I am building a go kart, with an arduino as the brain. I need to have a gearmotor to turn the front wheels left or right(the entire front axle turns). What kind of motor/controls would I use?
The kart could weigh up to 175 pounds with rider. The kart uses two 12v car batteries. I also need to make sure that the motor doesn't stall and draw a lot of current (the batteries might explode). One more question, how would I safely connect an arduino to the batteries while the batteries power a 250W motor (can I connect it to one battery even though two batteries are in series)? Thanks!
You need a suitable motor driver/controller, and you should power the Arduino from a separate battery pack (if you don't you risk it resetting at random if the main batteries are overloaded. They won't explode, but you could conceivable cause the plates to buckle on overload (this is rare in modern car batteries which are designed for bursts of 600A or so ().
[ incidentally car batteries are not deep-cycle, so not a great choice for traction application. Deep cycle batteries are designed to sustain power levels till nearly discharged and to last more discharge/recharge cycles. All lead-acid batteries are wrecked if you let them fully discharge (below 11.5V), you must store them fully charged, and charge after each use if you want them to last ]
Heavy duty robot motor controller unit is probably the best match for what you want. As for the mechanics that is another matter - again robot making forums are a good bet.
The power of motor depends on the speed and acceleration you want. I fail-safe off-switch is a required safety measure - if the electronics go wrong the driver must be able to cut the power. For a front-wheel drive go-kart I suspect the motor won't stall, it will spin the wheels!
You probably want to ensure that the torque at the wheels is enough to give an acceleration in the 0.5 to 1.0g range at low speeds and that the natural speed of the motor at 24V corresponds to a sensible top-speed for the kart - little exercise in mechanics there 
I'm sorry - I think you misunderstood. I want to turn left/right, not power the wheels. And I am not sure that the batteries are car batteries - They are from a razor electric scooter. They say "sealed lead acid 12v" so I assumed they were car batteries.
gizzmotronics:
I'm sorry - I think you misunderstood. I want to turn left/right, not power the wheels. And I am not sure that the batteries are car batteries - They are from a razor electric scooter. They say "sealed lead acid 12v" so I assumed they were car batteries.
You will need to figure out how much torque is needed to turn the wheels of your fully loaded cart, and the speed at which the wheels need to be turned left/right. You probably need to look into a linear actuator or some type of "monster servo".
Just looked into monster servos and linear actuators on pololu.com, and they all cost more than $75. Are there any cheaper options?
Below is a DIY linear actuator made from a B&D cordless screwdriver (~$10 at walmart).
gizzmotronics:
Just looked into monster servos and linear actuators on pololu.com, and they all cost more than $75. Are there any cheaper options?
First off - SLA (sealed lead acid) batteries are -not- car batteries; while there do exist "sealed" car batteries (that is, ones which you do not need to top off with distilled water every now and then), they aren't the same thing. SLA batteries are also known as "gel-cel" batteries; the electrolyte in them is in the form of a gel, and the batteries can be used in nearly any position. Car batteries, on the other hand (even sealed ones) are designed to only be used in one upright position, and must be secured properly to prevent tipping. The electrolyte inside (whether sealed or not), is a liquid, and can spill.
Regarding servos/linear actuators: If you want this project to be successful, you are likely going to have to spend some money. You could try to cheap out and use something like a window actuator motor, or a wiper motor from an automobile (or even a seat adjustment motor), but such motors aren't designed for the application you are wanting to use them for. As such, you may get lucky and have one work for a while, only to have it fail later (perhaps at a critical point).
Furthermore, understand that the actuators are only one part of the problem; the motor controllers (for the cart motor and the steering actuator) are likely not going to be very inexpensive either; just keep that in mind.
Finally - you need to be aware of, and design in, failsafes - both manual and automatic (and maybe even remote) into your system; for instance, you need to be able to kill the power if the linkage to the steering connection fails, or if the feedback potentiometer goes out of spec (or is disconnected in some manner, or a wire breaks, etc). You need to think about this part of your system first - before you build any of the hardware, and before you program any of the software. This isn't something you want to have as an "afterthought" or attempt to "bandaid" in at the last moment. Security and safety design is critical, and should be done at the earliest possible level of a project, to help ensure that it is implemented properly, and without faults.
gizzmotronics:
I am building a go kart, with an arduino as the brain. I need to have a gearmotor to turn the front wheels left or right(the entire front axle turns). What kind of motor/controls would I use?
Ah, confusing language, a steering servo for a go-kart doesn't "turn the front wheels", it moves the steering linkage (apparently usually a bell-crank for go-karts). Again you need to know the torque and maximum speed requirements before choosing a gear motor.
Thanks for the replies! I think I am going to build a rack and pinion steering. Will a smaller motor/servo be able to turn that?
gizzmotronics:
Thanks for the replies! I think I am going to build a rack and pinion steering. Will a smaller motor/servo be able to turn that?
Maybe, mabe not. vague requirements get vague solutions.
i have found that useing normal car steering for anything over 5 or 10 pounds is going to be expensive. The servos/linear actuators and controllers all have to be much beefier than you would think. For this reason Im going with tank style steering. Obviously not great on a gokart....
you might check out moddedpowerwheels.com as well as they have RCed a bunch and the biggest issue is always steering.
along with the servo/actuator or whatever you need a servo saver of some type to keep from stripping the gears the first time you hit the curb and whack your front tire.