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A rat rod is a style of hot rod or custom car that, in most cases, imitates (or exaggerates) the early hot rods of the 1940s, 1950s, and early-1960s. The style is not to be confused with the somewhat closely related "traditional" hot rod, which is an accurate re-creation or period-correct restoration of a hot rod from the same era.
Most rat rods appear "unfinished", regardless of their status, as only the vehicle's bare essentials are driven. 
Originally a counter-reaction to the high-priced "customs" and typical hot rods, many of which were seldom driven and served only a decorative purpose. The rat rod's inception signified a throwback to the hot rods of the earlier days of hot-rod culture—built according to the owner's abilities and with the intention of being driven. Rat rods are meant to loosely imitate, in both form and function, the "traditional" hot rods of the era. Biker, greaser, rockabilly, psychobilly, and punk sub-cultures are often cited as influences that shaped rat rodding.
The typical rat rod is a late-1920s through to late-1950s coupe or roadster with a big block chevy (rat) installed, but sometimes a truck or sedan. Many early (pre-World War II) vehicles were not built with fenders, hoods, running boards, and bumpers. The bodies are frequently channeled over the frame and sectioned, or the roofs are chopped, for a lower profile. Later-era post-war vehicles were rarely constructed without fenders and were often customized in the fashion of Kustoms, leadsleds, and lowriders; Maltese crosses, skulls, and other accessories were often added. The owner of the vehicle was typically responsible for most, or all, of the work present in the vehicle.
Recently, the term "rat rod" has been used incorrectly to describe almost any vehicle that appears unfinished or is built simply to be driven. Chopped tops, shaved trim, grills, tail lights, and other miscellaneous body parts continue to be swapped between makes and models.
Longtime rodding scribe Pat Ganahl took a broad look at the rat rod trend and had this to say:
"I see what are referred to as rat rods today comprising three elements: First are the traditional rods and customs. Those are cars built the way rods were built in the '30s, '40s, and early '50s, with a primary emphasis on low-buck and home-built, using period-correct components ranging from flathead to nailhead engines, wide whitewall tires to skinny blackwall bias-plies, and black primer to hand-rubbed paint.
"Second are what I personally call rat rods, as a positive term... They're artistic, fun, and sensational reinterpretations of late-'40s/early-'50s hot rodding as a culture that includes music, clothing, hairstyles, and tattoos. The cars are low, loud, chopped...with giant rear tires, lots of carburetors, open pipes, and tall gearshifts.
"Third are...what I derisively call crap rods today. They've always been part of the mix, unavoidably. I take exception to those who think that shoddy construction is somehow cool, or worse, those who know they can throw a bunch of junk parts together and peddle it as a cool rat rod to some inexperienced buyer who doesn't know the difference." 
The December 1972 issue of Rod & Custom Magazine was dedicated to the "beater", a low-budget alternative to the early car models that were slick and customized. Due to the beater's cheap upholstery, primer covering (instead of paint), and lack of chrome or polished metals, it has been considered a progenitor of the rat rod.
The origin of the term "rat rod" is the subject of dispute. Opinions regarding the term's origins were based in one of the following perspectives: The term first appeared in an article in Hot Rod Magazine, written by Gray Baskerville, about cars that, at that time, continued to be covered by primer; or, the first rat rod was owned by artist, Robert Williams, who had a '32 Ford Roadster that was painted in primer. However, Hot Rod magazine has verified the latter view. Gray's use of the term was in relation to "Rat Bikes," motorcycles that were assembled from spare parts, to be enjoyed and ridden, and not necessarily for the display of the builder's skills. It is believed that the term is likely to have originally been used in a derogatory or pejorative sense, as this remains the case among sections of the hot rod community; however, the term has also been adopted in a positive light by other parts of the sub-culture.
The origin of the movement itself is a little easier to pin down. In 1987, after many decades of building hot rods and customs, Jim "Jake" Jacobs gathered spare parts from his amassed personal stash and put together a ’28 Ford Phaeton in 28 days. It sported no fenders, wide-white bias-ply tires, a rusty Model A body on ’32 Ford frame rails, a chopped windshield and a shortened deuce grille shell with a small-block Chevy 350 and a ’39 Ford 3-speed.
