THE double clutching thread...
kjewer1
03-06-2005, 12:33 AM
Now that I've actually had a valid reason to double clutch, and have had some time to get good at it, I feel that I am now in a better position to explain this better than I have in the past. Hopefully this will be the double clutching thread to end all double clutching threads :) Hopefully the layout of this post makes it easy to follow the train of thought and the thought process used to understand this technique.
Some back story... The 1g I just bought for a daily driver has a completely banjaxed 3rd gear synchro. It grinds badly with every attempt to upshift to 3rd, and downshifting to 3rd is not possible. The previous owner found that by using a super heavy weight oil, its not so bad. I believe it's 120 weight. This will be important later. I figured its time to learn to double clutch, in an effort to put off the 800 dollar tranny repair expense ;)
Lets recap what I have posted in the past about double clutching. The point is to manually synchronize the speed of the input and output shaft in the tranny so the synchros don't have to synchronize anything. This is especially useful in trannys that either dont have synchros, or don't have synchros good enough for downshifting (Some racing gear boxes and truck gear boxes have trouble with this especially, by design). This is not to be confused with rev matching, which is to rev the motor to where it would be in that next lower gear before the clutch is let out, to reduce the shock and keep the balance of the car. But double clutching on the down shift MUST include rev matching, so they are sometimes confused. ;)
The technique, on the upshift, consists of clutching, shifting to nuetral (between gears), reclutching (let up, push back down) while between gears, and shifting into the next gear. When downshifting the technique is the same but while the clutch is UP in the neutral/between gears aprt of the shift you blip the throttle to rev match (bring RPMs up to where they would be in the next lower gear at this particular vehicle speed). So you can see that there is nothing fast about this process, with all of the added steps. It takes much longer than a normal shift, so forget everything you learned in the F&F movie ;)
The theory will have to come after some explaination of how the motor, clutch, tranny, and car are all related.
The clutch will engange/disengage the motor from the input shaft of the tranny. The input shaft is connected to the output shaft only when the car is in gear, and the output shaft always matches the wheel speed since its a direct mechanical link to the tires (via rear axles, rear end, driveshaft, tcase, or front axles, front diff, etc). So when the clutch is up, the input shaft rpm will match the engine rpm. When you are shifting but in between gears (nuetral), with the clutch in, the input shaft is not connected to either the wheels or the motor, and is allowed to decelerate due to friction and the drag from the fluid (and stop if you wait long enough). The output shaft its mated to however is still rotating, connected to the wheels. So the job of the synchros is to to match the speed of the two shafts so the gear in question can be engaged. The gears themselves are constant mesh anyway, but the synchro/slider/etc is what locks that gear to its shaft making it a part of the shaft. Check out www.howstuffworks.com for an explaination of this with pictures.
Now, the problem comes in when the clutch is down and you are between gears. The input shaft is not connected to anything and is free to slow down and maybe come to a stop. If there are no synchros, or they are not capable of matching the speed of the shaft to the respective gear (as in my case), you get that sweet ass crunch when you try to engage the gear.
The solution is double clutching. You are in second gear. You put the clutch down, shift half way to 3rd (in between gears, or nuetral), let the clutch back UP, which accelerates the input shaft back to engine rpm (which has now fallen a few hundred rpm), you put the clutch back down and push the shifter into third. If you timed all this right, the engine speed was where it should be for 3rd gear at your current road speed (rev matched), and the input shaft was right there with it thanks to the double clutch action, and the synchro/etc engaged perfectly with no sweet ass crunch.
For downshifting: You are in 4th gear at some rpm. Put the clutch down, shift halfway to third, let the clutch UP, blip the throttle to raise RPM (say 1000 rpm higher, whatever the rpm would be in 3rd gear at the current road speed, again "rev matching"). Clutch back down, push the shifter into 3rd. If you did all this right, the engine speed was where it would be in 3rd gear at the current road speed (if you are good with rev matching this will come naturally to you), the input shaft was at the same rpm since it was connected to the motor while the clutch was up when you reved it, and the shaft speeds match and it slides right in with no sweet ass crunch.
