Response to 2 threads

[jack miller]

I'm just catching up on some Forum reading.

I was bit interested in the braking thread which also evolved into the advanced performance driving school thread.

Regarding braking: I think one of the challenges in becoming more sensitive to braking in a race car is getting used to having your foot off the floor and thus controlling your whole leg enough to be sensitive with the pedal. In other words, at least for me, when I brake in a street car, the heel of my foot is resting on the floor and I'm usually just braking with my stiff work shoe and my foot is levered with my heel. In a performance situation, my thin driving shoe is completely off the floor so the braking motion (on and off) is with more than just my foot; it includes my leg. That's a bigger muscle that's normally not used to doing anything very subtle. Combine that with fatigue, the complication of heel and toe and you've got a lot to practice. And thus my small contribution to the conversation: practice braking on the street by keeping your foot off the floor board, and practice easing on and off the pedal.

As for advanced performance driving (APD) school, with some cooperation and possibly compromise of track time (or using lunch time?), we might be able to work something into our Streets events, since it has a straight that starts or finishes, depending on direction, into the skid pad. That could allow some pretty sick braking, rotation, drifting, etc., if you don't really care what comes afterward. We could do something single file during lunch for fun. Along the lines of the drifting contest that I proposed at SMt? Let's discuss it Friday night or here.

IMO, anyone who approached Spring Mt from the perspective of really focusing on improving their car control had an opportunity to get what I would expect intermediate drivers would want to get out of a APD school. I'm hoping Streets can provide a similar experience.

P.S. The DE guys should always set up the Bud Sweeper (CW) at the Q DE's. You can hang the car sideways there for quite a distance. Very, very good practice corner at the Q.

[Dan Chambers]

P.S. The DE guys should always set up the Bud Sweeper (CW) at the Q DE's. You can hang the car sideways there for quite a distance. Very, very good practice corner at the Q.

True. AND, we have to be really careful how we do that. Why? Ask Kim Crosser about that big CW sweeper at the top of the hill. To get it right is a dream come true at the Q. To get it wrong ......

[kary]

P.S. The DE guys should always set up the Bud Sweeper (CW) at the Q DE's. You can hang the car sideways there for quite a distance. Very, very good practice corner at the Q.

True. AND, we have to be really careful how we do that. Why? Ask Kim Crosser about that big CW sweeper at the top of the hill. To get it right is a dream come true at the Q. To get it wrong ......

It all depends upon when you choose to give up as to how severe or poor the results can be!

One of the things we need to teach our driver population is how your ABS brakes work when applied going forward with a sudden reversal of direction. It is rather shocking when you experience it for the first time and a bit counter intuitive as to how to combat the issue.

[Kim Crosser]

I resemble that remark! (In fact, it influenced my choice of tag lines.)

That sweeper was a lot of fun, and hanging it out in a 4-wheel drift at near top of 3rd gear (85+) was a gas - right up until I slid/spun off the track and did a little impromptu tree removal. I can remember thinking - "Cool - I will finally hit 4th gear (100+) on the top straight.)" (Followed shortly by "Hmm... but first I am going to eat all the cones up here.")

Having almost no effective braking while sliding backwards was a HUGE surprise. If it were to happen again, I would twist the wheel and try to get the car rotating, rather than just continuing to slide backwards. However, I did slide all the way past a light post, so intentionally spinning the car could have had worse consequences.

I think I would handle it much better now, but I would be worried about newbies in a sweeper that fast. For a DE/TT maybe, but that was awfully fast for AX participants who might only drive a track 4-5 times a year.

[glenn_993]

I can also attest to how ineffective ABS is while going backwards (Fontana AX 9/03). If it should ever happen again, (God forbid), I will grab a handful of parking brake.

[kary]

I may be reading these posts incorrectly so forgive me if I have.

But, it appears there is still a lack of understanding as to why the brakes with ABS do not work when going backwards. It is not the backwards part that is the issue. It is the fact that the ABS was engaged going forward and the change direction backwards while the ABS was still engaged. In that case the ABS is fooled into believing that it is no longer slipping so it realeases the brake even though you want it to stop. The remedy, and this is what we need to teach newbies in a school and at events, it is lift off the brake thus disengaging the ABS and then reapply the brakes (quickly). Now the ABS will not stop you from trying to brake.

