Difference between rev matching and double clutching?

[IcedKiwi]

Galveston said:

Sorry to be a pedantic bugger but the cut of the gears isn't relevant, it's the engagement system that matters. Straight-cut gearboxes (or helical actually) can have either synchromesh or dog engagement systems, but it's only dog boxes that require DDC for mechanically sympathetic down changes.

True - Dog clutches systems are still constant mesh, and synchromesh gearboxes should really be called constant mesh synchromesh but it gets a bit of a mouthful, and as all gearboxes are now constant mesh it seems appropriate to drop the constant mesh bit.
Sliding mesh are the other type where you don't have an engagement system and just crash the gears together, but they haven't been used for year.

[MC Bodge]

IcedKiwi said:

True - Dog clutches systems are still constant mesh, and synchromesh gearboxes should really be called constant mesh synchromesh but it gets a bit of a mouthful, and as all gearboxes are now constant mesh it seems appropriate to drop the constant mesh bit.
Sliding mesh are the other type where you don't have an engagement system and just crash the gears together, but they haven't been used for year.

Hence the name "Crash 'box"

[Cockey]

RobM77 said:

No clutch at all: Apparently this is possible (for example, if your clutch goes and you need to get home), but I've never actually had a clutch go (probably because I always rev match!), so I've never tried a downchange without the clutch, only ever upchanges (which is a standard technique with a straight cut racing gearbox, in fact it's kinder to such a gearbox to avoid the clutch on upchanges).

This used to be a bit of a party trick in my old (Williams-esque) Clio 16v. Funny enough it's the only car I've ever needed a new gearbox in, but I'm adamnent it was just a coincidence

[MC Bodge]

Doing clutch less shifts, up and down, in my first car taught me about the importance rev matching.

[Dave Holmes]

It is all very well to dismiss the need to learn double de-clutching because modern cars do not require it, but there is nothing sadder than to watch a modern driver get into an old classic and make scrambled eggs with the gearbox because he just doesn't appreciate how a gearbox works. In any case why put a strain on synchro-cones when you can keep 'em new by double de-clutching?

[slyelessar]

Interesting...

[Bigends]

The main difference between rev matching and double declutching ( on downwards changes)is that the box is in neutral when revs are applied - its a nice technique to learn and still occasionally have a play - makes for super smooth downwards changes even on a modern car

[RobM77]

Dave Holmes said:

It is all very well to dismiss the need to learn double de-clutching because modern cars do not require it, but there is nothing sadder than to watch a modern driver get into an old classic and make scrambled eggs with the gearbox because he just doesn't appreciate how a gearbox works. In any case why put a strain on synchro-cones when you can keep 'em new by double de-clutching?

I'm not sure that single declutching and rev matching puts much strain on the synchro on a road car does it? not relative to the life of the vehicle anyway. Surely with a single declutch and a rev-match you're only asking the synchromesh to take up the inertia in the gearbox (layshaft etc) and clutch components (friction plates etc) when the gears connect? That's not an onorous task for synchromesh designed to cope with inertia from an entire flywheel and engine spinning at a different rate to the target gearing.

I've always changed down in road cars using a single declutch and a rev match and I've never changed a clutch or changed synchro in a gearbox, despite doing a high mileage and typically taking cars to 150k miles or so.

[waremark]

Bigends said:

The main difference between rev matching and double declutching ( on downwards changes)is that the box is in neutral when revs are applied - its a nice technique to learn and still occasionally have a play - makes for super smooth downwards changes even on a modern car

The main difference is letting the clutch out while the box is in neutral to make it a DOUBLE declutch. What you describe is surely what many of us mean by rev matching

[IcedKiwi]

RobM77 said:

'm not sure that single declutching and rev matching puts much strain on the synchro on a road car does it? not relative to the life of the vehicle anyway. Surely with a single declutch and a rev-match you're only asking the synchromesh to take up the inertia in the gearbox (layshaft etc) and clutch components (friction plates etc) when the gears connect? That's not an onorous task for synchromesh designed to cope with inertia from an entire flywheel and engine spinning at a different rate to the target gearing.

