How Sequential Gearboxes Work

If you have read the HowStuffWorks article How Manual Transmissions Work, then you understand the inner workings of a typical manual transmission, and you know why manual transmissions use the standard "H" pattern in the shifter.

If you have ever ridden a motorcycle, you know that the manual transmission in a motorcycle is nothing like this. On a motorcycle, you shift gears by clicking a lever up or down with your toe. It is a much faster way to shift. This type of transmission is called a sequential gearbox or a sequential manual transmission.

Photo courtesy Derek Smith, Gruppe/5 Automotive 5-speed synchro box, ZF transaxle model 5DS-25-2 This gearbox has a custom-made sequential shifting mechanism that is pneumatically operated and computer controlled.

It turns out that most race cars use sequential gearboxes as well. A sequential gearbox gives the driver several important advantages that are very useful in a race car. We will discuss these advantages later in this article.

In this article, you will learn how a sequential manual transmission works and why this type of transmission is now appearing on so many high-performance vehicles.

In the Gearbox
How Manual Transmissions Work offers a basic understanding of the mechanisms inside a manual transmission. The five-speed manual transmission is fairly standard on cars today. It looks something like this internally:

There are three forks controlled by three rods that are engaged by the shift lever. Looking at the shift rods from the top, they look like this in neutral, reverse, first and second gear:

The "H" pattern allows you to move the shift rod between the control rods for the three forks and move the rods back and forth.

A sequential manual transmission works the same way. There is still a set of gear selector forks that move collars that engage gears. The only difference is the way the control rods are manipulated. The "H" pattern is eliminated and replaced with a different motion.

In a race car, the motion of the shift lever is either "push forward" to up-shift or "pull backward" to downshift. If you are in a gear and you want to go to a higher gear (e.g. from 2nd to 3rd), you push the shift lever forward. To go from 3rd to 4th, you push the lever forward again. To go from 4th to 5th, you press it forward again. It is the same motion every time. To drop back down a gear, say from 5th to 4th, you pull the lever backward. (You can see several videos showing the shift lever motion on the SEQSHIFT video page). On a motorcycle, you do the same thing, but instead of moving a lever back and forth with your hand, you move a lever up and down with your foot.

What these motions are doing is rotating a ratcheting drum. The drum looks like this:

Image courtesy Quaife Engineering Ltd.. Copyright Quaife.

You can see that there are grooves cut into the drum. These grooves can do one of two things:

• If the drum is located away from the transmission's gears, the grooves control standard control rods.
• If the drum is located next to the gears, the grooves directly move the gear selector fork, and no control rods are needed. This seems to be the more common technique because it has fewer parts and is more compact.

So, when you move the lever, it rotates the drum one increment (for example, 50 degrees). This rotation causes the rods or forks to move according to the grooves in the drum, changing the gears.

Photo courtesy Mike Challenger, Haydndesign Ltd Animation of six-speed sequential transmission

Because of the drum, you have to shift in sequence. There is no skipping, for example, from first gear to third. You must always go through second gear to get to third gear. It is the same when downshifting. The advantage of this system is that shifting mistakes are impossible. You always go to the next gear.

Advantages
Nearly every race car that has a manual transmission uses the sequential approach rather than the "H" pattern. There are four main reasons for this preference:

1. The sequential shift is quicker. For example, to go from 2nd to 3rd gear on the "H" pattern, you have to push the lever up, over and up again. That takes time. On a sequential gearbox, you simply push the lever up for every gear change.

2. The sequential shift is consistent. You do not have to think, "Let's see, I'm in second gear so I have to go up-over-up to get to third." You simply push the lever forward -- it's the same motion for every gear.

3. The hand location is consistent. With the "H" pattern, the location of the shift lever changes, so you have to think about where to put your hand depending on which gear you are in. With a sequential gearbox, the shift lever is always in the same place for the next shift.

4. The sequential shift has no surprises. If you mis-shift with the "H" pattern in a race (for example, down-shifting to 2nd when you meant to go to 4th), it is possible to blow up the engine. That can never happen with a sequential gearbox.

The other advantage is that the sequential shift lever takes up less space in the race car cockpit. You only need space for the forward/backward motion of the lever, not left/right.

This photo shows the gear shift lever of the Motorola Champ car. The gear shift lever is to the driver's right. The car has a sequential six-speed transmission, so instead of the H-pattern seen on a normal manual transmission, the shifter moves in a straight line.

Nearly all race transmissions use the sequential shift approach. The drum is rotated manually by a lever in the cockpit, or it is rotated by solenoids, pneumatics or hydraulics that are activated electronically. In the electronic case, the driver has a pair of paddle switches on the steering wheel to control the mechanism and never has to move his/her hands from the steering wheel.

Because of the advantages of the sequential approach, this type of transmission is starting to appear on cars in the high-end tuner market. A sequential manual transmission is not to be confused with a "tiptronic" sort of automatic transmission. The tiptronic system may duplicate the shift lever motion of a sequential gearbox. However, because a tiptronic transmission is an automatic transmission at its core, it still has the torque converter and usually does not shift as quickly.

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