Thursday, March 13, 2008 - 03:58 am, by: Peter Nitschke(Pen)
Neil, I read that post. As I understood it, your first sentence says that the requirements for vehicle (car) brakes is such that the maximum pressure required to stop the car in a defined safe distance, is equivalent to the pressure you could make just using two fingers on the brake pedal. That sounds fair and reasonable to me.
However, to say that you can't make more pressure in the system by using more effort on the pedal, defies physics - unless you have a release valve in the system.
Technically, the ABS system is a release valve, but it's activated by the change in rotational speed of the wheel, not by how much pressure is in the system. So if you had really sticky tyres on a hot road, you would require more pressure to activate the ABS compared to skinny bald tyres on loose gravel. The car would stop a lot quicker too.
So the limit to the amount of pressure you can apply is decided by the ABS, and will vary with the conditions. I haven't tested your theory, but I have my doubts that pressing the brake pedal with 2 fingers would get my ABS to activate under normal driving conditions, let alone with sticky tyres on a hot track.
With OEM pads, I can kind of understand your comment. After about 5 minutes on the track, it didn't matter how hard I pressed the brake pedal, the pads simply had lost almost all grip (with no warning) and I got to tour the paddock. But that is actually a limitation of the pad, not a limitation of pressure in the system. The ABS wasn't activating and acting as a pressure relief valve either, as the change in rotational speed of the wheels wasn't high enough for it to trigger.
With the Lucas pads however, towards the end of 30 minute sessions on the track, I was using both feet on the pedal and pulling hard against the steering wheel as the pads were by then losing efficiency - but they still kept working and I was able to stay on the track and keep driving hard. If I didn't use that much pedal pressure, I wouldn't have stopped.
So I would argue that you can make more pressure by using more effort, right up until the ABS activates. Basic laws of physics apply here.
The only other limit of pressure I have experienced, was many years and cars ago, when I had to brake hard and blew a brake hose. In that instance, the pressure on the pedal caused the pressure in the system to exceed the ability of the hose to stay intact. After that, it didn't matter how hard I pressed the pedal as it was on the floor, and yep, two fingers would have done the same job!
Peter do a advance driving coarse! As we Sydney members did one, yes Neil and many others. The instructor made us all do a brake test at 70km/h. Then he picked a car from the group and did the same test and pulled the car up in the same distance as us with just 2 fingers. As he said not much peddle pressure is needed to pull the car up. As shown with the two finger trick! As some people think you need to stand on the brakes to stop the car. I wish we could do that day again, as it was a great day out!
Peter Nitschke Junk Filterer South Australia UZZ30 UZZ31
Thursday, March 13, 2008 - 03:06 pm, by: Peter Nitschke(Pen)
Colin, that may well be the case normally. The situation I described however is when the pads are extremely hot and losing their grip.
I had to push harder and it did make a difference. Otherwise I would have just kept driving the way I was on previous laps.
Maybe 2 fingers would have been able to apply the same pressure, but even if I knew about this, I wasn't about to bend down and try it going into a hairpin bend from 180kph.
Friday, March 14, 2008 - 01:01 pm, by: Phil Gibson(Sciflyer)
Forget all this talk about track driving, hi-speed stops, etc that is just confusing the issue
Fact is its crazy to suggest that all street/normal driving use pads are equal
There is nothing wrong with looking at stock Soarer brakes (particularly V8 ones) and wanting to improve feel, bite and stopping power
There are heaps of pads suitable for daily driving that are better than OEM... on my last car i went from generic OEM specials to Bendix Ultimates which are definately street/daily use only *not* a track pad and the difference in brake feel and stopping power was huge
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