What separates real "race" cars from "modified street cars"? It's not horsepower...it's downforce. I was reading a race car aerodynamics book, and a typical dedicated race car will have a coefficient of lift from anywhere between -0.5 to a whopping -4.0+. A typical sports car is 0.3 to 0.5. That's positive lift, which means NO downforce. What this means is that those cars can take turns WAY faster than "modified street cars", and that's why they have significantly lower laptimes.

What I would like to do with this thread is to just brainstorm as to how to give the S2000 useable downforce.

What gives downforce? well, a front splitter, rear wing, and a flat bottom, and variations to a flat bottom such as venturi tunnels and diffusers. The easiest thing to do to the car is a rear wing, but that will upset the front to rear balance if there is no additional front downforce. The same holds true if only a front spoiler is added (one that works...not the OEM stuff). A flat bottom is very helpful and actually reduces the drag coeffecient while adding noticeable downforce. 30% of an F1 car's downforce is produced by the underbody elements, and that's with severe restrictions to what they can do.

So, what do you guys think about the feasibility of making a flat bottom tray to put under the car? Perhaps a low front splitter that can be easily installed for track days? I'm don't like that most of the rear wings out that produce downforce push down on the trunk lid. Imagine a useable rear wing that produced in excess of 300 lbs of downforce above 100mph pushing down on your trunk lid. I would imagine at least some severe depressions on the mounting points.

Well, before I keep rambling, let me see what you guys think of my ~radical~ idea for chosing aerodynamics over horsepower.

 

[QUOTE]Originally posted by Dum Dum Head
[B]What separates real "race" cars from "modified street cars"?

 

I sure would be interested in such a system. Development would be extremely expensive, I imagine, because of the required wind-tunnel testing to really make it effective. Matrial should be cheap, however. All that's needed is fiberglass.

GT wing: $497


Front canards: $445


Front splitter: $148


And flat bottom with rear diffuser: Maybe $1500?

Possibly could give great benefits. Possibly could be a waste of money. Hopefully the former.

 

Wouldn't adding downforce also make the car a good bit slower thereby negating the advantage of said downforce? Not trying to be confrontational, just a thought.

 

[QUOTE]Originally posted by johnyboy32
[B]Wouldn't adding downforce also make the car a good bit slower thereby negating the advantage of said downforce?

 

Wouldn't this depend on the track? I would think the speeds at, say, an autocross would be too slow in turns (or even in the short straightaways, for that matter) to offer any appreciable downforce?

 

In Formula 1, a very large aspect of preparing for a particular track is getting the aerodynamics down. Taking into account track configuration, car handling characteristics, engine power, and plenty more things I'm sure are there, the aero parts are continuously adjusted. Ever see a car come into the pits and one of the crew gives a front wing adjustment a turn or two? The commentators often talk about "high downforce" tracks and the like.

I'm curious about the shape of the bottom. Is it best to be perfectly flat? A bit concave to promote some vacuum? Or a bit concave to act like an inverted wing? I'm guessing either the first or third, by I'm no expert.

 

[QUOTE]Originally posted by Elistan
[B]I'm curious about the shape of the bottom.

 

Always nice to see people investigating what's possible.

Dumdum (I always feel bad when I call you that, even though I know its a bullet :-),

A friend and I have been mocking up some aero aids for the S2K. We plan on using the RM Racing front splitter because it is cheap, easy and seems to be reasonably well designed. Something that is fully adjustable would be better, and it would be better to have it closer to the ground, but I like to drive my S2K on the street :-)

In terms of the flat bottom, it really isn't that hard to do. We've just been debating on composite vs. metal. There are weight, strength, and temperature concerns. Additionally, we have to be careful about allowing airflow to critical components like the exhaust system, etc. Cost to do a flat bottom isn't that high once the design is down, and this is the least sensitive design issue since you just want to make it flat! :-)

The rear diffuser is a bit tougher. There is plenty of room under the rear of the car to add one, and fabricating one out of composite isn't that tough (I think one aftermarket company - not Mugen - has done one out of CF). However, deciding on the divergence angle is problematic as we really don't know what the air at the rear of the car is doing. I believe a shallower diffuser angle provides the best Lift/Drag ratio. Additionally, you want the diffuser as close to the ground as possible, which a street height S2K doesn't provide.

As mentioned though, the biggest issue is measuring the benefit. We can run basic 2d CFD code to model aerodynamics in profile, but that doesn't give us the whole picture, particularly at the boundary zones on the side of the car. We can also do string tuft testing with a video camera and chase car, but that only gives very basic information (we'll probably try this though). One thing that would give you a good idea of total downforce is to measure average suspension compression (on as smooth a road as possible) at a given speed. Since we know the spring rates, we can calculate total load then. Drag can be done with coast down testing, although that just tells you that you have more or less drag, not necessarily absolute numbers. The best check though is lap times, which require a consistent driver to pick up small changes. And more downforce, if it isn't balanced, may not make you quicker either. I have access to a wind tunnel at a university, but its a small scale one, so you'd have to develop a model, add your tweaks and then look at Reynold's numbers to assess relevance.

Something to consider is that the S2K has pretty mediocre aerodynamics to begin with. The rear end, according to Auto Motor Und Sport which tested it in a wind tunnel, generates 40 kg (about 88 lbs) of lift at 200 kph (125 mph). That's not bad, but not good. Even if you were to get zero lift front and rear at speed, the car would be quicker. Downforce is just extra gravy. In terms of the rear end, you wouldn't need a big wing to get downforce. In fact, a small spoiler, of less than 2" in height and an angle of 30-45 degrees, can actually reduce lift/increase downforce while not increasing drag at all on a typical car shape (Racecar Aerodynamics by, damn, I forget the author's name). A smaller, less agressive spoiler can actually reduce drag and lift at the same time (although it doesn't help as much on the lift as a bigger spoiler).

Anyways, keep searching, there is lots to learn from the racers out there. I suggest reading RaceTech and Racecar Engineering for more info. And college automotive engineering textbooks are great if you're really serious.

UL

 

A road car is fundamentally different than a race car in terms of the handling characteristics that you want in the car. One thing that the Skyline GT-R engineers said was that when they were designing the R34, was that they didn't want to overdo the downforce on the car.

You have to remember that downforce aids work primarily when the car is travelling in a path along its longitudinal axis. That is, all 4 wheels are gripping. So you're busting through a corner enjoying 200lb of downforce and you're flying. Then, all of a sudden, you hit a patch of reduced grip. Maybe a different pavement consistency or something. The rear steps out 15 degrees. Suddenly, the car is no longer travelling along its longitudinal axis. It's actually moving somewhat sideways. All of a sudden, the downforce drops by 100lb because the airflow over and under the aerodynamic aids has been reduced (due to the car moving sideways). Now, you're at a 15 degree angle and you've lost 100lb of force on the rear end. It slides out some more, eventhough you are compensating for it. All the downforce evaporates, and the car goes into a spin. Not fun.

So, in conclusion, you want a mild amount of downforce on a roadcar, and preferably aerodynamic aids that do not lose their total effectiveness if they are no longer perpendicular to wind direction.

I'd recommend you try out a rear difuser and see how you like it.