986 Forum - The Community for Porsche Boxster & Cayman Owners - View Single Post - Cursed by the double-clutch

David N. · 11-07-2006, 11:19 PM

Keep in mind the synchros are simply used to match speed between the drive shaft and the layshaft in the transmission, so they don't have much to do with rev-matching if you don't engage the transmission in neutral at some point between the starting gear and the ending gear.

Just think of when you shift to a new gear while the clutch is pressed in, the amount of resistance there is on the stick itself is a direct result of how much rpm difference there is between the driving shaft (what the wheels are doing), and the layshaft (a freely spinning shaft that goes from the open end of the clutch to the gearbox). The whole point of double-clutching is to use the engine to change the speed of that layshaft while in neutral so it matches the driveshaft when you shift into it in the new gear. Technically, if you could match it perfectly, you wouldn't even need to press the clutch the second time, you could just engage the dogs on the new gear with the clutch engaged.

I first got annoyed by synchro wear when I would shift from first to second really fast and notice a great deal of resistance. I compared it to a friend's 300zx with a 1-2 shift that was like butter, and wondered if my second gear was just worn or something. Then it hit me 1-2 has the largest general rpm drop, and what I was feeling was the synchros trying to slow down the layshaft so the dogs would engage. Wanting to spare that stress on the synchros, I figured I needed to think about a better technique. The solution for an upshift is simple, move the stick to neutral and wait a little bit for the layshaft to slow down by itself before shifting to the higher gear and voila, much better feeling 1-2 shift. But to downshift...you need to use the engine to speed up the layshaft while in neutral, ergo...double-clutching.

At the end of the day John V is probably right, synchros aren't gonna feel the pain. But it never hurts to know how it's working and what you can do

(BTW, the reason the 300zx stick felt like butter is because of a design that uses two synchros instead of one, nice little feature)

-David

11-07-2006, 11:19 PM	#13
David N. Registered User Join Date: Sep 2006 Location: Los Angeles Posts: 373	Keep in mind the synchros are simply used to match speed between the drive shaft and the layshaft in the transmission, so they don't have much to do with rev-matching if you don't engage the transmission in neutral at some point between the starting gear and the ending gear. Just think of when you shift to a new gear while the clutch is pressed in, the amount of resistance there is on the stick itself is a direct result of how much rpm difference there is between the driving shaft (what the wheels are doing), and the layshaft (a freely spinning shaft that goes from the open end of the clutch to the gearbox). The whole point of double-clutching is to use the engine to change the speed of that layshaft while in neutral so it matches the driveshaft when you shift into it in the new gear. Technically, if you could match it perfectly, you wouldn't even need to press the clutch the second time, you could just engage the dogs on the new gear with the clutch engaged. I first got annoyed by synchro wear when I would shift from first to second really fast and notice a great deal of resistance. I compared it to a friend's 300zx with a 1-2 shift that was like butter, and wondered if my second gear was just worn or something. Then it hit me 1-2 has the largest general rpm drop, and what I was feeling was the synchros trying to slow down the layshaft so the dogs would engage. Wanting to spare that stress on the synchros, I figured I needed to think about a better technique. The solution for an upshift is simple, move the stick to neutral and wait a little bit for the layshaft to slow down by itself before shifting to the higher gear and voila, much better feeling 1-2 shift. But to downshift...you need to use the engine to speed up the layshaft while in neutral, ergo...double-clutching. At the end of the day John V is probably right, synchros aren't gonna feel the pain. But it never hurts to know how it's working and what you can do (BTW, the reason the 300zx stick felt like butter is because of a design that uses two synchros instead of one, nice little feature) -David __________________ 1995 Silverado V8 - Green/Tan (FOR SALE) 2000 Boxster S - Ocean Blue/Graphite Grey 2002 GSXR750 - Blue/White http://www.darkoven.com/sig.jpg Last edited by David N.; 11-07-2006 at 11:54 PM.