2BWise

With the modern ABS systems on these cars you can stomp on the pedal as hard and as quickly as you want and the controller will work everything out. My guess would be that the instructors want to be very precise in not allowing their students to learn that because it is definitely a bad habit. The driver needs to apply brake pressure right at the limit of what the tire can withstand. With ABS it does it all for you. All the driver does is stand on the pedal as hard as possible.

Van

Bill, that's what Andrei did in my examples...

himself

Can you recreate this graph with both Gs and speed "vs time"? I find that trail braking and "squeezing" can be better interpreted using a time scale rather than distance scale. IMO distance view can skew the data analysis a bit. Also, graph shows blue braking later and much less (.5 G less), and carrying much less speed. It almost looks like there was slower traffic.

-td

2BWise

I would disagree. You have a steeper slope than the novice driver in your comparison.

My comment was meant to reinforce the fact that 67King made in that a full force apply to the pedal can have the ability to lock the front brakes before weight is fully transferred (shouldn't be possible if the brake system is proportioned properly but that's a different discussion). With ABS the driver apply rate is completely eliminated from the equation. The correct brake apply in a modern ABS car is as hard and as fast as you can move your leg. A bad habit if you ask me because if you learn on that and hop into a car without the driver is going to have a very difficult time slowing the car. That's why the confusion from the OP. The instructors are trying to teach the proper technique and not allow the drivers to use the ABS as a crutch to mask improper braking.

ProCoach

While time is a nice way to analyze individual performance, distance is the only way to compare more than one trace.

Also, distance does not change (the event happens at the same place on the plot) as it does with time.

ProCoach

Great example!

ProCoach

I'd want to see accompanying speed to make any judgement, but this is a good comparison of execution.

fleadh

I think it depends so much on the car, upcoming corner, and tires that you can't use absolutes in the discussion (do this, make it look like that, etc).

Not to mention, I wouldn't take any advice from anyone while you're demoing cars -- no matter who it is. Their job there isn't to make you faster, it's to make you safely enjoy the car and experience (which has no relation to going fast).

My 2cents. :-)

-mike

kurt M

Steeper G trace not brake pressure. Perhaps he can slow the car better than the ABS can if as was pointed out theblack trace was under ABS. Some ABS systems are better than others. Motorsports systems over street car based systems as example.

Good Thread!

Van

Here you go, TD. Like Peter said, time can throw things off, because, due to rate of speed, we're all going to arrive at different parts of the track at different times.

Same data as before, but with the MPH trace and the x-axis is time. David's car, the 911 targa is the most underpowered of the bunch - that's why he can brake later than the rest of us.

Bill, my gut feeling is that higher braking force can be achieved without engaging the ABS for two reasons: first, the friction from threshold braking provides more torsional force around the axle than a wheel that is having the braking pressure reduced completely for short amounts of time; second, the interruptions from the ABS releasing will upset the smooth transfer of weight to the front wheels, which allow them to do more braking.

However, I'm not an expert on that - and I don't even play one on TV... so we'll need others to weigh in.

J richard

^^^
Yes you can achieve higher brake pressures and improved braking without the abs application. By properly settling the chassis with the initial application the tires will be loaded properly and will not lock allowing you to outbrake the guy tahr just drives in and hammers the pedal and activates the ABS.

67King

Kinda like PSM. I wish PCA would support turning it off with new students. Really impedes proper technique from what I've seen as an instructor. I'd love to have ABS on my car, but I'm glad I learned to drive without it.

2BWise

Again I disagree here as well. Assuming the 997 is generally a stock vehicle the driver is not going to beat ABS. You can build more pressure, faster, with a full stomp on the pedal than the guy that drives under ABS. A modern controller can work closer to the peak of the tire than the driver can.

There is a serious downside to turning it off. Although, I fully agree that drivers should really learn to drive without it, so that they fully understand proper technique before jumping into a vehicle with the system on. Almost all modern cars rely on ABS even before the pedal starts pulsing under your foot. Most cars have electronic rear biasing now-a-days as well as functions that modulate pressure when you start turning the wheel.

ABS doesn't reduce all pressure, its only going to drop enough so that the wheel recovers to a lower slip level. An ABS controller will almost always be able to control the tire closer to its peak than the driver can, which will result in a shorter braking distance. The weight transfer is a function of longitudinal decel, so at the initial brake apply ABS will have limited affect on that rate until the wheel starts to go toward lock. The other advantage is that the controller can maximum rear braking while there is still weight on the rear and control the pressure down as it transfers forward.

Yes, acceleration won't be directly applicable to brake pressure across different vehicles, but for each vehicle acceleration and brake pressure are directly related. For the same vehicle, compare G or brake pressure, same result. For data acquisition you only really need to long one. If you know where you are using the brake you can back calculate the brake pressure to achieve it.

Some ABS units are better than others. I agree with you on that one, but depending on the vehicle and the purpose a lot of street units are extremely good. For the Porsche track applications it is likely very well done. The motorsports units get good performance because there are less functions active (no TCS, no ESC) so the compromises are far less since the focus is so narrow. But, the motorsport units will be derived from the same hardware and software that the street units are.

333pg333

This is a great example of what I'm talking about. Your trace seems to have very little squeeze. Just straight onto the pedal and achieving max pressure within a very short space of time/distance. This is the sort of trace that I've always thought we were meant to obtain. Hence, I can't see how this can be done by squeeeeezing? I just don't see how you can't be leaving seconds out there by squeezing...plus, it's actually harder to do of course. All I was doing coming into the braking zones on that day was thinking about gently initiating the initial cadence after being told I was too sudden. As has been mentioned, I think this emphasis on squeezing was definitely part of a demo day mentality but it's not like this was an isolated case of the squeeze being taught.

ps, thanks Van for the graphs. Much appreciated.

J richard

There is no way that a car driven in this manner will be faster than a properly driven car without ABS activation. If you are talking about ultimate braking distance yes, but for a car on the race track, not even a contest...