Shadetree Surgeon gives us a tour of "Fat Herta" by FNA Custom Cycles.

Rat rods are a common trend when it comes to custom cars, but motorcycles, not so much. They do exist, but they're not nearly as popular on two wheels as on four. The first bike that Eric Allard of FNA Custom Cycles built started out shiny, but he's never cleaned it. Rather, he's allowed its original shine to morph into surface rust, which perfectly augments the look of his rat rod BMW.

The bike is loosely based on a BMW R75, which donated its engine. Rather than BMW's traditional shaft drive, Allard used a most unconventional setup. The transmission connects to a differential for an ATV, one side of which has a sprocket for the chain drive to the back wheel. As if that wasn't wacky enough, the other side of the differential has a disc brake, which is how braking power gets to the back wheel—eventually.

The engine itself has a pair of Weber carburetors intended for a Volkswagen, one on top of each cylinder, with cone air filters pointing skyward, adding to the bike's unconventional look. The most significant visual aspect, however, is the front suspension, a leaf spring just under the headlight that the wheel connects to on both ends. This is the only suspension the bike has since it's a hardtail.

This is no show bike, however. It actually works in the real world. Despite its unusual appearance, Allard says it's actually the most reliable bike he has. He's even ridden it from FNA's home base in Lakeland, Florida, all the way to Miami.

In this video, Shadetree Surgeon gives us the grand tour of the rat rod Beemer along with Eric, explaining all of its numerous quirks and features. Then he takes us along for a ride on it to give us the full rat rod experience. Actually, he does it twice, because his GoPro died on him just after setting off the first time. I give him bonus points for authenticity. The same goes for FNA's custom rat rod Beemer, which Allard calls "Fat Herta." It very much looks the part, but it remains authentic in being fully functional as well.