RedlineMan

Hey Jack;

Re: Post #36;

As an aside, although I don't think you will ever hear any revisions of attitude toward full threshold trail braking for EVERY corner from Tim directly, I'm sure he must see by now that it doesn't work universally. However, I will give him credit for persisting to the point that the trailbraking notion was quite thoroughly dissected. Said persistence was not so altruistically intended, but a lot of healthy analysis was indeed the result in any event. Anyone paying attention should have learned from that that they themselves probably have some investigation of their own to do!

Along those lines, Larry frames it in an interesting way. Thinking of it his terms, I think perhaps I need to look at each individual corner, judge where my slowest point is, and see if I am giving up any speed by making that point too soon.

In most turns, my slowest point is turn in, and immediately after it's WFO. Easy to do with only 147 measly HP, but I do need to look closely at where my turn in speed could be too slow. In the Big Bend example, I simply had no grip whatever there last year (following 5 days of heavy rain). I know some of the time I am braking too much in the straight, but am on the gas heading for the 2nd apex. If I get it right I am braking on the straight and then dirt tracking all the way to the 2nd apex at steady throttle. A little left foot brake and maybe a quick throttle chop to rotate and back on full throttle for the exit.

There was actually so little grip last year, and it is so bumpy, that I found myself left-foot-stabbing the brake every time the front end washed out just to shift weight forward enough to steer again! Those following must have thought me nuts, or the owner of a failing brake light switch!

TD in DC

All of this discussion about the slow in fast out concept and the idea of the proper transition point between braking and acceleration has reminded me of an idea that has occurred to me during various DE lectures. Specifically, it strikes me that the concept of the "friction circle" is very useful as a learning tool, but probably not that accurate. [I am asking questions about this idea more out of intellectual curiosity than any idea about trying to change my driving style based on the answers I get here]

It would seem to me that the friction circle should really have a third dimension (a globe probably would not be appropriate) to take into account the effect the suspension has on keeping the tire patches in contact with the road -- which I think would take into account the idea of needing to drive smoothly so that your car remains "settled" to the greatest extent possible.

If you really only had the friction circle to take into account, it would be a lot easier in theory to drive as quickly as possible because you could simply use whatever traction is left over after making the necessary turning inputs -- assuming you have taken the line that lets you get through the corner the most efficiently (e.g., the line that lets you change direction without "wasting" any of your traction budget, like straightening out ess curves) -- for braking or acceleration as appropriate so that you are always on the border of the friction circle. However, in the real world, other factors may prevent you from actually reaching that "ideal." For example, you might go off the track despite accurately calculating the theoretical maximum available traction and relying on "real time" g-force and data sensors to ensure that you are not exceeding the calculated maximum available braking and acceleration forces if your suspension can't keep the maximum tire patch on the road. As such, the equation for available traction is not linear like the friction circle suggests.

It seems to me that the third dimension throws in the wild card idea of needing to ensure that you drive your car in a manner that allows you to get as close to the symmetrical "friction circle" ideal as possible, and your ability to do that will be determined in part by (1) your individual car's suspension (I imagine that some suspensions may handle tarmac irregularities better in a straight line than while cornering), (2) the track surface (bumps, defects, etc. . . ), (3) the geometry of the turn at issue, and, of course, (4) driver skill.

Once you take into account this "third" dimension, it might be that the friction circle actually becomes a friction "oval" with a shape that changes based on the particular corner and car at issue. I am not quite certain how to express this thought, but it seems that you might be able to exploit a non-symmetrical friction oval by manipulating the transition point between braking and acceleration much more than you could do so with a symmetrical friction circle.

All this goes to show only that (1) the fastest drivers are those who know how their car will react under any given circumstance and can adjust quickly enough to exploit that car's capabilities to the maximum, which is much easier to do if you know the track very well, (2) what might be right for one car in one corner might not be right for another car or corner, (3) DEs and Racing are not about perfect mathematical formulas and "calculating" the right way to approach driving, and (4) car control skills (i.e., feeling your car) are extremely important, and those who can feel what their car is doing sooner than others and have the skills and experience to adjust to what they are feeling will be faster. I obviously have a lot to learn, but am I thinking about this the right way?

