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Boro Box Pilot
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Discussion Starter #1
The basis of misunderstanding about double-clutching, rev-matching and heel-and-toe is a lack of understanding of the basic way the engine/clutch/transmission combination works. So take a deep breath and follow along.
What do synchros do? They are used to make the the layshaft match the gear that you are going to. To do the job in a car without synchromesh, you have to make the layshaft go the speed of the new gear yourself.

(Note, I have always just called the gears on the layshaft "the layshaft" so it is easier to visualize. This is a schematic description, not an analysis of which gear slides where on which shaft, or how synchros work, or what oil to use for them. So if you don't agree with the schematic concept I outline here, tell me where I went wrong, and we will look at it. Technical statements about gear movements on shafts or the like will be sent to the bit bucket. I know that modern transmissions are constant-mesh but it really does not change the concept.)

You have three rotating parts in series (1,2,3) with two ways to connect and disconnect them from each other (A B):

1 A 2 B 3

engine -> clutch -> layshaft -> gear selector -> various transmission gears

1) The engine speed is controlled by the throttle alone when the clutch is disengaged ("in"). It is equal to the layshaft speed when the clutch is engaged.

A) The clutch disconnects the engine from the layshaft when you step on the pedal.

2) The layshaft speed is equal to the engine speed when the clutch is engaged. The layshaft speed is related to the road speed when a gear is selected. Therefore, when the clutch is engaged AND a gear is selected, the engine speed is related rigidly to the road speed. (Nobody spins the wheels in this schematic outline!) When the clutch is disengaged AND the selector is in Neutral, the layshaft coasts down freely, regardless of either engine or road speed.

B) The gear selector disconnects the layshaft from the transmission gears when you select "Neutral", and connects the layshaft to a specific transmission gear when you select one.

3) The various transmission gears are being pushed around by the road speeding under your car via the differential and driveshafts. All of them All of them are spinning at different speeds in rigidly defined relationship to each other. If your tires are not slipping, the speed of the system is rigidly proportional to your road speed.

If you have working synchromesh: You are loafing along in Third gear and want to change to Second gear. When you put in the clutch and change the gear selector from Third to Neutral, the layshaft is going a particular speed that matched Third gear at the present road speed. If you do not change your road speed (your brakes are broke, say), the layshaft is going too slowly to match Second gear. As you approach the Second gear selection, the synchros will speed the layshaft up to the same speed as Second gear is now rotating. How do they do that? Ask a mechanic. They just DO it. I have a rough intuitive grasp of how they work, but it does not matter here.

If you have no synchromesh (Crash Box City): You are loafing along in Third gear and want to change to Second gear. When you put in the clutch and change the gear selector from Third to Neutral, the layshaft is going a particular speed that matched Third gear at the present road speed. If you do not change your road speed (your brakes are broke, say), the layshaft is going too slowly to match Second gear. (Sound familiar so far?) As you approach the Second gear selection, the gears go GRAUNCH.

What should you have done? This:


Push in clutch Layshaft equals Third gear speed

Select Neutral Layshaft coasts

Let out clutch Layshaft equals engine speed

Blip throttle Engine and layshaft speed up to Second gear speed and a
little

Push in clutch Layshaft coasts

Select Second No GRAUNCH if you have the speeds right. Ideally,
layshaft has coasted down to exactly Second gear speed

Let out clutch System is all locked up in Second.

This is double-clutching. It INCORPORATES rev-matching, or there is no point in doing it. Therefore, in my opinion, there is no reason to have to say "rev-matching" to describe this process. The rev-matching is the tricky part. How high do you rev it? Experience is the only teacher.

Does double-clutching do any good if you have a synchromesh box? Yes, it makes the synchros work less hard than they were designed to, so they will last longer. The synchros will make up for any errors.

Can you rev-match the layshaft without double-clutching? Yes. Don't use the clutch at all:


Ease the throttle Takes the load out of the system

Select Neutral Layshaft equals engine speed

Blip throttle Engine and layshaft speed up to Second gear speed

Select Second No GRAUNCH if you have the speeds right

This works exactly the same in a synchromesh box as a crash box. Since the layshaft is always connected to the engine, synchros will NOT help you. In fact, I think errors in this method can wreak your synchros. Normally, synchros only have to deal with the rotating mass of the layshaft. Now you are asking them to deal with your whole engine. They ain't gonna like it. You have to be perfect. This is what Rick Mears and many other disgustingly able drivers do. I couldn't do it on a bet.

