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The Secrets of F1 Seamless Shift Gearboxes Part2

By Eli (aka KineticKid)
May 19 2008

In November 2005, Geoff Willis acknowledged at the Motorsport Industry Association networking dinner that BAR Honda had used seamless shift gearbox during the 2005 season, and that it was based on the Weismann Quickshift transmission, first tested by Williams in 1993. This is not to be confused with the FW15C F1 CVT also tested by Williams in the same year.

Earlier in 2005, Race Car Engineering had spoken to Miles Ashcroft, head of design at Zeroshift Ltd in Mitlon Keynes, and confirmed that the company was in talks with a number of teams for a concept of an F1 gearbox that "demonstrated the ability to provide seamless selection of all seven gears in a unit as restricted as the smallest currently in use in F1 today."

The Zeroshift innovation lies in the design of the patented "hub" which holds the "bullets" that replaces the collars and dog teeth, respectively. Whereas in a conventional gearbox the collars slides along splines in the output shaft, the Zeroshift hub remains completely static relative to the shaft, and only the bullets slide in and out of slots on the outer surface of the hub. There are 6 bullets in each hub. Each end of the bullet has two faces: one flat, and the other "ramped." The faces are alternated on each end of the bullet. On each hub, there are 2 sets of 3 matching bullets.

zeroshift hub graphic:


When engaging first gear from neutral, the selector fork moves all six bullets towards the dog teeth of the first gear. (The selector fork is able to move the bullets via pressure applied to them through spring packs). Even though all six bullets come into contact with the dog teeth of first gear, only 3 of the bullets will lock with the faces of the corresponding dogs on the gear pair. This owes to the fact that when force is applied to the dogs in any one direction, only the "flat" faces of 3 of the bullets can lock with the dog teeth on first gear. Simultaneously, the "ramped" faces on the other 3 bullets will be making contact with the dog teeth, and will be "thrown out" as a result, since they are only being held in place by spring. This happens repeatedly, unless the throttle closes and engine braking takes over. When the torque reversal takes place on overrun, pressure is then put on the opposite faces of the same bullets which were previously being thrown out. In other words, the driven bullets become unloaded on overrun, and the previoulsy unloaded become the driven bullets.

On the upshift to 2nd gear, with the engine still accelerating, the selector fork applies pressure to move all the bullets towards the dog teeth on 2nd gear, but only the 3 unloaded bullets will respond the pressure of the spring pack and move towards the 2nd gear. The other 3 remained locked in the dog teeth of the 1st gear because of the torque applied upon them under acceleration.

At this point, both the 1st and 2nd gear pairings are engaged to the output shaft, and we are now running in 2 gears at once. Initially, the engine is now turning faster than the drive train. We will address how Zeroshift addresses the torque spike created in this situation a little bit later.

Now that the hub is clamped to the dog teeth on both 1st and 2nd, this means that 2nd gear is now overdriving first gear because of it's higher ratio. The torque holding 3 of the bullets to 1st gear is instantly transmitted to the dogs on 2nd gear, and the selector fork springs move the unloaded bullets towards 2nd gear as they prepare to become the driving bullets for 2nd gear in the case of overrun.

In order to smooth the aforementioned torque spike created as the gearbox instantly shifts to a higher ratio, Zeroshift considered a number of solutions. First was to momentarily inhibit the spark or fuel delivery, but this comes at the cost of a loss of torque to the rear wheels. Two more desirable solutions are to briefly slip the clutch during upshifts in order minimize the sudden increase in torque across the drivetrain, or to implement drivetrain dampers that would absorb this force instead. The eventual invention developed by Zeroshift was aptly named Flatliner, and involves electronic manipulation of the throttle and clutch.

The SSG first developed by BAR Honda was based on the Weismann Quickshift first debuted by Williams 15 years ago. This gearbox was by far the most compact in F1 history, in no small part due to the fact that the means used to connect the gears to the output shaft were located inside the center of the shaft, as opposed to being attached around it. As a result, the gear pairings could be placed directly adjacent to each other, since the mechanisms replacing the collars were inside the shaft, and the selector forks also normally appearing between the gears, would be foregone in favour of a selection system that runs through the open, front end of the output shaft. This significantly shortened the length of the box. While little is known about the members used to replace the collars, it is believed they comprised of one-way bearings. One way bearings rely on oil friction in order to lock as pressure is applied to them in one direction. When force is applied in the other direction, they spin freely. Due to this feature, a very complex, and as yet unknown mechanism was used to keep the selected gear engaged during overrun. Despite the solution used to overcome this problem, backlash was a significant drawback of the Weismann (ie. movement within the gear pairing before they are "meshed").

Weismann gearbox photo:

The above descriptions of the Zeroshift and Weismann boxes explain their function in their simplest form, as they have unquestionably evolved in their adaptation meet the demands of several years of intensive F1 power-train development. Surely, there have been as many questions raised as have been answered (For example: To deal with the torque spike created by Zeroshift, would one need to have rather closely stepped ratios, with an inadmissably high 1st gear ratio and inadmissably low 7th gear ratio?)

By delineating some of the concepts behind current gearbox technology, and contrasting it with tech used in other disciplines, we hope to have shed some light on a somewhat murky area of F1 technology.

*The first part of the article can be read at

Photos of gearboxes can be found at

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The Secrets of Seamless Shif Gearboxes in F1 Part2
Discussion started by BMW Sauber F1 (IP Logged), 19/05/2008 15:44
BMW Sauber F1
BMW Sauber F1
19/05/2008 15:44
The Secrets of Seamless Shif Gearboxes in F1 Part2

20/05/2008 20:30
Superb. Thank you.

Williams and proud of it.

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