Brian P

I think you are, TD.

The best way I've heard it described is that the "friction circle" does a great job of describing the limits for each individual tire. However, the simple fact is that we have to be concerned with the friction circles of 4 tires and maximizing the sum of those. The typical beginner/intermediate approach of deep, late braking maximizes the sum of the front two tires, but it wastes a lot of available traction in the rears. Keeping the car well balanced with smooth inputs ensures that all 4 tires can be maxed out.

Also, think about what the effects of deep, late braking are. As I mentioned, you are maximizing the friction in the front tires. This is great if you are doing straight line braking, but if you are doing heavy breaking into the turn, you are really using your front tires a lot and not doing much with your rear tires. When you consider that most Porsches are RWD, wouldn't you rather be getting the most grip out of the tires that actually make your car go faster?

JCP911S

Good discussion. Keep in mind that the cornering technique must be tied to the strategic position of each turn.

Turn 10 at Summit it THE classic Slow-in/Fast-out turn. An extra 1/2 MPH of exit speed will give you car lengths at the end of the straight. The key? Get the car rotated early, get the suspension set early and get on the gas early... which means light early braking is the key.

Big Bend at LRP is the classic Fast-in/Slow-out turn. The second half of the turn limits exit speed, so it is a throw-away. Cornering speed doesn;t really mean much here, so turn the first half ot the turn onto a braking zone. Take an early apex, brake late, and trailbrake all the way to the turn-in point of the second half of the turn. Disclaimer: This takes lots of practice and a delicate touch...

In both cases you act to "lengthen" the main straight... and speed down the straights is where low laptimes are created.

Bryan H

It has been my experience that most folks who trail brake allot over slow as a result of the weight transfer when going back to the throttle. The car doesn't feel like it will turn so they stay on the brake too long. They are correct, it won't stick in the corner beacuse the weight is changing from heavily front weighted to heavily rear weighted as brakes are released and throttle is applied. A few folks with a very good touch can make this happen correctly in some places but most can't. Slowing earlier and ballancing the car before it is turned will allow each tire to hold max corner loading earlier in the corner resulting in a faster total entry and exit speed. Since the car is ballanced it will feel more like slow in fast out. I believe trail braking has a place as a tool in the bag for certian situations but will be slow in a P car if used everywhere. Now if it were a Mustang that needs front weight transfer to turn....

Bryan

Z-man

Having just spent four wonderful days in a row at two tracks (Lime Rock & Pocono), and having had a few days to digest what went on inside my car whilst speeding along, I'd like to make some comments.

1. Entry attitude is vital. As stated by a few before me, I believe the attitude of the car at the entry of the turn is most important. I have finally seen the light! (thanks to Redlineman, Brian P, Bill Gilbert...and many others for your insight, patience and instruction...). In most cases, braking earlier is better. I am no longer part of the 'school of the late brakers.' Upon entering a turn, the car needs to be settled. By screaming into a corner, wacking that middle pedal as late and as hard as possible, and then turning in, the car is extremely unsettled: most of the weight is probably on that front outside tire that's screaming bloody murder! By moving back my braking points, and braking less, the car is more balanced upon entering the turn. And faster to boot! Now in racing, often folks will dive into a turn to either keep ahead of the pack, or to overtake a car in a corner. But that's apples/oranges.

2. Different apexes for different people. Pick one, and stick it! Yes, a late apex tends to be a safer way of taking a corner. Some argue that a late apex is also a faster way around a corner. But those are general comments, and such general comments work if all corners at all tracks were identical, and if we all drove the same car setup the same way! For DE events, if cones are used, they are often set up a foot or so later than what most consider to be an 'ideal late apex point.' An advanced driver should have enough seat time to determine what the ideal apex point is for his car and his driving style. Once that is determined, the goal should be to consistently hit that apex, and as Larry said, have the car in the correct attitude and pointed in the right direction. Turn after turn, lap after lap.