Now on to my experiences. I found it difficult to get the hang of this. At first it just took time to get my hand and legs to cooperate with this bizarre driving technique. Then its all about timing. I would get it right most of the time, but then it would crunch again occasionally. Turns out I was just getting lucky some of the time. I finally found that while upshifting I need to pause slighty before reclutching and shifting into 3rd. This lets the RPM drop a little further so it matches the road speed in that gear. I wait until 4000 rpm, clutch in, shift halfway, clutch up, pause, clutch in and shift. I say those last two steps at the same time because this is important! If I wait too long after pushing the clutch in to shift, it crunches. Lets think about why. Remember back to that 120 weight gear oil. Thats some thick shit, and provides a lot of drag on the input shaft when it is left to spin down. By taking too long after the clutch is in, the input shaft slows down too much and the speeds dont match any more, and it crunches. So I put the clutch in and push it into gear at the same time. Problem solved. With thinner fluid this may not be so critical.
Also note that the length of that pause will depend on the RPM I shift at, which is why I mentioned it above. The higher the RPM, the bigger the drop in RPM in the next gear. This may seem like stating the obvious, but its very important. Shifting at say 6000 rpm, that pause has to be a little longer, and this is where practice and experience comes in handy.
Downshifting was much harder at first, but I can now nail that too every time. Again, like with rev matching, the trick is in how much you rev it up. Rev too little, things dont match up. Rev too high, things do match up. But, if you rev too high, you can increase the pause ;)
Thats the basics. I'm surprised that I was able to learn how to do this in just a couple weeks, and I'm more surprised at how much I learned about tranny operation and how all these elements relate to each other. Things I always knew, but this validates and reinforces things I've known for years but had no actual experience with. Good times....
I can't feel my fingers anymore. I wouldnt even know they were still there if the carpal tunnel wasn't all flared up. This is going on the website...
Edit> The main reason for this is to get it into the archives so people can search it, hence the thread title. If anyone asks what double clutching is in the future I will be severely offended :icon16:
Some back story... The 1g I just bought for a daily driver has a completely banjaxed 3rd gear synchro. It grinds badly with every attempt to upshift to 3rd, and downshifting to 3rd is not possible. The previous owner found that by using a super heavy weight oil, its not so bad. I believe it's 120 weight. This will be important later. I figured its time to learn to double clutch, in an effort to put off the 800 dollar tranny repair expense ;)
Lets recap what I have posted in the past about double clutching. The point is to manually synchronize the speed of the input and output shaft in the tranny so the synchros don't have to synchronize anything. This is especially useful in trannys that either dont have synchros, or don't have synchros good enough for downshifting (Some racing gear boxes and truck gear boxes have trouble with this especially, by design). This is not to be confused with rev matching, which is to rev the motor to where it would be in that next lower gear before the clutch is let out, to reduce the shock and keep the balance of the car. But double clutching on the down shift MUST include rev matching, so they are sometimes confused. ;)
The technique, on the upshift, consists of clutching, shifting to nuetral (between gears), reclutching (let up, push back down) while between gears, and shifting into the next gear. When downshifting the technique is the same but while the clutch is UP in the neutral/between gears aprt of the shift you blip the throttle to rev match (bring RPMs up to where they would be in the next lower gear at this particular vehicle speed). So you can see that there is nothing fast about this process, with all of the added steps. It takes much longer than a normal shift, so forget everything you learned in the F&F movie ;)
The theory will have to come after some explaination of how the motor, clutch, tranny, and car are all related.
