The Great Double-Declutching Debate (Split from Tips)

NEW AND WANNABE DRIVERS (INTERACTIVE)
21

muckles:
There are so many different types of gearboxes on trucks that how could you really train a student to use them all?

Maybe if it comes up in conversation an instructor could explain the principle to a student, but most drivers are more likely to some across the various auto and semi auto boxes in their driving careers than a Twin Splitter or something similar.

Off on a tangent it amazes me that Haulage companies spend thousands on new trucks with the latest gearbox because it will save them money in fuel and maintainence, but fail to bother to teach the driver how to get the best out of it,

and also some drivers who won’t change change their way of driving because that’s the way they always did it and why should they change their driving because of some new fangled technology.
Of course they have no problems using thier mobile phone, DVD player, or sat nav or any other gaget that’s around, but they’re expert drivers so nobody can tell them anything about driving.

I’ve found all of that to be true. I couldn’t agree more on all points.

22

For as long as it takes me to get into the yard and pick our senior mechanic’s brain again on the subject, then yes. :stuck_out_tongue:

23

Is it possible this myth has come about from poeple DDCing really badly? e.g. revving the engine hard for an up change?

Or maybe confusing techniques and thinking of clutchless changes (again, shouldn’t do any harm if correctly executed but I don’t want to argue that one!)

But assuming the driver expertly performs the DDC and doesn’t force the lever into gear (if he’s done it right it should practically fall into place with little resistance) how can this damage the box?

Hopefully Lucy’s mechanic will resolve the argument before we’re all driving autos and it becomes irrelevant!

24

Tony1968:
Is it possible this myth has come about from poeple DDCing really badly? e.g. revving the engine hard for an up change?

Or maybe confusing techniques and thinking of clutchless changes (again, shouldn’t do any harm if correctly executed but I don’t want to argue that one!)

But assuming the driver expertly performs the DDC and doesn’t force the lever into gear (if he’s done it right it should practically fall into place with little resistance) how can this damage the box?

Hopefully Lucy’s mechanic will resolve the argument before we’re all driving autos and it becomes irrelevant!

Agreed on ALL points. (That’s just about my memory of the subject from when I was at technical college, but you put it better than I could.)
skorpio also seemed to me to be saying the same.

25

Lucy:
Sorry Zetor, but you are wrong on this one.

[best pantomime voice] Ohhh… no… I’m not…[/best pantomime voice]

:laughing:

26

dieseldave:

Tony1968:
Is it possible this myth has come about from poeple DDCing really badly? e.g. revving the engine hard for an up change?

Or maybe confusing techniques and thinking of clutchless changes (again, shouldn’t do any harm if correctly executed but I don’t want to argue that one!)

But assuming the driver expertly performs the DDC and doesn’t force the lever into gear (if he’s done it right it should practically fall into place with little resistance) how can this damage the box?

Hopefully Lucy’s mechanic will resolve the argument before we’re all driving autos and it becomes irrelevant!

Agreed on ALL points. (That’s just about my memory of the subject from when I was at technical college, but you put it better than I could.)
skorpio also seemed to me to be saying the same.

Yes, I know, I’m also just adding to my attempt at explaining gearbox internals on Wednesday. Just a poor excuse to try to keep this thread going until someone can convince me I’m wrong! It’s not easy trying to describe what goes on inside a gearbox and even pictures often just make it worse! Somewhere on the Internet there must be a decent animation…

27

Tony1968:
Yes, I know, I’m also just adding to my attempt at explaining gearbox internals on Wednesday. Just a poor excuse to try to keep this thread going until someone can convince me I’m wrong! It’s not easy trying to describe what goes on inside a gearbox and even pictures often just make it worse! Somewhere on the Internet there must be a decent animation…

You’re not wrong. I’m sure of that. The synchromesh bits are basically redundant if you DDC and match the revs properly. If you don’t use something, it doesn’t wear - ergo double-declutching does not wear synchromesh.

Contrast the howling protest that comes from the gearbox when someone tries to change down into too low a gear from too high a speed… now that is going to wear the synchromesh.

I too am eagerly waiting to hear Lucy’s mechanics explanation - I’m open minded and quite prepared to eat humble pie if needs be :slight_smile:

28

Tony1968:
… Somewhere on the Internet there must be a decent animation…

A quick google turned up some useful links:

Double clutching does 2 things. It makes your downshifts much smoother and it allows your gear synchros to relax:
http://www.se-r.net/transaxle_clutch/clutch_not_to.html

The Bottom Line It’s good for driving trucks, but also good for saving the synchromesh in a transmission:

There are plenty more but the theme is the same - double declutch on downshifts to make life easier for the synchromesh and extend its life.

