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Old 07-15-2010, 05:20 AM   #13 (permalink)
The Toecutter
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Rebellion - '16 KMX Framekit Custom electric velomobile
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Originally Posted by aerohead View Post
Those aftermarket hardtops pictured are beautiful,and way better aero than too-fast factory fastback.
The custom topped Spitfire instead of the factory GT6 would be a better choice, if you can find the top. The hard top would be a rare piece for that Spitfire and you would very likely have to make your own. Likewise, the GT6 rear end could be cleaned up with a fiberglass Kamm extension, and that would require less time than a fiberglass custom top and trunk to accomodate the Spitfire. The Spitfire MkIV/1500 DOES have an unearodynamic hard top that is more commonly available, and that has perfect potential for being modified to clean the aero as well.

The GT6 MkIII is probably more aero than the earlier versions of the GT6; Reverend Gadget used this body for a Spitfire conversion and it only needs 100 Wh/mile, both around town and steady 60 mph, using a relatively inefficient 6.7" ADC motor. The car is an extremely light conversion with LRR tires, definately less than 2,000 lbs.

The norm for Spit conversions, as long as they're using quality components and not scavenged forklift specials, is about 150-200 Wh/mile around town/60 mph, and that's without the efficiency tricks implemented by Gadget.

If you can find either a Spit or GT6, they're both excellent candidates for an aerodynamically efficient body with some work. You'll simply have to take different approaches to cleaning up the aero depending on which one you have chosen.

Originally Posted by chipX View Post
Thanks again for the replies.
Have found a piccie of a ex le-mans racer spitfire ( with a gt6 back end - supposedly good enough for racing aero )
Anyways , wanted to show you guys something interesting.
Look mid-way down the bonnet , its got a little deflector ?
Would you of thought this would be for " smoothing " the angle of the air hitting the windshield or what ?

These cars were hitting 134 mph from 109bhp 1.2 litre engines !
Thats probably around 70bhp at the wheels... not bad for 134mph
Ahh, the ADU4B, otherwise known as a GT4. It probably has more closer to 80 hp at the wheels counting brake drag, wheel bearings, and other small losses on top of the tranny/diff/joints; the manual transmission of the GT6 plus driveline are about 80% efficient combined, which isn't very good for a manual. Their loads are extremely low, as they have no power-anything except lights, dashboard, and radio. We will assume lights and radio are off(the ADU4B probably lacked the latter).

With 80 hp at the wheels assumed(brake drag, wheel bearings, transmission/driveline losses from 109 bhp), and assuming a mass of 730 kg, with old style bias ply tires with a Crr of 0.015, the CdA works out to 0.444 m^2 if 1.25 kg/m^3 is the air density.

This is also a CdA of 4.78 sq ft. If we assume a frontal area of 14.9 sq ft(GT6 had this), this is a Cd of 0.32.

This ADU4B would have a better CdA than a Honda Insight, and is a perfect candidate for a long range EV or fuel efficient and lightening fast hyper-miler compatible biodiesel rocket(I'd love an OM606 Mercedes diesel with bigger turbo and Myna tuned injector pump in an aeromodded Spit running grease someday... drool... but its internals would need upgrading to be suitable for the torque; heavier 2400 lb 240Zs tuned like this do 12s in the 1/4, imagine a sub 2000 lbs Spit tuned like this with appropriate gearing to reach 180 mph @ 5100 rpm, with that kind of body on not just think of the acceleration, but also fuel economy it would get if driven conservatively; you'd have a 60 mpg hwy car that could rip off 11s!).

I've once heard a Cd of the GT6 claimed as a 0.32 from a racer I spoke to years ago; maybe he was getting it confused with the ADU1B/ADU4B cars, as they are commonly mistaken as precursors to the GT6, and any Spit/GT6 does need lots of cleaning up around the bottom with regard to airflow. The Spitfire Mk I-III was quoted as 0.39 in a book titled "Streamlining and Car Aerodynamics" by Jan P. Norbye, and the MkIV/1500 Spitfire is quoted as a 0.42 on various internet sources.

There ARE fiberglass body kits for Spitfires/GT6s that are replicas of the racing body displayed in the above post. Click the link below:

Specs & Prices

There is another company that sells a front air dam for both spit and GT6 and a giant rear wing for the GT6(think Plymouth Superbird.

All of the products mentioned are expensive. If you're not rich enough to afford them, you're better off doing your own aeromods. They will be similar in effectiveness to the kit listed above(the kit is comparible to the ADU4B); in fact, a custom setup with attention exclusively to aero would probably do much better, although you may not have the weight reduction that could be provided by the above kit. There are some Spitfire/GT6 enthusiasts that have modified the stock hood to look exactly like the LeMans bonnet on the GT4 racers.

The GT6 would be less work than the others for diy modification, but it would not lend itself as much options as the Spitfire would give; the only effective path for cleaning up the aero for the GT6 without cutting off the roof is the Kamm style and deleting the rain gutters and applying fiberglass where necessary to re-shape the roof, but the spitfire would allow one to be creative due to it having no roof.

Regardless of which year/model of Spit/GT6 you buy, you can make them extremely good with aero, even the ones with ugly rubber front bumpers, and even on the cheap, if you're willing to put in the work.
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