Why you dont need to double-clutch
 

There was some small debate in the "Why this car is so much fun.." thread about double clutching. There were a lot of good responses. I emailed a good friend of mine who is an engineer at GM. He has worked on some very fun cars over there, including the Corvette. Aside from that, he is damn near a genius as he skated through college with a perfect Mechanical Engineering GPA at a top university. Anyways, this was his answer to our double clutch debate. I think its an interesting read:

quick answer:

First of all, the entire purpose of syncros is so that you don't have
to double clutch. That is why they were even created, if you always
double clutch it you don't need synchros. Synchos slow down the output
gear, input gear, and input shaft (layshaft) to match the speed of the
collers, output gears, and output shaft to linup the engagment between
the collers (locked to output shaft) and output gears (locked to input
gears). They do this by making frictional contact as you slide the
shifter over, but that is what they are designed to do. Unless they screwed
up their engineering design, they should not wear out from this and the
ones I've seen show very little wear. They aren't made of friction
material like clutches and brakes and are designed to last the life of the
vehicle. So, in a sense you would be saving your synchos by double
clutching it, but that would eliminate their entire purpose of existance,
destroying their spirit and will to live....

Longer Answer:

Look at this picture while reading, it will probably help, good write
up as well:

http://auto.howstuffworks.com/transmission5.htm

Inside a transmission all the gears are always intermeshed (reverse is
a little different). As an example for a 4 speed, there will 4 gears on
input shaft and 4 gears on output shaft, each pair of the gears we will
refer to as 4:1, 3:1, 2:1, and 1:1. All the gears on the input shaft
are connected to it, but they ride on bearings on the output shaft.
When you select a gear, you slide another ring that is locked onto the
output shaft up against the gears on the output shaft to engage that gear
to the output shaft, inbetween are the synchos. When these teeth do
not align, that is the grinding sound you hear. Synchros help align
these teeth so that you don't have to double clutch it. A typically 4
speed transmission will have 2 collars, 1 that can engage 1st and 2nd and 1
that can engage 3rd and 4th. This will require 4 synchros for each
gear as well. When in neutral, output gears are all floating, they are
not connected to anything but bearings around the output shaft and the
input gear, so there RPM is fixed based on the gear ratio. Manufacturers
do this different, in some cases the collars and synchors float until
they slide over as well, but this is the general idea.

Let's think about how everything is spinning in a made up tranny and
engine, in this case let's say first gear is 4:1, second is 3:1, third is
2:1 and 4th is 1:1.

Let's look at constant Engine RPM of 4000
Engine is spinning at 4000 RPM
clutch and input shaft is also spinning at 4000 RPM
4:1, gear on input shaft is spinning at 4000 RPM, gear on output shaft
is spinning at 1000 RPM
3:1, gear on input shaft is spinning at 4000 RPM, gear on output shaft
is spinning at 1333.33 RPM
2:1. gear on input shaft is spinning at 4000 RPM, gear on output shaft
is spinning at 2000 RPM
1:1. gear on input shaft is spinning at 4000 RPM, gear on output shaft
is spinning at 4000 RPM

In neutral stopped:
Output shaft is stopped, collers are stopped, synchos are stopped

Shift to first operation:
clutch in disengages input shaft, input shaft, input gears, and output
gears continues to spin from inertia (slows down slowly)
As you slide into first, the first gear syncho contacts both the coller
and first output gear, friction stops output gear, input gear, and
input shaft and finally collar engages first gear. As you let out clutch
everything spins up.
*note, in this case double clutching can not do anything since output
shaft and collers are at zero and engine RPM is never zero....

Now, you drive in first up to 4000 RPM and shift to second:
You push in clutch which disengages input shaft from engine, but input
shaft and gears continue to spin at 4000
RPM, 1st gear is at 1000 RPM and 2nd gear is 1333.33 RPM, output shaft
is at 1000. You slow engine RPM down perfectly (your good) to 3000
RPM. When you shift to neutral, the collar and 1st gear syncho disengage
from 1st output gear. As you shift to 2nd gear, 2nd gear is spinning
at 1333.33 RPM while the output shaft, coller, and syncho are spinning
at 1000 RPM. As you slide over the synchro touches first which slows
down 2nd output gear to match 1000 RPM (and sightly speeds up output
shaft, but we'll neglect that), which slows down input shaft, input gear
from 4000 RPM to 3000 RPM. As you let the clutch out, the engine
engages.

Now, if you double clutched it while in neutral, you would have slowed
down the input shaft and gears to 3000 RPM and the output gears to 1000
RPM, in which case the synchros and collers would match up perfectly.

