tuj

This is my first ABS car (I had a 96 Miata before) and after autocrossing my 8 for the first time last weekend, I am curious as to which is a better technique for hard braking in autox.

-hard application that triggers the ABS.

-threshold braking just below the point of ABS

Any opinions? Obviously not getting into the ABS is smoother, but in terms of overall stopping power, I was hard-pressed to really tell a difference between the two. My primary test was coming out of a fast straight at about 80 and having to brake hard for a 90 degree narrow left. I felt like I couldn't get to the edge of lockup and stay there without falling over into the ABS, unlike I could with my Miata.

Zaku-8

I'm curious, what is the reason not to trigger ABS? It's not like DSC where the computer holds throttle and interferes during a turn. It's just like if you were able to pump brakes with superhuman speed.

SDB

Actually it should be even better. It would require 4 brake pedals since ABS only pulses on the offending wheel or wheels. Only ABS can decide which wheel to pulse selectively, unless of course you do have the 4 brake pedal mod and have 4 feet.

tuj

Well technically the ABS is pulsing the brakes, so there are moments when the brakes are not working as effectively as they could. If you were perfect, you could threshold brake, ie brake just up to the point where the wheel would lock, but not quite go over. This should be the maximum possible braking of the car. Of course, the problem with this is that its pretty hard to do in cars without ABS.

I feel like the ABS cuts in a bit early. Almost like it starts working when it detects a wheel slowing faster than it should be, cutting in before the threshold is reached. My question is, for those with more track and autox experience than myself, in places where hard braking is needed, is it more effective to try to brake as close to triggering the ABS as possible, or just mash the pedal and let the ABS work for you?

ZoomZoomH

iTrader: (1)

i don't consider myself a great autoxer/track star (far from it :p ), but think of it this way: when your wheel locks up, it means you have overloaded the tire(s)' traction. All ABS/EBD do for you is it greatly reduces your chance of overloading the tires by intentionally *reduce* your braking power (aka the pulsing) so your tires have a chance to regain traction. If I was you, I'd think of ways to brake as little as necessary in order to get through a corner carrying as much momentum as possible :D

bmc

ABS is not as good as non-abs for straight line breaking. If you want to stop "Right now" ABS isn't what you want.

However, slowing down via ABS is faster than slowing down via "just off of ABS".

Plus, "stand on breaks" is much easier to reproduce than "modulate at this point in the pedal travel." Remember, consistancy is very important.

Imp

I agree with your other two points, but I don't totally agree with this one. ABS disengages the brakes. Anytime that happens, in theory, you're increasing your stopping distances. If you're right on threshohold of lockup, you're applying the maximum braking the car can take... any less and you'll stop in further distance. Any more, ABS kicks in, and again, further distance.

That said... how many people can actually work the brakes to 100% threshold in an auto-x environment, never mind daily driving.

--kC

bmc

ok, sure you are probably right, but consistancy of how a car breaks is far more important than the very small benifit you would get staying out of ABS.

John V

iTrader: (1)

In theory it's possible to stop faster without ABS. In practice, if you're close enough to the threshold you're going to cross it eventually - and once one wheel locks braking distances will increase dramatically, never mind the loss of control.

I'll take the consistency of a good ABS system any day.

The system on the base 350Z is a different story.

dmp

The absolute fastest way to stop is to lock up all tires. That creates the most friction. ABS won't help you stop FASTER - just help you to be able to steer whilst heavy on the brake pedal.

Imp

And expensive too! (Flat spotting).

Yeah, I know what you mean about the 350Z abs... ever been in a WRX? Worse!

The 8's brakes and brake system are 10x better than the Subarus (wayy too sensitive)

--kC

John V

iTrader: (1)

This is not true at all for pavement, macadam, or concrete. This is only true on surfaces like gravel.

