[JesseK]

In a WCD setup, if the chain is on the inside of the frame rail and the wheel is on the outside, wheel/axle flex is (almost) completely isolated from the chain alignment. Therefore there is a greatly-reduced chance the chain will come off. Sure, changing wheels is nice, but the big reason is minimizing risk on the most critical thing a robot needs to do during a match. It is why many COTS WCD products are manufactured the way they are. The details are easy to recognize if you know what to pay attention to. Here is the explanation:

During matches, drive train shafts flex under high stress (robot-robot impacts, high-torque driving, 2016 defenses, etc). Any CAD simulation shows that 1/2" steel round/hex shaft flexes at least a little. In the pits, the shaft will appear straight. Yet at the moment of impact, every shaft flexes. I forget the term that relates amount of stress to whether a material will return to its original form, but this type of thing is measurable.

It's important to analyze where a shaft flexes. In a WCD setup it flexes from the bearing block outward. If the chain sprockets are on the wheel, the sprockets will become misaligned during an impact. With #25 chain, this will nearly always cause the chain to jump the sprocket, and perhaps cause a sitting (or twitchy) robot. When a chain comes off like this, it is usually still attached at the master link.

There is also a major element of misalignment in assemlby of a sprocket to a dead-axle setup. If the bearings are not perfectly seated, or the bearing holes flex (like the do on ALL plastic VEX/AM/Banebot plastic wheels...), or the sprockets are offset from the wheel using plastic spacers, or if one of the attachment bolts is overtightened.... The list of "if x happens we are dead for the match" gets pretty large.

WCD Variants all have the same fundamental desirable feature: sliding bearing blocks that, when manufactured correctly (i.e., buy them...), come with some 99.9% guarantees. The two sides of the sliding bearing block are machined together to guarantee the bearing holes are perfectly concentric. Usually these bearing blocks have a flat face that sits flush with the frame rail as well. All of this means that all axles are parallel to each other, and are perpendicular to the frame rail. If the sprocket is isolated from any wheel impacts, then the chain is nearly guaranteed to not pop off, even under the highest of stresses - allowing us to use light & efficient #25 chain with nearly 100% reliability.

Now - in 2016 some well-known teams went to #35 chain due to the stresses of defenses on the chain itself. This is due to the fact that a wheel at full speed in the air, which then comes to a halt due to a 150lb robot 'landing' on top of it, will transfer all of the stresses directly to the chain. The #25 chain would literally break due to this stress, causing the robot to sit. This is not the same thing as a misalignment causing the robot to sit.