Motor Ratings: RPM with/without Load?

When a motor says it's rated to X RPM under load what "load" are we talking about? Is there a standard amount of torque associated with it?

I'm trying to choose a motor that would work well for an Arduino controlled car. I'm planning on a servo to control side to side on the front wheels and the rear wheels being connected to the drive motor(s). It'll be relatively heavy when it's done I'm sure, but I don't know how good of a motor I should be looking at. I want a decent amount of speed, but I think torque is more important at this stage so that I can get it started moving. I don't want to buy a kit because I can get the parts cheaper and I'd rather teach myself rather than just follow a few directions.

This one looks the most promising of all the ones I've seen because the datasheet has a bit of information: Toy DC Motor with Leads - ROB-09608 - SparkFun Electronics
I've taken the theory for torque, but never truly had to apply it. T = r X F, from this I would assume the radius described would be from the center of the motor to the edge of the wheel and the force would be applied by the motor to the system yes? How would I determine how much torque is necessary to start turning a wheel? Since I don't have the various weights of the other components how would I determine the necessary force?
In short I don't know how to ballpark it and advice as to how to do so would be greatly appreciated!

Read this...

http://forum.arduino.cc/index.php?topic=168147.0

Also, the motor's listed are really small they will not push a heavy robot. Try this…

http://www.trossenrobotics.com/store/p/5761-Motor-Mount-Wheel-Kit-with-Position-Controller.aspx

http://www.robotshop.com/parallax-7-2v-motor-bracket-and-wheel-kit-pair.html

Since I don’t have the various weights of the other components how would I determine the necessary force?

You need to come up with a estimated weight. Like 1-2 pounds, 5-10, etc. A heavy robot to me would be 25+ pounds. To you it might only be 5 pounds. We need some general range.

I don’t want to buy a kit because I can get the parts cheaper and I’d rather teach myself rather than just follow a few directions.

What do you mean by kit? Like a motor and wheel kit or a complete robot kit. Chance are that you will be better off buying the motors/wheels/encoder in a kit.

Right. With a kit, you probably do enjoy a package-deal discount vs. buying inappropriate this and questionable that from here and there, but either way you can learn as little or as much as you want to.
Doing with a kit doesn't preclude research past its presentation, amassing a dog's breakfast of parts doesn't make anyone into a genius, though I think that the latter does tend toward a certain frustrations borne of unmet expectations spiral with the "trying-to-learn-this-stuff" people.

Thanks for the replies guys. I think I will look into kits because it might be overwhelming to try to pick out the appropriate pieces as mentioned.

spyro76:
When a motor says it's rated to X RPM under load what "load" are we talking about? Is there a standard amount of torque associated with it?

Rated load. The continuous steady load the motor can handle without overheating, usually. The datasheet
should show this. Often you get a graph of rpm against load too, this is more useful.

spyro76:
Thanks for the replies guys. I think I will look into kits because it might be overwhelming to try to pick out the appropriate pieces as mentioned.

Rather than a kit, you might think about trying to use an old toy R/C car - perhaps something you already have, or something you can pick up at a thrift store. I'm partial to Goodwill, myself; in many cases, on a half-off Saturday, you can easily pick up a nice R/C toy car for under $5.00. Most of these -will not- have the remote control, but just check that the wheels turn, that it sounds like the motor is rotating with the wheels, that there isn't any stripped gears, etc - and that nothing seems broken with the steering (check that you can move the wheels left and right, and that the actuator seems like it is working mechanically, and that the wheels "return to center" after adjusting the centering knob). Also, make sure that the wheels are "round" (some of these toys have rubber with "flat spots" from sitting improperly).

If you need something larger, you might look into a "power wheels" ride-on toy (note: for this you will need to figure out a custom method to steer the front wheels, which will take some additional parts and design to make work). Sometimes you can find these at thrift stores (typically under $50.00).

If you need something still larger - electric powerchairs (with differential steering) can make fine robot platforms, but they are generally for the really advanced builder who has a bit of cash to spare. Costs in the used market is all over the map, but if you shop carefully and patiently, you can generally find such a chair for under $500.00 (the lowest price I ever saw was at a half-off day at Goodwill - the chair would have cost $20.00! The motors alone are worth much more than that!). Generally, though, you will need a truck to transport it, plus a couple or more people to lift it (and/or ramps). Also, you will likely need to invest in new batteries ($$$) and a charger ($$$) - but for all of that effort and money, you will get a platform that can haul around 200-300 lbs of robot...

Somewhere in-between all of this is another option (one that can be pretty pricey as well) - that would be a hobbyist R/C vehicle. Most of these are electric powered, and some can be fairly large. They tend to be easy to control with an Arduino (just use the Servo library for the most part). Unfortunately, depending on the model of the vehicle, they can be pretty pricey to start out with such a kit. Maintenance can also be a chore and has its own expense. That said, the parts are all standard...

Oh - a few other options to look at:

http://www.robotmarketplace.com/