Question about Rev Matching

[luan87us]

Originally Posted by the suicidal eggroll

The clutch only gets worn when the flywheel and clutch disk are spinning at different speeds and the clutch is partially or fully engaged. If the clutch is fully engaged and not slipping, no wear is happening because everything is moving as one. If the clutch is fully disengaged, then no wear is happening because there's nothing touching the clutch disk to cause any wear, regardless of how fast/slow anything is splinning. However constantly riding the clutch pedal will eventually wear out the throwout bearing.

Oh ok I got that. So that's why it's recommened to go into neutral with the clutch out at a stop instead of staying in first & holding the clutch in. I think I just opened my knowledge about manual transmission another level.

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[jadawgis732]

Originally Posted by luan87us

Wouldn't that actually wear out the clutch more since you're actually spinning the clutch disc at high rpm? I thought the better way to do it was the double clutch/heel-toe because it will let you increase your engine rpm without spinning the clutch. Sorry I'm a noob at manual transmission so lets me know if I'm wrong there.

That's my original question. Check the HowStuffWorks page I posted in the first question. I thought there'd be more wear and tear on the clutch with rev matching, but apparently I was wrong. Letting the clutch out in neutral alleviates this issue.

Originally Posted by pauli133

The clutch doesn't care how fast it turns. The clutch cares about how fast it turns in relation to the flywheel as it clamps (ie, whether or not it is slipping). If you approximately match the engine speed to the clutch speed (which, when shifting, will be the input shaft speed of the transmission), the clutch will have a very easy time as it clamps the two together.

And this is what I understood to be true, there is more friction the greater the difference between clutch speed and flywheel speed. However, when you engage the clutch (press in), my understanding was that the clutch speed went to 0.

Originally Posted by the suicidal eggroll

Rev matching is used to reduce wear on the clutch and make downshifts smoother. Double clutching takes that one step further to reduce wear on the synchros as well. As you pointed out, when the synchros start to wear down double clutching will become more and more necessary when downshifting.

Do synchros match engine speed and clutch speed? I read through the thread again and can't find the post you are talking about, where someone mentions synchros wearing down. I'm not even sure what they are.

[DansSpace]

Originally Posted by luan87us

Wouldn't that actually wear out the clutch more since you're actually spinning the clutch disc at high rpm? I thought the better way to do it was the double clutch/heel-toe because it will let you increase your engine rpm without spinning the clutch. Sorry I'm a noob at manual transmission so lets me know if I'm wrong there.

The clutch wears out because of the friction between itself and a flywheel spinning at different speeds. By spinning the flywheel up to the same speed as the clutch (Blipping the throttle for a downshift) you are causing less wear because the speed difference has been reduced to something much lower than it was before. The act of the clutch disk and flywheel spinning do not wear them out. It's the frictional force between them when speed differences are present. By rev-matching, you are smashing them together at the same speeds, and there's no friction and no wear.

[jadawgis732]

Originally Posted by DansSpace

The clutch wears out because of the friction between itself and a flywheel spinning at different speeds. By spinning the flywheel up to the same speed as the clutch (Blipping the throttle for a downshift) you are causing less wear because the speed difference has been reduced to something much lower than it was before. The act of the clutch disk and flywheel spinning do not wear them out. It's the frictional force between them when speed differences are present. By rev-matching, you are smashing them together at the same speeds, and there's no friction and no wear.

Okay, very basic question that I must be misunderstanding. When the clutch is engaged and one switches to a lower gear without disengaging in neutral, does the clutch speed go to 0 before letting the clutch back out in that lower gear?

[CrashTestRacer]

You guys are missing the point with clutch wear. It is a non-issue when shifting. You wear out the clutch when it is under load and driving the wheels.

The thing you DO wear out if you don't speed match is the synchro's in the transmission. The transmission is engaging different gears to the drive-shaft. Depending on which gear you choose those gears need to be spinning at the speed for that gear in order to mesh together. That is the job of the synchro - they are like little tiny clutches in the transmission that when you press on the shifter they spin that gear up or down to speed. The point of speed matching is to get the internals of the transmission spinning at the speed you need it to be spinning BEFORE trying to engage that gear - and then the synchro has nothing to do but let you shift into gear. Hence all the dancing with the clutch and neutral position. In fact - back in the day there were no synchro's and you had to speed match everything. That's why those old cars would do a lot of grinding.

If you press the clutch in (what I would call DIS-engaging it) The clutch and that "side" of the transmission will coast to a stop with internal friction as it is not connected to anything with the clutch in and the shifter in neutral.

[luan87us]

Do synchros match engine speed and clutch speed? I read through the thread again and can't find the post you are talking about, where someone mentions synchros wearing down. I'm not even sure what they are.

