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 (actually called 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. Another myth is that straight cut gears are more efficient than a helical gears; 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 is only one real reason, and that 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 (axles) to contain the loads; this can add significant weight to the transmission. Straight cut gears simplifies the transmission case design and also the types of bearings used. This allows for a very light weight transmission to be designed – and we know that weight is everything in a race car. The one negative of straight cut gears, and its a big one, is that they produce a lot of noise.
The above image shows the thrust loads (red) produced by helical gears. The transmission case and bearings must be designed to handle this load as well as the normal force (blue) of the gears pushing away from each other.
In many instances, the thrust loads produced from the helical gears can be limiting factor of a transmissions torque capacity. 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.
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.
Muncie M21/22 information source:
Gear types and efficiencies: