let's talk aerodynamics

[strat]

I have aspirations to run my Cavalier at a track day this year but I've noticed on some of the faster autox courses I've run that the rear end can be a little twitchy at speeds.

I know that the three basics are splitter, diffuser, and a rear spoiler.

just wondering what aerodynamic mods you guys are running to help get the car stick to the track.

[evomike]

Fix that in suspension

[Nick 95 6sp]

what have you done with alignment and corner balancing so far?

[strat]

car has -2.5 degrees in the front and -1.5 in the rear. no toe or caster adjustments as there is no way to adjust either on the cavalier.

I have not had the car corner balanced as I have not been able to find a local shop to do it...

[jpalamar]

I would do suspension/brakes/tires before any aero mods to a Cavi... I highly doubt your going to hit high enough speeds with it to make that much difference.

[jeffroevoix]

what tires are you running?

[oneday]

Originally Posted by strat

I have aspirations to run my Cavalier at a track day this year but I've noticed on some of the faster autox courses I've run that the rear end can be a little twitchy at speeds.

I know that the three basics are splitter, diffuser, and a rear spoiler.

just wondering what aerodynamic mods you guys are running to help get the car stick to the track.

You do not need any aero to help your car stick--unless it it a fully prepped SMF car. If you do put a wing on the car it will have to be bigger than this to do any good

...because that wing produced somewhere around 100 lbs of down force at 60 mph (if memory serves me correctly). And, you will have to have uber-stiff suspension to make use of the added downforce.

[the dual element wing on the car in my avatar produced 50-75 lbs of DF at 40 mph and 150+ lbs at 70 mph--but was also aided by a splitter and sleek shape of the Mclaren inspired body work]

Like others have already said, check your suspension components and alignment.

What spring rates are you running? Shocks? Sway bars? Tires?
Define "twitchy." Define "a little". And define "at speed".

Under what conditions is the rear "twitchy'? Braking, corner entry, corner exit, steady state corner, or a combination of these?

[strat]

Originally Posted by jpalamar

I would do suspension/brakes before any aero mods to a Cavi... I highly doubt your going to hit high enough speeds with it to make that much difference.

the car is superchaged with tein ss coilovers/strut tower bars/sway bars/upgraded bushings. brakes are baer 13" fronts/12" rears. it's not your typical cavalier.

I would expect the car is easily capable of triple digit speeds on the straight @ Watkins Glenn. the car has hit 65+ mph at the CPR autox events @ mid state airport.


oneday... car is currently running SM. going through a high speed slalom, the rear end started to over rotate. I know that there likely will not be transitions like that on a road course but I'm just looking at my available options as more speed won't necessarily require as hard a transition to likely end in the same result. (i know low speed downforce = BIG wing but I'm more concerned for 70+ mph speeds that the car should have no issues hitting on a road course)

spring rates are the standard tein ss for the j-body. 336 front/224 rear I believe... limited to what's available based on what's offered for the j-body but I am ultimately looking to upgrade to a more "sporting" option but it won't happen before the track day I hope to run this year) tires are hankook k110 v12 evos 245/40/17's on 17x8 fn01rc's. (which i'm looking to step up to the rs3's) car has addco 1" sways front and rear. control arn bushings/torsion beam bushings are poly and the chassis has some bracing (subframe brace, front/rear strut tower bars)

ultimately, i'd like to set the car up for track days and get a miata to autox.

[RedBowTies88]

if you're unstable at any speed >150mph its your suspension that needs addressing not aerodynamics.


actually at any speed in a strait line areo should be completely unnecessary unless the shape of the car itself yields lift...

[oneday]

Originally Posted by strat

going through a high speed slalom, the rear end started to over rotate.

That can be fixed with changing roll stiffness in the rear--either with swaybar changes or tire pressures. See more below.

Originally Posted by strat

I know that there likely will not be transitions like that on a road course
but I'm just looking at my available options as more speed won't necessarily require as hard a transition to likely end in the same result. (i know low speed downforce = BIG wing but I'm more concerned for 70+ mph speeds that the car should have no issues hitting on a road course)

If you get into that kind of transition on a road course where you are in a tank slapper you did something terrible to induce it and no amount of DF would have saved you.

Originally Posted by strat

spring rates are the standard tein ss for the j-body. 336 front/224 rear I believe... limited to what's available based on what's offered for the j-body but I am ultimately looking to upgrade to a more "sporting" option but it won't happen before the track day I hope to run this year) tires are hankook k110 v12 evos 245/40/17's on 17x8 fn01rc's. (which i'm looking to step up to the rs3's) car has addco 1" sways front and rear. control arn bushings/torsion beam bushings are poly and the chassis has some bracing (subframe brace, front/rear strut tower bars)

You are too stiff for proper handling with that car. The rear tires can't get enough weight on them to keep them planted. I would suggest getting to a test and tune day and undoing the rear sway bar and then tweek the tire pressures accordingly. Getting better tires (the RS3s) would certainly help too.

Originally Posted by strat

ultimately, i'd like to set the car up for track days and get a miata to autox.

Why not just get a Miata and set it up to do both? The Miata will be much more fun on a road course and probably more economical too.

