{"id":520,"date":"2010-12-24T23:45:04","date_gmt":"2010-12-25T07:45:04","guid":{"rendered":"http:\/\/www.rhoadescamaro.com\/build\/?page_id=520"},"modified":"2012-08-01T22:11:22","modified_gmt":"2012-08-02T05:11:22","slug":"cvd-2-a-lap","status":"publish","type":"page","link":"http:\/\/www.rhoadescamaro.com\/build\/?page_id=520","title":{"rendered":"CVD 2 -An autox lap"},"content":{"rendered":"

In CVD 1 we broke down the seven different things we ask our car to do in a lap. \u00a0In this chapter we’ll fully break down a complex autocross run. \u00a0If you’ve autocrossed a lot and understand what goes on on a g-g graph, this would likely be review for you. \u00a0If not…<\/p>\n

While no two cars or tracks are the same, some analysis might answer questions like, “What percent of the time does my car’s ultimate accelerative capability really matter?” or, “What percent of the time does my car’s ultimate lateral grip really matter?”, “How much of the time is my car’s extra width hurting, or maybe helping me?” \u00a0While I’ve spent plenty of time looking at event and tuning data over the past several years, I hadn’t previously performed this sort of analysis on an autocross course before. \u00a0While the results weren’t exactly surprising, it provides a way to put into perspective what we’re really doing out there most of the time.<\/p>\n

A lot of regions suffer from tiny or poorly-shaped lots, and unimaginative, or even worse, over-imaginative course designers. \u00a0One of the great things about Nationals, is the courses are always very good. \u00a0I’ve been seven times, driven fourteen courses (well, thirteen since one was flooded), and they’ve all been excellent. \u00a0Even at National Tours or Pro Solos you’ll occasionally come across a real stinker course, but for Nationals there’s really a lot of care that goes into things. \u00a0The course I’ve chosen to highlight is the West Course from the 2010 SCCA Solo National Championships, which as of this writing, should still be fresh in the heads of any autocrosser that ran it. \u00a0The course was designed by Roger Johnson, who many believe to be the best course designer in Solo. \u00a0Roger teaches autocross course design classes and has written a manual on course design – you can download it here<\/a>. \u00a0While all courses vary, this is a superbly representative sample of the sort of course one would encounter at a good SCCA event.<\/p>\n

The driver is yours truly, in a Super-Stock prepared 2004 Dodge Viper SRT-10. \u00a0While the run is far far from perfect, it is nevertheless one of the ones I’m least embarrassed to share. \u00a0The video is a useful aid, as the perspective should be familiar. \u00a0On that note – the best video IMO is the one that shows both your inputs, and the view forward with a decently wide angle. \u00a0Views that don’t show driver input, or show input but not the course, are not 1\/10th as helpful as one that show both simultaneously. \u00a0As you’ll see here, things happen way too quickly to try to split it up on video alone, the data is the key.<\/p>\n

High res video if you’d like to download (24.5MB): jrho_2010Nats_day2_run1<\/a><\/p>\n

Youtube version: http:\/\/www.youtube.com\/watch?v=pvlCyiME_Pk<\/a><\/p>\n

Here to follow are several screenshots of the analysis being performed. \u00a0Below this is the first. \u00a0On the far left is a track map (collected via GPS data from my DL-1 data acquisition system), illustrating by color, the lateral acceleration (turns left and right) generated around the course. \u00a0To its right is a similar map, this one displaying a similar output for longitudinal (accelerating and braking) accelerations. \u00a0Further to the right, is our now-familar g-g diagram, again in a shape resembling an acorn, though this one keeps to its edges a bit better than our 240’s acorn in chapter 1.<\/p>\n

Sorry for the high res, if you’re looking at this on a small screen you may need to scroll around a bit. \u00a0As an IT nerd by trade I just assume everybody is running 1920×1200 double-wide \ud83d\ude42<\/p>\n

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At this point I only have a rough start and finish line mapped out, so we only have a time. \u00a0This is perhaps a good time to mention, the value of a data acquisition system. \u00a0For most people, a time slip is all they get after a run. \u00a0All the variables about their car setup, how they chose to drive, the cost of any mistakes they made – it’s all wrapped up into just a single data point – a time. \u00a0 While time is what really matters in determining finishing position, there is a lot to learn from taking a look at everything that happened during the run.<\/p>\n

Below is the same run, but chopped up into the seven events (acceleration, skidpad, transitions, braking, launch, entry, exit) described in chapter 1:<\/p>\n

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Here, I’ve taken the run and divided it into 23 sections. \u00a0At each break, the car transitions from one state, or “event”, to another. \u00a0The time spent in each sector is tabulated at the bottom, and summarized in the little Excel window on the right. \u00a0While I didn’t spend a ton of time splitting these things down to the single-pixel or microsecond level, it still provides a good approximation.\u00a0 For those that don’t wish to view the big image, here’s the totals by event, for 59.18 seconds:<\/p>\n

