Anyone double clutch on downshifts? Why?

[matsuzaka]

On another forum there was a discussion about the reason for double clutching on downshifts. Variety of reasons were mentioned for using this technique.

I'm going to explain the procedure in case some of you are unfamiliar with this process: Let's say you are downshifting from third to second, you would depress the clutch once to go from third to neutral, then you blip the throttle to rev match while you're in neutral, then you depress the clutch a second time to go from neutral to second. Of course, this all happens very quickly, may be in a second or less.

I have always believed that the reason for doing this is to save the throw out bearing. Because if you blip the throttle while the clutch pedal is depressed, instead of being in neutral, you are spinning the TOB up to what ever RPM the engine is revved to, which is not good for it. It's possible this was more important in the old days because in some cases TOBs were not necessarily an actual bearing, just a low friction surface.

Any thoughts?

[BE911SC]

I was taught to double-clutch by my dad (in his '67 Healey) as a means to better mesh the gears and prolong the life of the synchros. My 915 downshifts MUCH smoother if I double-clutch it, especially with a good healthy stab of engine rpm. It snicks right into 2nd from 3rd every time. If I try to 'drag' it straight into 2nd from 3rd it takes a second or two for the 2nd gear synchro to do its job, which equates to me as wearing it down.

I don't think the TOB has anything to do with double-clutching other than its normal job.

[doublestuff]

My understanding is that by engaging the clutch (lifting the pedal) while in neutral and blipping the throttle, you synchronize (perhaps in a crude manner) the input and output shafts of the transmission, and this allows the gears to mesh with less wear on the synchronizers. In my experience, it seems to help downshifts go more smoothly, and sounds cool as hell (which is my primary reason for learning to do it well).
Check this youtube video on the internal workings of a manual transmission. While it doesn't specifically address double-clutching, it is very informative and may give you a better idea of what's going on.
Manual Transmission Operation - YouTube

[Scott Douglas]

Blipping the throttle with the clutch depressed won't spin up the gears in the transmission.

[GaryR]

If you think of neutral as being a "gear" you are simply shifting twice it to get to the next lower number gear.

[manbridge 74]

Fun.
Lost art.
Left leg exercise for us cable clutch guys (mine is a bit bigger from past Muncie abuse)
Smoother shifts
Trans lasts longer
Sound

Gotta get your pedals oriented better if you have smaller size feet.

[911SauCy]

Fun
Seems to shift better
Sounds great with FABSPEED components out back

[matsuzaka]

Yes, rev matching by blipping the throttle does give you the side benefit of saving the synchros. But the primary reason for doing that is so you don't upset the car on it's suspension during a downshift, I think.

The double clutching issue is a different matter. Do you know what I mean? Basically, if you are going to blip the throttle on a downshift, the alternative to double clutching is to blip the throttle while you have the clutch pedal down, therefore, wearing the throw out bearing.

[db_cooper]

Grand Prix - The best lap - YouTube

With a G50 or any late model manual transmission..you are wasting your time double clutching on downshifts..all brass syncros don't need it..with the steel syncros in a 901/915..now you are talking..a true racing transmission.

[RaGH]

Would double clutching be that more beneficial than say heel and toeing? I learnt to heel and toe ages ago and use that regularly even if i'm not braking hard to rev match. Would double clutching be that beneficial as it's still a form of rev matching but going through neutral as a 'gear' first?

[porsche930dude]

the reason we do it is so it goes into gear . like in my worn 915 2nd to 1st its a nesessity if your moving more than 5mph and the worn out synchros cant do their job anymore. they do it in race cars and tractor trailers because they have straight cut gears. atleast thats what iv gathered reading about it over the years

[Charles Freeborn]

Yes, but mostly out of habit from years of driving old cars with worn out synchros. My Porsche shifts like a dream.
-C

[LJ851]

Rev matching, heel toeing, or blipping the throttle before releasing the clutch (engagement) on a down shift only keeps the chassis more composed. It does nothing beneficial to a syncromesh transmission.

Double clutching properly greatly reduces wear on the syncros. In fact it is how you drive a non syncromesh transmission.

[mspirito]

The practice is meant to match the engine revs to the next lower gear. 30 mph in third gear = 2500 rpm
30mph in second gear is 3200 rpm thus matching the engine to the trans to the wheels

[KiwiJohn]

The practice is meant to match the engine revs to the next lower gear. 30 mph in third gear = 2500 rpm
30mph in second gear is 3200 rpm thus matching the engine to the trans to the wheels

Perfectly put.

[Noah930]

I double clutch and heel-toe regularly. Mainly because it's fun, there's an element of skill to it, and because it allows me to downshift quicker than slowly letting the clutch out. Compared to other cars I find it a little harder to do in the 911 on the street, because rarely do I step on the brake hard enough to get it depressed close to the level of the throttle pedal, but I still try to do it, anyway.

[Scott Douglas]

Originally Posted by matsuzaka

Yes, rev matching by blipping the throttle does give you the side benefit of saving the synchros. But the primary reason for doing that is so you don't upset the car on it's suspension during a downshift, I think.

The double clutching issue is a different matter. Do you know what I mean? Basically, if you are going to blip the throttle on a downshift, the alternative to double clutching is to blip the throttle while you have the clutch pedal down, therefore, wearing the throw out bearing.

Double clutching and blipping the throttle on a downshift are not the same thing, especially if you have the clutch pedal depressed when doing the later.

[catuck]

Double clutching during downshift is primarily for racing. Entering a corner while downshifting can upset the car. By the way, heal-toeing is how you do it. In old trucks and tractors, it is a method for downshifting with a non synchro transmission. I do it because my dad taught me to drive that way and its fun, so there's that too.

[911pcars]

"Double clutching during downshift is primarily for racing."

Uh no, or used to be. It can actually take longer to downshift in a race car in which case a serious racer will skip the double-clutch and merely rev-match. OTOH, serious racers also rebuild gearboxes as needed. You can also rebuild a worn out gearbox in your street car too. Up to you.

Unfortunately, the majority of POs missed/skipped the DC lessons and left it to subsequent owners to deal with remaining synchro life.

Sherwood

[nettles503]

My dad drove trucks in WWII, then a series of sports cars in the '50s and early '60s. When he was teaching me to drive a manual transmission, he insisted I learn how to double clutch. It's a lesson that's served me well when shifting a 915. Thanks Dad!