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Double Clutching

Discussion in 'E90/E92/E93 M3 (2008-2013)' started by echanda, May 4, 2011.

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    echanda

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    I need someone to educate me on this technique. I was fortunate to be at Lime Rock a few weeks ago for a wonderful track event sponsored by the BMWCC of CT. If you've never tried it I highly recommend it. My concern is that coming down the straight and into the turn I would downshift and every time the wheels would chirp (an awful feeling that I am damaging the vehicle). My instructor, who was beyond excellent, explained the concept of double clutching and heal toeing. I have been trying to perfect my technique ever since. However, I have come to realize that on normal roads, and in the constraint of legal speeds, the car shifts just as well when I don't double clutch as when I do. I know it is the marvelous german engineering in my M3 at work, but it didn't it work at Lime Rock. How does the car compensate for the spin down and how much of an rpm differential can it handle? Should I double clutch always, regardless?

    PS - As for heel toe I am helpless. Any hints on how to do that crisply?
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    Brian A

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    The technique you have to master is heel-and-toe downshifting. With a bit of practice, it gets easy and a lot of fun. Here is a link to a thread with a full descrption of how to do it:
    http://bmwcca.org/forum/showthread.php?t=7442

    I am surprised that your instructor was advocating double-clutching. My understanding is that it is only needed for cars without syncromesh transmissions (ie cars made before, say, 1920!!). I did it on a big dump truck I drove for a summer job, but that was because it had straight cut gears. After a while I just gave up on using the clutch at all, except for starting from stop.

    ForcedInduction guest

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    Double clutching definitely is not needed. Heel and toe is useful for matching the revs to the vehicle speed under hard braking. The instructor didn't want you to shock the tires and driveline by letting the clutch up with the vehicle and engine speeds mismatched. Blipping the throttle gets the engine revs up so they are close to the vehicle speed resulting in a smoother engagement.
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    steven s

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    Your revs are definitely too high.
    Heel/toe will help or you need to get your speed (or revs) down a bit more before shifting.
    You won't do any damage. I've been chirping my wheels for years until I finally got used to getting my speed a little lower.
    Still don't heel/toe, but I really need to practice more.

    In your case for now, reduce your speed a bit more, get your downshifting done in a straight line. You don't want to be shifting in a turn. You may need to be braking earlier also.

    I agree with everyone else, double clutching? Not necessary in this day and age.
    Not with a perfectly working transmission.
    And if this is one of your first DEs, just the thought of double clutching is way beyond what you need to be focusing on.
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    Deutsch Marques

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    Fear not about the inability to figure out heal-toeing. Despite what some say in the forums, not everyone with a stick here is a master of it. I've never been able to figure it out. :(

    Though I would love to find some way to finally be able to. I had dropped my car off at the BMW dealership for service once, and the older gentleman who brought me home in a manual E36 was heal-toeing like a pro. I was so envious!

    Granted my M has SMG, but I could make my everyday driving in the wagon much smoother if I could learn that technique.

    two30grain guest

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    I havent had the pleasure to do a track event, but I rev match under normal driving. I am so used to it now, I, out of habit, do it in every stick I drive, even when I am unfamiliar with the car. I rev match in every lower gear change except 2 to 1 (because i never go from 2 to 1, lol).

    There are two things I do. First, depress clutch, simultaneously apply throttle with a quick jab and switch lever, and then release clutch. This is good for dropping a gear for more power, or when coming to a stop where I have lots of space where the brake does not yet need be applied.

    Second, is an actual heel/toe, used for normal/more aggressive retarding of speed. With ball of foot on right side of break, roll foot to the right so that the other side of the foot quickly jabs the throttle, all while having the clutch depressed and making the gear change. Its easier than it sounds (after practice).

    I started by doing the first method I mentioned (sans heel/toe), but I wouldnt quickly jab the throttle. I would apply the throttle gradually after the clutch was depressed and hold it at a certain speed, then slowly let the clutch back in. Whether this is the right way or not, I dont know, its just how i started.

