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Common Tread

Why things are the way they are: Motorcycles and automatic transmissions

Jan 22, 2020

You don’t have to shift gears if you don’t want to.

The manual transmission is nearly dead and gone in cars in America. It would kind of stand to reason, then, that there should be some self-shiftin’ motorcycles available for sale here, right? In reality, there aren't that many and it's a topic that stirs up a lot of opinions. Just look at the fuss (500-plus comments!) Andy created when he wondered if motorcycles with automatic transmissions would bring in new riders.

So let’s take a closer look at motorcycles that don’t require the rider to handle the gear changes.

Is there an automatic motorcycle?

That’s a good question. Traditionally, in the automotive world, that term has described a hydraulic planetary automatic transmission, a very specific type of tranny that hasn’t been adapted to a motorcycle on a major scale, likely for a few reasons, most involving size and (lack of) return on investment of adaptation costs.

This distinction is important, because the self-shifters in the two-wheel world operate differently in a mechanical sense. Most successful designs are of fairly recent vintage, and even though many are described as “automatic” based on their method of operation, their function may or may not be transparent to the user. (I mean that the bike’s drive ratios change with little or no rider input — or they may not. It depends on the system!) Here’s a quick primer on current self-shifting motorcycles.

DCT
One clutch, two clutch, red clutch, blue clutch. Honda photo.

What's this DCT transmission I hear about?

DCT stands for “dual clutch transmission.” A DCT as used in most modern self-shifting motorcycles. It is actually really similar to a conventional manual transmission.

A DCT is basically two manual transmissions with two separate clutches housed in one box, but controlled by one shift drum. The transmission’s gears are split onto two shafts — odd-numbered gears are on one, and evens are on the other. The shafts usually run concentrically, to save space, as do the clutches.

The alternating-gears-on-the-shafts point is a pretty important concept, because it removes the drawback of the conventional manual, which is its constantly meshed gears. In a traditional transmission, only one gear ratio at a time can be selected. Because a DCT trans is basically two separate transmissions, the next gear can be “preselected,” allowing for lightning-fast gear changes.

DCT
Shafts and gears and such... where do they keep the wheelies? Honda image.

These are usually implemented in such a way that different gears are totally selected by the motorcycle, or optionally shifted by the rider — there’s usually a choice available. In both cases, though, clutch actuation is handled by the bike.

Here’s a very old Engineering Explained video delving into the mechanicals behind this style of transmission. Note that without a great understanding of how a conventional motorcycle transmission works, this video may not do a very satisfying job of relaying the points of difference.

Why does the transmission have to be so similar to a manual?

Short answer: It doesn’t.

It’s important to realize that in any modern auto/semiautomatic transmission, there’s computer power working in your corner to determine exactly which gear to select next. (If you’re in fourth gear, for example, is the next gear fifth or third? Various sensors monitoring throttle input, engine speed, and a few other parameters can help give the computer clues as to what gear is likely the next to be needed.) Beyond that, there may also be electronic solenoids selecting gears and engaging and disengaging clutch(es).

It’s these other bits and pieces that are handling the shifting. The DCT is just one platform upon which automatic shifting can be implemented. The computer and solenoids are just as important (if not more so) than the dual-clutch arrangement in the quest to get a motorcycle to change its own gears. A self-shifting motorcycle needn’t be a DCT — it just so happens that the compact layout has made it a viable option for motorcycle duty.

Yamaha FJR1300AS
Look. No clutch at all. The Yamaha FJR1300AS doesn't need one. Yamaha photo.

The Yamaha FJR1300AE/AS is a good example to bring up here. The clutch is electronically actuated and gear changes can be made with either a “regular” foot lever or by way of buttons. This isn’t a pure self-shifter, as the rider still changes gears, but it’s very close to a pure non-DCT semiautomatic (or “automated manual,” if you prefer) transmission. (And the gearshift lever is not mechanically connected to anything. It’s an electronic switch to actuate solenoids, which make the shift for you!)

The DCT is not inherently more suited to self-shifting than a transmission with a single clutch. In fact, this is the very reason Honda’s DCT has a few different modes — Drive, Sport, and Manual come to mind. The firmness of the shift and the shift points themselves differ, as well as whether the shift is actuated by the rider or the motorcycle!

The DCT is well suited to motorcycles precisely due to this flexibility. It is able to handle multiple gear ratios and changes, and those fast gear changes help retain the performance most motorcyclists have become accustomed to, without so much effort on the part of the rider.

The reason that “DCT” and “automatic” are used interchangeably, I think, is because Honda’s DCT is the first widely available transmission I am aware of that handles shifting without any more input than a standard epicyclic automobile transmission.

What about a CVT?

