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Double clutching

3K views 20 replies 15 participants last post by  VettTheRipper 
#1 ·
I know there are a lot of road-racers on this forum. :) Can someone describe the technique used for down-shifting? I was told that double clutching, with a small blip on the gas while in neutral, will allow one to match revs perfectly and get those really smooth shifts. Is this true? Anything I am missing / need to be aware of?

-Anil
 
#4 · (Edited)
anil - double de-clutching is common practice and is used to match pressure-plate RPM with clutch-plate speed. great for trail-braking and getting the RPM right for powering out of a corner all at the same time.

brake, clutch out and heel-toe blip the throttle while briefly in neutral, when revs peak with the blip, declutch.

with a bit of practice, the blip will be the right amount to match RPM's.

one tip, is that while you're learning and the RPM's aren't 100% matching, its better to declutch while the blip is decellerating the RPM than accelerating. declutching with an accelerating blip will cause driveline shock which of course it not good.

good luck

Boz.

PS. (edit) - you don't need to heel-toe while learning how to double-declutch. this is only needed if you actually want to brake while declutching. you can learn double-declutching technique before you begin the (more complicated) learning of heel-toe.
 
#5 · (Edited)
Anil -

(Edit - I was writing this as Boz posted, removing some redundant stuff from mine ...)

With the ratios in most street cars, my opinion is that it is easiest to learn by coasting down and double-clutching on the 5-4 downshift and the 4-3 downshift.

Basically -
clutch in
shifter to neutral gate
clutch out
blip throttle in neutral to get input shaft spinning
clutch in
shifter to desired gear
clutch out to match RPM to speed and gear as RPM is coming down

Once you get the hang of it, you don't need to double clutch and can simply blip the throttle as you are passing through. The ultimate is doing this without the clutch.

Just starting out, I recommend double-clutching because it gives a split-second to know if I've missed the gear (i.e. 4-1 would be bad). With the double-clutch, the synchros shouldn't have to work hard and the shift should come extremely easy ... almost as if the shifter is tugged out of your hand. If there is a lot of resistance, wrong gear.

If you are really wanting to be technical, you can straight-line interpolate required RPM at a given speed using the gearbox ratios. As an example, if you are in fourth (1.0 - 1) at 3000 RPM and need to go to third (ex: 1.3 - 1) then you need 3900 RPM, assuming you can shift instantly ... which doesn't happen, the car is costing down so you will want to synch the clutch as the motor RPM is coming down (relates to Boz point "its better to declutch while the blip is decellerating the RPM than accelerating. declutching with an accelerating blip will cause driveline shock which of course it not good").
 
#7 ·
Double clutching (at least in the ZO6)- a method of taking a smooth, simple process, and complicating it beyond what is necessary. Sort of the way upper management will often try to solve simple problems.:D
 
#9 ·
It's not *really* for the transmission, it's so that engine speed matches wheel speed when down-shifting and slowing for a turn. If they don't match, then the rear wheels lock and you try to go through the turn backwards. While this is amusing for the competition, it isn't a whole lot of fun for the occupants of the car going backwards. :cheers:
 
#10 ·
Re: You lost me...

Dreamin said:
One more time... why in the world do i need to double-clutch in the modern tranny with syncho's?? Isn't a heal-and-toe blip sufficient?
My tranny downshifts smoothly and cleanly at any RPM with a good blip timed perfectly when pushing the pedal in. I drove dueces on occasion during my military career (old covered wagon looking 6WD dinosaurs) and had to double clutch a lot. Some folks do double clutch their cars but the explanation as to why doesn't make any sense to me.
It's just my opinion but if I had to double clutch my Z or any modern tranny for that matter, it would be in the shop getting that POS tranny fixed.
 
