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Original Year
1980
# of pieces
662
Categories
Cars and Trucks:  Super Cars
Dimensions
57(L)x24(W)x18(H)
Features
Steering, 4 cylinder engine,
3 speed transmission, Adjustable seats,
Rear suspension, Differential
Other Views

8860 Auto Chassis

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


Set 8860, released in 1980, was the second generation auto chassis and improved upon the technology in the previous version, set 853, in almost every way (except size).  This set clearly represented the state-of-the-art in LEGO at the time.  It features a 4 cylinder reciprocating engine, a 3 speed transmission, a working rear differential, working rear suspension, rack and pinion steering, and adjustable and reclining seats.  It still uses a large amount of traditional studded construction, but far less than its predecessor.  The frame is built entirely of beams but, since it is only one layer thick for most of the length, it has poor torsional rigidity.  The large gray box in the front does not appear to serve any technical function, but acts as a counterweight to the cantilevered engine in the rear.

This set is a perfect example of function over form which, to me, is the whole point of Technic.  It is not beautiful in a stylistic sense (it does not even have a body), but the technical functions have been thought through in detail.  There were few improvements left to be made with this type of parts, and the rest of them would be included with set 8865.

All of the Technic colors (at the time) are represented in this set: black, light gray, red, blue, and yellow.  It also debuts a number of new parts:  a differential gear and its associated 14 tooth bevel gears, shock absorbers, and a new pulley wheel.

Features

Steering
The front wheels can be steered using a  wheel at the driver's position.  As can be seen in the computer generated image, the wheel drives an axle connected to an 8 tooth pinion gear via a pair of universal joints.  The pinion drives a pair of racks.  The racks use plates to attach to the arms of the steering mechanism.  The steering mechanism itself uses 6 traditional 2x2 swivels with 2 pair allowing rotation of the wheels about the vertical axis and another pair acting as steering arms attached to the rack gear.  Simple axles allow the wheels to rotate freely.

This is the first Technic set in which the steering employs the Ackerman Principle.  You can see in the images that the tie rod attachments are not directly behind the kingpins, but are one stud inboard.  The result of this is that the wheel on the inside of the turn is rotated more sharply than the wheel on the outside, which is exactly what is required for a turning in a proper circle without skidding.
steering    steering2
Click for an animation of the steering in motion.
Engine and Transmission
This set features a rear mounted 4-cylinder horizontally opposed engine, similar to a "boxer" engine but with one crank pin per pair of opposing cylinders.

The engine uses the offset axle holes of the 24 tooth gears to make a crankshaft.  Connectors placed end to end and clocked 90 form rods which connect to 4 specialized 2x2 square pistons.  The "cylinders" are actually square.   A belt on the rear of the engine drives a fan.

The engine also has a number of aesthetic features such as cooling fins (suggesting this engine is air cooled), a distributor, and a pair of exhaust manifolds.  There is also a bulkhead forward of the engine which may be either a firewall or a radiator.

Ahead of the engine is a 3 speed gearbox / transmission (see color coded computer image).  The green axle and 3 spur gears translate forward and aft.  The blue axle is driven by the engine and mates with the crankshaft via a 24 tooth gear (not shown).  The red axle mates with the 28 tooth ring gear of the differential via the new 14 tooth bevel gear.  In 3rd gear (pictured), the path to the wheels is (8/24) x (24/8) x (8/24) x (28/14) = 1:1.5 (overdrive).  In 2nd gear, (8/24) x (24/8) x (16/16) x (28/14) = 2:1.  In 1st gear, (8/24) x (24/8) x (24/8) x (28/14) = 6:1.

The gearbox is not synchronized in any way, so it is difficult to shift gears since they tend not to be aligned.

As you can see in the photograph, the shifter only allows two positions (2nd and 3rd).  1st gear is locked out by blocking it with a plate.  This is due to the fact the gear ratio is so low that the gear train cannot take the stress if the vehicle is pushed (engine turns 6 times for every turn of the wheels) and tends to skip.  If the model is motorized (and therefore driven from the engine instead of the wheels), the shifter is reversed to lock out 3rd instead, since 3rd gear would tend to stall the electric motor.
engine    xmsn
xmsn    xmsn
Click for an animation of the engine in motion.
Suspension
This is the first LEGO set with suspension of any kind.  It is primitive but effective independent rear suspension.  The entire wheel assembly rotates with a cantilevered lower control arm supported by a pair of new shock absorbers.  This  is a swing axle type suspension which results in large changes in wheel camber as the control arm is raised (see animation).

The wheel is driven through a universal joint.  Since there is only a single u-point, the rotational velocity of the wheel is not constant, and becomes increasingly erratic at larger angles.  Luckily, the sprng weight of the vehicle is low enough that the control arm is horizontal under normal load.

suspension    suspension
Click for an animation of the suspension in motion.

Differential
This is the first set with a real differential gear.  It 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 new 14 tooth bevel gears.  One is on each axle, and one planet gear in the middle allows the axles to turn at different rates.  This addition results in very smooth turns for this model.
diff    diff
Click for an animation of the differential in motion.
Seats
The bucket seats are among the most technical that LEGO ever made.  A pair of longitudinal axles support each seat and allow it to translate forward and back.  A rack gear runs parallel to the axles.  A pinion gear attached to the seat can be turned via a crank, driving the seat forward (see computer image and animation).

In addition to the ability to adjust the position of the seats, these seats also can be reclined!  A second crank turns a set of 8 and 24 gears, one on the base and one on the seat back.  A ratchet and pawl, spring loaded by a rubber band, keep the seat from falling back and lock it in any position.

There are no rear seats.
seats     seats
Click for an animation of the seats translating.
Click for an animation of the seats reclining.
Wheels and Tires
This set uses four foam 24x43 tires and wheels, the biggest foam tire that LEGO made.
Wheels
 


Other Views

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

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