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03-06-2013, 12:57 AM | #1 |
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Question about Rev Matching
Playing around on HowStuffWorks I saw this page, which provides a very good overview of clutch operation. The first little flash app on that page shows the strain put on the clutch by a flywheel. The faster spinning wheel, the flywheel, is connected to the engine. So when I rev match while downshifting, isn't the flywheel going to speed up and therefore cause more friction on the clutch? Isn't that bad? Could someone explain why I'm wrong.
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Last edited by jadawgis732; 03-06-2013 at 01:13 AM. Reason: I think NASIOC hyperlink colors should contrast better with standard text. |
03-06-2013, 01:07 AM | #2 |
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I can't answer this but want to know the answer too lol.
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03-06-2013, 01:18 AM | #3 |
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When you rev match by tapping the throttle. the flywheel will speed up to match the gear you are in. When you rev match properly, there is very little clutch wear because you're not slipping the clutch. It's just like when you upshift and let the RPMs drop to the right level before letting out the clutch, but in the other direction. Downshifting without rev matching requires you to slip the clutch because it forces the engine to match RPMs.
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03-06-2013, 01:33 AM | #4 |
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Need clarification. Engine speed != flywheel speed? And does the clutch match the road speed or what? I thought if I disengage the clutch the clutch is not spinning at all.
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03-06-2013, 01:37 AM | #5 |
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From what understand you need to bring the revs up according to gear ratio.
Ie... If you're at 3400 in second you need to bring it to 4800 to get into first. I am still trying to wrap my heard around it. And my feet. My brain shirts out when attempting heel toe. My muscle memory needs to learn it. |
03-06-2013, 02:44 AM | #6 | |
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Quote:
Your clutch disk is attached to your transmission input shaft, so when the clutch is engaged, your clutch RPM = flywheel/engine RPM. When your clutch is disengaged your clutch RPM is 0 (or at least spinning down towards zero due to friction/resistance of your tranny spinning with no input power. To 'rev match' you need to bring your engine to whatever RPM your next gear will be at for whatever speed you are going. When you upshift this happens naturally - if you time it right, as your engine will be spinning less RPM in the next gear up, and engine RPMs fall as you shift. To downshift you are in reverse of this so you have to match your transmission RPM to your newly needed engine RPM for the next gear down. So - for example say you want a 3-2 shift. You are going 25 MPH in 3rd at 2500RPM (for the sake of simplicity I am going to use round numbers) you need to now downshift by: Clutch in, out of gear, into neutral Clutch out, now linking the tranny any engine together, but out of gear Rev match (in this case rev to about 3500RPM) Clutch in when desired RPM is attained Select 2nd Clutch out This will allow you to get into second easier as the tranny/syncros are already spinning what they need to be for this selection - if you do this without the double clutch action you can actually HEAR the syncros working to spin up (at least I can in my noisy tranny in the STi lol) Hope that helps |
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03-06-2013, 06:58 AM | #7 |
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^Great description. He's got to be able to understand that.
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03-06-2013, 07:27 AM | #8 |
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bizzaro is right.
The flywheel is always at engine speed because it's bolted to the crank. When the clutch is pushed in, it is still connected to the transmission input shaft, so whatever speed the wheels are turning, (through gearing), that's what the clutch is spinning at. To get the clutch to go to zero, you'd have to have the clutch in and the tranny in neutral. When you rev match, you disengage the clutch and downshift, throttle to get the engine speed (flywheel) up to match the clutch speed at its new, lower gear speed (faster), then let the clutch out. The speeds are much closer together this way and you get less lurching when you let out the clutch. If you double clutch, you have an intermediate step where you are in neutral, leaving the clutch unpowered. After shifting to neutral, you let the clutch out, give it throttle to bring the clutch to engine speed as well as the input of the transmission and it's gear, then clutch in, shift and clutch out. This gives you gear speed matching which reduces synchro wear and makes the shift less jarring. |
03-06-2013, 05:58 PM | #9 |
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Oh, okay, so that's why people double clutch. Thanks!
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03-07-2013, 03:14 AM | #10 |
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Is this what people are referring to when they talk about synchros or syncromeshes- the ability to match clutch speed and engine speed simply by shifting into neutral?
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03-07-2013, 11:39 AM | #11 |
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Could someone possibly post a video of this with the STi transmission? I'm curious as to what engine speeds I might be looking at when it comes time for me to try this. Haha
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03-07-2013, 11:44 AM | #12 | |
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Quote:
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. |
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03-07-2013, 11:46 AM | #13 | |
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What RPM does the engine need to be for 2nd? Well shift to 2nd and see. It depends on how fast you're going. It's more about feel and timing than anything else, which you can only get with practice. Heel & toe just takes this one step further. In addition to all of the above, you are also on the brakes slowing down for the corner. So the process goes, use the left half of your foot to hit the brakes to slow down for the corner, push in the clutch, use the right half of your foot to blip the throttle while shifting to the lower gear, then release the clutch while still on the brakes. When you get to the corner and let off the brakes, you're already in the next gear and ready to accelerate out. Don't try this until you get your rev matching down. |
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03-07-2013, 12:14 PM | #14 | |
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03-07-2013, 12:18 PM | #15 |
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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.
