TitanDiddly
Guest
Right. In the early days, lot of combat robots(battlebots) used vacuum belts for grip.
-
We’re currently investigating an issue related to the forum theme and styling that is impacting page layout and visual formatting. The problem has been identified, and we are actively working on a resolution. There is no impact to user data or functionality, this is strictly a front-end display issue. We’ll post an update once the fix has been deployed. Thanks for your patience while we get this sorted.
Physics Projct: Electric Car
Page 2 - Seeking answers? Join the AnandTech community: where nearly half-a-million members share solutions and discuss the latest tech.
Right. In the early days, lot of combat robots(battlebots) used vacuum belts for grip.
KillerCharlie
Diamond Member
Oh man, my mousetrap car was easily the best. I used vinyl records for wheels. The body was hanging from eye-hooks with extremely low friction. I had a continuous gearing system on the axle - the part where the string wound around started out thicker and got narrower and narrower.
If you want this electric car to perform well, you definitely want to be able to switch gears for the two different parts of the competition.
LordMorpheus
Diamond Member
A little on DC electic motors that should be useful.
You get max torque at stall (0 RPM). However, at this point the coil is basically just a short and your current draw will be enormous. In addition, with no speed you get no power other than heat.
Torque decreases linearly with speed, and power being the product of speed and torque you find that that maximum power output of the motor is at 1/2 of its free (no load) speed. You will still be putting a fair amount of current through the motor (namely, .5*voltage/armature resistance) which can be above the rated capacity of the motor. Ten meters isn't very far, so you should be able to run the motor at it's maximum rated power for as long as you'd like in those ten meters.
I'm assuming you will be using a fixed gear ration (multiple ratios or a CVT would be overkill for this) so what you want to do is find the gear ratio that gives you maximum mechanical energy out of the motor over those ten meters. This is not a very difficult problem to solve for the exact gear ratio you want for a given motor.
You need to know the mass of the car, you need a rough approximation for frictional losses, both on the car and in your drivetrain, and you can solve for the gear ratio that will give you the fastest time.
say we have the following data about your motor and car:
stall torque = T_s
free speed = w_f
mass = M
friction (assuming coloumb only) = Fc
We want to find your gear ratio R (this will include the size of your wheel).
Using w(t) as the speed of the motor, we can find:
T(t) = T_s * (1 - w(t)/w_f)
The acceleration of the car will equal (R*T(t)-Fc)/M = A = X''
We know X'(t) = R*w(t), we can say that R*T(t)=T_s * (R - X'(t)/w_f)
so now we can say that (1/M)*(T_s * (R - X'(t)/w_f) - Fc) = X''
This becomes a simple linear first order differential equation that will give you the speed X'(t) as a function of time. Integrating this function (the constant of integration will be zero because you know that X(0)=0 (starting position) and X'(0) = 0 (standstill start) gives you position as a function of time. With this equation, set it equal to 10 (X(t) = 10) and solve for t, this time as a function of R, your gear ratio. Find the R that gives you the smallest value for t, and if you did it right it will be impossible for you to cover ten meters with your car and motor combination any faster.
When you start towing weight you increase both M and fc, which changes things. I'd be tempted to ignore this part and just smoke everyone in the drag race, but that won't impress anyone. If they'll let you change gear trains between heats make two with different ratios, one for racing and one for towing.
The towing part of the problem kind of ruins the fun because you can't optimize anything for it, it isn't specific enough.
You get max torque at stall (0 RPM). However, at this point the coil is basically just a short and your current draw will be enormous. In addition, with no speed you get no power other than heat.
Torque decreases linearly with speed, and power being the product of speed and torque you find that that maximum power output of the motor is at 1/2 of its free (no load) speed. You will still be putting a fair amount of current through the motor (namely, .5*voltage/armature resistance) which can be above the rated capacity of the motor. Ten meters isn't very far, so you should be able to run the motor at it's maximum rated power for as long as you'd like in those ten meters.
I'm assuming you will be using a fixed gear ration (multiple ratios or a CVT would be overkill for this) so what you want to do is find the gear ratio that gives you maximum mechanical energy out of the motor over those ten meters. This is not a very difficult problem to solve for the exact gear ratio you want for a given motor.
