Aero design question

[winkosmosis]

Originally Posted by evolutionmovement

The Miata's big problems are the truly awful gearing—the high ratios way too short and the low ratios are too tall to just swap the rear end out. If I do an aero Miata project (it's 2 designs for a Miata, a tandem 2-seat Impreza IF I can make it look marginally attractive, and a cessna-bodied 3-wheeler I'm deciding between), I'm swapping the transmission with a 2nd generation RX-7. I believe you can swap the bellhousing and bolt it in, but I need to look into it better. From there, I'd be covering the undercarriage, building an external hood vent for the radiator, extending the rear bodywork into a Kamm/boattail type thing, and either building an aerodynamic hardtop or covering the passenger side of the cockpit over, removing the windshield, and installing a sliding teardop aircraft canopy over the driver's side. I'm just reluctant to only have a single seat vehicle.

A lot of people think the first gear is too low, so it would allow a taller axle ratio. The rest of the gears, you just stay in each gear longer, and it ends up being about the same. I never really thought about that until someone on Miata.net experimented with much bigger tires to get the final ratio down. He ended up getting much better gas mileage.

[winkosmosis]

BTW there is a racing Miata out there with no windshield. I think it has a removeable tonneau covering the passenger side.

You would definitely need a rollbar though

[evolutionmovement]

The problem with just bumping up the FD on the Miata is that the engine doesn't have a lot of torque to compensate and you'd have to put really large tires on it to make a substantial difference in acceleration. If you drop from 4.1 to 3.9 on the FD with stock tires, you get 12.23, 7.36, 5.19 final gear ratios for 1, 2, 3 respectively. If you go from stock tires (~22.74 diameter) to, for example, 205/50-17 (Mazda 3 size, although I know the wheels themselves are incompatible) you end up with 12.86, 7.74, and 5.45, all shorter ratios still. And that's only figuring on dropping from 4.1 to 3.9. I would like to go to 3.636, but would have to lose way too much weight to compensate. Instead, the '87-91 RX-7 has shorter low ratios and taller top. The final gear ratio results without a FD change and stock tires are: 14.25, 8.21, 5.6, 4.1, 2.83 vs. the original ratios of: 12.86, 7.74, 5.45, 4.1, 3.34. As you can see, the RX-7 transmission allows for much better off the line acceleration and lower rpms in 5th.

My plan was to instal a standard-looking single roll bar behind the seat, where it should fit under the teardrop canopy before it tapers too much without having to design the canopy to be too large proportionately with the rest of the body. If not, I may try to incorporate it into the fixed vertical hoop that would make up part of the frame for the 3-piece windshield, but I'd rather not have to do that. Welding 2 steel bars in place of each door and front to back down the near-center of the original cockpit should easily banish the cowl shake that makes me generally hate open cars. I just wonder how much weight I would end up losing in the end since the Miata doesn't have a real lot of glass to lose and the passenger seat and top are probably minimal as well and be adding back some extra bodywork and the remote-activated power mechanism for the canopy. The idea was something like a single-cockpit D-Type Jag or Lotus 11 with an enclosed canopy (which I believe Lotus made one of with a canopy as a LSR car).

[theunchosen]

RK you have CAD now what package do you have. I have access to about a half dozen different packages and I was wondering what your sketches looked like. If you are worried about someone stealing the design and your thunder you can just printscreen if you want.

Although if you are like me and are not comfortable with your sketches(in my case ugly) being seen I understand completely lol.

[winkosmosis]

evolutionmovement, here is a thread on taller final drive experimentation MX-5 Miata Forum - 6 Speed, 3.636 Geared, NA Miata

BTW I don't think the RX-7 trans just bolts in. IIRC, swapping in an RX-7 rotary and trans is more difficult than swapping a V8, much less using the trans with the original engine.

And here is what I mean by staying in each gear longer to compensate for taller final.. Let's say at 30mph you'd normally shift to 3rd gear. Well now your rpms will be a little to low, so instead you stay in 2nd gear until 35mph. No performance loss, you might even gain in some situations because that 30-35mph is done at higher rpm

[rkcarguy]

^^A friend of mine turbocharged a older Civic that had the HF tranny in it. It was all he could find at the time. It actually ended up being really quick on the freeway because he could drop 2nd gear at 60mph and do one hard pull to almost 90.
It's been awhile since I played with mazda's but I know the Miata and 2nd Gen RX7 used the same rear end gears, and I think the transmission interchanged but you had to use the front bellhousing part from the Miata on the RX7 tranny.

Unchosen, I have autocad 2000 LT.
I'm going to scan them in here when I get them done, the basic shape and size of the car is done, but I have alot of components to measure and draw yet.
Our laser cutter can post process a program right off of a drawing so the more details I can get in there now the better. The parts will come pretty much ready to weld up and start bolting everything in without having to layout and drill a bunch more holes.

[theunchosen]

I was going to ask how you planned on exporting them to reality. . .I wish I had access to a laser cutter lol.

I've got access to Auto, Pro-E, Sketch Pro and a few others so whatever format will likely be good.

[rkcarguy]

Back, been busy with the baby and getting the shop wired up.
I haven't done much with the drawing, but did collect the last drivetrain piece I needed, picked up a 94 Civic EX coupe to install another engine/trans into, and is has a VX tranny in it!
My drivetrain is now complete with the JDM 3-stage lean burn D15B engine, fidanza flyhwheel, and VX tranny.
Someone just started making tubular chromoly civic front lower control arms which are very tempting, being the stock ones are cast steel and probably the most dense part of the entire drivetrain.

According to the mpg calculator, if I can cut the weight down into the 1400's and the CD to 1/2 that of the Civic with the VX drivetrain and LRR tires, 124mpg is possible at 65mph. It's interesting that between 60 and 70mph there is a huge difference in drag, as the drag forces are cubed and must be getting pretty nasty by that point. Also that last few mpg is really hard to get when playing with that calculator, it takes major weight/drag reductions to see a notable change in the mpg once in the triple digits...

[rkcarguy]

Just an update...been busy as usual, but have gotten to spend a couple evenings on cad. I am still working on the nose of the car and refining the overall shape so it has some curves and doesn't look like a modified dirt track car.
Initial weight calculations with aluminum, glass, engine, trans, axles, shift linkage, wiring, wheels, tires, suspension, brakes, lights, radiator, fluids, steering rack, pedels and hydrualics, +10% for fasteners, is totaling around 1320#'s.
I can lose maybe another 100#'s if I start replacing the factory honda parts with aftermarket or custom pieces $$$, but for now it's a start. The hardest part is the windshield. Our states DOT requires that homebuilt cars use a DOT approved glass windshield, which means I'm going to have to buy the glass, pattern it, and sorta build the car around it. A prius windshield is fairly light and doesn't have alot of curve to it......