Kettenkrad - German Motorcycle Tractor, WWII Tactical and Technical Trends, No. 19: February 25, 1943 (Lone Sentry)

The invasion of Crete revealed for the first time the use by the Germans of the motorcycle tractor for the purpose of hauling light, single-axle, open trailers or light guns. This tractor is known to be employed in the Middle East and, according to recent newspaper accounts, it is now to be seen on the Eastern Front. It is a suitable vehicle for accompanying airborne troops. Early in 1941, it was accepted as an army vehicle and received the number Sd.Kfz. 2 (Sonder Kraftfahrzug--special motor vehicle).

a. Body

The accompanying sketches show that the body (1) is a box-like structure made of pressed metal in two halves, and joined along a horizontal plane below the track-guards (2). It contains the driving position (3), the engine (4) and transmission, and a transverse seat (5) at the rear over the cooling system (6). The driver is seated on a saddle (7) mounted above the gearbox (8) and clutch housing (9), and has two rubber knee-pads fitted beneath the dashboard. On each side, the track-guards carry gasoline tanks (10) at the front while, level with the engine, the sides are built up (11) and contain on the left the tool kit and on the right the battery and fuze panel.

[Kettenkrad]

[Kettenkrad Schematic]

At each side of the passenger seat there is a light rail (12), while foot rests, rifle rests (14), and clips (15) are provided at the rear.

b. Engine

The power unit (4) is an Opel "Olympia model 38," gasoline engine, mounted towards the rear of the body. It is a four-cycle, four-cylinder unit developing about 36 brake horse power at 3,400 rpm.

Bore 80 mm

Stroke 76 mm

Engine capacity 1,478 cc

Compression ratio 6

Firing order 1 - 3 - 4 - 2

The engine body is in two main parts: the cylinder head, and the cylinder block and crankcase (both in one piece).

c. Crankshaft Assembly

This is supported in four main bearings. The pistons are of light metal and are fitted with two compression and one oil-scraper rings. The wrist or piston pins are full floating and are prevented from sideward movement by locking rings on each side.

d. Valve Operation

The overhead valves (one intake and one exhaust for each cylinder) are operated by pushrods and rockers from the camshaft, which is mounted in four bearings in the crankcase. The gasoline pump, tachometer, oil pump, and distributor are all driven from the camshaft.

Valve clearance (warm) Intake 0.2 mm (8/1000 in)

" " Exhaust 0.3 mm (12/1000 in)

e. Cooling

An impeller-type water pump, together with the generator, is driven from a master pulley on the free end of the crankshaft and circulates water between the engine and the radiator (6), which is located at the rear of the vehicle. A small water tank is mounted above the pump.

The laminated radiator is built in a large airduct (16), in which there is a fan (17) driven direct from the crankshaft. The rear end of the airduct may be closed by a flap (18) operated by a hand lever on the left of the driver.

f. Lubrication

Oil is pressure-circulated by a gear pump from the sump through a strainer and passes through the bearings of the crankshaft, connecting rods, camshaft, tappets, and valve rockers at a pressure of 30 to 45 pounds per square inch. Piston pins and cylinder walls are splash-lubricated.

In order that the oil pressure may not rise too high, there is an excess-pressure valve in the wall of the oil pump, and this returns some of the oil to the sump.

An oil cleaner of the metal-disk type is fitted. This is itself cleaned by turning the ratchet on the top.

g. Fuel-Supply

An Opel downdraft carburetor, with a large oil-bath-type air filter (19) on the air intake, is fitted. Gas is drawn from the tanks (of which there are two, each holding 5.5 U.S. gallons) and fed to the carburetor by a mechanical diaphragm pump of normal design, which is driven from the camshaft.

h. Electrical Equipment

Ignition is by Bosch coil and distributor, the latter being driven from the camshaft. A Bosch 75-watt generator, with voltage regulator, is mounted on the right side of the engine and together with the water pump is driven by a V-belt from the master pulley. A Bosch starter motor is also fitted on the right side. The 6-volt battery is mounted on the right side above the track-guard.

