[gamecrane]

I have retrofitted both gear motors from a Kidtrax BMW car to a beach cart I'm building. I only need the slow speed, 2.5 mph, or something close to that. Wired in series, I get the slower speed, but one motor runs slow, and almost stops when off the ground. I understand from what I've read that this is normal. How can I be sure both wheels are pulling when on the ground with a load? I just want to make sure both wheels are pulling for better traction through loose sand. Will always running the motors at slower speed cause them to burn out faster? Very simple set up, I only need FWD & slow speed for treks down the beach and back.

[wired]

running motors wired in series is like a limited slip differential. One tire grabs hard and the other tire will spin. Motors wired in parallel are like positive traction differentials. Both tires spin at same speed no matter what. The only thing you can do to limit speed is either choose a gearbox that runs at your desired rpm or run at a voltage that suits your speed. a variable throttle is a good option for your needs.

[Hammer-fm]

I'd call it akin to an open differential.

Regardless, if you want the motors to run slower but both always get the same voltage (which is the closest thing to something like a posi-trac), you will have to keep them in parallel and either retrofit lower-speed motors OR run a lower voltage to them. You could use a PWM controller or a strong buck converter to reduce the effective average voltage. However, doing this reduces the maximum amount of torque you can get and if your goal is for it to be self-propelled across sand or other high-drag surfaces this may not be that great.

If you went to a high-turn motor with a 8-10,000 RPM speed @ 12V, you'd get 2.5-3mph and likely retain more of the torque. For instance, this one: https://www.robotshop.com/en/rs-775-motor-7000rpm-12v-7613oz-in.html

The stall torque is a little over 2x that of the stock motor, and the speed is a little less than half. I don't know what size wheels you're using, but it would do ~3mph with a 22T gear (in a 21-22-23 gearbox) and 15.5" tires, and it would have a bit more torque than the stock Hurricane (15-tooth) gearboxes do.

There are better choices (I'd target 10-12k rpm), so that's just one example.