FormerH22a4

Quick question, if you don't downshift for a stop and simply cruise in for the stop, what is the best way to be gentle on the synchros? If your cruising in 4th and comming to a full stop, can you pop it neutral and coast to the stop, once stopped double clutch and put it in 1st. This is skipping gears isn't it?

I know it is better to be under power when slowing down but I am not there yet in terms of skill so keep that in mind, thanks.

xviper

If you're not going to downshift, you're better off to leave the shifter in the last gear you had it in. At least, if you did need to get going again for an emergency, a higher gear is better than no gear at all. Once you have stopped or almost stopped, moving the shifter into 1st will not harm anything as both your engine speed and road speed will be at a minimum. The "skipping" gears I was referring to is the immediate downshifting from a higher gear into 2 or more gears lower while your road speed has not had a chance to reduce. Doing what you do, there is no need to double clutch. Popping it into neutral with the clutch in while coasting to a stop pretty much isolates the engine from the tranny from the drive shaft. (The engine will return to idle, the tranny will slow to a stop unless your clutch is "grabbing" a bit, and your drive shaft will be turning at whatever road speed you have going.) All three are acting independent of each other. Your engine will "spool" down as will your tranny. The only thing that is slowest to catch up is everything aft of the tranny. Once you have slowed down to almost a stop, aft of the tranny will almost be stopped too.

FormerH22a4

thanks for clearing up the skipping of the gears. I will learn to downshift, I do it already but still not fast enough to do it 2 or 3 times smoothly before a stop from 60kms.

REDGSR

I just wanted to throw out some ideas about clutch and synchro wear with downshifting.
As far as I understand, there is a theoretical difference between simply rev matching and double clutching, but on most modern cars, double clutching is not necessary unless you really want to baby your transmission. So the diffrence between these two methods:

rev matching: disengage clutch, rev motor to proper RPM for lower gear, engage clutch, you will get a nice smooth downshift

double clutch: disengage clutch, put trans into neutral, let out clutch, rev engine to proper speed (rev match), disengage clutch again, shift into lower gear, engage clutch again, you should also get a nice smooth downshift

ok, so, whats the difference between the two?
We all know the powertrain is connected in this manner:

motor-->clutch-->primary gears-->output shaft (driven wheels)


For rev matching you have disengaged the clutch, effectively separating the motor from the rest of the drivetrain. Now you are reving the motor up to a certain RPM. So the motor is gong the correct speed for a lower gear, but since the primary gears are disconnected from the wheels and the motor, it has to use the synchros to get the primary gears to move the same speed as the output shaft(wheels). So in theory you are putting some wear on the synchros, but this is what they were built to do, and its not like you are using the trans to bring the motor up to speed (not bothering not rev - match while downshifting).

OK. so, in double clutching... By disenageing the clutch, putting the car in nuetral, and letting out the clutch, now the motor is separated from the wheels because no gear is selected, but the primary gears are still linked to the motor. Now, when you rev the engine, you are bringing the entire primary gear set up to speed. Now you press the clutch again, put the car in the lower gear, the primary gears are already moving the correct speed, so there is (in theory) there is no wear except the fact that the clutch has to bring the primary gears up to speed (from the first time you rengaged the clutch).

So double clutching is something that you can use, but in fact is probably just excess work and not really needed, even though probably slightly easier on the synchros.

If any of this is wrong, someone please correct me.

xviper

REDGSR: As far as I can tell, you are perfectly correct in your post. Double clutching would be the way to go for those who are really anal. Rev matching is just fine for everybody else. Keep in mind that when a new manual driver is learning, double clutching may be just too much to deal with. Also, when you are new to it, by the time you get the tranny into the next gear, your engine has slowed down to the point that the whole technique is a bit of a bust. There is enough stuff for a new guy to remember - "speed" won't happen till later. In our tranny, D.C'ing is only useful for downshifting into 1st at higher speeds.

boiler

[QUOTE]Originally posted by REDGSR
[B]I just wanted to throw out some ideas about clutch and synchro wear with downshifting.

fg23knight

okay, so to make it all clear, the difference b/n double clutching and rev matching is that, by double clutching we save the wear on the syncro from one gear ratio to another. Or rather, that is the only difference when downshifting by 1 gear. However, in order to downshift 2 gears say 4-2, we must (really should) double clutch since the wear on the syncro from going gear 4-2 will actually wear out the syncro.

and one other question I have: technically, even if you are good, would you downshift all the way say 4-3-2-1 ? or something like 4-3-2 is generally good enough?

Destiny2002

Err.. Uh.. How many times do I have to push the clutch before shifting from 6th into 1st at a red light?

xviper

[QUOTE]Originally posted by boiler
[B]

REDGSR, you may have missed an important

boiler

[QUOTE]Originally posted by xviper
[B]
Once you have pushed the clutch in and have disconnected the tranny from the engine, it doesn't matter that you are revving the engine before or after you shift gears.