If Straight Cut Gears Are So Great, Why Don't Street Cars Use Them?

For specific applications, such as motorsports, straight cut gears provide some advantages. However, for everyday passenger vehicles, they would offer a terrible experience, which is why most vehicles utilize helical gears. As the name suggests, straight cut gears feature teeth that jut out in straight lines from the center. Conversely, helical gears include teeth positioned on an angle, giving the look of a spiral as they rotate. This isn't the only difference between a racing transmission and a normal gearbox, as motorsports teams must ensure swift shifting speed and keep weight minimal to remain competitive.

The priorities of the motorsports world are far different from automakers manufacturing your average car. Often, one of the defining characteristics of a consumer vehicle is the quality of the ride or driving experience, which becomes a powerful selling feature. Straight cut gears produce significant amounts of noise during operation, akin to an aggressive high-note whine. In addition, a straight-cut dog box (a type of transmission used in racing) doesn't work with a delicate touch. Instead, it requires a confident amount of force to shift, and the driver must match vehicle speed with the appropriate level of engine rev to sync gear engagement. If not operated properly, a dog box can quickly get destroyed. Does this sound like a good idea for your average commuter? Heck, some of us here have just realized we've been using turn signals all wrong.

The reason most cars utilize helical gears is that they're easier to operate, don't emit a racket during driving, and provide a more refined experience. The spiral shape of helical gears allows them to reduce the impact of engagement and incrementally increase teeth contact as the gears rotate. This is essentially the opposite of straight cut gears, which go from zero contact to fully engaged, instantly. To get an idea of what a straight cut gear driving experience is like, YouTube channel slangen25 posted a video, "BMW M3 GTR on Nürburgring." Listen for the loud whine and the hard ca-chunk of the driver shifting, and you'll get a sense of why this wouldn't be an ideal daily driver setup for most.

The amount of contact area between helical gears is also greater than that of straight cut. This means the load from the engine is distributed across a wider area, helping to reduce stress on the components. Conversely, on a straight cut, the meshing between gears rests on a pair of teeth, focusing the load on a much smaller contact patch. Essentially, this translates into helical gears suffering less wear during operation and, in turn, providing a greater service life. Of course, longevity depends on you avoiding these driving habits that could cause more transmission damage than you think.

One of the advantages of straight cut gears is that their design not only is able to transfer more of the engine's power than a helical setup, but they also don't place an axial load on the shaft. With a helical configuration, the angled teeth create a force that pushes laterally, diminishing the total energy transferred. In contrast, straight cut gears don't generate any lateral force, instead transmitting all the power received without any waste. In addition, while the helical gear design is more forgiving, the gradual meshing contact between teeth generates friction, causing the system to lose energy via heat. While not noticeable on your average passenger sedan, race cars, on the other hand, are vying for fractions of a second in some cases, so every bit of power is crucial.

In addition, motorsports are highly concerned with weight, as more pounds force the engine to work harder, affecting performance. So, weight-to-power ratio is a critical means of improving lap times on the track. The helical configuration adds a lot more to the scale, as that lateral force mentioned earlier requires a bearing be installed to counteract this movement. Not only is this heavier than the straight cut gear system, which doesn't require additional parts, but it's also not ideal for extreme high-torque setups. If you apply tremendous amounts of torque to helical gears, they'll generate even more lateral axial force, which could literally cause the system to tear itself apart.