car talk says double clutching is bad?

[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).

[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

[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

[Edev]

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

Lol.

[lylefk]

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

[nando]

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

[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.

[newman]

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

[lylefk]

^ Cliffnotes?

[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?

[daniel]

Can someone explain exactly what double clutching is?

car in 3rd gear, 3000 RPM. push clutch in, shift into neutral. release clutch, which matches the transmission speed with the engine speed. rev up to 4000RPM, push clutch in, and smoothly, creamily slip the shift lever into 2nd gear, then let the clutch out.

its all about knowing the RPMs in each gear for a given speed so you can shift between gears and adjust the RPMs accordingly.

btw if you ever want to drive without the clutch, you do a similar thing, except you don't use the clutch.

[delfin]

Can someone explain exactly what double clutching is?

[nando]

The phrase you are looking for is heel-toe :p

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. ;)

[Wh33lhop]

I think knowing both methods would be beneficial. If you ever do any HPDEs, you'll want to know how to rev match. Just last week I was watching a video of an E30 with a seriously built engine, but the driver was just letting out the clutch to downshift and nearly spinning out on ever corner entry.. yikes!

The phrase you are looking for is heel-toe :p

There's a video of an engineer test driving the E92 M3 around some track chirping the tires on every downshift. I cringed.

Or there's this tool:



Fucking ugliest S38 swapped E30 on the planet whose owners abuses a slipping clutch and can't even heel-toe.

[Hallen]

I would suspect you are doing putting more wear in both the throw out bearing and the clutch by double clutching. In double clutching, you are engaging the clutch twice during each shift. The clutch and the flywheel will be spinning at different rates and every time the clutch squishes up against the flywheel, you are going to get wear.

Rev matching is the ticket. If you minimize the speed difference between the clutch and the flywheel and you minimize the length of time the clutch spends spinning against the flywheel, the less wear you are going to have. Think about it: you burn a clutch up by trying to feather the clutch while accelerating. You get more wear by doing that than you do if you rapidly engage the clutch causing a bit of a jerk (which can be eliminated with good rev matching). But the clutch spins less so you are doing as much to it. Anyway, that's just my take on it.