I don't believe the synchromesh is designed to cope with the flywheel and engine - That would basically be a clutchless gear change, and applying pressure to the gearstick until the speeds match and slot in. With a normal gearchange (even without a rev match) the synchro is only changing the speed of the layshaft and friction plate. When you rev match, the only difference is that the engine is brought to the same speed as the layshaft so when you bring the clutch up the two sides of the clutch are spinning at the same speed.
But agreed that the synchro is not going to wear out over the life of a vehicle unless perhaps you like changing into 1st while moving lots.
The only smoothness achieved with a double clutch is how easily the gearstick slots into place, the vehicle (and passengers) should be unaffected. It's the rev match that takes away any perceivable smoothness.

[trashbat]

Can anyone explain to a simpleton why, separate to rev matching, double declutching was ever required?

I think I know what a synchromesh is, something that uses friction to absorb the difference in gearbox and engine speed.

I still don't know why, if you could match speeds well, the intermediate neutral-with-clutch-up stage is needed.

[RobM77]

IcedKiwi said:

RobM77 said:

'm not sure that single declutching and rev matching puts much strain on the synchro on a road car does it? not relative to the life of the vehicle anyway. Surely with a single declutch and a rev-match you're only asking the synchromesh to take up the inertia in the gearbox (layshaft etc) and clutch components (friction plates etc) when the gears connect? That's not an onorous task for synchromesh designed to cope with inertia from an entire flywheel and engine spinning at a different rate to the target gearing.

I don't believe the synchromesh is designed to cope with the flywheel and engine - That would basically be a clutchless gear change, and applying pressure to the gearstick until the speeds match and slot in. With a normal gearchange (even without a rev match) the synchro is only changing the speed of the layshaft and friction plate. When you rev match, the only difference is that the engine is brought to the same speed as the layshaft so when you bring the clutch up the two sides of the clutch are spinning at the same speed.
But agreed that the synchro is not going to wear out over the life of a vehicle unless perhaps you like changing into 1st while moving lots.
The only smoothness achieved with a double clutch is how easily the gearstick slots into place, the vehicle (and passengers) should be unaffected. It's the rev match that takes away any perceivable smoothness.

That's what I meant - sorry if I wasn't very clear.

[IcedKiwi]

RobM77 said:

IcedKiwi said:

RobM77 said:

Surely with a single declutch and a rev-match you're only asking the synchromesh to take up the inertia in the gearbox (layshaft etc) and clutch components (friction plates etc) when the gears connect?

Some stuff

That's what I meant - sorry if I wasn't very clear.

Sorry - I read this sentence wrong I believe. The Synchro takes up the gearbox inertia. The clutch takes up the engine inertia when the gears connect.
NOT as I read the synchro takes up the gearbox, clutch, flywheel & engine

[IcedKiwi]

trashbat said:

Can anyone explain to a simpleton why, separate to rev matching, double declutching was ever required?

I think I know what a synchromesh is, something that uses friction to absorb the difference in gearbox and engine speed.

I still don't know why, if you could match speeds well, the intermediate neutral-with-clutch-up stage is needed.

You've basically got three independent components that can all spin at different speeds.
1. The engine
2. The gearbox input shaft (layshaft)
3. The gearbox output shaft (directly related to road speed)

When driving in gear everything is spinning at the right speed. When you push the clutch in you disconnect the engine and the layshaft. If you then blip the throttle, ONLY the engine changes speed. The layshaft stays spinning at the same speed. If you then change gear the layshaft and output shaft will still crunch because as far as the gears are concerned, nothing's changed.
By putting it in neutral and lifting the clutch, you can then blip the throttle which changes the speed of the engine AND layshaft. So now when you select the new gear the layshaft and output shaft are spinning at the right speed and slot together nicely.

