Why is it necesary to double clutch auto "crash boxes" but not Motorcycles

Question

Why is it necessary to double clutch old cars with crash boxes -- non synchro transmissions -- but not motorcycles?

Is it because motorcycles have close ratio gears?

Also, why do straight cut gears on motos not whine as much as cars? Just because they are smaller? Or does it have something to do with being sequential?

Answer 1

Bike transmissions are constant mesh; in other words, the two gears in every gear from first to top (4th-6th) are always in contact with each other.  When you shift, the shift fork slides the appropriate gear sideways, where upon "dogs" (cogs, thing of sliding your fingers on both hands together) on the side of the gear lock into an opposing set of dogs on the adjacent gear.  The input and output transmission shafts each have teeth machined into them and always have a gear engaged into them, that's where the power is transmitted.  All other gears slide on shafts and the path power takes through which ever gears is how the speed reduction occurs.

With a car tranny, the only gears that are in contact with each other are the two that are being used.  When shifting, all a syncronizer ring does is get the rotational speed of the "new" gear up or down to the speed of the gear it's being shifted into.  Many old cars with 3 speed on the column shift are syncronized only between 2nd-3rd and not 1st and 2nd.  That's why you can often get a crunch when shifting from 2nd to 1st.

As far as whining goes, apparently you've never ridden Kawasakis. Most are notorious for the straight cut, primary gear whine.  Close machining that makes the straight cut gear kind of egg shaped helps reduce whine, but straight cut gears are noisy, period.  Helical cut gears are much quiter but create side thrust that straight cuts don't.    

Answer 2

motorcycles do have synchros

Answer 3

it is not necessary to double clutch anything , except for semis and dump trucks

Answer 4

First- I have only worked over my own cycles-that said; I have never seen syncro rings on a motorcycle trans. They are shifted with clutchdogs. When one gear speed is close to the next, the loaded clutch dog (you load it with the shifter) will "pop" into gear. Double clutching the motorcycle helps.

    Straight cut gears in a car or truck can be 5" across and meshing with some small gear. Produces a high "gear speed" which can be readly heard inside a car or truck  --the noise of the smaller gears inside a motorcycle crankcase must compete with engine noise and wind in your ears.