    I think its important to mention, for the most part I dont worry about what the engine speed is, its just a quick jab, but when you first start, you will probably be paying more attention to it than you really need to.

    I havent read the previously posted link, but will now, that may have good info. Also, if you have a post '96 or so Bimmer, you might want to research the CDV (Clutch delay valve) and consider removing it.

    Cheers!
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    CRKrieger

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    All good and mostly correct information here. These are definitely two different things you're talking about. I won't spend much time on double clutching because it isn't such a necessary skill in a modern BMW with good synchros. I would certainly not make learning it a priority and I'd never advise a student to try learning both at once.

    Rev matching, though, could make the difference between driving and walking home. The concept is to get the engine speed to match the speed of the gearbox so, when you shift gears (either up or down), there is no lurch. I consider it a requisite to be advanced to be smooth both downshifting and upshifting. As an aside, I spent most of a weekend trying to convince an otherwise good student of mine that I didn't appreciate his bouncing my helmet off the headrest with every upshift, but he persisted. I finally gave up and got out of his car.

    Rev matching on upshifts isn't that difficult, but making your shifts imperceptibly smooth is an art form. A slight lift of the accelerator just before declutching, a quick shift into the right gear, and then catching the engine speed as it drops while you get back onto the accelerator is the key. If your passengers only hear, but don't feel, your upshifts, you're doing it right.

    Downshifting requires a bit more, but it, too, has the same goal: an imperceptible shift that doesn't upset the car or your passengers. To match your downshift revs, the engine has to be speeded up to match the gearbox speed. That's where heel-and-toe shifting comes in. I recommend you learn it on the street in stages.

    First, for a week or two, just get used to braking with the left side of your right foot. You want to get the brake every time and have your foot hanging over the accelerator at the same time. No slipping off. Don't worry about actually hitting the accelerator yet, nor about your shifts. You want the foot position down cold first.

    Your next step is to practice (only in light, or no, traffic) smooth upshifting and downshifting while maintaining about the same speed. No braking here; only shifting. This means you'll be upshifting normally (smoothly, as above) and downshifting using a little 'blip' of the gas to make it smooth. Work on that for a couple of weeks until you can go up and down through the gears smoothly. You should be able to easily go from high in second gear to third and fourth and back through third to second - all at around 30 mph. Around 40, you should be able to go from third to fifth or sixth and back.

    Finally, you get to try all at once. Start doing your downshifts while braking. That is, brake with the left side of your foot and use the other side to blip the gas as you did when you were only shifting. You will be delighted the first time you pull this off smoothly. As you spend another few weeks on this, it should become second nature. It did for me. I habitually downshift while heel-and-toe braking. This is not just a track skill. It almost always pays to do it on the street as well. This way, your car is always in the right gear for the quickest acceleration, should you need it.

    The last unanswered question is how fast you can go as you downshift. NEVER downshift into a gear when the car is going faster than you could drive in that gear while accelerating! NEVER, EVER EVER! That can lead to what we call a 'mechanical over rev'. When that happens, the tires, the gears, and the gearbox mechanically spin the engine faster than it's designed to go - above the redline. VERY BAD THINGS© happen then. Valve springs break. Valves break. Pistons get dented. Engines seize. Like I said, VERY BAD. So, as you see a skilled downshifter slow for a corner, the downshifts are progressive and in order. 5>4; 4>3; 3>2; 2>1 Each downshift must be at an appropriate speed or the engine cannot be revved high enough with the accelerator (as you must, to match the revs) and the tires and metal drivetrain parts will take over. If you're lucky, your rear tires will lose traction for a brief second and save your engine. If not, you could be buying an engine. That's why we don't want you to learn this at the track. For better track shifting, learn it on the street where the pace is slower and the shift points are not as critical. Good luck. ;)
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    echanda

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    First, thanks for all the advice. The comments here, and the referenced thread, have been very valuable. I drove about 200 miles today and had lots of opportunity to practice what I have read. The technique described here is so much easier than double clutching and I will give that a few weeks to sink in before attempting to heel-toe.

