2 powered back wheel and 2 idle front?

hello mates, my team and i was wondering if its ok to 2 motors on each one of the wheels in the back, while leaving the front wheels free spinning?
would it affect thing like
-center of rotation
-turning speed
-tilting with extended lift
-twisting axles when stalling

thanks in advance.

This would work fine. You can also chain the back wheels to the front wheels, so all are powered.

We use 2 motors on each of our front wheels on our current robot. We notice no difference between that and 1 motor on each front and back wheel. Except when you get stuck with your powered wheels in the air…

But this doesn’t happen too often.

Torque should not be significantly effected, although your traction to transmit that torque will be reduced with the surface area of contact from your drive wheels.

Center of rotation will be effected. Assuming your idle wheels are omni’s, then your center of rotation should be approximately between the two powered wheels. If your idle wheels are traction wheels, you are going to have a bad time.

Turning speed is mostly just effected by the distance between the drive wheels. Unless you make a narrower base, your turn speed should remain about the same.

Stability will only be effected by your contact foot print, which is not effected by idle wheels, however, if you somehow have the acceleration to pop a wheelie, it would be an issue.

Tilting should also only be effected by foot print with no regard to power or idle.

Twisting axles is possible, but would normally require some sort of poor gearing or programming decisions. You will have more torque on one shaft, but unless you are geared for a lot of torque externally, it still should not induce much twisting.

Axles should be fine, But a lot of torque on the plastic insert style wheels will turn the squares round.

Visual aide for change in center of rotation. Forgive me for using the center of mass symbol, it is just a nice clear marker of “here”.
2 Wheel Drive.png
4 Wheel Drive.png

Visual aide for idle wheel style. Not specified, but the driven wheels can be traction or omni, depending on your needs.

Edit to pictures, forgot to change/remove center of rotation.
2 Wheel Drive - Idle Omnis.png
2 Wheel Drive - Idle Tracs.png

We use four omnis on ours and we power only the front two and we still have a central rotation point. I was concerned about this when we were building it but we found it didn’t change.

Edit: This only happens if your weight distribution is very uneven

that is what his first figure shows


his figures do rely on the assumption of even weight distribution

This is correct. And negligible friction in the free wheels.

That is an interesting result. If there is odd weight distribution or friction prohibiting free spin of the wheels, this is possible, but i am surprised that there is no change at all. It should at least tend toward the new theoretical center, even if it does not fully reach it.

Ok this makes sense our weight distribution is not balanced between the wheels. Like 80% of the weight is on one side. Thats why we probably have some unusual turning point. Also it might not be precisely in the centre but its near enough we couldn’t notice.

We have done this in the past with a very light robot, 4 motors and it caused some wheel spin. I cannot remember if we were using Omni wheels or traction wheels. The wheel slip only happened when you accelerated.

I cannot give a direct weight that this will/won’t work since there are many robot specific factors.

I would not recommend this type of drive for this season because you need to cross the alliance bar to score in the 20pt zone. I’m not sure what your reason was to put your motors on just one wheel, but the easiest solution to get AWD is to chain the front wheel, as mentioned above.