Transitions: 24.27 seconds, 41.01%<\/p>\n

Skidpad: 23.25 seconds, 39.29%<\/p>\n

Entry: 5.16 seconds, 8.72%<\/p>\n

Pure Accel: 3.67 seconds, 6.2%<\/p>\n

Pure Braking: \u00a0.59 seconds, 1%<\/p>\n

And if you further boil those down you get these:<\/p>\n

Primarily turning: 80.3%<\/p>\n

Primarily slowing down: 9.72%<\/p>\n

Primarily speeding up: 9.99%<\/p>\n

This wasn’t forced or planned, but it works out to an awfully neat 80\/10\/10 split. \u00a0Now, one thing that’s for certain, is these values will be different for every car. \u00a0Even within the Super Stock class, there’s a lot of difference between a Lotus Elise, a Corvette Z06, a Porsche 996 GT3, and a Dodge Viper, even though all four seem to run really similar times on most courses. \u00a0This same analysis done for the Lotus, would show a higher percentage of the time in the Pure Accel stage, and less in Transitions. \u00a0The reason for this, is the Viper accelerates better than the Lotus, and it’s bigger. \u00a0As such, it gets through acceleration zones in less time, but then has to spend more time at its limit of transitioning\/slaloming, because it can’t get through that portion as quickly as the Lotus. \u00a0 The reason why the Lotus can slalom more quickly than the Viper we’ll cover later in the Transitions chapter. \u00a0Some of the time we spend at the limit of lateral and longitudinal accleration, those are the really fun parts of the course we’ll get into more later in the Entry and Exit chapters.<\/p>\n

You may also note that some legs of Skidpad or Transition have elements of accelerating and braking to them. \u00a0It is rare that you get to spend much time in a steady-state corner or constant-speed slalom, but such times do exist.<\/p>\n

Here are a few of the individual sectors broken out, to highlight where the car is on the g-g circle while in that state. \u00a0We’ll start with the simplest, acceleration. \u00a0This course had a nice fun acceleration zone right out of the starting corner, which the Viper ate right up:<\/p>\n

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I’ve included some extra data here, a “values” chart. \u00a0You can see that in the 1.42 seconds \u00a0spent in this sector, the Viper picked up 17.4mph, going from 30 to 47. \u00a0Over on the g-g graph, the dark section of line is way up above the normal upper portion of our friction acorn, with the longitudinal acceleration peaking around .73g – that’s because we’re in first gear, and we’re not pushing the car too hard laterally.<\/p>\n

Next up is \u00a0transitions – I’ve chosen the leg immediately following our acceleration run above. \u00a0You can see how the g-g graph picks up right where we were at the end of the acceleration run, and takes us into the left-right-left complex of transitons to follow. \u00a0In this series there’s some accelerating and braking, but the big story is the swing side-to-side of our g-g trace.<\/p>\n

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Time spent in this section: 6.08 seconds.<\/p>\n

Next up is a braking zone. \u00a0There really wasn’t a great example for solid straight-line braking on this course, and that’s not unusual in autocross. \u00a0I’ve chosen a brief section about 24 seconds into the video-<\/p>\n

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This is a pretty boring section to look at, but in .59 seconds, the car scrubbed off 10.5mph, and did almost 1.1g of braking, even though it was also being asked to do 1g of lateral acceleration too. \u00a0This almost should qualify as a corner entry event but hey it’s the closest thing to straight line braking here!<\/p>\n

Next is a corner exit phase, coming out of the skidpad we broke in to just before. \u00a0The thing that makes this a corner exit, if you hadn’t driven it, is shown in the values table – lateral accel goes from 1.156 to .912, and longitudinal accel goes from .208 to .587 – that shows a tapering off of cornering, and a gradual increase in acceleration. \u00a0Although in the g-g graph you just see the bolded line, in a corner exit, it goes from one of the sides, up and back towards the center.<\/p>\n

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Later we did a partial skidpad. \u00a0One of the visually more tricky parts of the course, was a nice constant-radius corner of about 100 degrees, at least in the Viper.<\/p>\n

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2.73 seconds spent here, all of it right at the limit of what the car could do turning left. \u00a0A little acceleration but the longitudinal is centered \u00a0mostly around zero.<\/p>\n

Last one we’ll cover, since there wasn’t a good launch, and the one we had wasn’t on the clocks, is corner entry. \u00a0I’ve chosen the entry into the “showcase turn”, the last corner on the course for this illustration:<\/p>\n

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As the brakes are applied, the g-g graph swoops down to max braking, then works its way around the friction acorn towards max lateral acceleration.<\/p>\n

The following graphic attempts to illustrate what happens in each event on the g-g graph:<\/p>\n

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While this shows corner entry going from max braking to max lateral, it doesn’t always have to be that way; a corner where you only have to lift, might cause a sweep from max accel to max lateral.<\/p>\n

Entire books have been written on the subject of race car data acquisition and analysis, and all of this is just scratching the surface. \u00a0We haven’t gotten in to throttle postion sensor, steering angle sensor, brake pressure sensor, suspension shock position sensors, etc. \u00a0Long story short, there is a tremendous amount to be learned both as a race car engineer but also as a driver, from the data. \u00a0Some people out there are talented enough to win championships as drivers or engineers without studying their data; for the rest of us mortals, it is a valuable tool that can help us close the gap on the “aliens” among us.<\/p>\n

Let’s combine what we now know about how the car moves around the friction circle with a few other key points:<\/p>\n