Question about rev matching in real life

[oohhh yeah]

That'll make it slip more and undo the good work you did in the middle of the change.

If you're downshifting to overtake right now, absolutely keep your foot in ("flat shifting"), but that's relatively poor roadcraft as all overtakes should be planned. That's for a different thread.

So flat shifting is faster, but it is worse for the clutch (slippage) is this correct?
Why would that slip more though? If you keep your gas in slightly then that means you can let out the clutch faster to compensate?

 

[Famine]

So flat shifting is faster, but it is worse for the clutch (slippage) is this correct?

Yes.

Why would that slip more though?

The purpose of the clutch is to bring the two shafts (gearbox input; engine crank) together at the same speed. If they're at different speeds (which they will be - you cannot perfectly match the angular velocity of both) it will slip. If the difference is small it will slip a little bit. If the difference is large it will slip a lot (if the difference is huge, it'll stall the engine as the road provides a lot more resistance than the flywheel/crankshaft). Revmatching is the act of changing the crank speed while you have direct control over it to match the input shaft speed controlled by the roadspeed so that the slip is minimised. Altering the crank speed beyond that causes more slip.

If you keep your gas in slightly then that means you can let out the clutch faster to compensate?

"Faster" is not a concern with rev-matching at any point. It's about smoothness and mechanical sympathy, not performance driving. If you're rev-matching, any notions of doing things faster should be put out of your mind - it's simply bringing the crankshaft to about the same speed as the input shaft while you have direct control over it so that when they're brought together again there is as little jerkiness as possible.

Keeping your foot in throughout makes it more jerky through over-accelerating the crank and making it faster than the input shaft so that when they're brought back together the clutch has to slip more. If you're interested in doing that to get more performance, you have no need to bother with rev-matching and you're wasting your time and effort doing so.

 

[wfooshee]

Extremely way overanalyzed!

If the car lurches when you let the clutch out, whether it lurches forwad or backward, then the throttle was wrong.

That's all. Do it differently next time, and you'll soon enough find the sweet spot.

Well i don't take my foot of the brake so i don't relly rev before i downshift, i just bring the clutch out slowly so i build RPM.

This will eat the clutch just as much as holding it slightly in while you rev the engine and slip the car to move. The heel-and-toe discussion you've been seeing is the ability to work the throttle while the brake is being used. It's called heel-an-toe but it's more often one side of your foot on the brake and the other side on the gas. Brake with the ball of your foot, and you should be able to rotate your ankle and blip the gas with the right sid of your foot. Bringing the clutch pedal up while the engine is coasting down from that blip is WAY better for the clutch then dragging the engine up from idle. Think: You're downshifting, which means the engine needs more RPMs than the previous gear, not less.

 

[oohhh yeah]

Yes.



The purpose of the clutch is to bring the two shafts (gearbox input; engine crank) together at the same speed. If they're at different speeds (which they will be - you cannot perfectly match the angular velocity of both) it will slip. If the difference is small it will slip a little bit. If the difference is large it will slip a lot (if the difference is huge, it'll stall the engine as the road provides a lot more resistance than the flywheel/crankshaft). Revmatching is the act of changing the crank speed while you have direct control over it to match the input shaft speed controlled by the roadspeed so that the slip is minimised. Altering the crank speed beyond that causes more slip.



"Faster" is not a concern with rev-matching at any point. It's about smoothness and mechanical sympathy, not performance driving. If you're rev-matching, any notions of doing things faster should be put out of your mind - it's simply bringing the crankshaft to about the same speed as the input shaft while you have direct control over it so that when they're brought together again there is as little jerkiness as possible.

Keeping your foot in throughout makes it more jerky through over-accelerating the crank and making it faster than the input shaft so that when they're brought back together the clutch has to slip more. If you're interested in doing that to get more performance, you have no need to bother with rev-matching and you're wasting your time and effort doing so.

What if you don't alter the crank speed beyond that while keeping right foot in? Will that cause more slip?

Also, I thought rev matching is the right way to do it for speed? So if I want to accelerate without abusing the car how should i downshift?

 

[Famine]

What if you don't alter the crank speed beyond that while keeping right foot in?

You will. As I discussed earlier, it's due to the nature of what the throttle actually does.

Go out to a car. Press the throttle gently while it's parked and in neutral. Notice how you can reach redline really quickly with only a tiny, tiny amount of throttle travel? That's because the only load you're accelerating is the crank and flywheel - no resistance means almost no power is needed to reach 100% of maximum revolutions. Now go out for a drive and try it in any gear. Notice how it takes ages to reach redline even with 100% throttle travel - and a longer time in each gear? That's because you're accelerating the entire car - lots of resistance means lots of power is needed to reach 100% of maximum revolutions.

So if you press your foot flat to the deck in, say, 4th at 40mph, not a lot happens. Clutch down, *engine screams blue murder*, pick 3rd, clutch up, *CLUNK*, car goes faster.

