Racing gearbox - a look inside

(Szerző: z. (Houdek Zoltán), utoljára módosítva: 2007.01.19.)

Those of you attending rally races must have already heard these words: async gearbox, straight-cut gearbox, dog-type box...These all mean the same thing. All the above words represent different properties and behaviours of the same mechanical solution.

What is a dog type gearbox?

Synchro drums (being used in street cars) use friction and therefore they need time to synchronize RPM of input and output shaft (or an internal shaft). This is needed for the proper connection of the gears. A dog-type gearbox used simple metallic locks instead of synchro drum. They "synchronize" shafts immediately, you can hear the loud clicking noise, mainly when racers put it into 1st gear at start line.

To use the dog type gearbox you need some physical power :-) but you do not need to use clutch. You can therefore switch faster. However if it is not set up correctly and precisely enough, it might stuck in any of the gears and you loose the given special stage.

What is a straight cut gearbox or gear pair?

In street cars gears do not have their teeth prepared paralelly with their shaft. This solution results in silent running while the connection produces more friction (more power lost). In this picture you can see normal gears.

Straight cut gears have parallel (to the axle) teeth, therefore they have less friction loss, they can handle bigger torques -- although they are loud. You can hear the same noise when driving in reverse. (Most street cars have straight cut reverse gear pair). You can recognize straight cut gearboxes of this sound.

Can you combine these techniques?

Yes, there can be non-straight dog-type, straight-cut with syncro, straight-cut dog-type. This latter is the most expensive and the most perfect. (Besides a sequential one, of course)

Photo

You can see the straight cut gears. You can also see the parts of the dog-type sync mechanism (upper shaft, between gears 1-2, 3-4, 5-6).