Sequential Transmissions and the Modern Automobile

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We would like to thank KAPS Transmission for their help providing photos for this article.

In discussions among automotive enthusiasts, terms like “sequential” and “dogbox” get thrown around to describe some pretty enticing pieces of machinery. However, a certain amount of confusion exists with these terms, and the misnomers revolving around the sequential transmission sometimes conflate ideas that don’t belong together. With all that misinformation circulating around, perhaps it’s time to shed some light on sequential transmissions, the forms they take, and the real-world benefits they offer.

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Sequential transmissions: misunderstood as they are prevalent. Photo credit: KAPS Transmissions

We live in an age in which visceral experience is less important than efficiency. This could not be better reflected in our choice of transmissions. Almost everything from a Ford Focus to a McLaren P1 puts its power to the ground with the help of an automated gearbox. In the case of the Ford, a dual-clutch box is there to reduce emissions: spark-cuts between gears improve economy and reduce carbon emissions. In the case of the Woking hypercar, a sequential is used to reduce lap times and, to an extent, improve safety. Both, however, rely on similar forms of technology to help achieve their own respective goals.

How the Sequential Functions

With both of the aforementioned cars employing paddles to select gears, its understandable that the term “paddle shifter” is implied to mean the object which actually does the shifting, but this is incorrect. In fact, the paddle shifter is just an ergonomic way to select gears, but has little to do with the gear change itself. Where they begin to differ is with the sequential transmissions they’re linked to – the component which does the actual brunt of the work.

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The tall, pump-action gear lever in this Ford Fiesta rally car allows for lightning-fast shifts, though the driver has to take his hand off the wheel to engage the next gear.

Like a basic manual transmission, a sequential features an input and output shaft. With the sequential gearbox, all the gears on the input shaft are considered fixed gears since they’re connected directly to the shaft. These gears all spin in unison. Similarly, the output shaft houses free gears, which spin as their own group but are mounted to the shaft on bearings which allow them to spin independently of the shaft speed. Thus far, the sequential operates much like a traditional manual transmission.

Grooves in the gear selector shaft help direct the gear selector pin into the proper location.

Grooves in the gear selector shaft help direct the gear selector pin into the proper location.

Where the sequential stands apart begins with the introduction of collars. The output shaft’s gears are punctuated by collars which can slide freely on a spline that is connected to the gear selector shaft. When the driver pulls a paddle or pumps a gear lever, a small ratchet arm turns a gear forwards or backwards, depending on whether they’re upshifting or downshifting.

Small grooves on the gear selector shaft then push the gear selector pin into the desired location on the selector shaft and, in turn, this moves the location of the gear selector fork. The gear selector fork then activates the collars which can couple or detach specific gears for the desired gear ratio.

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At the top of the assembly is the gear selector shaft and the gear selector forks which select the gears below.

This kind of transmission does not need to be shifted with paddles necessarily. Many sequentially-shifted cars use a gear lever which the driver pulls back to upshift or pushes forward to downshift. While this style of interface encompasses many racing cars, most motorcycles, and even a few street cars, the current trend is moving towards the paddle. With paddles on either side of the steering wheel, the driver can pull one to upshift and the other to downshift.

More often than not, the paddle shifter is part of a dual-clutch system — a form of sequential transmission which has gained plenty of steam recently. Ford and Volkswagen have been major proponents of this technology, which improves fuel economy and makes for a seamless shift and a smoother driving experience.

Direct-Shift Gearboxes and Dual-Clutch Transmissions

Nowadays, most cars under the Volkswagen umbrella utilize some form of DSG, or the direct-shift gearbox. Both the Volkswagen Golf and the Porsche 911 GT3 use this dual-clutch system which has proven to be as effective around town as it is on the racetrack. DSG boxes, known as PDK boxes within the Porsche fleet, are quite sophisticated and take a different approach to shifting but can be understood easily.

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Though complicated and heavy, the dual-clutch transmission seems to be the way forward for many auto giants.

Essentially, the dual-clutch utilizes two clutch packs which sit inside the clutch housing. This housing rotates with the flywheel. These clutch packs, which Volkswagen calls “K1” and “K2”, control the odd and even gears, respectively. It can lock one of the clutch packs and use one of two input shafts to link it to the transmission.

