Aero? Spoilers and Spliters.

Mostly, do I realistically need them. 

The car is the G35. It will see some track time in HPDE and TT. Realistic speeds in the 125 mph range at a guess. At the moment, it has nothing. Will aero do anything at those speeds? I can build a splitter to be removable, but I don't particularly want a huge spoiler on the back. How much is needed. 

Front. 

Rear.

One other thing. The under tray for the engine compartment is gone. Should I replace it. Will it help with air pressure building under the hood? 

I'll leave it to better qualified people to discuss mods, but definitely replace the undertray.

Any car that can legally run a splitter runs a splitter in their series. Do you NEED one for HPDE? That's a whole different question. 

According to steve, theres no application that DOESN'T benifit from aero other than agricultural. 

However, the benefits change with all the variables.

Step 1 is replace the factory stuff thats missing. 

Step 2 is ask steve.....

Definitely replace the undertray.  IME, they're usually pretty cheap from the dealer because they tend to get damaged a lot.

If you're competing and the rules allow it, then you probably need it to be competitive.  If you're just out there for fun, then it'll make you go faster, but if the car is still driven on the street then IMHO splitters are a PITA.  They hit on stuff in parking lots, they make the car ugly, and they attract unwelcome attention from people with colored lights on the roof.  They also tend to get significantly damaged with otherwise-minor off-track excursions.

What are you trying to improve with these potential aero mods? Higher top speed? Better high speed stability? Increased grip during cornering? 

There are members here with far more aero knowledge than me but this is what I think I know. In general, it seems like aero pieces usually require making a trade off. You may improve one aspect while hurting another. You should consider the types of tracks you'll run, and areas where you and your car will do well or struggle. Then you can choose whether you want to try and improve the areas that are already good, or sacrifice a bit there to gain improvement in the trouble spots.

In general, reducing drag will improve straight line speed and efficiency. Increasing drag can improve grip while hurting top speed and efficiency. Replacing the undertray should reduce drag, but adding a spoiler will likely increase drag.

So, if your G has power to spare on the long straights, but feels unstable in the corners, adding a spoiler might help the rear of the car feel more planted. If the car struggles to keep up on the straights, then adding a spoiler might help your exit speed onto the straight, while hurting your top speed near the end.

I am more interested in stability than I am absolute speed. One particular corner is of concern to me and that is the kink at CMP. A Z06 can go into it at 140 and while I don't think the G35 will be near that fast, it's going to be well above 100 mph. I've been through it in a Civic at 100 and it's rather pucker generating but a hoot when you do it right. I've also spun coming out of it at 100. That's extremely pucker generating. I don't want a twitchy car. If it costs me some speed to get it that's a sacrifice I'm willing to make. 

In reply to Toyman01 :

For stability at 125, I would be be hesitant to put a splitter on the front without a spoiler on the back. 

It has the potential to pull the nose down without the matching downforce at the rear, which can make it tail happy. 

Yes, aero can make a big difference at 125. 

Toyman01 said:

I am more interested in stability then I am absolute speed. One particular corner is of concern to me and that is the kink at CMP. A Z06 can go into it at 140 and while I don't think the G35 will be near that fast, it's going to be well above 100 mph. I've been through it in a Civic at 100 and it's rather pucker generating but a hoot when you do it right. I've also spun coming out of it at 100. That's extremely pucker generating. I don't want a twitchy car. If it costs me some speed to get it that's a sacrifice I'm willing to make. 

I know this corner well, I've spun through there in the wet with a Honda Fit.  One of the things to keep in mind is the undulation.

If you can find a factory piece easily, I'd replace the undertray... it should help you get better mileage, and will probably help cooling on the track.  While you're at it, I'd see about grabbing a "not perfect" rear trunk lid... that way you're less concerned about messing it up as you experiment.

