Double clutching?

[12eckless WRX]

Can anyone tell me what double clutching is? Sorry just getting into the racing scene...

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[VSG]

The "search" feature can be your friend!

http://forums.nasioc.com/forums/show...=double+clutch

[kovalov]

Originally posted by 12eckless WRX
Can anyone tell me what double clutching is? Sorry just getting into the racing scene...

Do not know the guy's name. But here his story is:

Driving with a Manual Transmission
I was born and raised in Europe and as such happened across an automatic car for the first time in my life only when I came to the States 9 years ago, at the tender age of 29 , i.e. I belong to the old school who learned to drive on cars with the 3-speed, non-synchronised gearboxes with the straight-cut gears. Which means that unless you master the art of precise rev matching (double clutching), you not only are not going to get the gears engaged, but you are also risking damaging the gearbox by grinding the gears.

Double Clutching

In the English language this term unfortunately describes the process of rev matching in both upshifting and downshifting. I'd like to distinguish between them. Let's use two different terms:

You do "double clutching" on the upshift:

lift off => depress the clutch => switch into neutral => release the clutch => depress the clutch => switch into a higher gear => release the clutch => step on the gas.

You do "double declutching" on the downshift:

lift off => depress the clutch => switch into neutral => release the clutch => [blip the throttle, increasing the rpms in order to match the higher rpms of the secondary shaft in the gearbox needed for the bigger DIA of the lower gear] => depress the clutch => switch into the lower gear => release the clutch => step on the gas.

Blipping the throttle to the precisely matched rpms is no doubt an acquired skill, but if done properly it affords a silky smooth downshift. One might argue that this technique is not needed on a modern synchronised street car in the everyday driving. Yes and no.

A case in point: you are in 4th doing 70mph on ice or on packed snow. Suddenly, you need to stop the car in emergency (a fruitcake in an automatic SUV thinks that she's invincible and plants herself solid into a snow bank, blocking the road with her ugly bath tub on wheels. I see this happen every winter). If you hit the brakes, you're history. You need to drop 2 gears and then brake with the engine. The trouble is, at 70mph the gearbox ain't gonna let you engage the 2nd no matter how hard you pull that stick. OTOH, you can do it easily if you know how to double declutch properly. This technique has saved my butt more times than I care to admit. I habitually double declutch at all times in any weather and although I would be first to admit that this is not always called for, this habit of mine is probably the only reason why I have never had a single gearbox breakdown on any of my cars. Ever. One more tip: always depress the clutch fully to the floor, enabling a complete disengagement. This saves your disk/basket.

Heel-and-toeing

It is a very good practice indeed and really helps on a turbocharged car, where you would otherwise come out of the corner off boost. I used it on a track up until I installed a blow-by valve on my 200. Now the turbo is just freewheeling under the lift-off and the turbine does not lose its momentum.

Why double declutch?

It makes downshifting possible if you're traveling at a high speed and need to downshift from 5th to 2nd and then to 1st in order to stop fast and prevent an accident on a slick road or if you are having a brake failure. Yes, you may lose your tranny by doing this, but I'd rather bust a couple of gears than kill either myself or someone else. Greatly simplified, in any gearbox there is a primary shaft, which is connected to the engine, and a secondary shaft, connected to the differential (and ultimately to the driven wheels). There's also a set of gears of different diameters interconnecting the shafts. For our purposes, let's consider just the 2nd and the 3rd of the 016 gearbox in my Audi 200TQ. The 2nd has the gear ratio of 34:16=2.125. The 3rd has the gear ratio of 35:24=1.458. Remember from Physics (Mechanics, the Kinematic Motion) that if the angular velocities (rad/sec) of two different DIA gears are the same, the tangential vector of linear velocity (m/s) on the teeth of a larger gear with the bigger circumference is higher than that of a smaller gear with the smaller circumference. Alternatively, if you want to equalise their linear velocities (as to prevent the gears from grinding at the moment of engagement), you need to change the ratio of their angular velocities as a reciprocal of their DIA over pi taken to one another.

Suppose you are driving in 3rd @ 2000 rpm. Now you're depressing the clutch and switching into neutral, hence disengaging the primary shaft from the secondary one. The secondary one continues to turn @ 2000 rpm because it is connected to the diff and the wheels and the car is still traveling at the same speed. Now you're trying to engage the 2nd and in order to achieve the same linear velocity on the lower gear, the primary shaft now has to rotate with the higher angular velocity by 2.125/1.458=1.457, i.e. at 2915 rpm. This means that when you downshift in order to keep the same vehicular velocity, you need to somehow increase the rpms on the primary shaft to 2915 while in neutral. That's what you do by blipping the throttle in between the downshifts. With a bit of a practice, you should be able to do the double declutching downshifts in under 2 seconds.

Using the same considerations, it is easy to show that for the rev matching on the upshift you need to decrease the angular velocity of the secondary shaft. That's a lot easier because all you need to do is make a short pause (by going through the neutral without blipping the throttle and just letting the rpms drop naturally due to the engine's internal friction).
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[VSG]

If you're not shifting fast, does double-clutching benefit on the upshifts? Cause it seems the syncros are lined up and stuff when I upshift.

[DO]

Double clutching is done when you don't have syncros. With a good syncroed tranny you don't need to double clutch.

[KIMUTAKU]

double-clutching saves ur syncros...it's fun too...

i dun think double-clutching during upshift is neccessary...i just do it when downshifting...(actually i do heel-toe...but not all the times)..

