Physics Projct: Electric Car

[savvy]

My sister asked me to help her with her physics project, and I thought that maybe asking you all would get her some ideas. Here are the rules for her project:

Goal: To build a car that runs on an electric motor that moves the fastest AND tows the most weight. You will be competing against your classmates to build a car that travels ten meters in a straight line the fastest. Then you will be competing to see which car can pull the most weight ten meters.

The Car:

1. You may use a prefabricated motor, these can be bought at Radio Shack, hobby stores, or electronic parts outlets, or you can use a motor from another device.
2. The wheel and axle on your car must NOT be an object intended to be a wheel or axle.
3. You may use whatever type of batteries you want, but the potential difference across the motor must not exceed 9V.
4. You must include some type of hitch that you can attach the weight sled to.
5. You will be competing in two competitions with your classmates. Part of your grade will be determined on how well you do in the competition.
6. As always, safety is the number one priority. If you do anything that makes your car potentially dangerous, you will not be participating.


I am sure some of you all have done a project such as this. What kind of motor did you use and what kind of materials? Any ideas are welcome, as well as where parts can be found. Thanks.

 

[AgentJean]

Originally posted by: savvy

2. The wheel and axle on your car must NOT be an object intended to be a wheel or axle.
.

What the heck is that supposed to mean? You're wheels and axels can't be a wheel and axel? WTH?

 

[SVT Cobra]

Make the wheels like a steam roller-More rubber on the ground=more grip.

Is this going to be on your normal wax floor? If so, do wax to wax for best grip, coat whatever you use for wheels in wax.

 

[MageXX9]

Since 9v is the largest, I'd go with that.

Make two different gearing systems, one for fast speeds, one for pulling, and switch them out for the corresponding competition.

 

[KMurphy]

How much is the cart supposed to weigh? If you really want acceleration power, you need a heavy battery. Look for 4 single cell bateries, 2 two cell batteries, or 1 four cell battery.

Here is a link to a 4 cell unit. It is not designed for motive power, but you don't need it to recharge repeatedly anyway.

http://www.apexbattery.com/interstate-p...eries-interstate-powercare-batter.html

If you go for a conventional store bought unit, parallel as many 9V batteries as will fit on your cart. Get a wheel chair motor designed for 6V. Do not get a motor with a higher voltage rating than the battery will supply; the motor's performance will not be good.

 

[funkymatt]

wth, we never got to do anything cool like this in school. lol

 

[Howard]

How much does she want to spend?

 

[drinkmorejava]

Keep in mind how voltage and current affect the motor speed and torque. If it's a decent motor, the manufacturer should be able to provide you with graphs.

 

[mercanucaribe]

Originally posted by: KMurphy
How much is the cart supposed to weigh? If you really want acceleration power, you need a heavy battery. Look for 4 single cell bateries, 2 two cell batteries, or 1 four cell battery.

Here is a link to a 4 cell unit. It is not designed for motive power, but you don't need it to recharge repeatedly anyway.

http://www.apexbattery.com/interstate-p...eries-interstate-powercare-batter.html

If you go for a conventional store bought unit, parallel as many 9V batteries as will fit on your cart. Get a wheel chair motor designed for 6V. Do not get a motor with a higher voltage rating than the battery will supply; the motor's performance will not be good.

She's building a science project, not a battelbot.

 

[Minjin]

Hmm...So there are no size limitations at all? What kind of weight are we talking about pulling? 2lbs, 20lbs, 200lbs? We definitely need more of a clarification on the wheel/axle thing. And whether or not you are allowed to make changes between competitions. I'm betting you can't do anything other than charge batteries. And speaking of batteries, they're going to want to use 3 banks of lithium primary cells in series. Lithium Ion would put you at ~7.2v and you want everything you can get. Nimh can be better when it comes to amperage output, but then you have to deal with 7 cells in series and making sure they are all equally charged.

 

[irishScott]

Originally posted by: AgentJean

Originally posted by: savvy

2. The wheel and axle on your car must NOT be an object intended to be a wheel or axle.
.

What the heck is that supposed to mean? You're wheels and axels can't be a wheel and axel? WTH?

