Braking

gprsh924 · 01-23-2006, 07:17 PM

Ok I've heard conflicting stories and I am looking for clarification. When braking (for a panic stop) in a stick shift car, will your braking distances improve if you downshift during braking versus just mashing the clutch and brakes and having at it? i do a mixture of both techniques and was wondering if anyone had any real knowledge on what happens?

singpilot · 07:33 PM

Anti-skid or not? Best stopping distance will be constant just-prior-to-point-of-lockup braking.

With ABS, the downshifting may confuse the driven wheels sensors, but I would think the max effort braking without the downshift would be the best.

gprsh924 · 01-23-2006, 07:37 PM

well im not really sure. i also read (i belive in motor trend) that on non-ABS cars, you dont shorten braking distances by jabbing the brakes, just maintain some steerability.

ernestedward · 01-23-2006, 07:59 PM

Quote:

Originally posted by gprsh924
well im not really sure. i also read (i belive in motor trend) that on non-ABS cars, you dont shorten braking distances by jabbing the brakes, just maintain some steerability.

That's true. ABS doesn't shorten the braking distance either - just maintains steerability. I believe what singpilot was referring to is what's also known as "threshold braking." It has to do with utilizing the different kinds of friction forces the tire is experiencing. Basically, it states that the most effective braking method is beginning with a quick, very hard jab that throws the weight of the car forward (and onto the front wheels) and then backing off slightly - to just before the point where the tires lock up. This method gives you the most favorable balance of weight/traction. I may have messed some of the details, but that's the idea.

gprsh924 · 01-23-2006, 08:01 PM

ok i think ive got that part of it down now, and i just need to learn what makes quicker breaking, downshifting and braking or just braking

VaSteve · 01-23-2006, 08:09 PM

Leave the car in gear. You will gain some braking effect from the compression of the engine. Only press in the clutch if you're spinning. This way the engine won't stall and if you catch the edge of the road or track, you won't zip off in another direction.

The real question is why are you panic stopping so much?

stuartj · 01-23-2006, 08:11 PM

As an instructor explained to me once...Brakes are doing what? (err, stopping you?) and engine is doing what (driving you forward...duh)

Threshold brake, two feet in.

One time, at Porsche camp, we did exercises with 996s where we were supposed to use the ABS on a wet skid pan. Some of us discovered it was possible to stop the car quicker threshold braking at the point of ABS activation. But in a panic, in an ABS car, just stand on the brakes as hard you can and let the system do the work while you steer seomehere else. Different skill in a non ABS car- but the gears wont help you.

singpilot · 01-23-2006, 08:17 PM

I still think the just braking is the shortest distance to stop (what you asked). Performance braking (being ready to accel again prior to stopping) would be to keep the engine revs up (by say... downshifting as well as braking). You could reverse the decel to accel most quickly that way.

Panic stop? Ernest is right. Get the tires to almost locked (keeping steering is a happy by-product here) and maintain that C of F until stopped. ABS systems do that for you with pulsed to lockup, immediate release, pulsed to lockup again, faster that you ever could. Modern ABS systems do it by individual wheels. Some systems do the wheels in pairs. If you were downshifting, and the drag of the downshifted engine, trans and driveline were added to the brake, the ABS (sensing impending lockup at the wheel) would release that brake (and maybe some others *paired* as well) until the traction spun the locking wheel (and it's speed sensor) up again. If the downshift wasn't going to allow that, you might be in for a surprise.

I am using my experience in being in full anti-skid once a week (typically) as a guide here. Stopping 50 tons of airplane on a wet or snowy runway gets you lots of ABS experience. Throw in slowing thru the hydroplaning speed, and a full stop has some interesting moments.

stuartj · 01-23-2006, 08:27 PM

Singpilot- the down shift issue is a real problem (got into a real argument with here about this once) because in a panic (or usually racetrack braking) its possible, if you stuff up a dwonshift, to briefly compression lock (chirp) the rears. This, coupled with the fact that the weight of the car is on the front wheels and off the rears, and you may be starting to turn in- is often enough to invoke the famous 911 pendulum lift off oversteer, and send you spearing off the road backwards....

Had an interesting chat recently with a pilot friend about ABS on his 747s..the difference being ofcourse, the engines arent driving the wheels.

singpilot · 01-23-2006, 08:32 PM

Stuart;

Exactly. I don't have to worry about spinning the tires UP. Or dealing with the torque (either direction) of an engine driven wheel. But the principle is the same. I say, do not downshift in a panic stop. You'll be busy steering and (non-ABS) modulating the brakes for max effort. Was what the original question was.

I didn't even want to go near the swapping ends of a 911 botched-downshift-induced excursion off-road.

stuartj · 01-23-2006, 08:42 PM

Err, no. You may have seen the results of same at BKK with a Red Rat 747-400 a few years back?

singpilot · 01-23-2006, 08:55 PM

Yes I did. Was there the night before. My reference to decellerating THRU the hydroplane speed? Touchdown, right into anti-skid in standing water. Above hydroplane speed, no prob. Decel into hydroplane speed, wheels stop turning, ABS senses skid, releases brakes, waiting for wheels to spool up again. Of course, they are floating on water. Aint gonna happen until you slow below the hydroplane speed.

