20 replies to this topic

[Kijima]

Some questions were raised after the first article about Hell and Toeing, and the term Double Clutching was thrown around. So perhaps an Idea of 'exactly' what Double Clutching is needed, (and by a few people here, requested) and how it differs from the Heel and Toe technique, and also why it is not just the same thing as Rev Matching.

Before we get into the technique of double clutching, it's important to discuss the 'whys' and 'how's' in this process.

Firstly, and let me say that this really is a very basic explanation on my behalf, and a lot of you reading this will already understand these processes, so if you are one of those people, you may find this a little elementary. But for the case of this making sense to someone new or not familiar with anything other than what they have been taught when they first got their license, I'll go into detail of the mechanics of it all.

So, we have three key components that are vitally important in the Drive of a car, and the mechanics of it all make up the engine, the gearbox and the wheels. There is an interface between each component of engine, gearbox and wheels which can be engaged or disengaged. The clutch is the interface between the engine and the gearbox, and the gear shifter is the interface between the gearbox and the wheels, and it's those components that we will concentrate on in this thread.

When a break and reconnection happens, it is more sympathetic to make sure the components are spinning together with similar torques (or forces) before engaging or disengaging them. Most drivers don't manage either interface these days as modern cars have Synchros which are doing the mechanical sympathetic work for you…But only to a point.

Make sense so far? Okay, that sounds pretty basic as an understanding of the principles involved, but what of Rev Matching and Double clutching?

Rev Matching

Rev matching is simply a method of getting the engine revs up or down to a more comfortable speed before you release the clutch. This is done by 'blipping' the accelerator pedal with the clutch engaged.

Not only does it sound cool (which by and large is why most people do it these days) it does feel 'smoother' when you have matched the revs to the next gear selected.

And here is where it gets interesting; it feels smoother because it IS smoother, because basically when you blip the throttle, you are manually getting involved in the interface between the engine and the gearbox. This is a big step forward in mechanical sympathy because the shunt that the engine gives the engaged gearbox is strong and carrying a lot of force, so by doing this you are lessening that shunt. All that Torque has to go somewhere right?

Double Clutching

Double Clutching is subtlety different though than just Rev Matching, how so?

Double Clutching differs in that it doesn't really help in a mechanically or sympathetic manner in regards to the engine and gearbox with shunting loads, but rather helps in the area of between the gearbox and the wheels.

While there are forces or load at play within the gearbox, the rotational inertia force of the gearbox is much smaller than the force of the engine. So double clutching really when all is said and done, it just helps reducing rotational inertia which aids the Synchros (if fitted) doing what they do best in matching rotational speeds.

So how do you do it?

Double clutching is when you take the car out of gear and raise the revs whilst you are in neutral. For example if you are doing 80kmh in 4th and you wanted to go into 3rd, you would press the clutch, push the gear shifter into neutral, release the clutch back up, match the revs to wherever they would be in 3rd at 60kmh, keep them there whilst you press the clutch back in, select 3rd and let the clutch back out. This process helps reduce wear and tear on your clutch/Synchros but it's not by huge amounts, nor do Manufactures state that Double Clutching is necessary, that after all is a Synchros job, and some high powered cars will have several Synchros on each gear just to make things feel more progressive, slicker and smoother.

Double clutching of old however, is used when you have no Synchros in a gearbox to 'synchronize' the speed of the two meshing gears. This was an 'art' many years ago, as older cars didn't have the technology of today carrying Synchros in their mixed bag of goodies. This 'art' of double clutching is still alive and well today because so many people do have older/restored cars that are without a Synchromesh Gearbox.

But for the most part of us, and since most moderns cars have Synchro boxes now days, there simply is no real need to bother with it, but in the case of learning something new, it is a real skill, and when learnt, it does feel smoother.

If you have EVER changed gears and while doing so you have you felt a little more pressure than usual or maybe feels overly stiff, maybe the gear shifter won't even go in the first time, but it only goes in when you press a bit harder, or press for bit longer. Double Clutching will work in that scenario every time, because the Synchros spinning the layshaft up to speed before connecting it to the wheels thus the layshaft is spinning at the right speed, its then that the gear will slip in with no effort at all, in fact is some cars it feels so positive it almost sucks the gear shifter into the next gate, and feels really smooth and 'snicky' when changing from gear to gear. So it's not a necessity, but it is a cool skill to have and you just never know when you will be asked to drive some old jalopy that doesn't have a Synchromesh Gearbox

That's about it for Double Clutching, but I'm sure you'll have questions or comments.

[Pettos]

Ahhh, so that explains why it doesn't go into first and reverse easily first time every time... because there is no synchro in first or reverse... like I knew that... but I never knew why... and why when I put it back into neutral then back into gear after reving the engine it slips into gear nicely...

Never had it explained to me thats all... I'm still learnin' this stuff..

Thanks Kij!!

[ZBravo]

I have double clutched all my cars and especially the old Bravo as when I was taught to drive a semi it has just force of habit...

[rosco]

But for the most part of us, and since most moderns cars have Synchro boxes now days, there simply is no real need to bother with it, but in the case of learning something new, it is a real skill, and when learnt, it does feel smoother.

Technical bulletins (Mercedes Benz) at work indicated that double clutching a syncromesh gearbox is not only completely unnecessary, but causes unnecessary extra wear because the clutch has to engage twice for each shift. It was specifically banned.

If the syncros are worn it helps, if there are no syncros it's essential, if it's a modern car with a sound gearbox it's a wank.

I'm guessing every P plater has tried it, I did...

