How do you down shift when multiple gears need to be dropped

[Jad]

I believe Kim question is very valid, though I can not give a definite answer as there are several parts of the drive train all doing their own thing. As I understand it, the synchros adjust the speed of the transmission. Thus, when they wear you get the grinding when trying to move the gear shift into gear on downshifts (while clutch is disengaged). This can be overcome by the old double clutching technique which is matching engine revs WHILE the clutch is engaged. Otherwise, rev matching just makes the shift smooth, but doesn't help synchros. My dad was from the old school and taught me to drive double clutching every downshift, but with modern synchros, most people feel this is unnecessary as rev matching is adequate.

Thus, with the rowing method, the synchros need to speed up the tranny by about 2K rpms for each gear versus skipping gears making the synchros speed up the tranny 5k rpms and unfortunately, 2nd is almost always the weakest (or most used) synchro. Thus, from my understanding, rowing is better for the synchros and Kims question deserves a real answer from an expert.

[kary]

I believe Kim question is very valid, though I can not give a definite answer as there are several parts of the drive train all doing their own thing. As I understand it, the synchros adjust the speed of the transmission. Thus, when they wear you get the grinding when trying to move the gear shift into gear on downshifts (while clutch is disengaged). This can be overcome by the old double clutching technique which is matching engine revs WHILE the clutch is engaged. Otherwise, rev matching just makes the shift smooth, but doesn't help synchros. My dad was from the old school and taught me to drive double clutching every downshift, but with modern synchros, most people feel this is unnecessary as rev matching is adequate.

Thus, with the rowing method, the synchros need to speed up the tranny by about 2K rpms for each gear versus skipping gears making the synchros speed up the tranny 5k rpms and unfortunately, 2nd is almost always the weakest (or most used) synchro. Thus, from my understanding, rowing is better for the synchros and Kims question deserves a real answer from an expert.

Jad & Kim, My point was that if I am slowing the car down in 5th gear with the clutch out, the entire drive train is slowing down as I brake from 140mph to 50 or 60 in the example I used. The gears and synchros are also slowed, so the differential you are talking about (5x) seems rather hard to believe given the car's drive train is engaged and the car is now traveling at 50 or 60 mph.

I to am willing to hear from an expert though no one has chimed in yet. Not sure anyone here know tranny's well enough to comment from an expert's oint of view.

[Jad]

[quote="kary"][quote="Jad"]

Jad & Kim, My point was that if I am slowing the car down in 5th gear with the clutch out, the entire drive train is slowing down as I brake from 140mph to 50 or 60 in the example I used. The gears and synchros are also slowed, so the differential you are talking about (5x) seems rather hard to believe given the car's drive train is engaged and the car is now traveling at 50 or 60 mph.

quote]

Kary, if you leave the car in 5th at 50 mph, the tranny is turning about 2k rpm (before disengaging the clutch and it drops), then you downshift to 2nd and the tranny must speed up to 5-6K rpm as you move the gearshift, which is where all the wear occurs even upon a perfect rev match which only effect the engine, not the tranny, at least per my understanding of how the inners of a transmission work.

[kary]

Jad, I push in the clutch more around 70 which means the gears are spinning a bit faster than one might think. Though I agree with your assessment of synchro speeds, I just think they cannot come down fast enough given the timing of it all and at least my gear box has done quite well just slipping right into 2nd gear without any binding or pressure.

Like you I am still waiting for some sort of expert to talk about this. I might look at Rennlist archives and see what has been said there.

[MikeD]

OK, all this talk about the "tranny being fine" leads me to ask: How do you know until it's too late?

Of course if you change the fluid regularly then you can get a hint if there are shavings. But even that is just a hint. A tranny with shavings can go for another 1K miles or 1 mile.

[kary]

OK, all this talk about the "tranny being fine" leads me to ask: How do you know until it's too late?

Of course if you change the fluid regularly then you can get a hint if there are shavings. But even that is just a hint. A tranny with shavings can go for another 1K miles or 1 mile.

Well, in June last year I had a new LSD put in. When it was apart one can see the gears and synchros. Everything looked fine. Additionally, if you are having issue getting into some gears that is certainly a sign of pending issues. There might be other issues but the best remedy I know if visual inspection and feel during shifting.

