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Old 4th March 2004, 05:48   #1
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Default Whats double clutching & whats the reason to use it

Hi, I am not much knowledgable about the mechanics of cars and dont know about this technique used while racing, why does one use and how does it help and do any of you use this while racing on the road or on a empty strip??


it was used in F&F..

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Old 4th March 2004, 07:33   #2
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Hey C4S,

Firstly, in my opinion the guys from F&F were talking crap... (or maybe there is a reason i am completely unaware of?)

Nowadays cars have syncromesh's on all forward gears, so this makes it pointless to double clutch. Let me try and explain why -

Let me just say once again i am going to simplfy all the processes so the concept can be clearly understood.

Lets say we have 4 main rotating parts
1 - Clutch
2 - Input axle to gear box (between clutch and gearbox)
3 - Output axle from gearbox
4 - the actual gear selector cog(s) (this is the simplified part). When u select a gear all you are doing is moving this "part" around, allowing it to make contact with different gears of different sizes, hence giving different gear ratios.

This is a diagram (just try and realize that the opening on the TOP where the gearlever is connected is connected to a lever in the gearbox that moves around the "moving part"


Lets do a hypothetical upshift from 1st to 2nd without doubleclutching (in a car with a syncromeshed gearbox -ie any car in the last 10++ years)
You are in 1st gear, the engine is at 5,000Rpm, you press the clutch and almost simultaneously release the accelerator.
Shift the gearstick into Neutral, and then continue to push it into second. (where the engine will only be revving at 2,500Rpm)
(Now the part that u are shifting in the gearbox was spinning really fast when u were in first and is still freewheeling very fast in neutral, so when u are pushing into 2nd this is where a syncro comes in....it is like a mini-clutch or think of it as a "Cushion" on the gear that basically gets the moveable peice spinning at the same speed so that when you push into 2nd gear it slides in without any problem.

Hope that was clear!
If you got that it should be quite simple to understand double clutching now - (remember u want to double clutch when there are no syncros)

ok, so you are doing the same 1st to 2nd upshift using dblcltching..
You are in 1st gear, the engine is at 5,000Rpm, you press the clutch and almost simultaneously release the accelerator.
Shift the gearstick into Neutral, and THEN RELEASE THE CLUTCH (this completes 1 "clutching").
By releasing the clutch and having stepped off the accelerator, now the engine is spinning at 2,500rpm which is similar to what is will be revving at in 2nd gear at this speed (u are controlling engine revs). Since the clutch is not pressed and the car is in neutral the "moveable peice" spins at a speed relative to that of the engine.
then PRESS THE CLUTCH AGAIN (2nd clutching) and continue to push it into second. (where the engine will only be revving at 2,500Rpm, which is the speed that the "moveable peice" is aldready spinning at!

Thats basically the basics of it!
As you can now see...double clutching in modern cars is just a waste of time (i cant think of a reason to.....) except sometimes when u are going a bit fast in 2nd and u try and put the car in 1st and it doesnt slot in (i assume u guys know what i am talking abt?) then u just leave it in neutral..leave the clutch...revv the engine...press the clutch and it slots straight into 1st!..there...you just double clutched. I guess in this scenario the syncros were just having a hard time bringing the "moving peice" up to speed??

Im sorry i didnt explain the way the gearbox works in any detail..but it is complicated...so find a good site with pics and.wait...here..this seems to be a great one.....Howstuffworks "How Manual Transmissions Work"
check out the whole site...you will learn a lot...and they have some awesomely helpful flash animations to explain things.

Gearbox's are truly amazing peices of engineering! just try in your mind to create one...even if you aldready know how they work and its still incredibly tough!

cheers.....
Rehaan.

Last edited by Rehaan : 24th July 2008 at 12:20.
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Old 4th March 2004, 07:36   #3
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Im not sure, but in cars without syncros, i dont think it was absolutely neccessary to double clutch unless u were taking the engine quite high before changing, hence causing large discrepancies between rotational speeds of parts.

cya
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Old 4th March 2004, 10:40   #4
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It makes a Ferrari sound good
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Old 4th March 2004, 11:17   #5
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hey guys, correct me if i'm wrong. Our BEST buses need double clutching, right???
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Old 4th March 2004, 12:07   #6
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yes - no synchromesh - frankly it is times our Tata / AL trucks moved from the ambassador age to the volvo age
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Old 4th March 2004, 13:07   #7
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Boy thats a superb explanation Rehaan (and how stuff works). I had a couple of doubts myself, Thanks for clearing that up.

GTO
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Old 4th March 2004, 14:09   #8
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I don't think we need to worry about double cluthing anymore. I'm happy as long as i can shift

The reverse doesn't have syncro right. Is that the reason why the RG makes that irritating sound when revved?

Regards...
Shan2nu

Regards...
Shan2nu
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Old 4th March 2004, 14:37   #9
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Hey Shan2nu,

Im not a 100% sure if reverse has a syncro or not........BUT

the noise you hear is because reverese gear is a "straight-cut" gear and not a "helical cut" gear.

Helical gears are the ones pictured above (ie my first post).....with the diagnolly "slanted" teeth. They greatly reduce noise, but are believed to be slightly weaker (this is debateable).

Hence, since reverse is straight cut u hear that noise. Also, in WRC if you listen carefully, some of the noise amongst the engine, exhaust and turbo is from the straight cut gearbox.

cheers
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Old 4th March 2004, 14:54   #10
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Ya i've noticed that in WRC cars. ven that hissing noise, that must be the turbo right?

Regards...
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Old 4th March 2004, 15:10   #11
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The turbo makes a sounds like a whistle.

If u listen to a octavia TurboDeisel go past u on a quiet night and u pay attention u will notice it. Any TC car for that matter! ;-P
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Old 4th March 2004, 21:24   #12
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THe WRC cars makes this hissing nose bcoz of two reasons
1. Turbo
2. MOdified airsuction ports
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Old 18th March 2004, 21:08   #13
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Yea the Turbo is letting of Excess Air through the Dump Valve. I love that sound
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Old 19th March 2004, 00:24   #14
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A simple explanation of Double Clutching would be... push the clutch.. put the gear in neutral...remove leg... then again press the clutch and put it in the appropriate gear... This method is done to prohibit Grinding between the gears ... and on cars that dont have a Synchromesh...
I hope my explanation is kinda correct
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Old 19th March 2004, 00:29   #15
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Quote:
Originally Posted by [b
Quote[/b] (v12 @ Mar. 04 2004,09:47)]hey guys, correct me if i'm wrong. Our BEST buses need double clutching, right???
The new BEST buses do have a Synchromesh... there are automatics also
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