Turbo23

iTrader: (4)

Well im looking to get my fc started in some time trial events here this summer, to hopefully gain experience for future possible road racing events. Well I know Ive seen cars with GT style wing and such. Since i still love to drive my car occasionally on the street, I at the moment do not want such a large wing.
Now Im sure many of you have seen the i beleive top gear or fifth gear video of the mclaren F1 road car on a track, and under heavy braking, a small spoiler/fin lifts up, to help stabalize the car by downforce, and drag. Ok well here is the idea.

On the old 86 rx7's they came with the black rubberized spoiler that comes right off the tip of the rear. (pic enclosed) My idea is to fabricate a system of lifting the spoiler under heavy braking running a brake line to the rear of the car, which would run into a piston. There would be a pressure regulator in front of the piston, so that it would only open under hard brake pressure. The 2nd pic enclosed is a "rough" look at what part of the fin would be lifting(colored in blue). The brackets attached to the fin from the piston would be adjustable, for a setting of 55-70 degrees depending on track size, and conditions. Almost the look of a nascar stationary fin on their cars for downforce, but would return to stock looking condition, when letting off of the brakes

Now the big question, would this actually be effective enough on a FC during hot laps? Im looking for a non flashy stockish modification. Since its the winter months, time is not a option, just looking to get your guys opinions on this.

Turbo23

iTrader: (4)

sorry for the amazing paint skills on all the pics, here is one from the driver side door view

Boswoj

Would definitely need to be a secondary system actuated possibly electronically by the stoplight switch. You would compromise brake feel and effectiveness if you add anymore pistons, lines, and switches to the actual hydraulic brakinbg system. From an engineering standpoint, this system would likely be useful as a "gee-whiz, look at that cool moving thing" than about true aerodynamic advantage. Aerodynamic effect varies exponentially with speed, not geometrically, and modern automotive brakes ae SO effective at turning kinetic energy into heat that any real benefit that you would get would be during the initial second of braking from very high speeds. SCCA GT cars are limited to perfectly flat duckbill spoilers like yours would be when "extended". Their primarly purpose is to add aerodynamic downforce in relatively high speed corners, not stability in braking. A good analogy are the aerodynamic downforce limits on Formula One cars - possibly the most aero-conscious automotive group in the world! It is conceivable that an F1 car might be able to make it through a corner on mechanical grip at 60, unable to make it through a corner without spinning at 80, and easily be able to go through with aero grip at 100. This is actually a huge exaggeration, but indicitive of the exponential growth of downforce as it passes through a "compressible fluid" (air) at speed. Cool that you are wrapping your brain around concepts like this, but this particular idea is likely to only be a good way to get the ricers to think you are cool. If you have more interest in things like this, I can probably scrounge up a reading list of related books with a little effort.

Turbo23

iTrader: (4)

related books would be great. I was looking at a book at borders, about circle track cars, and downforce. The 55-70 degree angle of the spoiler i found in that book for downforce. I do have a question as to why the mclaren f1 would have such a device on it? I know alot of super cars now adays have pop up wings for high speed road travel and such. Im just trying to find ideas for downforce, and design. Any other info would be great on these topics, or any type of small body modification for racing.

Black91n/a

Honestly, don't waste your time on it. If you want better braking then do a 5 lug swap with the 4 piston calipers in front and vented rear disks. You'll get much better braking with that mod then with some silly airfoil thing. Also you mention the possibility of future racing. Try a track day before you start modding the hell out of your car to see if you even like it. If you are sure you want to race and can afford it then keep in mind the class you'd be racing in, as the spoiler mod would be illegal in almost any class.

I've had my car on track with absolutely no mods other than an intake, exhaust and better brake pads and good brake fluid and it was fine (that was on hard all season tires too). I even passed some people.

You need to worry more about brake fade than aero. Get good pads and fluid and you should be fine.

My advice would be to save the money on modifications and put it into seat time at the track (get instruction, you'll learn SOOOOO much faster than if you do it on your own and you'll be safer) and you'll go a lot faster than if you spent that money on the mods.