Probably means that he can't go to some car dealership and buy wheels/axles. Has to build his own.

 

[RedArmy]

Originally posted by: irishScott

Originally posted by: AgentJean

Originally posted by: savvy

2. The wheel and axle on your car must NOT be an object intended to be a wheel or axle.
.

What the heck is that supposed to mean? You're wheels and axels can't be a wheel and axel? WTH?

Probably means that he can't go to some car dealership and buy wheels/axles. Has to build his own.

This is an electric car running at a max of 9v. I doubt that any 9v battery in the world could move the simplest wheel axle combo from a car dealership.

 

[jmcoreymv]

9V battery with boost converter?

 

[DrPizza]

the potential difference across the motor must not exceed 9V.

I don't see where it says "one battery."
Anyone else see a restriction on the number of batteries? You could have a whole fleet of 9V batteries, wired in parallel to give you a lot more available amps.

I don't see where it says you can't change gears between the two events... one gear ratio for speed, another gear ratio for torque. (And, if anyone protested that it isn't fair, even if it's not in the rules, it's a pretty simple argument that real trucks are built that way; i.e. 4WD low vs. 4WD high.)

Kinda weird; I just gave my physics class a similar project to do over the next month; except their cars had to be powered by a mousetrap.

 

[SVT Cobra]

Nice thinking DR Pizza!! Wire a couple of 9v's in parallel.


Gears also should be a must.

Also you need to get good grip on the tires like I said. And, what I believe the rule means is no toy tires/axle. As the teacher does not want someone ripping them off of an already made electric car, you need to make your own.

 

[Strk]

Originally posted by: SVT Cobra
Nice thinking DR Pizza!! Wire a couple of 9v's in parallel.


Gears also should be a must.

Also you need to get good grip on the tires like I said. And, what I believe the rule means is no toy tires/axle. As the teacher does not want someone ripping them off of an already made electric car, you need to make your own.

Yeah... It basically sounds like no going to your local toy store and using an R/C car out of the box.

 

[Tiamat]

You are going to have to get a motor that is capable of decent current. Buy some gear kits. For towing weight, you will have to play with the gear ratio so that torque is favored over speed.

 

[TitanDiddly]

What kind of scale are we talking here? Toy car? Golf cart?
If I were doing this, I would get starter motor and a car battery. Put a BIG wirewound resistor in series with the starter motor so it only gets 9v instead of 12v. This is much easier than finding <9v battery that will power a big motor. I'd use timing belts for power transfer/reduction- say 3:1 for speed runs and something much more(as much as viable) for towing. Essentially put two pulleys on each shaft(the starter and axle) but only have a belt on one at a time. The starter motor will probably have a built in solenoid, just use this for on-off. Use a 10m string for a deadman switch. Axle- just use a steel rod. Wheels- circular plywood with a groove cut in it, and a vacuum belt in the groove.

I'd consider myself highly proficient for this kind of thing- PM me with any questions.

 

[Fenixgoon]

Originally posted by: DrPizza

the potential difference across the motor must not exceed 9V.

I don't see where it says "one battery."
Anyone else see a restriction on the number of batteries? You could have a whole fleet of 9V batteries, wired in parallel to give you a lot more available amps.

I don't see where it says you can't change gears between the two events... one gear ratio for speed, another gear ratio for torque. (And, if anyone protested that it isn't fair, even if it's not in the rules, it's a pretty simple argument that real trucks are built that way; i.e. 4WD low vs. 4WD high.)

Kinda weird; I just gave my physics class a similar project to do over the next month; except their cars had to be powered by a mousetrap.

mousetrap car ftw... mine was the fastest accelerating , but it didnt go very far

 

[Bitek]

Pretty sure my electric truck would win this, if it didn't violate most of the rules

 

[SVT Cobra]

Originally posted by: TitanDiddly
What kind of scale are we talking here? Toy car? Golf cart?
If I were doing this, I would get starter motor and a car battery. Put a BIG wirewound resistor in series with the starter motor so it only gets 9v instead of 12v. This is much easier than finding <9v battery that will power a big motor. I'd use timing belts for power transfer/reduction- say 3:1 for speed runs and something much more(as much as viable) for towing. Essentially put two pulleys on each shaft(the starter and axle) but only have a belt on one at a time. The starter motor will probably have a built in solenoid, just use this for on-off. Use a 10m string for a deadman switch. Axle- just use a steel rod. Wheels- circular plywood with a groove cut in it, and a vacuum belt in the groove.

