Double-Clutching - Stupid Question Alert

[MuseChaser]

Thanks to all for helping me out with this. The post about input shaft being connected to the engine when the clutch was out was that knowledge nugget I was missing.

I still don't have any problem getting my car (2013 w/ 24K on it now) to downshift into 2nd or even 1st, as long as I get the engine revs up to the appropriate point first, w/out double clutching. Is it possible that some folks are underestimating just how high the revs need to be if you really want to downshift into first at 20mph (not that there's a reason to want to do that, at least that I can think of, at least not in normal real life public driving)?

Thanks again.. this has been enlightening.

Barry

[Tcoat]

Originally Posted by Ultramaroon

The syncros on my transmission did not hook up very well until they finally broke in. I went through a good 9000 miles religiously double-clutching my downshifts because spinning up the input shaft took way more effort than I was comfortable doing.

I'm at 18000 miles now and the syncros work beautifully. I hope they stay that way.

I still double-clutch into second if I miss my 1-2 shift when it's cold. And always into first if I'm moving at all. Just won't go otherwise.

I think there's some variation in the builds of this transmission because a friend's car will slip into first effortlessly up to 10 mph. Most other's are like mine - really picky with the lockout.

I am also one of the lucky ones. Cold has never had any effect at all on second, can downshift into first at 15 MPH (probably faster but afraid to try it) and other than the notchiness, which I like, I have never had any issue going into any other gear. Throw out is silent, no goose honk and shifting is a breeze. All with almost 50K miles on the original oil.
Reading all the issues here I sometimes wonder if mine is actually broke somhow but I am not going to mess with it.

[Ultramaroon]

Originally Posted by Tekno Virus

I'm not sure the physics behind double-clutching but damn does it help when the tranny is cold and I need to go to 2nd gear.

Gearbox is spinning through molasses when it's cold. As soon as you slip into neutral with the clutch disengaged, the input side literally grinds to a halt.

Upshifting quickly through neutral is not a big deal if you don't pause because the input side slows down to match speed.

Downshifting, or pausing in neutral is a different story because then the synchro of the target gear has to do the work of spinning up the input side through the molasses.

Double-clutching brings the input side back up to speed so it can sink into gear.

[Tcoat]

Originally Posted by Ultramaroon

Gearbox is spinning through molasses when it's cold. As soon as you slip into neutral with the clutch disengaged, the input side literally grinds to a halt.

Upshifting quickly through neutral is not a big deal if you don't pause because the input side slows down to match speed.

Downshifting, or pausing in neutral is a different story because then the synchro of the target gear has to do the work of spinning up the input side through the molasses.

Double-clutching brings the input side back up to speed so it can sink into gear.

Maybe I just shift fast enough that the input shaft speed is the same as if I had double clutched. If the input shaft "grinds to a halt" as soon as the clutch is press the first time the it should do the same the second time you hit the clutch, even if you did blip. Am I missing something?


Shaft 1,000 rpm
Hit clutch and shaft stops.
Shift to neutral
Blip to 1,000 rpm
Hit clutch and shaft stops.
Shift into gear

[Hotrodheart]

Even with synchos, I suspect that double clutching reduces wear. particularly on the synchos.


But the real reason is: Heel and toeing is a skill. It's fun when you get it right.


This comes from someone who's FR-S is an automatic.


But in the old days.....

[MuseChaser]

I know I just thanked Ultra for elaborating and explaining the issue, then thanked Tcoat for questioning the elaboration. I get what you're saying, Ultra, but I'm also as confused as T is about it; if the input shaft is disconnected when the clutch pedal is depressed, why would it stay spinning during the shift from neutral down to the desired gear (clutch depressed) anymore than it would any other time the clutch is depressed?

Not arguing.. just trying to figure this out.

Thanks again..

Barry

[Ultramaroon]

Originally Posted by Tcoat

Maybe I just shift fast enough that the input shaft speed is the same as if I had double clutched. If the input shaft "grinds to a halt" as soon as the clutch is press the first time the it should do the same the second time you hit the clutch, even if you did blip. Am I missing something?


Shaft 1,000 rpm
Hit clutch and shaft stops.
Shift to neutral
Blip to 1,000 rpm
Hit clutch and shaft stops.
Shift into gear

I think you're so experienced that you have your timing down pat. You don't pause long enough in neutral to feel the effort required after missing that golden moment.

Try this tomorrow morning. Get going in first and intentionally pause in neutral. Then feel if it takes any extra effort to engage second. You might not notice it with such a broken-in transmission but I'm betting you'll feel at least a little difference.

Taking into consideration that I upshift at about 2500 rpm when it's cold, recovering from a missed shift is more like this:

Input shaft 2500 rpm
clutch disengaged - input shaft still spinning while in gear
neutral - input shaft grinds to a halt
clutch engaged - blip to 2500 rpm (I don't actually watch the tach)
disengage clutch - drop into second before it has a chance to spool down
keep calm and drive on

You will also feel the increased force required to disengage first if you pause after pushing the clutch. Just drag from the molasses.

[Ultramaroon]

Originally Posted by MuseChaser

I know I just thanked Ultra for elaborating and explaining the issue, then thanked Tcoat for questioning the elaboration. I get what you're saying, Ultra, but I'm also as confused as T is about it; if the input shaft is disconnected when the clutch pedal is depressed, why would it stay spinning during the shift from neutral down to the desired gear (clutch depressed) anymore than it would any other time the clutch is depressed?

