What Are Straight-Cut Gears, And Why Do Race Cars Use Them?

There is no complex science behind what straight-cut gears are. It's exactly as it sounds, gears with teeth that run straight out from the center of the wheel, rather than slanting in a spiral. The latter is more like what you'd find in a typical everyday production car, and they're called helical gears.

In a normal road-oriented transmission, helical gears mesh smoothly and quietly, and this is because their angled teeth make contact gradually. Straight-cut gears, on the other hand, lock together all at once, and this results in a remarkably distinct high-pitched whine. It's the same whine you hear when reversing, as reverse gears tend to be straight-cut also. Picture that whine, but joining you everywhere you go, and you'll quickly realize why this is generally a race car-only application.

The main positive of using straight-cut gears in a race car is that they don't generate what's known as "axial load." This is when helical gears create a sideways thrust as their teeth slide against each other, putting stress on shafts, bearings, and the transmission casing. Straight-cut gears avoid this, and this means they are much stronger when under load and much more efficient, since less energy is wasted as heat or friction. It makes sense for race cars to use them then, as they can handle power and torque more easily without tearing themselves to pieces. This is not a case of when race car technology improved the road car though, as these gears are really best left for the track.

In motorsport, strength and reliability is everything — without it, you run the risk of not finishing the race. Straight-cut gears allow race cars to put significant amounts of horsepower to the wheels consistently, without destroying transmissions. By eliminating axial load, they save weight, reduce energy loss, and help to ensure the transmission stays in one piece for as long as possible. They're also simpler to manufacture and, when something inevitably goes wrong, they are less expensive to repair when compared with a traditional helical-gear transmission.

Race-prepared transmissions will often pair straight-cut gears with "dog engagement." This is a system which allows lightning-fast, clutchless shifts, and the result is that quick-fire, mechanical slam into the next gear you'll see in touring cars and the like. Just check out the video above for a prime example. For the driver at the wheel, it means less time wasted on executing the change, and more time stamping on the loud pedal. On the road, speed of shifts and being able to handle huge and constant supplies of power are less important, so smooth and almost silent helical-cut gears are a sensible preference.

Still, ignoring the downsides, enthusiasts and budding track-drivers love them. Classic Minis, for example, are often converted to straight-cut transmissions for performance builds, whether it be for a Goodwood racer on simply a fast-road build. For the most part though, straight-cut gears are much like racing slicks – brilliant for maximizing the performance of a car on track, but more than just a little overkill for school runs and grocery trips.