Ask the Geek: How Seamless Transmissions Work

Motorcycle Maintenance

Diagram of seamless transmission

Diagram Courtesy of US Patent Office

This diagram, seen in several patents related to Honda’s seamless transmission, shows the system of ratcheting pawls (red arrows) that lock each gear to the output shaft to transmit power. Pins inside the shaft extend outward to engage each gear, but when the pins retract and the next gear is selected, the pawls remain locked in place until the next gear is under load.

How do the seamless transmissions in MotoGP work, and are they that much better than a conventional transmission with all the latest electronics (including up and down quickshifters)? How long will it be before we get seamless transmissions on streetbikes?
Douglas Gibson
Tutwiler, MS

The only solid information we have to describe the inner workings of the seamless transmissions used in MotoGP comes from a series of patents related to a “multistage transmission,” several of which are attributed to Honda. All the manufacturers use seamless transmissions, and they may or may not be similar to the Honda setup—either because of corporate pride or to get around the patents.

In a conventional transmission, only one gearset can be engaged at a given time. This occurs when one of the gears slides along its shaft and dogs interlock with an adjacent gear. Because only one gear can be engaged at a time, and the gears must slide along the shafts, there is a finite time for each shift, and power must be cut during that time. The power cuts not only hurt acceleration on the straights, but they can also unsettle the chassis if a shift is made midcorner.

In the Honda layout, the input shaft and gears are essentially one unit, with the gears splined to the shaft and spinning at the same speed at all times. On the output shaft, all the gears spin freely, but inside each gear is a set of four pawls that can lock the gear to the shaft to transmit power. The pawls are pushed into place by pins that are extended outward from the center of the shaft by a set of rods inside; using a ratchet effect, two of the pawls lock into place under power, while two lock into place under deceleration.

When a shift is made to the next gear, the pins retract, but as long as power is still applied the acceleration pawls of the original gear remain locked in place and that gear will transmit power. Once the next gear is engaged and takes up the load, the shaft starts to spin faster than the original gear and the pawls disengage. Everything is reversed if the motorcycle is decelerating and the rider downshifts, with the deceleration pawls held into place until the lower gear is engaged.

Because there is no interruption in power, the shift is “seamless.” This saves a tiny bit of time on each straight, but according to riders in the series the main importance of the gearbox is the improvement in stability during gear changes midcorner.

It is very unlikely you will see this type of seamless transmission anytime soon on your streetbike. One reason is that, given the construction described above, neutral is difficult to build in. If, for example, you select neutral by disengaging all the gears, there is a chance that the pawls for a particular gear may still be locked into position, and even with the clutch pulled in at a stop there may be enough drag in the transmission to prevent them from disengaging. All MotoGP bikes equipped with a seamless transmission do have an extra lever on the handlebar, reportedly related to the use of neutral.

Another issue is cost. A conventional six-speed transmission has about 80 individual parts, including the gears, shafts, shift drum, forks, shims, and circlips. A seamless transmission as described in the Honda patents looks to have about three times that number, with 16 individual parts in each gear alone related to the pawl mechanism. Some of those parts are very small and under very high load; reportedly the Honda MotoGP transmissions require service every day at the track by a dedicated HRC technician.

For street use, the performance benefits of a seamless transmission would be very slim compared to a quickshifter-equipped conventional transmission, especially when the costs are taken into account.


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