Why does "Double Clutching" help change gears ?

[DOCTORBILL]

Today I was stopped at an intersection and couldn't get into first gear when the light changed.

I tried to force it and "ground some coffee"...while the person behind me thinks,
"What a dufus brain...!"

I usually pull the stick back into second when that happens and then it easily slips into first.

This reminded me of "Double Clutching" which truck and race car drivers often do when
they downshift, or press the clutch in twice just before going to a lower gear while rounding a corner.

Now that I have learned a smidgen about manual transmissions, I am supposing
that double clutching spins up the input shaft to higher rpms so that the syncros
match up better and allow the lower gears to slip into place.

Am I correct ?

When the gears 'grind', is that the gears themselves or the brass syncros grinding ?


Last question - what is "Speed Shifting" - without using the clutch ?

Sounds like a really good way to blow the transmission all to Hell...!

DoctorBill

[drdisque]

when the transmission "grinds" it's the gears, and it means the synchros aren't doing their job, either because you've asked too much of them (such as trying to drop into 1st at 40 mph) or because they're worn out.

[brivers]

1. You almost answered all questions yourself. Double clutching is a way speeding up or slowing the input shaft to make the gears mesh up while shifting. If your upshifting you do it when the motor slows down to the approximate rpm that it will be when you shift to that gear. When downshifting you have to rev the motor to the approximate speed it would be when it goes into the lower gear.
2. Its the gears grinding.
3. Speed shifting does ruin transmissions. Although if you were to loose your clutch pedal, (as in the cable breaking) you could do this relatively safely by using above method, only no clutch. Upshift; let engine idle down, downshift; slightly increase rpm. Kick out of gear to stop. Turn off engine, shift to first gear and start motor to take off.

[Johnny Mullet]

I always "blip" the throttle before downshifting to match RPM's and to help save my synchros.

[flashlight]

Double shifting in cars with synchro gears is completely unnecessary as it puts extra wear and tear on the clutch. Double shifting was originally used to keep the gears from grinding before synchronizers were commonplace.

[Rooy]

Originally Posted by flashlight

Double shifting in cars with synchro gears is completely unnecessary as it puts extra wear and tear on the clutch. Double shifting was originally used to keep the gears from grinding before synchronizers were commonplace.

Not completely unnecessary. On downshifts, especially in spirited driving, it saves synchros so they don't have to work so hard. Wear on the clutch will be minimal. You're just disengaging and reengaging it one extra time, no slipping.

I always double clutch and rev-match downshifts. Driving a car with worn synchros, it's necessary. On upshifts, simply pausing in neutral for a second or two before selecting the gear will let the RPMs fall to where they need to be.

[Woodie83]

Originally Posted by flashlight

Double shifting in cars with synchro gears is completely unnecessary as it puts extra wear and tear on the clutch.

All the wear on a clutch is from starting out from a stop. There is little to no load on the clutch when changing from one gear to another. Besides, our clutches are more sturdy than our synchronizers, and far easier to change.

I often shift without the clutch, just for entertainment. I rarely do it on downshifts as that would be hard on the synchronizers, but I upshift without it all the time.