Double Clutching - Mercedes Benz SLK Forum

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#1 Old 05-23-2007
 
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Double Clutching

I was wondering, my friend was teaching me how to drive manual and now he is teaching me how to drive a manual vehicle quickly. To my understanding, double clutching is when you disengage the clutch and shift into neutral, rev the car to appropriate rpm at which the car should be in the next gear, disengage the clutch again and select the next appropriate gear. My friend is telling me that double clutching does not involve any "rev-matching." Can someone who knows what they are talking about tell me what double clutching is?
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#2 Old 05-23-2007
 
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50 years ago the gear boxes have been not synchronizid, you switched the gears by double clutching. (double clutching = one power push between the change of two gears)

Im using it while the engine is cold because the gears are switching better and I love the sound of my Piecha exhaust. But it is not necessary. Modern gear boxes do not need it.
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#3 Old 05-23-2007
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Gero is correct a modern manual does not need to be rev-matched for a smooth shift because it has synco mesh to do that for you but double-clutching does involve the gas.

You either shift into N and allow the engine speed to drop off allowing shifting from 2/3 or blip the throttle to increase the engine speed for a shift from 3/2.

As said its not really necessary on a modern box its used more on trucks and racecars where syncro's would be inefficient.

It appears your friend might not have a good enough grasp on the concept yet mate as double-clutching has always been used to rev-match to enable smooth shifts.
 
#4 Old 05-23-2007
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Does your friend have an Audi TT? The TT has some trick gearbox with two clutches, where one gear is always on-hand...
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#5 Old 05-23-2007
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Originally Posted by in2dwww View Post
Does your friend have an Audi TT? The TT has some trick gearbox with two clutches, where one gear is always on-hand...
You mean the DSG right.

Real cool box
#6 Old 05-23-2007
 
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According to my friend rev matching and double clutching dont fall in the same catagory. He says rev matching is used for downshifts and double clutching is used for upshifts...
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How does he rev match without "double clutching" ?
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#8 Old 05-23-2007
 
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BTW Mc, your friend should show you the double clutching or rev matching (what ever he says) in the rallye style. The former real good Rallye drivers, Röhrl, Munari, Toivonen, Vatanen and others touched the brake with the head of the shoes, while the rear was touching the throttle. That way they were faster in the curves: breaking into it, double clutch/switching and puching the throttle without taking the right foot from the two pedals!
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#9 Old 05-23-2007
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In some of the books/information on driving a car on a track, it mentions "double clutching", where when down shifting, you clutch in/out of gear to neutral while braking and then clutch in/out to the next lower gear while either heel/toe blipping or rev matching for the speed so your tires don't "jump" on you.

My last track experience, I found this wasn't that much of an issue, since MB tranny seems to work really well in these environments, although I tried to rev match, a couple of times I didn't and the everything seems to be just fine (i.e. I didn't spin off the track).

On older cars, especially "muscle" cars, double clutching was another way to race your car, by letting the clutch slip to keep your rev's high and keep from bogging the engine, but anyone who had a SOHC european and/or japanese engined car knew, the SOHC could beat the "muscle" cars based on our light weight and ability to rev the SOCH engine quickly.
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#10 Old 05-23-2007
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Quote:
Originally Posted by OWL-SLK View Post
BTW Mc, your friend should show you the double clutching or rev matching (what ever he says) in the rallye style. The former real good Rallye drivers, Röhrl, Munari, Toivonen, Vatanen and others touched the brake with the head of the shoes, while the rear was touching the throttle. That way they were faster in the curves: breaking into it, double clutch/switching and puching the throttle without taking the right foot from the two pedals!

Isn't that just called 'heel-toe'?


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Heel-toe? Never heard...Im sorry
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#12 Old 05-23-2007
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Quote:
Originally Posted by in2dwww View Post
Does your friend have an Audi TT? The TT has some trick gearbox with two clutches, where one gear is always on-hand...
Try a Wilsonian, Epicyclic Pre-selector Gearbox - used in Armstrong Sidleys and Daimlers - you use the shift lever to select the gear you 'think' you want next, then hit the clutch at the time you want to shift (could be several minutes later), and it drops into that gear - bizarre, but works!!






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#13 Old 05-23-2007
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Quote:
Originally Posted by OWL-SLK View Post
Heel-toe? Never heard...Im sorry
from: http://www.nasaproracing.com/hpde/heelandtoe.html

