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Stupid shifting question

Discussion in 'Technical Q&A' started by Birdman, Nov 25, 2003.

  1. Birdman

    Birdman F1 Veteran

    Jun 20, 2003
    6,683
    North shore, MA
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    THE Birdman
    Ok everyone, I am going to have to just admit that this is a really stupid question, and it will reveal my lack of car knowledge. Nonetheless, here it goes...

    What is the point of double shifting and why is it done?

    Thanks!

    Birdman
     
  2. Birdman

    Birdman F1 Veteran

    Jun 20, 2003
    6,683
    North shore, MA
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    THE Birdman
    What I meant to ask was about double CLUTCHING, sorry. See, I'm so clueless I can't even ask the question right!!
     
  3. The technique originated in cars that did not have syncronized transmission. When shifting gears one gear is turning much faster than the other gear. A syncro has teeth which lock the two gears to the same speed (as the whole unit is rotating on a slider).

    In today's modernized cars the transmissions all have syncros, but the theroey is still the same: 1) depress clutch move gear shift from first to nuetral. 2) release clutch to start the input shaft spinning (while revving engine with gear selector in nuetral. 3) depress clutch and move selector from nuetral to second (while both shats are spinning a the same speed).
     
  4. Birdman

    Birdman F1 Veteran

    Jun 20, 2003
    6,683
    North shore, MA
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    THE Birdman
    So in a modern transmission (and I'm making an assumption that a 308 has a "modern" transmission) is there a good reason to double clutch?

    I have heard people say that you are supposed to double clutch a Ferrari, but I don't know why.
     
  5. AJS328

    AJS328 F1 Veteran
    Owner

    Apr 23, 2003
    7,518
    New Jersey
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    Augustine Staino
    Birdman,

    I am not sure that it is entirely necessary but I have noticed that when done properly it can make shifting gears considerably smoother in certain situations. My car likes it on 3rd to 2nd downshifts. :)
    -Augustine
     
  6. I have to double clutch my 355 from first to second until it warms up. If I do not double clutch it either will not go into second (so I just go straight to third).

    Yes your f-car does have syncros, so in theroey, once your tranny is warmed up you are merely matching shaft speeds making the job of the syncros (soft metal) easier with less wear.

    Other than cold 1-2 up shifts, or really hard downshifts, I have never heard of f-car dirvers double clutch on a everyday basis.
     
  7. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    6,335
    Think of it like this:

    A transmission has an input shaft and an output shaft. The output shaft is connected to the rear wheels (via differential) and will turn at a rate proportional to the road speed. The input shaft is connected to a clutch. When the clutch is engaged, the input shaft turns at engine speed, and when disengaged, it freely turns.

    Gears are selected by means of a dog (a gear-like device with teeth machined to grab the selected gear) and a spline to transmit the force to/from the shaft. Gears and shafts have rotational inertia, so does the drive line and the engine.

    When the input shaft is not rotating at the same speed as the output shaft and the shifter trys to engage a gear, the dogs bite into the selected gear and make a distince snick sound. Alternately, the dogs can miss biting the other gear and a horible grinding sound ensues.

    A synchronizer is a device that uses the pressure of the shifting dog onto the selected gear to spin the dog up/down to the speed of the output shaft and ensure that the snick occurs not the grind. Since the synchro relies of friction to spin the dog up or down (matching revs) it can wear out. As such a synchronizer is just like brakes and clutches in that it relies on friction (e.g. wear) to perform its job. The more work you make the synch perform, the shorter its life.

    Double deClutching is a means by which the driver can perform the work of the synchronizers. While the gear selector is between gears, the clutch is released and the throttle is pressed to rev match the input and output shafts. At this point, the selected dog can engage the driven gear without disengaging the clutch, but most praticioners of this method press the clutch pedal to minimize noise and maximize their sense of accomplishment.

    So, one double deClutches to take load off the synchros, and the engagement teeth on the dog. If one practices long enough on this method, it takes but a fraction of a second longer than a single declutch and is much easier on the transmission parts.
     
  8. Dr Tommy Cosgrove

    Dr Tommy Cosgrove Two Time F1 World Champ
    Rossa Subscribed Owner

    May 4, 2001
    28,469
    Birmingham, AL
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    Tommy
    You have to do it all the time with an Alfa's 2nd gear.
     
  9. Birdman

    Birdman F1 Veteran

    Jun 20, 2003
    6,683
    North shore, MA
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    THE Birdman
    Wow, this is good stuff! Thanks guys, this is why I love FChat. I learn so much here! I have noticed that when cold, my 308 doesn't shift as easily. I'll try double clutching to see if it helps. You think I shift this gated shifter slowly now, wait until you see me do it when I have to orchestrate two clutch disengagements per shift!

    Birdman
     

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