Mystery Sidecar Rig

I know a little bit about motorcycle history. I read a lot, and I've been an active motorcyclist since 1974. So, I always find it interesting to see a bike I just can't reconcile to what I know.

Henderson was a major brand of American motorcycles during the early 20th century, known for their powerful and reliable inline-4 engines. Schwinn, who built motorcycles under the Excelsior name, bought Henderson in 1917 and produced the Henderson bikes in Chicago.

I spotted this one, sporting a sidecar, on the street near my home. Oddly, though, it has a V-6 engine in it.

I don't know if this is a vintage shoehorn-style conversion, or if it was done recently...or if it is a factory model to which I just can't find any reference. I really didn't want to poke around too closely on someone else's unattended machine, so I didn't get a really good look at the engine.

Here's a closer shot of the engine. Note the exposed rocker arms.

It's a fine-looking motorbike, and I especially dig the dual-saddle arrangement for carrying a pillion passenger.

Anyone have any idea as to the origin of this rig?