3. To throttle steer or NOT to throttle steer? At Lime Rock, I was using throttle steering around Big Bend to get my car pointed in the right direction at the 2nd apex. (I double apex that turn). I was also using throttle steering (just a slight lift) to get the car to turn in at the Left Hander that follows Big Bend. A friend of mine who just 'graduated' from the National PCA DE instructor's school pointed out to me that I didn't need to lift as I was doing in the left. He advised me to keep on the throttle and that the attitude of the car would be fine at the apex point. He was right. It's a no brainer: by staying on the throttle instead of lifting, I was able to go through that section at a greater clip. So: while throttle steering helps setup the car for the apex, and is a useful tool, sometimes there are better, faster ways to get the car pointed in the right direction. Why lift when not necessary?

All fun stuff - I'm really glad the DE season is upon us!

Sorry for rambling,
-Z-man.

Geo

Friction Circle theory is just that.... theory. The fact is, the "circle" is likely not a circle at all and more importantly, the edge of the circle is totally undefined. Sure you can assign a value to it, but it's just theory. It's really a theoretical point of reference.

The point of what I wrote above is that since it's just theory, don't get sucked into making more complex by worrying about 4 theoretical circles when one will do nicely, thank you very much. If you get caught up in dealing with 4 individual theoretical circles you tend to lose the real point which is overall grip. Yes you have to manage all four corners for max grip, but that's overall grip. Don't make it a circle jerk (bad pun intended).

JCP911S

I define "trailbraking" as applying the brake while turning the wheel. It can be hard or easy depending on what is needed for a given turn. It has two benefits:

First, it extends the brake zone allowing you to brake later without destabilizing the car.... so it is "free" braking.

Second, it smooths the transition from full brake to full cornering. Braking compresses the front suspension. Cornering compressent the outside front suspension. BY lifting off the brake as you feed in more steering you can keep the outside front suspension in full compression during turn in. As you continure to turn, weight transitions from the inside front to the outside rear as the car pivots arount the outside front tire.

IF you brake in a straight line. Lift off the brake then turn you have to compress the outside front suspension twice. So trailbraking helps stabilize the car.

Friction circle theory enters here at the tire only has 100% grip. If you use 80% for braking, you can only use 20% for turning, so as you dial in more slip angle you have to ease off the brake... 20/80, 40/60/, 50/50, 70/30 etc.

In practice, you simply coordinate steering and brake to balance the car on the outside front wheel... as long as the sum of brake and steering is constant the car will be absolutely balanced on the outside front.... so you just go by that.

Brian P

Completely agree. My point is that a person can make an argument that sounds really nice from a friction circle theory point-of-view (like heavy and deep trailbraking) and in reality, all that person is doing is reducing overall grip.

I believe that the reason overall grip is reduced is that they have effectively reduced the size of the friction circle on 3 of the 4 wheels, so overall grip (the sum of the friction circles) has gone down.

JCP911S

Stuff like Friction Circle are models to help you understand how all the factors relate and what the tradeoffs are. I think that helps you take a more thoughtful and logical approach to learning and developing your technique.

But bottom line, execution in still in your *** and your wrists and your fingers.... balance, eyesight and touch really count for alot in putting all the theory into practice.

Larry Herman

It's all about balance. When I am braking down to the transition point in a turn, I am balancing the deceleration with the turn-in. I want to slow just enough where the front-end will still turn-in and start the car rotating (a lot for tight turns, a small amount for mid-speed turns, and almost none for sweepers), but the back won't immediately try to spin. Then, depending on the turn, I am still on the brakes waiting for the car to rotate to give me the proper attitude towards the apex. At that point I pickup the gas to plant the rear tires and stop the rotation, and set sail for the apex. This whole scenario occurs in no more than a few seconds for each turn.

I find that if there is a chance for me to spin, it is usually in the beginning of the turn, where I am making this transition, and both ends of the car are at the absolute limit of their grip at the same time. Once I am under acceleration, it is usually just a matter of hanging on, and keeping the front of the car ahead of the rear, and not running out of road in the process.