CO2-powered metric racer car competition | help please?

[sophiasxxn_134]

TL;DR Summary: Goal: Design and build a CO₂-powered metric car that is fast, lightweight, aerodynamic, and tracks straight.

Requirements:
- Body length: 200–300 mm
- Axle height: 7–9 mm from bottom
- Width at axles: 35–42 mm
- CO₂ pocket wall thickness: ≥ 3 mm
- Body thickness: ≥ 10 mm anywhere
- Wheelbase: 105–270 mm

Evaluated mostly on speed.

Project is to design and build a CO₂-powered metric car that is fast, lightweight, aerodynamic, and tracks straight, out of a right-angled wooden block (measurements are currently escaping my mind right now). The block of wood I am supposed to be using is a right triangle. (the lengths are a, b, and c, just for reference. a is the shorter side and b is the longer side. b is intended to be the bottom of the car. c slants downwards from a to b.)

I guess this is kind of an accurate depiction of the side view, but the real thing is just shorter.

My instructor specifically stated b should not be changed too drastically as that's where we'll drill a hole 5 cm long for the CO₂ cartridge. obviously I can sand down above and below the hole but not too much.

some staple requirements:

• Body length 200-300 mm
• Axle hole 7-9 mm above the bottom of the car
• Body width at the axles 35-42 mm
• Co2 pocket wall thickness 3 mm minimum on all sides
• Body thickness ≥ 10 mm at any point
• Wheel base (axle center lines) 105-270 mm

What is the best design to attain maximum speed, distance and fuel efficiency?

 

[berkeman]

Welcome to PF.

Since this is your schoolwork project, you must do the bulk of the work. What research have you been doing so far, and what have you found? Can you post links to the reading you've been doing about this?

Also, are you allowed to ask for outside help on this project?

 

[sophiasxxn_134]

Some links:
- https://www.bolton.ac.uk/blogs/the-most-aerodynamic-vehicles-ever-made
- https://carvibehub.com/what-is-the-most-aerodynamic-shape/
- https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/aerodynamic-forces/
- https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/boundary-layer/

(credits to the owner)

When I was looking through images of some of the fastest land vehicles, trying to find similarities, the front end was almost always pointed (see: https://www.hotcars.com/fastest-land-vehicles-world/). Some other sources said the front end should be like a sphere and not too sharp, so I'm confused as to which is better. In the display cases at my school, they show each semester's fastest car, and for almost all of them, the sides of the body arc inwards smoothly before reaching back outwards to accommodate the wheel (similar to the third and fourth drawings above that I found while researching).

Also, in regard to your second question, I've emailed my instructor and he says that so as long as I build (not really build more like sand because he's going to cut it) the final prototype by myself or with someone in my class, he's fine with it. The main part of the project is to research about which designs are best for our cars, and to use that knowledge when we work on it.

To clarify, I have put forth effort to understand the topic assigned to me. The part I am so far confused on is whether the front part of the vehicle should be pointed or shaped like a teardrop. There were no textbooks or digital resources provided, and I've tried searching up ways to reduce drag, but the results were split. I do, however, apologize for not realizing the need to use the homework template, as I'm rather new and there were no messages on the home screen directing me of any kind.

 

[berkeman]

Thanks for the extra info -- that helps a lot to show that you are putting effort into this schoolwork project. That is important to us here when we help with schoolwork questions.

What is the best design to attain maximum speed, distance and fuel efficiency?

In the display cases at my school, they show each semester's fastest car,

So I'm a bit confused. Can you say more about how your design will be evaluated? When you say "fastest car", how were they tested? Is it like a scale-model standing 1/4 mile drag race where terminal velocity determines the winning car? What are typical winning speeds in this contest?

(Also, you use the term "metric car", but I don't know what "metric" means in this context. Could you say more about what that means? Thanks)

 

[gmax137]

What class is this assignment (is it physics, engineering, woodworking, or??) And what grade level (high school, university, or ??)

