what angle to build splitter?
 

Im getting ready to make a fiberglass molded splitter for my IT-s FC this weekend and wanted to get some advise about what angle to build it at relative to the ground. The splitter will not be adjustable in the end, and I pretty much have to get it right the first time.

thanks for any advise

On a non-adjustable splitter I would think the angle should be positive rake at static ride height. How much positive rake would depend on how much rear squat your car will see when accelerating as well as how much rear squat under top speed. The object would be to make sure you never create lift with the splitter.

Scientifically you can use shock travel indicators on the rear during some high speed runs and accelerations out of corners. This would tell you haw much squat you will see. Non-Scientific way would be to remove your shocks and springs from the rear and lower the car with a jack until you reach bump stops or maximum squat and then mount the splitter horizontal to the ground.


-billy

What are you trying to accomplish with the splitter? Downforce, or just better air control through the coolers? IF it is the latter, just make it parallel to the bottom of the nose of the car.

whts up....i have a fiberglass spliter on my car and i belive u can either get from van stenberg or speed source..

heres a pic of my Rx-7

thanks guys for the info, I am attmpting to make a splitter to both direct air, as well as create some downforce and clean up the airflow under the car a bit. Most of the pro touring cars seem to be running angles of 8-10 degrees, and as they see higher speeds than I will, I was thinking of runing something in the neighborhood of 10-12.

What do you guys think?

damn..on a side note I just realized that this is quicklly becoming a Florida discussion..Im probably 5 -10 minutes from Miami lakes

if your tht close 2 miami lakes pm so you can come by and check out my set up on my car

we have been working on a Carbon Fiber splitter for the FC that fits a s5 and s4. This gives 8 inches of air that would have passed under your oil cooler. Should be for sale around december ish for about 100 bucks. Most of these splitters and lips are just for looks and do nothing for performance

While I'm certainly no engineer, I don't think I would go that aggressive with the front splitter. That is more drag to overcome. We don't have nearly the HP or Torque of the Pro Touring cars. We also don't have as much top speed potential to require the downforce.

Another point, the greater angle of attack will require a stronger splitter as it will be subject to increased loads. You will not be able to simply run a few bolts into the stock holes and clips in the bottom of the front bumper cover.

If you are building your own splitter, plan on using it to a cooling advantage also.

Jim

I use one that is parralel to the ground, it creates alot of DF, before my wing, you could see the cars attitude(angle) change on the straights. It is quite large, but the point is agree, not to go "too" aggressive. My car is in an unlimited class, and the spliter is part of a partial flat bottom. Thwe wing, and splitter in combination reduced my top speed on the straight at thunderhill by ~5-7 mph. This is in a 450+ rwhp car @ ~130 mph. Carl

so If I toned it down a bit and still went with something around 8 degrees. so you guys think it would be too much. I was essencially trying to create something that was more or less flat, however I dont want it to work against me as the rear squats.

Bukwild, that looks great by the way..is the nose CF too? keep us informed on when these go on sale

Here are some pics of mine. Double click the images to view full screen. http://www.teamfc3s.org/forum/member...tos&car_id=562

Profile is essentially the same. I have since learned that I should not have made the cut in the center. The reduction in surface supposedly dramatically decreases DF. I will be testing this next season with a larger splitter w/out the nice looking cut-out. The one on my car is a sheet of AL, and goes all the way back to the front crossmember (about 4 feet) i then have vertical returns wich direct all the airflw through the FMIC, and coolers. Carl

df,
Remember, you will already have approximately a 2-3 degree rake to your car anyway. Now, add 8 degrees of spoiler on top of that. I think that would be too much. I think if you make the flat of the splitter parallel with the bottom of your car, the 2-3 degrees you get will be enough to channel the air and provide plenty of downforce.
My splitter stops at the leading edge of the front wheel opening. ITS rules dictate that, even though you may see several cars with the splitter going back to the cross member. Carl is lucky, he doesn't have to deal with some of the silly IT rules.

Think of it analytically. What problem are you trying to solve by adding a splitter? a high speed push??? cooling??? aesthetics??? a little of each. The trade off is drag. The solution is create as little drag as possible while still solving your problem.

Carl,
Thanks for reminding me how much i enjoy staring at your car. :)

jim

I built a splitter for my FD a while back to use during trackdays. I notice no difference in front end handling, but I sure do get more air through the nose. Cooling wise it really works.

Mine wasn't scientific other than "that looks about right" :p: It's parallel to the bottom of the nose.

https://www.rx7club.com/attachment.p...postid=2312775

If you go too aggressive on the splitter angle you can have flow separation which will cause increased drag and decreased downforce. This is bad. The pro's have analytical computer models and test in wind tunnels, they know what they're doing, so try to stick with what they're doing, but don't just do something because you see others doing it, you have to know why. More isn't always better!