#11 ·
What is desribed above is true double clutching. Even racers do not do that anymore. The most common technique is:
Clutch in
shift into nuetral
blip gas
shift into lower gear
let clutch out
Doing this eliminates the driveline shock and skidding of the rear wheels, ( not good as you are at the limit of the tires)
This is what they teach at Russell Racing and I have used this technique in Formula Fords, Formula Russell's and Spec. Ford Racers. None of these cars have synchros in the transmissions.
It also helps to be smooth and not upset the car. Smooth is fast.
 
#13 ·
Re: Re: You lost me...

VettTheRipper said:

My tranny downshifts smoothly and cleanly at any RPM with a good blip timed perfectly when pushing the pedal in. I drove dueces on occasion during my military career (old covered wagon looking 6WD dinosaurs) and had to double clutch a lot. Some folks do double clutch their cars but the explanation as to why doesn't make any sense to me.
It's just my opinion but if I had to double clutch my Z or any modern tranny for that matter, it would be in the shop getting that POS tranny fixed.
Well, not really, you engine speed and the trans output shaft are synch'd, but you're still forcing the input shaft to accelerate/decelerate by using the synchros.

Doubleclutching is going to save the synchros from wear. Thats what it buys you.
 
#15 · (Edited)
Re: Re: Re: You lost me...

mgarfias said:
Doubleclutching is going to save the synchros from wear. Thats what it buys you.
This may be true but letting your car sit in the garage will save your syncros also.....it's overkill in the extreme sense. It's the equivelant of changing your oil every 500 miles so your engine will last longer. :D

Drive it .....like ya stole it.....till ya break it:cheers:
 
#16 · (Edited)
In the Z06, what double clutching/declutching does is let you compress the number of actions done, that you would normally do straight line sequentially.

I learned double clutching/declutching in the service. (on vehicles up to 45 ft tractor trailer rig) The principles of why you should be double clutching/declutching are sound. One difference between say a tractor trailer rig and a z06 is that the diesel rig can, does, and should engine brake, while on a Z06 there are very few instances where I would engine brake.

The invention of syncho's made a straight downshift an option, but can be dicy, for a lot of reasons. The truth is if you don't drive on the edge, you dont really need to learn. However, you will never be on the edge unless you learn double clutching and declutching as part of the heel and toe procedure. But the fact of the matter is that it does save on the syncho's and more importantly gets you physically and mentally prepared for the transitions.
 
#18 ·
I double clutch about 90% of the time when down shifting out of habit. My 62 still has the original syncros in it!

Anyway the primary rule in racing is that you must finish to win. The easier you are on the hardware, the better your chances are! This is most important in other than drag events where multiple laps are required.

The Z06 doesn't really heel toe well, it's more like side to side. First time I tried it like I do my 62 I almost put myself through the windsheild, the Z06 has power brakes dooooh!!!
 
#19 ·
Cool philosophy Ripper. I feel the same way, that's why I have been learning to autocross. I do however sometimes have problems keeping the rear end stable while downshifting. I have not tried any of the methods described above. Wonder if it would help in agressive driving on the autocross track?
 
#20 ·
Yeah for sure! If you downshift without the proper balance you can throw the car dynamics out easy! ie your experience with your rear end being unstable.

When you are downshifting there are limited to the downshift!

For example if you skip gears say like 6 to 4 or 6 to 3rd the real limit just from an engine point of view is our redline. On an upshift if you over rev you have the rev limiter. On the downshift you DO NOT have a rev limiter. So you can OVER REV your engine!
 
#21 · (Edited)
I used to run with a group of guys in California during my military daze and we used to practice our motorcycle racing skills on the twisty roads of the mountains of NorCal. We used a technique called "riding the pace". It consists of never ever using the brakes" (i know sounds scary) But it forces you to be more aware of gear selection and RPM control. When you first start doing it you need to go slow and you need to be familiar with the road.

What this does for your driving skills is incredible and speed over the course gets impressive with practice. This technique was useful in learning to race MC's since braking at the wrong time could be disastrous. It really emphasized throttle/clutch/RPM control. Learning proper downshifting was also a key element. Timing is everything when downshifting using throttle control vice double clutching. I'm just so used to it that it's second nature.
 
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