Now for the downshift, you speed up to 4000 RPM in second so output
shaft, output gears, collers, and synchros are at 1333.33 RPM while input
gears and input shaft are at 4000 RPM.
you push in clutch to disengage input shaft and rev engine to 5333.33
RPM. When you shift to neutral, 1st output gear is spinning at 1000 RPM
while the coller and synchro are spinning at 1333.33 RPM. As you slide
over the synchro once again contacts the first output gear, speeding it
up to 1333.33 RPM and the input gears and input shaft to 5333.33 RPM.
Now when you let out the clutch, the engine engages. As you can see,
it is very simliar to upshifting and if you double clutched it you would
save the different speed contact between the synchro and 1st output
gear.

Another option, power shifting!!
Your cruising at 4000 RPM and want to shift to 2nd and your cool....
You come off the gas perfectly to remove any force in driveline, you
can now easily pull the shifter into neutral. At this point engine RPM,
input shaft, input gears are all spinning at 4000 RPM. 1st gear, 1st
synchor, 1st coller and output shaft are spinning at 1000 RPM. 2nd gear
is spinning at 1333.33 RPM. You now perfectly adjust the RPM to 3000
RPM. Now, the input shaft and input gears are all spinning at 3000 RPM,
1st output gear is spinning at 750 while the coller, synchro, 2nd
output gear, and the output shaft are all spinning at 1000 RPM. As you can
see, you can now easily slide into 2nd gear without any frictional
contact betweent the clutch or synchros; Your so cool you can wink at the
hottie noticing your bad ass self now...

So, if you are really concerned with clutch or synchro wear, you should
powershift if your good enough....but, if your not, the jerks you feel
are the synchros and collers/gears catching suddenly....

Excellent technical overview of how a manual transmission works. Deserves a sticky in the Tech Garage.

Won't somebody think of the synchros!?!?!

Not a Mazda transmission, they don't follow the standard rules. :mad:

???

Don't understand where you are coming from.

I think Mazda transmissions are the smoothest shifting pieces I have ever experienced.

Nice clutch feel short throw from the factory never miss a gear or even gnacsh a dog tooth.

You want crappy shifting try a early 914.

You're both well-intentioned but miss the whole reason to double clutch...
 

nobody double clutches or heel and toes to save the transmission -you do it so you can use the brakes on all four wheels - instead of the transmission that drives the rear wheels, in an RX-8 - to transfer weight from the rear of the car to the front during cornering. I always knew, from the age of 12 when I learned how to drive a 4-speed Datsun 510 sitting in my Dad's lap, that modern manual transmisions were all sycnromesh, at least in the forward gears, that the real reason to double clutch and/or heel and toe was to make gear changes without affecting the chassis dynamics of the car. It allows you to only shift the weight of the car by accelerationg with the throttle or braking. Nothing else.

It's also a fun way to go down thru the gears.

Isn't this a Fuji transmission?

Explain more please....I am interested to hear a more scientifc arguement for this..

It's an AISIN 6 speed box. Very similar to that of an S2000.

Never ever had an issue with Japanese made transmissions. I think they are much better than most over-rated European made tranny. Just my opinion.

Fuji... is the name behind Subarus.

The idea is to rev match the engine to the rear wheels when going into a lower gear.

do this experment:

at 50 mph, go from 5th gear to second gear with out double clutching.
notice how the rear wheels squeeled and partialy locked up.

now dot go from 5th gear to 2nd gear double clutching( which you rev the engine while the clutch is out and the gearbox is in neutral)
notice no wheel lockup.

If you are doing 125mph on the long straight at Sebring and you go into 3rd with out using the "double clutch" technique ( short term for Heal toe double clutch downshift" you will wind up spinning into the wall since you lost traction in the rear.

The syncros are intended primarly for upshifting and so an untrained driver can downshift on the street. Double clutching is not necessary for street aplications, however it is good practice for when you take your RX-8 to the track.

Mario Andretti doesn't double clutch. But he drives a "crash box" that doesn't have syncros to get in the way. He is still rev matching, and that is the most important part of downshifting, particulary at speed.

Bob Bonduraunt and Skip Barber both have excelent courses on this subject as well as, Late Apexing, weight transfer, trail braking, and the sort. My opinion is one of the best mods and longest lasting mods you can do to your car is take on of these courses. It has saved me from many accidents since I have much more control of any car I drive.


John

Your expiriment is pretty pointless, as i doubt anyone here is arguing that some sort of rev matching is needed on downshifts, just as you said. You can do the same thing with just rev matching (clutch in) and the wheels wouldn't squeel. I think this topic is moreso about rev-matching vs double clutching if anything.

If a driver can rev-match upshift and downshift very well, I don't see why he or she has to double-clutch since a good rev-match doesn't produce unnecessary weight shift.

Agreed. Rev matching != double clutching. You don't need to double clutch to rev match.

Thank you. Two different things.

Rev matching is the pourpose o double clutching. By not double clutching and "rev matching" you have rev mached the input shaft to the output shaft. you still havn't rev mached the rest of the transmision. double clutching gets the gears and center shaft matched to the output shaft. On a gearbox with syncros, double clutching makes for a smother shift, which is key for the track. A good rule I learned at Bob Bonduraunt, is,
" In order to speed things up on the outside you have to slow things up on the inside."
This principal works well on more then just racing.

