Main gear shock cords (bungees) are made of natural rubber encased in woven cotton. The cotton casing provides some friction between wraps and adds to the shock absorbing capability. Inferior, non-aviation cord does not stretch properly and if used, may result in bent gear and fuselage tubing.
Cords are affected by time, landing cycles, UV and oxidation. Shock cords need to be replaced when they allow the main gear ends at the center of the fuselage to sag under load, or when defects are found such as frayed cotton casing, narrowed cord ("necking") or wear at contact points. Useful life is estimated at five years. For best results, both cords should be replaced at the same time.
After 1993, Avid changed the cords to accommodate the increased gross weight of the Mark IV. These cords are 90" long and require 7 wraps.
Original Avid shock cords had aluminum crimped ends. Replacement cords may be fabricated by the homebuilder with a simple fixture and safety wire.
The fixture stretches and holds the cord loop while it is wrapped tightly with safety wire at the same position as the aluminum crimp the factory used. When the loop is taken off the fixture, tension is released, the cord relaxes back to its normal size and the loop is locked in place. In use, the loop ends never see as much stretching as they do when it was on the fixture and the safety wire was applied.
- Cut aircraft shock cord, 3/8" diameter, to the correct length. To achieve 90 inches and 7 wraps, the cord should be cut to 114".
- Fold one end of the cord into a loop over the top peg of the fixture. Use a tie-wrap to hold the loop in place on the peg. Double check the loop size and overall length of the cord.
- Mark the cord with a Sharpie marker where the safety wire wrap needs to start. This line also serves as a visual check that the loop didn't slip when put in the fixture or stretched.
- Optional: Brush Scotch 3316 structural adhesive between the cords to prevent slipping.
- Using the vice-grip tool, grip both sides of the loop as close to the mark as possible while still being able to pull the cord far enough back to latch the vice grip tool onto the fixture's bottom peg. This is usually several inches away from the mark due the force required to manually stretch the cord. Tie wrap each side of the loop to a jaw of the vice grip tool below the clamp.
- Sitting on the floor, using your heels to hold the fixture, pull on the vice grips hard enough to stretch the cord and latch the vice grip tool on the bottom peg.
- Double check the marks to verify that the cord hasn't slipped.
- Cut a ?" length of .040 safety wire. Wrap in a figure 8 around both cords at the mark. Twist.
- Tightly wrap safety wire around the bungies to form a clamp to hold the loop end together.
- Remove the bungie from the fixture. Wrap the short end near the finished loop with masking tape and carefully trim off the excess cord with a sharp box cutter, taking care not to nick the cotton outer wrap.
- Repeat for the other end of the cord.
Note: this demo was done on scrap cord, real wraps would be tighter more closely wrapped.
Elevator control tube interferenceEdit
To prevent interference with the elevator control tube, some builders enclose the tube with a one foot length of 1" PVC pipe (trigear), or weld in a piece of tubing to hold the shock cord away from the control tube.
Replace rubber hose around main gear end.
Two person installation methodEdit
Measure overall length, loop center to loop center. Divide this length by the number of wraps (7). For a 90" cord, this is approximately 13".
Starting at one end, mark the cord at 6-1/2" (the first wrap will be half a wrap, or half of 13"), then every 13". The last mark will be about 6-1/2" from the other end.
From inside the fuselage, hook one loop around a fuselage peg. Route the cord down and around the gear and back up, leaving it loose.
Lying underneath, use a spark plug puller tool to grasp, pull down and stretch the cord until the first mark is at the bottom, on the gear leg tubing. Hold the cord tight against the tubing with your thumb, while the second person working from above pulls up the slack and holds their end tight, feeding cord down to you for next wrap. Pull until the second mark is at the bottom, feed the cord up and have the second person pull up the slack. Continue until all the marks are in line at the bottom on the gear leg tubing, then hook the remaining loop onto the other side of the fuselage peg.
Safety limit cableEdit
plastic tubing to cover
rubber band or covered hair elastic band to take up slack