My Aluminum Comp Schwinn (not recommended for motorizing) requires the dreaded tensioner to reroute the chain up & over the chainstay - some bikes need that redirect or the chain will hit...
Hehe, I see why you don't recommend that bike. But like you said Barly, on some bikes you simply have to re-route the chain.
KC- It's not so much an issue of frame design as jut the usual chain stretch we all experience with a new chain as it wears in.
Actually chain doesn't stretch.
Here is an excellent read on it on Diamond Chain's site
http://www.diamondchain.com/files/Maint_Guide.pdf
Most of the initial chain 'stretch' we see on the kit bikes is caused by the big sprocket wearing in.
That is complete after a 100 miles or so.
If you are putting 300 miles a week on it, just give this a try for giggles and grins.
Buy a KMC 410H Heavy Duty 'Freestyle' or similar chain, ~$13 at the bike store.
Size it up with no tensioner so your wheel is as far forward in the frame keyway's as possible.
Your rear sprocket is already broke in but will get a little new wear as will the new chain, but one rear wheel adjustment should fix that after a hundred miles or so.
You don't even have to worry about a pedal side tensioner, the derailer takes care of that.
I suspect you are having to constantly adjust the chain tension
because of the tensioner, not 'chain stretch'. If chains were that inefficient and wore out that quickly there would be tensioners on everything from motorcycles to pedal bikes, and chains would be wearing out like flashlight batteries don't you think?
What the heck right? It's cheap to try, and easy with your gear bike.