Grab a chair... here's another Tome.... :-)
You innocently asked...
>I understand how the engine & tranny are
>connected, et cetera. But I really don't understand how the *INSIDES*
>of a manual transmission work.
I put this first 'cause understanding how a manual tranny works might help
understand how Heel and Toeing and Double Clutching work.
---------
Manual Tranny 101:
Since this is 101 and not the graduate class I've simplified things a bit.
This is a description for your basic 4sp RWD manual transmission. FWDs are
a bit more complicated, as are 5sp Overdrive units, but the concept is
identical.
There are basically 3 shafts in a manual transmission. There is the input
shaft, the output shaft and the idle gearset (also on its own shaft)
The input shaft sticks out the front and is splined. The clutch disc
(which sits between the pressure plate and the flywheel) slips onto this
spline. When the pressure plate (which is bolted to the flywheel) is
released, it squeezes the clutch disk between itself and the flywheel. The
input shaft is then driven through the spline at the same speed as the
engine.
The output shaft sticks out the back and is splined to receive the end of
the driveshaft. Through the driveshaft, its spline and the rear axle the
output shaft always turns with the wheels proportionally to road speed.
The input and output shafts are in line with each other. In top gear (no
overdrive) the input and output shaft are mechanically connected (I get
into how below), the tranny is delivering a 1:1 gear ratio, and the engine
speed is the same as the driveshaft speed.
The idle gearset lays alongside the input shaft and output shaft. It
allows gear ratios other than 1:1. It is driven by the input shaft; each
has a gear on it that are constantly connected to each other.
The key to achieving different gear ratios is how the input shaft is
connected to the output shaft through the idle gearset...
Along the idle gearset are other gears. All of these gears are literally
part of the idle gearset; one piece of metal. (It is hollow and spins on a
metal shaft through sets of roller bearings.) Each of these other gears is
a different diameter and is matched with a mating gear or the proper
diameter on the output shaft. The gears on the output shaft spin freely on
the output shaft, also on a set of roller bearings.
The key to having different gear ratios is to figure out a way to
mechanically connect one of the gears spinning on the output shaft TO the
output shaft itself. Once this connection is made, the power from the
engine will go:
Flywheel -> clutch -> input shaft -> input shaft gear ->
idle gearset gear mated to the input shaft ->
idle gearset gear spinning the selected output shaft gear ->
Output shaft gear of choice -> through the "locking mechanism"
betweem the output gear and the output shaft->
out the output shaft to the driveshaft -> through the rear axle to the wheels.
OK... so what is this "locking mechanism?"
It is the gear selector and syncro set, and it sits on the output shaft...
Along the output shaft of the transmission, you will find the following:
gear (say 1st)...syncro...1st/2nd selector...syncro...2nd gear
The 1st/2nd selector consists of an inner and outer sleeve. The inner
sleeve is splined to the transmission output shaft. The outer sleeve is
splined to the inner sleeve, and has a slot on the outside diameter. The
shift fork fits into this slot. As you move the gear selector between 1st
and 2nd, you are moving the shift fork up and back, thus moving the outer
sleeve of the 1st/2nd selector up and back over the inner sleeve.
The 2nd gear synco is a brass ring which slip fits over the output shaft.
At one end it has cone-shaped teeth that match the spline between the inner
and outer sleeve of the 1st/2nd selector, pointed toward the selector. At
the other end, the synco is cone-shaped, and fits by friction up under a
matching surface on the 2nd gear.
Second gear also slip fits over the output shaft. It has two sets of
teeth. The large set are helical, and are connected to the input shaft via
the idle gearset. The small set are straight cut, and are the same pattern
as the teeth that connect the inner and outer sleeve of the 1st/2nd
selector, and of the syncro.
When you select 2nd gear, you are sliding the outer sleeve of the selector
over the matching teeth of 2nd gear. This mechanically ties the input
shaft to the output shaft:
Input shaft -> idle gearset -> 2nd gear -> outer sleeve of selector
->
inner sleeve of selector -> output shaft.