Turbo23

iTrader: (4)

i already have all the necessary brakes modifications, just tossing around another idea for better braking stability.

aussiesmg

iTrader: (1)

McLaren F1 is no RX7, it was until last year the fastest production car ever built, and the downforce generated is used for cornering at speed not braking. the slower you go the less effect the downforce will have on your car.

Take Formula 1, on a fast track with high speed corners they use more downforce on a slow track with slower corners they use less....so at less speed, less impact upon handling. Nobody uses these for braking aids.

DaveTurnerMotorsports

A local autocrosss and time trial competitor here in San Diego designed such an electronic system for his S2000, and then incorporated a non-movable wing on his STS autocross cars, both with repeatable beneficial results even at lower autocross-speeds. The idea works, it just needs to be realized with much larger wings to be effective at lower speeds.

Turbo23

iTrader: (4)

thats why I was looking at something alittle smaller for track running. And for the mclaren f1, yes it is very fast, a 400hp FC is also not a slow car. The little spoiler on the f1 only comes up under braking, which is why i refered to the videos of it on a road course, and they stated that its purpose was for stability under braking. I know andrew mckee's autox rx7 had what dave is talking about. I was looking for something moveable, since well I really dont want the nascar look.

RETed

You're talking about active aero?
Isn't that **** kinda illegal?

Anyways, one of the Japan videos did a comparison on different types of wings on an FD, and you had to get the wing way above the roof line to start making significant downforce.
I don't think messing with the Sport wing position will make a significant difference unless we're talking about hundredths or thousanths difference at the national run-off's level???


-Ted

T_Racer

iTrader: (1)

Also keep in mind balance. If this wing were effective, you would suddenly have more traction at the rear which could do to things in my thinking. One, cause understeer as the front end suddenly has less traction than the rear due to the increased rear aero. Second, from the sudden added downforce, maybe overwhelm the rear tires, causing oversteer. Just throwing some ideas around, but maybe balance it with a good splitter in the front. The McLaren mentioned here definitely has more aero designed into it than the FCs, without the active rear wing, with the addition of the wing completing a total package. The splitters are easy to make, cheap for the most part, and can add some good aero for not a lot invested. As a whole picture, most FCs are making less power, less mechanical grip and other areas that I can't think of right now, where more could be gained with alignment corrections, better parts, ie. shocks, swaybars, etc. and the such on the 7.
Not trying to kill an idea, think it might surprise us all, but just make sure to think through all the ideas to cut off any problems before you start to get discouraged.

Cheers,
Travis

C. Ludwig

iTrader: (27)

It's a waste of time. Spend your time and money working on you as a driver. For any amatuer the greatest amount of lap time you'll ever gain is through driver improvements. You haven't even tracked the car yet and you assume it to be unstable under braking. You need experience not gadgets.

Boswoj

Mr Ludwig is correct. The single biggest improvement that you can currently make to your performance vehicle is the driver. Don't get lured into the idea that your car needs every go-fast gadget in the world. You will become much faster by investing your money in some track time, even if it is in a stock Yugo!, than spending that time in your garage adding questionable aerodynamic aids.

Couple of comments on the info in some of the replies:

1) Braking instability has a tendancy to be much more about brake bias then it does about induced aero grip. Installing a simple cockpit adjustable Wilwood or Tilton front-to-rear brake bias adjuster and experimenting a bit with the settings will give you a much better understanding of how to settle the car down under braking.

2) It doesn't surprise me that the McLaren has an extendable spoiler, becasue it was built to a peculiar set of compromises that your car wasn't. The problem with supercars is that the very wealthy people who buy them expect them to have certain attributes which are not completely compatible. High downforce and high top speed are design specifications that are completely at odds with each other. In order to attain the extremely high speeds advertised, the car had to be made extremely aerodynamically "slippery". When adding aero downforce, you also add drag - and even though it has a big, powerful BMW lump under the cowling it won't win the battle for top speed against the exponential growth of aerodynamic drag at those kind of speeds.

