Lego Technic 1991

The Flex System
1991 EU Technic catalog
6 new sets, including the 8838 Shock Cycle and 8856 Whirlwind Rescue, as well as a new Idea Book were released. Lego also included an extra instruction book with the 8024, 8815 and 8820 to build a meta set from these parts. Although this had been done in other themes before, it was the first in the Technic theme.
There were several new parts released, including new motorcycle wheels and large shock absorbers as well as an entirely new system of flexible cables and cardan joints.
US patent 5,733,168
The Flex System: The new Flex System consisted of thin, flexible rod-shaped cables with coupling heads, larger flexible sheaths and connectors that could be attached with axles or ball joints. When used together with the flex tubing the flex cable allowed for the transmission of force much like a Bowden cable. The small size and flexibility allowed it to be used where traditional gears and cross axles would be too bulky or impractical.
The original patent application also made a claim for using the flex cables as a means of stiffening static constructions, however this was never fully realized in any Technic models. The use as a Bowden cable, however, was utilized in a number of models, most notably as a helicopter or airplane control mechanism. The system remains one of the more elegant Technic construction methods and has always been underutilized by Lego.
US patent 5,360,364
The Cardan Joint: The cardan joint consisted of a spherical ball with axial grooves, a loose-fitting ring with ball attachments as well as a guide ring with carrier pins. The pins of the guide ring meshed with the grooves of the cardan ball, allowing the transmission of torque but also allowing the guide ring to pivot about the sphere. The loose fit of the ball attachments allowed them to remain static as the rest of the assembly spun and be controlled by a flex cable. Not only did this allow for 3-axis control of a helicopter rotor but also formed the basis of a constant-velocity joint. Another example of the increasingly sophisticated specialized elements Lego was now developing.
8838 Shock Cycle:The Shock Cycle featured both front and rear wheel suspension, accurate steering and a 2-cylinder chain drive, The new shock absorbers and hollow rubber tires, along with the new engine elements resulted in a quite realistic model. As Lego continued to release new, highly specialized elements the designers were beginning to create very realistic models, both in terms of function and form. The Shock Cycle would serve as the basis of a number of motorcycle models over the next decade.
1991 sets:
8074 Universal Set w/ Flex System8810 Cafe Racer8815 Speedway Bandit8820 Mountain Rambler8838 Shock Cycle8856 Whirlwind Rescue8891 Designers Idea Bookmeta Racer (8024+8815+8820)

Six new models were released in 1991, many of which incorporated fairly complex new systems of parts.  There were two motorcycles, both of which used the new head tube to achieve a realistic rake on the fork.  The larger motorcycle also had a chain driven V-2 engine and both front and rear suspension using the new shock absorbers.  A small go-cart featured a one cylinder engine and rack and pinion steering.  A small Jeep-like truck had rear suspension and steering.  A large universal set showcased the new flex system with 4 complex models.   Finally, a large helicopter, the largest to date, was packed with features including the flex system and a new cyclic system for the main rotor.  As if this wasn't good enough, the helicopter also had a deployable winch and retractable tricycle landing gear.

Chief among the new features was the flex system.  It included flexible cables of various lengths along with ends which could be snapped on and would attach to either an axle or a ball joint.  Additionally, there were sleeves for the cables which allowed them to be routed through structure and used as a push-pull cable (Bowden cable).  When combined with a new ball gear similar to a Constant Velocity Joint, this system made for a fairly realistic cyclic motion on the large helicopter.  The flex system is a fascinating concept which was rarely really used to its fullest potential.  It was only ever included in 15 sets, many of which used it for the same type of rotor motion as the 8856.  One reason for the lack of use might be the fragility of the cables.  The cables are made of a different type of plastic which, while flexible up to a point, can easily end up with brittle fracture if bent too far.  This is especially a problem when removing the ends.  Tugging too hard can easily shatter the cable and repairing them is not easy (but possible; I broke and repaired one while writing this year's section).  Later, different ends were introduced which made it easier to attach and remove them.

Also new this year were several motorcycle components including a wheel and tire, a head tube, and a larger set of shock absorbers.  Combined with the reciprocating engine and a drive chain, this made for a motorcycle which was quite functionally realistic.  The wheel was only ever used on motorcycles, and only came with 4 sets as of 2008.

Parts:
Flex System
A new flexible push-pull cable system (Bowden cable) was introduced.  It could be used as a standard link, for transmitting force around corners, or simply as a diagonal brace in a truss.

• The central item is a flexible plastic cable with a small neck on either end used for clamping.  The cable has been available in a variety of lengths over the years from 3 studs long all the wy to 33.  They are fragile and easily broken, so finding a replacement can be pretty difficult.  Be careful with them.  They were almost always light gray.
• The other item is a hollow cylindrical tube which is also flexible but not brittle like the cable.  This served as a sheath for the cable, guiding it through the structure and preventing buckling under compression loading.  This tube was later also used as an extension and connector for pneumatic tubing.  They were almost always dark gray.
• The end connector has an axle hole and a slot for the cable.  The slot has a tab which grabs the necked portion of the cable.  The cable snaps into place when inserted from the side if done with care.  There is a second end connector which has a slightly different hole to snap onto ball joints.  The regular end is dark gray and the ball end is black.  Both were later replaced with different versions which were easier to install.

Shock Absorber
The new shock absorber has a much longer stroke than the old one (which remained in production).  Over the years, it was mainly used on motorcycles, but also found its way onto a couple of the very large vehicles as well.

Head Tube
The motorcycle head tube has a 2x2 brick on one end and a yoke on the other end which allows for a fork rake of approximately 25 degrees.  This part was only ever used once in a set for a purpose other than as a motorcycle head tube.

Triangle
The somewhat misleadingly named triangle is a 1/2 thickness link which is 5 studs wide at the bottom, 1 stud wide at the top, and 3 studs high.  5 of the holes are round while the two lower corners are slotted to lock axles.  This part has become pretty standard in most Technic sets from this point forward.

Motorcycle Wheel and Tire
A new large motorcycle wheel and tire were introduced.  The wheel has 6 spokes and a cruciform hub.  It only ever came in white in 4 Technic sets.

Rotor System
A brand new rotor system was introduced which allowed the rotor to mimic the motion of a cyclic.

• The heart of the system is a spherical ball gear with 8 teeth and an axle slot running along the central axis.  The part was rarely used except in a couple of helicopters and once as a CV joint.
• The 4 blade rotor has a central recess to allow insertion of the ball gear.  There are 4 small pins evenly spaced around the inside of the recess which lock onto the gear, forcing the rotor to rotate with the gear, and therefore with the driving axle.  Since the gear is spherical, the rotor can pivot freely on the other two axes (torsional constraint only).
• The final part of the rotor system is what I will call the swashplate.  It has 4 equally spaced ball joints and a hollow center which snaps around the rotor sleeve.  The idea is for the rotor to spin within this, but for the swashplate to remain stationary.  It accomplishes this by grounding itself to the structure using one or more of the ball joints.  One or more of the remaining ball joints can be coupled to a cyclic control to allow the rotor to tip both side-to-side and forward-back.  This isn't quite how a real helicopter cyclic works with each blade changing pitch as it rotates, but it certainly gives the Technic helicopters a realistic feel.  It was only included in 2 sets, both of them sweet.

1991 sets:

• 8856 - Whirlwind Rescue
• 8838 - Shock Cycle
• 8810 - Alpha Racer
• 8815 - Speedway Bandit
• 8820 - Mountain Rambler
• 8074 - Universal Set with Flex System

By Isodomus and Technicopedia