The idle gearset is also turning the other gears, but this is OK since,
like 2nd, they slip fit on the output shaft. The only mechanical link to
the output shaft is through 2nd via the selector sleeve.
So what does the syncro do? It provides a mechanical bridge between the
outer sleeve of the selector and the matching gears on 2nd gear. As the
sleeve encounters the syncro, it engages it easily since the teeth are cone
shaped... designed for this. Then the syncro helps to spin up the 2nd
gear to the same speed as the selector sleeve via the cone shaped, friction
fit between the syncro and the gear. This allows the sleeve to then slide
over the matching portion of 2nd gear without grinding off teeth. As you
use the transmission, the friction surface and cone shaped teeth of the
brass syncos wear and become less effective. Eventually, you have to
replace the syncros...
There will be a test on this next Tuesday... :-)
>Oh.. one last question. Could somebody (Steve?) please explain double
>clutching to me and its {dis}advantages?
>
>Recently I've been double-clutching and rev-matching downshifts and
>noticed that when I rev-match right using this method, the shifter goes
>into the gates in a much smoother manner. Is this because I'm
>pre-synchronizing the synchros with the rev match so the notchiness
>(normally caused by the synchros) is eliminated?
The only disadvantages I can think of are
1. if you're concentrating on heel and toeing you might not be
concentrating on the other aspects of driveing... where you're going; are
you going too fast...
2. while Heel and Toeing you have less of your foot on the brake pedal and
if you suddenly have to hit the brakes HARD your foot might slip or not
generate enough force.
There are no mechanical disadvantages for the car; in fact there are
advantages...
Here are descriptions of each:
HEEL AND TOE DOWNSHIFTING (And REV MATCHED DOWNSHIFTING at no extra cost)
Here's a description of the technique of Heel and Toe from a teacher at a
performance driving school:
=-=-=-=
Downshifting - In order to accelerate quickly out of a corner, the car
must be shifted to the appropriate lower gear before the corner.
Naturally this should be done as late as possible, preferably at the same
time that you are braking for the corner.
This presents a small problem. If you are braking and shifting, your
right foot is on the brake, and your left foot is on the clutch. Without
throttle input, the engine RPM's will drop quite low and when you let the
clutch out, you will 'pop' or jerk the rear wheels. This sudden jerk acts
as an additional brake, but could cause the rear wheels to exceed their
traction limit and cause a skid, to say nothing of the potential damage to
either the engine or drivetrain! The solution lies with "heel and
toeing". This technique during downshifting helps us achieve smoothness,
while eliminating possible clutch slippage and extending the life of the
transmission.
This technique is one that should be thoroughly practiced until it becomes
second nature. It can not be learned on the track. Here's how it goes:
1) As you begin braking with your right foot, depress the clutch with your
left foot. (so far, just like normal). 2) With the clutch all the way
in, two things happen: move the stick shift into the desired lower gear,
and rotate your right foot so that the ball of your right foot is still on
the brake, and your heal is over the gas pedal. 3) Now "blip" the
throttle to raise the RPM's by about 1000 - this matches the RPM's. 4)
Now release the clutch, and ease off the brake.
Here's a real life example. You're going down the back straight 100mph,
4th gear, and 6000rpm. Ahead is a 90 degree right turn coming up quick,
and the exit speed of this corner is 60mph. Exiting the corner at 60mph
in 4th gear doesn't work, so you have to downshift. Just before the
corner, brake in a straight line. In just moments you've slowed to 80mph,
and you should now depress the clutch. Now rotate your right foot, shift
into 3rd gear, and blip the throttle. While you are rotating your foot
and blipping the throttle, you are still slowing down, and you are now
going about 60mph. When the tach reaches about 5500rpm, let the clutch
out smoothly, and ease off the brake. That's it. Now go out on the
street and practice! (you can practice 3rd to 2nd gear downshifts on the
streets - 3rd gear and 45mph into a 2nd gear, 20mph corner.) This example
only works if you are turning 6000rpm in 4th gear at 100mph and 5500rpm in
3rd gear at 60mph, but you get the idea.