[kary]

Some research on Rennlist here are some threads:

http://forums.rennlist.com/rennforums/s ... ar+synchro

http://forums.rennlist.com/rennforums/s ... ar+synchro

http://forums.rennlist.com/rennforums/s ... ar+synchro

[Kim Crosser]

I googled some other posts (non-Porsche). The only consistency is inconsistency.

Guess what - there are two relatively even schools of thought on "skip shifting" vs. "rowing".

The arguments in favor of rowing are:
a. Less chance of a missed shift
b. In "correct" gear if you need to engage power
c. Reduced wear on synchros

The arguments in favor of skip shifting are:
a. Less wear on clutch and other tranny parts (1 clutch engagement vs. 3-4 with rowing)
b. Less work for driver while decelerating and/or trail braking
c. Modern synchros don't need any help - wear is not an issue

One real interesting post described practicing matching revs without using the clutch. It pointed out that with modern synchros, if you get the revs close to the right speed, the gears will easily engage/disengage without clutching. http://www.se-r.net/transaxle_clutch/clutch_not_to.html
NOTE - please do not blame me if you try this and wind up needing a new transmission.

[Dan Chambers]

Here's another perspective.

At one of the DE/time Trials at Fontana I was loosely/carefully experimenting with the issue of 5th to 2nd at Turn 3.

1. Rowing through gears, good. Engine and trans seemed to like this just fine (opinion, since the car doesn't actually have emotions).
2. 5th to 4th to 2nd, seems "pretty" good, not a lot of protest from the engine or gearbox; takes a little bit of concentration... hope no one dives the corner.
3. 5th to 2nd, ooooh gotta match those RPM's just right.......

then I tried 5th to neutral... blip the throttle to spin the trans input shaft..... then clutch in, blip throttle and shift to 2nd. OOOooooooooh the gearbox and engine really liked it. It's a tiny little more fuel and concentration, but really easy, smooth shift.

Anyway, after thinking about it as it relates to safety and getting out of jams, I'm going to more or less stay with the "through the gears," row, row, row, your trans method. I figure, I'm always in a gear to go back on the throttle if I need to....even if I have to go off-track. In the 5th to 2nd method, I could be half way through the braking/decelleration event when "Joe-Bozo" comes screaming into the apex early, and requires me to straighten out and go back on throttle .....fishing for the right gear to get me out of a jam. I only like to fish from the rails of a boat, not with a gearshift lever in my hands.

Just my thoughts on safety.

Dan'o

[Dan Chambers]

One real interesting post described practicing matching revs without using the clutch. It pointed out that with modern synchros, if you get the revs close to the right speed, the gears will easily engage/disengage without clutching. http://www.se-r.net/transaxle_clutch/clutch_not_to.html
NOTE - please do not blame me if you try this and wind up needing a new transmission.

Kim:

I once had to limp home in my '68 VW with a broken clutch cable in high school. With the syncros working, I only had to worry about 1st gear, and wearing out the starter at stop signs and red lights. Up-shifting required taking the rev's high enough...waiting... lift, then shifting up. Even 'down-shifting' was possible by going 3rd-neutral-rev-2nd, etc. The syncros seem to work great in that, and P-car transaxles. I think you hit on a good point.

Dan'o

[David Ray]

Watching the video (best part of the post) :Ever notice how that chip in Hans' windscreen looks like a Starwars pod racers he's chasing....sequential shifting is a nice thing to have.

[kary]

here is Tim's video:

http://www.group9motorsports.com/videos ... view.MSWMM

[martinreinhardt]

Kary, Do you have this video in mpeg or quicktime format?

[Tim Comeau]

Ok guys. Here's how I do it. (Thanks, Kary.)
This was part of some footage being put together for the instructional video. We were checking camera angles, lighting and such.
* I misspoke during the second set of downshifts. I believe I said 5th to 3d when I meant 5th to 4th.*

As you can see, the downshifting can be done quite quickly and it should never, IMO, take longer than the time needed for braking. Any longer and you're spending time inefficiently. The 2 should always be directly proportional, that is, if you need to be in a lower gear exiting the turn, you'll need to slow down for a longer period of time. Make sense?
I'm getting ready for Fontana this weekend with NASA so I don't have time right now, but I'll try and get some other "at speed" video to help make some ideas more clear.
I've already learned something.

[kary]

I just posted the file as Tim provided it to me. I could not open it either but realplayer has an extension for it.