if an accident is imminent. or your going into a corner too fast. do you pump the brake quickly, or do you you pump it slow but hard?

 

If by "pump" you mean "depress" then I understand your question.

Depends on whether you have ABS or not.

No ABS, Depress with authority 99.999 percent of braking potential of both the brakes and tires. Lock-up offers less traction generally speaking.

Yes ABS, Slam those puppies.

my 2 cents.

 

With today's tire technology, a locked tire probably generates the best stopping, BUT you loose the tire with flat spot.

Always brake in a straight line so that ALL of the grip will go to braking.

Once its in a spin, it's both feet it. (Brake and clutch). If you are headed in the direction you want to go, keep the tires locked. To change direction, release brake and relock.

 

There's another option that comes something short of what I think of as "pumping the pedal." True threshold braking - where you are applying as much pressure as possible without locking up - is actually managed by your toes and foot bones. I try to teach students in HPDE type settings to use their ankles for the last few mm of pedal travel rather than the large muscles in their legs.

K

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Knestis &raquo;</TD></TR><TR><TD CLASS="quote">There's another option that comes something short of what I think of as "pumping the pedal." True threshold braking - where you are applying as much pressure as possible without locking up - is actually managed by your toes and foot bones. I try to teach students in HPDE type settings to use their ankles for the last few mm of pedal travel rather than the large muscles in their legs.

K</TD></TR></TABLE>

that's good advice!

makes so much more sense to use smaller more precise muscles to manage a precise action!

you dont hear anything like that in common driver's ed

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Grumpy &raquo;</TD></TR><TR><TD CLASS="quote">With today's tire technology, a locked tire probably generates the best stopping
</TD></TR></TABLE>
Hmm... mu values for static friction are still always higher than mu values for kinetic friction... is there something else going on?


Contribution to thread- I've been told that braking by focusing pressure on the ball of your foot can allow for more precise pedal control since there you have a greater concentration of nerve endings in that area.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by WRXRacer111 &raquo;</TD></TR><TR><TD CLASS="quote">
Hmm... mu values for static friction are still always higher than mu values for kinetic friction... is there something else going on?

</TD></TR></TABLE>

Do you speak engish??

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by WRXRacer111 &raquo;</TD></TR><TR><TD CLASS="quote">
Hmm... mu values for static friction are still always higher than mu values for kinetic friction... is there something else going on?
</TD></TR></TABLE>

Generally, yeah, but there a few materials that are exceptions. Tires aren't one of them though as far as all the data I've seen claims. The coefficients are pretty close though. A tire reaching maximum force is a combination of static and sliding friction anyway, so it's all about maintaining the proper slip ratio to produce the maximum braking force (per Milliken).

Threshold braking is the way to do it, but a good ABS system can pulse fast enough to produce this. I don't stand on the pedal in any case.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Grumpy &raquo;</TD></TR><TR><TD CLASS="quote">

Do you speak engish??</TD></TR></TABLE>

BAHAHAHAHA

I imagine it has been a long time since you've had physics class, so I'm going to assume you have no idea what he's talking about... If you do, sorry for the redundancy.

Mu (as seen below) = coefficient of friction = the "level" of friction


When an object is static (standing still) it requires more force to move than if it were kinetic (moving).

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by MichaelJComputer &raquo;</TD></TR><TR><TD CLASS="quote">

When an object is static (standing still) it requires more force to move than if it were kinetic (moving).</TD></TR></TABLE>

And to be completely clear as mud, a spinning tire is (mostly) stationary with respect to the ground and a locked tire is moving ("kinetic") with respect to the ground.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Grumpy &raquo;</TD></TR><TR><TD CLASS="quote">

Do you speak engish??</TD></TR></TABLE>

Sometimes. Engineer-ese frequently gets in the way though.

Two kinda interesting articles I just dug up- the graph is kinda useful
http://www.visionengineer.com/mech/abs.shtml
http://www.visionengineer.com/ref/friction.shtml

PS- the one thing that really creeps me out about ABS is that when you DO spin... it doesn't let you lock the tires when you go both feet in. I'm not a roadracer, but I figure it's easier to avoid a spinning car that picks one direction and heads that way instead of potentially looping back into my path.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by MichaelJComputer &raquo;</TD></TR><TR><TD CLASS="quote">

When an object is static (standing still) it requires more force to move than if it were kinetic (moving).</TD></TR></TABLE>

Isn't that Newton's first?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Grumpy &raquo;</TD></TR><TR><TD CLASS="quote">Isn't that Newton's first? </TD></TR></TABLE>

Newton's first law: Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.

 

The statement about Mu, friction, is true, that a locked tire will always have less traction than an "almost locked" tire. Pnuematic tires create their highest grip in the 10 degree/6% (I believe) slip range. So to confuse things even more, a "perfect" braking has the tire actually doing 6% slower than the ground, pretty cool huh?