Once hydroplaning, and until that slowing below point, you pray that there is no crosswind that you have not compensated for (with aerodynamics), you'll quickly increase the reverse thrust (if you already are not maxxed out), and try to not slip off of the crown of the runway. In the old days, you'd pray for a deep puddle. Modern runways no longer have those.

The big rat hydroplaned right off of the end of that water covered runway in Bangkok. They reduced reverse thrust on the initial landing roll because they got sideways (crosswind, asymetric reverse, or slipped off of the crown, or a combo of all three), and wanted to get straightened out before resuming reverse thrust.

Oops. First accident in the history of the airline. Sad deal.

There is an old saying... Given only two choices, I'd rather take the grass at the side of the runway at 90 knots than the fence at the end of the runway at 40 knots.

We've got a lot of guys out there that have forgotten that saying.

stuartj · 01-23-2006, 09:13 PM

I was on it. Spilled my drink, ice cubes everywhere.

Just kidding. But i did arrive there the next day- it was very sorry sight indeed. There was lot more to that incident, but you're in the trade so i guess you read all the literature. Interesting tho, that plane was apparently a write off. But it was repaired-Qantas have still never lost a jet hull.

Stuart

M.D. Holloway · 01-23-2006, 09:16 PM

I think I'd rather take the train...

ianc · 01-23-2006, 09:22 PM

Downshifting won't do a thing for you, and it's probably the last thing you want to be splitting your attention trying to carry off properly in a panic stop situation.

What will dictate your stopping distance is the amount of friction between your tires and the road. The brakes will easily be able to exceed this if you just stomp on them, so that is the goal in threshold braking.

I also don't agree that ABS will not shorten your stopping distance. In a perfect world, that would be so, since you would be perfectly modulating the brake pedal just as the ABS would. In our world, the ABS can sense imminent lockup much more accurately than you can, and will be able to pulse the brakes to attain this. I'm not saying that a skilled driver can't attain comparable stopping distances without ABS, but for the rest of us in a panic stop situation, ABS is a damned good bet.

ianc

stuartj · 01-23-2006, 09:45 PM

Quote:

Originally posted by ianc

I also don't agree that ABS will not shorten your stopping distance. In a perfect world, that would be so, since you would be perfectly modulating the brake pedal just as the ABS would. In our world, the ABS can sense imminent lockup much more accurately than you can, and will be able to pulse the brakes to attain this. I'm not saying that a skilled driver can't attain comparable stopping distances without ABS, but for the rest of us in a panic stop situation, ABS is a damned good bet.

ianc

I concur- in a panic stop stand on the ABS as hard you can and steer.

I was referring earleir to a perfect world situation (on skid pan). Personally I think ABS is THE most significnat real world safety improvement in modern cars.

VaSteve · 01-24-2006, 05:41 AM

Quote:

Originally posted by stuartj
As an instructor explained to me once...Brakes are doing what? (err, stopping you?) and engine is doing what (driving you forward...duh)

Engine is only driving you with the gas on.

Try this w/o panic braking:

Come to a point and let off the gas and see how far the car rolls. Do not brake. It will evetually stall but don't worry about that.

Do it again but this time shift into nuetral at the same point.

Which rolls further?

Some instructor you must have had.

stuartj · 01-24-2006, 01:15 PM

Quote:

Originally posted by VaSteve
Engine is only driving you with the gas on.

Try this w/o panic braking:

Come to a point and let off the gas and see how far the car rolls. Do not brake. It will evetually stall but don't worry about that.

Do it again but this time shift into nuetral at the same point.

Which rolls further?

Some instructor you must have had.

Yeah, he wasnt muh good.

What you say is quite correct, but consider this- under hard brakes that the car is decelerating at a greater rate than would be provided by engine braking alone (such as you descirbe)- its true to say in these circumstances while the brakes are retarding quite rapidly the car, the engine is still trying to drive it forward.

Cheers

VaSteve · 01-24-2006, 01:32 PM

Quote:

Originally posted by stuartj
Yeah, he wasnt muh good.

What you say is quite correct, but consider this- under hard brakes that the car is decelerating at a greater rate than would be provided by engine braking alone (such as you descirbe)- its true to say in these circumstances while the brakes are retarding quite rapidly the car, the engine is still trying to drive it forward.

I'm not saying use engine braking alone.

Add the threshhold braking and the engine braking and you should stop in shorter distance than the middle pedal alone. Unless you were still on the gas (or your gas pedal was stuck down) the engine is not naturally "driving the car forward."

ABS actually increases your stopping distance. The grab-release-grab-release ABS effect increases the braking distance during the release. True threshold braking is before the ABS comes on.

Panic stops and track driving are two different things however.

gprsh924 · 01-24-2006, 01:37 PM

I had suspected that on a true panic stop it would be best to just work the brakes.

And to VaSteve, the main times im panic stopping is when i dont know im coasting down a hill and the dumb woman on her cell phone rolls through the stop sign midway down the hill, pulls out in front of me, causing me to slam the brakes and miss her rear bumper by a foot or two