Aero- Windshieldless vs fastback

Let's say you were building race car, and were pretty unlimited on class restrictions, etc. You can modify the body at will.

You choice is to cut the windshield, or build a fastback. A choice like this:

But you can also add aggressive aero at will- side skirts, splitter wing, diffuser, whatever.

Limited budget.

The low windshield version punches a smaller hole, but the fastback is smoother, and gives better opportunity for maintaining the airflow and the laminar layer.

The goal is speed. Road racing, autoXing, maybe occassional straight shots (1/4 mile?). Top speeds probably not to exceed about 100 mph, generally more like 70. Lots of corner carving.

Without the advantage of an available wind tunnel, which would you choose, and why?

Fastback simply because I think it looks better. I plan to buy a Factory Five Daytona/65 Coupe once my midlife crisis sets in.

Yeah, I was trying to avoid the appearance question, since everyone will have a different opinion on that.

Speed is primary goal. Looks secondary.

Open top/low windshield. It will feel a whole lot faster than a closed car and you should be able to build it lighter. So, put a V8 in the Mumpkin and cut off the windshield.

SVreX wrote: Yeah, I was trying to avoid the appearance question, since everyone will have a different opinion on that. Speed is primary goal. Looks secondary.

Well then fastback, because it has the potential for less aerodynamic drag, all other things being equal. Which means more speed for a given power level and better high speed stability.

70mph? Aero doesn't factor into it much, at least not wind resistance. That's on the cusp of being able to really use downforce, though. So I say fastback just so you can put a giant wing over the middle of the car without totally buffeting the driver.

Or, hmm, I wonder if you could make a downforce-aerodynamic Lexan windshield, and go for the roofless option.

Adding cornering to the equation means you will be slowing down a lot. They key then is not in making the fast parts faster but making the slow parts faster, even if it means shaving a little performance off of the top end. Corner speed carries through the entire straight, while top end speed is only useful right before braking, and all that.

Chop the top on the fastback.

In reply to Knurled:

Aero vs downforce. Interesting distinction.

Yes, you are correct. My primary purpose for the aero components is to create downforce.

100 horsepower or 700 horsepower? Given your speeds, I would guess not 700. Chop the top: it's lighter and has less frontal area. Use a tonneau top to smooth airflow. Compensate for loss of rigidity with the roll cage.

David

Look no further than the golden age air racers. Rosco Turner's open cockpit Weddel-Williams was converted to closed cockpit the following year with almost no other changes. He picked up 20+ kts in speed.

Exposed roll bar tubes are really dirty, aerodynamically speaking. And you can certainly get useful aerodynamic downforce at 70 mph. So I guess it's really going to depend on the rest of the body - if it's a low waistline, you'll have a lot more exposed roll bar. So I'd go with fastback as well.

There's a practical consideration - it's a lot easier to build an open car.

At speed under 100 mph. aero is not that critical. Down force, unless you have a huge wing, is also not that available.. Lightness and handling are more important. Plus sufficient power.

Without knowing the exact context of the application, I'd give up 10mph at the end of the straight if I could keep 10mph through every corner. The nature of the decription sounds very autocross-like, and the quickest autocross cars sacrifice all sorts of theoretical speed in order to get cornering speed.

I agree that at 70 mph, more critical things to consider are probably CG height, total mass, and such. The roadster probably wins there.

To my recolection, roadsters often have a pretty terrible Coef. of Drag relative to enclosed cockpits. But, they do have a smaller frontal area, generally.

Since drag force is directly proportional to both frontal area and CD, it's just a matter of the relative magnitudes.

In general, I would expect that the CD goes down by a larger percent that the frontal area goes up, when switching from a roadster to a fastback, but it will depend heavily on the specific design of each.

If you can make a fastback with the same frontal area as the roadster (or close to the same), then I think there's no question about which will be better.

For some practical comparisons, I would suggest looking to motorsports. For example: I don't know much about the rules, but my impression is that LMP cars typically run closed cockpits when possible, even though it's a higher frontal area. I would expect that to be a good indicator that it's a better payoff.

While LMP cars operate at much higher velocities and the payoff is more exaggerated, the trend doesn't change. ie: if it's better at high speeds, it's still better at low speeds, there's just not as much improvement.

Generally, the wake of a fastback should be much cleaner too, so if you're going to run a rear wing, the fastback will probably allow you to run a more effective design (higher downforce, lower drag) vs. the roadster.

One thing you might not have considered is that the enclosed car is a lot more comfortable to drive on track. Less buffeting, less fatigue, etc. It may not be an issue for you. But I've driven open and enclosed cars on track and it makes a big difference. The presence of a back window makes a big difference too, even with the side windows down. Back glass = smoother and more comfortable.

LMP cars also tend to sacrifice ultimate performance in favor of minimal drag for best fuel economy. At least, this was the reason given for the spate of cars flipping off the track a while back.

Fastback, in case it rains. Keep your driver comfy and he'll be faster.

Stricly using aero numbers, the fastback is the way to go. All you have to do is see how many race cars have roofs compared to the ones that don't, especially at tracks like Le Mans.

Appleseed wrote: Look no further than the golden age air racers. Rosco Turner's open cockpit Weddel-Williams was converted to closed cockpit the following year with almost no other changes. He picked up 20+ kts in speed.

Much like the Group 44 Triumph Spitfires (which spawned the GT6 body), IIRC.

SVreX wrote: Top speeds probably not to exceed about 100 mph, generally more like 70. Lots of corner carving. Without the advantage of an available wind tunnel, which would you choose, and why?

My answer and why;

https://www.youtube.com/watch?v=voelMATidc0

In reply to Hasbro:

Very compelling answer!

unlimited rules and budget? I would build it like a roadster.. but add the aero of a jetfighter's cockpit to keep the driver from getting beaten up and to clean up the air over the open hole the driver is sitting in.

Best of both worlds

In reply to mad_machine:

I said unlimited rules and LIMITED budget!

iceracer wrote: At speed under 100 mph. aero is not that critical. Down force, unless you have a huge wing, is also not that available.. Lightness and handling are more important. Plus sufficient power.

I put a road race wing on my Miata and cut my lap time by 2 seconds on a track with a 48 mph average speed. My peak speed on the track - on a long sweeper that is purely grip-limited at my power/weight level - went up from 70 to 75 mph. That's about the same change in lap times I saw by adding 200 hp on that track.

Aero is still critical at 70 mph and lower. Anyone who doesn't believe that will lose to someone who does.

SVreX wrote: In reply to mad_machine: I said unlimited rules and LIMITED budget!

depending on your limits... canopies seem relatively cheap.. if you are wearymicrobe