This summer we are experimenting with non-adjustable wheel positions, and chains inside our side rails.
The first rail we made had nominal C2C distances. While it is functional, the chains sag enough to touch the inside of the rail tube, making the drive assembly noisy when running. For the second tube, we took a WAG (embarrassing to admit, for engineers), and increased the C2C distance about .008". This resulted in saggy chains again. After that, we did what we should have done from the start, and actually measured the C2C distance of tensioned chains, instead of just guessing.
We made this test setup, first using the chain from our experimental drive train, with a nominal C2C distance of 10.75". Note that the sprockets, bearings and shafting are all 1/2" hex type, all from VEXpro. There are slip fit tolerances between these parts that require the C2C distance of the bearing pockets to be greater than nominal to achieve a tight chain. Also note that the neither the hex holes in the bearings nor the hex holes in the sprockets are truly concentric. This leads to visible wobble in the OD of the sprockets, when they spin. This causes the chain tension to vary, and is a source of vibration and cyclic fatigue to the overall drivetrain.
To test whether any C2C variations were related to the length of the chain, we also tested a nominal 4" C2C chain setup.
The chain tension was set by anchoring one of the bearing blocks, pulling the chain "finger tight" and locking down the other bearing block. We then spun the chain by hand to observe that it could run free, and hand checked the chain tension the same way we do in the pits, to be sure it was tensioned comparably to our competition drive trains. Then we removed the chains, sprockets, shafts, and upper bearings. The upper bearings have a tight slip fit. To remove them, it was necessary to insert the end of an axle shaft and wiggle it around, working the bearing loose. This left the two bearing blocks with the bearing pockets exposed.
We then used the milling machine edge finder and the dimensional readout to find the C2C distances (we actually measured to the left sides of both bearing pockets).
Here are the results:
We concluded that the increase in C2C distance was due to the tolerance stackup of the parts, and not tightly related to the length of the chains. We plan to incorporate a .019" to .020" delta to our nominal C2C distances in future designs.