Last but not least, one more link. Weekended somewhere dull? Don’t forget your meccano set. Amaze and amuse your friends with a working synchromesh model:
http://users.actcom.co.il/meccano/synchromesh.html

29

When I was on coaching, we were taught DDC for one of the reasons Zetorpilot quoted above, that is for a smoother downshift. Thereby giving greater “comfort” to your passengers

30

Ok, I’ve been getting more and more intrigued by this one, as I’ve always been taught NOT to DDC a synchro because it damages it - the first time I remember quite clearly because I was working for that General and Ag firm and had been driving Twin Splits and Roadrangers for them continuously, until one day they let me loose in their single FH12, after a half hour lecture on not blipping the throttle etc etc etc.

Anyway, having been told the same so often since, and seeing such a split in opinions here, I went and did some brainpicking with the two mechanics I most trust in order to try and get a resolution and some concrete details…I tried to put some of the counter-arguements as well, to be totally fair about this. Here’s the results:

EXPERT ONE - 60yr old Fleet Manager with 40 years experience on all types of vehicles, both HGV and Non-HGV.

Does DDC-ing a Synchro box cause damage? - “It certainly wouldn’t do it any good.”

In what way? - “It would cause premature wear.”

How? - "By forcing the synchros to do more work. "

But surely by matching the revs you are taking work away from the synchros? - “No, you are just forcing them to do more work on the opposite “side”,”

EXPERT TWO - 50(ish)yr old Deputy to the above with about the same experience on the same types of vehicles. More inclined to take the time to explain stuff properly.

Does DDC-ing a Synchro box cause damage? - “I wouldn’t of thought so. How can it? It’s just pointless because the synchro cones do it for you.”

In what way? - “Well, all the synchros do is slow down the gears so that they go straight in.”

How? - “Well, you have like rings, or cones, which will slow the spinning cog down to match the engine. On the downchange, they slow the cogs on the engine side down instead, so blipping the throttle just does what the synchros would have done for you - it’s pointless. Especially considering that the synchros don’t do anything until you move the gearstick, so DDC in itself won’t do anything at all other than giving you a sore leg. It’s old Grandad driving.” (doesn’t beat about the bush doesn’t our Steve!)

So when you get an old, tired, synchro box that needs the odd blip here and there to get it to behave, that’s not a result of people DDC-ing it in the first place? “I don’t see how it can be” (repeats the above in more detail, but you get the jist) "If anything, it would wear out the clutch with all the pointless de-clutching. Not the plate, obviously, but the clutch pack. So from that point of view I wouldn’t do it - that and the fact that it’s pointless Grandad driving. "

So what does make a synchro box get tired like that? “The same thing as wears out all gearboxes - drivers leaning on the gearstick as they’re going along, usually.”

So…Between these two, the jury’s out - although I have to say I’m far more convinced by Expert 2 at the moment, if only because he was able to back up his arguement with factual explanation. To be fair to Expert 1, though, he was up to his armpits in bulk box liners and white powder at the time of “collaring”, so didn’t really have the time or oxygen to get into the whole thing on a deeper level…

This is really fascinating me now. I’m really glad it’s come up, because it is, I admit, far too easy to just be told the same thing over and over by different people and take that as gospel. I do still remember clearly De Saint’s explanation, complete with diagrams, but it has sadly disappeared into the PDA archives (they’ve upgraded since then) and the man himself seems to have gone offline. I’m on hols until next Wednesday, but will start collaring yet more random technicians on my return and see if I can dig up anything else…As I said, it’s fascinating me now, so a big thankyou to Dave and Zetor above for giving me the impetus to actually question the details of this! Jeannie the Jinx is bound to break down and land me in a dealership soon, so I’ll have a whale of a time working my way through them all…ahem… :blush: :wink: :laughing: :laughing:

The clutch-pack wear intrigued me. Perhaps that is actually the main reason why the practice is discouraged on heavier vehicles, and just telling drivers “stop it, you’ll [zb] the synchros” is easier in a busy yard than going into a full explanation…who knows…

I still think it should be taught after basic training though. Bearing in mind that they teach “brakes to slow, gears to go” these days, it makes more sense to just get trainees to slow down enough on their brakes for an easy downchange, rather than contradicting that policy by then making them double-declutch and do the work through the gears. Otherwise surely it will only serve to confuse the issue, which brings us back to overload.

