car talk says double clutching is bad?

[newman]

I never double clutch, but it's not a bad idea, even with a modern synchromesh transmission.

Let's say that you're traveling at 30mph, = .5 mile/min = 31680 inch/min in your 325is

Stock tire size is 195/65-R14 which has a circumference of 75.3 inch

At that speed the axles are turning at 31680/75.3 rpm or 420.7 RPM

Final drive on the car is 2.93, so the driveshaft and ouput shaft are spinning at 1232.7 RPM

Now, in both of the following scenarios, we will assume that during the time the car is shifted, the car's forward velocity does not change.

Let's say we want to downshift from 3rd to 2nd.

5th gear is 1.4:1 so engine RPM at 30mph is 1725

4th gear is 2.2:1 so engine RPM at 30mph is 2710

Without double clutching:

1)When the clutch depressed the driveshaft, which is "pushed" by the rear wheels remains constant at 1232 rpm.

2) The driver shifts into neutral

3) The input shaft of the transmission, the clutch friction disc and all of the synchros all continue to spin at the engine rpm equal to 30mph in 3rd gear (1725 RPM) due to momentum or begin to slow down due to friction. There is nothing driving these parts at this time.

4) While moving the shifter towards second gear the driver "blips" the throttle to 2710 RPM

5) The driver then moves the shifter into second gear.

6) The driver releases the clutch, the car continues smoothly.

No wear and tear, right? Wrong.

Let's think about what just happened. The clutch friction disc input shaft were spinning at 1725 RPM at the start of the example, and 2710 at the end of the example, when and how did they speed up?

When the driver "blipped" the throttle? No. When the driver blipped the throttle the clutch was disengaged from the motor. Revving the motor has little effect on the speed of the friction disc/inpushaft/synchros.

It happened when the driver moved the shifter from neutral to second. The synchros are basically little friction rings that rub together to force the input shaft to rotate at the same speed as the output shaft before the gears engage.

Imagine trying to stop a bench grinder that you just turned off and is coasting to a stop by sticking a screwdriver up against it... that's basically what the synchro has to do.

Granted, synchros are MADE to do this, but they still can wear out. This is exacerbated by a 2 or 3 gear downshift where the differential of the input and output shafts are much greater.

Now imagine this scenario with a double clutch maneuver.

4a) After step 4 and before step five the driver releases the clutch pedal momentarily. Since the friction ring and input shaft are now coupled to the engine, they spin up approximately to the appropriate 2710 RPM. Then the driver depresses the clutch, which disengages the friction/input shaft, but momentum keeps them at roughly the same speed.

5) Now when the driver shifts from neutral to second gear, the synchros do not have to do much work to make the input shaft and output shaft synchronous.


This MAY cause more wear on the clutch parts, but probably not much.

What would you rather replace, the clutch or the synchros?

[lylefk]

^ Cliffnotes?

[newman]

read the post before it for cliffs, read my post if you can spare an extra 120 seconds...

[daniel]

^ Cliffnotes?

it was explained very complexly, but basically it says you are wearing out the synchros when rev-matching, and you are not so much when double-clutching.

one must remember that when the car is in neutral with the clutch engaged, the transmission is spinning at the same speed as the motor. so revving the motor also increases the RPMs of the transmission. if you are in neutral and you disengage the clutch, rev up the motor, then let the clutch out, you are putting stress on the clutch to make the tranny speed match the motor speed.

[nando]

Or, just not worry about it because the synchros in your BMW transmission will outlast the car. ;)

[lylefk]

I'm just too hungover to deal with that right now...lol

[Edev]

I'm just too hungover to deal with that right now...lol

Lol.

[naplesE30]

I never double clutch....to much work.. in my old probe I didnt even use the clutch except when starting out. I HATED THAT CAR

[wolfgangstbd]

i usually do my down shifting while braking. let the rpms drop a little then heel toe and down shift. feels like my car is an auto and its quick and most important of all fun. if double clutchinh make driving fun then do it. i imagine all the premature wear you would put on it is just a couple of days in the end. also ive seen getrag 240/260's in working order for like a 150

[Dillinger]

No need to double clutch these transmissions. Did you learn to drive on a tractor? Hell I don't even use my clutch after 1st or 2nd (speed shift).

[george graves]

i've been double clutching for a few years now...

For a few years you say....Have you been doing it, say - since about 2001?

[daniel]

For a few years you say....Have you been doing it, say - since about 2001?

why? are you saying "few" = 8? i equate "few" to mean between 3 and 5. "several" = 6-9.

i've been rev matching for 4 years, double clutching for 3.

[Danny]

For a few years you say....Have you been doing it, say - since about 2001?

Before that he was granny shifting.

[Wh33lhop]

Before that he was granny shifting.

lawl

Gotta love the guys who equate double clutching to powershifting. Thanks, F&F.

[Wh33lhop]

that's the common name for it, but since I don't actually use my heel, rev matching is a better term. It's more like pinky toe/ball of foot. ;)

I guess. How about toe-toe? ;)

Generally around town I'll use that method, but when I'm really railing on it I'll stab the throttle with my heel. Thing about BMW's bottom-hinged gas pedal though (while I love it) is that you get less leverage on the bottom of the pedal than at the top, so if you want to effectively heel-toe you've got to stab it really hard or you've got to rotate your foot more and stab it more towards the top of the pedal.

Kind of a pain.