[strat]

I doubt the miata will be more economical. I supercharged my cavalier for 1500. i'd be looking at 3 times that to supercharge a miata. ultimately, i've had the car since new and I still have alot of plans. I hate to leave a project unfinished.

i'll have to play a bit with suspension setup... i would have never imagined that somebody would tell me the car is too stiff with such low spring rates. keep in mind the rear sway bar is more of a torsion beam stiffener than a true sway bar. (the center of the bar actually sits up in the torsion beam, not mount to the frame)

I do have alot more plans for the car that should help make it a better handling car (irs out of late model n-body with a custom coilover setup) but that's down the line.

[oneday]

Originally Posted by strat

I doubt the miata will be more economical. I supercharged my cavalier for 1500. i'd be looking at 3 times that to supercharge a miata.

You never mentioned you had to s/c the Miata...speaking completely from a consumable standpoint, Miatas cost very little to track...very light on tires, brakes and gas.

Originally Posted by strat

i'll have to play a bit with suspension setup... i would have never imagined that somebody would tell me the car is too stiff with such low spring rates. keep in mind the rear sway bar is more of a torsion beam stiffener than a true sway bar. (the center of the bar actually sits up in the torsion beam, not mount to the frame)

It's not that the spring rates are necessarily too stiff, but when combined with suspension travel, motion ratios, friction circles, and roll stiffness...they are not effective.

Autocross Secrets

Get someone with more experience to drive your car. See how it handle for them. I've driven a few people's cars that complained of poor handling, and it was mostly due to poor technique.

[strat]

I was talking more along the lines of building the car to where I would like it to be. parts tend to be a lot more expensive for imports vs. the cavalier. (granted the selection is not as large...)

I have already talked to one of the top drivers in my region about driving the car. I plan on having him take it for a spin once I get my new clutch/flywheel/lsd installed. current clutch is on the way out.

[strat]

also, took a look at the autox site you posted. seems counterproductive though to set the car up for oversteer on corner entry for road course. I would think the car should be setup more neutral so I'm not trying to catch the rear at high speeds in corner entry.

again, trying to set the car up for a track day which was what brought about the whole aerodynamics thread to begin with.

[oneday]

Originally Posted by strat

I was talking more along the lines of building the car to where I would like it to be. parts tend to be a lot more expensive for imports vs. the cavalier. (granted the selection is not as large...)

That's part of the beauty of Miatas...the shear number of them on the roads and tracks make parts super easy to find and cheap to buy. Plus the wealth of knowledge about set up and upgrades makes making them fast (relatively so) and handle amazingly as easy as searching the Googles.

Originally Posted by strat

I have already talked to one of the top drivers in my region about driving the car. I plan on having him take it for a spin once I get my new clutch/flywheel/lsd installed. current clutch is on the way out.

Good to hear...a true LSD will also help handling.

Originally Posted by strat

also, took a look at the autox site you posted. seems counterproductive though to set the car up for oversteer on corner entry for road course. I would think the car should be setup more neutral so I'm not trying to catch the rear at high speeds in corner entry.

Sure, which is why I suggested disconnecting the RSB...:-)

Originally Posted by strat

again, trying to set the car up for a track day which was what brought about the whole aerodynamics thread to begin with.

Adding rear DF to a FWD car is tricky, especially considering the archaic and rudimentary rear suspension the Cav has.

[strat]

i'd be looking at doing front splitter/diffuser/rear spoiler not just adding downforce to the rear but again, just weighing my options. I'd shoot for balancing out the downforce as much as possible. (I'm also not sure what the car is going to do past triple digit speeds as I don't like to drive that fast on public roads... i'm guessing the front end will get light cuz let's face it, it's an economy car that's governed to 105mph in stock trim)

archaic is an understatement... that's why i'm looking at ultimately ditching the torsion beam rear and going IRS. sadly, my attachment to the car is part of the reason that I'll buy a miata as a second car to play with. part of me doesn't like to see a project go unfinished and I do still have alot of plans for the car. part of me likes to play with a platform that doesn't get alot of serious attention (who doesn't like wings, mufflers, and altezzas). and still another part of me goes wtf are you still dumping money into this car. LOL

unlike some of my buddies that run autox, i'm not afraid to have a better driver give me tips on car setup and driving technique. anything to help improve my setup, ability and times.

[oneday]

I think the point that most started with still holds true, regardless of your desired intentions: Get the suspension sorted before you dive into wings and things. Besides the inconvenience of adding DF (aero can be a bit much for getting around in the world [speed bumps, sidewalks, driveways, etc]) DF really is a dark art and can cost $$$ to do right.

On top of that advise: Are you planning on doing anything competitive with the car besides the occasional AX? If so (and even then) what are your class limitations for aero?

[strat]

I run SM as is so I think it's pretty well open but I'll have to double check the rulebook.

I plan mostly longer autox courses (like midstate airport in the cpr region) and the occasional track day. that would be after I aquire a miata though. until then, the car will be my autox car which I run for points in the nepa region.

[strat]

looks like splitter, rear spoiler, and cannards are legal in SM (with surface area and mounting restrictions)

most local events are pretty leanient on the rules... now if I decide to run a national event and place high, I may run into issues. either way, from the quick glance, i believe that any plans I have would fall well within the rulebook limitations.

i still plan to do some more sorting with the suspension first though.