We have newbie questions...

This is our second year with the vex iq bots and my kids were wondering…when you use the Omni directional wheels on a 2 motor driven bot, are the Omni wheels connected to the motors or to the free axles?

The free axles.

Quite often the front and back axles would be connected by gears (like the Clawbot design) or by sprockets & chain. In that case the omnis could either be on the front or back, but most drivers prefer them on the front.

Jrp62 reports what I’ve seen, the omni wheels are usually on the front of the robot. The only exception would be for a robot that is exceptionally front-heavy, then I would drive the front wheels and put the omnis on the back.

Exactly. It makes most sense to have the omnis on the lightest end of the robot as it will naturally try to pivot round the wheels on the heavier end. But as Jpr62 says, this is usually the free wheels rather than the driven wheels as you most likely do not want side-to-side slip on your driving wheels.

It really just depends on how the team wants the robot to handle. I would recommend having them try both front and back.

If you look close, one of the two World Championship robots from last year (team 323 from Indiana) had a 4-omni-wheel base, where the robot could freely roll sideways.


Thanks for the input! We have tried them on both the front and the back and since the bot is kind of top heavy, (it’s a mix of scoop, and linq) the kids decided on putting the wheels on the back free axles. It now drives great but won’t go up the bridge easily now. Any thoughts on how to fix this new problem?

That’s a classic robotics engineering problem. Two solutions: back up the ramp (so the powered wheels are at the rear as the robot climbs) or add some gears to transfer power to all four wheels at once. Here is a video of a VRC robot that would only climb in reverse: https://youtu.be/rqovGM8Tl7Q

is there a video around to show how to do the gear thing? I’ve never seen that before.

The clawbot in the VEX documentation uses a center gear to link the front and back wheels.

I see how thats possible but our kids question was how does it turn if the motors are in the middle?

Your question isn’t clear. It’s still tank drive, so if left side is stopped and right is forward, the robot turns left.

Or if you were asking how the robot moves on the clawbot with the motor in the middle and the wheels attached via gears the answer is the motor gear meshes with the gears on the wheels and drives them.

Note, if you need a longer drive train than

O - M - O

you can put idler gears in the middle to extend the wheelbase

O - o - M - o - O

you just can’t do this:

O - o - M - O

you need to have idler gears on both sides of the motor to keep the wheels moving together.

And yes, you can go wild and crazy:

O - o - o - M - o - o - O

Hope this helped. If not, try asking your question again. Sorry about the character art.