Heel toe Downshifting
Why is “Heel toe” important? Remember, as you approach the traction limit of your tires, anything that takes traction can cause the car to slide. Have you ever downshifted and released the clutch too quickly and felt the car jump as the engine RPM’s were forced up to match the cars speed? Kind of like tapping the brakes. Think of this, if you are driving at 70 mph in 4th gear, at 4000 rpm’s and shift down to 3rd, your engine rpm’s will go up, to say 5200 rpm’s. You can accomplish this by easing out the clutch, until the engine is forced up in rpm’s by the car. This works, but it is slow, hard on the clutch and transmission synchros, and uses up some of your traction to force the RPM’s up. The alternative is to match the engine speed to the transmission speed (in the lower gear). This can be done by pushing in the clutch, blipping the throttle, selecting the lower gear and releasing the clutch. The problem is, downshifting is done at the same time we are braking. Guess what, we have two feet and three pedals to operate simultaneously! The Heel Toe technique solves this problem.
“Heel toe” is a misnomer. It can be done in many ways, depending on the pedals in the car, and the anatomy of the driver. Although it can be, it is not usually done with the heel and toe. The process is commonly done by placing the ball of the foot on the right side of the brake pedal, and while holding consistent brake pressure, the side of the foot rolls onto the throttle, “blipping” the throttle. Depending on your anatomy, and the pedals, it can be done any way that allows the brakes to be used while the throttle is blipped.
Heel Toe cannot be done smoothly unless two things are done:
1) The pedals must be matched. Normally this is done by adjusting until the brake and throttle are even in height, when the brakes are pressed on. The pedals must also be properly spaced. In my car it required adjusting and bending the gas pedal until I got the match I needed. In many cars, the pedals have some range of adjustment, making the process easier. One thing to remember, as you adjust the gas pedal, make sure that there is a mechanical stop for the pedal. If you rely on the stops in the carburetor or injection system to stop the motion, you will probably bend or break something as you try to squeeze a couple more horsepower out of the pedal. Also, make sure the linkage allows the butterflies in the carburetor to be fully open when the pedal hits your mechanical stop.
2) The technique must be practiced. Do not come to the track, with the intention of learning to Heel Toe. Learn the technique on the street, and practice it until it is second nature, before trying it at the track. If your street car is different from your track car, and your street car has a manual transmission, set its pedals for Heel Toe, and learn the technique. Try to get the pedal arrangement similar for both cars. If you must learn the technique in your track car, make it low on your priority list. When driving the line is second nature, you are comfortable in traffic, you’ve got all of the corner stations figured out, start working on it.

Heel Toe is not a required skill at your first event or two, as a matter of fact, you don’t ever have to learn it. It is a tool that will make you a smoother driver (ie. faster!), and you will be easier on your equipment. It’s a tool to add to your arsenal of skills as your high performance driving becomes more polished.
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#14 Old 05-23-2007
 
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Hey guys this is my friends complete response, is it correct?

Rev Matching and Double clutching is NOT the same.

When you rev match, you just downshift and "blimp" the throttle and go the designated RPM for that gear. you DO NOT go to Neutral and then back to gear to rev-match.

For example when I go from 4-3.
Say I'm at 4K rpm in 4rth gear, and I need to be in 7K rpm for 3rd gear.
So when I down shift from 4rth to 3rd, I just tap the gas and let the revs go to 7k and then drive, this eliminates the jerk that will happen if you just straight up down shift.

Double Clutching is normal shifting with an added step, you go in to neutral and then to next or previous gear.

"Double clutching is also used for downshifting which some like to call "rev-matching""
Thats rev matching when double clutching. People do this when they are going like 50mph in 5th gear ( like 5-2), and then need to accelerate,
so you double clutch and rev match to 2nd gear for the needed acceleration.
Instead of granny-shifting, which can hurt and wear your clutch really quick, you double clutch rev match.

the whole point of double clutching is to make your shift smoother, a lot of drivers still use that when up shifting, especially from like 1-2. then theres a big ass jerk, cause your dropping a **** load of revs
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#15 Old 05-23-2007
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Sounds correct to me....
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#16 Old 05-24-2007
 
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Sure glad they make automatics when I was younger my feet and hands were more co-ordinated than now and my mind and hearing made all this shifting seem easy but now just reading what it takes to shift -- all I can say is wow have fun while you can
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#17 Old 05-24-2007
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I haven't read the entire thread in full detail but let me see if I can help.

Heel-toe down shifting and rev-matching your downshifts is still common today but mainly used on the racetrack as stated in the post above as to not upset the car.

True double clutching requires you to use the clutch twice per downshift. This used to be mandantory before transmissions became fully syncronized many decades ago. Some racing "dog boxes" do not have syncros and so you either grind them or you double clutch. It is very close to what you are saying in that you have to go to neutral before going to a lower gear but key is you have to let the clutch out in neutral and then blip the throttle. This speeds the imput shaft in the transmission up to the speed it should be at for the lower gear.

The difference in the two being that on heel-toe downshifting you are simply trying to avoid having to let the engine come up the the desired rpm to match the tranny and on double clutching you are trying to bring the imput shaft up to the speed of the output shaft.

Hmm trying to think of an easier way to explain it. Basically double clutching is not used anymore since we have transmissions with often multiple syncros per gear change. In the old days you couldn't just push the clutch in and select a lower gear and let it out and have the motor come up to speed. If you did that it would grind and therefore the need to double clutch.

Hope that wasn't covered above somewhere and I just wasted a bunch of peoples time.
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#18 Old 05-24-2007
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I will say that the 7spd is the best auto I have ever driven (which isn't saying a whole lot). I think I am missing my 6spd stick more and more everyday. I like how you can drive in comfort but then if you are driving aggressive it will hold the current gear when going around a corner etc.
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#19 Old 05-24-2007
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Quote:
Originally Posted by MCFaisal View Post
Hey guys this is my friends complete response, is it correct?
Double clutching and rev-matching may not be the same, but you still need to rev match when double clutching.

Double clutching may not be needed, but it's easier on the synchros and makes for a smoother shift. I double clutch when going into a lower gear, 2nd or perhaps first when needed. The double clutch downshift into 2nd is actually my favorite shift.

Double clutch:
1. Off power, Clutch in, shift into neutral
2. Off clutch, blip throttle to match revs
3. Clutch in, shift into gear
4. Off clutch, on power

It may seem like a long chain of events, but when you get the hang of it, it is really done quite fast.

For normal driving situations, I just rev-match downshift. Heel and toe isn't really necessary or cannot be done properly, unless you're hard on the brakes coming into a corner.
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#20 Old 05-24-2007
 
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so everyone agree's double clutching and rev matching are NOT the same?
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