I am trying to build a 4-wheel moving platform with differential steering on the rear wheels. Initially, the front 2 wheels were on the same axle, however it refused to steer, just go in a straight line. Then, I used a front swivel wheel, worked better but would not follow the same path from forward to reverse. Then used 2 front swivel wheels, same problem. The swivel wheels could turn 360 deg, not sure if this was the cause. Any thoughts or suggestions?

 

Normally you drive the front wheels with a single swiveling castor on the rear.

 

You could also steer the Front-Wheels with a Servo.
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Another alternative is to couple each front wheel to the wheel behind it and drive it like a tank.

 

AKA = "Skid-Steer"

You can also Motorize all 4 Wheels, and Steer all 4 Wheels.
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Four wheels in a diamond pattern. Single front and rear wheels on castors, and power the other two.