Help me to choose motors, please.

Hello.

I'm building a car. It has mecanum wheels. The weight of the car is 1.5kg, the weight of each wheel is 250grams, the diameter of each wheel is 10cm. wheels are printed on 3D printer. I;m also using two drive shields Pololu DRV8835 (http://www.pololu.com/product/2511). I have four Pololu micro metal motor reductors 50:1. The car goes forward and backward when the speed is set to maximum. At the half of the max speed the car doesnt move but there is a sound from the motors that show that they are trying to do that. I can turn the car as a non-mecanum wheel car (to wheels forward, two wheels backward when the speed is maximum). But mecanum wheels allow the can to move aside. This my motors can't do even when the speed is max. I'm thinking of buying Pololu2214 (http://www.pololu.com/product/2214). But I don't know will it be enough. How can I calculate that? I tried different suffaces at home.

I’d like to add. When I’m holding my car (wheels are in the air) the wheels turn as they should.
When the speed is set to 3/4 of max the car moves also, but slower of course.

We need to know what motors you are using, and what you are using to power them.

Have you measured the voltage from the power source when it is under load? Is it holding up?

First step is determine the speed and force (or angular velocity and torque at the motor shaft). Power is the product of speed x force or angular velocity x torque. Then choose motors with enough power, and gearing to get the desired top speed.

Tiny micrometal motors are not many watts, perhaps about 1, so if your mechanics needs 10W (say) to overcome friction and reach desired speed you have the wrong motors.

I'm using Pololu 50:1 Macro Metal motor reductor(6V). And I'm powering it with four AA Batteries. I didn't measure the power under load.

Yesterday I found this site http://www.robotshop.com/blog/en/drive-motor-sizing-tool-9698

And it seems I can use Pololu 2218 (http://www.pololu.com/product/2218). With six batteries pack of course. What do You think?

A DC motor moves depending on the voltage that is given to it. My current motor moves only whem I supply it with voltage 75%-100% max voltage(max=6V). And I can't increase the weight of my car any more because it does not move even at 100% voltage supply. I want to increase the weight of my car and make my car move at 10%-100% of max voltage supply (or at least 50%-100% voltage supply).

Lyubomyr: I'm using Pololu 50:1 Macro Metal motor reductor(6V). And I'm powering it with four AA Batteries. I didn't measure the power under load.

Then how do you know your problems are not power related?

Now you're saying macro metal - perhaps you should provide a link to the actual gear motor you are using...