With rev-matching you'll never want to poke the throttle beyond 25% travel in the middle phase. 25% throttle travel will do bugger all in any gear except first and you should never change into first gear while you're moving.

Also, I thought rev matching is the right way to do it for speed?

That would require you to ignore every post I've made in this thread where I've said it is nothing to do with performance driving.

So if I want to accelerate without abusing the car how should i downshift?

Normally.

If you need to change down a gear to accelerate it right now you're doing it wrong - it's incredibly poor roadcraft. Change down to the appropriate gear and keep it at the same speed. Then accelerate. Two separate actions.

 

[homeforsummer]

Well i don't take my foot of the brake so i don't relly rev before i downshift, i just bring the clutch out slowly so i build RPM.

There's nothing wrong with that, it's what I do most of the time too, since most of the time you aren't going quickly enough to damage the clutch or upset the balance of the car. Heel/toe is more useful in harder driving because when you're changing down you tend to want to do it quickly, and letting out the clutch quickly from high revs will put a jerk through the drivetrain and upset the balance of the car, where rev matching minimises this.

At normal driving speeds, letting the clutch out slowly works well most of the time.

 

[oohhh yeah]

You will. As I discussed earlier, it's due to the nature of what the throttle actually does.

Go out to a car. Press the throttle gently while it's parked and in neutral. Notice how you can reach redline really quickly with only a tiny, tiny amount of throttle travel? That's because the only load you're accelerating is the crank and flywheel - no resistance means almost no power is needed to reach 100% of maximum revolutions. Now go out for a drive and try it in any gear. Notice how it takes ages to reach redline even with 100% throttle travel - and a longer time in each gear? That's because you're accelerating the entire car - lots of resistance means lots of power is needed to reach 100% of maximum revolutions.

So if you press your foot flat to the deck in, say, 4th at 40mph, not a lot happens. Clutch down, *engine screams blue murder*, pick 3rd, clutch up, *CLUNK*, car goes faster.

With rev-matching you'll never want to poke the throttle beyond 25% travel in the middle phase. 25% throttle travel will do bugger all in any gear except first and you should never change into first gear while you're moving.







Normally.

If you need to change down a gear to accelerate it right now you're doing it wrong - it's incredibly poor roadcraft. Change down to the appropriate gear and keep it at the same speed. Then accelerate. Two separate actions.

Makes sense. But what if I'm careful and I only poke the throttle a tiny bit, so the crank speed doesn't spin too fast? Then it should be fine to keep the foot in right? If this is the case, then if I'm good enough with the throttle I can keep it in while disengaging the clutch and I can keep it in while engaging the clutch in the next gear down right? If I do it right it shouldn't be bad for the clutch?

And what would "normally" be like?

 

[Famine]

Makes sense. But what if I'm careful and I only poke the throttle a tiny bit, so the crank speed doesn't spin too fast? Then it should be fine to keep the foot in right?

When unloaded (clutch pedal down), the engine needs almost no throttle to spin faster. When loaded (clutch pedal up), it needs loads, more in higher gears and less in lower gears for the same speed. There isn't a single throttle position you can keep that suits your original gear, your unloaded engine and your new gear.

And what would "normally" be like?

If you film your feet as you drive along, you'll notice that you always feed out the throttle as you feed in the clutch and then feed in the throttle as you feed out the clutch - and that after a gearchange you're always providing exactly the right amount of throttle to maintain the engine at the new crankshaft speed for the same roadspeed.

Or: throttle up while clutch down; gear; clutch up while throttle down.

 

[homeforsummer]

While I appreciate the OP's desire to further his knowledge and Famine's patience in essentially saying the same thing in a myriad different ways in response, is there any particular reason the OP seems absolutely intent on persevering with a driving habit that is neither good from a skill point of view nor a mechanical sympathy perspective?

There's clearly a right and a wrong way of doing things, so why the obsession with the wrong way?...

 

[oohhh yeah]

While I appreciate the OP's desire to further his knowledge and Famine's patience in essentially saying the same thing in a myriad different ways in response, is there any particular reason the OP seems absolutely intent on persevering with a driving habit that is neither good from a skill point of view nor a mechanical sympathy perspective?

There's clearly a right and a wrong way of doing things, so why the obsession with the wrong way?...

What? Rev matching is good mechanical symphony.

What would be the "right" way then?

 

[oohhh yeah]

When unloaded (clutch pedal down), the engine needs almost no throttle to spin faster. When loaded (clutch pedal up), it needs loads, more in higher gears and less in lower gears for the same speed. There isn't a single throttle position you can keep that suits your original gear, your unloaded engine and your new gear.





Or: throttle up while clutch down; gear; clutch up while throttle down.

For examples sake let's just say I'm god and I can get the throttle right. Would that be worse for the clutch?

And isn't downshifting "normally" bad for the clutch? Because I thought the point of rev matching is for mechanical symphony. Why would you not want mechanical symphony every time?

 

[jtv90069]

probably the best way of understanding how it is done best is from watching the footwork of the best drivers.