3) Sudden aero downforce on the rear wheels cannot overwhelm the rear tires and cause oversteer. There are some obvious dynamic evvects that we won't get into, but friction is simply a coefficient times the "normal" (as in perpendicular to the friction surface) load. Added load means added grip, up the the point that the rear suspension reaches the end of it's travel, bottoms out, and the tire sidewall becomes the spring rate for the rear. Then it slides - quickly and violently!

4) Splitters add SOME aero effectiveness. A great deal of study has gone into automotive ground effects and for street driven cars, splitters are nearly useless. There is too much dependancy on things like ride height and stability to make air dams and splitters very effective. Something, yes - but small. Slightly better when used in a racing application, but at the club level they are still not major aero.

5) Do definiely agree with the balance issue. A lot of testing and even wind tunnel time would go into making an aero device safe. During braking a large portion of the weight shifts to the front, giving the front brakes the majority of the work to do. Assuming that you could build a system that actually made a significant amount of aero under braking, instead of having a fairly constant weight transfer that allows steady pedal pressure, you would have to learn to modulate pressure to adjust for aero load coming off the car at a rate exponentially faster than speed was coming off the car. Formula one drivers have become experts at this as they have 4000 - 7000 more pounds of downforce at the time they START braking then they do at the end of their braking.

6) Lastly, at a couple of the tracks thta we race on up here, I'm hauling my 1st gen with stock small bearing brakes and race pads down from 140+ every lap. With the bias adjusted properly, it stops straight and FAST. If your car scares you that much with braking instability then there is something wrong with the car or the driver that is much more important to fix than thinking up aero gimmicks!

I'll wok on that reading list!
Good Luck

speedturn

iTrader: (1)

You don't have enough volume in your brake master cylinder to be operating any other cylinders; your existing brake calipers will use most of your master cylinder volume . Just wait until you get a warped rotor or over heat your brakes and you will see that I am right.

The roof of the RX-7 is so high, and the rear hatch slope angle is so steep, that you will not get any clean air on that short center section of spoiler. Sure, standing it up will make a few more pounds of drag to assist braking, but it will not be noticeable below triple digit speeds. The rear of the McLaren has much smoother airflow.

To check the airflow over your existing rear hatch window and rear spoiler, try doing a simple woof tuft flow test. Get some bright yarn, cut into 5" lengths. Tape it every 6" all over your rear hatch window and rear spoiler, and then go out and drive with an observer video taping the tufts as the wind flows over them. You will find out the the wind flow is really dirty back there; poor

T_Racer

iTrader: (1)

[QUOTE=Boswoj]3) Sudden aero downforce on the rear wheels cannot overwhelm the rear tires and cause oversteer. There are some obvious dynamic evvects that we won't get into, but friction is simply a coefficient times the "normal" (as in perpendicular to the friction surface) load. Added load means added grip, up the the point that the rear suspension reaches the end of it's travel, bottoms out, and the tire sidewall becomes the spring rate for the rear. Then it slides - quickly and violently!


I also agree with Ludwig, Seat time is irreplaceable compared to dumping money into parts that you are unsure you need. But to address Boswoj, I think I am right though. Let me explain quickly. I am also taking some assumptions in that assuming the wing idea would work more than reality that it is in too much dirty air over the roof and hatch. But in thinking of this, if the rear tire is at the limits of its adhesion circle, or friction circle, or grip circle, what ever, if added force is added that it could cause just what you are saying, quick and snappy oversteer. On the good side, at least you could see oncoming traffic and who is about to smack you. LOL But I also agree, that we are ignoring many variables in this whole braking scenairo, as in weight transfer, suspension geometry changes through increased caster/camber different from stock, blah blah blah.
To put it simply, I love innovation, but in this case, stating that you are getting your car out there for the first time, put money into entry fees, good tires, some suspension adjustability pieces to allow you to change things around to create more stability and allow you to change and experiment with these, and you will be money in the bank ahead. Of course we are talking about racing, so racing and money in the bank are two seperate things.

