The primary gears in an engine are between the crankshaft and the clutch, and typically have ratios in the range of two or three-to-one. Because this increases the engine’s leverage over the clutch, the clutch must be somewhat larger and heavier than if it were spinning at crankshaft speed.

There have been a few engines designed with their clutches mounted on the crankshaft. The first that springs to mind is the many BMWs boxer twins with a car-type clutch on the engine’s flywheel. This works perfectly well, but the mass of the clutch disc is responsible for the audible KLAK you hear at upshift.

Yamaha’s YDS-2 two-stroke parallel twin carried its little clutch on the left end of its crankshaft. This was workable at moderate stock rpm, and in non-racing use.

During a racing start, the rider pins the throttle and controls the drive with the clutch. At the beginning there has to be clutch slip to allow the engine to spin high enough to make the power for quick acceleration. During this slip phase of a start, the clutch acts as a power divider. At first, the bike moves slowly, so most of the engine’s power goes into friction - heating the clutch. As the bike gains speed the clutch slips less, and finally with the clutch fully engaged, slip stops and all the engine’s power (minus the usual losses) is being to accelerate the bike.
Trouble was, the engine-mounted clutch was designed for the lower torque of the engine, as opposed to that delivered to the gearbox, multiplied times the primary ratio. A racing start pushed that little clutch’s temperature up so high that it was common to blue or warp one or more of the steel plates.

After a time with the crank-mounted clutch, Yamaha moved the clutch to the usual position, on the big primary gear that drives the gearbox. They also – possibly with regret – made the clutch larger in diameter to make it more able to deal with the steep temperature rise of racing starts.

I remember seeing Steve Baker changing the primary gears of his TZ250 at Daytona. Seems his engine was making such good power that it needed the taller primary ratio of the TZ350. Sigh!

Now jump to Superbikes in the 1990s. At the end of the 1980s some bikes were behaving badly, squatting down at the rear as the rider throttled-up to exit corners. This squat took weight off the front, making it push. Running wide as a result of this was an invitation to being passed on the inside.

People tried things until they hit upon something that worked – raising the swing-arm pivot axis just a bit. This used the powerful tension in the drive chain to generate a stronger lift effect, countering the squat.

Then came a terrible discovery. Changing rear sprockets might bring back the squat. The underlying reason for this was that the anti-squat effect came from the angle between the upper run of the drive chain, and the central plane of the swing-arm.

Once a team had found a set-up that banished squat-and-push, they didn’t want to mess with the gearing. It had become THE SOLUTION. Urgent requests went to Japan, asking for a different way to change the gearing. Please, please make us primary gears with alternate ratios, so we never have to change the rear sprockets!

No problem – soon boxed sets of alternate-ratio primary gears appeared. A bit later, the chassis of new models seeking Superbike homologation were being manufactured with adjustable-height swing-arm pivots. And in the background, Ohlins had circulated a set of instructions on how to work out squat-anti-squat numerically.

Another effect appeared as well – designing new-model engines with their gearbox shafts vertically stacked, rather than the old way – one in front of the other. This allowed swing-arms to be made longer.

Why bother? The longer the arm, the smaller the angle through which it rotates in a given rear suspension deflection. Deflecting the suspension (or changing rear ride height) changed the angle between the upper chain run and swing-arm – possibly enough to bring on a light case of squat-and-push. In general, engines with their clutches ‘way up high have vertically-stacked transmission shafts and swing-arms nearly 24 inches long. This is their story.

There was a more direct way to deal with squat-and-push. During a certain era, the Ohlins techs would get requests for radically stiff rear springs. This was brute force anti-squat – make the rear suspension so stiff that it couldn’t squat.

Now that people understand the geometry of squat-and-push, alternate-ratio primary gears are no longer necessary, and there is even a WSBK rule against them.