The answer I gave you is the reason Bob Bonduraunt gave me and why that was the second thing we learned on the track. (The first was late apexing.)

Djseto.

Your avitar is a bike. double clutching a bike or a car with a sequential gearbox is imposible.
When at the threshold, rev matching is not a bad idea.

As a side note going from 5th to 2nd real fast without double clutching can break the syncro. A friend of mine did that in my FC RX-7. He went from 2nd to 5th instead of 3rd by mistake. From that point on, untill I rebuilt the gearbox, I had to double clutch upshift to go into 5th. This technique is the same for Trackter trailers, since they do not have syncros and the high mass of their gears.

John

moreover, the heal and toe technique has nothing to do whether you just rev-match or double clutch. Its simply a techique you use to brake and blip the throttle at the same time. Do can either double-clutch or not with heal and toe.

I never said I double-clutched on my bike. Where did that come from? I have more than enough track days and racing schools under my belt to know that. That being said, I am aware of all the racing school topics you talked about like trail-braking, late apexing, ect. when it relates to motorcycle racing. Oh, and that is ME in my avitar! Zoom Zoom on 2 wheels!

I did not say you double cluched your bike. but since you ride I could understand why you think double clutching is not necessary. I wish the 8 had a sequential gearbox. It was origeonly going to have a sequential gearbox.

Smoker.

People use the term "double clutch" to refer to heal to double clutch downshifting. The purpose for "heal toe" is to rev match while double clutching, single clutching, or crash boxing, on the strait so you are in the proper gear for the exit of the turn. The only time you double clutch downshift on the track is when you are "heal toing" or "bliping" the throttle, which realy is using the side of the foot. We also know you should aviod shifting in a turn as the resulting weight transfer can result in a spin.

John

p.s. That looks like you are having a blast in your avitar. I am in Athens GA on an overnight right now and just stopped in the bike shop here looking at the GSXR 1000. My wife would kill me if I came home with one. I miss my 750.

I am not an expert, but my understanding is as follows (using http://auto.howstuffworks.com/transmission5.htm as a terminology reference):

Rev Matching: After engaging the clutch and selecting the new gear one adjusts the engine RPM a so that, when the clutch is re-engaged, the flywheel RPM (driven by the engine) will closely match the clutch disk RPM (driven by the driveshaft through the transmission gears). This is done to avoid a dramatic and uncomfortable acceleration or deceleration of the drive train and rear wheels due to the sudden engine loading on the transmission's input shaft.

Double Clutching: After engaging the clutch and placing the transmission in neutral, a momentary engagement of the clutch (and engine reving if downshifting) with the intention of causing the transmission gears to spin at roughly the same RPM as the driveshaft, allowing you to then smoothly engage the driveshaft collar on the desired transmission gear without grinding. This is required for transmissions without synchronizers. This is not needed on transmissions with synchronizers as it is the synchronizer's job to smoothly accelerate/decelerate the desired transmission gear to driveshaft speed prior to driveshaft collar engagement.

Heel and toe: A single-foot brake/accelerator pedal technique, done when braking and down shifting at the same time, to allow the driver to blip the throttle to Rev Match (see above) the engine while continuing to exert force on the brake pedal. See http://www.edmunds.com/ownership/how...2/article.html for more detail.


[Edit: Actually, I believe I was wrong with the double-clutch comment "unless you shift at exactly the right RPM for the two gears you are transiting". If you don't have synchronizers you always need to double clutch.]

Alright, I'll throw a question in here -

First - I'll clarify that I simply mean double-clutching because the stubborn beast refuses to go into gear. I'm not talking about heal-and-toe or downshifting for cornering. I'm simply talking about trying to upshift one gear because it's time.

I'm not a grade-a shifter, but I consider myself more than sufficient. Every manual transmission vehicle I've ever owned has simply refused to go into a gear once or twice for no apparent reason (not rev'ing high, not down-shifting, not much of a stretch for rev-matching). Happened most often on the 1976 Honda Civic in which my dad taught me to drive. He taught me to double-clutch it to get it into gear and occasionally I've done that on other Hondas, Toyotas, Subarus and once or twice in the 6 months I've owned my RX-8.

According to the technical explanation above ,the synchros are *supposed* to mean double-clutching is *never* necessary, but in practice I've found it's *occasionally* necessary. Why would that be?

Maybe your synchro is acting up once in a while therefore if you opt for double-declutching, it shifts again.

yeah, but double clutching is fun............:))) Besides its so intinctive in me that I dont even think about it anymore when downshifting.---shift to nuetral, blip the accelerator just enough to match revs, and slam it into gear...............

My 5th gear syncro was cracked and in order to get into the gear, without grinding the gear, I had to use the double clutch technique you had to use.

Quite posibly your situation, your clutch was not disengaging completely, or the gearbox was cold. I have had that happen to me before.

John