The above suggestion does work on boxes with no synchro on 1st and 2nd, by the way. I tried it only last night on my 1967 Landrover, which I normally double-declutch, and even the knackered 2nd gear which jumps out as quickly as you take your hand off the lever stays in quite happily if you slow her down enough first. Bloody thing then stalls because the mixture’s a bit iffy, mind…but that’s another story… :blush: :blush: :blush: …Point is, the gears go in, and there is no need to DDC it in a gearchange exercise situation even with out the lower synchros. :stuck_out_tongue:

Ps. Apologies to anyone stuck behind the Landrover behaving oddly in Lazenby High Street last night. Never let it be said that I’m not prepared to look a complete ■■■ in order to check my facts. :grimacing:

31

Good reply Lucy, you obviously asked the question very carefully…

I can’t agree with Expert one but if he was too busy to explain that’s understandable.

I almost totally agree with expert two. I’ve been arguing the case that DDC won’t do any harm and he seems to go alog wirth that. I can’t totally agree with the fact that it won’t do any good because I think if you alwys did it your sychro would last forever but that’s a small point.

I guess the term “clutch pack” is being used to describe the whole clutch assembly. In this case DDC would have no effect on the clutch plate but would double the wear rate of all the actualting components - release bearing, fork pivot, slave cylinder, pedal bush, drivers kneee etc. In the case of the release bearing the wear would be small compared to sitting at traffic lights with the pedal down.

I can also believe that with the wasted fuel the overall cost of DDC may well outweigh any saving in W&T on the gearbox and totally agree that if learners no longer need to demonstrate it it shouldn’t be taught.

I knew you had a Land-Rover but didn’t realise it was a proper old one! I learned to DDC when I bought a 1964 IIa in the 80’s. With a bit of practice you can change transfer box ratios on the move too but that’s more involved and if you’re short you have the added complication of not being able to see over the spare tyre whilst moving the lever! Given that many people on here have never experienced a “crash” box may I suggest they make an offer for the series one that’s been sitting on my drive for 5 years waiting for me to do the MOT work?

This is an interesting thread - just a pity we seem to have hijacked something that was supposed to help learners!

32

Good point Tony…Better split this little debate off into it’s own thread…I’ll leave it in this forum for now, though, since the whole thing started as a conversation about whether trainees should be taught to DDC - plus can you imagine the carnage if the whole of the Pro Drivers’ Forum steamed in with their two ha’penny worth? :open_mouth: :wink: :laughing: :laughing: :laughing:

33

I was saying to somebody the other day who knew all about these black arts…double de-clutching and clutchless changes…that I’d be interested to learn how to do them but on the one hand I’m not likely to need to know them, and on the other I doubt I’ll even be in a position to be shown it.

As a matter of interest I find the gearboxes on out DAF CFs (4 over 4) quite heavy going sometimes…especially first thing in the morning…would either technique improve that? I certainly don’t rush my gear changing either.

34

The term “brakes to slow, gears to go”, or rather, the converse of it, whatever that might be, primarily originates from the days of less efficient compressors and pre-dates Spring Brakes. Similarly, with the expression that, “you go down a hill in the same gear as you would go up it.” (I suggest) originates from the days when (although engine braking was important) there was a necessity to keep the engine ‘revving’ in order to maintain air in the tanks.

I must say that I am also at a loss to identify how DDC’ing can damage a gearbox, and I’ve stripped and changed the baulk rings in many a (car) gearbox.

I’ve even, at a weekend, found that I needed a replacement clutch master cylinder, and driven around to local scrapyards with the defective part for the vehicle I was driving, lying in the passenger footwell. :slight_smile: Clutchless changes are merely a coordination of of ear and throttle.

We used to have a mixed fleet of ERF’s. Some with the Eaton twin splitter, others with a more ‘conventional’ gearbox. The synchro’s on the Eaton were a waste of time, and therefore ‘clutchless’ changes became the norm. Changing to a ‘non Eaton’ vehicle, particularly towards the end of a long day, I often found myself doing ‘clutchless’ changes in those.

All learnt from the days of driving a Moggy 1000 van, with too little leg room, where it was easier to adjust the revs than re-orientate one’s leg.

