Yes, that is a problem we haven't been able to address. Bike manufacturers are pretty good about honoring warrantees. However, I'm sure they would be leery of our product and they have no reason to work with us.

So far, the tandem adapter hasn't hurt any captain's frame. Neither has the Xtra Cycle, which also extends the rear triangle. We have done the "cover-our-rear" thing by:
limiting use of the tandem adapter to mountain and hybrid bikes.
no carbon fiber frame parts or seat posts on the captain's frame.
limiting combined weight of captain and stoker to 350 pounds.
no department store captain's bikes or stamped rear drop-outs.

People who have tried the tandem adapter say the frame is as rigid as most mid-range tandems and our MSRP is $550.