What the hell?  My girlfriend reads this forum sometimes you know, you bastards. 

regarding the speed of the input shaft, Is this why transaxle synchros always go so quickly, because it's got all the momentum of the driveshaft to speed up/slow down?  I've asked a couple of people with Porsche 944s (they're also rear transaxle aren't they?) and they've never had probs with synchros.

Most definitely, probably picks up more DDs as well. 

I thought that transaxle cars had a much smaller flywheel, as the driveshaft itself adds to the flywheel effect - is this true?

Scott do you own shares in the Syncro company, If you only DD when the box starts to grind you have already started the process that leads to rebuilding the box. I have DD since I first started driving on every change down, except on the few cars I could do clutchless changes. (PS do not do cluchless changes in 116's, if you don't get it perfect you end up with a broken box, just ask Alan Jones he destroyed heaps of boxes in the ingus car, Bondy never broke 1).

If you want long life from your Syncro's then DD, my 116 GTV has done over 440,000kms and has never had a synco put in it, I also have never had to replace the diff, a common failure on Transaxles. It would be fare to say that the car was driven with mechanical simpathy all its life, but was always driven hard with a 180hp motor, it was use for sprints, hammered over 10's of thousands of km of mountain passes, including gravel and even used for recey and setups no rallys, dirt not tarmac.

Anthony

I'm a bit late to discussion, but I will add my two cents worth ...

I always understood that double-declutching was when you didn't use the brake ie changing up gears in a non-synchro box (or a truck) or using the gears to slow you down when downchanging.

I always understood hee-toe as incorporating the brake pedal into your double-declutch.

In the first instance I double-declutch, or use the gears to help slow me down when I'm driving my road car. Often you find yourself in a situation in the road where you've slowed down in traffic (coasted) without using the brakes and need to select a lower gear, so I just blip the throttle, double-declutch and make a smooth gear change.

In a racing situation you never you the gears to slow down - that is what the brake pedal is for!  As a few have said before in this thread, you will always be on the brakes long enough for you to double-declutch on the downchange. Ok, I might double-declutch every gearchange - I will often go straight from 4th to 2nd for Honda Hairpin at Phillip Island, but I will always double-declutch - its smoother and easier on the machinery. And as club racers we have to look after the gearbox - no one can afford to get then rebuilt every second race meeting...

Honestly I would be very surprised that anyone in our form of racing would not double-declutch, whether we are using synchro or dog boxes.

have a look at the "foot-cams" they sometimes deploy in the V8 Supercar coverage. All those cars use 6 speed Hollinger dog-boxes. You will see that they flat change up the gears without using the clutch (though some drivers have taken to using the clutch for across the gate changes - 2nd to 3rd and 4th to 5th. When they change down gears all the drivers with one exception are using the clutch to select the gears and are thus double declutching on their heel-toe changes.

You may ask who that one exception is? well as far as I know Greg Murphy is the only V8 driver who left foot brakes, which means he isn't using the clutch at all and must deploy what we consider in this thread to be the true "heel-toe." I'm not quite sure how he does it (I believe it's a common practice where sequential boxes are used, eg old Lola Champ Cars from 2006 and before) but he must be good at it as I reckon it would be tough on the gearbox...

Jim, I think we are getting stuck on semantics, or if you prefer terminology.

As I said I never use the gearbox to slow myself down on the track, but I do use the gearbox to "help" me slow down on the road and limiting use of the brakes - usually I'll double declutch down from 4th to 3rd and only incorporate the brakes for a heel and toe change back to second as I'm coming up to an intersection.

Maybe that is why (on my 01 Honda Integra Type R with 110K) I'm onto my second clutch but still on my first set of front brake rotors... 

I’m struggling a bit with this concept

The way I see it is if collar and gear are speed matched (double-declutching) then the only wear occurring on the synchro-rings is from the transverse movement of the collar, however if the speeds are not matched (normal shifting) then the synchro-rings get the same wear from the transverse movement plus the additional wear from the rotational movement.

If this is the case, which I don’t disagree with, then how does reducing this speed difference as much as possible (double-declutching) wear the synchro out twice as fast?

Any chance of a further explanation Big Trev?

Sorry Trev I still don’t follow, why are you pushing against the synchro at this point in the sequence,  During the time at which the gearbox input shaft is re-connected to the engine (clutch pedal up then clutch pedal down) the gearbox should be in neutral-> no pressure on the selector forks-> no pressure on any synchro rings.  It’s only after the input shaft has been disconnected from the engine (clutch pedal down) after the speed matching has been completed that the synchro should begin to make contact

As long as the gearbox is in neutral during the speed matching process I still can’t see how this creates extra wear.  If this was the case then wouldn’t just revving your engine in neutral whilst stationary wear out our synchros as well?