### Author Topic: Building an Electric Car - How to optimize?  (Read 1350 times)

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#### Schlayer

• Full Member
• Posts: 57
##### Building an Electric Car - How to optimize?
« on: February 24, 2015, 09:26:27 AM »
I have a little project to do for a segment of a competition between high schools. For this event, we need to make an electric car using a specific motor and it may only be powered by 1 or 2 standard 9-volt batteries. The car is scored based on how quickly it can drag a sled (a cut open cardboard box) carrying various provided masses (up to a total of 5kg) a distance of 5 meters. The scoring uses the following equation:

distance * (masses + mass on sled) / travel time

Basically, I need a way to find out the optimal gear ratio to use with this motor : http://www.pitsco.com/Motor_385
Any help would be appreciated!

#### mklrobo

• Supreme Robot
• Posts: 547
• From Dream to Design at the speed of Imagination!
##### Re: Building an Electric Car - How to optimize?
« Reply #1 on: February 24, 2015, 11:31:15 AM »
Cool! I am on your side!
lighter 9 volt battery, or equivalent? Find the lightest material you can to make
your electric car, as that can be translated to pulling power.
What is the projected wieght of the car?
What is the diameter of the wheels?

Keep me posted....

#### Schlayer

• Full Member
• Posts: 57
##### Re: Building an Electric Car - How to optimize?
« Reply #2 on: February 26, 2015, 11:16:58 AM »
The problem my friends and I have is that we can't know the coefficient of friction between the cardboard box and the floor. We are allowed to compete with up to five vehicles, so I think at least one vehicle is going to be super lightweight and designed to floor it (pun intended) with the cardboard sled carrying a fairly light load and just try and run as fast as possible. I'm trying to make my own vehicle one that moves relatively slow but pulls a lot of weight. Right now I'm intending to use some scooter wheels I found lying around - I think I recall they are 73mm in diameter. I decided to make my own car a bit less weight conservative, as we will need traction to pull a heavy load I expect it may actually need to be weighted down itself to pull a heavier load (>/= 2kg???)
As we are limited to 'standard' 9V batteries (the rectangular block type) I think we're pretty much stuck with the weight of typical alkaline batteries. For our lightest vehicle, the builder used carbon fiber arrow shafts (hollow but strong and rigid) and made a 4 sided frame, and plastic attachments to mount the axel shaft in. That one will be the fastest by far I hope.
The car I built has a wooden square frame with two sides mounted with down-facing aluminum L beams, to reduce friction with the axels. Everything is secured with steel bolts and hex nuts, and my axels are 5/16" thick stainless steel. I built it to be really sturdy in case we need to put a lot of weight on the frame for traction, but it's still not more than around 4-5 pounds by itself. Again, the motor we need to use is the same for every team, so there wasn't a way around that. I'll see if I can't show you some pictures of the frame of my car as-is.

Thanks for the assistance, mklrobo!

« Last Edit: February 26, 2015, 11:22:47 AM by Schlayer »