bob_dallas

I know it's OT for this thread but low speed is probably an understatement for that one - rotation sure does help there. I can't believe that you can't get a TT to rotate there - that sucks.

I can't wait to get back up to Hallet in October. Are you going to Octoberfast?

ColorChange

Joey:

The article is titled “Understanding trail braking” on the front cover. While it does discuss line choices, the real point is maximizing g’s in the appropriate direction. I also think you are missing something. Your comment

“A more useful comparison, IMHO, would've included segment times using various brake-throttle transition points on the same line, i.e. Am I faster when I get on the gas right after turn in or closer to the apex? The "Going Faster!" book by the Skip Barber people covers this issue in some detail and points to some of the keys for making this determination:”

Doesn’t make sense to me. If you are on the same line, there is only one way to drive it to maximize g’s. You do not have the option of getting on the throttle earlier or later, it’s all defined by the line you have chosen, and maximizing g’s along that line, to which there is only one solution. I agree with the rest of your comments.

Jim, if you read the article, the fastest line through a hairpin is actually slower at the apex (lat g max) than the classic line but this difference is more than made up by being so much faster coming into the turn by trail braking and accelerating earlier coming out.

Bob? What has contradicted my instincts? This article fully supports what I have been saying, at least I think so (nicer attitude offered for you).

Bob & prg: I agree with your comments on PSM kicking in if you get yaw going. What I have been experimenting with (only have about 2 hours track time in the tt) is to do the trail braking fairly well hooked up (this keeps psm away), and then using brake release and the throttle to control rotation.

SundayDriver

Quite the contrary. There would be an intinite number of solutions while driving on the edge of the friction circle. I can be all the way to the side, maximizing lateral g's, or I can reduce those in trade for longitudinal g's. In a fairly static sense, that is exactly what trail-braking is all about. Do I want to give up cornering force for braking. Similarly, do I want to give up cornering for acceleration.

In the real world (inside the car, not on the computer or magazine article) you have to deal with balance, transitions and changing conditions. Thus the solution you choose for one lap often isn't the best solution for another lap and certainly not another day.

You should also realize these articles are usually based on real world data from purpose built race cars. (Now I have not read that particular article so I am generalizing). In other words, data from cars that may do 3.5 g's at turn in, but only slightly more than 1.5 by the time they get to the apex. These are also very stiffly sprung cars that CAN apply huge amounts of brakes under cornering without upsetting the balance. Case in point, there are places where I do huge trail braking in the Stohr but could barely touch the brakes in a production car (it would throw the balance so far off I would be fighting the car).


Can you elaborate? You put forth the theory that the fastest way around a track was to threshold brake to the apex of almost every corner. I have not yet seen the article, but I would certainly like to know what information supports your theory.

Thanks

joey bagadonuts

I'm not sure I understand your assumption about throttle application. How does the line define where you apply the throttle? In any corner you can get on the gas right after turn in or closer to the apex, right?

If you're saying that there is only one optimal brake-throttle transition point which will yield the fastest segment time, then I would agree. The challenge is trying to determining where that is.

J.G. Rausa

http://www.enter.net/~tdu/Places/Hil...ley/index.html

Interesting thread because of a recent event where I had trouble carrying speed through the hairpin shown in the link to pics above. I drove through quicker in the rain than when it dried out the next day where I proceeded to lock 'em up, understeer, and almost go off the road. Maybe this would have been an approipriate turn for some trail-braking? Opinions on an appropraite "dry" line requested.
Thanks,
Joe

ColorChange

Joey:

The key here is on the same line. Once you define the line, there is only one way to reach the fc and stay on that line. You can have different throttle or braking points and be on the fc, but these would lead to different lines. I am making the assumption that conrer entry speed and postition are the same.

