Generally used only in motorsport applications, straight-cut gears are an interesting alternative to a conventional gearbox setup. Known for their distinctive scream and inherent lack of usability, this is a transmission setup that most of us will never come into contact with.
What are they?
The name explains it all really, the actual teeth of the gears point straight out from the centre point of the gear instead of forming a helical shape like standard gears in almost every other road car on the planet. Instead of the teeth curling nicely in a spiral format around the centre axis of the gear, they protrude outwards; more like the sprocket on a motorbike.
How do they work?
The main advantage of using straight-cut gears is that they produce no axial load. This ‘thrust force’ is generated by the sliding contact between the teeth of helical gears. This lateral force is applied to the input shaft of the gearbox, which in front-wheel drive configurations will then convert through to the driveshafts. This greatly restricts the amount of torque that can be applied through the gears before failures on other components occur.
So straight-cut gears effectively allow much larger powertrains to be placed in a vehicle without risking the output shafts and other bearings tearing themselves apart, producing a larger safety factor within the transmission itself.
Are there any other advantages and disadvantages?
A straight-cut transmission is inherently more efficient than a helical gearing system. The axial load produced by helical gears only detracts from the output energy from the transmission along with an increase in friction and therefore losses of energy due to heat occur.
Straight-cut gears are also much easier to assemble and produce less-catastrophic failures when they do go wrong due to their simple structure. Heavy-duty transmission casings and shafts have to be used with helical gears for the transmission to cope with the additional axial load, therefore straight-cut gears save heaps of weight which is extremely important in a motorsport setup.
Drawbacks come in the shape of their convenience and operation. They naturally create a hell of a lot of noise; helical gears mesh together in small segments at a time while straight-cut teeth are in contact with each other fully and for a longer period of time which makes for a very distinctive whine. The operation of the gears is also fairly brutal when straight-cut; the gears appear to ‘slam’ into place after each change and can make pulling away smoothly or getting a decent launch difficult.
Technically, on a tooth versus tooth comparison, a helical gear can carry a larger load seeing as it is diagonally positioned on the gear (assuming tooth size is equivalent). This means that it distributes the forces being applied on it much more efficiently compared to a weaker, vertical tooth.
Despite virtually all cars these days using standard helical gearing systems, there are still applications within motorsport that use straight-cut gearing. Back in the prime of sports car racing (1950s and 60s), Stirling Moss and Steve McQueen drove Austin Healey ‘Sebring’ Sprites to 3rd and 9th place respectfully at the 4 Hours of Sebring, a race specifically for cars under 1000cc. These little pocket-rockets sported lovely little four-speed straight-cut gearboxes and one such car was restored faithfully by CT favourite Fuzz Townsend on the last series of Car S.O.S.
Many motorsports enthusiasts these days will convert their cars to straight-cut gears for their efficiency when driven hard, as you can see (or more like hear) below through this converted BMW E36 320i racecar.
I wouldn’t rush onto eBay to look for a straight-cut gearing conversion for your daily driver, but it’s certainly something to think about if you have a spare trackday weapon!