You need to know the mass of the car, you need a rough approximation for frictional losses, both on the car and in your drivetrain, and you can solve for the gear ratio that will give you the fastest time.
say we have the following data about your motor and car:
stall torque = T_s
free speed = w_f
mass = M
friction (assuming coloumb only) = Fc
We want to find your gear ratio R (this will include the size of your wheel).
Using w(t) as the speed of the motor, we can find:
T(t) = T_s * (1 - w(t)/w_f)
The acceleration of the car will equal (R*T(t)-Fc)/M = A = X''
We know X'(t) = R*w(t), we can say that R*T(t)=T_s * (R - X'(t)/w_f)
so now we can say that (1/M)*(T_s * (R - X'(t)/w_f) - Fc) = X''
This becomes a simple linear first order differential equation that will give you the speed X'(t) as a function of time. Integrating this function (the constant of integration will be zero because you know that X(0)=0 (starting position) and X'(0) = 0 (standstill start) gives you position as a function of time. With this equation, set it equal to 10 (X(t) = 10) and solve for t, this time as a function of R, your gear ratio. Find the R that gives you the smallest value for t, and if you did it right it will be impossible for you to cover ten meters with your car and motor combination any faster.
When you start towing weight you increase both M and fc, which changes things. I'd be tempted to ignore this part and just smoke everyone in the drag race, but that won't impress anyone. If they'll let you change gear trains between heats make two with different ratios, one for racing and one for towing.
The towing part of the problem kind of ruins the fun because you can't optimize anything for it, it isn't specific enough.
mugs
Lifer
Hah, funny... we had to do mousetrap cars in high school too, and I used CDs for the wheels. Our competition was based on how far the car went, not how fast it would go. I just used one axle, two wheels, and attached the mousetrap to the axle. Trip the mousetrap, it hits itself and starts rolling. Other people got their car to go 20 feet or so, mine hit the other end of the gym and could have kept going. The CDs had no rolling resistance.
It means that you have to use something creative, like CDs or something else for the wheels.
Thanks for all the replies. Let me clear some things up:
It's going to be on the school tile floor as the road surface.
There are no size limitations at all. (But common sense says it would probably need to be the size of an RC car, like the models you would buy at the store.)
She wants to spend probably around 40 dollars total. (Her teacher said the most expensive component would be the motor.)
The wheel and axle thing was confusing to me too. She said that you had to use something that is creative (like CDs as wheels).
She did say that there are a lot of "loop holes," so as long as they don't explicitly violate the rules, she can do it.
It's funny, but she said that all her physics projects have absolutely nothing to do what the class is learning about.
I myself don't know much about batteries and such, so the people that have asked me questions regarding batteries have already confused me. 😛
I think that she is allowed to switch out batteries for the competitions. But I will have her check with her teacher.
For all the questions that I did not answer, I will check them with her to be certain.
TitanDiddly
Guest
College level, I presume? I don't see a high schooler spending $400 on a project.
I stand by my earlier design reccomendation- if possible, get the starter from a diesel motor. They're generally much stronger because of the high compression.
I stand by my earlier design reccomendation- if possible, get the starter from a diesel motor. They're generally much stronger because of the high compression.
ultimatebob
Lifer
Which competition are you trying to win... The speed one, or the towing one?
If you're going for both, you'll probably need a multiple speed transmission.
If you're going for both, you'll probably need a multiple speed transmission.
Evadman
Administrator Emeritus<br>Elite Member
No no no. No gears. Way way way too complicated and costly. Everyone is trying to over-engeener this thing. I see 2 cheap and fast ways to do this depending on just how crazy you are. Either way, the batteries can be the same; a 7 or 9 volt drill. Use either a drill motor (if you really really want gears use a drill that has a 2 speed gear box like a dewalt) or a car starter as a motor. A drill would be cheaper.
For axles use a 1/4" steel bar from any hardware store. Cut to length and epoxy on some lawn cart wheels. For sprokets, fond an old mountain bike and take the 5 sprokets off the rear tire. Use the smallest on the motor, biggest on the axle. break the chain (with chain breaker or hammer) to make it the right length.
In other words:
Step 1: find drill
Step 2: find Bike
Step 3: disasemble bike, drill
Step 4: reassemble according to above
Step 5: test to find best sproket to use
Step 6: Win, become boy magnet
Step 7: go out with evad
Step 8: dump evad, go out with someone on the football team
For axles use a 1/4" steel bar from any hardware store. Cut to length and epoxy on some lawn cart wheels. For sprokets, fond an old mountain bike and take the 5 sprokets off the rear tire. Use the smallest on the motor, biggest on the axle. break the chain (with chain breaker or hammer) to make it the right length.
