Difference between rev matching and double clutching?

Difference between rev matching and double clutching?

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Yadizzle1

Original Poster:

684 posts

124 months

Thursday 30th April 2015
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As the title says, I was wondering what the technical differences would be between rev matching, say from 4th into 3rd, and double clutching?


SK425

1,034 posts

148 months

Thursday 30th April 2015
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Yadizzle1 said:
As the title says, I was wondering what the technical differences would be between rev matching, say from 4th into 3rd, and double clutching?
They're different things. 'Rev matching', the way the phrase is usually used, refers to matching engine speed to road speed before re-engaging the clutch after a gear change, so that there is no clutch slip, no jerk, no sudden change in revs as the clutch re-engages.

Double declutching is the process of: clutch down - gear to neutral - clutch up - adjust revs to be appropriate for new gear - clutch down - gear to new position - clutch up. Another way you might think of it is that you kind of change gear twice - first into neutral and then into the new gear you want, and in between those two you adjust the revs to be appropriate for the new gear. What you're matching there is the speeds of the two sides of the gearbox, so that when you engage the new gear they synchromesh doesn't have any work to do. Hopefully, if you've got the rev adjustment right then you will also achieve a rev matched gear change in the first sense.

Putting some round numbers on it to illustrate, imagine your car does 2000rpm at 40 in 4th and 3000rpm at 40 in 3rd...

Rev matching:
1. Clutch down
2. Move gear lever from 4 to 3 and at the same time raise the revs from 2000 to 3000
3. Clutch up, free from slipping, jerking and sudden rev changes

Double de-clutching:
1. Clutch down
2. Move gear lever from 4 to neutral
3. Clutch up
4. Raise the revs from 2000 to 3000
5. Clutch down
6. Move gear lever from neutral to 3, feeling no resistance to the movement
7. Clutch up

Does that make any sense?

Yadizzle1

Original Poster:

684 posts

124 months

Thursday 30th April 2015
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Thanks SK, so double clutching isn't really neccessary on modern cars with synchromesh gearboxes? Does that mean I'm safe to rev match down through gears to achieve a smooth downshift?

R_U_LOCAL

2,676 posts

207 months

Thursday 30th April 2015
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Different things, yes, but not incompatible with each other.

You can rev-match when single-clutching and you can rev-match whilst double-clutching. On my orignal advanced police course in 1995, double de-clutching was still being taught. Not because the cars required it - they all had fully synchromesh gearboxes with no real need for double-clutching.

The reason it was still taught was to encourage students to slow their gearchanges right down - a slower change is generally a smoother change and the extra pause in the neutral phase gave students more time to match revs on both up and down-changes, and also gave them a more definite phase to consider as part of the planning process on the approach to hazards.

By the time I was instructing in the early 2000s, double-clutching was no longer taught as a matter of course, but I would use it as an instructional technique occasionally when I ever had a student who continually rushed their gearchanges.

The best way to describe the technique is "doing the hokey-cokey on the clutch pedal".

In-out-in-out (shake it all about).

Mechanically, it's almost completely unecessary in modern cars, but can, very occasionally, help with a change down to first on the move, when accompanied with a healthy rev-raise.

Not a bad skill to learn and practice though - particularly if you ever get the chance to have a go in a vehicle with an old fashioned crash (non-synchro) gearbox.

ETA - everything you need to know about gearchanging and rev-matching can be found here:

Zen and the art of changing gear

Edited by R_U_LOCAL on Thursday 30th April 22:50

Toltec

7,159 posts

222 months

Thursday 30th April 2015
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Some cars do not like going into first gear while moving above a few mph, my old impreza for example, double declutching would let me change down into first where a rev match would not.

Seldom needed though, uphill into hairpin bend or driving around a circuit were about it.

Edited by Toltec on Thursday 30th April 22:54

Yadizzle1

Original Poster:

684 posts

124 months

Thursday 30th April 2015
quotequote all
R_U_LOCAL said:
Different things, yes, but not incompatible with each other.

You can rev-match when single-clutching and you can rev-match whilst double-clutching. On my orignal advanced police course in 1995, double de-clutching was still being taught. Not because the cars required it - they all had fully synchromesh gearboxes with no real need for double-clutching.

The reason it was still taught was to encourage students to slow their gearchanges right down - a slower change is generally a smoother change and the extra pause in the neutral phase gave students more time to match revs on both up and down-changes, and also gave them a more definite phase to consider as part of the planning process on the approach to hazards.

By the time I was instructing in the early 2000s, double-clutching was no longer taught as a matter of course, but I would use it as an instructional technique occasionally when I ever had a student who continually rushed their gearchanges.

The best way to describe the technique is "doing the hokey-cokey on the clutch pedal".

In-out-in-out (shake it all about).

Mechanically, it's almost completely unecessary in modern cars, but can, very occasionally, help with a change down to first on the move, when accompanied with a healthy rev-raise.

Not a bad skill to learn and practice though - particularly if you ever get the chance to have a go in a vehicle with an old fashioned crash (non-synchro) gearbox.