I'd consider myself highly proficient for this kind of thing- PM me with any questions.

How will plywood get any grip?

 

[Bitek]

I'm pretty into electric RCs, so I'll give my advice.

Since its for a school project, I'm guessing the scale is not going to be too big. 10M is short anyway. 1st things 1st, what the budget?

You may use whatever type of batteries you want, but the potential difference across the motor must not exceed 9V

This is a bit odd way of saying it, but does that mean they will be testing the batteries by a voltmeter before the start? The easiest battery to use would be a 6 cell NIMH batt (7.2V), but these come off the charger around 1.4V/cell even tho they are rated as 1.2V per cell. Alkalines are higher @ 1.5V/cell, but drop more V under load, so use this to your adv. Unlimited budget? Get a 7.4V Lipo (lithium polymer) battery and rule them all. 8.4V off a charger and no V drop under load. Careful w/ these tho, they catch fire if you are an idiot w/ them. See Towerhobbies.com to start. (Note that batts are cheap in smaller sizes, but require special chargers, I don't know what your setup is, so...)

If they are just going to go by the battery rating you could go upto 7 cells NIMH, but by voltmeter you are looking at maybe only 5. Those big 9V alkaline batts you see would also be above 9V by a voltmeter, so you want to get clarification on this point. Resistors don't drop voltage, they only limit current. You would need a voltage regulator for this, but its a PITA, runs hot, and would be a big power loss. Best to just select the proper batt.

Secondly, as far as motors go: if you are looking at a 2-3 lbs model, I would go for a 540 or 550 sized motor. You can go on Ebay to find these motors, either from drills (get a 7.2V or 12V motor, not too big) or from powerwheels replacements, or look in the RC section for "E-Maxx replacement motors." Example

For models under 1 lbs, you are looking at more a 380 or 280 sized motor. Harder to find, but look again at ebay or electronic shops like allelectronics.com.

How are these things to be controlled? I would get a RC manual speed control (MSC) off ebay or so. Drill parts also have something like this in the trigger assembly. They are dirt cheap, and you could use a servo and radio if you wanted to be l33t, else turn it manually and go.


Also, note that is said axels and wheels had to be "non-intentional," it didn't say tires. You could use sticky rubber or foam RC tires FTW.

Best help I can give with the limited details.

 

[mugs]

Originally posted by: AgentJean

Originally posted by: savvy

2. The wheel and axle on your car must NOT be an object intended to be a wheel or axle.
.

What the heck is that supposed to mean? You're wheels and axels can't be a wheel and axel? WTH?

You can't take the wheels off a toy, for instance.

 

[SVT Cobra]

Using a sticky adhesive on the tires would help it get grip and help towing.

 

[JW310]

Originally posted by: SVT Cobra

Originally posted by: TitanDiddly
What kind of scale are we talking here? Toy car? Golf cart?
If I were doing this, I would get starter motor and a car battery. Put a BIG wirewound resistor in series with the starter motor so it only gets 9v instead of 12v. This is much easier than finding <9v battery that will power a big motor. I'd use timing belts for power transfer/reduction- say 3:1 for speed runs and something much more(as much as viable) for towing. Essentially put two pulleys on each shaft(the starter and axle) but only have a belt on one at a time. The starter motor will probably have a built in solenoid, just use this for on-off. Use a 10m string for a deadman switch. Axle- just use a steel rod. Wheels- circular plywood with a groove cut in it, and a vacuum belt in the groove.

I'd consider myself highly proficient for this kind of thing- PM me with any questions.

How will plywood get any grip?

I believe that's what the "vacuum belt in the groove" would be for. The rubber of the vacuum belt would provide traction. Given the description he gave, that'd be my best guess.