It seems a lot of people are interested in transmissions with straight cut gears due to their use in high performance applications. Because these gears are typically designed for racing they usually have the option to be dog engaged – which is a synchroless design. Few people are familiar with dog engaged transmissions since they are only in full race transmissions and are usually priced out of the weekend racers budget.
First, some background:
If you are an automotive enthusiast, you should know that modern manual transmissions are of a constant mesh design and have a basic understanding how the typical manual transmission works. If you don’t, this has been covered ad nauseum by many people and I suggest you watch the video on the bottom of this page. You should also specifically know what a dog gear is which can be found here. (To get straight to the point, an image of dogs taken directly from that site here. The dogs are the teeth on the face of the gear. This is for a motorcycle, but it’s all the same.)
Most people know that modern transmissions use synchros even if they don’t exactly know how they work. Synchro’s are a type of clutch used to match the rotational speed of a gear to the next gear it’s going into. The synchro actually uses the force the driver is applying to the shift lever to create the friction required to match the speed of the gear. In modern cars, the designs of synchro’s have become so refined that very little effort is needed to push the shifter into gear. But, sometimes they can be noticed in a situation of high mismatch; for instance, when downshifting into first. In this situation, you will notice more effort is required to move the shifter into gear; and, if you listen closely, you may be able to hear the gears spinning up as you press on the shifter.
So that’s the basic gist of how a synchromesh transmission works.
The Dog Engaged Transmission:
Dog gears are a bit different than the teeth that are found on a synchromesh trans. I’m not sure where the name comes from because they do not resemble the teeth on an actual dog, at least the dogs I have seen. As shown above, the dogs are teeth cut into the face of the gear. Typically, these teeth on a dog engaged gear are cut very wide and there is a lot of ‘play’ when the gears are meshed together. The wide spacing between the teeth makes shifting more forgiving and easier to connect.
Because there is no synchronization of the gears, this job is entirely left to the driver. The driver has to be careful and rev match every shift, because if they don’t, the dogs can become damaged as they bounce across each other. Even when a shift is executed properly, it may be noisy and cause some discomfort. Also, down shifting becomes complicated and can require double clutching to rev match properly. As you can expect, this can get pretty annoying on a daily basis and why auto manufactures never looked back after the synchromesh was invented.
So why would you ever want a synchroless transmission?
Even though a dog box can be very unforgiving, they have some advantages on the race track. The main attraction is very quick clutchless shifting. The dogs on a dog engaged transmission are spaced wider apart and allow for a small amount of rpm mismatch. This makes for a clunky and noisy shift, but who cares in a race. Clutchless shifts are accomplished by simply letting off the throttle for a split second, and once the load is off the transmission, the shifter is simply thrown into the next gear. No synchromesh box can match the speed of these shifts. Another benefit is that dog boxes require virtually no force to be put into gear since there is no synchro’s to operate.
Take a look at this video to see the beauty of the system:
There are a couple parts of this video where he looks pretty nervous. I wonder how many takes they had to do…
Food for thought:
How well you can shift has a large effect on the lifespan of a dog box transmission. Typically, these transmissions don’t last as long as a synchromesh box due to the abuse the dogs take.
H-Pattern Dog boxes were run in most forms of professional racing, including Formula 1, until the paddle shift came around. This was the only option for a fast shifting transmission for many years.