By aggressive angle do you mean that the leading edge of the splitter is higher than the trailing edge, or the trailing edge is higher than the leading? And uh for that matter, what angle do you want the splitter to be at anyway? Even if the leading edge is higher than the trailing edge it will still force air down and it will go below the car at a higher velocity because its squeezed, which would create DF, wouldnt it?

Im still trying to figure out splitters myself, I dont plan on being able to seriously track race anytime soon but Im planning on building a FC for HPDEs, and Id just want a splitter up front to help with cooling and as a little safety insurance, along with a wing. I dont want to end up catching air and flying off a track.

Well, actually, I think I should ask if theres any necessity for DF for safetys sake taking a street car in a HPDE, granted you already have coilovers which would drop the car a little. I know anecdotally a well set up splitter has even helped (though it might be psychosomatic) cars in autox's, but would a wing and splitter be a safety margin at all?

As far as the build, Im gonna find a cheap shell and go w/ a V8 or a 2.3T (I can get shit VERY cheap for either, its a budget thing, not hating on rotaries!). LACK of power or torque wont be an issue - but I was wondering if I should even care about aerodynamics with the money Id save or if I should put it into the tires, brakes, and coilovers/struts more.

Sorry to ask so many questions at once, but I got a lot to learn, and this section seems to have a lot of people who know their stuff. :bigthumb:

You want the leading edge of the splitter lower than the trailing edge at something like 5-7 degrees. This causes downforce by causing a pressure reduction under the noze of the car.

The stock car won't fly away or anything, there's lots of nearly stock bodied racers out there and lots of stock bodied cars have been on the track and have stayed earthbound.

You don't need downforce for "insurance", it can make you faster though.

Oh, that makes sense. You want it so that at maximum rear weight transfer its still raked positive (is positive rake the same as positive caster?), right?

And well, coilovers that lower the car a lil more than stock ride height also takes some of the risk of a draft getting under your car and causing lift away, right?

Thanks!

Getting the front of an FC to stick is the least of your worries. There is a reason every ITS RX-7 I'm aware of runs without a rear sway bar.

As light as the car is and as stiff as the springs/shocks are, I dont see why it would really need a huge rollbar.

Some theory behind tuning Ive heard is that with racecars its better to just stiffen up the springs and use less rollbar. Hell, Ive heard the Ultima GTR doesnt have em. Street cars want to (generally) use softer springs all around but rollbars... in a full race car its not so important.

BTW, is ITS generally FB or FC? Both have very different rear suspensions - I wouldnt compare tuning a watts linked solid axle (or a PHB if they converted) to a semi trailing arm :P

Splliters are *supposed* to be horizontal. When you drive you build up a high pressure "bubble" of air in front of the car and by putting a horizontal splitter there, the pressure will push straight down on it. There is virtually no increase in drag with a horizontal splitter. The only reason to make it angled is if you have a smooth underbody (that is low enough to the ground, ~5-10cm) so that you can take advantage of ground-effects. By angling it, you would reduce the amount of air passing underneath the car, and thus increase the pressure difference between the bottom and top of the car. Pick up an aerodynamics book sometime, they can really teach you a lot if you take the time to read them.

Greg

hey man if you think tht u just run stiffer springs and no sway bar at all then you need to go back to suspension school or attend a skip barber racing school....ITS is mostly FC......FB is IT7 suspension is different on both cars FB you have solid axle and FC is independent rear......most guys in ITS running FCs are running anywere from 400-475spring rate up front with a stiffer swaybar.....and 300-375 spring rate in the rear with no sway bar........with koni double adjustable shocks and hypercoil springs......suspension is everything on a race car do some more research be for you give your .02..... :dark:

I actually run 600lb fronts, and 400 lb rears.

Not to interupt this great conversation ...but what is the conversion factor for the units regarding lb and kg when indentifing spring rates?

Sideways...
Try this, it is very useful.

http://www.convert-me.com/en/metric_conversions_en.html

ty :) ...still couldn't find the right conversitions though. i suppose i'll do it the systematic way with cancellations.

lbs/in to kg/mm i think.

Its kilogram/mm vs lbs/inch, right?

"In order to convert kg/mm to lb/in do the following. Multiply the kg/mm amount by 56. So if you kg/mm spring rate is 6 kg/mm Front and 5 kg/mm Rear your lbs/in would be 336 lb/in Front and 280 lb/in Rear. Pretty Simple."

That seems to be the basic conversion factor, and more convenient than "55.9974146". :P

I also found a nifty lil calc here.... http://www.proshocks.com/calcs/anglefirst.htm might be useful if youre ever calling up someone and spitting out your own spring rates you want wound and happen to know all the math.

Good thread!

Just as ernestgj says, a splitter to me is always horizontal (parallel to the ground). Once you start putting an angle on it you now have a spoiler. Splitters and spoilers function quite differently.

So, whats the specifics on a front spoiler, then? And uh... what angle do you build it at so that on WOT it doesnt have the leading edge higher than the trailing edge to keep it just as a splitter? Or, do you just stiffen up the rear end of the car a lot?

