Is double clutching advisable? I recently bought a
1968 Jaguar E-type coupe in extremely good condition
and I noticed that the brakes and the accelerator are
^^^^^^ ^^^^^^^^^^^
very close together. I assume this was done to aid
double clutching.
What do the brakes and accelerator have to do with double clutching? :-)
You're making a common mistake, confusing double-declutching with
a heel-and-toe downshift. The two are distinct, but can be combined.
What you've observed in the gas and brake pedal is an orientation
that helps with heel-and-toe. (And THAT'S a misnomer; it's actually
ball-of-foot-and-little-toe.) At the simplest level of abstraction,
double-clutching is done for the sake of the synchros in the gearbox,
while heel-and-toe is done for the rear tires. That is, the clutch
business may help you get the stick into the next gear, while the
heel-and-toe portion of it helps you get down the road more smoothly
(which means faster and in better control).
The two pedals are so close that if
you're wearing wide shoes they brush against the pedal.
Heel and toe, then. Here's the scoop.
Why to do it:
- When you are in gear, there is a direct linear relationship between
the speed at which the wheels are turning and the speed with which
the engine is rotating. Accelerating and braking while in the same
gear keeps this relationship linear and smooth.
- When you change gears, this relationship is forced to make a
relatively large jump to take the new gear ratio into account.
That is, the relationship between wheel and engine is forced
to make a discontinuous adjustment from the old engine speed
to the new engine speed -- the road speed is decreasing in a
fairly linear manner, but the engine is now operating at a
different gear ratio.
- If this large jump occurs in too little time, shock is applied
throughout the driveline; the most dangerous form this shock takes
is when the engine's compression strokes apply sudden and violent
engine braking to the driving wheels. If the difference between
the old wheel RPM and the new wheel RPM is great enough, the tires
will break free from the roadway as surely as though you locked
the brakes. If you are negotiating a corner at this time, a spin
is almost inevitable.
- A heel-and-toe downshift brings the engine up to an RPM that is
consistent with the new gear ratio and the car's linear deceleration
at the driving wheels. Done properly, the heel-and-toe downshift
retains traction while also preparing the engine to accelerate or
decelerate without applying shock to the driveline.
How to do it:
1. Find a position such that you can [anybody wanna hear a tirade
about how engineers over- and misuse "such that" and why this is
an example of the correct usage? :-)] apply full pressure to the
brake pedal with the ball of your foot and still manage to blip
the throttle with the right edge of your shoe (sort of up where
the little toe joins the sole). Search for this position while
the car is not moving.
2. When you wish to change down into a lower gear while braking,
move your foot into the position and apply the required amount
of pressure to the brake pedal with the ball of your right foot.
Press on the clutch with your left foot and hold it in.
3. Select the lower gear while holding the clutch in.
4. Then use the right edge of your foot to press lightly on the
throttle pedal, just enough to bring the revs up. Then get off
the gas so that the engine slowly loses RPM.
5. While the engine revs are up (several hundred or even a couple
thousand RPM higher than when you first hit the clutch), let
the clutch out smoothly. The fast-idling engine will pick the
RPM which the current gear ratio requires, and you will avoid
shock to the drive wheels.
That's the technique in a nutshell. Most often you use the
heel-and-toe downshift technique to pick a lower gear as you
are on your way into a corner; the idea is to have constant
torque through the corner and to be able to use the engine's
power to start accelerating smoothly at the exit. I find that
I often heel-and-toe in a straight line, in traffic; as I come
up to a stoplight that's just changed, for instance, but where
thre are three or four cars ahead of me, I will often drop the
car one or more gears while braking, so that I will be in the
best gear to accelerate when the traffic clears.
Note by the way that using the engine to slow the car is not
the goal here. In fact, if you can feel the car slow down as
you let the clutch out, it means you're not braking hard enough
or you haven't let the engine come up to a high enough RPM level.
Brakes are cheaper to replace than clutches (well, maybe not on
an E Type :-).
I'd like to utilize all the sports car feature of this
car to the fullest and double clutching seems to offer
great promise. So can someone explain to me how you
double clutch? I think I know what it is but I just want
find out for sure before I grind the hell out of the transmission.
Should I try this out on a rental car first?
Double-clutching, if you're still interested, means that you press
the clutch once in step 2, above, and then another time just before
you perform step 3. The general consensus on double-clutching is
that it is not necessary on modern gearboxes in good condition.
It used to be necessary to synchronize the rotational speeds of
the gearbox internals (and in some cars it still is). It doesn't
hurt anything, except maybe your clutch linkage and the pedal
bushings and the like. It's just not generally necessary.
Heel and toe, on the other hand, is good in any car because it gives
you a greater control over the relationship of the engine to the
drive wheels, lets you decelerate more smoothly, and puts you in a
better position for accelerating out of a corner. In short, it's
a big part of using all the sports car features of ANY vehicle to
their fullest.
Remember, if you spend money on a hot-rod part for a specific car,
any performance enhancement that it gives you goes away when you
drive another car. Money and time spent on driving technique is
the high-performance item you can take with you no matter what you're
driving.
--Scott
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