Utility Cycling - Xtracycle for someone with bad knees

In two completely different accidents (one involving ice, one involving rock climbing) I have managed to dislocate both knee caps. In theory, my knees are mostly back to functional, but the one place I have really noticed problems is in bike frame geometry. I'm fine on my vintage Raleigh sports, but anything mountain bike shaped is impossible to ride more than a block or too.

Now my daughter is now four and I was planning on converting my Raleigh to a xtracycle for child transport, but I just heard back from xtracycle that it should be a 1980s or newer mountain or road bike. So.... does anyone have experience converting something city bike shaped or am I just SOL? Alternately, do you have a favourite kid transporting bike that might be easier on my knees? (not "comfort" geometry. For me that is even worse)

Thanks

ETA: it's just the raleigh frame, I have all new components

I think this is a pretty advanced fit issue, which specifically has to do with the position of the saddle relative to the cranks. This is most commonly expressed by the seat tube angle (ST).

Your Raleigh Sports probably has a fairly relaxed 72º ST, which is common on MTBs, so I don't think, unless you're riding a rather small frame (they tend to build those with steeper STs), that geometry is the issue, and without knowing which frame you've ridden that caused pain, it's impossible to say with certainty one way or the other.

My guess is that moving the saddle further rearward of the cranks on the troublesome MTB you've ridden might yield some clues. By doing so, you'll be reducing the degrees of flexion in the knee when you're applying power, which may be the source of the problem. If you can get that bike again, try pushing the seat all the way back on the rails, or if it isn't fitted with a layback seatpost head, get a seatpost with layback so that you can get more backward adjustment out of the saddle.

Of course, other factors may be your forward reach on the bike and how your weight is centered over your knees. Handlebar height and reach are important here, and if I were to guess at another fix for the offending MTB, I'd suggest a shorter reach stem with more rise, to move your weight back onto the saddle.

Like I said, it gets complicated, but I don't think that ruling out MTBs in general makes much sense. It's about getting the right sized frame and setting it up right.

I hope that helps!

Maybe a Trek Transporter Plus.. they come with an Electric motor rear wheel..
to help the pedal with a little boost.

My Extrabike has Shimano cranks with Biopace rings which are very knee friendly... I have a leg that does not work at 100% so the good leg works harder on clinbs and I really feel it when I am packing bigger loads and today I added an electric assist to give me that little boost when I need it.

One would not know they were riding a bike that weighs 100 pounds when you have that little switch under your thumb... :)