The input shaft is made up of two concentric shafts. The outer clutch is attached to and rotates with the inner shaft, whereas the inner clutch is attached to the outer shaft. Two output shafts carry the gears, with one output shaft carrying first, second, third and fourth gear, and the other output shaft carrying fifth, sixth and reverse. Basically, since gears have to be shifted sequentially, the clutch packs act to engage the next gear before it is actually shifted into. 

The architecture of the sequential gear stack allows for compact packaging and "preselected" gears.

The architecture of the sequential gear stack allows for compact packaging and “preselected” gears.

The dual-clutch system offers sublime smoothness during gear changes all while shifting very quickly, and while much of that performance is due to the aforementioned construction, much of it is due to the computer which predicts the next shift based on driver input. If the driver is accelerating and redline is approaching, the computer will recognize this and engage the next higher gear collar, in effect “preloading” the next gear.

Similarly, when heavy braking occurs and the computer anticipates a downshift, it will preselect the next lower collar. When the paddle is pulled and the shift occurs, the clutch engagements overlap, in effect, transferring drive at a progressive rate from one to the other and not interrupting the power delivery. 

The dual-clutch transmission offers smooth, seamless shifting and remarkably quick gear changes. Though the Volkswagen product has its detractors, its prevalence has led to its refinement as the years go on. With electromagnetic sensors on the shift selectors and its predictive computer, it may be one of the more sophisticated gearboxes out there, but its weight and complexity has made it less than ideal in certain racing situations, and for this reason sequentials with a single-file gearset are still the most popular in racing applications.

As Sean Ritchie from Weddle Industries says, “Simplicity and overall strength are better for what we’re after with our transmissions.” Weddle Industries distributes the Albin transmissions found in most Trophy Trucks, so it’s fair to say he knows what he’s talking about.

Dogboxes

Sequential transmissions are usually dogboxes, but not every dogbox is sequentially shifted – some are shifted like an h-pattern. Essentially, dogboxes lack synchromesh gears: those little helpers which make a transmission quiet, smooth-shifting and docile. Synchros work through a cone-shaped clutch attached to a gear, which, when pressure is applied, allows for synchronization of speeds between the input shaft and the gearset shaft speed before the two are locked. With this relatively complex arrangement, shift time increases, gear sizing is limited and the clutch pedal needs to depressed for every shift.

Without synchros, the dogbox has more room for gears with stronger, larger teeth known as “dogs.” These heavy-duty teeth can handle plenty of power and take a thrashing. However, the lack of synchros can make engagement a rough affair if done incorrectly. Simply put, the best way to shift is as quickly as possible.

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This Hewland dogbox can be easily recognized by its large, uncomplicated gears. These gears happen to be straight-cut, a feature which produces a distinctive whining noise.

This shift speed is feasible for several reasons. Most dog boxes use the clutch for starting and stopping, but shifts rarely require the clutch to be engaged fully. The dogbox will shift smoothly with a quick lift of the throttle or, for the mechanically sympathetic, a brush of the clutch. Dogboxes also allow for preloading, where the driver applies a small amount of force to the gear lever in the direction of the next desired gear.

This minute amount of pressure will force the lever into the next gear as soon as the throttle is lifted or the clutch feathered, and shift times will be reduced even further. Without the engagement of the synchros slowing down shift times, lighting-quick shifts can be had, but a bit of timing and technique is needed to pull it off well.

Critically, dogboxes lack the smoothness and drivability most motorists want from a daily-driven street car. The constant vigilance required is a little too thought-consuming to be comfortably driven by most. Though there are a few purists who maintain dogboxes are perfectly streetable, it’s fair to say their advantages are best experienced on the racing circuit.

Racing Advantages

With most sequentials, mis-shifts are less likely though a certain amount of coordination required. If that much-needed lift and the pulling of the gear lever isn’t timed correctly, the shift can be rough, jarring and cause the car to buck. Certain sequentials, however are capable of cutting spark between upshifts – thus negating any need for timing. All the driver needs to do is mat his foot and pull a lever or flick a paddle. With nose-to-tail driving, a mis-shift can cost a position, or worse, cause an accident.

