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Conference terri::cars_uk

Title:	Cars in the UK
Notice:	Please read new conference charter 1.70
Moderator:	COMICS::SHELLEYELD

Created:	Sun Mar 06 1994
Last Modified:	Fri Jun 06 1997
Last Successful Update:	Fri Jun 06 1997
Number of topics:	2584
Total number of notes:	63384

697.0. "double declutching" by DOOZER::PENNEY () Thu Jul 27 1989 03:21

Any views on double declutching?

I do it 90% of the time regardless of car (BX usually). Long-term habit. 

Only exception is if car has a freewheel.

Feel it can only prolong life of synchromesh, but...

a. Suspect modern synchro typically strong enough to cope with no ddc for
   100K miles assuming that changing of gears is non-violent, and of oil 
   periodic.

b. Some years ago Ford issued specific recommendation against ddc: "likely 
   to damage g/box".

   - ???

Own guess on b: Maybe what Ford were getting at was that whereas with crash
gearboxes you had to ddc, accurately, to get a silent down change (silence
being your indicator of having done it correctly), with modern g/boxes you
have no such aural indicator, and the chances of achieving a correct ddc
are not that high;  plus (continuing my conjecture on what Ford meant) a
bad ddc is worse than a bad single dc. 

Anyone know more about the Ford piece?

If above is what they meant, I'm confused ('cos in terms of harmful effect 
on gearbox alone I think they're wrong).  If it's not what they meant I'm
still confused ('cos I can't think of anything else they could have meant).

I reckon the order of merit is:

	Minimum					   Minimum
	gearbox					*clutch linkage
	 wear					    wear
	  1  <----	accurate ddc	      ---->  =2
	 =2  <----	inaccurate ddc	      ---->  =2
	 =2  <----	accurate sdc	      ---->  =1
	 =2  <----	inaccurate sdc	      ---->  =1

					[*i.e. broken rhd BX clutch cables]

Comments?

(Hoping not to have to re-educate the rest of the family - all double
declutching like yo-yos, dammit..) 

	- Richard

T.R	Title	User	Personal Name	Date	Lines
697.63	This seems as good a topic as any...	IOSG::MARSHALL	Harry Palmer	`Wed Aug 22 1990 13:17`	12
	How does synchromesh work? The inside of a gearbox is about the only part of a car I haven't had the "pleasure" of poking around, and Mr Haynes' diagrams don't reveal the dark secrets of synchro cones and the like. Keep it straightforward: ie standard constant mesh box with selector forks sliding the synchro bits along the mainshaft. I always thought synchromesh was a sort-of-combined-dog-and-friction-clutch inside the gearbox, but now I'm not so sure. I've got a nice shiny new Sierra box waiting to go in the Moss, but I don't fancy taking it to bits just yet... Scott
697.64		ANNECY::MATTHEWS	M+M Enterprises. Thats the CATCH	`Wed Aug 22 1990 16:55`	20
	I'll give it a go. I've rebuilt a Moggy 1000 gearbox about 15 times, so I have a good idea of where all the bits go, but not necessarily how they all work together. The basic problem when changing gear is to get the gear cogs connected to the engine side of the gearbox, to rotate at the same 'speed' as the cogs connected differential (or propshaft) side of the gearbox. The selector forks move the cogs inside the box when you change gear. The rotational speed is corrected by the synchromesh. As the cogs come together they come into contact via a conical face. As the cones come into contact (one inside the other, one on eack cog), friction causes the slower moving cog to increase speed. As the cones get nearer, dogteeth on the cogs come into contact to make sure that the cogs are well aligned and don't slip when the gear change is complete. Well, thats my version of things ... not sure if it is all true, but it is probably close enough. Mark
697.65	Not quite I'm afraid	HAMPS::LINCOLN_J	John, Hampshire House, Basingstoke	`Wed Aug 22 1990 17:16`	17
	More wrong than right I'm afraid. In a synchromesh gearbox the gear wheels are in constant mesh at all times, except perhaps reverse. However some of the gears are allowed to run free on their shafts and the synchromesh cones, as described in .-1 are used to synchronise the speed difference between the wheel and it's shaft before engagement of sliding splines which actaully form the drive. The inertia of the parts involved is much less with this system so it works easier. You really need to study a real one to get to grips with the concept. Also many gearboxes have baulk rings to ensure that engagement doesn't occur without synchronism being achieved. -John (who's rebuilt two boxes)
697.66		ANNECY::MATTHEWS	M+M Enterprises. Thats the CATCH	`Wed Aug 22 1990 17:23`	2
	.... just goes to show that you don't need to know how it works to get all the pieces back in the box :-) :-)
697.67	How many re-builds? :-)	IOSG::MITCHELL	Elaine	`Wed Aug 22 1990 17:59`	3
	.... yes, but if you re-built it so many times, sounds like they didn't go back in the right place in the box..... :-) :-)
697.68		ANNECY::MATTHEWS	M+M Enterprises. Thats the CATCH	`Wed Aug 22 1990 18:12`	6
	No, all the pieces were in the right place. The problem was that the first motion shaft spigot bush was missing, so the first motion shaft was flapping around in the flywheel. When I dropped the clutch, the first motion shaft flexed and ripped all the teeth off it ... Mark