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Gimp (Forum Supporter)
Gimp (Forum Supporter) UltraDork
5/4/21 2:34 p.m.

So, over the winter, I (in theory) improved my rear spoiler - https://grassrootsmotorsports.com/forum/aerodynamics-presented-nine-lives-racing/prepared-rear-spoiler-how-badly-am-i-compromising/176944/page1/

Well, the rules for the front of the car have changed, and despite the current price of plywood, I'd like to take advantage of the new rules.

So, here is what I currently have, based on the previous rules (lots more details here - https://grassrootsmotorsports.com/forum/build-projects-and-project-cars/1981-camaro-c-prepared-build/59501/page31/):

 

The most significant limiting factors that are no longer an issue are all related to size.  Previously, the splitter couldn't stick out past the bumper as viewed from above, and couldn't extend any further back than the opening of the wheel well.  The new rules change that, and add a few more options (emphasis mine).

The standard OE front spoiler or a non-standard front spoiler/splitter may be used. If a non-standard front spoiler/splitter is used it must comply with the following requirements: Shall be installed parallel to the ground (within ±3° fore and aft) and may extend a maximum of 6” (15.24 cm) forward of the front bodywork/fascia as viewed from above. Splitters may not extend rearward past the centerline of the front wheels axles. No portion of the splitter may be wider than/extend beyond the widest part of the
front bodywork/fascia from a vertical line drawn at the center of the front axles forward on the vehicle. The splitter and canards may have endplates.
The endplates may connect the splitter and the canard. The splitter and canard endplate total surface area is limited to 100 sq. in. (645.2 cm2) for each side. Canards are allowed and may extend a maximum of 6” (15.24 cm) forward of front bodywork/fascia as viewed from above. No portion of the canard may extend past the widest part of the front bodywork/fascia as viewed from above. Canard area will be measured in the same manner as wings using Section 12.10. Canard area may not exceed 1.2 sq. ft. (1114.8 cm2). Openings are permitted for the purpose of ducting air to the brakes, radiator, and/ or oil cooler(s); equal openings may be placed in the standard lower front panel directly behind openings placed in the spoiler/splitter. The spoiler/splitter may not function as a wing. This allows a vertical airdam/spoiler above a horizontal splitter.

So this leaves me with a few options and a lot of questions.

Question 1 - Front splitter profile.  Right now, the leading edge of my splitter follows the shape of the front bumper, because that was the rule.  Now, I can extend up to 6 inches forward of the bumper (more on that later), which opens up the shape of the profile.  I've seen a few different shapes, and I understand why some are like that, but I wasn't sure if there is an advantage to a particular shape.

  • Rounded - looks pretty and is doable within my limits
  • Geometric - gives more surface area, looks cool, and allows for more clearance when going around things
  • Flat/squared off - pretty extreme and can look either cool or weird.  Possible to pull this off, but I would have to do some measuring

 

 

Question 2 - Length.  The shape chosen will dictate some of this, but I'm now allowed to stick out 6" further than before.  on most modern cars, this would be a no  brainer, but as you can see the front of my car has a lot of "overhang" and already has a significant amount of "shelf" before the air dam, depending on where you measure (9" at the ends, and 11" at the center).  Does adding another 6" of "stick out" to that surface really do much for me, downforce-wise, at this point?

 

Question 3 - Overall width.  I can now be as wide as my front fender flare at the center of the wheel - that's pretty wide.  To me, going wider means more of a chance of hitting cones.  Is there a significant advantage to going much wider than I am now?

 

Question 4 - End plates.  I'm allowed to run splitter end plates, with a maximum surface area of 100sq inches (combined with any canards).  I know I want end plates, but I'm not sure on the size and placement.  I've seen a few recommendations of no taller than 4 inches - I'm great with that, but do I put them at the front of the splitter to gather the air there, or place them at the back, closer to the tire? 

What about end plates with a stepped design vs flat?

 

Question 5 - Canards.  Frankly, I don't even know where to start with these and really don't know if I should bother for this application.

 

The only thing I'm sure of at this point, is I will be extending the splitter further back to the wheel centerline to clean up the air there.  The rest is pretty open. 

 

I'd love to hear everyone's thoughts on where to go with this thing.  I'll be starting soon, as plywood isn't getting any cheaper.  Thanks!

stafford1500
stafford1500 Dork
5/4/21 4:45 p.m.

I am going to reread this post a few times and I will put something together that may help point you in the right direction.

stafford1500
stafford1500 Dork
5/4/21 6:17 p.m.

Let me see if I can get your questions answered/directed in the same order you asked them and maybe a few other ideas.

