I'd check out a Dyna LowRider. Lighter bike with good seating position and low so it feels like you are sitting in the bike and not on it.

 

I have a really bad back. Neurosurgeon said "you have the back of a 95 year old man". I'm in the same boat. I'm contemplating selling my Street Glide for a FXDC. I had a 2011 FXDC with forward controls, and that was one of the most comfortable bikes I've ever owned. Of course everyone's back troubles aren't in the same area, but in my case, that Super Glide Custom was fantastic for me.

 

if this is a serious question you need to define "bad back"

what part of back, constant or intermittent, broken, bent, what....

without more information my answer would be exercise bike or no bike at all.

this from a guy who broke his back and chest in a motorcycle accident long ago.

 

A hospital gurney with a chainsaw engine...

 

Two things that may be counter intuitive. A firm seat like a Corbin, or similar will help more than a plush seat. The firm seat supports you and doesn't flex or give as much which makes your back work to keep you in position. Bars are a key to comfort. Get your arms up and your back straight. Looking like a parachute riding down the road with your shoulders slumped and back rolled over is going to make any bike uncomfortable.

 

Honda goldwing

 

Goldwing, or Indian. Both ride smooth.

 

 

Any bike with forwards with torture your back. Even mids on Harleys are too far forward to keep your spine in good posture.

 

New heritage or lowrider would be good. There is the road king too