Just idle curiosity, and a couple of brief Google stabs didn’t yield anything of note…
…have any of the major (heck, or even minor) US auto manufacturers ever made motorcycles? I’d imagine they wouldn’t find the market lucrative since the big money has always been in enclosed vehicles, but several non-US manufacturers (BMW, Honda, and Suzuki) all seem to find it worthwhile.
Didn’t Honda and Suzuki start out as motorcycle manufacturers that branched out into automobiles in the 70s and 90s respectively?
Few that I can recall, with no additional effort to research it. 
In Yurp, Maybach (now part of VW) got its start making bikes, not cars.
Loads of foreign and local bicycle manufacturers, yeah. Like Indian, which got its start in 1904. BMW, which still holds on to both branches. Daimler’s first vehicle was a tricycle which, these days would be licensed as a motorcycle, so the difference has, for 110 years, been a technicality based on the number of wheels.
BMW, Honda, and Suzuki are motorcycle companies that expanded into the enclosed vehicle business.
I remember the first BMW I saw. It had a two cylinder horizontal engine simular to their motorcycle engine, except it had been converted to a water box.
The early Honda cars were small and I believe they also had two cylinder engines.
I do not know of any US car maker that made a motorcycle.
Googling, I see that GM and Segway teamed up this year to make a two wheeled vehicle.
GM used to own 20% of Suzuki, a company that makes motorcycles.
Then there’s the Famous Ford Flathead, which I had never heard of. I’m not sure whether it was a custom cycle with a Ford V8 engine or what.
Studebaker built a few prototype motorcycles in the 1940s, but they never went into production.
I see that there are a lot of motorcycle manufacturers. Maybe the industry is intrinsically more competitive than the 4 wheel market. So, while some motorcycle manufacturers might move into cars, there would be little interest in the reverse direction.
Ooo! Chrysler released the Tomahawk V10 - a total of nine of the concept vehicles were sold by Neiman Marcus.
Thanks for the Googling. Interesting stuff-- the Tomahawk V10 looks like some unholy cross of a Tron lightcycle with a Terminator. I’d forgotten about the GM/Segway announcement, which I’d seen a while back. Not sure if it’s a motorcycle, but it certainly does have two wheels.
The flathead Ford motorcycles on that site look like they’re custom made by the site owner, but they show one of my ideas-- car manufacturers preferred developing large engines which were impractical for use in motorcycles-- to be incorrect. The bikes seem just fine with that size engine.
Just a bit of a nitpick, but Maybach is owned by Daimler, not VW.
BMW actually started out making aircraft engines. Their blue and white roundel logo was intended to depict a spinning propeller with the sky as the background.
The car Snnipe 70E mentions was most likely in Iseta, which started out as an Italian design licensed for production by BMW.
One company that actually DID start with cars, and then turned to motorcycles was the German company DKW…DampfKraftWagen, literally “Steam Powered Car”. The car side of the business merged with three other companies (I forget which) to form the four linked rings that are the Audi logo. The motorcycle end of the business continued on it’s own.
They were very successful prior to WW-II. After the war they ended up in East Germany. The name was changed to MZ (or MuZ depending) with several V-1 (AKA buzz bomb, AKA Doodlebug) scientists. These folks applied their knowledge of acoustic resonance to 2-stroke exhausts, and the tuned pipe was born. Since Mr. Gorbachev tore down that wall, MZ as been struggling to gain a foothold in the world market. They build scooters and several rather nice motorcycles. In the last decade, their presence in the US has been devastated by the falling dollar.
Why, Yes, I do own a MZ motorcycle, how did you guess?