Can you rev-match the layshaft to the transmission gears by throttle - blipping with regular single clutching? No. The layshaft is not connected to the engine while the clutch is in. Those that recommend throttle-blipping rev-matching are right that it gives a smooth transition when they let the clutch out, but it does absolutely nothing to match the layshaft and does not save your synchros any work at all. (See the way to get around this in autocrossing below.)

OK, here is the hard part. Racing. You are NEVER loafing along in Third gear. The scenario for double-clutching changes: You are blasting along in Third gear and want to change to Second gear for the exit of the corner you are approaching. When you put in the clutch and change the gear selector from Third to Neutral, the layshaft is going a particular speed that matched Third gear at the present road speed. When you brake hard for the corner, the layshaft is going some speed that may or not be right for the speed Second gear will be turning when you are ready to use it. Classic driving style is to double-clutch into Second while braking. This requires heel-and-toe to blip the throttle to rev-match the layshaft. The only reason that this is the "classic" method is because they used the engine braking in Second to help out their lousy brakes.

So here we come to the in my opinion part, where I tell you why I think all the above work is not necessary. In autocrossing with modern brakes, engine braking is counterproductive to slow down for a corner. It upsets your brake balance and does not really help very much in slowing the car down. So instead of revving the engine way up to make Second while you are still going fast, wait to select Second until you are going the entry speed for the corner. The synchros iron out the small difference between the layshaft and Second gear. So instead of revving the engine way up to make Second while you are still going fast, wait to select Second until you are going the entry speed for the corner. The synchros iron out the small difference between the layshaft and Second gear.


Push in clutch and brake together Layshaft equals Third gear speed,
coasts down a bit
Select Second near end of braking The car has slowed, layshaft
coasts while you are still braking, gears
are slowing, no need to hang about in
Neutral. No GRAUNCH, the speeds are
close and the synchros work.
Off brakes and on throttle Here is where you do the rev matching so
that there is no big jerk when you let
out the clutch
Let out clutch It is still well before the (late) apex.
Add power Well, this what you came here for. Go!
Mind the attitude and unwind the wheel.
Let out clutch It is still well before the (late) apex.
Add power Well, this what you came here for. Go!
Mind the attitude and unwind the wheel.

You will note that the actual downshift is one move, and each foot has only one job to do. I am convinced that this method is the second-fastest way to do this. Rick Mears will be able to do the left-foot-brake, no-clutch method faster, due to the lack of transfer time of the right foot from brake to throttle. But since most of us cannot pull that off at all, I think my method is the fastest for non-gifted mortals. I always say to make the synchros work for a living, but if you don't shift too soon, they don't work very hard.

This works very well with a streetable car having a normal flywheel and synchros in an autocross situation. The reason it will not work as well in road racing is that the braking time is longer. A light flywheel that allows the engine to rev down faster is not going to help either.

---------------------------------------
does this mean my current method of downshifing is incorrect?
say i'm in 3rd...push in the clutch, blip the throttle, drop it to 2nd. done. there is no jerk. but according to this article, it is still hard on the syncros...true? do ppl do what i do or do u guys do what the article does(IMO it's quite a bit slower no?)
 

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Boro Box Pilot
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Discussion Starter #2
aww, no one with insight? :sad:
 

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Funny, I use the method he recommends (shifting down before accelerating out of the apex) but I have been trying to get used to downshifting earlier (on approach to corner) because I thought that was what I was 'supposed' to do (to take advantage of engine braking). I've also tried double-clutching but it seems WAY too inefficient and doesn't seem to make any difference, but I guess that's because I have a 'modern' manual transmission and no double-clutching is necessary, according to that article.

I guess I'll go back to the way I was doing it all along. :wink:

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<font size=-1>[ This Message was edited by: Bishamon on 2002-06-17 12:20 ]</font>
 

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i read that article, or something like that when i was back in high school. took me a while to understand but after knowing exactly how it all comes together is a bonus (read: take automotive classes)

explaining the above article to someone is very difficult.. i should print that out and hand out copies to those wanting to know.
 

