Difference between rev matching and double clutching?

[hygt2]

james_gt3rs said:

There's one use for it for me. When taking a rolling first gear at 5mph in traffic, I'll DDC and it slots into first gear nicely. A lot of drivers just force it into first at 10mph and that wears the synchro.

+1

Also when you do a block change, such as 6th into 3rd, 2nd or 1st coming off the motorway. Again, double de-clutch reduces the synchromesh wear.

Changing a clutch is still relatively less expensive than refurbishing a gearbox.

[Brian Trizers]

Really? In that case the Volvo S60 I recently parted with after 12 years and 140,000 miles from new should have been a wreck, in the gearbox department at least. In that time I must have done thousands of block downchanges (and hundreds of block upshifts too) and double-declutched exactly twice, when I'd read something about it and tried it out of curiosity. OK, I very seldom selected first on the move because if the car was moving at all it would pull away in second, but it never occurred to me that a block change required more than one pump of the clutch, either for smoothness or for longevity.

Is this something left over from a mechanically fragile earlier era? The Volvo's first clutch, incidentally, lasted 114,000 miles, until the slave cylinder spilled fluid all over it. The mechanic who replaced it reckoned it was only about half worn, so I don't think I'd been abusing the clutch to spare the gearbox.

[BertBert]

IcedKiwi said:

I don't think there should be any friction between them with the clutch fully depressed. If there was and you had clutch drag, the layshaft would be spinning when the vehicle is stationary and you'd be using the synchros to go into 1st every time. And on a lot of cars there's no synchro on reverse so you would get a crunch.
You'd be trying to change the engine speed every shift using the synchros which I imagine would burn through them pretty quickly. The synchros are pretty small and I doubt they'd have enough friction to breakaway the clutch if it was dragging.

Although I'm prepared to be corrected

If you've ever changed a clutch I think you'd agree. But it doesn't take much drag to spin up the layshaft.
Bert

[hygt2]

Brian Trizers said:

Really? In that case the Volvo S60 I recently parted with after 12 years and 140,000 miles from new should have been a wreck, in the gearbox department at least. In that time I must have done thousands of block downchanges (and hundreds of block upshifts too) and double-declutched exactly twice, when I'd read something about it and tried it out of curiosity. OK, I very seldom selected first on the move because if the car was moving at all it would pull away in second, but it never occurred to me that a block change required more than one pump of the clutch, either for smoothness or for longevity.

Is this something left over from a mechanically fragile earlier era? The Volvo's first clutch, incidentally, lasted 114,000 miles, until the slave cylinder spilled fluid all over it. The mechanic who replaced it reckoned it was only about half worn, so I don't think I'd been abusing the clutch to spare the gearbox.

Depends on how much progress you want to make. Also, all S60 have low pressure turbos, except for the T5, which has good low-down torque. I find if I need quick acceleration out of tight corners, especially uphill like zig zag hill or Welsh mountain roads, I need the N/A cars (MX-5, MR2, NSX, GT3) to have at least 4,000 rpm to be in the power bend. As such, if you want to select first very quickly, you would need DDC so 1st would go slot in and engage at 4,000 rpm+, or you have to wait a long time for the gear stick to slot in, as well as wearing the synchromesh.

Of course, if you have a LOT of torque off idle, you wouldn't need to. But these cars are far and few in between. The only car I find with sufficient torque below 1,700 rpm is a TVR Griffith 500 ...

[Mr2Mike]

MC Bodge said:

Hence the name "Crash 'box"

And these are the only boxes that require double de-clutching. Dog boxes do not, and (working) synchromesh boxes do not. The only thing it achieves on these boxes is making your gear changes twice as slow.