Original Year
# of pieces
Other: Motorcycles
Steering, front suspension,
differential, V-4 engine
Other Views

8857 Street Chopper

iso revolve
Click the image to download the LDraw file of this model.
Model by Eric Albrecht

This tricycle released in 1993 is one of the biggest motorcycles in the Technic line and the only large scale 3 wheeler.  It features front steering and suspension and a rear V-4 reciprocating engine.  The rear tires are uncharacteristically huge, but it turns out that there is a reason for this.

Unlike a lot of Technic models, this motorcycle is aesthetically detailed rather carefully.  The liftarms used for the fuel tank serve no functional purpose.  There are foot pegs under the front fork.  The seat is made from plates and hinges like the original Expert Builder models, as are the fenders and the rear wing.  The chain on the engine serves no technical function either, nor do the hoses on either side.  There's a round headlight nestled between the front forks.  All of this detailing makes for a very good looking model which does not detract from its functionality.


The front fork can be steered using the handle bars which are constructed of toothed connectors and axles.  The fork pivots around an axle which passes through the head tube part.  A pair of triangle parts are used as triple trees.  The handlebar angle and is fixed using toothed connectors.  The fork rake angle is set by the head tube.  As you can see in the computer image, the rake angle is further increased by placing the head tube at an angle using crankshaft elements.

Like a real chopper, the rake angle is very extreme which requires a very large turning input to get a reasonable turning radius and results in riding on the side of the tire.

steering     steering
LDraw file courtesy of Benjamin Wendl.
Click for an animation of the steering in motion.
The V-4 engine is made from cylindrical engine elements and is located under the seat between the rear wheels. It is driven by the differential.  The 28 tooth ring gear drives a 14 tooth bevel gear.  Next a 24 tooth spur drives an 8 tooth pinion connected to the crankshaft, resulting in a ratio of 6:1.
The crankshaft is offset 1/2 stud from center, giving the pistons a stroke of 1 stud.

There is also a chain which runs between the 24 tooth gear at the bottom and a 16 tooth gear on the top.  This top axle does not do anything but presumably represent a cam shaft.
engine     engine
LDraw file courtesy of Benjamin Wendl.
Click for an animation of the engine in motion.
There is a differential gear between the two main wheels to allow for easy turning on a powered axle.  The diff incorporates a built-in 28 tooth ring gear which can work either as a bevel or a spur, similar to the 24 tooth crown gear.  It is made to house 3 of the 14 tooth bevel gears.  One is on each axle, and one planet gear in the middle allows the axles to turn at different rates.
differential    differential
LDraw file courtesy of Benjamin Wendl.
Click for an animation of the differential in motion.
The front suspension is a girder type.  Instead of having compressible fork tubes, the forks are actually rigid.  Instead a 4-bar linkage is formed between the forks, the triple trees, and some 1x4 side links.  A spring at the bottom of this mechanism supports the weight.  As front brakes are applied, this type of suspension will cause the wheelbase to shorten and the forks and headlight to move upwards toward the handlebars.
suspension    steering
LDraw file courtesy of Benjamin Wendl.
Click for an animation of the front suspension in motion.
Wheels and Tires
The front wheel is the same motorcycle wheel which had been used in the previous bikes 8838.  It's large diameter and narrow width make it an ideal front tire.

The rear wheels are enormous, wide, and flat.  They are clearly far too complex to have been designed for this motorcycle, a fact which would be confirmed in 1994.  Only two models ever used these wheels.  The center hole in the wheel is actually round and does not have a cross slot, so it does not grip the axle.  If you look closely you can see that there is a bushing inside the hub.  Ever since its introduction in 1977, the bushing has had 4 flanges at one end which allows it to be placed in-between studs on a plate.  This capability was rarely ever used.  In this case, those same flanges grib tabs on the wheel hub which locks the wheel to the main axle.
Wheels    Wheels

Other Views

Side View
side view
Top View
top view
Front View
front view
Back View
back view
Computer Render

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