I'm about to purchase a Downtube 8FH (full suspension, internal geared hub). Then I noticed that the maximum recommended rider weight was 195 lbs. I weigh 210 lbs. But curiously, the same bike with the 9 speed cassette is rated for 245 lbs, so it doesn’t appear to be the rear suspension. And the 8H, with the same geared hub, is rated for 225 lbs. So it doesn't appear to be the geared hub. What gives? Why is the bike I want rated for only 195 lbs? The only thing I could guess is that under compression the chain will tighten since the single pivot is not on the same center as the bottom bracket. A heavier person hitting a big bump could perhaps snap the chain? I would think the suspension travel would take care of that and the only thing that would happen is the suspension would bottom out sooner for the heavier rider.
Kudos to Yan at Downtube for a prompt and thoughtful response:
The internal hub FS chain length varies as the shock engages. Higher weights risk to much chain slack if it is not set up for each rider. Hence not allowing over 195lb riders relieves the possible chain slack issues. The rear derailleur eats up the slack, hence that bike does not have the same issue.
I have had many riders well over the 195 limit on the FH ( one friend is around 250lbs ), however they set the chain length for their weight. So far no issues.
So my hunch was right; varying chain tension because of the change in in the distance of the chain line as the swingarm travels in its arc. Proper chain tension for a riders actual weight should allow the 195 lb. minimum to be successfully exceeded.