i. Starting Equipment

Normally the engine is started by a self-starter controlled by a pull-knob on the dashboard. A crank is also provided and, for use, is inserted in an opening in the radiator grill at the back of the vehicle (just above the trailer coupling) and pushed through to engage with the crankshaft.

j. Transmission

Transmission is through an Opel multiple-spring, single dry-plate clutch, mounted on the flywheel, to a 3-speed-and-reverse gearbox (8), which also incorporates an auxiliary gear box, giving high and low ratios so that in effect 6 forward speeds may be obtained. A long gear-shift lever (20), held near the top in a gate (the H-shaped aperture in which the gear lever operates) on the dashboard in front of the driver, gives the main gear selection, while a shorter lever (21), to the rear of the first, gives selection of high and low ratios for road or cross-country travel. A hinged latch which covers the "reverse" part of the gate prevents accidental engaging of that gear, while an extension of the latch beyond the hinge makes its removal an easy matter.

The vehicle must be stationary while changing ratio in the auxiliary gearbox.

The speedometer is driven from the gearbox.

k. Differential

The differential (22) is of the controlled spur-gear type and incorporates two steering brakes (23), one for each track. These are internal expanding, and in order to increase their braking efficiency the drums are not directly fastened to the axle shafts, but are driven at considerably greater speed from the differential spur pinions through a set of gears.

l. Sprocket

From the differential, the drive passes through the steering brakes (23) and metal couplings (24) to the final reduction gears (25), and thence to the sprockets (26). Each sprocket is a narrow twin-rimmed wheel, of which the inner and outer rims are shod with rubber pads (27) (12 per rim) to form a continuous tire. The pads have the same inside curvature as the rim, but are flat outside. Adjustable rollers (28) (12 per sprocket) are fitted between the rims to act as teeth and to engage the track. An internal expanding brake, which is foot operated, is mounted inside each sprocket.

m. Suspension

(1) Front Wheel

The front wheel (29) is a pressed-steel disk type with a 3.5- by 19-inch tire (tire pressure 34 lbs. per sq. in.) and mounted in a pressed steel, motorcycle front-wheel fork (30) of conventional pattern. The springing (two vertical coil springs) (31), with controllable friction-disk shock absorbers (32), and the steering column (33) are very similar to those of a normal motorcycle.

(2) Track Assembly

The track assembly, which is of the usual type for half-track vehicles, consists on each side of a driving sprocket (26), four equal size, double-rimmed bogie wheels (34 and 35), and an idler wheel (36). Of the bogie wheels, the odd numbers (34) (from the front) are narrow wheels with radial spokes, while the even numbers are wider, pressed-steel disk wheels. The former run between the rims of the latter, the whole bogie and idler system being set rim to axle.

The bogie wheels are mounted on bell cranks (37) fastened to torsion bars, which pass across the body of the vehicle in crosstubes (33) of circular section and are anchored in the opposite side. The torsion bars of the corresponding bogie wheels of the two sides are carried one above the other (39) in the same cross tube.

The idler is simply another narrow-rimmed bogie wheel and is carried on an eccentric arm (40) which can be adjusted by a screw rod (41) passing through a bracket on the body. By this means the position of the idler can be varied, and hence the track tension may be adjusted. The bogie and idler wheel rims have thin, solid rubber tires. There are no return rollers, the track returning along the tops of the bogie wheels.

n. Track

Each track is made up of 40 forged steel links (42) (fig. 1) joined together by a bolt (43). These link joints are lubricated from oil chambers (44) which also, in part, form the tongues of the track links and pass between the rims of the bogie wheels. Above the oil chambers and track bolts, rubber shoes (45) are mounted. These are easily replaceable, being retained by 4 screws (46) only.

[Kettenkrad Track]

o. Steering

The motorcycle tractor is steered by the front wheel, by handlebars (47) in the normal manner of a motorcycle, and for sharp turns, by the steering brakes, operating on the tracks.