The sliding friction of a tire is much lower than the kinetic friction, and that doesn't take into account the overheating that will happen when a tire is locked up.

dmp

It's true for 'most conditions'. (shrug). ABS isn't a way to stop your car quicker - again, it's purpose is to allow people who PANIC when emergency-braking, so they don't lose their ability to steer the car

John V

iTrader: (1)

No. It's not true for most conditions. Your information is just patently false.

The fastest way to stop is to brake each wheel to the threshold of lockup. This is impossible to do repeatedly. ABS is a good compromise.

Locking the wheels isn't even close to the fastest way to stop. I don't know where you got the idea that it was.

http://www.sportrider.com/ride/146_9510_rss/
http://www.msgroup.org/forums/mtt/to...p?TOPIC_ID=153

dmp

More info:

http://www.nhtsa.dot.gov/cars/proble...absbrakes.html

A relevant quote:

"Do cars with ABS stop more quickly than cars without? ABS is designed to help the driver maintain control of the vehicle during emergency braking situations, not make the car stop more quickly. ABS may shorten stopping distances on wet or slippery roads and many systems will shorten stopping distances on dry roads. On very soft surfaces, such as loose gravel or unpacked snow, an ABS system may actually lengthen stopping distances."

dmp

You are giving motorcycle links?

John V

iTrader: (1)

TRUE.

But nowhere does it say that a locked tire stops faster, so I'm not sure why you used that quote as support for your argument that a locked tire stops faster, because it does not. ABS may lengthen stopping distances in theory because there are times at which the brakes are not being applied. Furthermore, ABS effectively applies and releases the brakes rapidly - the best way to stop is not to force the tires to do sudden things. The fastest way is to threshold brake. Unfortunately, we haven't figured out how to detect how much force it takes to keep a tire BARELY rolling, so ABS is our best option.

John V

iTrader: (1)

I ride, so I have more motorcycle links than I do car links. Motorcycle tires act the same as car tires.

More:

http://avstop.com/Stories/Tires.htm

Relevant quote:

In discussing braking and tire damage, something new pilots may not think about is how bad locking an aircraft's brakes is on stopping an aircraft. A locked, skidding wheel assembly is not efficient in stopping an aircraft. Properly applied rolling friction on a properly inflated tire is what stops a tire in the shortest distance.

dmp

...but i've only seen ONE car do a 'stoppie' :D

I'm not sure where I got the idea about locking up the tires - but i'm looking for suppoting information.

dmp

That quote was used to defend the 'other' half of my statement, re: the purpose for ABS in the first place = 'NOT to shorted stopping distance" = ABS won't make your car 'stop' in a shorter distance.

dmp

Here's one bike-related link - I wish the guy would go into detail:

http://www.msgroup.org/Contrary/NEG085.html

John V

iTrader: (1)

"Consider the source."

This guy has no idea what he's talking about.

Had to look back and check, but I never said ABS gives the shortest stopping distances. I said it gives the most CONSISTENT braking and is very close to the shortest distance in practice. Theory is just that - theory.

Of course, you can verify this for yourself. Find an abandoned section of road and do some tests with and without ABS. It'll become obvious which is the faster way to stop pretty quickly.

John V

iTrader: (1)

More:

http://ecom.toyota-gib.com/English/V...04%20Terms.htm

Brake Lockup – In braking, lockup describes the point at which a tyre starts to skid in an emergency stopping situation. A tyre’s maximum braking force is developed when it is on the verge of lock-up, so a car’s shortest stopping distances are produced when its front and rear tyres are held just short of lockup. Anti-lock brakes (ABS) prevent wheel lockup, giving the advantage of simultaneous steering and braking ability (a locked wheel cannot be steered).

ULLLOSE

iTrader: (1)

So where does it say the fastest way to stop is locking up all four wheels? btw for the most part no stock production brake system will lock all four at the same time. Due to a nearly 70% front brake bias you will almost never see stock rear brakes lock up in the dry, only the front will do it. I guess you must think drifting is the faster way around the track too.

John V

iTrader: (1)

I'm glad somebody else is with me on this.