Another Autocross Line Question

I believe that the key to skilled autocross driving - besides important things like looking ahead, proper braking, smoothness - that the ultimate key is driving the proper line on the course. Those other things will come with practice and experience. Finding the racing line, on the other hand, is more about theory, and will never be perfected without full understanding, either intellectually and/or intuitively.

So I come to the GRM collective with another question about the proper autocross line on the course I will be running this weekend, or just general thoughts on LINES in general.

I know it's difficult to pin down the correct line without seeing the course, however, a map can still help us talk about the general theory, which is what I'm trying to understand.

http://www.houston-bmwcca.com/wp-content/uploads/attachments/autocross/maps/2016/03-2016.jpg

^ Course map here. The green line looks mostly correct to me except for at the beginning. I'm familiar with the concept of late-apexing to set up for the next corner or to maximize straight away speed. What I don't understand about the green line is why you would setup for the left-hander in the slalom so far to the left, straightening the approach. Why not cut diagonally from the second pointer cone, straight to the cone wall on the right, then apexing the left hander a little later. Is it about washing out wide to setup for the slalom? Is it because smooth arcs albeit with more distance is better than less distance but with tighter arcs? How the hell can you tell which to do?

Does anybody have any thoughts about learning the line? I'm operating on little more than a vague understanding of late apexing.

Course map for ease of viewing...

I don't think the green represents the Best line, just A line. I mostly disagree with the green line at the end where I see no reason to drop down toward the left curb before finishing. Seems to me that staying high would be the shortest path and no speed to be gained by dropping down.
Instead, I think the green line at the end is tying to accentuate the fact that to exit the course you will need to hug the left curb but this move toward the left curb could happen after the timing light.

This looks like a very short course. I'll bet lap times of 30-35 seconds.

My thoughts exactly on the line at the end, JohnRW. Thanks for the embed. : )

Maybe I can frame the question differently. Regardless of the line on the map, what would YOU do?

I'm probably making this needlessly complicated. The real question is whether to approach the slalom wide to maximize speed or cut the corner, driving straight to the second slalom cone.

c0rbin9 wrote: cut the corner, driving straight to the second slalom cone.

Then it just becomes a lane change / chicago box instead of a slalom.

MCarp22 wrote:

c0rbin9 wrote: cut the corner, driving straight to the second slalom cone.

Then it just becomes a lane change / chicago box instead of a slalom.

That. Cut it short.

Then in the middle where the figure 8 crossover is, the green line swings out wide to make the left. Cut that short too.

My rule of thumb is always straightest and shortest line, but I never have a car with any real power.

You are asking a wide open question. You did not tell us what vehicle you will be competing in. FWD, RWD, AWD, horsepower, suspension, LSD all factor into choosing a line. Also, is the course a perfectly smooth surface with no drain grates, no cracking pavement, to off-camber turns, etc. Sometimes we choose a line not because it is the best, but it avoids gravel/sand being placed in our line by someone following a different line.

If you narrow your question somewhat and gives us some variables that you can control, we can offer some guidance.

I still don't think you can have a precise line discussion based on a course map only. It'll change when you walk the course.

The fast line (and setup, IMO) is the one that allows you to get on the throttle the earliest.

Two things I see.
1. This is the shortest course I've ever seen at the police academy. I know the layout is limiting, but I've never seen them use so little of the facility.
2. If you have to pick one corner to get right, make it the one before the big right hand sweeper close to the end. After you pass under the stoplight you have essentially a bent three cone slalom. You need to stay ahead on this and cleanly backside the cones. If you get late and struggle on the last cone, you'll really struggle to get the car pointed in the right direction into the sweeper. That will be a fast part of the course and getting that one bit wrong will cost you speed and time all the way through the finish.

RedGT wrote: I still don't think you can have a precise line discussion based on a course map only. It'll change when you walk the course.

I will keep this in mind. Be like water.

RedGT wrote: The fast line (and setup, IMO) is the one that allows you to get on the throttle the earliest.

This is helpful. To my thinking, getting on the throttle early means using the max width of the track to straighten out corners as much as possible, and probably late apexing.

mazdeuce wrote: Two things I see. 1. This is the shortest course I've ever seen at the police academy. I know the layout is limiting, but I've never seen them use so little of the facility.