Cheers
Travis

Turbo23

iTrader: (4)

Thanks for the input you guys. I know seat time is THE most important thing, I was just thinking of the idea. Im not scared of my car during braking. I do not have any track experience as of the moment. Well alittle beaver run course runs, but thats all. And on a couple of long back straights where i live, i pratice 60-0 and 100-0 braking. So thanks for the input, hope you see some of you at the local NE tracks

Boswoj

T_Racer :

Think of this as a 3 dimensional system, not a 2d one and you will see what I mean. You mention the friction circle - do you fully understand it? For vehicle dynamics purposes, it is typically used as a measure of total loading that a given object (a car) can take in a horizontal plane before transitioning beyond the available friction forces to a non-directionally stable sliding condition. The idea is a simplified view of grip that says a given tire, or joined set of tires has the ability to resist sliding up to a certain force. Once the resultant of those forces exceeds the resisting friction forces, the car exceeds it's friction circle and slides. In gross simplification it is used to indicate that if you already have a tire loaded to 80% with cornering forces, and you wack open the throttle and add 50% of it's capacity resisting tire spin in acceleration you end up with a tire loaded at 130% - outside it's friction circle and sliding. It really doesn't apply to aero loading, which is what makes it so appealing. Here is an example:

Car #1 - 1000 pound car + 1000 pounds weight added to the car for more grip. .20 coefficient of static friction. 2000 pounds x .2 = 400 pounds of "grip" resisting cornering forces (centripetal acceleration)

Car #2 - 1000 pound car + 1000 pounds aero downforce added to car for more grip. .20 coefficient of static friction. 2000 pounds x .2 = 400 pounds of "grip" resisting cornering forces (centripetal acceleration)

Which car goes around the corner faster? Car #1 needs to accelerate TWICE the mass around the corner using the same amount of tire grip. Car #2 will leave the #1 car for dead! Now, you say "then let's take that darned weight off of car #1!" By doing so, both cars now weigh the same but you just took half of the "grip" away from car#1 as well. The point is that adding downforce adds grip. If you can do so without adding weight then a much faster corner can be expected, because in your terms the friction circle just got bigger for car #2! I'm not just picking this stuff off of the top of my head - I'm an engineer and thats just the way that it works.

T_Racer

iTrader: (1)

Bos, not to highjack this thread, but you obviously have a very deep understanding of this. I am just trying to get it through my hard head. I do totally understand your whole reply, and it really helped me out on a few things that I was unclear of before. And I have been thinking of the friction circle as 2d, without factoring in the weight, or aero weight as addint to the grip factor. I also see why the aero weight car is accelerating "aero" weight, not mass weight, hence the difference in performance in the two theoretical scenarios.
But, is it not true that a certain tire, or set of tires, has a certain amount of grip? In my elementatry thinking, you could set Anna Nicole Smith, at her heavy weight, on it and add more grip through mass weight, equal to aero weight, whatever. You can only load a certain tire to a point, before the tire will slip from a stable grip state to an unstable sliding state. True? You could add 14 wings, but if you overload the tire, it is out of its range of maintaining a constant radius and will slide.
I am not trying to argue a point, just understand. I believe we are on the same page, and my thinking does jive with what you are saying, just mine comes out with a few errors, and non-coherently.
Thanks, good luck to all. Thanks Boswoj, has been very informative. Time to go back to the bathroom for more quality time with Fred Puhn.

Cheers
Travis

P.S. Turbo 23, is that in reference to the ford engines? I have two Merkur XR4s. Gotta love the ole pinto lump in there.

DamonB

I can't find a good pic of a 3 dimensional friction circle right now. Basically a car with much aero on it sees its friction circle expand as speed increases. This would be a friction circle in the shape of a 3D cone with it's small tip pointing down. The vertical axis (z axis) represents speed so as speed increases the area inside your friction circle increases. Interestingly you then have to deal with the center of aerodynamic pressure vs the vehicle CG so the friction circle also changes shape as speed increases.