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03-07-2013, 12:34 PM | #16 | |
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Quote:
Then when the clutch surface is reconnected to the flywheel surface (foot lifting off the clutch), the two surfaces are spinning at nearly the same speed so there will actually be less "wear and tear" on both the fly wheel and the clutch surfaces. |
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03-07-2013, 12:37 PM | #17 |
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Huh, when I heel toe/rev match (or this is at least what I thought I was doing) I push in the clutch, shift to the lower gear, then brake with my toe as I rev up with my heel so that there is no "jump" or braking effect when I let the clutch back out. I never knew about letting the clutch out in neutral first, before shifting to the lower gear. Or is this only when shifting into second and first???
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03-07-2013, 12:48 PM | #18 |
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So if I do not double clutch while down shifting, that is bad?
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03-07-2013, 01:00 PM | #19 | |
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I typically only double-clutch when going down 2 or more gears. If I'm just going 4th to 3rd or 3rd to 2nd then I don't bother, but if I'm going 5th to 3rd or 4th to 2nd then I do. |
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03-07-2013, 01:00 PM | #20 |
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Don't get confused with heel/toe and rev matching. Heel/toe is nothing more than using the brake while you are using the accelerator (for rev matching into a corner, for example). Forget about heel/toe for now.
It does not wear out the clutch at all because everything will be spinning the same speed when the clutch is engaged. The point of this is not to necessarily save your clutch, either - you can burn that guy up during launches if you want, but up/down shifting is a non-concern. The point is to drive smoothly and to save your transmission from having to do the speed matching using internal synchro's. I think eggroll is missing a step in his description. Part of the process is to let the clutch out to engage the engine and transmission (in neutral) so that when you blip the throttle to your new speed you are bringing the transmission up to speed, too, and not just the engine. This is really the essence of speed matching double clutching. You will find that when you match the speed correctly that the gear will slide right in with no resistance at all. You should be in the habit of applying virtually NO pressure to downshift (or upshift for that matter). The resistance is the synchro working. Basic rev-matching: Step 1 - Push in clutch (dis-engage engine) Step 2 - Shift to Neutral Step 3 - Let clutch back out (engage engine and transmission) Step 4 - Blip throttle (engine/clutch/transmission up to speed) Step 5 - Push in clutch Step 6 - Shift to new (lower) gear (LIGHT pressure!) Step 7 - Let out clutch and accelerate When you blip the throttle, go for something higher than the speed you need to give you time to shift into gear after things slow a little, don't be scarred to rev a bit high. Just put gentle pressure into the next gear and wait for it to tell YOU when the speed is matched - and it will drop right in. Advanced rev-matching: ( Do not try unless you are confident!) Step 1a - Push in clutch without letting up on throttle (or full throttle) Step 1b - Light pressure on shifter to neutral - will pop out when ready Step 2a - Let clutch back out (engage engine and transmission) Step 2b - Wait a moment for engine to get up to speed Step 3 - Push in clutch, off throttle (or if you're insane - don't) Step 4 - Shift to new (lower) gear (LIGHT pressure!) Step 5 - Let out clutch and accelerate There is also a stupid-advanced version where you don't use the clutch in step 3-5 at all which does a good job of showing you that if everything is truly matched, then you can go into gear without disengaging anything if you're timing and speed matching is impeccable. Last edited by CrashTestRacer; 03-07-2013 at 01:06 PM. |
03-07-2013, 01:02 PM | #21 | |
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03-07-2013, 01:03 PM | #22 |
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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.
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03-07-2013, 01:08 PM | #23 |
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I've found that in my 2010 wrx it's pretty difficult to double clutch. This is due to how far the clutch travels in my car. To rev match easier in my car I:
Engage the clutch As I start lowering my gear from high to low, I blip the throttle When the gear lever is in the correct gear I disengage clutch and press on the gas normally If done correctly your car should not jerk, and it should not change speeds. I only heel-toe when I'm braking during a turn and need to drop in gear before I exit. |
03-07-2013, 01:30 PM | #24 | |
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And yea it's very hard to do rev matching in my 2013 WRX because of how far the clutch travel. You will have to be very quick to get it right. I tried double clutch a few time going from 5th to 3rd and I missed half the time lol. |
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03-07-2013, 01:39 PM | #25 |
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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.
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