[ScoobaruRS]

Why do you have to let off the clutch and then press it again when double clutching? Because I made a habbit to rev the engine to 2000 rpm (shifting at 3000) before I let off the clutch. I guess I'm doing the same thing only I: 1) Accelerate to 3k 2) Let of Gas 3) Press clutch 4) Upshift 5) Rev to 2k 6) Let off Clutch...Is this bad?

[DO]

The reason why you let out the clutch in neutral is to match the engine speed to the wheel speed, inside the tranny. Don't bother with double clutching. You don't need to double clutch a car, unless the syncros are gone, I repeat, you don't need to double clutch a car with good operating syncros. I do heel and toe for downshifts, this is way more important then double clutching.

The only reason to double clutch is to match engine and wheel speed. This is was syncros do. Don't stress yourself for double clutching, unless you plan on driving a big rig.

[ScoobaruRS]

Well, I meant that it was a subconsious habbit. Mainly because I didn't like the sound (or feel) of the synchros spooling up when I shift. I guess I was just asking what the point of pressing the clutch twice was. (Lifting off and then pressing again) Double clutching is way to much work for me just to elongate the lifespan of my synchros. Plus I don't think I'm that coordinated with my feet.

[Chuck H]

If you don't double-clutch, then you aren't saving the synchros any work. All you're doing is rev-matching to make the clutch engage more smoothly when you let it out.

Don't take that as a criticism -- rev matching is much better than just cramming it down a gear and sidestepping the clutch again. But the only way you save any wear and tear on the synchros is if you let the clutch out again in neutral so that changes in engine speed also change the transmission input shaft speed.

[ScoobaruRS]

Oh ok. I guess thats what I was doing was rev matching. I don't really notice a difference with the clutch in or out in Nuetral. I think I just had the wrong idea about how it all works. It makes sense tho, thanks.

[Frederf]

The point is to shift into neutral (clutch out), then get the revs up. The clutch must be out in neutral to get the maximum number of parts spinning the correct speed.

That way when you shift again (downshift 99% of the time) the maximum number of parts in the tranny are already spinning the correct speed.

They are right that since you have syncro's and heel toe would produce similar results. But the WRX's syncros aren't bulletproof so I double clutch on at least half of my downshifts.

[jhong567]

so when you double clutch and not rev the rpms high enough, how bad is it for your tranny? That jerking motion bothers me if i mess up. And at what rpm do you guys usually downshift using double clutch in normal traffic? thanks.

[Chuck H]

You have to develop a feel for it after you get used to shifting the car. I'd start out with normal shifting and just letting the clutch out more slowly (basically slipping it slightly) on downshifts, especially if this is your first manual transmission car. Once you get comfortable enough with shifting that you don't even think about it any more, then it's time to learn the more advanced techniques like double-clutching.

The RPM you need to rev to depends on which gears you're shifting between and how fast your going. For normal slowing, I usually end up downshifting around 2000 RPM, and revving it to somewhere in the ballpark of 2800-3000 RPM for the lower gear. It doesn't have to be exact.

However, if I'm on the highway, and want to drop from 5th to 4th or 3rd, it obviously takes more revving to match the speeds. If you want, you can sit down with a chart of the gear ratios and figure it out for any combination of gears and engine RPM, but that's a LOT of work and just getting a feel for it works just as well.

[GoodFinder]

Thanks for the dialog. I'll have to give this a try. Maybe it will help with my decel noises.

GoodFinder

[Gspot]

...this is the most confused I have ever been reading these forums. Whats the difference between heel-and-toe and double-clutching? ...What is heel-and-toe?

[Subie Gal]

double clutch is a must to get the Impreza into R...

R is not synchronized


Jamie
www.subiegalracing.com
SubieGal Rally Posters available now
*******************
www.allsubaru.com

[Chuck H]

Originally posted by Gspot
...this is the most confused I have ever been reading these forums. Whats the difference between heel-and-toe and double-clutching? ...What is heel-and-toe?

Double-clutching is stepping on and releasing the clutch pedal twice per gear change. You step on the clutch, shift from gear A into neutral and let the clutch out again. If you're downshifting, rev the engine to the RPM it will be in once you're in gear B. Now step on the clutch again, shift from neutral to gear B, and let the clutch out again.

Heel-and-toe is a method for combining braking with double-clutch downshifting at the same time. The theory is basically the same, except that you're trying to work 3 pedals simultaneously with only 2 feet. You're on the brakes the whole time, and then you perform the double-clutching procedure described above. The only difference is that you have the ball of your right foot on the brake and you twist your ankle sideways so that the outside of your foot can press the gas pedal to bring the revs up without letting up pressure on the brake with the ball of your foot. That one takes a lot more practice, and is really only necessary on the race track.

For instance, at NHIS, when you're going off the oval up the hill onto the road course, you're transitioning from a fairly high speed straight to a tight uphill corner. You need to be hard on the brakes to slow down for the corner, but you'll bog the engine something awful in the corner if you don't downshift. And if you downshift without rev matching, you're going to upset the car's balance when you let the clutch out and engine braking causes undesirable weight transfer to the front wheels. So with practice, you can be braking near the lockup limit and still get down to 2nd gear so that when you let off the brakes on entry to the corner, the car actually goes instead of just choking and dying.

[Gspot]

I was just trying the double clutching on the way to school. I think I'll stop trying before I run into something. Too hard.