pic: 4908 Prototype West Coast Drive



Simple CAD model of a prototype chassis team 4908 is working on developing. Uses 4in Colson wheels and a two speed 3 cim DS. Also has sliding bearing blocks for setting chain tensions.

It looks like the gearbox output shaft is direct-driving the center wheel. If so, is there enough room to back out/remove a gearbox after the whole thing is assembled without taking the frame apart? The spacing between the back sides of the gearboxes seems tight (but then again, I can’t measure it).

Why the 3CIM, dual speed transmissions? Are you simply using it as a “worst case scenario” placeholder in this design, as it occupies more space and weight than any other option? Or are you seriously considering using it during the upcoming season?

I fully understand both 3CIM transmissions and dual-speed transmissions for drivetrain, and have used both in the past. However, a 3CIM dual-speed transmission strikes me as an exercise in overkill. The improved acceleration (when compared to a “standard” 2-CIM single speed) and pushing power are accomplished with either solution. Similarly, with the additional CIMs you can creep your top speed above a KoP transmission while retaining superior torque without the need for shifting. However, when you incorporate BOTH the 3rd CIM and the dual-speed, you get the downsides of both. You have the additional weight, additional space usage, the need for pneumatics, the usage of additional PWM outputs, the use of additional speed controllers, the additional current draw, and the additional financial cost. For only a minor performance improvement over either a 2-speed or 3-CIM single speed, you’re tacking on a lot of additional costs. You’re also inviting the risk of tripped circuit breakers if you’re not careful about your power consumption in high gear (especially if you’re not traction limited) and programming when you shift gears.

That being said, your general design looks fine. It’s pretty hard to truly screw up a WCD design. You may want to reduce the lengths of your shafts on the outer wheels, so you have less overhang behind your sprocket. You may also want to spend some time planning out your bumper mounts, since they aren’t always trivial for WCD solutions. You could also consider adding some cross-bracing behind the transmissions to reduce the cantilevered loads they’re placing on your drive rails and output shaft. Also, be mindful of what types of gussets you’re using (as they aren’t shown in the picture), and their interactions with the sliding bearing blocks.

This is actually the rationale I’ve seen behind shifting with 3 CIMs. If you want a faster high gear, it’s how you avoid tripping breakers (and/or brownout) if you end up in a situation where pushing is your only way out

Back when we used the cRio, this was more viable. Now that we have the RoboRio you need to exercise much greater care to prevent costly brown-outs. You NEED current sensing in your code for this to be worth it, and probably some form of auto-shifting to get the best use out of your power available. When we did 6 CIM in 2014 all we had to worry about was popping the breaker. We would not do another 6 CIM with the RoboRio without a lot of thought about how to implement it well.

My team (2471) competes in an off-season competition called bunnybots every year. This year one of the major components of the game is going around a race track. I realize this isn’t a normal drivetrain application, but I thought you might be interested in a data point or two.

Both robots are running 2016 legal electrical systems. One is 4 CIM and 2 mini, geared for roughly 18fps free speed (Mini CIMs are compensated for in gearing). This robot usually doesn’t brown out in a pushing match because the 2 mini CIMs are in their own transitions and usually spin out. However we have not tried a lot of pushing match senarios.
The other bot is running a 6 CIM two speed drive geared for 9.5fps and 24.5fps free speed. It has auto-shifting code but no current monitoring yet. This robot only browns out when the battery is too low (aka 12v rested after ~8min of driving like crazy). One thing to keep in mind is that both robots are about 80lb without bumpers or battery.

Thanks everyone for the feedback! All of your comments have been helpful, and I will take this information back to my team to discuss it.

Thanks, you likely saved our team a lot of trouble by letting us know these downsides. When designing this chassis we were unaware that we should be concerned about these factors.