-first post- Need help with motorizing a pedal car

[PainGypsy]

Fitst off, I have been reading this forum for a couple weeks (just getting into electronics) and think it is the best resource on the web I have seen yet. So thanks in advance <p>I bought my daughter a Kettle brand petal car and have set out to motorize it. It is very light-weight and well designed. It weighs about 25 pounds and my daughter weights about 35. Chain drive 10" tires it has a 2" diameter 12 tooth chain sprocket on it's rear axle. I want it to go at least 5mph. That means (if my calcs are correct) a rear tire rpm of arround 170. Now the math that I have no clue on and what I need help with:<p>How to coalculate Horse power of a motor.
How much HP and Torque will I need?
and some other things I can't think of at the moment. <p>The components I think I am set on is some sort of mosfet speed controller, 2 12v batteries (serial or paralell I'm not sure yet, depends on motor) and a motor with sprocket. I believe I can handle finding the batteries. It's the speed controller that I can't seem to find, all are far lower amperage than I think I need. But I need to be sure of the motor before I choose a speed controller. Any help in calculating and or finding the proper motor would be greatly appreciated. Then I would be glad to piece together or buy a mosfett speed controller, which I may need more help on. I'm not affraid to solder.<p>Thanks Alot!<p>>> Forgot to mention I want it reversable as well <p>[ March 29, 2003: Message edited by: PainGypsy ]</p>

[bwts]

hi PainGypsy<p>Electric wheel chair or golf cart motors are the thing ur looking for have a look at http://www.rotomag.com/ they motors for electric vehicles etc. But I would look for second hand or scraped myself (cheapskate that I am).<p>B)

[Chris Foley]

May I make a suggestion, and I hope you don't take offense. This is the type of project where something can go wrong, and you might really regret it. For instance, the MOSFETs might short out, providing full speed/torque to motor, child panics, does a brain freeze for just long enough to float into traffic, &c. Or, the car tips over, the batteries split open, and oops, they're lead/acid, and I hope it doesn't splash on somebody's face. Or an obstruction snags the kid's other foot from the open bottom of the pedal car, and she can't remove her foot from the accelerator. A dress or shoelace gets snagged in the chain. Or any number of other rather ominous, changed at least two lives (yours and hers) in a second and there's no turning back the clock possibilities. If you wanted to try for a humongous RC unmanned (or un-girled) vehicle, it might be a different story, but that's a somewhat more complex project.<p>Look. If you really need to do this, at least take a look at prior art. A lot of good engineering goes into those Barbie cars, if for no other reason than to prevent Mattel from losing their shirts in lawsuits. Take one apart, read the manual, try to find a service manual and peruse that, and do everything you can to at least be aware of the safety features of these things. Talk to an engineer about your project. But I'd definitely recommend using your intelligence and energy on something else.

[josmith]

There is a motor that is popular with robot builders called an "EV warrior" From what i understand they are a surplus item that sells for around $20 and produces 1hp. That's enough power to lift the car and passenger vertically at 5mph. YOu'll have to do a search for a supplier.<p>I have to agree that it is you're kid you're putting on this thing so be carefull!

[PainGypsy]

Thank you all for the great advice. I love my daughter very much and closely supervise her even if she goes outside, much less if she is riding her pedal car or bike. I have thought about all of the contingencies that you have mentioned andmore. <p>I have a friend who is a master sheet metal worker and I will be enclosing the chain and making a "floor board" of sorts. As far as the betteries I am unsure where I will mount them for safety reasons. One thing I have considered too is a OOPic getup with some fail safe code. Maybe an ultrasonic range finder collision prevention and a mosfet failure shut off. One thing I am relatively sure of is this car wont flip over.<p>I truly appreciate all of the advice and concerns posted here! I will check into the advised motors. The RotoMag and EV Warrior are both I have been seen online. <p>Do you guys know of any place on the web that would have formulas for HP and battery life and things when volts and amps are known? I've never been schooled in electonics :-\<p>Thanks again

[josmith]

The Battlebots forum on Delphiforums has lots of good information on batteries, motors, powertrains and related calculations.