..besides that fact that we can talk about it later to act cool

I have always driven with just simple heel toe rev matching and a single clutch motion. this seems to work fine to smooth out downshifts. Any gurus that care to enlighten me on the nuances both technical and mechanical? I need to be convinced that it is worth learning this skill . A in depth analysis of what is going on inside my transmission is ectremly welcome.

 

With modern transmissions, don't bother. I had a non-synchro Bug (1957) that needed a double-clutch downshift to first, to get the gear shafts spinning in synch before it would go into low, so I learned how to do it at an earlyl age. A later Beetle had a crapped out 3rd-gear synchro so it came in handy there, to avoid the graunch going from 4th. Gearboxes are good enough now that it's not a big deal.

K

 

You'll probably get differing opinions on this one...

I double-clutched all my downshifts. Then I went to Bondurant school. There, Buddy Rice (no0w in F3000) said to me, "Why are you doing that? Stop it." So I did.

The purpose primarily is the following:
1. Easier on a transmission's syncros
2. Easier to shift a car with failing/failed syncros

Reason it does this is the throttle tap when you're in neutral with the clutch pedal *off* the floor speeds up both the input and output sides of the transmission. That way when you put the clutch in to shift, the gears attempting to engage each other are, theoretically, spinning at the same speed or real close to it. This is the job that syncros do for you in most modern transmissions.

I and many others contend (and I've got the high-mileage but still-functional transmission to back this up) that syncros in today's cars are good enough that double clutching is unnecessary. That said, one situation where I think it could help is in an autocross, where you're going from 2nd to 1st gear at fairly high RPMs. Of all the syncros in my transmission, the 1st gear one was the only one showing noticable, visible wear.

HTH, someone who really knows transmissions feel free to correct any lies I just inadvertently told...

 

the only way to properly spin up both the input and output shafts is with the clutch engaged, hence why the blip is done in neutral with the clutch engaged.

Personally, I believe it to be a skill worth having in the toolbox although not needed all of the time.
When I heel toe downshift, I have found it to be much smoother to do the blip in neutral (even if the clutch is still engaged) and then put it in the lower gear, goes in nice and smooth although not a true double clutch. Putting it in the lower gear right away (Before the blip) takes too much effort and doesnt feel right to me.

Check out Chapter 6 in Going Faster, mastering the art of race driving

 

Personally, I've noticed that downshifts are smoother when you double clutch. Like someone already said, just rev matching won't spin up the input shaft in the transmission, so you still get a little bit of a jerk. If you're really interested, you can check out this link:
http://auto.howstuffworks.com/transmission1.htm

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by krshultz &raquo;</TD></TR><TR><TD CLASS="quote">
Reason it does this is the throttle tap when you're in neutral with the clutch pedal *off* the floor speeds up both the input and output sides of the transmission. </TD></TR></TABLE>

the trans output shaft rpm is directly praportional(sp?) to the rpm of the wheels. the output shaft is mechanicaly locked to the differential either through a gear or pinion shaft on transaxles or a driveline in rwd.

the reason for double clutching is to match the input shaft and output shaft rpm so the trans doesnt grind going into gear. if you have a trans w/o syncros then you need to beable to double clutch as stated before.

 

Thanks for all the input and links, guys. I usually do the blip as I go across nuetral on the box with the clutch in, cheating toward bliping as I pull out og the higher gear. That has always seemed to be butter smooth, but it must be because the slack I leave is being pick up by the synchros.

 

Well, I guess now's the time to ask:

Other than the fact that it's an extra skill to learn (if you haven't already) and it could be one extra skill to screw up, why not double clutch?

I started doing it in the ITR for autocross runs. I know that the synchros in the ITR tranny can do it for me (although as mentioned above, not very well on the 2-1 shift) and I know that the 88 CRX tranny can do it for me as well. However, the basic fact remains that my transitions are smoother when I double clutch and I feel like I make less mistakes, not more, when I'm on track. It's not like there isn't plenty of time...the CRX ain't no formula car under braking.

Is there a legit reason for an instructor to say "stop that?"

 

Travis,

I'm with you. I double-clutch downshifts every time. I find there is plenty of time to go through double clutching in the braking zones. For me, I would probably have to spend time relearning downshifts because not double clutching would upset my rhythm.

I don't know if there is an advantage to not doing it if you already do it and your not spending any brain dollars on it.

Gary
Sheehan Motor Racing
http://www.teamSMR.com

 

double clutching is just something that would happen instinctivly(if you have good driving skills)its not something you normally need to do..but when racing in traffic there is alot of things going on and that means you gotta do alot of things..well you know

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by travis &raquo;</TD></TR><TR><TD CLASS="quote"> It's not like there isn't plenty of time...the CRX ain't no formula car under braking.</TD></TR></TABLE>

I'll cautiously agree with that while on track. However if you can spend the time double-clutching in autox while maintaining maximum traction... you're a better man/woman than me.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by maxQ &raquo;</TD></TR><TR><TD CLASS="quote">
you're a better man/woman than me.
</TD></TR></TABLE>

I really don't think it takes all that much extra time. It's not like I think through each step and pause in neutral, I just double tap the clutch as the shifter moves through neutral. There's no real pause, and I don't bring the clutch pedal all the way up or down since the engagement doesn't have to be full when in neutral (since there's not a big load on the friction material).

-travis, who's not a man/woman, but has met a man/woman and he/she was creepy

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by travis &raquo;</TD></TR><TR><TD CLASS="quote">
I really don't think it takes all that much extra time.
</TD></TR></TABLE>

Define "all that much extra time". If I underbrake in a particular zone (total optimum time on brake 0.25 seconds) for an extra ten feet because I'm double-clutching, I give up a half second braking early.

Likewise, because I so rarely brake in a straight line, the more time my hand spends off the steering wheel the more time I'm losing...

Not disputing your claim, maybe I just need more practice...


Andy - who considers himself lucky to get back in first gear at all during an autox run...

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by maxQ &raquo;</TD></TR><TR><TD CLASS="quote">Define "all that much extra time". If I underbrake in a particular zone (total optimum time on brake 0.25 seconds) for an extra ten feet because I'm double-clutching, I give up a half second braking early.

Likewise, because I so rarely brake in a straight line, the more time my hand spends off the steering wheel the more time I'm losing...
</TD></TR></TABLE>

If you get a total of 0.25 seconds for braking, I personally would weigh the benefit of shifting at all. You're not going to get much more than a quick stab. If you're going in slow enough to need first gear on the exit and you only stab the brakes, that means your entry speed must be pretty close to a first gear speed already. If that's the case, I would be tempted to make the gear change on exit without any rev matching.

Anyway, to answer your question, I would say if there's time to heel-toe and shift, there's time to double clutch. The time difference is the difference between slamming the shifter vs. dragging the shifter. The pause in neutral is almost no pause at all. For that reason, there's also no extra time with your hands off the steering wheel. Either way, you have to reach over and pull the shifter.

 

i double clutch because when i become a professional driver the transmissions in those cars don't have synchros. why not do it now? just kick your left foot twice real quick. doesnt take that much time at all.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by racerdc5 &raquo;</TD></TR><TR><TD CLASS="quote">just to confirm, is double-clutching the same as heel and toe downshifting, except you are in neutral and OFF the clutch as you rev match, and then press the clutch to get it into the lower gear?

</TD></TR></TABLE>

Yes, except it doesn't necessarily have to be done under braking.

 

The same technique can help you get the car in reverse if it is being stubborn when you are trying to back out. Blip the throttle (slightly) with the clutch out, then push the clutch in and slip it in gear. Just like butta...