Yeah, hopefully they give us lots of runs.

mazdeuce wrote: 2. If you have to pick one corner to get right, make it the one before the big right hand sweeper close to the end. After you pass under the stoplight you have essentially a bent three cone slalom. You need to stay ahead on this and cleanly backside the cones. If you get late and struggle on the last cone, you'll really struggle to get the car pointed in the right direction into the sweeper. That will be a fast part of the course and getting that one bit wrong will cost you speed and time all the way through the finish.

Thanks. This is the kind of observation I'd like to be able to make.

Just an observation. I have no idea why they stay so far away from the apex of that corner in the dead middle of the map, after the slalom. Seems it would be much faster to cut that curb.

Well, I think you want to backside/late apex the three-cone wall coming out of the corner to setup for the sweeper, which means going a little wider in the kink. Otherwise you'll be way behind on the entry to the slalom. THAT far out is probably excessive, but I can't see going all the way to the curb either.

EDIT: Hmmm, not so sure. I'm thinking it might be better to actually early apex the kink and allow the car to hook around the three-cone wall. Distance vs speed.

c0rbin9 wrote: Maybe I can frame the question differently. Regardless of the line on the map, what would YOU do?

What is my car good at? What is my car bad at?

Minimize the time lost in the disadvantageous spots in a way that plays to your strengths.

There is not "one" racing line that fits every car because every car is a little different. What car are you running and what modifications and tires does it have?

Also, especially on an autocross course, you cannot take the course and analyse it one corner at a time because you often have to sacrifice a corner to gain speed in the next few corners.

Also, the map doesn't tell you the road condition. Unless you have high-end Penske shocks, the bumps on the course can have a big influence on the racing line.

I am no help at all....

Knurled wrote:

c0rbin9 wrote: Maybe I can frame the question differently. Regardless of the line on the map, what would YOU do?

What is my car good at? What is my car bad at? Minimize the time lost in the disadvantageous spots in a way that plays to your strengths.

Dunno, just pick a hypothetical car and discuss the line for that car. I'm not looking for specific concrete details on what to do, more theoretical discussion.

FWIW, car is a 1997 Nissan 240SX. Automatic, bone stock, all season tires. The auto transmission probably means taking a wider than usual line on some corners if not downshifting to first.

In reply to c0rbin9:

Prior to the event(s), practice left foot braking and using the brake to build a little load in the drivetrain near the center or apex of the corner to help launch the car at the exit.

Use one of the advantages of the automatic to help you.

Manually shifting it usually helps prevent early upshifts as well.

One of the things I'm still working on is staying on the gas (all the way on the gas, mind you) until I absolutely have to brake, avoid coasting and you'll drop time from your runs.

With that car, I would try a very smooth line with the biggest radius you can get. All season tires really don't like to be over-driven so watch your entry speed.

And like Stefan said, shift manually to prevent an up-shift at the wrong time.

But I disagree with the left foot comment. In my experience, it's always slower on an autocross course especially for a RWD car.

It's just a line on a map. It can give you a vague idea, but until you walk it, you won't know for sure. Cones may not be as diagrammed. Pavement may have issues, turns can be off camber etc. You can overthink this stuff. Practice/seat time will cure most things.

In reply to fanfoy:

I should have clarified: The left foot braking practice (on the street) is to get used to using his left foot on the brake while using the gas as well as he exits the corners. Braking styles vary depending on the driver, car and course so feel free to try different things until you find what works for you and the car.

I agree that smooth is better with all-seasons. Pump up the air pressure to 40-50psi (whatever the max inflation pressure is for the tires) and look as far ahead as possible on the course so you can keep your inputs slower and smaller.

In reply to Stefan (Not Bruce):

More air pressure? I thought less air pressure would allow the tire to displace itself on the road more, giving you a bigger contact patch and thus more grip. Why run MORE psi?

Waaaaait a minute....240SX? Were you at the last SCCA event at the police academy a couple of weeks ago? If you were, I was your rookie chief for the day because the usual one was out of town.

Moreso than lines, I would say braking is THE area where most people can find time. It is the hardest part of autocrossing and the most difficult to master. Especially since most people think they do it correctly. After autocrossing for 10 plus years, I did a Skip Barber class and had an instructor totally change my braking. I still wouldn't say I do it well, but it dropped a large amount time.

Lines are difficult in that they can vary between cars. There are a couple of areas I might run a different line, especially around the "D", but until you see the actual course, it's just theory. All in all, it looks pretty good though.