Jake drove it to Pleasenton, CA and entered the Goodguys' annual West Coast Nationals with no bodywork, paint or interior (other than what was needed to be functional and legal). He parked in full view of many high-end, top-dollar customs and proceeded to pull out a bucket of paint and some brushes. Many spectators stopped to watch, and a few even joined Jake as he painted the car. 
The Jakelopy was intended to remind people that hot rods were supposed to be accessible. They were supposed to be a hoot to build and drive.
Gray Baskerville described the Jakelopy in the 11/’00 issue of Hot Rod as being "finished...two years before the first rat rods appeared."
Frames from older cars or light trucks are sometimes preferred for rat rod conversions due to the chassis that is used for these types of vehicles—the chassis type provides a sturdy base for subsequent alterations. Older cars in poor condition are often advertised as candidates for rat rod conversions and, in some cases, the owner will purchase a custom frame, or design and build it himself/herself. In other cases, a rat rodder may use a small pick-up chassis, such as a Chevy S-10, to insert into an older car body, in order to create a vehicle that features the look of a classic rat rod, while also maintaining the reliability of a modern vehicle.
Rat rods often appear unfinished and, at most, primer-only paint jobs are applied; satin, or matte, black and other flat colors are also common. “Natural patina” (the original paint job, with rust, blemishes, and sometimes bullet holes, left intact); a patchwork of original paint and primer; or bare metal, in rusty or oiled varieties, with no finish at all are some of the other finishes that may be used—such finishes honor the anti-restoration slogan that "it's only original once". Contrary to the aesthetic of many car builders, rust is often acceptable and appreciated by rat rod owners. Owners with a pinstriping brush will often apply free-hand pinstriping to their rat rods.
Early low-budget hot rods were often long term "works in progress" and as such final finishing treatments (such as metal prep, paint, and trim) remained in the future, and the Rat Rod imitates this aesthetic.
Interiors of rat rods can range from spartan to fully finished, though this is typically the final phase of construction. Mexican blankets and bomber seats form the basis of many rat rod interiors, and most are designed to be functional without many comforts; although, this will vary in accordance with the owner’s taste.
Though a variety of engines may be used, the most common engine type that is used in rat rods are: Flathead V8 engines, early Chrysler Hemi engines, or more modern small block V8 engines from any manufacturer (Chevrolet is a common choice of small block engine). Straight-8s straight-6s, straight-4, and V6s are also fairly commonly used in the construction of rat rods—these engines may exhibit varying displacements and modifications. While diesel engines are occasionally used, these engines are rarely fitted with emission controls, as such a feature was not part of the original construction, or the feature was not required under special license.
A beam axle is the most commonly used type of front suspension, due to its appearance when exposed without fenders on a vehicle with open front suspension. Independent front suspension is rarely used and most rat rods use a 1928-1948 Ford I-beam axle, with a transverse leaf spring. Although any solid axle can be used in a rat rod, the Ford axle has been preferred for years due to the availability of spare parts. "...9-inchers are the most used rearends in nearly every form of racing and most high-performance street vehicles..."
Spring types in the front and rear can be transverse, parallel or coil setups—parallel is not used as frequently as the more common single-spring transverse setup and coil springs are still occasionally seen even though this spring type is less popular for aesthetic reasons. Rat rods will often be built with airbag suspension, thereby allowing the driver to raise and lower the car; this can be a useful feature due to the extremely low ground clearance of many rat rods.
In many cases, the front suspension is mounted a considerable distance forward of the radiator, a practice that may be derived from the construction of early drag racing cars.
Many traditional hot rodders and restorers loath the rat rod trend, summing the entire movement up with words like "cheap" and "talentless". In general, the sentiment among critics is overwhelmingly dismissive and some times overtly negative.
Hot rodder and freelance journalist Brad Ocock said of the rat rod trend: "There's a huge difference between rat rod and beater. A beater has potential. A rat rod is something someone threw together to make a statement, and usually that statement is, 'I don't know how to weld. I had a bunch of crap lying around and realized there was enough to put together a car but didn't want to put any effort into it.'"
However, despite such attitudes in many areas of hot rodding, over the last ten to twelve years rat rods have become more and more accepted at car shows and in the custom car culture in general, with many car shows either including sections for rat rods, or beginning events directly devoted to them and aimed at wider audiences than ever before.