Not arguing.. just trying to figure this out.

Thanks again..

Barry

Ah, but it is still being spun while it's in gear. It's that pregnant pause in neutral that gets people on cold mornings. Timing is everything.

I might have answered the question already but it's worth emphasizing.

[Tekno Virus]

I don't know why going to neutral first then any gear would take the load off the synchros although it does.

Once the clutch is depressed, the shafts are still spinning and the gears are constantly meshing however there should not be any torque going through any of the shaft in single clutch or double clutch, so it begs the question why does going to neutral first make shifting a whole lot easier on the synchros?

If the clearances aren't as tight like in a Honda or VW, yeah, you can shift in a zippy, but man, this car on a cold day, double-clutching is the only viable way of shifting for the first few minutes or so.

[MuseChaser]

GOT IT, I think! The input shaft is spinning anytime the gearshift isn't in the neutral position; the clutch has no bearing on the input shaft, and the input shaft stops spinning only when in neutral, whether or not the clutch is engaged. The clutch connects the output shaft to the input shaft, but has no direct relation to the input shaft. Yes?

If so, then that clears things up quite a bit.

Gotta love this place.. thank you.

Barry

[Ultramaroon]

Originally Posted by MuseChaser

GOT IT, I think! The input shaft is spinning anytime the gearshift isn't in the neutral position; the clutch has no bearing on the input shaft, and the input shaft stops spinning only when in neutral, whether or not the clutch is engaged. The clutch connects the output shaft to the input shaft, but has no direct relation to the input shaft. Yes?

If so, then that clears things up quite a bit.

Gotta love this place.. thank you.

Barry

Awww... nope.

The clutch connects the engine to the input shaft.

The input shaft is connected to the output shaft at a ratio of the gear chosen.

...unless it's in neutral.

[MuseChaser]

Sigh.. maybe it's getting too late for me think straight. Thanks for your patience... let me try this again...

Originally Posted by Ultramaroon

Awww... nope.

The clutch connects the engine to the input shaft.

OK.. now I'm back to where I ORIGINALLY thought things were. When the clutch is depressed, there is no force from the engine applied to the input shaft.

Originally Posted by Ultramaroon

The input shaft is connected to the output shaft at a ratio of the gear chosen.

....whether or not the clutch is engaged.. in other words, if the transmission is in third gear and the car is coasting (clutch disengaged, no connection to the engine) at 40mph, the input shaft is still turning at whatever rpm it would turn if the engine was connected and powering the car at 40 mph, yes?

Originally Posted by Ultramaroon

...unless it's in neutral.

.. because when the transmission is in neutral, THAT'S what disconnects the input and output shafts, not the clutch. The clutch disconnects the crankshaft/flywheel from the input shaft.

If you let the clutch out when the tranny's in neutral, the input shaft spins at the ENGINE rpm instead of the tranny rpm, but has no effect on the output shaft because the tranny is in neutral. This keeps the input shaft revs up to better match when the transmission is cold just prior to eventually shifting into the desired gear.

Have I got it right now?

If not... I'm going to bed anyway..<grin>.

In any case, Tcoat's question, and mine, still remain. Once you push in the clutch, the input shaft is disconnected from the engine and will continue slowing until you get the gear selected. How is that any better than rev matching and shifting through neutral without undue delay? I guess I'm so used to keeping the engine and the wheel revs synced for shifts that I've never really noticed a problem.

Am I closer now?
Thanks again.

Barry

[Ultramaroon]

Originally Posted by MuseChaser

Have I got it right now?

If not... I'm going to bed anyway..<grin>.

Yup. That's it!

Originally Posted by MuseChaser

In any case, Tcoat's question, and mine, still remain. Once you push in the clutch, the input shaft is disconnected from the engine and will continue slowing until you get the gear selected. How is that any better than rev matching and shifting through neutral without undue delay? I guess I'm so used to keeping the engine and the wheel revs synced for shifts that I've never really noticed a problem.

Am I closer now?
Thanks again.

Barry

Sorry, I'm lost here. We can pick up tomorrow if you like. Basically, as others have said, there is no need to double-clutch any modern synchronized transmission. It is merely an exercise for us obsessive-compulsive types to satisfy our need to feel like we're doing everything we can to preserve a few grams of bronze and hopefully get a few thousand extra miles out of the tranny.

That being said, it's a fun and interesting thing to learn. Next step will be to smoothly up and down shift entirely without the clutch. You know, so you don't have to wait for a tow truck when the hydraulics take a shit.

[Tcoat]

Originally Posted by Ultramaroon

I think you're so experienced that you have your timing down pat. You don't pause long enough in neutral to feel the effort required after missing that golden moment.

Try this tomorrow morning. Get going in first and intentionally pause in neutral. Then feel if it takes any extra effort to engage second. You might not notice it with such a broken-in transmission but I'm betting you'll feel at least a little difference.

Taking into consideration that I upshift at about 2500 rpm when it's cold, recovering from a missed shift is more like this:

Input shaft 2500 rpm
clutch disengaged - input shaft still spinning while in gear
neutral - input shaft grinds to a halt
clutch engaged - blip to 2500 rpm (I don't actually watch the tach)
disengage clutch - drop into second before it has a chance to spool down
keep calm and drive on

You will also feel the increased force required to disengage first if you pause after pushing the clutch. Just drag from the molasses.

You may be onto something because I don't pause in neutral on any shift. I will try that since it is bloody cold this morning.