Hi folks!
Can someone explain the the pro’s and Con’s of rev matching when changing gear. How does it reduce clutch wear, if indeed it does?
As far as I know with modern synchro gearboxes there is no need ( I stand to be corrected ).
It does however come in handy if the clutch goes and you need to get it back without a clutch.
That is the way I see it, surely with a synchro box the slower the friction plate engages the flywheel the better. If the flywheel is running fast because you have just induced revs then surely the friction plate is more likely to have induced slip? I think!
If you are meaning ‘blipping’ the throttle between gear changes, then the DSA (and indeed anyone concerned about fuel economy) discourage this. New gearboxes, or more to the point clutches, negate the requirement for this.
On the subject of gears (i’ve got my driving instructor head on now) using the gears to slow the vehicle down is also discouraged. You should use the brakes to slow the vehicle and then select a gear to match the engine and road speed. 
Again, economy is the aim.
Agree with your view, however it was explained to me by an LGV Instructor that inducing revs and then engaging the clutch after a gear change reduces clutch wear. I don’t see how for the reasons I explained earlier.
Surely by engaging the clutch onto a high revving flywheel clutch wear must increase, the tempreture certainly will!
marcustandy:
If you are meaning ‘blipping’ the throttle between gear changes, then the DSA (and indeed anyone concerned about fuel economy) discourage this. New gearboxes, or more to the point clutches, negate the requirement for this.
Unless its an autotrans/i-change/etc, on downshifts the gearbox software blips the throttle electronically to sycronise the speeds of the different components about to engage or mesh for the reasons of reduced wear and tear.
The R&M package for a Merc costs less with autotrans for this reason.
CM:
… explained to me by an LGV Instructor that inducing revs and then engaging the clutch after a gear change reduces clutch wear…
On the marking sheet the Examiner uses during a driving test is a section that reads:
Correct use of:
Accelerator
Gears
Clutch
Brakes
Although a driver on test is allowed 15 driving faults, no more than 3 of then can be for the same thing. If the throttle was ‘blipped’ between every gear change there would certainly be more than 3 marks in one box.
Manwell:
… the gearbox software blips the throttle electronically to sycronise the speeds of the different components …
It is true that this happens but it is an exact amount as worked out by the software. However, it has been deemed (and tested) that humans can’t do it efficiently enough for it to be beneficial to the longevity of the associated components.
marcustandy:
]It is true that this happens but it is an exact amount as worked out by the software. However, it has been deemed (and tested) that humans can’t do it efficiently enough for it to be beneficial to the longevity of the associated components.
I’ll agree to differ on that one, I agree blipping the throtttle for super smooth gear changes should be the last thing on the mind of someone on a test in a vehicle they’ve only driven for few days. But ask any of the eaton twin splitter buffs how they drive, and how often they replace the clutch. 
marcustandy:
CM:
… explained to me by an LGV Instructor that inducing revs and then engaging the clutch after a gear change reduces clutch wear…On the marking sheet the Examiner uses during a driving test is a section that reads:
Correct use of:
Accelerator
Gears
Clutch
Brakes
Although a driver on test is allowed 15 driving faults, no more than 3 of then can be for the same thing. If the throttle was ‘blipped’ between every gear change there would certainly be more than 3 marks in one box.
Manwell:
… the gearbox software blips the throttle electronically to sycronise the speeds of the different components …It is true that this happens but it is an exact amount as worked out by the software. However, it has been deemed (and tested) that humans can’t do it efficiently enough for it to be beneficial to the longevity of the associated components.
Thanks for this, I wholly understand the rules governing driving tests but if this Instructor is teaching his students to do this to save his clutch he must have a high failure rate, at no gain to him or his vehicle, and I feel vindicated, I have been doing it right all this time.
Brakes are for braking, gears are for going , so you dont use the gearbox to slow down.
There is no need to ‘blip’ the throttle between changes if you have a modern syncro box.
The eaton twin splitter is not a modern syncro 'box and needs carefull use to get the maximum from it.
Iveco and Mercedes use a non syncro 'box in their autos which is why the software blips the throttle to match the speed of the compoments and a human finds this very differcult to achive.
Read the Advanced drivers bible ‘roadcraft’ directed at Police drivers but has some usefull tips.
Fire appliance driver, IAM and RoSPA member 
zebadee:
Brakes are for braking, gears are for going , so you dont use the gearbox to slow down.
There is no need to ‘blip’ the throttle between changes if you have a modern syncro box.
The eaton twin splitter is not a modern syncro 'box and needs carefull use to get the maximum from it.
