Non-shrinking mold material for one-piece race vehicle 1

[doginabathtub]

Hi,

I'm working on shell for a race vehicle made from carbon fiber. The shell is roughly torpedo shaped and surrounds a driver laying with head forward. The course consists of a downhill free-rolling section as well as an uphill section where the vehicle is pushed by runners. Here's some pics of the race:

http://www.tcpulse.com/2005/04/15/carnival05/buggy1/

In the past, we've made the shell in 2 pieces using a honeycomb carbon laminate floor pan and a carbon top section reinforced with foam ribs all vacum bagged and cured at room temp.

We'd like to make our shell stronger by making it in one piece. Originally we were hoping to make a mold out of blue polystyrene foam, but learnd that it shrinks significantly under vacum.

I'm wondering if anyone could suggest another material to mold with or of a technique we might be able to use to get a one-piece body.

Thanks in advance for any help!

-Bill

 

[alexit]

I've seen a CF man-powered submarine that was wound to a tapered form to create a very thin almost conical shell (that was also very strong.) Then they make a CF hemisphere using same technique and bond together.

I wonder if this "spinning" could make your shape (except open on the large end...)

 

[doginabathtub]

hmm, that would be an awesome solution, though i have no idea where one would get ahold of that kind of equipment.

also, can winding create non-circular cross sections?

thanks for the insight,

-Bill

 

[amorrison]

One design for ideas

http://courses.washington.edu/uwsub/images/Uw1.jpg

Google images – human powered submarine

http://images.google.ca/images?q=human+powered+submarine&svnum=10&hl=en&lr=&start=0&sa=N

There was also a writeup in “Popular Science” about 12 years ago.

 

[alexit]

Yes, winding can create non-circular but you must have proper contact points. Imagine two radii one large one small arranged like "c)" (but with large intersecting small at ends.) As long as there is no "undercut" you should be able to spin this profile (with draft to the form so you can slide shell off.)

 

[gwolf]

Doginabathtub,

Hugely entertaining project, I love this sort of stuff. I have a few points which you might find useful:

1) Your quest for lightness is not very important compared to aerodynamics and rolling friction. If your runners can already push the vehicle as fast as they can run then there is no point in reducing the weight any more. The point being that in the absence of wind drag, and rolling friction (which I don't know much about), and humans having to push, it doesn't actually matter how much the vehicle weighs - ask Galileo.

2) Your vehicle looks to me (from the aeronautical field) like it isn't a very efficient aerodynamic shape. However, I never have to deal with vehicles in ground effect. You could try looking at "Low Drag Bodies" on the net which may be useful, these profiles have been used for glider fuselages for decades. Some may say that at the speeds which your vehicles travels at, aerodynamics are not important but thay are wrong. These vehicles can get up to 60mph on surprisingly modest gradients. This is fast enough to make a significant impact.

3) Check out the Goodwood festival of speed soapbox challenge on

http://www.goodwood.co.uk/fos.

There are some very serious cars like yours there. I have also seen videos of this race and stability, handling, and safety become much more important than you would think. I have seen cars with larger wheels (10-18") crash badly on corners because the vehicle couldn't take cornering forces.

Hope this is of help.

Gwolf.

 

[davidinindy]

"The shell is roughly torpedo shaped and surrounds a driver laying with head forward."
You're telling me there is a person in that thing? Doesn't look big enough!
How are those things steered? I didn't see much explanation on the site, other than they roll down the hills, and are pushed up.

David

 

[doginabathtub]

gwolf,

thanks for the suggestions! we're in the process of doing some cfd analysis on some better shapes. we're considering carving a multi part wooden positive mold that would be able to be slipped out afterwards.


davidinindy,

haha, yeah, there's a person in there. our drivers are most commonly small girls (basically the shortest students we can find). the most common type of steering is an ackerman system with the front two wheels where the driver steers with a bar in front of their head (superman style). we've gone to solid axle steering for simplicity in alignment.

 

[davidinindy]

Holey cow... I'd like to see the girls that can fit in those things. They're college girls right? You don't go rade the local kindergardens for recruits? LOL Still seems unfathomable.

David