Project: Chinese Wildfire 250-C

[Ptero]

Borrowed froim DoinTime at the Yamaha R1 Forum
Drag Coefficient/Aerodynamics of Cars Versus Bikes : Yamaha R1 Forum

Total drag (Dt) and drag coefficient (Cd) are two completely different but related things. Cd is equal to the coefficient of parasitic drag (Cdp) plus the coefficient of induced drag (Cdi). Cdi is minimal on motorcycles though so it can be effectively disregarded.

Total drag (in lbs) can be found by multipyling the Cd, dynamic pressure* (q), and the surface area (S).

* dynamic pressure is made up from the density ratio (local air density accounting for local altitude, temperature, and barometric pressure measured against standard density) multiplied by the air velocity in knots squared. The resultant of that is then divided by 295.

The formula looks like this:

Dt=Cd(q)S

As you can see drag is determined by five factors. Cd, surface area, local pressure, local temperature, and air speed.

As the above poster mentioned, high end sports cars generally have Cd numbers in the mid 0.3s. Motorcycles however have Cd numbers close to 1.0 depending on the model bike, size of rider, and riding position. In looking at the formula you should be able to deduce one major point. Drag is increasing exponentially with speed.

Plugging in some generic numbers show this better.

Variables: Standard atmosphere (sea level, 15 degree C, and a barometer of 29.92), Car at .35 Cd. Bike at 1.0 Cd. Car surface area at 19 sq. ft. Bike surface area at 7 sq. ft. (surface areas and Cds are approximate numbers).

50 kts

Bike Dt = 59.32 lbs

Car Dt = 56.35 lbs

100 kts:

Bike Dt = 237.3 lbs

Car Dt = 225.44 lbs

150 kts:

Bike Dt = 533.90 lbs

Car Dt = 507.20 lbs

Conclusion:

In the case of cars vs. bikes the bikes generally have triple the Cd but 1/3 the surface area so Dt remains about the same regardless of speed. Cars kick the crap out of bikes at high speeds because at high speeds the predominant factor is aerodynamics. As speeds increase the rate of acceleration decreases and the superior HP/weight ratio that gave the bike superior acceleration at low drag speeds cease to be the deciding factor. At high speeds where rates of acceleration are minimal you need horsepower to overcome the drag factors. As we have seen, aerodynamic drag is approximately equal so whoever has the most horsepower wins.

[hyperyaris]

Totally cool! Just be careful of those turns, with only three wheels.

[Ptero]

You've got that right. I actually hate conventional trikes for their handling but I was looking for a 250cc trike that I could afford. I love reverse trikes, however, and also have a Trimagnum that wants to get together with the Northstar engine in the 98 Caddy out in back. Did you know reverse trikes like the Trimagnum have skid pad ratings as good or better than a Corvette?

[Christ]

Handling characteristics are changed greatly when you go from four wheels to three.... why do you think autocrossers take tight corners late? To get that back wheel off the ground.

[evolutionmovement]

Most of my designs are reverse trikes. I've loved them since I first saw a Morgan almost 20 years ago. For the way I use a vehicle, they're nearly a perfect solution.

[aerohead]

Ptero,the weight looks real good.Frontal area is that of a CIVIC.I'd guess in the neighborhood of Cd 0.40.-------- If you have a chance,get to a public scale and weigh the Wildfire with only its front wheel on the scale.I like Vetter's idea of the tail.We need to have a sense of where the center of gravity is for stability.If you get popped by a strong sidewind gust,you want the car to weathervane,turning the car into the wind,cancelling the yawing-moment,not pushing the nose around where the side catches even more air,causing you to go out of control.You might be able to use a bathroom scale,one wheel at a time.---- If she were mine,I'd mount the seat as close to the floor as I could and cut as much roof and sides away as possible,substituting a bubble canopy with DOT approved laminated safety glass and working windshield wiper in front of your face.Looks like a really fun project!-- Keep it brightly colored as visibility will be an issue with Ken and Barbie out there driving near you.

[Ptero]

If you get popped by a strong sidewind gust,you want the car to weathervane,turning the car into the wind,cancelling the yawing-moment,not pushing the nose around where the side catches even more air,causing you to go out of control.

I'd put tractor brakes and yaw props on it, but with Craig's tail it will just blow off the highway and I won't have to worry about weathervaning. The thing I like best about it is that if I ever build the rigid dirigible I've always wanted, I can use it as the control car and then, after I've moored, I can release the clamps from the roofrack and drive into Lakehurst. Hey, I can use the tail as a rudder!

Seriously, I will post the weights when I start work.

[aerohead]

Originally Posted by Ptero

I'd put tractor brakes and yaw props on it, but with Craig's tail it will just blow off the highway and I won't have to worry about weathervaning. The thing I like best about it is that if I ever build the rigid dirigible I've always wanted, I can use it as the control car and then, after I've moored, I can release the clamps from the roofrack and drive into Lakehurst. Hey, I can use the tail as a rudder!

Seriously, I will post the weights when I start work.

Ouch! I think the helium mine near Amarillo,Texas has about played out now.If you have to use hydrogen,stay away from that aluminum-powder /nitrate dope on your fabric.We don't want to hear about the WildenBurgh Disaster!

[MetroMPG]

Originally Posted by Ptero

Borrowed froim DoinTime at the Yamaha R1 Forum
Drag Coefficient/Aerodynamics of Cars Versus Bikes : Yamaha R1 Forum

I think if you plug the numbers into the aero/rolling resistance calculator, you'll find the motorcycle example they used will have less total drag than your 3-wheeler (mostly due to its higher Cd than the car example they used).

Anyway, it's academic. You've decided this is your project.

[hyperyaris]

When the single wheel is in the rear, the traction is much better than when it is in the front. And it really is not a matter of tire traction. It is a matter of physics making the entire cabin topple over.