A rear spoiler will be the easiest thing to add.  Yes, it'll create more drag, but that's probably a reasonable trade off to make at a track like CMP.  VIR, will have higher speeds, and so might not need as larger a spoiler to have the same stability.  With a secondary trunk lid, I'd fab up something relatively short and angled ~20-30deg "laid back" from a vertical reference line drawn from a horizontal reference defined by the wheel hubs.  With that you can add shorter/long "span" spoilers (i.e. side to side dimension) and shorter/longer height to tune the rear downforce as you learn what you like and easily account for any front end downforce you might add.

If you can't source an undertray, then it'd probably easier to fab up a plywood splitter for the front.  Check out klodkrawler's e46 thread for inspiration.  Just keep in mind, that you'll need at a minimum a spoiler to balance it out, and the rake of the car front-back will impact downforce balance front-rear.

The main difference between spoilers and wings is that wings can create comparable rear downforce for a smaller drag cost.

edited: to add klodkrawler's thread linkage

There is a company that makes a aluminum undertray. It's on the way. It should also help reinforce the front bumper and give me a rigid place to mount a splitter should I go that direction. 

In reply to Toyman01 :

I've gone through that kink at 110 (indicated, maybe optimistic) in the LTD. I lifted. I'm a pansy. I also didn't spin out. It was scary even lifting.

Dusterbd13 said:

According to steve, theres no application that DOESN'T benifit from aero other than agricultural. 

However, the benefits change with all the variables.

Step 1 is replace the factory stuff thats missing. 

Step 2 is ask steve.....

Dusterbd, thanks for the confidence in me...

ToyMan: Before you do anything beyond replacing the splash shield with your new aluminium undertray, you need to find out how balanced the car is at speed.

The effects of even small aero bits at 120mph can be significant, in both drag and balance. Most people will not feel a change in downforce, but the change in balance can be felt at really small increments. If you are trying to build the car for track days as a secondary to daily driver duties, use the secondary decklid mentioned a few posts up if/when you decide to add rear downforce. The spoiler size height/width can be easy tuning tools, but you can also adjust angle to make smaller/easier at track adjustments. If you lay the spoiler all the way back (level with the deck lid, Pro-Stock sort of spoiler) the downforce/drag impact will be pretty small, with a fair balance change.

Make a splitter that can be replaced/removed easily with telescoping tubes and cross bolts at the front fascia. Dive planes can make short work of balance adjustments, but are draggy.

My estimation is the corner you are concerned about would prove to be loose in this car. Plan to add rear before you add front.

Later, Steve

In reply to tuna55 :

If you came out of 8 correctly in the Civic you had to lift or it would hit the Rev limiter in the kink. Like in the middle of it. That's bad in a fwd car that is already at its limits. Very exciting. Almost enough to make you wash your race suit. 

Without a beefy wing or diffuser out back, just a front splitter is likely to noticeably unbalance the car. Honestly, for a mostly street car that's going to see occasional track time I wouldn't bother with building a bunch of removable aero.  

You aren't going to win anything in HPDE and that car isn't going to be competitive in any TT class without a host of stuff that's going to make it suck on the street. 

If you're worried about the speeds through that particular corner, I'd just back your speed down a bit.

The one thing a spoiler is good at is not stalling when you get a bit sideways - like a G35 wants to. If you run a wing, go with more area and less angle of attack (6* instead of the peak at 12*). A lot of people make spoilers from Lexan so that they can be seen through, but it'll be heavier than an equally stiff one made from aluminum.

If the primary concern is not spinning, I wonder if a suspension solution is the way to go. Is your suspension stock? Have you considered making a tuning change eg. Change a sway bar ?

If I was in the market for spoilers, I would want something developed for my chassis by somebody who had the time to test it and prove it out, and came with a front splitter that was proven to balance out the rear downforce.

With my skill set, the "easy" cornering pickups come from adjustable damper rebound and camber at both ends, and usually more spring rate.  15% of rebound either way for me makes the difference between a car that will turn and a car that will slog.

This thread gas sent me back down the rabbit hole of research of miata aero for the challenge car.....

So, thank you for the diversion!

