bestbooties:
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This item has realy got me revved up,if you’ll excuse the pun.I can’t believe what I’m reading!Mercedes synchro rings are no different to anyone elses in the way they operate.
Revving up as you change down,IS not as hard on the synchro’s as their purpose in life is to bring the freewheeling gear on the mainshaft to the same speed as the sliding gear,by means of friction.
Revving the engine brings the speed of both gears closer,so REDUCING the amount of friction required to bring them to the same speed,BUT,this does require DDC,as the clutch has to be engaged in neutral when you blip the accelerator to speed up the gear trains close to matching speeds.
Revving up on changdown without DDC WILL make the synchros work harder,but you will not have to slip the clutch as the engine will be going at the speed required for the gear you are now in.
So,in a nutshell,driving with a sychro 'box the way you used to with a “crash” 'box is treating the synchro box with kid gloves,it will never wear out!
Synchro 'boxes were designed to make driving easier so drivers didn’t have to be dead right every gear change,plus you CAN make faster changes,when hill climbing for instance,whereas years ago with a "crash 'box you couldn’t change up going uphill,there was a good chance you would have stopped altogether before you got into that next gear.
Next time your truck with a synchro 'box goes in for a major service,have a look at the gearbox oil as its drained.all that gold dust you see in the oil is what has worn off the synchro rings.
Spot on IMHO. We were also advised by Mercedes that on our new trucks we should not blip the throttle as it wastes fuel. This does of course sound good to the boss, and does not take into account who will be supplying and fitting the more rapidly worn out clutch when we all “drag” the engine revs when changing down by slipping the clutch.
To be honest, the wind direction and strength makes more difference to fuel consumptio, as might the weight of the driver
Pop Larkin:
And, if DDC causes damage, why do synchro boxes need less effort to shift the lever when DDC is used than when pushed straight through? Surely the extra effort must be caused by friction and the more friction there is the greater the wear and tear?
My point exactly.You will find on a new motor,if the gearchange is a bit “baulky”,or reluctant to drop into the next gear,DDC will make it so much easier.
As I said on my last post about the “gold dust” that comes out with the gearbox oil,I don’t suppose anyone has done a survey as to how much is regarded as normal wear,or how much is deemed that someone has been giving the sychros some stick!.
I had one ocasion when I was on M/E of a mate of mine with a Volvo F88 290,lost 3rd and 4th gears in the main 'box whilst in Saudi.He was towed the whole way from one side of Saudi to the other and told to meet me down by the Dead Sea in Jordan where we had to load drilling equipment back to the UK.This was when I was on for Chapman and Ball,and being as I was the fitter/driver,if anyone had problems while abroad,within reason,they had to get to me,or I had to get to them.
Anyway,while my mate loaded my trailer,I took the top off the gearbox to find that the bronze shoes at the ends of the selector forks,that sit in the groove of the synchro unit,had sheared off and fell in the bottom of the 'box.
I was able to get spares from the Volvo agent in Amman,but before fitting them,I was able to get my hands round the gear trains to reach the bottom of the 'box and retrieve the broken parts,and I thought I’d struck it rich with the amount of “gold dust” in the bottom!
This was a case of someone who knew how to punish a gearbox.
What’s the matter?Have I brought this thread to a grinding halt or have you all run out of gears?Or maybe we’ve flogged this subject to death.Or maybe we’re waiting for an adjudicator?
bestbooties:
Or maybe we’re waiting for an adjudicator?
I’d suggested that right at the top of the page. I think we’re going to have to wait until Lucy has run out of people to accost. She may be gone for some time…
bestbooties:
I think it’s only the 3 of us that have have come to any sort of common agreement isn’t it?
Must be more than that. Looks like we need a poll to see how many think DDC is good for the gearbox and how many are wrong?
I bought a diesel Fiesta new way back in 1989 and when I sold it with 160,000 miles on the clock it still had the original clutch and gearbox. Every gearchange was DDC or clutchless when I was in lazy mode. If DDC damages gearboxes, I just ain’t convinced, so my vote would have to be in favour of DDC.