=-=-=-=
Here's mine from a previous post:
The idea of heel and toe downshifting is to allow maximum braking force
-while- selecting the lower gear. Consider that you are braking hard
coming into a corner. If you now use your feet just on the brake and
clutch as you downshift, you will generate a lot of engine braking as you
release the clutch. With the tires already at a limit under heavy braking,
this engine braking can cause the driven end (front in FWD...) to lock
up, thus loosing traction. By matching revs, you select the lower gear
without upsetting the car's balance. And now, you can accelerate out of
the corner. :-)
As an aside, you may not necessarily have to match revs -exactly- as you
release the clutch. The idea it to prevent engine braking forces. As you
prepare to release the clutch, you can blip the throttle with the right
half of your foot to a point -above- the matching RPM. Then, as rpms fall,
release the clutch. As long as you are a bit above or equal to the
matching RPM, you will be OK. There should not be a significant amount
change to the car's balance generated by bringing the engine rpm -down- to
the needed speed. This modified technique still requires good timing and
coordination, but you don't have to concentrate as hard on the required
RPM.
I find that the rev-matching idea underlying the technique of heel and toe
downshifting is useful even when I'm not burying myself into a corner... I
always Rev Match when I downshift (preparing to pass, for example) as a
method of minimizing driveline shock.
Also, please be prudent if you decide to attempt to learn these techniques.
Rely on -your- judgement, not my opinions...
=-=-=-=-=
I use it all the time for the practical purpose of prolonging the life of
my clutch. I can't believe how many people downshift into a lower gear by
basically just dropping the clutch and FORCING the engine rpm to rise up.
With heel and toe, you tap the gas while simultaneously braking (all with
the right foot) to "pre-raise" the engine rpm before dropping the
clutch.
If you are just downshifting in preparation to pass or the like, you may
not need to really Heel and Toe because you aren't on the brake. You can
Rev Match the downshift by tipping into the throttle a bit when declutching
so that the revs are matched.
In each case, the result: the clutch engages with the engine (and thus the
flywheel) already matching the same rpm. In principle, if you were
perfect at it, there would be no clutch slippage at all. Hence: no clutch
wear. Try it, heel and toe and rev matching become second nature very
quickly. Save your clutch and have fun downshifting at the same time!
DOUBLE CLUTCHING
Recall the basic idea behind Rev Matching (and Heel and Toeing)...
minimize driveline shock by matching the flywheel speed to the clutch disc
speed as you engage the clutch. Double Clutching takes this one step
further. It matches the input and output side speeds of the transmission's
syncros. So ideally you would not need syncros. In fact many traditional
race cars with straight cut gear boxes do not have syncros and you have to
double clutch to drive them.
Here is Rev Matching (Heel and Toeing just adds the brake...):
1 Clutch in
2 Select lower gear
3 Tach up to engine RPM that will occur when the clutch is released
<minimizes clutch wear and engine braking>
4 Clutch out
Here is Double Clutching
1 Clutch in
2 Tranny to neutral
3 Clutch out
4 Rev the engine in neutral to spin up the transmission input shaft speed
to get the engine near the rpm that will occur in the lower gear.
(now both sides of that gear's synco are spinning at the same speed)
5 Clutch in
6 Slip into the gear
<minimizes syncro wear>
7 Tach up to engine RPM that will occur when the clutch is released
<minimizes clutch wear and engine braking>
8 Clutch out
Note that if you double clutch downshift in a corner you achieve the same
purpose with repect to minimizing engine braking as if you heel and toe
downshifted. Double clutching takes more time and is more difficult to
master, so if your syncros are in good shape, heel and toeing is easier.
I don't bother to double clutch.
I rev match and heel and toe all of the time.
---Steve Cutchen O- | As published in Houston Chronicle Sports
scutchen@airmail.net | http://web2.airmail.net/scutchen