transmission (DCT) for large-displacement two-wheelers. The Honda VFR1200F, a V4-engined sports-tourer, was the first machine to be equipped with a DCT, which could be operated in three modes – two fully automatic riding modes (D mode for regular riding, and S mode for high-speed 'sports' use), and a 6-speed manual mode, which allowed riders to change gears manually, via handlebar-mounted buttons, without needing to use a clutch.
This setup offered the best of both worlds – on long commutes, on straight stretches of highway and at more or less constant speeds, riders could just choose the fully automatic D or S modes, and completely forget about having to change gears. On the other hand, when riding across twisty mountain roads and/or other kinds of challenging terrain, riders could revert back to manual mode, thereby gaining more control over the engine and the transmission, but still without having to go through the hassle of using a clutch. The system was as foolproof as possible – it offered smooth upshifts and downshifts, you couldn't 'stall' the engine and it offered better overall efficiency as compared to regular manual transmission. Over the last few years, DCT has been fitted to various other Honda motorcycles of 700 cc and above, with varying levels of rider acceptance. Mechanically, Honda's DCT for two-wheelers is quite competent, but in some cases, riders have been reluctant to relinquish the control that comes with a fully manual transmission.
Coming to its internal workings, Honda's DCT has two independent clutches – one works gears 1st, 3rd and 5th, while the other is for gears 2nd, 4th and 6th. The result is very fast, smooth and seamless shifts. With an onboard computer controlling the gear shifts, based on parameters like road speed, engine rpm and engine loads, DCT is always 'primed' for action, pre-selecting gears in advance and taking full advantage of the engine's power and torque curves. With dual input shafts, an in-line clutch design and all hydraulic circuitry placed beneath the engine cover, Honda's DCT is also a very compact design that uses a simple shift mechanism based on a conventional shift drum.
In actual operation, as soon as the DCT's ECU detects the need for a gear change, the system prepares for the transfer of drive power between gears by starting up the rotation of the gear to be changed to. The DCT's two independently actuated clutches also play an important role here, ensuring the smooth transfer of rotation speed from one gear to the next, with virtually no let-up in the drive force being transmitted to the rear wheel. Hence, especially when changing gears under hard acceleration or braking, the dual-clutch setup ensures smooth engagement/disengagement of gears and keeps power delivery seamless. By minimising the number of unnecessary gear shifts and by optimising the shifting pattern either for efficiency (in D mode) or for outright performance (in S mode), the DCT helps a rider get the best out of his machine at all times. Not just that, DCT even offers a modicum of engine braking – while there is no manually operated clutch here, closing the throttle and/or downshifting via push-button operation provides engine braking similar to what's provided by conventional transmission, which helps when a rider is trying to slow down from very high speeds, and during high-speed cornering manoeuvres.
While CVT and DCT offer fully automatic operation, these (especially CVTs) are often not suitable for very powerful, high-performance bikes. DCT can still be optimised for all kinds of usage, but many riders simply prefer the higher degree of control provided by manual transmission. Hence, in recent years, OEMs have looked at ways of using advanced electronics to improve and enhance the traditional manual transmission. So we now have various electronic assist systems that make conventional two-wheeler transmission easier to use, while enhancing performance at the same time. One such example is Yamaha's YCC-S setup found on the FJR1300A touring bike. While the gear shifts still have to be performed in the conventional manner, the bike does not have an actual clutch lever at all. Instead, the clutch is operated electronically, with the ECU controlling the engine's ignition timing as well, for smooth, seamless shifts.
While YCC-S is a system that's been optimised for relatively relaxed usage (given the FJR's intended usage) and is not meant for more focused, high-performance machines, OEMs are not ignoring sportsbike riders either. Take, for example, Ducati's electronic quick-shifter (DQS) or Aprilia's quick-shifter (AQS), which are race-derived systems that allow full-power upshifts and downshifts, without the rider needing to manually operate the clutch. There's also BMW's Shift Assistant Pro system that works in a manner similar to that of Ducati and Aprilia machines, but is much more touring oriented. With such systems, the ECU senses pressure on the gear shift footpeg and depending on the action required, either reduces power for a fraction of a second (for upshifts) or automatically 'blips' the throttle (for downshifts) for smooth gear shifts.
An important part of such performance-focused electronic quick shifter systems is also the 'slipper clutch,' which is now found on most high-end sportsbikes. Also known as a 'back-torque limiter,' the slipper clutch uses an integrated freewheel mechanism that's designed to reduce or eliminate 'wheel hop' induced by engine braking, when a rider simultaneously brakes aggressively and downshifts from high speeds. The slipper clutch works by partially disengaging (or 'slipping') under hard braking and downshifting, thereby preventing the rear wheel from trying to 'drive' the engine faster than it normally would under its own power. With the pronounced engine braking effect with bigger, more powerful two-wheeler engines, slipper clutches help the transmission work more effectively, prevent wheel hop and thereby reduce the chances of loss in traction.
Unlike four-wheelers, where the conventional manual transmission is on a steady decline and may disappear entirely within the next few years, two-wheelers are likely to retain the traditional manual transmission in the foreseeable future. Yes, CVTs will continue to do duty on small capacity scooters and some bigger bikes will see the adoption of DCTs that can, depending on intended usage, provide ease of use, greater efficiency, improved fuel economy as well as optimum performance. However, DCTs are not necessarily the future for all two-wheelers. Riders demand 'control' over their bikes' power delivery and gear shift pattern, and that is still best provided by the conventional manual transmission.
However, slipper clutch systems and electronic quick-shifter systems will continue to evolve and are likely to eventually filter down to more mainstream machines over the next few years, providing greater ease of use without compromising on performance. Indeed, OEMs like Honda and Yamaha are already working on completely seamless electronically operated gearboxes that will allow full-power upshifts and downshifts, without the rider needing to use the clutch (the clutch will still be there, but will be operated electronically.) With current systems, power delivery is interrupted, even if only for a fraction of a second, but with next-generation systems, even that brief lapse will be eliminated. As always, the quest for improvement will never stop.
TEXT: Sameer Kumar