In reply to Dusterbd13 :

yeah, there's a lot more public info out there on miatas

I cracked my Katz open, in part because it was right there under my keyboard... and was reminded of the quote:

"rear spoilers are highly dependent on the shape and distance from the rear window"

unfortunately, I... a) don't have Hucho, b) all the data in Katz is pretty specific based on tests from SAE papers (which I don't have access to)... so it'd be a little difficult to extrapolate it out "paper napkin style" for this application.

i had a decent sized decklid spoiler on my car and it was mostly neutral handling but understeering at high speed, (120). i designed, built and installed a spiltter and that helped at 120+, but then it started to oversteer on low speed (40-60) corners.  i went back to the drawing board and built a wing, and installed that as well. the car is much better now. braking got better. splitter and wing angle are both adjustable and i have not yet started to tune that. but i am now considering a diffuser and dive planes based on next seasons results with aero tuning. 

JBasham said:

If I was in the market for spoilers, I would want something developed for my chassis by somebody who had the time to test it and prove it out, and came with a front splitter that was proven to balance out the rear downforce.

The problem I'm running it to is 90% of the stuff available for this chassis is, Yo, Hawt Carbon Bling Splitter Dude!!. The G35 definitely attracts the form over function crowd. Separating the wheat from the chafe has been fun. 

The factory wing/spoiler on my ZX2SR seemed to keep things stable in it's performance range.

Blocking off the grille as much as possible may have helped too.

The aero I run (typical wing/splitter combo) might be considered extreme by HPDE standards and tame by Time Attack standards.

Prior to running aero I expected to feel some massive difference at high speed corners but it's less like moving from ok summer tires to 200tw super sticky summer tires and more like making a shock or swaybar balance adjustment.

The car is absolutely faster with the aero (to the tune of around 2 seconds) but it's more of a thing where you go "huh, last time I drove here without aero I was sort of chasing the back end, now it's stuck"

Similarly, the handling was about perfect at our last event and we lost the splitter in day 1 due to an off. The car was still managable without a splitter on day 2 but it was a bit pushy on high speed stuff, the corners where I'd normally be able to just lift off and turn in required a bit of brake to get enough bite to turn in.

I'd guess that you could probably fab up a basic splitter and mount from home depot racing for around the same cost as that aluminum panel. There are lots of ways to do it and how I did it is already shared above. I'd 2nd the idea of getting a 2nd trunk if you decide to do this. I'm not sure how the G35 is but on the e46 loosen 2 bolts, remove 2 more and the trunk is off. Swapping between trunks is a 10 minute job. might be worth considering so you can keep a street trunk and a race trunk. The splitter could stay on full time without much issue as long as you don't make it extend forward too much.

Oh one other note, If you spend some time building a splitter out of plywood there's a fairly good chance it will be far more robust than the factory underbody cladding. Our splitters (I believe we're on #4 now) have survived some very heavy shunts going off track and protected the much more valuable things above them. The flimsy plastic undertray BMW provides would've been destroyed.

AnthonyGS said:

Good call on the undertray.  If you dad add aero keep in mind wings work a lot better than spoilers.  Spoilers add drag and a smidge of downforce.  By comparison wings add scads of downforce and a smidge of drag.  You don’t need math, wind tunnels and a lot of conjecture to figure it out.  Chapparal figured it out from the humble surroundings of Midland, TX.  My dad crafted very effective aero parts in the shop without the aid of a calculator or wind tunnel too.  I used to put rear wings on slot cars when I was a kid too amd they work great.  That was long before I learned about Reynold’s numbers too.  

I wouldn't testify in court, but I think you are mixed up on this one. The way I understand it is that spoilers reduce drag (and reduce lift, not generating downforce), and that wings add lots of downforce at the expense of drag.

That's why F1 cars run much smaller wings at high speed tracks to REDUCE drag and increase top speed. Then at slower speed tracks like Monaco run the highest downforce package (largest wing) of the season to create as much downforce as possible, ignoring drag, because it's a much lower speed track.