ETA - everything you need to know about gearchanging and rev-matching can be found here:

Zen and the art of changing gear

Edited by R_U_LOCAL on Thursday 30th April 22:50
Thanks Reg, that link and your explanation was really helpful. I'm trying to practice double clutching but I can't do it without revving both times I shift essentially! I guess a bit more practice is needed.

R_U_LOCAL

2,676 posts

207 months

Thursday 30th April 2015
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Yadizzle1 said:
Thanks Reg, that link and your explanation was really helpful. I'm trying to practice double clutching but I can't do it without revving both times I shift essentially! I guess a bit more practice is needed.
Forget double-clutching for now. Just stick with single-clutching, but slow your changes down. Remember there are two actions in a gearchange, not just one. You take the car out of gear and then you put it in the next gear. Introduce a slight pause in the neutral phase (only half-a-second) during which you can control the engine revs to match engine speed to road speed.

A slight (but not complete) lift when changing up and a gentle squeeze to raise the revs when changing down.

Take your time, keep practicing and it'll soon become second nature.

MC Bodge

21,552 posts

174 months

Saturday 2nd May 2015
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Rev matching makes things smooth and mechanically sympathetic. It marks you out as somebody who considers car control.

Rev matching under braking marks you out as an individual and somebody who considers, and cares about, car control and balance.

Combining all of the above with double-de-clutching marks you out as a connoisseur who is most attractive to members of the opposite sex and extremely popular at parties.
-I also do the above because i like to learn new skills, don't want to lose them and because I can.

hygt2

419 posts

178 months

Sunday 3rd May 2015
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I do double de-clutch with heel and toe on down change. I think this is more from a habit from my old Sierra with worn synchromesh in the gearbox. Certainly not necessary when I got to a Scorpio or Omega. Definitely not necessary in my MR2 or my 996T but I still do it as I find the gears slot in easier.

MC Bodge

21,552 posts

174 months

Sunday 3rd May 2015
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hygt2 said:
I do double de-clutch with heel and toe on down change. I think this is more from a habit from my old Sierra with worn synchromesh in the gearbox. Certainly not necessary when I got to a Scorpio or Omega. Definitely not necessary in my MR2 or my 996T but I still do it as I find the gears slot in easier.
Not necessary, but satisfying and a good skill to have.

Hubris

156 posts

136 months

Sunday 3rd May 2015
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I find heel and toeing a nightmare with the over-servoed brakes in my current car.

MC Bodge

21,552 posts

174 months

Sunday 3rd May 2015
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Hubris said:
I find heel and toeing a nightmare with the over-servoed brakes in my current car.
But when you do master it you have achieved a whole new level of sensitivity.

davepoth

29,395 posts

198 months

Sunday 3rd May 2015
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Being able to pop the car down into first in traffic smoothly is worth the entry price of DDC alone, I reckon. It does make everything feel so much smoother - people have commented on it when they've been a passenger, so it must make quite a bit of a difference.

I think it's also a lot more sympathetic to the car - once you're moving the clutch might as well be an on/off switch if you're doing it properly. I have a bit of a suspect clutch on my DD so it's helping to eke out a few thousand miles extra.

Yadizzle1

Original Poster:

684 posts

124 months

Wednesday 6th May 2015
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Hubris said:
I find heel and toeing a nightmare with the over-servoed brakes in my current car.
The first few times I tried heel toeing I nearly sent myself through the windscreen! Fortunately used to how sensitive the brakes are in my car. Can even left foot brake without scaring passengers now laugh

RobM77

35,349 posts

233 months

Thursday 7th May 2015
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Rev Matching: Performed during a gearchange (usually a downchange), this is the process of changing the revs of the engine manually to match the revs of the new gear that's been selected, rather than letting it forcibly happen when you let the clutch up. For example, if the engine is revolving at 4,000rpm in 4th gear and you want to change down to 3rd gear, then that might require 5,000rpm, so rather than just letting the clutch up and the drivetrain taking on the higher rpm immediately by being forced from the driven wheels upwards, the driver raises the revs of the engine himself/herself before engaging the lower gear, so that the drivetrain is ready for the new gear. There are two reasons that rev matching is done: a) mechanical sympathy, because the drivetrain doesn't like being spun up forcibly b) dynamic sympathy, because the drivetrain's reluctance to spin up will cause a pull/drag on the driven wheels, reducing their grip and in extreme circumstances (heavy braking and/or wet or snowy conditions) it can actually lock the driven wheels (normally if they're lightly loaded, for example in a FE/RWD car when braking).

There are three ways of performing this 'rev-matching' technique:

Single de-clutching: This is like a regular gearchange, but with a rev-match as described above, so whilst the clutch is down and you're moving the gear lever, you press the throttle (either gently or in a blip) to get the new rpm before letting the clutch pedal back up again. This is what most rev-matchers do, whether on the road or race track. I do this with every downchange in both my daily driver and track car (and in a virtual car on PC sims!) and it makes me cringe not to.

