Synchros and double clutching

[C M Smith <cmsmith@domain.elided>]

There seems to be a bit of confusion about shifting as it was then and 
shifting as it now is.

Old non synchro boxes used gears that moved on splined shafts. When you 
shifted gears you really did. Of course, the correct term is changing 
gears, especially in a synchro box. You had to synchronize the gearset 
speeds to get the gears to shift at all. However, the teeth were large and 
shaped to encourage them to slip together, indeed, that is the basis for 
the synchronised box. The task of transmitting drive was separated from the 
task of connecting the different gear ratios. Now it is the dogs that have 
the shaped teeth and the gears themselves are cut for optimum strength and 
noise reduction.

Synchro boxes are constant mesh transmissions. When you change gears you 
lock the shafts to the selected gearset with a synchroniser and a dog. The 
dog is splined to the shaft and has teeth that match the teeth on the 
gearset. The synchro speeds or slows the dog to match the speed of the 
gearset (the gearset speed cannot be changed significantly as it is 
determined by the speed of the entire set of gears which in turn depends 
upon which set was last engaged to the shafts) when the speeds are 
sufficiently matched the dog engages into the teeth on the gearset and 
drive is connected.

Now there are variations on this theme. Newer gearboxes use a cascading 
design to reduce the internal inertia and overall length of the box, but 
the principles are the same for al synchronized boxes.

There are other types of modern boxes, using just dogs with big sloppy 
teeth and no synchros to allow sequential selection of gears, no skip 
shifting is possible. Motorcycles and rally cars use these.

So, double clutching is no longer necessary, and as one post has it, unless 
you are driving really hard you need not heel and toe either. But it's fun 
and "historically" correct to do both, even though the method was 
originally developed for a completely different gearbox mechanism.

Cheers


Michael Smith
Calgary, Alberta,Canada
91 Alfa 164L
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