:stuck_out_tongue:

35

Lucy:
So tell me Dave…

…In between the poor sods trying to master the gearchange exercise with a whole extra range that they aren’t familiar with…get used to the size of the vehicle and it’s blindspots…learn a whole new set of road positions and turning circles…suss out the reverse…and iron out all the bad habits they’ve picked up driving a car/rigid/whatever…all in around 5 days, if that, and with the pressure of expense and a test at the end of it…

…where exactly are our beleaguered students supposed to find time and concentration for a lecture on when and when not to DDC.has come up in the post above, without causing COMPLETE[/i] overload?
I guess beleaguered students would have to find the same time as before. I had to learn DDC because until you had learned to, you couldn’t change gear unless you had one of those new-fangled F86’s or F88’s.
My guess is that it isn’t DDC that damages gearboxes, it is poor DDC that does. Learning to DDC on synchro boxes masks any defiency in technique - I’ve seen loads of drivers DDC, or think they have when in reality they’ve done nothing more than dip the clutch with an arbitary blip of throttle. There’s more to DDC than that if everything is to be matched up.
As for pressure of training costs? That’s a joke. Training is cheaper today than it’s ever been! OK, it’s still a sizeable chunk of cash, but when I learnt it cost me £175, and that was when earning £30.00 a week was top money, between £20 and £25.00 was more normal, £50 would buy a good car and it would cost £35 a year to insure it, and for the current price of a pint you could get well lashed and still have change. At the time I was running my own Transit, charging 11.5p per mile and making enough I could afford my HGV licence. Had I been working for somebody, the £175 would have been as far away as a mortgage is to first time buyers today.
And if anybody accuses me of being old - they are likely right, or do I just feel it!

36

Lucy:
It does. It forces the cones to work harder, which in turn prematurely wears them. De Saint over on the PDA posted a very comprehensive explanation at one point (he’s a wagon mechanic by trade) but I can’t seem to find it at the moment, may have dropped off into their archive. That was the jist though, and is bourne out by experience.

Sorry Zetor, but you are wrong on this one. Will keep searching for better info for you.

It can’t - the cones are only coming into contact when the speeds are matched, so less wear takes place than if you don’t DDC.

Think it through - it’s obvious really :laughing:

Also DDC or even part DDC (blipping throttle when changing down) causes less clutch wear when changing down as the speed of the clutch plates will be matched and the resulting gear change will be smoother. IMHO even car drivers should be able to DDC (I was before I ever touched a lorry) - you may need it one day.

37

Pop Larkin:
I guess beleaguered students would have to find the same time as before. I had to learn DDC because until you had learned to, you couldn’t change gear unless you had one of those new-fangled F86’s or F88’s.

Very true, but then there was a whole lot less traffic on the roads and flat-bed vehicles were standard. There was also no requirement as to the maximum number of gears. Now we have extreme congestion and box vehicles with a minimum of 8 gears as a legal requirement, so the overall amount of information is about the same. I originally learnt to drive myself in a 1959 BMC FG350, btw. :wink:

My guess is that it isn’t DDC that damages gearboxes, it is poor DDC that does. Learning to DDC on synchro boxes masks any defiency in technique - I’ve seen loads of drivers DDC, or think they have when in reality they’ve done nothing more than dip the clutch with an arbitary blip of throttle. There’s more to DDC than that if everything is to be matched up.

Now that may well be the answer to all of this. Correct and effective DDC takes time to master, certainly more than 5 days in a synchro’d vehicle…hence attempting to teach newbies in this way is a bad move. It would also explain why so many of us have been warned off it on the grounds of “gearbox damage”. :bulb:

As for pressure of training costs? That’s a joke. Training is cheaper today than it’s ever been! OK, it’s still a sizeable chunk of cash, but when I learnt it cost me £175, and that was when earning £30.00 a week was top money, between £20 and £25.00 was more normal, £50 would buy a good car and it would cost £35 a year to insure it, and for the current price of a pint you could get well lashed and still have change. At the time I was running my own Transit, charging 11.5p per mile and making enough I could afford my HGV licence. Had I been working for somebody, the £175 would have been as far away as a mortgage is to first time buyers today.

There I’m forced to disagree. Think about it…

£175 divided by an average wage of £25 (for easy reckoning) makes training cost equal to 7 weeks wages - and that was when you could go straight to Class 1 without messing about with a rigid test.

Now, assuming you don’t need multiple retests, the average training cost to Class 1 probably works out at about £2500 - mine cost me nigh on that nearly 10 years ago - so with an average wage of about £250 for 8 hours a day at the generous rate of £8.30 (ish) an hour, your looking at 10 weeks wages.

So how is it cheaper then ever? :confused:

And if anybody accuses me of being old - they are likely right, or do I just feel it!