Tire grip increases with an increase in tire load (whether through weight transfer or downforce) but that increase in grip is not at a 1:1 ratio with an increase in load. Grip increases at a lower rate than the increase in load. This directly explains why we always attempt to keep all four tires doing an equal share of the work. Why we corner weight the vehicle, why we want the CG as low as possible, why we want as little weight transfer as possible, why we use sway bars etc.

T_Racer

iTrader: (1)

All good info. Damon that cone explanation helped visualize things for me, thanks. I guess in my theory I am using gross oversimplification, but my point in my original post about sudden aero load of a tire that is at its limit, it would increase grip. But my oversimplification says that you can have a 100 sqft wing over the rear of the car, but a sears all weather family truckster tire can only develop so much grip. You could hang the wing of a 747 off that car, upside down of course, and you will only have so much grip with that tire. For this size wing, George Bowland's creations could help visualize. I guess I was assuming too much in this whole thing, not thinking all of the variables through. I still believe my thinking is right, just GROSSLY oversimplified and understated things. But please feel free to add more, I am soaking this up like a sponge. I believe after this, Scuderia Ferrari should hire me as their wind tunnel and tire specialist. LOL

Cheers,
Travis

DamonB

We have to make sure we understand the difference between friction circles of tires and friction circles of vehicles. A tire on a test rig will generate a nice round friction circle with nearly equal grip in every direction. A vehicle on four tires reacts differently however. Let's use a rwd vehicle as an example. In braking this car has four tires that contribute (though mostly the fronts due to weight transferring forward), in cornering it has four tires that contribute (though mostly the outside tires again due to weight transferring to the outside) but for acceleration there will only be two tires available at any time since only the rear tires are powered. Because of this friction circles for cars are nearly always more elliptical; the car generates greater cornering and braking forces than it can acceleration.

Here are two friction circles my car generated. The top of the circle represents acceleration and you can see the car cannot fully exploit that area of the tire's grip because it only has two tires that contribure to accelerating the car, whereas there are four that brake and corner:



So back to the grip of a single tire vs the grip of an actual car with four tires. As we increase load on a tire that tire offers an increase in grip (though grip increases at a rate lower than the increase in load; it's not 1:1). As we increase load on the entire vehicle and its four tires (downforce) grip increases. As we increase the load to greater and greater amounts the rate of increase in grip vs load continues to be less and less, but grip always does continue to increase. Grip will to continue to increase until you overheat the rubber (and thus lose grip because you're outside the tire's ideal temp range) or the tire mechanically fails (splits, bursts etc).

Let's say a given vehicle is cornering at a given speed. When a vehicle is cornering the tires essentially see two forces. One force is the load on the tire (vehicle weight, weight transfer, downforce or a combination of all) which acts in a vertical direction. The other load is a horizontal one. This load is a resistive force to the centripetal acceleration of the vehicle. If the centripetal force overcomes the resistive force of the tires then the car slides to the outside of the corner. One way to increase load on the tires is simply to make the vehicle heavier. However this adds mass to the vehicle and thus means there will be a greater amount of centripetal acceleration when the car corners since the car is now heavier. We've increased the vertical load on the tire but in increasing the vehicle's mass we've also asked the tire to resist the extra force of the now heavier vehicle when it corners. If we use aero downforce however we can increase the load on the tires without adding mass to the vehicle. We get more vertical load (grip) but have no additional horizontal load (mass) to deal with. We then exploit this by making the car travel around the corner faster until we overwhelm the tires again. Everytime we add additional downforce we will continue to corner faster until we overheat the tire, burst the tire or run out of horsepower to overcome the drag or roll the car.

When a tire slides (is saturated) during cornering it is always due to the fact that the resistive horizontal force of the tire was overwhelmed by the centripetal acceleration of the vehicle. We cannot saturate a tire with downforce. Since downforce merely pushes the tire vertically against the pavement there is no component present to push the tire to the side and make it slide. We can always make the vehicle slide if we continue to increase speed (and thus centripetal acceleration) around the corner but the tire didn't slide due to increased downforce. The tire always slides because it was overwhelmed by the mass of the vehicle.