Edited by rosco, 19 August 2008 - 04:25 PM.

[rosco]

Ahhh, so that explains why it doesn't go into first and reverse easily first time every time...

First has syncromesh, it's just that it's at a dead stop. You'll notice it's worse when the oil is cold. Reverse has none.

My last car with no syncro on first was a '66 Morris 1100, so by the '70's they all would have had it.

[Kijima]

Rosco's on the money with both points, first one to a degree as it depends on the makeup of each gearbox andf this point of extra wear versus rotational speed matching is a conundrum to say the least, and you'd only find out who is right after well over 100,000kms of driving I'd reckon.

But Double clutching on a new car is not warranted, hence why I made the comment about it earlier, the only difference here is that Rosco was around when the majority of cars didnt have a synchromesh. ( pay's to be an old bugger eh rosco only kidding mate, plenty of respect going in your direction)

[rosco]

But Double clutching on a new car is not warranted, hence why I made the comment about it earlier, the only difference here is that Rosco was around when the majority of cars didnt have a synchromesh. ( pay's to be an old bugger eh rosco only kidding mate, plenty of respect going in your direction)

I like being an old bugger, I've earned every grey hair I have... I learned to drive in a Vauxhall Viva and did my licence test in an FE Holden. Then I moved up to an AP5 with push button auto behind a 225ci slant six (I wonder how many of these youngsters will recognise that).

Like Dave I have driven crash boxes and it's a hard habit to shake once you have become used to it (I wasn't calling Dave a wanker!). In my case it has been Road Rangers and reversed Spicer 5/6/7 speed boxes in coaches. Mmmm, warm fuzzy feelings for 400+hp of V8 GM two stroke diesel.

One day I want to live on a remote mountain top and hand out wisdom to pilgrims.

Edited by rosco, 20 August 2008 - 10:18 AM.

[ZBravo]

Rosco I didn't take any malice towards your post....when I drive the old buses from the musuem its a must to double clutch as the old 1949 Leyland Double Deckers take a knack to steer....

I find that in the 2 I don't do it as much as I did in the Bravo as that had a 50pound clutch and as a commerical vehicle and far from a street racer it felt comfortable.

the gearbox in the 2 is sweet and light and easy to rush without mistake....

The last crash I drove was a MCI 8 like the one below for a private company....moonlighting is such fun sometimes.

[rosco]

This was my baby... I was sad to see it go! Holy begeezus that thing could fly, it had the big V8 GM and no worries about speed limiters. I did seem to have a habit of damaging tacho cards though...

[ZBravo]

Rosco we better stop with the bus/coach pics or the flames will start...

[rosco]

Rosco we better stop with the bus/coach pics or the flames will start...

It was one of those rare Mazda Dennings...

[Kijima]

To late, both of you are old fossils and are showing it! Anyway, back on topic guys.

[turbo brian]

Double clutching of old however, is used when you have no Synchros in a gearbox to 'synchronize' the speed of the two meshing gears. This was an 'art' many years ago, as older cars didn't have the technology of today carrying Synchros in their mixed bag of goodies. This 'art' of double clutching is still alive and well today because so many people do have older/restored cars that are without a Synchromesh Gearbox.

But for the most part of us, and since most moderns cars have Synchro boxes now days, there simply is no real need to bother with it, but in the case of learning something new, it is a real skill, and when learnt, it does feel smoother.

Also good for 'modern' cars with thrashed, worn, or damaged gearboxes.

a car that is regularly tracked will have a prefered power gear and more often than not, this gear's syncro will be worn, making smooth gearchange difficult or almost impossible at normal street driving conditions, which is where double-clutching will assist in getting the gear in without sounding like you have left half the gearbox back at the last corner.

I have confronted this issue many times, and simply clutch in, shift to neautral, clutch out, clutch in, shift to next gear, clutch out will work as smooth as the day the car drove off the showroom, yet the gearbox would be in need of some serious TLC.

Something to keep in the back of the mind for the possible day when you get a car that simply wont change gears clean, try double-clutching rather than a granna shift and she will go in like it was new again.

B.

[ZBravo]

To late, both of you are old fossils and are showing it! Anyway, back on topic guys.

Listen hear young fellow its Mr Fossil...

[rosco]

To late, both of you are old fossils and are showing it! Anyway, back on topic guys.

Listen hear young fellow its Mr Fossil...

If we keep this up someone will buy us a pair of rocking chairs for xmas. You know, back in the days before the interwebs were discovered we had a thing call a life... that was even back before the first microwave!

[ZBravo]

back when double shuffle was all the rage..

[Dr. Teeth]

Do they even sell rocking chairs now? Or do I have to dig one up from a museum? lol

[rosco]

back when double shuffle was all the rage..

Indeed! They could really boogie...

[Shagger]

so... when young vinny in fast and the furious said " Granny shiftin' not double clutchin' like you should" how does double clutching improve performance in racing terms. Wouldnt the act take forever?

[rosco]

so... when young vinny in fast and the furious said " Granny shiftin' not double clutchin' like you should" how does double clutching improve performance in racing terms. Wouldnt the act take forever?

Vinny also seemed to have about 35 gears and a throttle that could always be pushed that little bit further down.

It doesn't take long, and usually you don't need to use the full travel of the clutch; often it's only a few mm between being engaged and not. It's all in the timing. The advantage is in the strength of the gearbox.

A non-syncro box is generally built to take a lot of punishment but would be a PITA to use on the road for the average person.

A syncro box is far easier to use but the syncros are a weak point when it comes to absorbing punishment.