Bit of both, I suspect. :grimacing:

38

gardun:

Lucy:
It does. It forces the cones to work harder, which in turn prematurely wears them. De Saint over on the PDA posted a very comprehensive explanation at one point (he’s a wagon mechanic by trade) but I can’t seem to find it at the moment, may have dropped off into their archive. That was the jist though, and is bourne out by experience.

Sorry Zetor, but you are wrong on this one. Will keep searching for better info for you.

It can’t - the cones are only coming into contact when the speeds are matched, so less wear takes place than if you don’t DDC.

Think it through - it’s obvious really :laughing:

Also DDC or even part DDC (blipping throttle when changing down) causes less clutch wear when changing down as the speed of the clutch plates will be matched and the resulting gear change will be smoother. IMHO even car drivers should be able to DDC (I was before I ever touched a lorry) - you may need it one day.

Which is precisely why I embarked on the fact-finding mission above, which has so far proved inconclusive. Pop Larkin’s post may well have hit on the answer, though… :bulb:

(As a side note, though, the cones do not only come into contact when the speeds are matched in a synchro box, it is the cones which do the matching - see explanation further up the thread as given by one of the mechanics I quizzed. :wink: )

39

gardun:
It can’t - the cones are only coming into contact when the speeds are matched, so less wear takes place than if you don’t DDC.

I think you’ll find that it’s the cones that achieve the matching of shaft speeds, and undue force from the driver that wears the cones prematurely.

gardun:
Think it through - it’s obvious really :laughing:

WADR gardun, a true understanding is needed first. That’s why I waited for comments from people with more expertise than myself. :wink:

gardun:
Also DDC or even part DDC (blipping throttle when changing down) causes less clutch wear when changing down as the speed of the clutch plates will be matched and the resulting gear change will be smoother.

I believe that “the speed of the clutch plates” is just another way of saying “shaft speed,” (singular) since the pressure plate spins at the same speed as the flywheel (engine,) and the friction plate spins at the same speed as the first-motion (input) shaft. The only wear taking place, when these two are mis-matched for speed, is on the clutch assembly. For that reason, I think you’ll find that “blipping the throttle” with the clutch pedal depressed can prematurely wear the plates, achieves nothing, and is wasteful of fuel as others have noted.

If I understand correctly, the wear we’re discussing is on the syncros. This is usually caused by a driver using excessive force to overcome the speed mis-match between the input shaft and the layshaft. On a downward gearchange, any “whine” from the gearbox and is a sure indication of syncro abuse. (LGV driving tests can be failed for this :wink:) The “crunch” of gears is normally caused by syncros too worn, either by mileage or abuse, to be able to perform their function any longer ie. they can’t match the shaft speeds, which otherwise would allow a driver to perform a smooth gearchange.

gardun:
IMHO even car drivers should be able to DDC (I was before I ever touched a lorry) - you may need it one day.

Back to the LGV driving school wagon, and remembering that most driving school wagons are at least “second life,” we usually had to teach DDC so that the candidate could demonstrate the competencies required by the DSA driving test. Once qualified, and with the ability of being able to DDC, a skilled driver can get significantly more mileage from a “tired” gearbox quite safely. So it’s true gardun, “you might need it [DDC] one day.”

40

Those of us who took their tests when DDC was a requirement must surely admit that it would’ve been easier if we didn’t have to do it. At the time I was driving a SII land-Rover so knew how to do it, I’d also had clutch cylinders fail in MGs and minis so was OK with clutchless changes but my problem was remembering NOT to DDC when just changing a half gear (splitter). I’d have preferred not to have to do it. Also, as you say back then all I had to do to go from car to class 1 was take a class 1 test. No need to do a class 2 first, no theory, no hazard perception, less traffic…

I think you’re reading things into Gardun’s post - as I read it he’s simply saying that by DDCing you’re making the cones come into contact at the same speed, thus they’re not doing anything. He’s not actually claiming that they don’t come into contact. The cones still come into contact, they just don’t do anything.

I like Pop’s comment about Synchro hiding bad DDC technique. I’ll admit that with the vintage trucks I’ve had to pull over and start again once or twice because I’ve got myself in such a mess I’ve been unable to get ANY gear! In a synchro you’re always going to get something, even if it’s 3 gears out from what you need!

I can also see the point that car drivers may need to know this one day but we couls also argue that they should know how to jump start a car, change a tyre, change a blown bulb etc. If people call the AA to change a tyre are they really going to put the effort in to limp a car home for repair? Personally I’d limp home and spend an hour getting mucky and £10 to fix the problem. most people I know would rather keep their hands clean and pay £100+ to have someone else to do it. These days everybody wants to be on the B ark!