This little critter was responsible for putting a lot of the world on wheels back in the early 1950s–including the United States. It was a very basic 125 two-stroke single of German design, and a darned good one, all part of the reparations package that the Allies imposed on Germany.
You start a war, then you lose the war, and all those countries that spent lots of money defeating you want some sort of payback. In this case the right to copy your motorcycle. Which eight countries did, including the U.S. (Harley Hummer) and Britain (BSA Bantam).
Back up a bit–to 1916 when a Danish engineer in Germany, obviously smitten by the Stanley Steamer, decided to build an automobile called a Dampf (steam) Kraft (power) Wagen (car)–which became the initials of his company. Not surprisingly, it failed, but more conventionally powered cars were successful.
He also turned his attention to smaller vehicles, like motorscooters and motorcycles. These appeared in the early 1920s and in-line with the DKW company’s initials were called Das (the) Kleine (little) Wunder (wonder). They were not very wondrous, but were inexpensive, with two-stroke motors using the standard technology of the day.
By the late ’20s DKW was a major player in the motorcycle industry, producing more than 40,000 bikes in 1928 alone.
Conventional two-stroke technology meant getting the fuel into the combustion chamber using the cross-flow system, the descending piston opening up portals on each side of the cylinder, with the incoming oil and gas mixture helping to push out the burned fuel. Functional, but not very efficient, as some of the new fuel went out with the old.
This was improved when somebody developed the asymmetric deflector piston, which had a ridge that would send the new fuel upward rather than directly toward the exhaust port, creating more efficiency.
In 1929 a German engineer, Adolf Schnuerle, developed the loop-scavenging system. This had two intake ports, one on each side of the exhaust port angled so the incoming fuel would head for the far side of the combustion chamber and assist in pushing out the old stuff. With no need for a deflector, this made the piston lighter.
The RT125, with the RT standing for ReichsTyp, or National Model, was a good design, reliable and cheap. In 1932 DKW, which was also making cars, merged with Audi, Horch and Wanderer to form the AutoUnion. The four circles we often see on current Audis can be seen in several places on the bike.
In the ’30s DKW is said to have manufactured more motorcycles than any other company in the world, having two-strokes of various capacities as well as many racing bikes, like the supercharged rotary-valve 250 of 1938.
The war came and went, with the RT125 constantly produced by the Germans, mainly for military use. At war’s end the old factory was on the Russian-controlled side of Germany, but some enterprising DKW people who had sensibly fled west set up a new factory in Ingolstadt.
As soon as the Allies allowed them they began producing the first postwar RT125, labeled with a W to show it had been manufactured in the west. A mag/dyno ignition/lighting system required no battery, but the horn was a squeeze-bulb affair.
The 123cc engine used an alloy head and an iron barrel, having a bore of 52mm, stroke 58mm, and a deflectorless piston. With a compression ratio of 5.9:1 it was said to generate 4.75 horsepower at 5,000 rpm. Gas and oil were mixed in the 9.5-liter (2.4-gallon) tank at a 25 to 1 ratio, and sent into the engine via a Bing carburetor. Primary power drove a chain to the clutch, then through a three-speed gearbox to chain final drive.
In 1952 the RT125/2 appeared, with power upped to 5.6 horses. Ignition on this /2 model pictured was via conventional battery-powered coil, with six volts powering the both lights and horn. Turn the petcock on, tickle the carb, push the plunger key down in the headlight shell and kick the left-side starter.
The engine unit was bolted into a simple rigid frame made of steel tubes. The top under-tank tube angled back toward the rear axle, splitting into two after the center post. A single front tube cradled the engine, and then split to go back to the rear axle.
At first the 125 used the pre-war girder fork, but in late 1950 changed over to a telescopic fork…which did not have any damping qualities. The new fork increased the 155-pound weight by some 20 pounds. Wheels were both 19 inchers, with 2.50-size tires. And rather small 4.5-inch drum brakes, but probably adequate as the top speed was about 50 mph. Distance between the axles was 49 inches.
The gas tank did incorporate a small toolbox, barely large enough to hold the tools required to fix a flat tire. A solo saddle for the rider was a mere 26 inches above the ground, with a luggage rack on the rear fender, to which a pillion saddle could be fit.
In 1954 came the 125/2H, with rear plunger-type shock absorbers for those who wanted to pay the extra money. The last DKW 125s came out the factory door late in 1957…though the company continued producing larger capacity motorcycles for another 10 years.