The clutch will engange/disengage the motor from the input shaft of the tranny. The input shaft is connected to the output shaft only when the car is in gear, and the output shaft always matches the wheel speed since its a direct mechanical link to the tires (via rear axles, rear end, driveshaft, tcase, or front axles, front diff, etc). So when the clutch is up, the input shaft rpm will match the engine rpm. When you are shifting but in between gears (nuetral), with the clutch in, the input shaft is not connected to either the wheels or the motor, and is allowed to decelerate due to friction and the drag from the fluid (and stop if you wait long enough). The output shaft its mated to however is still rotating, connected to the wheels. So the job of the synchros is to to match the speed of the two shafts so the gear in question can be engaged. The gears themselves are constant mesh anyway, but the synchro/slider/etc is what locks that gear to its shaft making it a part of the shaft. Check out www.howstuffworks.com for an explaination of this with pictures.
Now, the problem comes in when the clutch is down and you are between gears. The input shaft is not connected to anything and is free to slow down and maybe come to a stop. If there are no synchros, or they are not capable of matching the speed of the shaft to the respective gear (as in my case), you get that sweet ass crunch when you try to engage the gear.
The solution is double clutching. You are in second gear. You put the clutch down, shift half way to 3rd (in between gears, or nuetral), let the clutch back UP, which accelerates the input shaft back to engine rpm (which has now fallen a few hundred rpm), you put the clutch back down and push the shifter into third. If you timed all this right, the engine speed was where it should be for 3rd gear at your current road speed (rev matched), and the input shaft was right there with it thanks to the double clutch action, and the synchro/etc engaged perfectly with no sweet ass crunch.
For downshifting: You are in 4th gear at some rpm. Put the clutch down, shift halfway to third, let the clutch UP, blip the throttle to raise RPM (say 1000 rpm higher, whatever the rpm would be in 3rd gear at the current road speed, again "rev matching"). Clutch back down, push the shifter into 3rd. If you did all this right, the engine speed was where it would be in 3rd gear at the current road speed (if you are good with rev matching this will come naturally to you), the input shaft was at the same rpm since it was connected to the motor while the clutch was up when you reved it, and the shaft speeds match and it slides right in with no sweet ass crunch.
Now on to my experiences. I found it difficult to get the hang of this. At first it just took time to get my hand and legs to cooperate with this bizarre driving technique. Then its all about timing. I would get it right most of the time, but then it would crunch again occasionally. Turns out I was just getting lucky some of the time. I finally found that while upshifting I need to pause slighty before reclutching and shifting into 3rd. This lets the RPM drop a little further so it matches the road speed in that gear. I wait until 4000 rpm, clutch in, shift halfway, clutch up, pause, clutch in and shift. I say those last two steps at the same time because this is important! If I wait too long after pushing the clutch in to shift, it crunches. Lets think about why. Remember back to that 120 weight gear oil. Thats some thick shit, and provides a lot of drag on the input shaft when it is left to spin down. By taking too long after the clutch is in, the input shaft slows down too much and the speeds dont match any more, and it crunches. So I put the clutch in and push it into gear at the same time. Problem solved. With thinner fluid this may not be so critical.
Also note that the length of that pause will depend on the RPM I shift at, which is why I mentioned it above. The higher the RPM, the bigger the drop in RPM in the next gear. This may seem like stating the obvious, but its very important. Shifting at say 6000 rpm, that pause has to be a little longer, and this is where practice and experience comes in handy.
Downshifting was much harder at first, but I can now nail that too every time. Again, like with rev matching, the trick is in how much you rev it up. Rev too little, things dont match up. Rev too high, things do match up. But, if you rev too high, you can increase the pause ;)
Thats the basics. I'm surprised that I was able to learn how to do this in just a couple weeks, and I'm more surprised at how much I learned about tranny operation and how all these elements relate to each other. Things I always knew, but this validates and reinforces things I've known for years but had no actual experience with. Good times....
I can't feel my fingers anymore. I wouldnt even know they were still there if the carpal tunnel wasn't all flared up. This is going on the website...