What a very interesting thread!!! all newbies should be made to read this.
On a personal note, I have always used DDC and when possible clutchless changes. My XF doesn’t like the clutchless changes very much though so I rarely use them now. But I have tried not using DDC and as Pop was saying, I find the changes much rougher especially downshifts.
One of the advantages in knowing these skills came into fruition when I broke my left ankle 15 years ago. With my lower left leg in plaster, I managed to drive my old Cortina Mk4 from Kingston Upon Thames down to the south coast to my parents holiday home. Used first to pull away, managed to get to the A3 without stopping and then drove all the way using clutchless changes lolol
Seeing as how this debate has taken many twists and turns I thought I would ask our friends at EATON transmissions … and here is the definative reply
Interesting conversation. No doubt a subject of debate and in need of
clarification, let me see if I can make sense of this for everyone.
Lets start with the purpose of the synchronizer. The synchronizer is
intended to bring the main shaft gearing up to speed with the main shaft
allowing smooth engagement of the clutching teeth, which will transmit
torque thru that gear. The main shaft will always be spinning at the
road speed of the truck. When the master clutch is disengaged (pushed
to the floor) the engine is disconnected from the transmission allowing
the synchronizer to do it’s job. In the 1.5 seconds it takes the driver
to move the lever to the next position all of this is happening. This
is a little summary of what happens mechanically during a shift, now
let’s clarify the debate if double clutching damages synchronizers.
Let’s think about the shift using the double clutch method. Your driving
down the road in 6th gear, you see a red light ahead and you begin to
slow down. You want to use the engine to slow you down, so you depress
the clutch, move the lever to neutral, re-engage the clutch, and begin
disengaging the clutch again while moving the lever into 5th gear.
Right? Remember this, when you begin disengaging the clutch with your
foot it takes a certain amount of time for total disengagement and speed
differential between the input shaft and engine to occur. In most cases
when a driver begins depressing the clutch for the second time he
already has the lever in motion to the next gear, pushing up against the
synchronizer, before the clutch has totally disengaged. What does this
mean? It means the synchronizer is trying to match the engine to the
road speed. Guess what. Then engine RPM is not going to change and the
synchronizer is going to suffer. The synchronizer is only designed to
bring the input shaft and main shaft gearing up to speed with the main
shaft, it will not overcome the engine. Now, this may be a very quick
event and will not immediately destroy the synchronizer but it will
cause accelerated wear. Can a very patient driver use the double clutch
method without causing increased wear? Yes. But are we always this
conscious of our shifts? To be safe, single clutching is best for
synchronized transmissions.
So what the man from Eaton is saying is that CORRECTLY executed DDC will not damage the synchro but doing it badly will?
I think this reflects what a number of us have already pointed out.
The same is also true for “crash” boxes, even more so but I guess the difference is that you KNOW you’re doing it wrong with one of those wheras a synchro will just work harder to hide your mistake from you.
In most cases
Can a very patient driver use the double clutch
method without causing increased wear? Yes. But are we always this
conscious of our shifts? To be safe, single clutching is best for
synchronized transmissions.
[/quote]
As has been said, good DDC isn’t what damages gearboxes - it is bad DDC. By why a patient driver? Once the skill of DDC has been mastered the timing becomes natural for the circumstances - it has nothing to do with patience at all, just maintaining a skill level. Seems a bit like asking if a patient driver can get round a roundabout without falling over to me. Or has the skill in driving been dumbed down to patience?
The problem is Pop even the best driver gets tired, I cut my teeth on a Mandator where DDC was compulsory to have any chance of getting any sort of gear change, get it wrong and many times it would involve stopping and starting again. even after driving it for a couple years and having learnt that trucks little foibles occassionally I still missed a change.
It is those misses that will increase the wear on a synchro box and even the best DDC driver in the world will get it wrong at times…
in a perfect laboratory condition world using DDC would not cause any extra wear, in the real world it does !!
Point taken Rikki. But tiredness also applies when not using DDC and I’ve seen enough drivers forcing against the synchro and crashing gears to not be convinced that DDC does any more harm than not using DDC.
