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Double Clutching
Ray Pipkin/Ove Kvam

More On Double-Clutching...

Posted By: Ray Pipkin
Date: Sat, 1/7/2000, 12:13 p.m.
In Response To: heel-toe shifting is good: (Raj Dev)

There seems to be some confusion between double-clutching and the "heel & toe" method on downshift. For starters, one can either double-clutch or not while using the "heel & toe" method. Without double-clutching, tapping the accelerator in the middle of the gear change with the clutch pedal depressed causes engine speed to match road speed when the clutch is released. However, because the clutch disk and transmission are not being driven when the accelerator is depressed,this method does NOTHING toward bringing the clutch/transmission up to output speed.

In the "heel & toe" method WITH double clutching, the accelerator is tapped in the middle of the downshift with the clutch pedal out and the gear momentarily in neutral. By this method, BOTH the engine and the clutch/tranny are brought up to speed before the downshift is completed.

In an automotive transmission, almost all of the rotating mass is on the input side. That is, (1) the input (or fourth) gear which drives the countershaft, (2) all of the remaining four drive gears (in a 5-speed) plus reverse drive gear, (3) the countershaft, (4) the countershaft gears (in constant mesh with the drive gears), as well as (5) the reverse driver gear and shaft, are PERMENENTLY ganged to the input shaft to which the clutch disk is splined, and rotate with it. Only the synchronizer drums are splined to the output shaft and rotate with that. All of this rotating mass, along with the clutch disk, must be brought up to speed before a downshift can occur.

If double-cluthching is not used, than the clutch/tranny-to-output speed is equalized by the momentary slipping of a synchronizer ring on its mating surface. (Depending upon the transmission, the synchro ring can either rotate with the driven gear and mate with the drum, or rotate with the synchronizer drum and mate with the driven gear.) The ring (truncated cone, actually) is made of either bronze or brass so as not to be of the same material as the piece with which it mates, and it is often grooved to let the oil squeeze out during shifting.

I am not a racer, but from what I understand, hard racing can fry the synchro rings, and many racers make a habit of double-clutching. Furthermore, non-BMW racing trannys like Hewland or Staff (spelling?) do not have synchronizers at all. So, while I would agree with Randy Walters and Schem325i that double-clutching is not necessary for normal street driving, I think double clutching saves considerable wear and tear on the synchronizer rings under extreme racing conditions.

Upshifting is usually not a problem, although here too, double-clutching (without the accelerator tap, of course) reduces the work required of the synchronizers. I said that upshifting is USUALLY not a problem because, to reduce tailpipe emissions in some of the newer BMWs (1999 & 2000), the engine rpm does not immediately drop when the accelerator is released. This effect is quite noticeable on upshifts.

This was a good thread. More meaty than most.

Ray Pipkin




Subject: Re: It's redundant

Author: Ove Kvam
Date: Wed, 28 Feb, 13:46 GMT
In Response To: Re: It's redundant by Vic 85 325e

Okay, I'll explain a downshift from 3rd to 2nd. Rmemeber that you will apply some light braking force with the right foot big toe during the entire procedure:

1: Clutch in
2: Gear lever to neutral
3: Clutch out
4: Blip throttle to speed up input shaft
5: Clutch in
6: Gear lever to second gear
7: Rev match with throttle to anticipated RPM in second
8: Clutch out

You use the outside of the right foot on the throttle pedal, and the entire procedure is done in about a second when you are used to it. It is quite difficult the first time! You don't have to release the throttle pedal between step 4 and step 7, and you can start revving up the engine when step 1 is finished to shift faster. If you are good at it, you can even skip step 1 and 3 by getting the timing right. You can take the gear lever out of gear when there is zero load in the drive line without using the clutch. You know you have done it right when the gear lever slips right into gear without resistance, and you can't feel any jerking in the car during step 8.

The method of shifting with rev matching and no clutch does not wear anything if you can actually pull it off. It is however very difficult. I guess you want me to explain the double declutching. In a gearbox, the cogs are always spinning together, but only one set of cogs are connected to the shaft at any given time. The synchros are used to match the speed of the cogs and the shafts so that they can be locked together. By double declutching you match this speeds manually so that the synchros don't have to. That is why it just snaps into gear with no resistance. If you depress the clutch, the input shaft will eventually stop. The syncros will then have to spin it up again unless you help it.



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