The main part of the project is to research about which designs are best for our cars, and to use that knowledge when we work on it.

So physicsforums can help you but we're not going to do your research. What do you think determines the car's speed?

In the display cases at my school, they show each semester's fastest car

Do they tell you how fast they went? This could be an important bit of info!

When I was looking through images of some of the fastest land vehicles trying to find similarities, the front end was almost always really pointed

Do you think your car is going to go that fast? What is important at very high speeds may not be so important at lower speeds.

I think this could be a fun project!

 

[sophiasxxn_134]

Can you say more about how your design will be evaluated? When you say "fastest car", how were they tested? Is it like a scale-model standing 1/4 mile drag race where terminal velocity determines the winning car? What are typical winning speeds in this contest?

One way my teacher has mentioned to measure my final model is through wind tunnel testing. It's unfortunate, but my teacher specifically stated "Some will go fast, others will go slow." There really is no other way to put it. It's just whoever gets to the Once the due date arrives, I'm inclined to believe there will be a track set-up outside the classroom (supposedly around 10-15m based on the particular day, based on previous classes). I'm sure there will be some kind of evaluation to ensure our models follow the guidelines. Unfortunately, no information was posted about how long it took for some of the winning models to finish the track.

Also, in this context, "metric" simply means all the measurements and rules follow the metric system.

 

[berkeman]

Unfortunately, no information was posted about how long it took for some of the winning models to finish the track.

It seem strange that your instructor is not making clear evaluation rules/guidelines/procedures available for this project. It is hard to do design optimizations without clear goals set forth. @gmax137 and I are both asking what sort of speeds are involved because it makes a difference in how to optimize your design. For example, at most practical speeds on a 10m track, air resistance will be extremely small, so making the lightest car will give the best acceleration and highest speed...

 

[sophiasxxn_134]

What do you think determines the car's speed?

I'm taking Engineering in the 9th grade, for the second semester. If we're talking about what speed is in context of this project, it would be the amount of seconds it will take for the car to reach the end of the track. I think some aspects that are important to consider are air resistance (for instance, you wouldn't want a flat front part because they cause turbulence; incorporating a streamlined shape would be better), a balanced weight with the mass' center a little towards the back, and straight tracking.

Do you think your car is going to go that fast? What is important at very high speeds may not be so important at lower speeds.

That's a really interesting thought, and one I hadn't touched upon. Thank you for making me aware of it.

 

[sophiasxxn_134]

It seem strange that your instructor is not making clear evaluation rules/guidelines/procedures available for this project.

I realize that, but I don't think it ever raised problems before. The only provided resource is a one-page document introducing the project and stating the specifications. There is a section on evaluation, however, it only says there would be a rubric given to us once we start building our model.

 

[berkeman]

Whelp, I would probably go for making the model as light as possible and not worry about the aero, given what you have said so far. So I would spend my time drilling holes all over the model to remove as much material as possible and still let it be structurally okay to bear its own weight. But that's just my take...

 

[sophiasxxn_134]

Whelp, I would probably go for making the model as light as possible and not worry about the aero, given what you have said so far. So I would spend my time drilling holes all over the model to remove as much material as possible and still let it be structurally okay to bear its own weight. But that's just my take...

Thank you for your help nonetheless, and I'll make sure to keep that thought in mind.

 

[gmax137]

I would concentrate on ensuring the car runs straight. And minimize friction in the wheel bearings / axles. Look into pinewood derby winners.

 

[Lnewqban]

Welcome, Sophia!
Engineers do not re-invent things that have been developed for a while.
They learn about what others have done, and try to improve it.

Just look up CO₂ dragsters.

 

[berkeman]

Just look up CO2 dragsters.

Nice find! I wonder if that's what the OP's competition will look like.

 

[gmax137]

Looks like there's some kind of string guide to keep them going straight?