When does the shape of the front bumper play a role in this issue? Like comparing some aftermarket bumper covers with stock. Those with big openings I assume would allow too much air to pass through and not enough air to remain in that pressurized area pushing down on the splitter.

Nihil - I think someone mentioned that earlier, by way of shock travel markers. It is at the point when the rear squats no more that the leading edge will not increase above the trailing edge and it will be horizontal at WOT. So it is more car specific to lift the lift of the front end generated by the engines power and stiffness of the springs. Am I understanding this correctly?

All cars lean back on acceleration, drag race cars pull the front wheels up off the ground actually. But yeah on a race car for circut racing you dont generally want that much, as that would mean it would understeer like hell on the throttle... heh. I guess you just gotta play with it and get it right.

A far as bumper shape, if it sticks out enough and its strong enough it will push down on the front of the car, but youre right.. most rice crap is just for looks. I guess for safetys sake you oculd just give it a little bit of a positive rake.

Front spoiler, ie Formula 1 or Indy car are inverted wings. Like an airplane they have an angle of attack. The negative lift (or downforce) is proportional (quadratic function, not linear) to the angle of attack. The drag is also, but that's another issue. Formula 1 produces the wings so that at WOT the angle of attack will be close to zero (you don't need front downforce when you are accelerating hard, do you). When the car is at constant velocity (nose not pitched either up or down) it has a different angle of attack. Finally, when braking it has the maximum angle of attack which means they get the most downforce on the front tires. So basically, you want to make sure that the angle of attack is greater than or equal to 0 degrees when you are at wide open throttle. Probably the easiest way to do this is to take a video of your car launching and running through 1st gear (you only need 1st gear because this produces the most squat). Pause the video on the screen and whip out a protractor to measure the angle between your car and the ground. Notice that the car will squat a lot on launch, this angle isn't important because there is no air flow over the car so a wing is useless at this speed.

Greg

Oh yeah, to clarify spoilers/wings: spoilers are continuously connected to the car while wings have an air gap between them and the car body.

Greg

anyone care to share why ITS cars use stiffer front sway bars and remove the rears?

-Ben Martin

I've found an online source for some excellent aero articles. The site's down right now, but I'd think its only temporary.

http://www.advantage-cfd.co.uk/Newsl...-Articles.html

The articles about front airdams, splitters and diffusers are V14N10, V14N11, V14N12.

Not trying to be a smart a$$, but mainly because that seems to work. Starting with my ITA 240 sx, I tried several different spring/bar set ups. With a rear bar, I could get the car loose, but the loose was very "snappy" and not easily controlled. A long time RX7 (who we shall refer to only as Hollywood) racer suggested I try losing the rear bar and reset around that set up. It worked. It was also loose, but much easier to drive than with the rear bar.

I have a similar set up on my ITS RX7. It works. Big, speedway style adjustable front bar. No rear bar. Several different theories on spring rate, but most are similar.

If you want more, pm me or you might start a new thread. This post will get lot here.

On the bar setup, the main difference is the field of use. There is a huge difference in balancing a car for average speeds over 85mph and balancing a car for autocross. You run the big front bar and no rear bar to do exactly what one would think that would do, tighten the car up. Like Jim said, that's what works.

actually I did my final year thesis on CFD vs. Traditional testing and found that a splitter will not increase drag in any way noticiable. It will assist by creating suction and removing the large bubble of dead air in front of the car which in fact is a big wall of air that slows you down. as long as the body can withstand the DF produced without being rip out, and if you flatten your floor out it will assist even more. keep your splitter at zero as best as possible and keep it well withing yout existing bumper.
I used FLUENT and windtunnel testing as my test method and found that the more (faster) you can move the air over your car the less the drag will equate to.
try to avoid bubble seperation and heavy boundary layer.
But over all i think you will find a splitter with a wing and diffuser is all you need for any motorsport.

I would not mold the splitter out of fiberglas. Splitters take a lot of abuse, and fiberglas would break very fast. I studied the Audi prototypes up close, and they make their splitters out of what appears to be a dense grade of 3/8" plywood. I was told that they adjust ride height until the splitter grinds really bad on the pavement bumps, then they raise it up a little until it grinds just a little bit each lap.

5 weeks ago, I built a splitter just for one race. I usually race in a class where splitters are illegal, but for one SCCA regional road race I ran in the Super Production class which allows unlimited aerodynamics. I made a 15/32" thick plywood splitter and screwed it to my front undertray / air dam. The track, Nashville Superspeedway, had very rough pavement transistions, and the splitter took some hard abuse and still looked great at the end of the weekend. Next time, I would make one out of thinner 3/8" or 11/32" plywood.

On the GT Car forum, they recommend using an expensive polymer sheet called Garolite
http://www.gt-racecar.com/forums/top...?TOPIC_ID=1566

thats what i was originally planning, a thick ass plywood lip, and just shave the front to make it angled for airflow a little, but sounds like i can go skinnier and not even bother w/ the front anymore, i really liked the idea of a brushed or polished aluminum ankle chopper ;)