pic: 2168 2016 Drive Train Top View

[lnex1357]

“My team built our first custom drive base this year (as opposed to a kitbot), and were considering doing it from here on out. We faced problems with our wheels angling, even though we had side plates, and our chain tension.
How did this drive train work for you guys in competition? Did you find problems with your wheels angling (Stance) causing your your chassis sag at all? And lastly what chain did you use and how'd that work out?”

So in general, this DT is nothing more than a standard WCD with 10 pneumatic wheels and a sheet metal belly pan. The only a-typical design feature is #35 chain and associated modifications to the WCP single speed transmission plate spacing. That being said some lessons learned post the 2016 competition season, that are applicable to your questions.
“Wheel sag” is an interesting one. Admittedly I’m curious as to the details on that one. What we did experience was deformation in our DT rails. This deformation was seen in upward bending in each of our bearing block pockets. A number of factors contributed to this which include:

• Material Selection: We initially used the 0.1” wall Vex Pro tube stock and later at Championships switch to 0.125 wall. In most applications I would hold the opinion that the Vex Pro would suffice to do the job. But in this game it was not the ideal choice.
  
• Our pocket “ears” the slots for the bearing block bolts to slide in and out of as the attached chain is tensioned were milled out too large due to manufacturing defects by our sponsor, i.e., we had to mill our slots to 3/8” wide to fix the errors which allowed the shock load on the bearing to be absorbed directly by the material above the bearing block and not shared by bearing block mounting bolt. Overall going forward we will use exact center to center spacing for our bearing blocks based on an exact chain length with pockets only large enough to bolt the blocks directly to the tube rail as opposed to making pockets to support sliding the bearing block for tensioning.

As I said earlier, we used #35 chain and sprockets. We never experienced any failures with this design decision. However, it did present challenges with respect to sprocket selection while trying to keep the chain above the belly pan and also required longer standoffs and mounting bolts to generate the required spacing in the WCP single speed transmissions we used.

“This is very nice. What size and type of wheels are those? They look like 4in colsons, but I feel like they are bigger than that. This is very similar to what my team wants to make as an off season rebuild.”

We used 6” pneumatic “zig zag” ties from WCP. These tires provided great traction on the terrain defenses, but experience a much higher ware rate than the diamond tread available from WCP, e.g., if we weren't taking risks we should have replaced ours after each event. This is because when inflated they crown at the center as opposed to the diamond tread which remains mostly flat. Additionally, because of this dynamic the zig zag tread introduces more scrub and creates challenges with respect to software controller gains that are constantly required to change as the wheels ware in order to maintain performance specifically this year in the instance where we were using the camera in concert with the DT for auto targeting. One other thing of note is the diamond tread tires are not as tall as the zig zag tires and thus the zig zag tires appeared to do a better job of absorbing the shock loads imparted by traveling over the defenses. In general, I personally do not recommend pneumatic tires unless the game requires them because of the wheel scrub you inevitably deal with. We use colsons religiously and only used the pneumatic wheels this year to absorb the shock load and create a high grip on some of the terrain defenses. One approach to this can be to do what teams like 1678 and 177 did which was to have their drop center wheels as colson wheels and the remainder pneumatic.

“Which sprockets and gearboxes are you using?”

We used 12T #35 sprockets from Vex Pro through the entire DT and 3 CIM single speed WCP WCD transmissions. Next year we will be building our own custom transmissions again for arrangements and ratio goals, but in general have no issues with the WCP transmissions. One thing we did learn however like many teams this year is to stay away from the 11T w/12T center distance pinions from Vex Pro.