    Your first day on the track is so overwhelming and its hard to absorb all the information the instructors pass along, but every one of them was awesome and I can't believe how much I learned that day. As a result of my clutch braking (I believe that's what it is called) my in car instructor gave me an overview of rev matching concepts and the various techniques (including double clutching). I think where I got off track is from a book picked up at the track that indicates the following:

    "When the engine isn't directly connected yo the input shaft of the transmission, which would be the case if you let the clutch out, the blip isn't as effective at speeding up the input shaft. There is some drag going on between the motor and the input shaft, and a blip, even with the clutch depressed, will speed up the shaft some. The blip is much more effective, however, if you positively connect the engine and input shaft by letting the clutch back out in neutral."

    I had been following this guidance. Now that I re-read the chapter I realize that they were discussing racing transmissions.

    I can't wait to get back to Lime Rock once I've mastered this! :)

    two30grain guest

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    like previously stated by others, thats really what synchros are for, and they do the job well!

    I tried to practice double clutching but gave up early on, as after awhile I failed to see the point (on the street anyway). the track maybe different.

    Good luck at the track next time, I am interested to see how you did.

    Hopefully I can do a track day soon also!

    Cheers!
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    CRKrieger

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    Makes a big difference with my non-synchro tractor gears ...

    [IMG]
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    echanda

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    The original SUV! ;)
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    Brian A

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    Dude! Cool ride!

    What kind of swaybars you using???? With that kind of staggered wheel sizing, can you dial out all the understeer?
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    Satch SoSoCalifortified

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    Heal and tow

    Part of the problem is our terminology. "Heel-and-toe" downshifting sounds like a nifty idea until a novice tries to actually manipulate pedals with his (or her) heel and toes. . . .

    In my car, it's mostly ball-of-the foot-and-side-of-the-foot activation. That is, I bring up the revs with my foot on the brake by rolling my ankle to the right so the side of my foot catches the accelerator pedal. It helps if the pedals are arranged nicely; in my E30, I screwed an inverted-L-shaped block to the accelerator so I could catch it at just the right position.

    But that's for rally work, where a smooth, fast downshift is a critical element of the game.
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    Zeichen311

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    Needs a tricolor M stripe and ghost flames. I bet Satch can hook you up. :D

    cwbiii guest

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    Looks like an 8n or 9n... used to use one of these occasionally as a teenager on the farm.
    Looks like an international in the background.

    Chuck
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    echanda

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    I used to collect baled hay with one of these. It had no brake pads in the right wheel, so you needed to be very strategic when you came down hills with a full load. It was better than the other choice we had. It had no seat, so shifting was an adventure (double clutch or regular). :)
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    CRKrieger

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    The 9N (1939 series) was all grey. The 2N (1942 series) was red & grey, aka "Redbelly", like this, but a lot more rare. Both of those had three-speeds. The 8N (1948 series, with a 4-speed) is what I have, but this is not my tractor. It's just a photo I snagged online. Mine is very similar (headlights off, sitting on a shelf; brush guard) with this stock paint scheme. I'm leavin' it this way 'cause I don't like no green or red tractors. :p That, and I'm not painting my elders. It's a '50 model while I'm a '51 ...

    It does have a bit of an understeer problem if you're in deep enough mud or you've got a plow dug in deep behind you. No antiroll bars, though because, being bolted directly to the transaxle, the body can't roll in relation to the rear axle. :D

    The owner's manual actually discourages shifting on the fly by telling you to just start out in whatever gear you're using. Still, a gearbox is a gearbox, synchros or not - so you can double clutch shift it. ;)

    Oh; and Satch, I actually DID learn to heel and toe shift. The 2002 accelerator is anchored on the floor (actually, so's the brake & clutch!), so it was located where I could hit it with my heel. I taught myself, then modified the technique when I got into cars with suspended pedals.

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