>terry, reverse is a non-synchro gear. going into first before engaging
>reverse may eliminate this grinding. the first gear engagement slows
>the gear speed and allow the reverse idler wheel, the gear that changes
>the direction of the output (main) shaft, to engage with a minimal
>amount of strain. ted
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Just to add to that, in most fully synchronized four/five speed
transmissions, the reverse gears are straight cut, never spinning (at least
the idler gear anyway) until you engage reverse, with only a beveled
leading edge to help them engage. Unlike the forward helical cut gears,
which by the way are always engaged and spinning with the laygear or
cluster. Yep, its true. Those forward gears themselves are all spinning,
and at different speeds no less, on the output shaft when the clutch pedal
is not depressed and the engine is running. You need to have everything in
the transmission stopped, or not rotating, to smoothly engage reverse. The
actual "engagement" of the always spinning forward gears is done with a set
of little pointed dog teeth on the engaging(?) side of the gears, and the
sliding synchro sleeve. This is what actually connects the selected
spinning gear, by way of these dog teeth to the output shaft, through the
laygear and corresponding gear ratio, using the synchro (which is actually
a cone clutch) to slow things down so the dog teeth don't gnash when
sliding that selector sleeve. That poor synchro has to slow down (or speed
up to match) not only the selected gear, but also the first motion shaft
and the clutch driven plate, to the speed of the output shaft so it has a
formidable job. It's the second gear ratio and the amount of work it has
to do that makes second gear so hard on synchros. *Usually* the reason
second synchro goes first is that second gear is spinning the (third)
fastest speed relative to the output shaft so it does the most work.
That's because the car is usually in motion or at speed, especially when
downshifting when the engine is revved fairly high. Why isn't first
usually to go since it's spinning fastest of all (and the gear itself has
the most mass to slow down/speed up) relative to the output shaft you ask?
Well basically, it's used much less and under less stressful conditions.
When at rest selecting first is usually done after things have slowed down
a bit. The engine is idling 800-900 rpm relative to the output shaft which
should be at 0 rpm That and the fact that most(?) of us don't down shift
into first (while winding the engine out) is why that synchro isn't the one
to go first. Anyway, sorry for the winded lesson as it were, it's
sometimes helpful to explain *why* we are advised to do something, like
engaging fourth before selecting reverse, so one has a better understanding
of just what is happening and so we all (or at least myself) don't sound
like a bunch of kooks :-) When you select a forward gear first, before
sliding it into reverse, you are actually stopping the cluster from
spinning which has the old straight cut, spinning gear machined into it.
The cluster gear or layshaft is ALWAYS spinning when the first motion shaft
is turning, i.e. whenever the engine is running and the clutch pedal is not
depressed. You just stopped the cluster from spinning using of one of
these synchros so that engagement of the, also not spinning straight cut
reverse idler gear, isn't done with a spinning cluster gear.
By the way, there are some trannys that do have a synchro on reverse. The
Mazda five speed in my truck actually has one -
I'll be signing off in a couple days for about a week (to go visit the
spousal unit's relatives in the middle of nowhere Colorado), so every one
have a Happy New year. See ya all in 2000!!!
Barry Schwartz (San Diego) bschwart@pacbell.net
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