The synchros are for your gears not your clutch or flywheels. The synchro basically match the speed of the collar to the speed of the spinning gear so it doesn't grind when you change gear. That's why Double clutch or rev match was required back in the day to change gear otherwise you'll keep hearing the grinding noise lol

[the suicidal eggroll]

Originally Posted by jadawgis732

Okay, very basic question that I must be misunderstanding. When the clutch is engaged and one switches to a lower gear without disengaging in neutral, does the clutch speed go to 0 before letting the clutch back out in that lower gear?

If the tranny is in gear, the clutch disk is spinning at the same speed as the transmission input shaft, which is the speed of the wheels multiplied by the gear ratio. This is the case whether or not the clutch is engaged or disengaged.

Bottom line: If the tranny is in gear, clutch speed ~= wheel speed.

[chapstien]

There are miles of threads about this.

[chapstien]

Originally Posted by the suicidal eggroll

Rev matching is used to reduce wear on the clutch and make downshifts smoother. Double clutching takes that one step further to reduce wear on the synchros as well. As you pointed out, when the synchros start to wear down double clutching will become more and more necessary when downshifting.

This.

Also, the point of double clutch rev matching is to get the transmission input shaft spinning at the appropriate speed demanded by the output shaft for the intended gear. Sure the synchros do the job, but if you are driving a car that is known for wearing out synchros necessary for making 2-1 gear shifts possible, why not learn to do it and not have to replace them. Even if you only did it 50% of the time, they would last longer.

[the suicidal eggroll]

jadawgis732,

In a very basic sense, the synchros are like small clutches that sync up the tranny input shaft (and therefore the clutch disk) with the drive gear. When you go to select a new gear, the synchros are what speed the clutch up or down so the new gear speed matches the drive gear speed (the wheel speed).

Say you're cruising along in 4th. Engine RPM = flywheel RPM = clutch disk RPM = input shaft RPM = 2000 RPM. With a gear ratio of 1:1, that means your wheels are also spinning at 2000 RPM. Now you want to shift to 3rd.

1) You clutch in
2) The engine/flywheel is now disconnected from the clutch disk and starts to slow down
3) The tranny is still in gear, so the clutch disk, tranny input shaft, and wheels are all still spinning at 2000 RPM (unless you hit the brakes and slow down the car, let's ignore that)
4) You shift to neutral
5) The clutch disk and tranny input shaft are now disconnected from the output shaft and wheels, so the clutch disk and input shaft start to slow down
6) You press the gear lever against the 3rd gear gate
7) Synchros start to mesh, which accelerate the clutch disk and tranny input shaft up to the wheel speed (2000 RPM) times the 3rd gear ratio, call it 1.5:1, which would mean the clutch disk and tranny input shaft are accelerated by the synchros to 3000 RPM
8) The mesh is complete, and the shifter pops into gear, the wheels and tranny output shaft are still spinning at 2000 RPM, the clutch disk and tranny input shaft are spinning at 3000 RPM (because of the 1.5:1 gear ratio), and we've spent so long talking about this that the engine and flywheel have dropped down to idle
9) You start to release the clutch, as the pressure plate and flywheel smash the clutch disk between them, you get a lot of friction (because the flywheel is spinning at 750 RPM and the clutch disk is spinning at 3000 RPM). This friction slows down the clutch disk while simultaneously speeding up the flywheel, until the two meet somewhere in the middle, call it 2700 RPM. Your engine, flywheel, clutch, and tranny input shaft are now spinning at 2700 RPM, while the tranny output shaft and wheels are spinning at 1800 RPM because of the 1.5:1 gear ratio. What you've done is use the car's forward momentum to speed up the engine by way of the clutch, which has the effect of slowing the car down, as well as wearing down the clutch, and pissing off all of your passengers because it felt like you just hit the brakes for no reason.


All that make sense?

Rev matching is where you blip the throttle during steps 6-8, so that when you get to step 9, the flywheel is already spinning at the same 3000 RPM as the clutch, so you can drop the clutch quickly without slowing down the car or causing excessive wear.

Double clutching is where you release the clutch between steps 5 and 6 while you're in neutral, so the engine, flywheel, clutch, and tranny input shaft are all linked. You then blip the throttle to get all of them spinning up to 3000 RPM, push the clutch back in, and then move on with step 6. Since the clutch disk and tranny input shaft are already at 3000 RPM, the synchros have basically no work to do, so wear on them is very little and the tranny pops into gear with little effort.

[bizarro252]

Originally Posted by the suicidal eggroll

This is only true if the tranny is in neutral. As Jack pointed out, if the tranny is in gear, then clutch RPM = tranny RPM ~= wheel RPM.


Rev matching basically comes down to this. When the clutch is disengaged and the tranny is in gear, your flywheel RPM = engine RPM and your clutch RPM = wheel RPM (times gear ratio). Rev matching is using the throttle to speed up the flywheel to match the clutch RPM, before releasing the clutch. If you do it right, there is zero wear on the clutch because the two are already spinning at the same speed when you engage the clutch.