Double de-clutching: As above, but the blip is performed with the car in neutral and the clutch up. This connects the engine to the gearbox and spins the shafts in the gearbox up as well. It's usually done either just for that extra bit of satisfaction, or when driving a car without syncromesh gears (so with gears that are straight cut like Lego gears). The only times I've ever done this are when driving 1930s classic cars or when driving a racing car slowly, coming into the pits for example.

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).

HTH

ETA: We might as well define heel and toe whilst we're at it. This is a technique that enables rev-matching whilst braking, because obviously if you want to rev-match in a normal car whilst you're braking then both feet are already being used (left one on the clutch, right one on the brake) and unable to blip the throttle, so you simply blip the throttle with the side of your foot instead. I assume that the reason it's called heel and toe is that most class pre-1950 cars have a throttle way down below the brake, so you use your heel to blip instead of the side of your foot as in a modern car. The two things worth mentioning about H&T is that not all cars have brakes and throttle spaced nicely for it (although most sports cars and RWD sports saloons do - it's the mark of a car designed for keen drivers), and not all cars have a firm enough brake pedal to be used as a fulcrum, because some variation in toe pressure on the brake is inevitable if you're blipping the throttle at the same time. Most modern cars have evolved to have very sensitive brake pedals, but again, most sports cars and RWD sports saloons have resisted this as much as possible - notably Porsche and BMW, although even they're bowing to modern convention of late...

Edited by RobM77 on Thursday 7th May 12:26

jhfozzy

1,345 posts

189 months

Thursday 7th May 2015
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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).
I used to commute between RAF Woodbridge and RAF Wattisham (either side of Ipswich) it was a 45 minute drive.

At the time I had a snotter of a Suzuki Swift which I bought off a mate for £200. One day, my clutch cable snapped about ten minutes into the journey and I had nothing in the car to fix it with at the time.

I drove all the way to work along the A12 and A14 with no clutch, stopping the engine at lights / junctions, putting it into first and starting the engine when I needed to go. I was very relieved when I pulled up at the gates without having a crash or being pulled by the police.

Fixed it in work with a solderless nipple (snigger) and it lasted me another six months before I traded it in.

Dr Jekyll

23,820 posts

260 months

Thursday 7th May 2015
quotequote all
jhfozzy said:
I used to commute between RAF Woodbridge and RAF Wattisham (either side of Ipswich) it was a 45 minute drive.

At the time I had a snotter of a Suzuki Swift which I bought off a mate for £200. One day, my clutch cable snapped about ten minutes into the journey and I had nothing in the car to fix it with at the time.

I drove all the way to work along the A12 and A14 with no clutch, stopping the engine at lights / junctions, putting it into first and starting the engine when I needed to go. I was very relieved when I pulled up at the gates without having a crash or being pulled by the police.

Fixed it in work with a solderless nipple (snigger) and it lasted me another six months before I traded it in.
I take your Woodbridge to Wattisham and raise you central Birmingham to Stafford in the evening rush hour. Then returning next morning to put the car in for a previously booked service and having to explain to the mechanic why he was having trouble driving my car into the workshop.

Changing gear is not that difficult, especially up the box. But moving off from rest probably does the car no good at all. I did hear a story about a police advanced student whose rev matching was so good he didn't notice he was hitting the floor mounted dipswitch instead of the clutch.

Bigends

5,412 posts

127 months

Thursday 7th May 2015
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I was taught to drive as an 18yr old Police Cadet then went onto standard and advanced training. We were taught to double de clutch on down changes right from day one,so just did it naturally from then on. Made for a much smoother change if done properly with the engine revs already matched to the speed of the gearbox internals. Dont do it now, but still can, quickly and smoothly. In the case of clutchless changes - we'd often practice driving without the clutch on those long boring night shifts and had no problems changing up or down -only problem was stopping at junctions. In an emergency, you can engage first and crank the car along on the starter till youre up and running again.

Galveston

708 posts

198 months

Friday 8th May 2015
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RobM77 said:
Double de-clutching: As above, but the blip is performed with the car in neutral and the clutch up. This connects the engine to the gearbox and spins the shafts in the gearbox up as well. It's usually done either just for that extra bit of satisfaction, or when driving a car without syncromesh gears (so with gears that are straight cut like Lego gears). The only times I've ever done this are when driving 1930s classic cars or when driving a racing car slowly, coming into the pits for example.
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.

RobM77

35,349 posts

233 months

Friday 8th May 2015
quotequote all
Galveston said:
RobM77 said:
Double de-clutching: As above, but the blip is performed with the car in neutral and the clutch up. This connects the engine to the gearbox and spins the shafts in the gearbox up as well. It's usually done either just for that extra bit of satisfaction, or when driving a car without syncromesh gears (so with gears that are straight cut like Lego gears). The only times I've ever done this are when driving 1930s classic cars or when driving a racing car slowly, coming into the pits for example.
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.
No, not pedantic at all, thanks for the correction. yes