Edit> The main reason for this is to get it into the archives so people can search it, hence the thread title. If anyone asks what double clutching is in the future I will be severely offended :icon16:
viet_boiiiii
03-06-2005, 02:24 AM
So how do i double clutch? is it hard? Can u teach me? LOL jk. Thanks for the info man i had a blast reading it cause i've learned alot in the 5 mins it took me to read this. This should either be stickied or just rememberd ^^ Thanks again GSXracer!
Nayr747
03-06-2005, 02:26 AM
Damn, that's one nice thread. I learned a lot. So the heavy weight oil is bad or good for double clutching/ bad gear? Since it slows the input shaft down so much wouldn't it be bad or do you just have to be quicker on the clutch in-shift?
JoeWagon
03-06-2005, 02:41 AM
Kevin, what about the time between neutral and getting into gear? I'm thinking of why disconnecting the input shaft (clutch) in a normal shift that takes .2 seconds will slow the input shaft any more than the double clutching's neutral to gear shift. Seems like there isn't much more time involved going from 2nd to 3rd than neutral to 3rd, if time is the main issue because of a slowing input shaft.
kjewer1
03-06-2005, 03:24 AM
When you shift at the same time you put in the clutch the shaft isnt going to slow down nearly as much as it does on a normal shift. It may seem fast, but the 2-3 especially takes some time. Looking in the datalogs, it typically takes me 50% more time to shift 2-3 than 1-2 or 3-4. For normal transmissions this doesnt matter either way, since the synchros work.
Also consider that if you do this properly there is no need to use the clutch the second time. ;) I still use it to prevent further wear. But with the 2g I probably spent a combined 3 months at least driving with no clutch at all over various busted hydraulic events. The process is the same just no clutch. With no clutch being used the input shaft will always be at engine speed, so the only thing lieft is the timing, either in the pause in nuetral when upshifting, or the rev match on the downshift. IF you get it wrong with no clutch you stand to lose a lot more. The synchros have to deal with the whole weight of the engines rotating assembly rather than just the input shaft ;) But I'm probably OT now...
Also consider that if you do this properly there is no need to use the clutch the second time. ;) I still use it to prevent further wear. But with the 2g I probably spent a combined 3 months at least driving with no clutch at all over various busted hydraulic events. The process is the same just no clutch. With no clutch being used the input shaft will always be at engine speed, so the only thing lieft is the timing, either in the pause in nuetral when upshifting, or the rev match on the downshift. IF you get it wrong with no clutch you stand to lose a lot more. The synchros have to deal with the whole weight of the engines rotating assembly rather than just the input shaft ;) But I'm probably OT now...
JoeWagon
03-06-2005, 03:56 AM
After riding around with my trucker friend and being amazed at him going through a bunch of gears without the clutch... I tried it too. Suffice to say I didn't actually get it because I was worried about the synchros health. If you can do this, I'm impressed.
kjewer1
03-06-2005, 04:06 PM
It took a couple days before I was good at it. Later on, I could upshift any gear without a grind. Downshift remained hit or miss though ;) The only part that really sucked though was having to use the starter to get the car moving, since there is no way to go from neutral to first with the motor running, with no clutch.
musicman181
03-06-2005, 09:32 PM
when i try i always get the grinding sound. i really want to learn this technique, but what am i damaging in the process. is it worth it?
kjewer1
03-06-2005, 10:36 PM
You are wearing your synchros when it grinds. I dont think its worth risking the tranny to learn how to do this, instead, wait until your clutch or tranny shits the bed like mine did and you have no choice but to learn. ;)
BoostedSpyder
03-08-2005, 02:23 AM
every once in a while i will let a friend drive my car. it is amazing to me how much i play with the gas while regular driving and shifting is going on. seeing someone else stall and miss shifts and not be able to downshift because they have no idea about rev matching [i don't do much double clutching, even after F&F ;)], it just amazes me. my car is a bitch to drive! i really have never realized it untill now! all the little things that i do that make that car seem as smooth as a baby's ass is a ton of work. and trying to explain how to drive it from one light to another is damn near impossible...
