Aerodynamic efficiency: Narrow body, wheels outboard vs wider body enclosing
 

My project is a small reverse trike for 1 person for highway travel, probably using a 125 to 250 cc engine and weighing between 300 and 500 lbs, cruising typically at 55mph, and getting over 125mpg I expect. I’d like to get maximum mpg, though if there would be no more than a few mpg difference I'd probably go with narrow body (front wheels outside) as it would be a little easier to build and take less material.

The narrow body would result in less area. The front wheels would have small fenders close to the tires and probably something like moon disks to reduce drag. Drag coefficient would be worse due to some exposed front suspension as well as front wheels. Example is the HyperRocket, though it’s bigger and more toward sport-car.

The alternative is to make the body wide enough in the front to cover the front wheels and suspension – width would need to be enough to accommodate wheels turned and height enough to accommodate suspension travel, though just behind my head (roll bar and body tapering behind) would remain the highest part of the body at about 30 inches off the ground. There’s no need for more than 24 inches width to accommodate my body. Example is the California Commuter, though it’s lower, lighter and probably a little better aerodynamics than mine will be.

I plan to keep the front wheels small – possibly 10 to 12 inch scooter wheels with 3 inch tires, so maybe 15 to 18 inches high and around 3 inches wide.

Any thoughts about which approach would provide the least drag and how big the difference might be will be appreciated.

Thanks,
Scot

http://ecomodder.com/forum/showthrea...ole-26239.html

I went with the enclosed wheels option. Not because I was convinced it was better, but so it would provide side by side seating. If it were not side by side then the design might follow your idea. My vehicle will weigh about 1100 pounds with similar mileage and horsepower amounts. Had your weight specifications been crucial I might have chosen the same option as yourself, but the additional concern for me was crashworthiness.

regards
Mech

Yes, all good points. If you want side-by-side seating then you'll be wide enough to enclose the front wheels, and crash-worthiness is certainly a consideration - but not one I'm attempting to address. At 300 lbs. (target), I'm not going to come out well in a crash involving a 3000 lb. SUV! I'm willing to accept that for the advantages of light weight and minimizing aerodynamic drag.

So, for my project, there is still that question, which provides the best aerodynamic efficiency? Narrow body with front wheels outboard which gives less area but worse drag coefficient? Or wider body, enclosing the wheels, having more area but better drag coefficient?

For the record, my Forester weighs 3,152.6 lbs, and is not considered an SUV. Plenty of sedans weigh that much or more.

It depends on what you come up with. You have to have estimates for both configurations then compare.

IMHO, the real world contains too many strong crosswinds to make only looking at the 0 deg yaw condition worthwhile. I also think separate pontoon fenders have the disadvantage in crosswinds. *warning* I have no empirical case studies of this.

I think the open wheel/narrow body would get the least wind resistance, but that depends on factors like how nasty is your suspension going to be, and how narrow the body will be in relation to how wide the tires will be.

Ugly weighs in at about 3100lbs, our new SUVish vehicle weighs 4500lbs, and that's after the 500lb diet the factory gave it over the previous version.

Design your vehicle to go under an SUV in a crash. The SUV will rollover and make use of it's 20 airbags.

I'd go with open wheels as it's easier to fabricate and nothing beats seeing your own wheels so you can accurately position a vehicle on the road.

which
 

For extreme simplicity you might compare your open wheel monoposto at Cd 0.15 at it's frontal area,compared to the fully-enclosed body at Cd 0.11 with it's frontal area.

Hot rodders at bonneville found in the 1950s that an enclosed body has lower drag even though the frontal area is larger.

Without a comparison of your 2 proposed designs and the Cd and frontal area figures, any advice would be speculation.

I'd copy this design as closely as possible, were it me :D
http://www.diecastsociety.com/wp-con...03-640x320.jpg

Kidding aside, do what's easier/cheaper. The most important thing is that you build, and the difference between the 2 designs will likely be negligible.