June 1, 2005

Gearbox query  

A driving question: can anybody explain to me the benefit of double clutching when upshifting on a manual transmission car? This page lingers on the difference between double clutching on the upshift and downshift, but doesn't describe any benefit to the former; this page mentions only what seem to be obscure maintenance reasons for doing it. Am I correct in thinking that the only real performance benefits to double-clutching are in smoothing out the downshift?

Comments
Ted  {June 2, 2005}

When I used to drive a manual I double clutched all the time. Up and down. Although I would on the downshift more often. I primarily got into the habit on the downshift in anticipation of braking. I like to cruise in neutral until I determined exactly my engine need then shifted to exactly the gear I wanted. This was often to the scoff of my wife who determined that somehow my being in neutral while traveling 40 mph was somehow inherently dangerous and I should be in gear. But I liked to Drop from 4th to 2nd or up from 1st to 3rd. I felt it gave me more control over my needs. I felt it gave me a performance boost plus saved gas as I cruised in neutral on the downshift.

Hugo Zoom  {June 2, 2005}

I don't know the answer to your question, but I can recommend two texts that discuss double clutching: The Technique of Motor Racing by Piero Taruffi and The Art and Science of Grand Prix Driving by Niki Lauda.

Years ago I remember reading an account of Al Holbert shifting without using the clutch at all in a Can-Am race, maybe circa 1980, because the clutch had stopped working. He managed to finish the race in the top 5.

I was a stupid teenager with no thought of whether I was causing any damage to my car, so, naturally I had to try it. There is a name for the technique, but I forget it.

What you do is accelerate up to the speed at which you want to upshift, or slightly faster, then you let off the gas until the vibration you feel in the shift knob goes away, at which point the drivetrain is "telling" you that the rpms of the input and output shafts of your transmission are in sync, hence no vibration, and you just shift up. It works if you have an agile shift linkage that allows you to shift fairly quickly, whereas if you have a rubbery front-drive cable linkage it may take too long and by the time you get into the upper gear the revs will have dropped too low(crunch). Whatever you do, don't force it.

The opposite is to slow to the speed you want to downshift at, then let off the gas, when you feel the smoothness set in you shift into neutral, then give it a little gas and downshift.

NOTE: the upshifting is relatively easy to master. I'll admit I never really got the hang of the downshifting, although I still regularly slow down by just braking then shift out of gear when the smoothness sets in when I'm slowing to a stop without clutching.

the usual caveats apply.


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