Ya I almost edited after he pointed out that flaw - but I didn't, if my post is confusing I can rewrite

[jadawgis732]

That explains so much. I used to engine brake because someone told me and I was a moron. I got into that habit, and whenever I engine braked I never rev matched, so I'd hear this high pitched whine when I started to let the clutch back out in a lower gear without rev matching. That's the clutch speed coming up to parity with the engine speed. I also understand now, why I sometimes have trouble getting my shifter into a lower gear than would be comfortable for the engine (It feels like it's blocked), and have to wait a second. That second doesn't make me lose much speed, it's just the synchros doing their job.

The only part now is for me to break old habits. And it's SOOOOO hard. Whenever I take a phone call I'm not thinking about optimal clutch operation anymore, and I revert to my old style. Whenever I'm finding an album to play, f* the clutch because I'm a non-multitasking momo. I have a feeling it's going to take years to get used to driving a clutch correctly, just because I've spent like 6 years operating it incorrectly.

I've also found that "blipping" isn't enough, you have to be quick with the gear change following the blip. I blip, and cause the engine speed to rise 1500RPM but by the time I start letting the clutch out, that 1500RPM is almost entirely lost. There must be a lot of friction on the flywheel or something to make it lose momentum that quickly.

Also, not to sidetrack but an engine speed of 2000RPM in fourth (pretty close to 1:1 ratio) doesn't equal 151MPH. I found it by:
235/45/R17 tires have a 80" cicumference
80 inches/revolution * 2000 revolutions/minute * 60 minutes/hour = 9,600,000 inches/hour
(9,600,000 inches per hour) / (12 inches/foot) / (5280 feet/mile) = 151.51MPH.
Thought I was doing it wrong until I checked http://www.csgnetwork.com/tirerevforcecalc.html
Use 151.51MPH, 17" Tire, 4.16" tire body size (sidewall) and you'll have close to 2000RPM.
What am I missing?

[bizarro252]

Originally Posted by luan87us

Oh ok I got that. So that's why it's recommend to go into neutral with the clutch out at a stop instead of staying in first & holding the clutch in. I think I just opened my knowledge about manual transmission another level.

This is mostly an issue with the wear on the throw out bearing as far as I understand it, sitting at a light with the clutch pressed in causes excessive wear to this bearing and is basically not recommended to hold the clutch in if you are going to be sitting for any length of time.

[the suicidal eggroll]

Originally Posted by jadawgis732

Also, not to sidetrack but an engine speed of 2000RPM in fourth (pretty close to 1:1 ratio) doesn't equal 151MPH. I found it by:
235/45/R17 tires have a 80" cicumference
80 inches/revolution * 2000 revolutions/minute * 60 minutes/hour = 9,600,000 inches/hour
(9,600,000 inches per hour) / (12 inches/foot) / (5280 feet/mile) = 151.51MPH.
Thought I was doing it wrong until I checked http://www.csgnetwork.com/tirerevforcecalc.html
Use 151.51MPH, 17" Tire, 4.16" tire body size (sidewall) and you'll have close to 2000RPM.
What am I missing?

I left the final drive ratio out of the example to keep things simple. The final drive on the 5 speed is around 3.9:1, which drops your calculation from 151 mph to more like 40 mph. Also those were just example gear ratios, our actual gear ratios are different. On the 5 spd, 4th is 0.972:1 and 3rd is 1.296:1.

[jadawgis732]

Originally Posted by the suicidal eggroll

I left the final drive ratio out of the example to keep things simple. The final drive on the 5 speed is around 3.9:1, which drops your calculation from 151 mph to more like 40 mph. Also those were just example gear ratios, our actual gear ratios are different. On the 5 spd, 4th is 0.972:1 and 3rd is 1.296:1.

Ahhhh, thought so.

[jadawgis732]

Does anyone remember the first Fast and Furious when Vin Diesel says "not double clutching like you should be?" That was the first time I heard the term, and I always associated it with going faster. They were just drag racing, weren't they? How did car buffs not call them out on this?

Pretty much my point is that those movies were awful. There's a drinking game where you watch one of the movies and every time you hear a corny line you drink. You're totally incapable of operating a motor vehicle afterwards.

[bizarro252]

Originally Posted by jadawgis732

Does anyone remember the first Fast and Furious when Vin Diesel says "not double clutching like you should be?" That was the first time I heard the term, and I always associated it with going faster. They were just drag racing, weren't they? How did car buffs not call them out on this?

Pretty much my point is that those movies were awful. There's a drinking game where you watch one of the movies and every time you hear a corny line you drink. You're totally incapable of operating a motor vehicle afterwards.

Car buffs...and even car casuals should have called them on this, and they do haha.

I think the line was even worse, something along the lines of "Granny shifting, not double clutching like you should" and yes, in a drag race there would be no double clutching needed lol

[chapstien]

Originally Posted by bizarro252

This is mostly an issue with the wear on the throw out bearing as far as I understand it, sitting at a light with the clutch pressed in causes excessive wear to this bearing and is basically not recommended to hold the clutch in if you are going to be sitting for any length of time.

Yes45