In other words:
Step 1: find drill
Step 2: find Bike
Step 3: disasemble bike, drill
Step 4: reassemble according to above
Step 5: test to find best sproket to use
Step 6: Win, become boy magnet
Step 7: go out with evad
Step 8: dump evad, go out with someone on the football team
TitanDiddly
Guest
In that case, borrow a 7.2v or 8.4v drill, chuck up a rod with a pulley on it. Build from there.
Black88GTA
Diamond Member
I actually had to do a similar contest in high school, although there were some differences. We had to build an electric car that would pull a weighted trailer up a ramp, with working headlights, powered by 1 AA battery.
I built an AWD car, using two gearboxes with interchangeable gears. I used R/C car wheels and very soft, grippy tires and custom made axles out of a brass dowel rod and then threaded the ends. The gearboxes and battery holder were attached to a piece of balsa wood. I built the whole thing on a Mercedes 500SL hardtop convertible plastic model, and painted the trim and wheels gold 😀 It looked like a pimp daddy monster truck. I still have the car too, you don't just throw something like that out!
I ended up taking first place in design, although only 6th in the overall running. There were roughly 45 entrants.
It was a while ago, but I think I used these gearboxes. That site has tons of stuff available for what you are doing...I wish I had something like that when I was building mine. It took me forever to find those damn gearboxes, and they were expensive. There's a whole list of them here. Free shipping if you spend over $25, too!
Good luck...
I built an AWD car, using two gearboxes with interchangeable gears. I used R/C car wheels and very soft, grippy tires and custom made axles out of a brass dowel rod and then threaded the ends. The gearboxes and battery holder were attached to a piece of balsa wood. I built the whole thing on a Mercedes 500SL hardtop convertible plastic model, and painted the trim and wheels gold 😀 It looked like a pimp daddy monster truck. I still have the car too, you don't just throw something like that out!
I ended up taking first place in design, although only 6th in the overall running. There were roughly 45 entrants.
It was a while ago, but I think I used these gearboxes. That site has tons of stuff available for what you are doing...I wish I had something like that when I was building mine. It took me forever to find those damn gearboxes, and they were expensive. There's a whole list of them here. Free shipping if you spend over $25, too!
Good luck...
wheels=CD with one of those rubber seal thing around the edge. axel= piece of small metal tubing, like what an antenna from RC control is. solder a makeshift pully thingy on it or something... even a sprocket then put a gear on the motor
power? not sure. small elec motor.. look into slot car motors... 1/32 or 1/24 scale use big motors but i ong thing there would be enough power... check it out. run battery thru something to amplify its power.. etc.. w/e
dont know a ton maybe my wheel idea helped. seen a kids at school win first place with those wheels
power? not sure. small elec motor.. look into slot car motors... 1/32 or 1/24 scale use big motors but i ong thing there would be enough power... check it out. run battery thru something to amplify its power.. etc.. w/e
dont know a ton maybe my wheel idea helped. seen a kids at school win first place with those wheels
TitanDiddly
Guest
I'd still prefer the starter motor though. You can get one for $20 at a junkyard. Pulleys would be cheap too, especially if you told them it was for a school project. Serpentine belts are a few bucks each at car parts stores. A car battery you can borrow. Some pipe for the axle from home depot, wood scraps for the frame/wheels are free by taking old pallets apart, or just buy it. It's cheap. The worst part of it is that you need engineering skills- the hardest thing would be making a groove in the plywood wheels for the vacuum belts to sit in. You could avoid that by gluing/stapling rubber on instead. One source of rubber is inner tubes.
Oh, and make it front wheel drive. You don't want it popping a wheelie and smacking somebody in the head. Just make sure you get lots of weight over the drive wheels.
I wish I were doing this project.
TRENDING THREADS
-
Discussion Zen 5 Speculation (EPYC Turin and Strix Point/Granite Ridge - Ryzen 9000)- Started by DisEnchantment
- Replies: 25K
-
Discussion Intel Meteor, Arrow, Lunar & Panther Lakes + WCL Discussion Threads
- Started by Tigerick
- Replies: 25K
-
Discussion Intel current and future Lakes & Rapids thread- Started by TheF34RChannel
- Replies: 24K
-
-
