high-speed braking
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Joined: Feb 2001
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From: You wish
Couple of comments.
First, if you have more traction than braking force, as discussed previously, and are travelling at high speeds in a high gear, engine braking can contribute to a shorter stop. When you letoff the gas at speed, the ECU turns off the fuel injectors, so now the engine becomes a inertia driven compressor - and that inertia comes from the forward motion of the car, translated to the rotation of the drive wheels. If you ever get a chance, see how much torque it takes to rotate the crankshaft using a torque wrench. As you near the top of a compression stroke, I guarantee you it is significant. Now imagine that torque beign applied back through the drivetrain at 7000 rpm.
Someone else mentioned that leaving the clutch engaged meant you had to slow down the engine along with the car and that decreased available braking force. This is only true if your braking rate, as translated to the engine through the gearbox, exceeds the normal spindown rate of the motor.
For example, the F20C doesn't take more than 3-4 seconds to go from 8000 rpm to idle if you rev it up and then let off the gas in neutral. Let's say that the normal spindown rate is therefore 2000 rpm/second. Now let's say we're travelling at 120 mph in 5th gear, or about 7500 rpm. We have a sudden need to stop as quickly as possible (perhaps a restroom break? :-). Apply the brakes full force and let's _assume_ that you cannot generate lockup/ABS. In 5th gear we do about 16 mph/1000 rpm. So if you can slow down, using the brakes alone, at more than 32 mph/second, then you should declutch. However, such a rate equates to over 1.4 g of deceleration, which you aren't going to pull without race tires, big brakes and some downforce.
At a lower speed, in say 3rd gear, we'd only need to slow at 20 mph/second or faster to exceed the engine's normal spindown rate which equates to just under 1g - this can be done on street tires with the stock brakes, so declutching here would be a good idea. Of course, as others have pointed out, you should really be heel-toe downshifting as you slow - but you probably wouldn't be panicking then either.
Also, if you don't declutch on an emergency braking run, when the engine reaches rpms near idle (i.e. as you're nearing a complete stop) the fuel injectors are turned back on again, providing forward motive force. You'll still stall the engine, but that little shove might push you that one foot further that you didn't have. I admit I've been guilty of not declutching in some panic stops, but you really should try provided you have your wits about you (which might mean it isn't a panic stop then, eh?).
UL
First, if you have more traction than braking force, as discussed previously, and are travelling at high speeds in a high gear, engine braking can contribute to a shorter stop. When you letoff the gas at speed, the ECU turns off the fuel injectors, so now the engine becomes a inertia driven compressor - and that inertia comes from the forward motion of the car, translated to the rotation of the drive wheels. If you ever get a chance, see how much torque it takes to rotate the crankshaft using a torque wrench. As you near the top of a compression stroke, I guarantee you it is significant. Now imagine that torque beign applied back through the drivetrain at 7000 rpm.
Someone else mentioned that leaving the clutch engaged meant you had to slow down the engine along with the car and that decreased available braking force. This is only true if your braking rate, as translated to the engine through the gearbox, exceeds the normal spindown rate of the motor.
For example, the F20C doesn't take more than 3-4 seconds to go from 8000 rpm to idle if you rev it up and then let off the gas in neutral. Let's say that the normal spindown rate is therefore 2000 rpm/second. Now let's say we're travelling at 120 mph in 5th gear, or about 7500 rpm. We have a sudden need to stop as quickly as possible (perhaps a restroom break? :-). Apply the brakes full force and let's _assume_ that you cannot generate lockup/ABS. In 5th gear we do about 16 mph/1000 rpm. So if you can slow down, using the brakes alone, at more than 32 mph/second, then you should declutch. However, such a rate equates to over 1.4 g of deceleration, which you aren't going to pull without race tires, big brakes and some downforce.
At a lower speed, in say 3rd gear, we'd only need to slow at 20 mph/second or faster to exceed the engine's normal spindown rate which equates to just under 1g - this can be done on street tires with the stock brakes, so declutching here would be a good idea. Of course, as others have pointed out, you should really be heel-toe downshifting as you slow - but you probably wouldn't be panicking then either.
Also, if you don't declutch on an emergency braking run, when the engine reaches rpms near idle (i.e. as you're nearing a complete stop) the fuel injectors are turned back on again, providing forward motive force. You'll still stall the engine, but that little shove might push you that one foot further that you didn't have. I admit I've been guilty of not declutching in some panic stops, but you really should try provided you have your wits about you (which might mean it isn't a panic stop then, eh?).
UL
Joined: Feb 2001
Posts: 1,045
Likes: 1
From: Chicago
Okay, points about being ready on the throttle and all are all valid and acceptable. In my post, I was really trying to make sure that everyone understood that the limiting factor is tire/road contact. Since the S2k (as well as almost any modern car and certainly in the case of any car with 4 wheel ABS) easily has the braking power to lock up the tires at speed, the point of engine braking is not about faster stopping but feel, vehicle dynamics, and emergency manuvers (which if you note I mentioned way up at the top, the very first reply to this thread).