I look forward to continuing our discussion over a beer. I'll buy the first round.

mitch236

Trail braking......... hum, what to say? I like these threads but the reality is that, without fail, every advanced student that I instruct enters too slow. Once you learn the line, you work on exit speed. That seems to be where most of my advanced students are at. Now the idea of trail braking or late braking enters their minds and that is where I have to educate them. Instead of working on braking, they should be practicing their entries. If you enter too slow (and most of us do), you can never regain the speed lost. Now is where unnecessary G-force creation occurs. Because you realize you could be traveling faster, you accelerate during the entry phase and create unecessary G-force and still don't regain the lost speed. Trail braking will make this worse. Until you can enter with all the speed your car can handle and still not affect the mid-turn and exit phase, you should forget about learning trail braking. Sort of like trying to sprint before you can walk properly.

mitch236

I think this is wrong. Once the line is defined, you could be be on the fc, but the direction of the vector could be changed. That is why I liked the statement
The vector must be pointed in the most useful direction.

joey bagadonuts

CC,

One last thought and then I'll save the rest for happy hour.

Keep in mind that grip is not a constant. Since grip is a function of weight, any change from brake to throttle will impact the amount of grip available at either end of the car. Therefore, you can turn in on the same line and go faster if you can increase the amount of grip available at the end which is slipping.

ColorChange

Mitch, No. By definition, on the fc means you are at the max lat and max long g's the tires can handle, all the way along the line. There is only one way to stay on that line and the fc. One and only one. You will have to exactly trade lat and long g's to stay right on the line. If the vector was off from exactly what was required, you would be off the line. There is no flexibility when you are on the fc if you are constrained by a given line. Make sense?

JCP911S

Trail braking serves two purposes.

The first is to push the braking zone into the entry of a turn to allow you to brake later, and thus stay on full throttle longer. The second is to settle the nose of the car to help tranisiton to full lateral accelleration... this is why drivers will often give the brakes a light tap going onto a corner (usually with the left foot). Dragging the brakes as you turn in helps smooth the entry to the turn.

Secondly the amount of trail braking depends on where the turn is in the track as well as the geometry of the turn. In a high speed sweeper you will trail brake less... in a tight hairpin, you will trail brake more... for the reasons stated many times above.

On turns at the end of a straight, you should typically apex early, and trail brake hard to extend the straightaway and "throw away" the second half of the turn. At the Glen I trail brake hard all the way through the first part of the bus stop which allows me to brake at the 300 mark rather than the 400 mark. Exit speed is meaningless here.

On turns leading to a straight, you typically take a late apex in order to get on the throttle as soon as possible. Turn 1 at the Glen I do virtually no trail braking as I want to get the car rotated quickly and get on the gas. However if Turn 1 led to a bunch of low speed squiggles, I would trail brake hard at that turn.

mitch236

Maybe I am wrong but the way I see it is that the fc does not care whether you are accelerating or braking, turning or straight as long as there are forces acting on the car and the sum of all the forces equal the maximum available traction. So you could enter a hypothetical turn under acceleration at the fc or you could enter the same turn under signinficant braking and be at the same fc. Which is faster depends on many things but both scenarios would maximize fc.

JCP911S

Bottom line yes you cannot exceed the adhesion limits of the tires, but lets not get to "G-Sum" again.

Good drivers know that fast laps come from maximizing speed on the straights not speed through the turns. This is based on the right combination of braking turning and accelleration. Certainly you want to use all the adhesion the car has to give, but how you allocate that to maximize time spent at full throttle is really what separates the good drivers, and its really really hard.

I would guess that the average experienced PCA driver is using maybe 60-70% of the car's capability right at the point of turn-in compared to the pros who use 95% plus. Thats where the pros earn their $$$

SundayDriver

One more time, unless you want to avoid the hard quesitons...

I can drive a line at neutral throttle and the max lateral g's.
I can then go back and drive the exact same line, but a bit slower while acceleratiing.
I am on the friction circle in both cases and on the same line in both cases.
There are multiple solutions as described above because I have an intinite set of possibilities between lateral and longitudinal g's.
I think the other posters to this thread understand that and are also questioning you about it.
Are you going to ignore the question/assertion that there are infinite solutions or perhaps you have me on your ignore list. Or maybe you are just waiting until you provide the proof (as promised) of your g-sum theory.

SundayDriver

100% correct. That is why it is a circle,not a point. It is up to the driver to determine (perhaps with help from data or a coach) what part of the circle to be on. Or sometimes, not to be on the circle because of the reality of vehicle dynamics or a bump or something else going on.