Black91n/a

Weight does matter a lot. Extra weight will give extra grip, but fore a 100% increase of weight you may only get 80-90% more grip, meaning less cornering speed. Less weight is always good for performance.

T_Racer

iTrader: (1)

I think my IQ has gone up like 10 pts after reading all this. I now see, with Damon's explantation, that I didn't look enough into my answer and thus, was wrong with my initial impression of adding aero and suddenly pushing the car out of its grip circle. This has been a great excercise in understanding grip vs mass vs aero induced drag/downforce. I really appreciate everyone helping with this, especially Bos and Damon. I tell you what, I think us rotarheads are some of the smartest, collectively, in the whole auto loving group. LOL Thanks

Travis

Boswoj

T Racer - If my posts seemed arguementative, it was not intentional! I love taliking about this stuff. Like many engineers, I am likely better with applied physics than I am with interpersonal relationships! I think that it is great that you are making the effort to reach out and actively TAKE education on this subject - that is the hallmark of the best students!

As mentioned earlier, for example purposes we have been viewing VASTLY simplified models for the sole reason of illustrating gross concepts. There are huge amounts of dynamic behavior in vehicle systems. Vast amounts of study have gone into the tire to surface interface alone! Most people go out and get into their daily driver and have no idea what a miracle of engineering and physical metalurgy that they have the privledge of operating. I started driving when there were still BAD cars! Now, even the bad cars are outstanding cars! Anyway:

It seems like Damon and I are often on the same page technically, as we end up filling the cracks in each others explainations! One of Newtons laws of motion states that a body tracelling at a constant velocity in space will tend to continue travelling at that velocity in the same direction unless a force is applied to it to MAKE it change. We haven't even scratched the subject of slip angles at the contact patch yet, but again - using gross oversimplification - your car is a body at a steady speed going down the road. The white line down the middle of the road that you are travelling is the x-axis, and the cross street that you want to turn left on at the next light is the z-axis. The weight of the car and any aerodynamic downforce acts downward along the "Y" axis which is parrallel to the telephone poles at the side of the road. When you turn the wheel, you are introducing a force vector at the wheels that simultaneously decelerates you in the x-axis and accelerates you in the z-axis. You decelerate from 60 mph "X" to 0 mph "X" at the same time that you are accelerating from 0 mph "Z" to 60 mph "Z". Since Force = Mass x Acceleration, it is possible to calculate the amount of force that is required to accomplish that. Since both velocity (The change of distance over time) and acceleration (the change of velocity over time) are TIME based you can now see that the quicker that you want to get through a corner, the LARGER the force that must be transmitted through the contact patches of your tires. That force equals the load of the car times the coefficient of static friction. That is the only force resisting the car's desire to continue on in a straight line, and accelerates it along another path. That is why cornering forces are typically quoted in "G's", as a "G" is a unit of acceleration equal to the pull of gravity at sea level, which is 32 feet per second per second. It is one of the few acceleration measures that makes logical sense to non-technical people for no other reason than it is what they FEEL every day.

There are many more interesting subjects that develop within this line of inquiry, like what a tire really looks like when it is rolling down the road, multi-spring rate dynamic systems and it's relationship to frequency damping, and downforce to drag ratios for vehicle systems .....................

fun,fun,fun. This is why I race - because it allows me not only to form all of these nice theoretical systems in my mind, but also to see them operate first hand in a real world application.

j9fd3s

iTrader: (3)

our informal test was to put a day old del taco hamburger on the rear spoiler of my fc after sevenstock 6. we then proceeded to put the fc on a trailer and tow it to racing beat, on the freeway the del taco burger stayed on the spoiler about half way there, where it finally was dilodged by a bump.

theres been some debate as to weather theres no downforce at all or theres enough to hold a del taco hamburger down, but the likely answer is just that theres no air flow over the wing anyways.