Originally a counter-reaction to the 50s “Hot Rod”, a label recently applied to undriven cars and 
super high priced “customs” or “trailer queens”. The “Rat Rod”‘s beginning was a throwback to the Hot Rods of the earlier days of Hot Rodding, built to the best of the owner’s abilities and meant to be driven. Rat Rods are meant to loosely imitate in form and function, the “Traditional” Hot Rods of the era. Biker, Greaser, Rockabilly, and punk culture is often credited as influence that shapes of Rat Rodding.
The typical rat rod (an early 1920s through 1950s Coupe or Roadster): Early (pre-World War II) vehicles often have their fenders, hoods, running boards, and bumpers removed. The bodies are frequently channeled over the frame, and sectioned, or the roofs chopped for a lower profile. Later post-war vehicles are rarely constructed without fenders and are often customized in the fashion of Kustom (cars)s, leadsleds, and low-riders. Maltese crosses, skulls, and other accessories are often added. Chopped tops, shaved trim, grills, tail lights, and other miscellaneous body parts are swapped between makes and models. Most, if not all of the work and engineering is done by the owner of the vehicle.
Recently, the term “Rat Rod” has been used to describe almost any vehicle that appears unfinished or is built simply to be driven, whether or not the vehicle would have been customized or even existed during the 50s.
Paint and Finish
Many Rat Rods appear unfinished with primer paint jobs being common. Other finishes may include “natural patina” (the original paint with rust and blemishes intact), a patchwork of original paint and primer, or bare metal with no finish at all in rusty or oiled varieties. Contrary to tastes of many car builders, rust is often acceptable and appreciated by a Rat Rodder.
Interior
Interiors of rat rods vary from fully finished to a spartan, bare bones form. Mexican blankets and bomber seats form the basis of many rat rod interiors. Most are designed to be functional without many comforts although this will vary with the owner’s taste.
Drive Train
Though a variety of engines may be used, the most common are to be found in a Rat Rod are Flathead V8’s, early Chrysler Hemi engines, or more modern Small Block V8’s from any manufacturer, especially Chevrolet. It is not uncommon to see straight-8s straight-6s, straight-4s, V6s or even Diesel engines. These engines may exhibit varying displacements and modifications.
Most Rat Rods are rear wheel drive, with an open Driveline. The rear-ends are typically passenger vehicle pieces, as are the Transmission (mechanics). The Ford Banjo rear-end is popular, as is the “Quickchange” type as used in many early hot rods.
Suspension
A Beam axle is commonly accepted as the only type of front suspension that will look right when exposed without fenders on a vehicle with open front suspension. Independent front suspension is discouraged, Most Rat Rods use a 1928-1948 Ford I beam axle with a transverse leaf spring. Although any solid axle is acceptable, the Ford axle is preferred due to the availability of spare 
parts.
Springs vary from transverse, parallel and coil setups in the front and rear. Parallel is not seen as frequently as the more common single-spring transverse setup, though both are used commonly. Coil springs are often deemed unsightly without fenders, but are still occasionally seen.