When the shifts are executed properly, however, the speed gained on certain sections of track pays huge dividends. When flying around a bumpy off-road course, sometimes having a sequential shifter way to ensure shifts. As Sean Ritchie remarks, “For the off-road racing side, it’s harder to find a gear with an h-pattern while bouncing around.” Merely pulling back on a shifter when being jostled around is the most effective way of keeping acceleration constant.

Standing by the side of a racetrack and observing, it’s easy to recognize the uninterrupted thrust of a car equipped with a sequential transmission. Compared to a car which lulls as the clutch is depressed, the sequential-equipped car accelerates as if it has twice the horsepower. Generally speaking, the more power a car has, the more valuable a quick-shifting transmission becomes, since the shorter shifts take, the more time is spent putting all that power to the pavement. 

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While flying around on an off-road rally course, having a sequential gearbox makes finding that next gear much easier.

While quick shifts and improved acceleration are easy to imagine, there are some benefits which might surprise the uninitiated. For one, not having to use the clutch to downshift allows for more left-foot braking. This allows the drive to use both feet to shift the weight and put the power down quickly. In a low grip situation where the car’s attitude has to be constantly managed, this is an major advantage. It also allows for some brake-boosting, where the right foot prods the throttle while braking to keep the turbo spooled. Two-footing also saves time: switching one foot from pedal to pedal takes a few tenths of a second.

Because the clutch is typically used only for stopping and starting, the driver is given the option of using his left foot to brake mid-corner.

In the case of paddle-shifted gearboxes, the driving experience is much safer since the driver can keep their hands on the wheel at all times and shifts are near-immediate. At speed, having to depress the clutch and change gear conventionally can lead to oversteer due to the rear end becoming unloaded for a prolonged period of time.

With a modern paddle-shifted gearbox, there is virtually no weight transfer off of the rear wheels while shifting, so the balance remains constant and some of those high-speed slides can be avoided. Countersteering at 100 mph with one hand takes incredibly quick reflexes, some serious strength, and isn’t advised for the novice driver. For these reasons, some assume that paddle shifters have taken the skill out of racing. 

Many armchair experts love to assert that modern racing is child’s play without heel-and-toe or gear skipping. Nonsense. Though the challenge of getting the perfect downchange has gone out the window, most drivers could do that in their sleep. In fact, many drivers see shifting as a obstacle in way of the real challenges of driving, which are making apexes, metering out the power, and braking late.

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Despite what the naysayers carp about, shifting with a paddle does not take much of the challenge away from driving.

After delving into the finer points of modern transmissions, it seems that what a seasoned racing driver and the average motorist want from a gearbox is quite similar: smooth, quick shifting and as little annoyance as possible. In the case of  so many economy cars these days, a dual-clutch setup works to improve economy: keeping it in the right rev-range and turning every drop of gas into forward momentum. Though, it has to be said that the addition of these quick-shifting gearboxes makes the Ford Focus seem so much faster than its horsepower might suggest. 

While cars get faster and faster, it’s obvious that the need for safety and drivability is just as important, and these needs are always reflected in the transmission’s design. With transmissions getting more and more sophisticated as time rolls on, the continuously-variable transmission might become commonplace in a few decades.

Though that may lead the world forward in terms of economy, the purist in all of us will proclaim that there’s something thrilling about rifling through the gears, hearing the engine wail on a downshift and the feeling that, somehow, it is all under your control. Let’s hope that whatever solution the future holds, it strikes a happy medium. 

Article Sources

About the author

Tommy Parry

Tommy Parry has been racing and writing about racing cars for the past seven years. As an automotive enthusiast from a young age, he worked jobs revolving around cars throughout high school, and tried his hand on the race track on his 20th birthday. After winning his first outdoor kart race, Tommy began working as an apprentice mechanic to amateur racers in the Bay Area to sharpen his mechanical understanding. He has worked as a track day instructor and automotive writer since 2012, and continues to race karts, formula cars, sedans, and rally cars in the San Francisco region.
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