Drag effects at low speeds?

[aerohead]

Originally Posted by examorph

Me and a group of people are making a small car which is powered by an electric motor for a race, I am having a hard time deciding on a design for the bodywork, after researching I thought the best design would be a tear drop since many say it has the lowest drag coefficient of any natural shape, what are your thoughts on this?

Some extra information: The cars maximum speed will probably not go over 30mph, the front and rear wheels have to be in-line.

An additional question, we are using bike tyres for the wheels on the vehicle, is it worth covering these up or at such low speeds will the drag not be noticeable?

The lowest drag forms are 'streamlined bodies of revolution.'Or 'teardrops.'
'Half-bodies',derived from streamlined bodies of revolution produce drag coefficients on the order of Cd 0.08 in ground proximity.When wheels are added the drag goes up to Cd 0.12-.13.With the narrow bicycle tires you might see on the order of Cd 0.10 if wheel fairings were employed.
If your team is attempting to stretch the range of the battery pack the 'teardrop' form could help with that.
In the Sunrayce '95,a 'torpedo'-shaped 'TNE-3',3-wheeler from Northern Essex Community College was achieving 150 mile range at 55-mph (US statute miles) on a single charge,equivalent to over 350 mpg.
This car was about 3.84 times as long as it's body diameter with Cd 0.10.It would be worth checking out.

[drmiller100]

what does the race track look like?

Will you accelrate to 30 mph, then maintain forever, or is there start, slow down, speed up?

Below 30 mph, weight often has more drag then wind. So, minimum weight is obviously a BIG goal.

From there, whether to add the weight of the body or not depends on the course. If you are usually below 15 mph, it might be tough to justify a body.

[Frank Lee]

Bodyless, below 30 mph, they will LOSE.

[Varn]

miller: I really doubt if the weight is that important on track bike. I am not trying to argue but to point out my perspective as a 60+ year old cyclist.

Aero is everything except for power available. On my upright bike, a 17 pound carbon Trek road bike, I can push it to about 28 mph for a short burst. but am comfortable at about 18-20 mph on a smooth road with little wind. On a simple, mostly faired recumbent (weight about 36pounds), I am comfortable at 22-25 and can go about 35-37 in a sprint.


If you have the option, put pedals in your electric cart. You will win.
I remember a solar contest in Japan in 1994. A VARNA was sent over with a small solar panel on the top and it kicked every other solar bikes butt. The rider was Myhee. He had just set the 24 hour record of 633 miles and was hot. I rode with Myhee in Michigan 25 hour in 2004 and spent a lot of the ride talking and hearing about his exploits. At Michigan he carried a bugle with him and every time I saw him during the night I was rewarded with a tune.

[Grant-53]

I recommend these two books:
High-Tech Cycling by Edmond Burke
The Leading Edge by Goro Tamai
I make my bike fairings from either 4 mm Coroplast or 1" hex mesh bonded between two layers of 5 mil plastic supported by angle aluminum and hard nylon tubing.

[examorph]

Wow, so many replies and some very good information! Thank you, now, I have a new question, length vs height vs width?
I have changed some things around on the car to reduce the length of it by almost half! but it has added a bit to the width but the material used to compensate for the width is no where as near as much as the material saved on length, also I have took the advice the majority of you have given and covered the wheels up.
so in conclusion, is it worth increasing the height and width to compensate for the reduction in length or should I really not worry as much about length and try keep the car as slim and low down in height as possible?

Also, one last question, is there any disadvantages of allowing the airflow under the vehicle?

[aerohead]

Originally Posted by examorph

Wow, so many replies and some very good information! Thank you, now, I have a new question, length vs height vs width?
I have changed some things around on the car to reduce the length of it by almost half! but it has added a bit to the width but the material used to compensate for the width is no where as near as much as the material saved on length, also I have took the advice the majority of you have given and covered the wheels up.
so in conclusion, is it worth increasing the height and width to compensate for the reduction in length or should I really not worry as much about length and try keep the car as slim and low down in height as possible?

Also, one last question, is there any disadvantages of allowing the airflow under the vehicle?

*For an 'enclosed-wheel' car,the air 'see's the car and it's reflected mirror image below the ground.
*For this 'ground-reflection' situation the air likes a car which is 5 times as long as it's height,or 2.5:1 for the body and reflection.This is demonstrated with the Aerodynamic Streamlining Template.
*If you're doing an exposed wheel design,you might want to treat the body as an aerodynamic section instead of a body of revolution,where the majority of air goes around the body.With sections,the lowest drag is reached when the length of the body is about 3.8 times the body width,just as with the TNE-3 car mentioned earlier.
*With faired-in bicycle wheels you're looking at Cd 0.10.
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*you're trying to minimize frontal area,reduce unnecessary body surface area (and it's friction),and slip in with the lowest coefficient of aerodynamic drag by shrink-wrapping the body around the driver and mechanicals while respecting aerodynamic minimums for attached flow.
*length of the body will be dictated by whether you're going to do a streamline body of revolution or streamline section.
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As to the under body flow,it appears that some of the 'special' shapes fair no better than bodies with 'normal' ground clearance (as the TNE-3 car).
*You've probably seen NUON's NUNA series of solar racers.They have exceedingly low drag coefficients.For a battery-only EV this shape might not have the interior packaging capabilities you'd need.You should check these cars out if you haven't already done so.
Sounds like a really fun project!

[examorph]

Hello, I am back, this project has not been started yet and I have been very busy lately so have had no time to do much extra work on it but now I finally have a bit more time to spend thinking about how I should design the car.
I have modelled the car and have ran some wind tunnel tests on it and got some results but dont really know how to read them....the wind tunnel test results show me the drag coeficient (what I wanted) but this is on a graph over time, as time pass's the drag coeficient changes, the velocity of the air going over the vehicle also changes, this is strange because I set the value for the air speed and it should be constant, so I dont understand why the drag coeficient and air speeds are changing, any ideas?

The graph I see is the exact same graph as the one shown at the bottom of this picture:

http://gfxspeak.com/wp-content/uploa...6541711650.jpg

and here is the 2d slice air flow image I see constantly changing with time:

http://gfxspeak.com/wp-content/uploa...6541170698.jpg

Please note, these images were from google and are not of the car I have designed, I will get some images of that soon.

[examorph]

Need 5 posts

[examorph]

1 more post

edit: sorry about that, please continue from 2 posts up.

Thanks