Why do manual transmission cars "whine" when you run them in reverse gear?

forward gears are all "helical" slanted and curved....strong teethies

 

reverse gears are all straight cut, cheaper but noisy...

 

(straight cut gears are something like 8% more efficient than helical)

(one reason yamaha motorcycles always had a distinctive whine)

forward gears are all "helical" slanted and curved....strong teethies

 

reverse gears are all straight cut, cheaper but noisy...

 

(straight cut gears are something like 8% more efficient than helical)

(one reason yamaha motorcycles always had a distinctive whine)

Helical gears are actually much weaker than straight cut, as the contact surface is really a patch that moves from one side of the gear to the other as the two gears mesh. But because of this gradual, moving contact point they are quieter.

 

Straight cut gears have contact along the entire length of the tooth all at once, providing a stronger mesh. But because the entire length makes contact at once it crates noise vibration from the contact.