To actually try to answer the original question, you want to SQUEEZE the brakes on "quickly." If you "jump" on the brakes, the fronts will likely lock first/early because the system is generally balanced to include the weight transfer to the front wheels. Smacking the pedal will put to much pressure to the front before the download is there to support it. So there must be a "ramp rate" of pedal pressure, as in a line (data acq actually shows a fairly straight line, though on street tires it might be more of a curve). That said, the ramp rate should be just shy of lockup from the moment you go to the brakes (before weight transfer lockup will happen with less force) and increase as the weight transfer happens.

To take a different angle on it, we need to state that the brake balance is static. We'll say 80% front, 20% rear. With the car rolling at a constant speed, we'll say the car has 60% of the weight on the front, 40 on the rear. The moment you go to the brakes you can only use 3/4 of the ultimate potential (60% of the weight on the front/80% of the brake line pressure) As the car transfers weight to the front you will be able to increase the pedal pressure until there is 80% of the weight on the front tires.

That's probably enough for now...

 

I love the Road Racing/Autocross forum because you guys actually give awesome advices and inputs.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by EM1b18cR &raquo;</TD></TR><TR><TD CLASS="quote">I love the Road Racing/Autocross forum because you guys actually give awesome advices and inputs. </TD></TR></TABLE>

a real tech forum? no way

No ABS for me I guess I better start practicing that threshold braking

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by EM1b18cR &raquo;</TD></TR><TR><TD CLASS="quote">I love the Road Racing/Autocross forum because you guys actually give awesome advices and inputs. </TD></TR></TABLE>

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Knestis &raquo;</TD></TR><TR><TD CLASS="quote">There's another option that comes something short of what I think of as "pumping the pedal." True threshold braking - where you are applying as much pressure as possible without locking up - is actually managed by your toes and foot bones. I try to teach students in HPDE type settings to use their ankles for the last few mm of pedal travel rather than the large muscles in their legs.

K</TD></TR></TABLE>

so basically dont take my heel of the floor and pump the peddle repeatedly and swiftly?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Boostage &raquo;</TD></TR><TR><TD CLASS="quote">

so basically dont take my heel of the floor and pump the peddle repeatedly and swiftly?</TD></TR></TABLE>

No. Threshold braking is a learned thing, and I contend that even with today's advanced ABS, best case, it is a better way to slow a car down. By definition, once ABS has been engaged, you have reached the threshold of the tire's traction in the given environment. Staying just on this threshold isn't a repeated pump of thepedal so much as it's a feel thing. You get to a point where you can just tell that the tires are about to lock up and you modulate your braking accordingly. It's not easy and is one of the toughest things to learn in motorsport IMHO.

Now for street driving...if you've got ABS, mash the pedal to the floor as hard as you can and let the computer do the work. You can probably do as good or better threshold braking but in a panic situation, it's best to concentrate on avoiding the thing you're trying to avoid, rather than trying to use "proper technique."

If you don't have ABS the situation becomes trickier. I don't know anything about this coefficient of friction business but I do know this. If disaster is imminent (as in, you're 50 feet away from certain impact going 50mph), locking up all four wheels will reduce your ability to steer the car to zero. That's what ABS does for a living, and if you don't have it, think before you just bury your foot in them.

Hope that makes some amount of sense...

 

Threshold braking is the way to go, regardless of if the car has ABS or not. ABS does not give you maximum braking; it helps, but it mostly keeps you in control under hard braking and saves some unneccessary wear on your tires. I have one car with ABS and one without, and threshold braking stops both of them much better than just stomping on the brake pedal as hard as I can like a newb. I find that ABS makes threshold braking much easier and with only small consequences if I give it too much pressure, but it can be a pain in the *** to do on my non-ABS Honda. Of course, it's still a million times better to at least attempt to threshold brake than to just lock the tires up. You lose about 30% of your stopping power when you lock the tires up. You lose some stopping power when you slam the pedal in an ABS equipped car too, but I don't think it's nearly as much, and there's no skidding.

 

and how do I threshold brake?

 

If you want to know how to brake stuff, you really ought to be talking to Chad and Karl....Ducking....

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Backmarker &raquo;</TD></TR><TR><TD CLASS="quote">If you want to know how to brake stuff, you really ought to be talking to Chad and Karl....Ducking....</TD></TR></TABLE>


hmmmmm.....you better not have a tow hook installed....cause you are getting bump drafted!!

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by EM1b18cR &raquo;</TD></TR><TR><TD CLASS="quote">I love the Road Racing/Autocross forum because you guys actually give awesome advices and inputs. </TD></TR></TABLE>

lol

Yea, this is the only forum (most of the time) when I can actually talk geeky and people will actually follow, and the same goes the other way around.

 

The goal is to apply as much brake as you can without locking up the wheels or going into ABS. It's not quite as easy as it sounds, but it's still not all that hard.