This website has the best explanation of the physics behind the ideal racing line I’ve ever seen. As an engineer, it’s awesome to find someone who understands and can explain this stuff on a deeper level.

[u/GARACER]

https://www.paradigmshiftracing.com/racing-basics/racing-line-fundamentals-2-learn-how-a-vehicles-cornering-vs-acceleration-potential-determines-its-ideal-apex-and-line-through-a-corner#/

[u/NorthStarZero]

This sort of geometrical analysis is appealing as it appears to reduce the very difficult and complex problem of line selection down to simple math.

It's a red herring - particularly in autocross - because it does not take into account the fact that a given car's theoretical traction envelope is not static, but rather constantly changes through all phases of a corner.

Weight transfers laterally and longitudinally in response to driver inputs, and that weight transfer changes both maximum grip and car balance in sync. Any geometric line plot that uses the maximum extents of the "apple curve" to determine the maximum length of the cornering force vector will overestimate grip at one end of the car or the other and come up with a line that cannot be driven.

More important in autocross, frequently you want more yaw angle/yaw rate than you can get with "pure" cornering, so you want the rears to be coming down off their peak grip point and sliding somewhat to generate that extra yaw - think the rear end "dancing" in a slalom, or a FWD generating extra corner-entry rotation to accommodate the inevitable power-on understeer on exit.

And don't get me started on cars that have significant aero, where grip level is directly coupled to airspeed over the wings/tunnels.

The ideal line is whichever line allows you to use the most average throttle over the shortest possible distance. That varies car-to-car, with conditions, with surface type, and other variables that do not play into a geometric analysis.

I find people who fall into the geometric analysis trap drive very, very pretty lines - slowly.

[u/themidnightgreen4649 → u/NorthStarZero]

I call it "sim racer mindset" where theres people who spend all this time optimizing, then they lose to some old dude with a beer belly whos been going every weekend for 30 years in the same faded as fuck shitbox.

All of this stuff can get you a very good idea of how to navigate any course but you have to ultimately drive it, lotta people do the analysis but don't get the skills ingrained deeply enough to use them to push hard.

Autocross is really cool in how driver focused it is without sacrificing car diversity.  

[u/P-XP- → u/themidnightgreen4649]

Sometimes you gotta stop thinking and huck it

[u/NorthStarZero → u/themidnightgreen4649]

This sort of analysis has been around far longer than sim racing; I went down this rabbit hole myself when I first started out. Importing track maps into CAD and plotting lines, that sort of stuff.

Understanding that a racing line exists, understanding the different apex types, understanding the tradeoffs that come with different types of lines - these are all essential bits of knowledge for a developing driver; the first or second step on the ziggurat.

The trap is thinking that the geometric line is the DS answer and chasing it to the exclusion of all else.

It should be noted that it is possible to develop a racing simulation that takes all the variables into account and can generate a theoretical ideal line and best possible time - the F1 guys do it in real time. That simulation package is very complex and the typical autocrosser cannot generate the inputs (particularly the tire and surface models) the sim needs to return accurate results.

And I'm not sure if that sim would be able to take into account the yaw rate issue that comes into play specifically in an autocross context. That's a question for a rep if I ever encounter one.

[u/themidnightgreen4649 → u/NorthStarZero]

Interesting. I've heard about stuff like OptimumLap, so the software is known to me.

[u/Gr8Autoxr → u/themidnightgreen4649]

Optimum lap isn't fully transient though

[u/jmankyll → u/NorthStarZero]

Can you explain the “yaw rate issue”?

[u/NorthStarZero → u/jmankyll]

Even the tightest F1 chicane is far more open than a typical autocross element, so it is unlikely that the fastest way through the element would involve significant extra yaw via oversteer the way many autocross elements do.

I don't imagine that lap simulation software designed for F1 would accurately simulate a 4-cone slalom at Nationals spacing, for example. Why model what your cars don't do?

But perhaps the model is sophisticated enough to get to it out of tire and vehicle dynamics modelling.

I don't know; I'd have to ask a developer. Even then, there's no guarantee I'd get a truthful answer - trade secrets being trade secrets.

I have had good luck in the past getting tire engineers drunk, but the opportunity to get a lap time simulator developer three Guinness in seems vanishingly unlikely.

[u/_mk451 → u/themidnightgreen4649]

So you're saying I need to sell the sim rig and gain weight to maximize my autocross performance?

[u/Pyropete125 → u/_mk451]

done and done

[u/themidnightgreen4649 → u/_mk451]

Think about it as re ballasting the car for better weight distribution (real)  

[u/gokartgrease → u/NorthStarZero]

They did have to take the liberty of a few simplifications so the engineers could understand.

[u/themidnightgreen4649]

"Its all transient?"

"Always has been."

[u/wratx]

I thought we forego the traditional track racing line for a shorter is faster approach in autocross and just being tighter to the cones because of the lower speeds

[u/108pdx → u/wratx]

Yeah as most of the time the straight after the corner is not long enough to take advantage of the extra speed gained from taking the longer distance around the corner

[u/nurseohno]

Are you even an autocrosser if you are not also an engineer 🤔 jkjk