The synchro basically changes the speed of the layshaft which with the clutch down and in neutral is basically spinning freely.

Hope that makes sense

Edited by IcedKiwi on Wednesday 17th June 10:05

Edited by IcedKiwi on Wednesday 17th June 10:10

[trashbat]

Yes it does - thanks

[RobM77]

IcedKiwi said:

RobM77 said:

IcedKiwi said:

RobM77 said:

Surely with a single declutch and a rev-match you're only asking the synchromesh to take up the inertia in the gearbox (layshaft etc) and clutch components (friction plates etc) when the gears connect?

Some stuff

That's what I meant - sorry if I wasn't very clear.

Sorry - I read this sentence wrong I believe. The Synchro takes up the gearbox inertia. The clutch takes up the engine inertia when the gears connect.
NOT as I read the synchro takes up the gearbox, clutch, flywheel & engine

You're quite right, I was indeed getting myself confused! Now that I've re-read what I wrote it was indeed wrong. I'm not used to thinking about road car gearboxes. In normal use in a road car the gears are of course usually engaged with the clutch friction plates seperated from the flywheel, so the synchro only ever takes up the inertia in the gearbox. The difference with DDC is that halfway through the gearchange you reconnect the engine to the gearbox with the car in neutral and then do the blip to raise engine revs, which spins up the gearbox internals so the speeds match and put less onus on the synchromesh to engage the gears.

[BertBert]

It would be a mistake to consider there to be be no friction between the components of the clutch when it is fully depressed. Thus the rev match blip made in neutral will have the effect of spinning up the layshaft and easing the life of the synchros.

So you find with the nice neutral blip on the downchange, pushing the gearlever into the lower gear is easier (and if it is a positive attribute Reg?) faster.

Bert

[IcedKiwi]

BertBert said:

It would be a mistake to consider there to be be no friction between the components of the clutch when it is fully depressed. Thus the rev match blip made in neutral will have the effect of spinning up the layshaft and easing the life of the synchros.

So you find with the nice neutral blip on the downchange, pushing the gearlever into the lower gear is easier (and if it is a positive attribute Reg?) faster.

Bert

I don't think there should be any friction between them with the clutch fully depressed. If there was and you had clutch drag, the layshaft would be spinning when the vehicle is stationary and you'd be using the synchros to go into 1st every time. And on a lot of cars there's no synchro on reverse so you would get a crunch.
You'd be trying to change the engine speed every shift using the synchros which I imagine would burn through them pretty quickly. The synchros are pretty small and I doubt they'd have enough friction to breakaway the clutch if it was dragging.

Although I'm prepared to be corrected

[RobM77]

Of course we shouldn't lose sight of what's actually necessary in a normal modern road car, and I don't believe DDC rev-matching is, beyond personal satisfaction of course. If you drive a car until 200,000 miles, then clutches are seen as consumables by most people in that lifespan, but if you SDC rev match then it will significantly extend the life of a clutch (for example, I've never changed one). SDC rev match downchanging is also dynamically sympathetic and could actually be seen as essential in many powerful road cars, especially FE/RWD ones with lightly loaded driven wheels, or any car in the snow or slippery wet conditions.

The other thing that interests me is the wear on the clutch actuation mechanism if you DDC, because you're operating the clutch twice as much as normal. I have no data to support that though, so it's just a pondering.

[james_gt3rs]

trashbat said:

Can anyone explain to a simpleton why, separate to rev matching, double declutching was ever required?

I think I know what a synchromesh is, something that uses friction to absorb the difference in gearbox and engine speed.

I still don't know why, if you could match speeds well, the intermediate neutral-with-clutch-up stage is needed.

There's one use for it for me. When taking a rolling first gear at 5mph in traffic, I'll DDC and it slots into first gear nicely. A lot of drivers just force it into first at 10mph and that wears the synchro.