This is just an announcement to anyone who may be using the 2011 C-Base shipped in the KOP. The wheel mounting holes on ours aren’t centred along the frame which makes our wheels tilt on one side and elevated on the other.
We tested out our drive base today, which we saw no prevalent issues with until we fired her up and realised that the drive wheel on one side had no traction. When we looked at it in the workshop, we noticed that one of the wheels was tilted and the other side was elevated.
I just checked the assembly instructions and saw no warning of this which lead to us having to rebuild our robot tonight, but this is just a slight alert to prevent this from happening to other teams.
You’ll want to make sure all the holes are lined up either all high, or all low depending on how elevated you want your wheels to be.
This is by design - if you line up the center wheel holes so that they are below the outside wheels, your middle wheel will be lowered to facilitate easier turning.
This is actually by design. Though it is odd that there isn’t a mention of it in the manual. The center wheel is purposefully 1/4" lower than the outside wheels. Having the center wheel lower makes the robot “rock” to either the front or back side, so that only 4 wheels are touching at any time. This drastically shortens the wheelbase and makes the robot much much easier to turn than if all 6 wheels were even or if you were going with 4 wheel drive.
I believe the C-base is designed for a 6WD with a dropped center configuration. This is an INTENTIONAL feature of the drive base because it will allow your robot to gain the traction of a 4WD, without the turning difficulties that arise from that. If you search CD for “6WD Dropped Center” or “West Coast Drive” you will find many examples and discussions about this type of drive train.
Feel free to ask any questions you may have here as I’m sure myself or many others would be happy to add some clarification for you.
-Brando
Edit: Beat twice in the time it took to type up my response!
This year, our drive system is going be 4 traction wheels in the back of the robot, and 2 omni wheels in the front. They are all going to be powered by one motor on each side (by linking chain.) Will the dropped center wheels be a problem?
No, but the omni wheels are really not necessary for your setup. You could use 6 traction wheels and achieve the same effect you are trying to get with the use of omnis.
This is a great lesson for you guys! It’s a real eye-opener to the intricate details in the design of things. Hopefully it won’t take too much to fix. While you put it back together, pay attention to the details of how you put the drive train together – the wheel, their spacing, everything should be a perfect mirror of the other side.
Doing so increases quick maintainability, efficiency, and resolves drive train skew when attempting to drive a straight line.
Even if you are not using the 6wd center wheels and are going with a four wheel drive setup you MUST make sure that all four side frames are setup with the center hole down, because the other axle holes are also offset from the centerline.
I understand the analogy and the physics behind the design. This thread was just implemented in order to give other teams a heads up in case they were wondering why their drive base was misbehaving.
As for the quick rebuild, it was really exciting to have a time of such critical thinking and delegation. We have a lot of new members on the team this year and this was probably their FIRST troubleshooting experience.
Overall, it was a great night for the whole team, one to remember.