Submitted: Saturday, Apr 20, 2013 at 20:26
ThreadID: 101793 Views:2156 Replies:5 FollowUps:13
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We have an Isuzu 600 with a syncro 6 speed box at work , that the synchros are a bit scratchy. I advised a hirer to double clutch as it shifts heaps smoother. He told me thats a bad practice with synchro boxs as it crushs the synchros. Seems a bit strange to me, but as we have some learned mechanics on here was looking for there opinion.
Cheers Pete
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Reply By: Rangiephil - Saturday, Apr 20, 2013 at 20:34

Saturday, Apr 20, 2013 at 20:34
The idea of a double declutch on a non synchro box is to equalise the speed of the gears before they mesh.
The job of synchros is to do the same.
I cannot see how double declutching could hurt synchros and I have never seen this in practice or ever heard anyone suggest it before in my 48 plus years of involvement with cars, and I am old and grew up when " 3 on the tree" was common.

Regards Philip A
AnswerID: 509407

Reply By: Axle - Saturday, Apr 20, 2013 at 20:57

Saturday, Apr 20, 2013 at 20:57
I don't think it would hurt to double clutch either Pete,.. another way is to pull to neutral pause a bit and then go into gear, every gearbox seems to have its little thing though.

Cheers Axle.
AnswerID: 509411

Reply By: Ross M - Saturday, Apr 20, 2013 at 22:14

Saturday, Apr 20, 2013 at 22:14
Double clutching doesn't affect the synchros as it allow time, as does a pause, for the relative spinning parts to be somewhere near the same speed so the synchro has less work to do.
Since gear ratios are matched to engine operation the engine speed will be somewhere near the desired speed and letting the clutch out before the second press ensures the input shaft of the gearbox hasn't slowed too much and made things worse in relative speeds. This should allows the synchro to take up the challenge with the minimum of stress.

Works for all the synchro buses I drove. Essential on the non synchro/crash box which the doubling is trying to emulate on the synchro box.

Hirer may not know which side the butter is on.
The only thing which makes synchros work real hard is trying to force them to attempt a gear change by just pushing on the stick, worst thing you can do.
Most people expect truck boxes which are three times heavier (or more) than cars to change zachary the same. Due to the spinning inertia masses involved some adjustment of the brain has to occur to allow gears to slow down before changing.

Two fingers pressure on the gearstick is all the pressure a change should need if the operator is aware of what they are driving. With just that pressure you can feel when the box has speed matched it's internals and is ready for the engagement.

Drag race changes just don't/won't happen.

Ross M
AnswerID: 509415

Follow Up By: Gnomey - Sunday, Apr 21, 2013 at 09:08

Sunday, Apr 21, 2013 at 09:08
Agree completely. Doubling saves wear on the synchro rings so is actually good for the gearbox, especially on the downshift. I've done it on all my manual vehicles since the first gearbox I helped to rebuild.

Knowing how things work can save you a lot of money because you have a better idea of the difference between use and abuse. Simple idea that used to be common sense.

FollowupID: 787306

Reply By: The Bantam - Sunday, Apr 21, 2013 at 10:05

Sunday, Apr 21, 2013 at 10:05
The argument that double declutcing a syncro box being bad practice is a sound one.
In fact you may find you fail your driving test if you double clutch on a syncro truck.

The argument is that if the syncro box is shifted "properly" this should represent minimum wear on the syncros.

If however the box is double clutched.....because there is no imperitave of matching the revs correctly like there is in a crash box.....the revs may not in fact be matched properly and this may accelerate wear in the syncros.

Technicaly it most definitely is "bad practice" to double clutch a syncro box.

NOW...back to the real world.....the problem with syncro boxes in trucks is people drive em like cars......instead of managing road speed to gear selection in advance and giving the gearbox sufficienet time for the syncros to work.

Commonly the divers push the boxes thru the gears with insufficinet time for the syncros to work and come steaming up to corners and stuffing the stick in the slot and hope.

This buggers syncros in very short time.

The instructor that taught me to drive crash boxes, had a mate that managed a fleet of medium rigids driven by tree looppers.....these guys are not drivers, and the toll on the gearboxes was and endless frustration.

SO...syncros buggered....what do you do.....ya double clutch.......there is no choice.

But don't be surprised if the boss does his nut, if he finds you double clutching his brand new syncro truck.

AnswerID: 509441

Follow Up By: Ross M - Sunday, Apr 21, 2013 at 10:19

Sunday, Apr 21, 2013 at 10:19
If double declutching is done for a crash box to match speeds then it also works for a synchro box too, exactly the same principle. It therefore is not a bad practice but may help considerably with tired synchros and give them less work to do and assist their action.

