In <5506341721111995/A06651/KSCSC/119BAC620400*@MHS.KAMAN.COM>, MACEDONIA wrote:
>Just by the fact that I'm asking this question you can tell that I've never
>driven a car with a non-synchro gearbox before. What is the "heel and toe"
>technique? I probably should know this because I'm restoring a 1966 Mark I MGB.
>But, since its pretty much stripped down to the shell, I figured I have plenty
>of time to find out!
>
Well, first, the 66 has synchro in all but first gear, so the
opportunities for "heel and toe" will be very limited. It is rare
that one has to drop into first while on the fly. The concept is also
known as "double clutching". The idea is that for a smooth shift, the
the clutch disk and input shaft must be going at about the correct
speed for the gear that you are going to shift into, which in turn is
determined by the speed of the car and the ratio of said gear.
Normally this is handled by the synchro rings. If you are
downshifting, pressure from the shift lever engages the input shaft
with the ring associated with the gear you are shifting into. This is
a sliding engagement that gently results in the rotational speed of
the gear you are shifting into being transmitted to the input
shaft so that when the engagement dogs strike, the two are going
about the same speed and you get little or no clash.
Without synchro rings, there is no mechanism for such a smooth
transition. You the driver are responsible for matching the input
shaft speed with the approximate speed of the gear. When
accelerating, this is fairly easy. The idea is that each subsequent
gear has a lower ratio, so you need a lower input shaft speed than
that for the gear you are leaving. This is done by depressing the
clutch, shifting out of the lower gear (higher ratio) and waiting a
bit until the clutch (now free-wheeling) slows down, then slipping
into the next gear. This is why you get so mad at great-uncle Henry
when he is driving because he waits so long between shifts. If you
are in a hurry, the input shaft can be lowed down faster by shifting
out of the lower gear and into neutral, then re-engaging the clutch.
The engine slows down faster than the clutch alone. At the
appropriate point, one then depresses the clutch again and shifts into
the higher (lower ratio) gear. This is the classic form of "double
clutching" (or as certain citrus-infatuated colleagues might say,
"double de-clutching"). I would note that in the Phantom series of
Rolls-Royces, which were crash-box equipped (no synchros) until 1934,
there was a clutch brake that engaged when the clutch was depressed to
mitigate the nuisance of double de-clutching.
Well, that is all for shifting up. What about shifting down? The
opposite principle applies. You need to speed up the input shaft when
going into a lower gear. This is done by shifting out of the higher
gear into neutral, releasing the clutch, blipping the throttle,
depressing the clutch, and slipping into the lower gear in an Elysium
of silence (if all goes well). One might note that this involves a
fair number of moves executed rapidly if in the heat of battle. One
might also note that most downshifts are associated with decelerations
of the car itself (like into the corner, drifting on all four if Santa
likes you). This leads to the tautological circumstance of being
required to depress the clutch and brake and blip the throttle
simultaneously, hence to the procedure of "heel and toe". Whether it
is the heel that hits the brake and the toe that hits the throttle or
vicey-versey depends on the relative pedal height and the flexibility
of your joints.
This procedure is trickier than it sounds in practice, and leads to
incredible admiration for early competition drivers. Remember kids,
don't do this at home.
A. B. "Tried it, can't do it" Bonds
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