More common configurations would be 2 colsons + 1 omni (front or back), 1 colson + 2 omnis (front and back), or 2 colsons + 2 omnis (front and back), or all Colsons. But as long as you have the center wheel or wheels dropped below the front and back ones, any of these should be fine (including the one you posted).
Yes, unless you’ve got omnis on all corners you need a center drop.
If the tubing is thick enough you can bore the 1.125" holes (NOT 1.25") directly in to it, but most teams will opt to use some form of bearing blocks. They don’t have to be versablocks specifically though.
Careful on (1). There is always scrub friction, and sometimes you get lucky. 115 in 2016 had no center drop on a wide drivebase and suffered greatly. We had to add a drop in the end.
If you want to do no drop, put omnis in all 4 corners to be safe. Ideally, do a small 1/32" or 1/16" drop anyway.
Depends on your drive dimensions and preferred driving characteristics. The longer the drive base, the more likely it is you may need a center drop. The wider the drive base, the less likely.
If you get your preferred center-to-center distance dead on, you can probably survive without versablocks. But if you haven’t previously experimented with chain/belt to know the preferred c-to-c for your chain/belt selection (accounting for wear/stretching), then Id suggest versablocks to enable dynamic tensioning.
I would recommend a center drop (at least 1/8" overall)
Also, don’t stick bearings straight in the tube unless you can actually get a clean fit. If there’s wiggle room (like from an inaccurate hole saw) then your cantilevered wheel axles will suffer.
1/8th wall thickness is enough for bearings to be fitted into a drivetrain, however keep in mind that you’ll need additional chain / belt tensioning (unless you guess the C2C accurately) because the axles are fixed location.
Couldn’t you make do without a center drop with only 2 Omni wheels (say Omni’s only in the front)? Depending on some other aspects of your drivetrain. I’ve never tried it and a center drop might be better.
There’s a paper from 2003 titled “Drive train basics” that overviews the physics of turning. It’s a bit hard to find with the CD redesign, but I’ll try to link it later.
Try looking up the VexRobotics CAD files from “drivetrain examples” to get exact measurements of the center drop but yes you will want center drop for your drivetrain on each side so that your turns can be easier and so will your performance from what I’ve been told about west coast/drop center 6 drives. Bearings will be better than drilling holes so that nothing slides inside, gets busted or bent, acts as a brace when you put the bearing in.
Me team has used WCD with precision machined 2x1 tubes and belts/pulleys utilizing WCP Belt calculator with fantastic results the last 3 years.
Would definitely recommend not using bearing blocks if you have the means to machine 1.125" pockets to a fairly tight diameter/position tolerance. Belts have been very trustworthy, low friction, and good for auto when using the spacing calculated by WCP down to +/- .0005".
In 2017, we had a particularly fun-to-drive 6wd drivetrain that used WCP SS gearboxes each powered by 2 cims and 1 minicim, geared to +18 fps with 4" omnis in the corners and 2 4"x2" colson wheels. This allowed us to do J-turns and robot drifts which was helpful for gear cycles (and super fun).
Definitely never run 6 cim/ 4 cim+2 minicim unless you have low cf wheels like colsons and considerable center drop, or omnis in the corners.
I’m actually surprised a lot more teams do not use 2x1 WCD with belts.
You’ll find a CAD and full set of tech drawings here.
1/8 centre drop, configurable for 4" wheels
Vexpro 3-mini-cim BS (we found this gearbox to be pretty reliable. Bolting a churro between the gearboxes eliminates any issues of sag. Gear ratios for 4" wheels can be found in the drawings)
Overall size: 30.5 x 28 (fits, I think, the last 4 or so seasons of robot rules)
wheel pitch of 10.25 w/ #35 chain (we found #35 to be a little too much. We do not recommend the chain setup u see in our CAD, b/c of weight. 10.25 is not C-C, but we found we could get away with quite a bit of slack with chain. Masterlinks are not recommended. Consider using vex chainbreakers.)
We like to use bolts and washers instead of clip-rings to hold in our axles. We really enjoyed the easy and fast maintenance and repair. (instead of using lock washers, loctite thread locker is recommended to keep bolts in their threads)
That’s just the tip of the iceberg. If u have more questions, feel free to ask.