Iveco and Mercedes use a non syncro 'box in their autos which is why the software blips the throttle to match the speed of the compoments and a human finds this very differcult to achive.Read the Advanced drivers bible ‘roadcraft’ directed at Police drivers but has some usefull tips.
Fire appliance driver, IAM and RoSPA member
Many thanks, understand this, however, we seem to have moved away slightly from the original question which was:
How does rev matching save clutch wear, that is, increasing the revs to match that level they were at when you originally ‘dipped’ the clutch, immediately prior to re-engaging the clutch and restoring drive?
zebadee:
Brakes are for braking, gears are for going , so you dont use the gearbox to slow down.
If so, why are trucks fitted with engine/exhaust brakes, they work more efficiently higher up the rev range which means changing down a few cogs.
Of course it all depends on who is paying the bills for brake and clutch relines, my trucks means I use the engine and its built in brake for initial braking and the service brakes for fine control, I think your comment applies more to the drivers of emergency reponse vehicles and tear [zb]'s
Manwell:
zebadee:
Brakes are for braking, gears are for going , so you dont use the gearbox to slow down.If so, why are trucks fitted with engine/exhaust brakes, they work more efficiently higher up the rev range which means changing down a few cogs.
Of course it all depends on who is paying the bills for brake and clutch relines, my trucks means I use the engine and its built in brake for initial braking and the service brakes for fine control, I think your comment applies more to the drivers of emergency reponse vehicles and tear [zb]'s
Very good point, I was always taught use engine braking, save the service brakes for when you must use them and with minimum fade. I guess this last point is now somewhat diminished on more modern vehicles, disc brakes, etc, still exists but not such a problem, Yes / No?
Without getting to technical, the theory was, to get a smooth gearchange and to reduce wear in all components, engine, gearbox and clutch, the revs had to be matched as closely as possible but in a modern truck this is,t necessary.
This was in the days of non syncro gearboxs and trucks without computerisation.
However, things have moved on, with a little black box controlling part or all of the engine management, with quicker reactions from the various component parts and making the driver a steering wheel attendent who has very little input into the machine he’s supposed to be controlling.
If an instructor is telling you that you should ‘blip’ the throttle between gearchanges, then he’s wrong. You dont and for more than 1 reason. You dont ‘blip’ the throttle before you switch off ( as you used to) and you dont rev the engine when you have just started it from cold.
Hope this helps but if you want a more technical explanation just goand read some of the old Commercial Motors or an old drivers handbook.
CM:
Many thanks, understand this, however, we seem to have moved away slightly from the original question which was:How does rev matching save clutch wear, that is, increasing the revs to match that level they were at when you originally ‘dipped’ the clutch, immediately prior to re-engaging the clutch and restoring drive?
Syncromesh ensures that the gear teeth are turning at the correct speed before allowing them to mesh, but there is nothing apart from the skill of the driver to syncronise the differing speed between the flywheel connected to the engine and the clutch connected to the gearbox. If they are turning at different speeds some wear will take place as friction will occur until their speeds are equalised. You know when you’ve hit it right because when you let the clutch out your gearchange will be nice and smooth with no jerks and hardly any change in engine note
Its all down to choice, whether you take pride in your driving and have any mechanical sympathy with the machine you are operating, or you are just paid to do the job
Manwell:
zebadee:
Brakes are for braking, gears are for going , so you dont use the gearbox to slow down.If so, why are trucks fitted with engine/exhaust brakes, they work more efficiently higher up the rev range which means changing down a few cogs.
Of course it all depends on who is paying the bills for brake and clutch relines, my trucks means I use the engine and its built in brake for initial braking and the service brakes for fine control, I think your comment applies more to the drivers of emergency reponse vehicles and tear [zb]'s
Changing gears to use the exhaust brake is not using the gearbox as a brake. You simply increase the speed of the engine which in turn increases the amount of exhaust gas available for the exhaust brake to work more efficiently.
Brakes are for braking, gears are for going is standard advanced driving for any vehicle.
Thank you, got that, obvious when you think about it. 
zebadee:
Changing gears to use the exhaust brake is not using the gearbox as a brake.
Correct.
zebadee:
You simply increase the speed of the engine which in turn increases the amount of exhaust gas available for the exhaust brake to work more efficiently.
Correct. 2 out 2 = 100%
Gee thanks 
Manwell,
Am I correct in thinking that you run your own truck? I dare say you know better which is the cheaper option; more fuel or brakes wear & tear.
The info I have given (and it would seem, Zebadee) is based on ‘by the book’ methods. I’ll be the first to admit that ‘by the book’ isn’t always the ‘best’ option all of the time.
When I have dvr trg students for any cat, one of the first things that I tell them is that I am not there to make them a ‘good driver’; I am there to get them ‘through the test’. It is actually two different things!!