Two take-off rings (48) at the bottom of the steering column are connected by rods to arms on the brake crosstube (49). These in turn actuate levers, the ends of which are joined by means of adjustable rods to the steering brakes (23).

The steering brakes come into play for turns of over 5°, corresponding to a movement in either direction of about 1 3/4 inches at the ends of the handlebars.

p. Brakes

The footbrake, which is located on the right, and the handbrake (50) both operate an internal expanding brake mounted in each sprocket.

q. Driver's Controls and Instruments

These consist of the following:-

(1) steering handlebars with right hand throttle twist grip;

(2) main and auxiliary gear selection levers;

(3) a footpedal (51) on the left, operating the clutch;

(4) a handbrake (50) mounted on the left of the gearbox cover;

(5) a footbrake on the right;

(6) a radiator shutter control on the inside of the body, left of the driver, -

together with starter button, ignition and lighting switches, tachometer, speedometer, odometer, oil-pressure gauge, and water-temperature gauge.

r. Modifications for Tropical Use

A German document details several modifications which are made to the motorcycle tractor to fit it for use under tropical conditions.

(1) Engine

The oil filler cap has a linen hood tied over it, and the breather pipe has a steel-wool filter held between two pins near the bottom. For cleaning purposes, the filter may be removed after the lower pin has been extracted.

(2) Cooling

In order to provide sufficient cooling, the fan is driven at 1.4 times the crankshaft speed.

(3) Fuel Supply

The Opel carburetor is replaced by a Solex model, believed to be of the duplex downdraft type. A filter is incorporated between the gas tanks and the pump.

(4) Air Filter

The "Knecht Tornado" air filter fitted to the normal vehicle is replaced by a similar type of oilbath filter, incorporating a mechanical precipitator on the inlet side.

(5) Electrical Equipment

A new generator is fitted, while a solenoid-operated starter motor replaces the mechanically-operated type fitted in the normal model. The starter push button is located on the right side of the body level with the driver's seat. The distributor is enveloped in a linen bag.

(6) Transmission

Both main and auxiliary gear levers have linen hoods tied to them to cover their points of entry into the gearboxes. The breather holes in the gearboxes, the steering brakes, and the stub axle housings are all covered with cloth hoods.

(7) Tracks

For special purposes, tracks in which the links have extension plates (52) welded to their outside (figs. 2 & 3) are provided. These are probably for use in very loose sand or swampy ground.

(8) Brakes

The covers of the track brakes have extension plates welded to the upper halves to prevent the entry of sand as much as possible.

(9) Controls

The throttle twist grip is covered with a linen sheath which is tied to the grip at each end and to the handlebars, with sufficient free cloth allowed between these fastenings to permit full movement of the twist grip.

(10) Additional Equipment

A 0.4-gallon container for distilled water, and a gallon tank for radiator water are fitted.

A length of wire (16 to 20 feet) and a 13- by 16-foot tarpaulin complete the special tropical equipment of the motorcycle tractor.

s. Further Particulars

(1) Dimensions

Length, over-all	9 ft	0 in
Width, over-all	3 ft	3 in
Height, over-all	3 ft	11 in
Width between tracks	2 ft	8 in
Wheelbase, from center of front wheel to center of track	4 ft	5 in
Length of track in contact with ground	2 ft	8 in
Belly clearance		9 in

(2) Weight

	Without load		2,690 lbs
	Loaded		3,470 lbs
	Axle load, front wheel		120 lbs
	Load on tracks		3,248 lbs

(3) Performance

	Maximum speed on roads,
	At 3,000 rpm		38 mph
	At 4,000 rpm		51 mph*
	Trailer capacity		1/2 ton (approx)
	Maximum gradient, loose sand
	Without trailer		24° (45% or 1 in 2.25)
	With trailer		12° (20% or 1 in 5)
	Depth of water forded		1 ft 7 in
	Fuel capacity		9 gal
	Gasoline consumption,
	On roads		17 mpg
	Cross-country (approx)		12 mpg

* This speed, it is stated, is only to be attempted in exceptional circumstances.