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Boro Box Pilot
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Discussion Starter #7
On 2002-06-17 15:42, trueno92 wrote:
i read that article, or something like that when i was back in high school. took me a while to understand but after knowing exactly how it all comes together is a bonus (read: take automotive classes)

explaining the above article to someone is very difficult.. i should print that out and hand out copies to those wanting to know.
well as far as i know, my current method of rev-matching when downshifting is OK since i have a modern transmission. just that it doesnt stop the syncros from working as hard, they still have to work as hard because i am not syncronizing the speed of the layshaft with the rest of the parts!!
but if this is so, how come there is no jerk when i downshift? ah....syncros do that...
now i'm even more confused....
um so...just simply depressing the clutch, blipping the throttle and bringing it down a gear still hurts the tranny??
 

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the key is to get the input shaft and drive shaft at the same speed, hence reving the engine before you let up the clutch on the downshift.


PLEASE DON'T DO:
pushing in the clutch
taking foot off gas
engine is now at idle
force lever into lower gear
let up clutch.

that is not really correct engine braking.

DO:
CLUTCH
REV MATCH TO THE DOWNSHIFT GEAR'S RPM THEN SHIFT INTO LOWER GEAR AND THEN LET OUT CHUCH ONCE THE REVS ARE UP.

THIS SAVES YOUR TRANNY AND MAKES YOU A BETTER DRIVER. NOT TO MENTION EASIER ON THE DRIVE TRAIN.
 

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MECHANICAL PROOF:

input shaft/output shaft are approximately at the same speeds as each other after the rev, so putting the lever into gear is much easier on the tranny since the input/output shafts are at teh same speeds.
 

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Boro Box Pilot
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Discussion Starter #10
On 2002-06-17 18:37, trueno92 wrote:


DO:
CLUTCH
REV MATCH TO THE DOWNSHIFT GEAR'S RPM THEN SHIFT INTO LOWER GEAR AND THEN LET OUT CHUCH ONCE THE REVS ARE UP.

THIS SAVES YOUR TRANNY AND MAKES YOU A BETTER DRIVER. NOT TO MENTION EASIER ON THE DRIVE TRAIN.
i do that, but according to the article this doesnt save any work for the syncros...
it says to push in the clutch, shift to neutral, let out the clutch, blip the throttle, press in the clutch and put it down a gear... but WHY?! that's soooo slow. i just do as u said...press in the clutch, blip the throttle to match the revs of the lower gear, and drop it down a gear..then let out the clutch...
who the heck just lets out the clutch without rev matching it lol
 

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what i described was a good way to downshift,

double clutching is getting the shafts at the same speed BEFORE you are going into gear.

either way, they both save your syncros.
 

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operation boro thunder
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man luckily i went on howstuffworks a few days ago, otherwise i would have NO CLUE what you guys just said... :sad:

and the moving diagrams there make life a lot easier

question tho: what IS rev matching and "blipping" the throttle?
 

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TN Track day addict
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On 2002-06-17 22:56, project trueno wrote:
man luckily i went on howstuffworks a few days ago, otherwise i would have NO CLUE what you guys just said... :sad:

and the moving diagrams there make life a lot easier

question tho: what IS rev matching and "blipping" the throttle?
u have to be in a stick car to explain that, otherwise, you'll still be confused.
 

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Boro Box Pilot
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Discussion Starter #14
On 2002-06-17 21:32, trueno92 wrote:
what i described was a good way to downshift,

double clutching is getting the shafts at the same speed BEFORE you are going into gear.

either way, they both save your syncros.
ok good, all i needed to know is that my method of downshifting saves my syncros.
thank goodness
 

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hmmmm i think i can understand most of what you guys are talking about. so rev mathching is the key to smooth double clutching?
 

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i always thought it was bad to gas and clutch at the same time...so his method of neutral, rev, drop was what i'd follow.

also, since i don't have a manual...i'd guess by rev matching it means each gears own RPM for a certain speed?(kinda guessing when ur driving then?)
 

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Ok, I don't drive a manual, but I don't understand why when you're racing would you put it into neutral in the middle of a corner? It's my understanding that if you want the best traction possible, you want to be IN gear because you need to control the throttle throughout the corner. Being in gear also saves your brakes.
 

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just want to clarify: so it IS possible to shift entirely without using the clutch in a car with a syncro gearbox? I can get it to shift out into neutral without the clutch, but never yet dared to try shifting into the next gear without it.
 

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operation boro thunder
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i guess it IS possible, but it'll hurt your transmission, its kind of like grabbing something while its spinning, it'll hurt your hand....best if you disconnect the engine from the tranny first, then change gears, the clutch will last longer
 
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