The other fact is that brakes are less efficent, ABS or not, when going backwards so we cannot misunderstand that fact with the ABS issue.

[Kim Crosser]

Kary - I think this needs further input from other technical sources.

I got a different input from Steve Grosekemper when we discussed why there was so little braking force when going backwards. Steve indicated that the design of the brakes (much more braking surface/force in the front wheels) coupled with the weight transfer when sliding backwards results in much less braking force backwards. (I hope Steve can join in and correct any misstatement here.)

Both Steve and the Pioneer chief Porsche technician (forgot his name) told me that the ABS doesn't "know" whether the wheels are rotating forwards or backwards.

Doing a little web research, the ABS sensors have an electromagnetic sensor that measures the "teeth" in a "tone wheel" passing the sensor. As each tooth passes the sensor, the sensor generates a square wave output. The "pulse rate" from the signal output determines the measured "wheel speed". There is no difference in the output signal whether the wheel is rotating forwards or backwards - the sensor is only measuring how rapidly the teeth are passing.

If this is true, then releasing and re-applying the brake won't have any effect (other than extending the slide a little bit).

Perhaps we could get someone to donate their car for some definitive testing?

[kary]

Kary - I think this needs further input from other technical sources.

I got a different input from Steve Grosekemper when we discussed why there was so little braking force when going backwards. Steve indicated that the design of the brakes (much more braking surface/force in the front wheels) coupled with the weight transfer when sliding backwards results in much less braking force backwards. (I hope Steve can join in and correct any misstatement here.)

Both Steve and the Pioneer chief Porsche technician (forgot his name) told me that the ABS doesn't "know" whether the wheels are rotating forwards or backwards.

Doing a little web research, the ABS sensors have an electromagnetic sensor that measures the "teeth" in a "tone wheel" passing the sensor. As each tooth passes the sensor, the sensor generates a square wave output. The "pulse rate" from the signal output determines the measured "wheel speed". There is no difference in the output signal whether the wheel is rotating forwards or backwards - the sensor is only measuring how rapidly the teeth are passing.

If this is true, then releasing and re-applying the brake won't have any effect (other than extending the slide a little bit).

Perhaps we could get someone to donate their car for some definitive testing?

This is all very interesting about the ABS, but regarding the brake efficiency (degradation) going backwards is well know. Check my last sentence above about brake efficiency.

Now for the ABS thing. I believe your words to me (when I arrived on the scene of the accident) were "the brakes did not work at all" when you were going in reverse, correct? Well, I have had the same situation and have found through experience that if you apply the brakes going forward and engage ABS and reverse directions the brake do not work as well as if you lift off of them and and reapply in the same situation. This could have to do with the way brake pads wedge when applied so if you lift and re-engage the pad in the other direction it bites better than if it were engaged going forward and then switches backwards. Maybe technically speaking the ABS is not involved, but then why do they have racing ABS that does lock up in these situations?

[Kim Crosser]

I believe I said that the "ABS did not work at all", and indeed, at that time I thought I had an ABS failure - I had no noticeable vibration in the brake pedal during the long slide.

If you recall, there were two long straight stripes of rubber all the way from the curve to the tree - so we knew that the brakes were working enough to scrub off a fair amount of rubber along the way.
Also, none of the tires was flat-spotted, so the ABS must have been doing something - I just couldn't feel it.

For quite a while after the "incident" I thought the long slide was an ABS failure, or that ABS didn't work right in reverse. Steve and the Pioneer technician (and some others) convinced me that it was just a consequence of sliding backwards - the brakes just don't work as well.

Your comment on the pads wedging is interesting, but I am not sure I buy it completely. Since the ABS should be releasing and re-applying the brakes in that situation, why should manually doing the same thing improve your braking?

Several people have commented about the lack of braking force when going backwards, but no one else (Dente, Grosekemper, etc.) has suggested that pumping or releasing/reapplying the brake would have shortened the slide.

Perhaps we could get an ABS/brake expert to jump in here and settle this question?

[kary]

All I know is that brakes wedge differently going forward versus backwards. I know that I can lay black stripes down on the road without using the brakes or flat spotting tires. And I know if I engage ABS going forward and leave it engaged until I go backwards I have zero braking power. I also know I have not hit anything by lifting and re-engaging