You'll find them on lots of scooters and in some snowmobiles, too. A CVT, or continuously variable transmission, is a bit different than a conventional automatic or manual transmission in that there are no discernible “steps” between gears. Rather, the input and output shafts have cone-shaped pulleys that allow fluid changes in gearing ratio — it’s continuously variable! It may help to think about a CVT’s gearing as a ramp, and a conventional manual transmission or DCT as a set of stairs. (When you hear one of these in action, it sort of sounds like the bike is accelerating through one long, continuous gear. That’s because it is!) Here’s a video that shows how the Reeves drive, the most commonly used setup in scooters, operates. (Please note I do not in any way endorse the narrator’s pronunciation of “centrifugal.”)

Due to the way the sheaves in a simple Reeves CVT operate, a drive belt works well to transfer power from the input to output shaft. This is appropriate for the low torque these machines develop. Generally, though, a belt doesn’t work well for high-torque applications due to the lateral strength the belt must have in addition to its tensile strength. Notably, the Aprilia Mana, an 850 cc motorcycle making 76 horsepower, bucked that trend and made fairly good use of this style of CVT.

That said, more powerful machines often make use of a CVT that operates in a very different way. Motorcycles like Honda’s DN-01 have been equipped with hydrostatic transmissions, a type of CVT which is a bit more robust than the Reeves-type shown above. This type of transmission works well, but can be costly to manufacture and thus to purchase. You may be familiar with this type of transmission if you have a lawn mower, as they’re used there often, but don’t let that application fool you — these transmissions can be constructed to transfer lots of power. Fluid in these units must be kept very clean in order for the transmission to function well hydraulically. That's an important consideration in motorcycles, as the engine oil is often shared with the transmission.

Here’s a good video on how hydrostatic CVTs work. This transmission is from an ATV, but the principle for motorcycle use is exactly the same.

What was the “Hondamatic” that Honda used to make?

Honda indeed made a transmission called the Hondamatic. (Actually, they made a few, because their automatic transmissions in the car went by the same name. Remember the hydrostatic CVT from the video earlier? That, too, shared the same moniker. Confusing!) The 1970s-era motorcycles fitted with Hondamatic transmissions weren’t quite automatics. They were actually two-speed clutchless manuals. The motorcycles still had a foot shifter, but there was just one gear change to manage, and manual actuation of the clutch was removed.

On a bit of an editorial note, I’d say that Honda has done more to contribute to self-shifters than anyone else. Part of that is likely due to their size as the largest manufacturer, part is likely due to their automotive branch sharing R&D with the motorcycle division, and part is also just a dedication to make motorcycling accessible for people from all walks of life.

What about no transmission and no shifting?

Gear shifting can indeed be automated, but for some motorcycles, eliminating it is on the table, as well. At the time of this writing, what are arguably the most and least technical motorcycles share a common bond — they lack transmissions. At the simple end of the spectrum, we have the humble minibike. The simplest minibikes have a simple centrifugal clutch and one gear, which is why they’re a bit limited in their climbing ability and top speed. (Some are also fitted with Reeves-type CVTs, too, and others have a primary and secondary drive to achieve better gearing relative to a chosen tire diameter.) It’s basic, but these machines work surprisingly well relative to their cost.

Mini
No shifting necessary. Photo by Lemmy.

At the cutting edge of technological breakthroughs, we find the electric motorcycle. Many of those don’t have a transmission due to electric motors’ ability to supply nearly all of the torque they make across a much wider envelope of operating speeds than an internal combustion powerplant.

For a motorcycle that's powered by an internal combustion engine and is expected to operate efficiently at a wide range of speeds — think everything from rolling through the parking lot to keeping up with freeway traffic — a transmission with multiple gears is a necessity, however.

Will automatics in motorcycles become popular like in cars?

Automatics obviously rule the U.S. automotive market, but globally more cars are still sold with a manual transmission. For now.

Motorcycles are different and people ride them for different reasons than they drive a car, especially in the United States. Manual transmissions work well and most people are happy with them, so there's not a lot of pressure on manufacturers to come up with something new.

The tide could turn, however. Traditionally, manual transmissions offered more gear ratios, at least potentially better efficiency and faster shift times. That's now changed. Seven-, eight-, nine-, and 10-speed transmissions are now somewhat common in automobiles. It would take quite a bit of driver skill and effort to make good use of all those ratios manually and there wouldn't even be a performance advantage. Exotic supercars made by Porsche and McLaren now have automatic transmissions. Ferrari doesn’t even make a car with a manual trans.

So improved performance could tip the balance toward getting riders to want to let the motorcycle handle the shifting. Automobile drag racers learned that auto trannies were the secret to consistency a long time ago. (That raises the question about MotoGP. DCTs are not permitted in GP. Instead, you’ll find the seamless transmission, which effectively uses a locking pawl system to achieve the same gear preselection that a DCT accomplishes. The seamless is hopelessly complicated to build and service, it seems, but clearly the race results make ‘em worthwhile!)

Is shifting gears a feature or a bug? Well, any beauty that may exist in shifting is, if anywhere, in the eye of the beholder. If selecting your own gears is not to your liking, though, you’ve got more options to avoid it now than ever before in motorcycling history.