Hi Ben,
I think Art Pfenninger (sp) discussed this a couple of days ago, but
I'll try to give an outline of the technique for you.
If you envision gears turning and the teeth having to fit together for
engagement, you see that they must be turning at the same speed for the
teeth to merge properly or at all. Synchronisers in the gearbox
"synchronise" the speeds of the gears so that the merging (engagement)
can take place.
Before they were invented, common, then universal, it was necessary for
the driver to synchronise the gear speeds. That's fairly easy and
intuitive on an upshift, that is, the driver knows that the engine speed
will be lower in 2nd than it was leaving 1st, and the engine speed drops
somewhat during the time of the shift, and the gears mesh.
Now consider the 2nd to 1st shift. The engine speed has to be increased
before the gears will mesh. Thus the driver has one more thing to do with
his feet.
Clutch is pushed down, throttle foot remains steady on the gas. Gear
selector is pushed into neutral. Clutch pedal is let up. Throttle foot
remains steady on gas or presses down to raise engine speed. Clutch is
pushed down a second time; gear selector is pushed briskly into 1st.
Clutch pedal is let up again.
So you tried it, and what happened? It may have been a flawless,
noiseless, downshift. Congratulations. It may have been a grinding
shift, but you got it in. OK, but why did it grind? Because the two
gear speeds were close, but not exact. By the way, one wants to make this
a quick, positive shift. Grinding the gears is not good at all, and a
rough shift (within reason) is going to put less metal loose in the
gearbox than a prolonged butcher-please-grind-me-a-pound shift. From here
on it's practice, practice, and more, until it's mastered.
With a synchronised gearbox, the gearbox is doing most of the
adjustment of the gear speeds. Your double-clutching makes its job easier
for shifts between synchronised gears and reduces wear on the little
pieces. It's not required, but it will result in smoother shifts and
impress passengers.
As late as WWarII, racing cars rarely had synchronised gearboxes. It's
simpler to make robust gears that are "straight cut" for a given gear
strength, given the metal technology of the time. Drivers, then, had to
do their braking and double clutching simultaneously. Their hand/eye/foot
coordination must have been something to behold. A "missed" shift meant
that you came into a corner too fast, with brakes that were probably
fading, and in neutral, all in all, a bad combination.
Bob
On Tue, 17 Mar 1998 19:20:00 -0500 Benjamin Ruset <bruset@home.com>
writes:
>> It's the name for the technique of pushing the brake pedal with
>one's
>>toe while simultaneously pressing the throttle with the heel. The
>idea
>>is that one can bring up the engine speed for a smooth (preferably
>double
>>clutched) downshift while slowing the car with the brakes too.
>
>Prepare yourself for the "I don't know what the hell I'm doing"
>question-o-the-day!
>
>What is double clutching? I've driven sticks before (quite well...)
>but I'm
>not
>sure if I have ever double-clutched or not.
>
>
>BEN RUSET - bruset@home.com (note the new address)
>Safety Fast & MG Cars Webring - http://www.infi-pos.com/~oasis
>
>
>
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