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Hey guys, the Evo will be my first foray into manual transmissions and I was hoping someone could clear up a question I have about double clutching. I understand how and why it's done, but I'm wondering why the "double" is necessary. Why can't you just press clutch --> shift to neutral --> blip throttle --> shift to lower gear --> release clutch? What happens differently between being engaged in neutral and being disengaged? I've asked people as well as Google but can't find a good answer.
 

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From everything I've read, double clutching is totally redundant on cars with synchros. As long as you rev match as described, should be fine.
 

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Some further reading tells me that having the clutch engaged (pedal not pressed) while rev matching will allow the input shaft to be rev matched also, which reduces the stress on both the clutch disc and the synchros. Doesn't seem to be really needed though in everyday driving.

Got the info here:
http://www.wisegeek.com/what-is-double-clutching.htm
 

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From everything I've read, double clutching is totally redundant on cars with synchros. As long as you rev match as described, should be fine.
Exactly right. Just rev match and move on. Double clutch rev matching can be a bear if you cannot heal-toe well, not to mention the possibility of newbs throwing the shifter into gear during one of the 2 clutch engagements.
 

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Discussion Starter #6
Some further reading tells me that having the clutch engaged (pedal not pressed) while rev matching will allow the input shaft to be rev matched also, which reduces the stress on both the clutch disc and the synchros. Doesn't seem to be really needed though in everyday driving.

Got the info here:
http://www.wisegeek.com/what-is-double-clutching.htm
Is the input shaft fixed to the engine or transmission?
 

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You can double clutch a down shift to get a smooth transion and have no lung forward right before taking a corner. Again not really needed, but it does allow for a smoother and faster down shift. You can also upshift with it as well to keep RPM high for a shift up, HORRIBLE ON A CLUTCH!! usually done when racing on occasion. Just my 2 cents from what ive heard.
 

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By double clutching, you can synchronize the shaft speeds in the transmission before doing the shifting, thereby doing the job of the synchros. It is less important in a synchronized transmission, but doing so, you significantly reduce the stress on the synchros. Since our transmission's synchros are not very strong (I presume due to packaging issues; not much space there for a hefty transmission ;)), double clutching could significantly improve your tranny's life.

Since I had 4th gear grinding issue with my '05 STi in year 2005, I always double-clutch for down shifts.

Oh well, do whatever you want :)
 

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I think maybe he just wanted a way more simple explanation like:

Engine (Crankshaft) -> flywheel -> Clutch -> Transmission Input shaft -> Transmission -> Driveshaft/diffs -> wheels

When you press the clutch you disconnect the engine from the transmission by seperating the clutch from the flywheel. When you shift into neutral you disconnect the transmission from the input shaft.

When you change gears, the speed of the input shaft has to change somehow with that gear change. If you downshift (for example) then you know you have to rev the engine to match the vehicle speed. But how does the input shaft to the transmission get accelerated before trying to engage with the new lower gear? The evo has synchros, as was mentioned, that as you move the shifter into place act as a friction surface between the input shaft and the gear and accelerate the input shaft to the same speed so the gears can mesh. Double clutching is where you instead re-engage the clutch while in neutral, rev the engine which will now also rev the transmission input shaft, dissengage the clutch so you can now shift into the new lower gear and the input shaft is roughly the same speed as the gear you are selecting. This, as mentioned, takes stress off of the synchros as they don't have to do the work of accelerating the input shaft (the engine did).

That said, as far as i know it's completely unecessary in the evo. the synchros can handle the acceleration of the input shaft just fine.

But for fun, if you want to feel this. Next time you are sitting letting the car warm up, put it in neutral with the clutch engaged. Then press the clutch pedal and push the shifter into first. If your on a smooth flat surfaceyou should be able to feel the car roll a little bit forward. This is the energy in that input shaft being transferred through the synchros to the transmission and then to the wheels.

This is also why sometimes it feels like the shifter just falls into place when doing a normal upshift. It's because that transmission shaft will natural slow down to to friction and if it's speed matches the gear you are trying to shift into then it will slide in very easilly.

You can also feel this if you break suddenly from a higher rpm and both dissengage the clutch and shift to neutral right away. Well that input shaft to the transmission will be free spinning at the higher speed and slowing down (due to friction). So if you then try to slot it into first with the car travelling at a slow speed and the clutch still dissengaged, if you apply just a little bit of pressure, when that input shaft slows down to match the vehicle speed for that lower gear, then all of a sudden the gear shift should just fall into that gear. hardly any pressure or force required.
 

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The synchor system has to accelerate more than just the input shaft - it has to speed up the clutch disk(s) also. So there can be some significant mass involved.

Other than double-clutching can be slightly faster in a synchromesh gearbox than single clutching, there is little reason to do it other than as practice for when you might drive a real race car (with dog gears and no synchros).
 

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i double clutch in trucks or it wont go in gear on the downshift it will just grind and vibrate the hell up yer arm lol.

Double clutching also wastes time, and isn't practical in cars especially when dragging.

Granny shifting is the way forward in an evo. What ever granny shifting means??? normal/common way of using the clutch i take it??
 
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