Many car enthusiasts are familiar with the high pitch whine a transmission with straight cut gears makes, but not many people know why race cars use straight cut gears.

Most people think that straight cut (spur) gears are stronger than helical gears and that’s why their used. While generally race transmissions with straight gears are very strong, its not because of the straight teeth on the gears. What may shock you is that straight cut gears are actually weaker than a same sized helical gear. The reason for this is quite simple: helical gears have more teeth meshing and thus have a greater load bearing surface area. The notion that straight cut gears are more efficient than a helical gears also exists; but the difference in efficiency is negligible with both spur and helical gears having about 98-99% efficiency.

So why are straight cut gears used in race transmissions? Well, there are a few reasons why, but the main reason is because straight gears don’t produce any thrust loads. Its very easy to build strong gears that are either straight cut or helical, so that’s not the problem. The problem is when you start putting a lot of torque through helical gears, the thrust loads become so high that you need a very strong transmission case and shafts (heavy) to contain the loads. Straight cut gears simplifies the transmission case design and also the types of bearings used. This allows for a very light weight and compact transmission to be designed – and we know that weight and packaging is everything in a race car. All of the other benefits and negatives to straight cut gears take a back seat to this.

In many instances, the thrust loads that helical gears make can be limiting factor of the torque capacity for a transmission. That is, the case fails before the teeth on the gears do. By installing an aftermarket straight cut gear set into a case that came from the factory with helical gears, it is possible to drastically increase the load capacity of the transmission. Depending on the application, straight cut gears may not be necessary because it would increase the transmission capacity well beyond the power the engine outputs. Simply reducing the helix angle of helical gears can increase transmission capacity and keep the noise at a more acceptable level. This was the case with the famous Muncie M22 4 speed that was prolific in many GM muscle cars. Even though that transmission used helical gears, the helix was at a shallow 20 degrees compared to 45 for the M21 (the low performance version of the same trans). The M22′s gears did make some noise which earned it the nickname “Rockcrusher”.

Interestingly, there is a gear design called a double helical (or herringbone gears) which cancels out the thrust loads; but, I’m not aware of any car transmissions that use this design.

These gears are from an industrial application. The larger gear is over 12′ tall (3.65m)

I’m not sure if these types of gears are suitable for a car transmission since all of the double helical gears I have seen are very wide. There’s probably not enough of a benefit to justify the extra cost and manufacturing difficulty in using this type of gear.

If you like this post, be sure to check out my post on Dog Engaged transmissions. “Dog Boxes” or “Crash boxes” as they are also called, are high performance race transmissions that typically use straight cut gears. These transmissions provide clutchless shifting and are the fastest transmissions outside of twin clutch paddle shifts.

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Muncie M21/22 information source:
http://442restoration.homestead.com/files/muncie_4_speeds.htm

Gear types and their efficiency:
http://www.roymech.co.uk/Useful_Tables/Drive/Gear_Efficiency.html