[MechEng83]

25 chain is very unforgiving for misalignment, spacing, and flexing of the structure. We were constantly dealing with chain jumps on our 2012 robot that used 25 chain.

35 chain is much more tolerant to each of those items, but significantly heavier. Our 2011 robot didn't have traditional tensioners, but slotted holes for the external wheel mounts that allowed us to slide the wheel until the chain was "tight" but not stretched. We never had to adjust the entire competition season.

You shouldn't design the center distance to be slightly larger than the expected chain length, because assembly will be nearly impossible and will put high tension in your chain. Slightly less distance will allow the chain to sag and will cause some amount of hysteresis (different response forward and backward) in your drive system. Again, 25 chain will be more likely to jump a tooth or fall off in this case compared to 35 chain, both due to the roller center distance, and the width of the teeth.