Crab steer rover

###Design
I want to make a rover that can drive like a car, have zero turn radius, and only using 2 servos for direction control. The idea was to steer left and right wheels separately, but front and back wheels synchronously. By this way, the space between left and right wheel is totally free to mount anything high and low. The rover can also go sideways, but only sideways. It cannot go omnidirectional.

The servo steers two wheels at once using pulleys and strings. I used nylon. Assembling, adjusting the tension, and maintaining proper orientation of the wheels was hard.

Each wheel is driven with a N20 DC gear motor, the 300rpm variant. The motor is stationary, and torque is transmitted with bevel gears. Each motor must be driven separately like mecanum wheels.

Each wheel strut is held on the chassis by a bearing, using 3mm steel balls.

###Printing
chassis x2, one mirrored
middle_frame x2
bearing_inner x4
ball_cage x4
wheel_strut x4
motor_holder x4
pulley x6
servo_pulley x2
gear1 x4
gear2_wheel x4

###Non printed parts
N20 gear motor, 300 rpm x4
SG90 or MG90 servo x2
Additional servo mounting screws or similar sized x4
3mm steel balls x24
Nylon string, 0.5mm(?) thickness
M3 screws, length 6mm, 10mm, 30mm x...
Rubber strips for wheel grip
Board, wooden or acrylic or cardboard, for mounting electronics
Controller with 4 separate DC motor drivers (equivalent to two L293D or L298N but don't recommend those, I used 4x RZ7886)

###Post Processing
Use m3 hand tap to make threads for some holes. (Which? Examine the blend file...)

How to assemble the bearings:

1. Hold the outer bearing part vertically.
2. Put 6 balls on the outer race, clumped together.
3. Put the inner bearing part.
4. While holding both parts, distribute the balls roughly, using tweezers or toothpick.
5. Orient the whole thing horizontally.
6. Distribute the balls evenly.
7. Put the ball cage.

###Evaluation
Video of it running around: https://youtube.com/shorts/hL1gf4FBLTY?feature=share
Maybe driving accuracy can be improved using gyro, but for now I don't continue the project and leave it here for anyone interested to develop further.