It will not accelerate any wear on synchros because it's sole purpose is for speed matching.
FollowupID: 787317

Follow Up By: The Bantam - Sunday, Apr 21, 2013 at 11:07

Sunday, Apr 21, 2013 at 11:07
The official line IS that double de-clutching syncro boxes is BAD PRACTICE.

Without exception manufacturers will not recommend it.....even Eaton the people who make Road Ranger boxes....they make syncro and auto boxes not recommend double de-clutching syncro boxes.

As I say when you drive a crash box.....if ya revs are not properly matched you know straight away and in no uncertain terms.

In a syncro box unless you are a matamatical genius and a technician with the tacho & have no way of knowing with accuracy that the revs are matched in a properly working syncro box.

Almost every change will either over reved or under reved...a poor diver may indeed put more strees on the syncros by far, double de-clutching than not.

In the real word.......most likely...unless the truck is a couple of months old.....the syncros will be ya double clutch.

I have recently spent some shifts with some young blokes ( brothers) in removals vans......the family have owned these trucks from new....none of them double clutch the syncro boxes even their old Hino with over 400 000km still shifts smoothly.
Even the 60+year old family retainer who still drives his sons B doubles from time to time does not double clucth these trucks.
These trucks have been properly driven since new and always single clutched.

If the syncros are in good is best practice not to double the clutch......pause changing with light pressure on the stick is all that is required.

FollowupID: 787322

Follow Up By: Ross M - Sunday, Apr 21, 2013 at 12:48

Sunday, Apr 21, 2013 at 12:48
As far as the synchros are concerned they don't know the clutch was pressed previously and it therefore is exactly the same as changing normally with care and finesse. The only thing the double does is try to ensure there is little fall away of the revs to the synchro doesn't have to work as hard.

The same type of locking dog ring is being engaged in both systems, the only difference is the synchro blocking the action until revs are equalized.
FollowupID: 787340

Follow Up By: The Bantam - Sunday, Apr 21, 2013 at 13:41

Sunday, Apr 21, 2013 at 13:41
The thing you are failing to account for is that in baulk ring syncro systems.
the syncro ring has two functions.......indeed it is there to "baulk" or stop the gear engaging till the input and output are syncronised.

But the second and equally important function of the syncro ring is BY FRICTION and under pressure from the shifter spinning the gear to be selected into syncroistion with the output.

While the clutch is down the front of the gearbox is free to be spun to the syncronised speed.......when the clutch is up it is not.

The problem and the excessive wear comes from the failure to accurately match the revs and causing more friction than there would be normally shifting.

AND there is a complication.
Because there is a baulk ring in the way and it may not be aligned every time....double clutching in a syncro box necessarily requires an over shoot in revs to slip the syncro ring till the teeth engage regardlles of syncronisation speed....if you get the syncroisation spot on at least some of the time the gear will not go in.

This is why syncro boxes in any condition do not drive anywhere near as well clutchless as crash boxes do.

If you have driven a crash box and not used the clutch for shifts....lots do.....get the syncroisation even close and the gears will snick in.......and then try to limp home a syncro box with a broken clutch cable or busted may now grasp why clutchless shifts in syncro boxes are so hit & miss.

AND, one of the common reasons for double clutching syncro boxes is impatience.....some of the heavy syncro boxes are wide spaced and slow to shift.....people will try to hurry the box by double clutching and invariably do a bad job of matching the revs....result more wear on the syncros.......these boxes will not be hurried and the only thing that will give a good shift is patience.

The fact remains that all the manufacturers of syncro boxes do not recommend double clutching...and certain regulators, institutuions and a lot of fleet owners will very much frown on the practice.

Do I double clutch syncro boxes.....even my own......of course I do....when necessary.

But on a syncro box in good condition it can and is argued that it is not necessary, of no benifit and it can easily be argued that it producess excessive wear.

BTW.....this is not a matter of opinion....I am presenting what is manufacturer recommended and what the standard argument is against double clutching syncro boxes.

FollowupID: 787346

Reply By: pop2jocem - Sunday, Apr 21, 2013 at 10:31

Sunday, Apr 21, 2013 at 10:31
Double de-clutching on the upshift without accelerating the engine and double de-cluching with an appropriate increase in revs when down shifting is actually helping the synchro rings do their job. A truck type gearbox has much heavier components than a car so the synchros have to work a little harder. The synchro rings are of course heavier in a truck but still need more time to do their job.
Having said that if the synchros are in good condition and the shifts are not hurried double de-clutching should not be necessary, however the synchros will not be damaged by doing so. In fact you may increase the life of them and all other components by doing so. Hurried shifts will "crush" the synchros, not "double clutching".

AnswerID: 509445

Follow Up By: J.T. - Sunday, Apr 21, 2013 at 14:10

Sunday, Apr 21, 2013 at 14:10
Good arguments for and against.

Drive an auto and problem solved. A good auto for those trucks is a Daimler 6446 Se shormaster auto. Very strong and smooth.

