I love to hear that teams are doing some work in the off-season.
From just the information you’ve given me, I’d say either use bigger wheels or a different gearbox. However, if you could send some pictures and/or CAD drawings of what you’re trying to do, we could help you out much more.
I don’t have access to CAD atm, but I think the only one that would present that issue is the 12:84 version.
The only remedy I can think of, if you’re absolutely sold on using THAT gearbox, is to use larger wheels, a different reduction, or to not direct drive (though it wouldn’t really be a WCD anymore…)
Otherwise, using a different gearbox is probably the best way to get around this.
As far as 1 speed transmissions go, the AndyMark 3 CIM Toughbox might be a good replacement, as would this year’s KOP gearboxes. (3 CIM drive is probably a bit much for a t-shirt shooter anyways…) Just note that you’ll probably want the long hex output shaft.
West coast drives have a very specific type of gearbox, which has the gearbox’s output shaft directly connected to the center wheel (if it is a 6wd). It also has chain/belt running through the center of the gearbox (usually).
We built a west coast drive this year using the WCProducts 3CIM single speed WCD gearbox variants. Notice how the output shaft is to the bottom of the gearbox and there is enough room to place a 4in wheel on the output shaft and not have the gearbox touch the ground?
If you are set on using that specific gearbox there are ways to get around the gearbox touching the ground by chaining the output shaft to the center wheels, but then the drive would no longer be strictly speaking a West Coast Drive. Or you can always use bigger wheels.
As you discovered, you’re not going to be able to direct-drive off of that gearbox with 4’’ wheels.
Your options are pretty straightforward - you can either use bigger wheels, mount the gearbox elsewhere and link it with belt or chain, or you can use a different gearbox. The COTS options that will work with 4’’ wheels, as far as I know, are all two- or three-stage.
If you have the machining capabilities and really want a singe-stage reduction and direct-drive center wheel, you can make your own gearboxes. Be aware that the ratios you can get with a single-stage reduction and 4’’ wheels might not be high enough for comfort in an extended pushing match with a heavy robot and high-traction wheels, so if you’re looking to have pushing power you might want to go with a different gearbox anyway.
Edit: Upon re-reading, I noticed that this is a t-shirt robot, so pushing performance probably isn’t too important. Still, the advice stands if you want to transfer this design to an actual competition robot.
Here’s a goofy out-of-the-box suggestion: use the small wheels to drive larger pneumatic tires mounted directly below them. That’ll give you ground clearance without changing the gear reduction. The larger tires will also work better on non-flat surfaces, in case you want to use the t-shirt robot outside (perhaps during a football game).
I think that would still cause a reduction like gears, due to the radius difference.
Just use a two-stage gearbox. It’s not that much more expensive to buy a two-stage gearbox than to buy a one-stage.
Alternatively, you can make your own gearboxes. It isn’t frightfully hard to do given a mill/ xy drill press or CNC or sponsor for cheap. Designing gearboxes is fun too. You just mix and match vex gears to suit your needs.
From experience, I would advise against using that specific gearbox. One of the internal bearings slides out quite easily and can cause the drive shaft to fail on the snap ring groove. If you haven’t already purchased it, switch to another VEX gearbox like the WCP 3-CIM single stage, or the Vex Pro 3 CIM Ballshifter. Otherwise your options are to get bigger wheels (good idea for a t-shirt cannon anyways IMO), or to raise the gearbox and drive all the wheels with chain or belts. While this removes direct drive capability inherent to WCD, the other aspects of a WCD will remain the same without a dramatic change (with the exception of COG, which will be higher )
As mentioned before, machining your own gearboxes is also possible. My team ran a single stage 3-CIM gearbox geared for 16 FPS in a direct drive setup. They were very over-built, but rock solid throughout the season.
Keep in mind that by increasing yor wheel diameter 50% like that, you will need to increase the secondary gear diameter 50% as well to keep the same speed. So your gear would still be scraping the ground. Plug the numbers into the JVN design calculator and you’ll see what I mean.
We used this gearbox this year (7:1) and it was our best drivetrain ever! Best speed and torque. That being said, I would suggest changing the gearbox! To much power and 3 CIMs is too much for a demo bot.
If you DO decide to use these i would suggest supporting the other side of the axle. On Saturday at CT State Champs we broke the axle clean in half (after 3 competitions, and many demos)