Clark
Clark
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Joined: Dec 2000
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From: Lynchburg
I'm not sure I agree with the statement that you can lock ABS brakes. In my experiences they did as they were designed to do, not lock up. Last week while coming home with my wife from picking up a pizza, we were doing about 65mph when a deer (hugh buck) ran out in front of us, I put in the clutch and jambed the brakes on with everything I had, and the brakes did not lock. The antilocks worked perfectley and quickly, we stopped so quick that the pizza box that my wife had in her lap opened up and landed on the dash. What would be the point of an ABS system if it didn't work after a certain speed, I would think that at very high speeds you would want it even more.
P.S. we missed the deer, thank God...
P.S. we missed the deer, thank God...
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Joined: Nov 2000
Posts: 1,349
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From: Ocean City
[QUOTE]Originally posted by Clark
[B]
That being said, and particularly with anti-lock brakes, the absolute fastest reliable way to make an emergency stop is to put in the clutch and stomp on the brakes
[B]
That being said, and particularly with anti-lock brakes, the absolute fastest reliable way to make an emergency stop is to put in the clutch and stomp on the brakes
Joined: Feb 2001
Posts: 1,045
Likes: 1
From: Chicago
Nick,
I'm not saying that you can lock up ABS, I'm saying that your brakes are capable of locking up the tires (hence the need for ABS to come in and keep you from doing so). More to the point of my statement is that since you have 4 wheel ABS, your fastest braking should be achieved by braking so hard that all 4 wheels are trying to lock up, with ABS preventing that. In this situation, each individual tire is braking at it's maxiumum (since one tire may have less grip than another due to road condition, weight distribution, etc.).
cmnsense,
Actually, most modern ABS systems use "impending lock" rather than actual wheel lock-up in their operation. In a modern ABS system, there is a wheel speed sensor that is processing wheel speed in increments of several times per second. This data is fed to a computer that tries to predict when your wheel will lock up, and then ease braking pressure so that this doesn't actually happen.
The next statement I make is very dependent upon the ABS system in question, but here goes. You can actually increase your stopping distance in an ABS equipped car by trying to modulate impending lock yourself (note, this is all a very theoretical statement, doesn't really happen in the real world). Here's the example: you as a driver feel impending lock, and with superb timing you ease back on your brake pedal pressure accordingly. A milisecond later the computer processes and senses the same impending lock and eases back your brake pressure for you, hence you are now braking even less because your superbly calibrated rear end just fooled the ABS.
In older versions of ABS, this really did happen on occasion when somebody driving on snow or ice tried to pump their brakes as if they were in a non-ABS car. Sometimes it would happen that the pumping would be such that the ABS would keep backing off brake pressure just as the driver kept trying to apply it.
sorry this post was so long, I'll try to keep it shorter next time.
Clark
I'm not saying that you can lock up ABS, I'm saying that your brakes are capable of locking up the tires (hence the need for ABS to come in and keep you from doing so). More to the point of my statement is that since you have 4 wheel ABS, your fastest braking should be achieved by braking so hard that all 4 wheels are trying to lock up, with ABS preventing that. In this situation, each individual tire is braking at it's maxiumum (since one tire may have less grip than another due to road condition, weight distribution, etc.).
cmnsense,
Actually, most modern ABS systems use "impending lock" rather than actual wheel lock-up in their operation. In a modern ABS system, there is a wheel speed sensor that is processing wheel speed in increments of several times per second. This data is fed to a computer that tries to predict when your wheel will lock up, and then ease braking pressure so that this doesn't actually happen.
The next statement I make is very dependent upon the ABS system in question, but here goes. You can actually increase your stopping distance in an ABS equipped car by trying to modulate impending lock yourself (note, this is all a very theoretical statement, doesn't really happen in the real world). Here's the example: you as a driver feel impending lock, and with superb timing you ease back on your brake pedal pressure accordingly. A milisecond later the computer processes and senses the same impending lock and eases back your brake pressure for you, hence you are now braking even less because your superbly calibrated rear end just fooled the ABS.
In older versions of ABS, this really did happen on occasion when somebody driving on snow or ice tried to pump their brakes as if they were in a non-ABS car. Sometimes it would happen that the pumping would be such that the ABS would keep backing off brake pressure just as the driver kept trying to apply it.
sorry this post was so long, I'll try to keep it shorter next time.
Clark
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Joined: Nov 2000
Posts: 1,349
Likes: 5
From: Ocean City
Maybe I should have said relative wheel/tire slow down instead of lock.
Jammin' the brakes on the stook in snow leaves mini skids in the tracks, they must be locking, and overcomming static friction. Its just that when you break that it's harder to regain.
Jammin' the brakes on the stook in snow leaves mini skids in the tracks, they must be locking, and overcomming static friction. Its just that when you break that it's harder to regain.
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