Crash gearboxes or dog boxes.

Having gone through all of that business about synchromeshes, it's worth mentioning what goes on in racing gearboxes. These are also known as crash boxes, or dog boxes, and use straight-cut gears instead of helical gears. Straight-cut gears have less surface area where the gears contact each other, which means less friction, which means less loss of power. That's why people who make racing boxes like to use them.

Normally, straight-cut gears are mostly submerged in oil rather than relying on it sloshing around like it does in a normal gearbox. So the extra noise that is generated is reduced to a (pleasing?) whine by the sound-deadening effects of the oil.

How a crash gearbox works

But what is a dog box? Well - motorbikes have been using them since the dawn of time. Beefing the system up for cars was the brainchild of a racing mechanic who wanted to provide teams with a quick method of altering gear ratios in the pits without having to play "chase the syncro hub ball bearings" as they fell out on to the garage floor.

Normal synchro gearboxes run at full engine speed as the clutch directly connects the input shaft to the engine crank. Dog boxes run at a half to a third the speed of the engine because there is a step-down gear before the gearbox. The dog gears in a dog box also have less teeth on them than those in a synchro box and the teeth are spaced further pressed bearing apart. So rather than having an exact dog-tooth to dog-hole match, the dog teeth can have as much as 60° "free space" between them. This means that instead of needing an exact 1-to-1 match to get them to engage, you have up to 1/6th of a rotation to get the dog teeth pressed together before they touch each other and engage. The picture on the right shows the difference between synchro dog gears and crash box dog gears.

So the combination of less, but larger dog teeth spaced further apart, and a slower spinning gearbox, allegedly make for an easier-to-engage crash box. In reality, it's still quite difficult to engage a crash box because you need exactly the right rpm for each gear or you'll just end up grinding the dog teeth together or having them bounce over each other. That results in metal filings in your transmission fluid, which ultimately results in an expensive and untimely gearbox rebuild.

But it is more mechanically reliable - it's stronger and able to deal with a lot more power and torque which is why it's used in racing.

So in essence, a dog box relies entirely on the driver to get the gearchange right. Well - sort of. Nowadays the gearboxes have ignition interrupters connected to them. As you go to change gear, the ignition system in the engine is cut for a fraction of a second as you come to the point where the dog teeth are about to engage. This momentarily removes all the drive input from the gearbox making it a hell of a lot easier to engage the gears. And when I say 'momentary' I mean milliseconds. Because of this, it is entirely possible to upshift and downshift without using the clutch (except from a standstill). Pull the gear out of first, and as you blip the throttle to get the engine to about the right speed, the ignition is cut just as the gears engage.

Even the blip of the throttle isn't necessary now either - advanced dog boxes can also attempt to modify the engine speed by adjusting the throttle input to get the revs to the right range first.

Of course even with all this cleverness, you still get nasty mechanical wear from cocked up gear changes, but in racing that doesn't matter - the gearbox is stripped down and rebuilt after each race.