nice write-up! and the howstuffworks.com site should be requered surfing for all newbs...
nice write-up! and the howstuffworks.com site should be requered surfing for all newbs...
kjewer1
03-09-2005, 07:46 AM
Tell me about it. This was especially true when I was venting the BOV on the stock MAS with some of my fancy gagetry... Other poeple would have a bitch of time trying to drive it. And after going to the MAFt I was reminded how the other half live... I'm all set with that BS now, I'm too old :D
lostmypicks
03-20-2005, 12:21 AM
ya my dad was a trucker and he did it for 21 years he taght me how to do it. I still have a little truble but my dad can do it without even trying or thinking about it
I am just glad that I was taght earliy
I am just glad that I was taght earliy
KE.street.styler
03-08-2006, 09:52 PM
I don't mean to be a $H1T or anything, but in all honesty; who here really thought that double clutching in a race was a GOOD idea if they didn't have to? Yeah, some racing gear boxes are designed so you have to double clutch. But I mean average-bloke with a new whatever out messing around with his friends.
I had a 93 Del Sol with a burned up clutch and no syncro's for 2nd or 3rd, and I still raced it. So I know I've double clutched in a race before, and I've lost everytime I double clutched.
Lets get a poll going, seriously. Who has double clutched while racing?
I had a 93 Del Sol with a burned up clutch and no syncro's for 2nd or 3rd, and I still raced it. So I know I've double clutched in a race before, and I've lost everytime I double clutched.
Lets get a poll going, seriously. Who has double clutched while racing?
blk_srt
03-08-2006, 10:12 PM
How about you dont bring back a year old thread and shut up?
SLoe
03-08-2006, 10:27 PM
Let's do a poll on which n00b can bring up the oldest thread. pr beat me to ya :( I bagged like 3 n00b13s this week.
blk_srt
03-08-2006, 10:31 PM
WTF is with all the noobs lately? Its like they all come out of their ricer hibernation or something:disappoin
Blackcrow64
03-08-2006, 10:33 PM
WTF is with all the noobs lately? Its like they all come out of their ricer hibernation or something:disappoin
:rofl:
Ricer Hibernation... I like that... hehe...
:rofl:
Ricer Hibernation... I like that... hehe...
SLoe
03-08-2006, 10:35 PM
OK, let's not whore up Kevin's good post anymore. Let it die again. Over and out.
defiancy
03-08-2006, 10:37 PM
Really what the hell is up with this week and old threads being brought up. Holy crap. Lock this thread.
kjewer1
03-09-2006, 06:15 AM
Well, in reality, the reason I post these things is so they can be discussed. So I don't mind old threads like this being brought up again for further discussion since thats the whole point, though the motives in this case may be less than ideal :) I have plenty of patience for the noobs, I was one too once. ;) I'd rather see KE post, and if no one is interested no one replies, rather than 8 more posts about how old the thread is.
To answer the post, I don't think that double clutching was ever though to be beneficial when upshifting in a race, aside from fast and furious, which we will leave out of this thread for my sanity's sake :) But in road racing double clutching while downshifting with certain gearbox absolutely is beneficial. It can even be the difference between staying on the track or ending up in the woods... Since you don't want to upset the cars balance going into a turn. For just about everyone in this forum (street driving/drag racing modern cars) it really doesn't apply, but it can still be educational to discuss it. :)
To answer the post, I don't think that double clutching was ever though to be beneficial when upshifting in a race, aside from fast and furious, which we will leave out of this thread for my sanity's sake :) But in road racing double clutching while downshifting with certain gearbox absolutely is beneficial. It can even be the difference between staying on the track or ending up in the woods... Since you don't want to upset the cars balance going into a turn. For just about everyone in this forum (street driving/drag racing modern cars) it really doesn't apply, but it can still be educational to discuss it. :)
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