Presented by Nine Lives Racing
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9/24/22 10:30 p.m.

I've been lurking here for a while, mostly following along with the LS1 Kart build. I have my own Kart based on a C4 Corvette. I autocross and do HPDEs with the car. The car is just under 54%/46% weight front/back so downforce in the back is important. The bottom of the car is pretty flat with the exception of the drive shaft tunnel between the front and rear wheels. I'm considering building a rear diffuser to get more downforce. Considering that the bottom is flat, and doing some search on how diffusers work, my understanding is that built properly it should do something...but I'm also fully aware of how ugly the airflow around the top of car is. 

Even if the air above the diffuser is extremely turbulent, will the principals of a rear diffuser work as long as the airflow below the car is relatively clean?

cheese New Reader
10/26/23 9:03 p.m.

I couldn't find my old account info so here I am with a new one. As the weather has turned moist here in the PNW, it's become apparent that my Polestar 2 has some weird aero quirks based on the most lo-fi flow vis of seeing how water drops run off during and after driving. I can't find any studies or cfd on this car so I'm turning to the like-minded dorks at GRM to help me sort this out.

1, it seems that there's a separation edge at the rear seam of the glass roof, so the rear glass doesn't stay clean while driving. With no rear wiper this is a pain. The shark fin seems to fully detach the center section, and flow appears to slip over the angled C pillar creating a long dead zone with an attached region between. The decklid also seems to be fully stagnant as water drops can be present even after a run at freeway speed. Would VGs around the orange locations help keep flow attached down the rear glass and potentially onto the deck lid? 

2, the rear camera is fixed and exposed right below the bumper edge, above the license plate. It suffers from rear soiling and collects dirty water rendering it useless. After a whole lot of amateur research about rear soiling and wake turbulence, it seems like the only way to keep this area clean is to feed it clean air. The Polestar 2 has a mostly flat underbody and relatively clean, flat diffuser, but very little in the way of aero devices to control wheel wake— it doesn't even have front wheel air curtains. Should I look at creating some fences or strakes in the underbody to direct cleanish central air up or something else? 

I'd like to minimise the wake of the car overall to keep drag low and maintain range. Unplugged offers a 'high efficiency' longtail spoiler for a couple Tesla models which have a similar rear glass and decklid shape:

They even have colours for directors!

Sadly I can't find anything similar for the P2 and wonder if it's because the inherent aero design places the entire rear of the car in a wake zone. 

What say you, GRM? 

stafford1500 GRM+ Memberand Dork
10/27/23 6:13 a.m.

For the rear glass clearing, I would start by taping the seams across the roof and rear glass as a first step. Use something thin like packing/cellophane tape. If that is better you may be able to fill the seams with a silicone or rtv. Adding vortex generators would be a second test. They will add drag locally, that's how they generate a vortex. The rear antenna could be smoothed at the base with tape to see if that fixes the center dead zone (that's probably the one that you notice most). The outer dead zones are likely tied to the subtle roof rail edges. Trying to reduce any lateral steps on the back third of the roof should have an impact on the outer dead zones.

For the rear facing camera, there is no good solution other than adding a dedicated duct/hose/blower across the face of the camera. The entire rear fascia is in a turbulent low pressure wake zone and at centerline the camera may actually have flow directed straight at it from the rear in most driving cases. The wake rolls around the sides, deck, lower bumper and tries to fill back into the camera area, with pulsating vorticies. Look up VonKarman vortex street for more info on that.

The extended deck spoiler may help with overall drag reduction, but the camera fouling will get worse.

camopaint0707 Reader
10/27/23 7:12 a.m.

OP do you fact check the stuff johnny says?

cheese New Reader
10/27/23 1:28 p.m.
stafford1500 said:

start by taping the seams across the roof and rear glass

that makes sense and is an easy start point. I don't know that I can do much about the angled C pillar edge, there isn't an obvious chine there or anything but there is a kink line in the body metal rather than a curved edge. 

I figured a duct for the camera would be the only effective option, so I may investigate if there's space above the diffuser/inside the bumper for a duct, and a spot within the bumper itself for the outlet. The latter can probably be a 3d printed nozzle and maybe use a commercially available NACA inlet in the diffuser itself. Not the ideal source but in a passenger car perhaps the only unobtrusive one.

Tom1200 PowerDork
12/28/23 11:32 p.m.

So I am trying to get the last bit out of my Datsun I vintage race.

The CD is a horrendous .42.

My thoughts:

Drop the side mirrors.

Air dam; I have a cheap flat piece of plastic. The lower valance curves down and back. My thought is the air dam should curve down as low to the ground as possible within the rules.

Undertray: the rules only allow for engine cover/tray. The currently lacks one. Will adding one make any difference?

Radiator Opening; would fairing the sides of the opening help or is the air simply tumbling through the grill so it won't matter?

Finally the I have the vent openings at the base of the windshield wide open; does this help, hurt or doesn't matter?

QuasiMofo (John Brown)
QuasiMofo (John Brown) GRM+ Memberand MegaDork
5/3/24 8:23 p.m.

FWD aero. 

Patient is a 2012 Chevrolet Cruze 1.4T GRM $2000 Challenge car currently undergoing a severe weight loss program. Original weight was 3100, target weight is 2250.

Car will be 1.5" lower than original with spring rates at about 475 front and 300 rear, running 225/50r16 tires (200tw). Improved boost control, E85 and bigger injectors plus an aggressive tune will move it around the course. 

My vision initially included a large splitter and an adjustable wang but it dawned on me that the rear may be able to utilize a simple adjustable spoiler. Bonus I have a pallet of .190 aluminum and polycarbonate that failed quality inspections and I picked up for about $50 at our outlet store. 

What target size for splitter and spoiler should I start with?


Poor_sh GRM+ Memberand New Reader
5/27/24 2:39 p.m.

Ok so, I'm running a beat up 99 miata which has a few gaps between panels/fenders and especially the front of the hood and top of the bumper cover. Is it even worth adding aero pieces? I'm sure tape can do a lot for gaps right, but is it enough?

sleepyhead the buffalo
sleepyhead the buffalo GRM+ Memberand PowerDork
5/27/24 5:38 p.m.

having panel gaps is primarily going to generate drag (yes the turbulence isn't going to help... but it'll be a minor impact), which is the functional equivalent of saying "I don't think my car has enough power for aero".

Professional Awesome found ~2sec on a ~2min lap with aero, without any optimization.  ymmv, but I've seen 2sec referenced a number of times, and the side benefit of the car being/feeling more planted/stable.  so, if the rules allow it, and your budget can afford it... yes it's worth adding "aero".

KyAllroad MegaDork
7/17/24 3:23 p.m.

In the search for cheap aero I'm repurposing a big carbon wing with wicker bill onto the Zoomboni. 

Given that drag at autocross speeds isn't really a problem how aggressive can I get with the wing angle for maximum downforce?


 Allso, is there any research showing that spoilers (like CSP builds) function as more of a "droge chute" and work more to keep the rear of the car behind you than particularly pushing it down into the pavement?

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