  • Question 1 - Front Splitter Profile: The profile you are going to want is the one that keeps you from hitting cones as you corner, gives you enough front downforce to balance the rear you have, and does not bottom out under braking or heavy aero load. The geometric shape is not too different from your current splitter at least style wise. The rounded version does not really give any distinct aero advantages other than it will more closely match the pressure profile ahead of the car. The flat/squared off version is going to be the path for maximum front downforce.
    • The rules also mention an angular tolerance of +/-3degrees. A nose down attitude will gain front downforce at high splitter gaps, but you want the splitter to end up close to parallel when you are at low splitter gaps, typically under braking. Low splitter gaps in most cases means less than 1" clearance.
  • Question 2 - Splitter length:  Since the new rules allow up to 6" of extension beyond the forward most point on the car, you are looking at the ability to increase the splitter area outside the car by ~350 sqin (includes the extra material from centerline out to the bumper corners being extended to the max). You are also going to be able to add ~300 sqin between the front wheels. That is a whole lot of area to work at even a fraction of a PSI of suction.
    • More length is going to mean more bending loads. you are going to need more bracing for any extra length. You may wind up with the corners getting worn down under heavy braking cornering. That would help drive your shape discussion above.
    • The extra length does two major things: it provides more surface area for the realtively high pressure on the top surface compared to the lower pressure on the bottom. This amplifies the effect and shifts the aero balance forward pretty significantly.
  • Quaetion 3 & 4 - Overall width & End plates: Increasing the width, like you mentioned will impact you cone contact cornering abilities. The area out board of the fascia is not going to be massively important unless you can get more fore-aft angle than allowed by the rules. You may want to consider the geometric shaping of the ends to the full width allowed and tie in the end plates and extending the wheel arch down to the splitter. This will give a large area of high pressure on the splitter and increase the extraction for the bottom of the splitter/wheel opening.
  • Question 5 - Canards (French for Geese): Canards or Dive planes can be very effective at creating front downforce, but are typically very high drag. They would have to be mounted at some pretty significant angles (~30deg from the ground) and need some curvature to keep the edge vortex spooled up on the bottom  surface. That vortex is very low pressure and is the source of the drag because you are working the air very hard away from it's lazy course of going around the car. They typically do not need to be mounted with the trailing edge much higher than the center of the front wheel, since the wheel opening shape angle starts to roll off above that area. The splitter endplate mentioned in #4 above will give a similar result if it is made taller than it is long. Dive planes are usually the first casualty during cone impacts! That leaves you with less downforce and parts to repair/replace. I would leave them off until you get to a point of limited front grip.

That pretty much covers your questions. Now lets consider some other things.

  • What is your balance currently? Balance is going to be the main deciding factor in how much more splitter you really need/want. If you need more front then extra splitter is the right direction. If you never have understeer, more front is not going to help until you get more rear grip. The extra splitter material on the back edge is going to provide you some more front. You may want to design a maximum area splitter and another that is half way between max and what you currently have. Try all of them at a test/event and see what you feel in the changes. A test and tune day would be great for this. You will also find out what the contact limits are for the current setup (you will probably find the corners get worn pretty badly on the max size).
  • Pretty much any adjustable feature you can build into a splitter will give you a way to tune the front downforce. If you can make the adjustment a sliding feature, you may be able to hot-swap differnet extensions for the tuning. Try to make those sliders part of the support for the extensions (the supports will also act as fasteners to keep the extensions attached to the car).
  • You did not see it here but, you may be able to take advantage of the high forces involved and use some spring loaded supports to get the splitter angle nose down at high speed (autocross relative) and returns to flat-ish at low speed. You have old valve springs right? This will be considered a moveable aerodynamic device, so you decide how deep you want to go. There are plenty of other ways to make a reasonably stiff flat structure move in ways to benefit downforce. Time for reasearch...
  • For fine tuning, use the same slider adjustment type of idea for the rear of the splitter. Since it is further aft, it will be less of a brute force change and move balance less for a similar downforce created at the front edge. 
  • You may want to consider materials other than wood for some of your splitter structure. Wood is a great material for those high wear areas, but fiberglass, metal and carbon fiber should be considered. They can be designed/tuned to flex in directions that benefit you (seems like I said that already...).
  • Leading edge radius. You want one. It needs to be pretty significant and kept clean. This edge sets up the flow for the entire under surface. If you can get a full 3/8" radius from the top to the bottom, great. If you have to give up on one side, sacrifice the top edge and get as much as you can on the bottom edge. The leading edge radius is less important as you get further away from the center, mostly from the inside edge of the tires to the outboard edge of the splitter.

That should give you plenty of reading/thinking material. Put you ideas back up here when you start to get a plan together.

Gimp (Forum Supporter)
Gimp (Forum Supporter) UltraDork
5/5/21 7:47 a.m.

Thank you. This is an incredible response with a lot of think about.  I'm going to try get some of those thoughts together and I'll report back, but I wanted to make sure to acknowledge your help first!

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