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Old 11-19-2018, 12:08 PM
  #41  
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Iíll weigh mine next time off. Itís really thin carbon fiber. You can see pinholes though it in places.

The shape really helps make the wing work. The downside is the points penalty in NASA. Itís not insignificant!
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Old 11-19-2018, 05:34 PM
  #42  
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Originally Posted by Occamís Racer View Post
.376 Standard top, closed windows
.395 Fastback, open windows
.424 Standard top, open windows
.462 Convertible top down
I made a car performance calculator a while ago, while it does have some limitations, it is probably quite useful for this. I'll preface this by saying my launch model isn't very good and in my elapsed time on straightaway calculations, I can't get it to account for shift times (so I'll keep same number of shifts for both cars). Also, it was some time since I wrote it, so I hope I've not made any glaring errors with my data input....

I used the numbers above, with 18sqft frontal area to create 2 cars,
Car A, Cd=0.376, CdA= 0.63 standard top, closed windows
Car B, Cd=0.462, CdA= 0.77 convertible top, windows down

I assumed 2400# weight (with driver), 6 speed with 4.77 and a 140whp dyno that was used as an example for an S1 Supermiata power graph (this might not be exact, I ought to use data in tabulated form rather than reading off a graph). As a sanity check on the data, I calculate car A to have a trap speed of 95mph in the standing 1/4. Does that tie up with anything people have measured?

Both cars were calculated to be able to reach the gearing max of 118mph, but the graph below shows that there main difference is available acceleration at higher speeds. (the higher drag car is the solid lines)



At 60mph, car A has 3.0m/s/s of acceleration available and car B also has 3.0m/s/s available.
At 100mph, however, car A has 1.2m/s/s of acceleration available but car B only has 1.0m/s/s of acceleration available.

Trying to put this into context, assuming both cars having an exit speed of 60mph onto a straight with 500m before the braking zone,
Car A will travel that 500m in 13.07s, arriving at 105.6mph
Car B will travel that 500m in 13.18s, arriving at 103.8mph

If I made Car B1, which is car B but lightened to the extent to take the same elapsed time on the straight, I'd have to remove 52kg(115lb)!!! to get the time down to 13.07s. This car would arrive at 104.8mph. This probably isn't representative of a kind of parity weight though, while they'd spend the same time on the straight, the 52kg lighter car would make time everywhere else and on all straights with a lower top speed. If I make the straight only 200m long, then the equal elapsed time weight reduces to 30kg (66lb)

These results are quite interesting and while it does suggest that there is performance to be gained from lower drag, it does need to be considered if adding parts, eg a fastback, whether the weight gain for the drag improvement is worthwhile. The information I am lacking is how much time would be lost everywhere else from increased mass. Does anyone have this?

I can run more tests through the calculator. Is there anything anyone would like me to try?
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Old 11-20-2018, 04:27 AM
  #43  
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I love calculations but what about a real world test? I can do this, but it costs time and money and Iíd ask for some financial help. Iíd also like input on how to do this correctly. I have two Miatas with a few top options and I live near Watkins Glen International, which is a fast enough track. I have an AIM solo for telemetry.

My street car is a 93 with 106 hp. It has the factory hard top. I could also fit the Treasure Coasf chop top. So for this car I could test open, vs chop vs hard top. I wish I had a Project-G soft top, as that could be interesting as well.

My 1994 race car makes about 150 hp and has the fastback. I can also remove the rear portion which leaves the chop top. And I could remove the top. So for this car I can test open vs chop vs fastback. I would like to test a factory hard top as well, but it wonít be easy to mount because the rear deck is now changed to support the fastback.

I guess I start a fundraising effort and book a two day HPDE. Are you interested enough in the results to put money towards this? What would the test day look like? What concerns do you have?
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