FollowupID: 787350

Follow Up By: pop2jocem - Sunday, Apr 21, 2013 at 15:33

Sunday, Apr 21, 2013 at 15:33
Spot on Greg,

Removes the "opinions" of the nut behind the wheel who has it firmly fixed in his mind how a gearbox should be treated.

FollowupID: 787362

Follow Up By: Rockape - Sunday, Apr 21, 2013 at 16:59

Sunday, Apr 21, 2013 at 16:59
now you know why I now drive an auto. I started getting bills from the shire for removal of all the gearbox parts lying on the road. They even had a special clean up Australia contingent to follow me.

Many transport operators are now going to the autos because of bad operators and the fact the bloody things are smarter than us.

I can remember driving all syncro F10 and F12 Volvos where you would wait what seemed like an eternity for the syncros to say, yes boss we are happy now. Volvo were also big on no rev matching due to saving fuel, problem was by the time the syncros said yes boss, you would have to bounce a couple of gears.

Drove another logging truck where you would change at 1800 revs and then have to let the engine die to 1200 before you could change. *&%$&.

I still sometimes double back when I have a senior moment but it is getting harder with the auto. I say bring back crash and joey boxes. I miss banging my elbow on the back of the cab when splitting gears (sorry that should read destroying gears)

OT. Pop for a good laugh have a listen Dallas Steele singing "The Sequel To Brown Dog"
FollowupID: 787370

Follow Up By: pop2jocem - Sunday, Apr 21, 2013 at 17:52

Sunday, Apr 21, 2013 at 17:52
G'Day RA,

I spent a fair few years fixing the results of letting a supposedly "experienced operator" loose in the cabs of everything from A to Z in trucks from 5 to 105 tonne, and of course testing them afterwards.

Present company excepted of

I then spent more years than I care to remember overhauling and testing Alison (now ZF I believe) auto and powershift boxes. Boy, did that make life a whole lot easier on the trucks in general and the owners wallet as well.
Apparently the new boxes are basically a manual but shifted automatically. Never driven one of them myself (now retired thankfully) but I am told they combine the driving ease of an auto with all the advantages of a manual box.
Ain't progress marvelous. (;-))

FollowupID: 787372

Follow Up By: Rockape - Sunday, Apr 21, 2013 at 19:33

Sunday, Apr 21, 2013 at 19:33
you will love this. I worked for a father and son partnership many years ago and this is how it worked. Well it wouldn't work that way today.

Employment rules. If you bugger it and it is your fault, then this is the deal. You will assist in the repairs and you won't be paid until it is going.

They were fair people and they were the rules. Hell you learn real quick.

As for the new stuff. It has one problem. It doesn't have voice recognition. It has no idea of the Australian language or it's finely tuned use of bad language.

May all your knuckles be repaired, now you are retired from the skinning business. LOL.
FollowupID: 787379

Follow Up By: pop2jocem - Sunday, Apr 21, 2013 at 20:20

Sunday, Apr 21, 2013 at 20:20

Yeah nowadays the employer would be responsible for sending the goon who destroyed his truck to a counsellor in case his delicate psyche had been damaged by the experience. Then a complete re-training program followed up by more evaluations to make sure he didn't become an axe murderer because his mother didn't breast feed him. And all of that at full pay and paid for by the employer. Many years ago a swift boot up the derrière would have achieved the same result in half the time and at no cost.

The knuckles are slowing healing, the callouses are softening and I very seldom start the uncontrollable shakes at the sound of a GM under full noise. AND I DO MEAN NOISE. The doctors say that although eventually the black stains ingrained into my skin will fade the hearing is a gonner.

Keep on truckin' mate and think of better days ahead.

FollowupID: 787386

Follow Up By: fisherPete - Tuesday, Apr 23, 2013 at 21:45

Tuesday, Apr 23, 2013 at 21:45
Yes spot of fellows, our new 900 has a AMT box, absolute dream to drive. Only problem now is drivers think they are driving a big sedan, and do not allow adquate space when turning into driveways etc.
Cheers Pete
FollowupID: 787574

Follow Up By: The Bantam - Wednesday, Apr 24, 2013 at 19:48

Wednesday, Apr 24, 2013 at 19:48
What is being posted above reflects the realities in heavy transport.

Mostly in heavies,( heavy rigids and heavy combinations) you will find plenty of crash boxes and a rapidly increasing number of autos but very few syncro boxes.

The syncro boxes simply cost too much to own.

The local quary operator here runs one of the largest all Mack fleets around with last time I heard 40 ish truck & dog combos......they tell me he is only buying autos these days and almost the whole fleet is now auto.

He is the top shelf tipper drive a bloke will get around here..late model trucks and autos.....mmmm choice.
Even though he has almost all autos.....he instst on employing drivers with recent crash box experience.

FollowupID: 787652

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