[JamesCH95]

Originally Posted by Vikesrock

Not sure exactly what you mean by this. The tractive force the robot can exert is the sum of (normal force*coefficient of friction) for each wheel. The total normal force for all the wheels is fixed and is the weight of your robot. By lowering the center wheel you redistribute the normal force from 2 of the wheels (assuming they are entirely off the ground) across the other 4. If your center wheels have a higher coefficient of friction you will gain tractive force by doing this, not lose it.

Vikesrock beat me to the punch!

Another reason to drop the traction wheels is that their tread is much more compressible than the average omni wheel. If they are at the same height, the traction wheel won't get maximum grip.

One more reason to drop the traction wheels is that their tread will wear faster than the omni wheels, so they will still be able to drive the robot after some use. If they are not dropped then they'll wear out and provide less and less traction.

My team has used dropped-center traction + 4 omni wheels on two robots that won Xerox design awards, clearly it's a valid option.