[2348humanplayer]

Really, it depends on what kind of drivetrain you have. Our team had separate methods.

If the wheels didn't need to worry about being bent (as in, they can just be a straight line) we used the simple method of just getting the chain, wrapping it around the two sprockets, and seeing which link to break. We broke the right one, and used the master link to join them. Simple.

However, there may be a time when you can't make the chains go straight to the wheels. We had this issue in 2010, when we needed our robot to be able to traverse the bumps, but a straight link for the chains would grind on the bumps, causing damage to the field and the robot. To fix this issue, we basically had the chains go across the robot, parallel to the frame, before bending down to the sprockets. However, if we did this method, measuring the chain the old way would be incredibly difficult. That is why we made tensioning devices to help keep them taut. What we did is we made a mount to go under the robot, putting two of the hard white plastic spools on them. They were attached to the underside of our robot by two bolts (we always use extruded aluminum for our robots, we just put a couple of cut bolts into the groove for placement, held in place by lock nuts and washers). if we ever needed to replace chains, we simply released the tensioners by sliding them towards the center, loosening the tension so it was easy to remove the longer chain. When replacing it, we made the chain long enough where it would at least reach the two and be able to have enough give. We strung them over the tensioning spools, then slid them out until the chain was taut, and bolted it into place. The wheels were still able to turn with our direct drive, and the chains were well out of the way.

But, in all honesty, that's a very odd circumstance, like going over tall obstacles or having tiny wheels and big sprockets. The easiest thing to do is just dead reckon it by making the chain the right size and measuring the links needed. If anything, a slightly loose chain is better than an overly tight one in any case.