Where to buy bungee / shock cord

[Luked 71]

Searching through the archives, it sounds like there are different grades of bungee cord. I checked the normal places (copied from the web sites):

 

Aircraft Spruce:

• 4 mm = 0.15748 inches
• 6 mm = 0.23622 inches
• 8 mm = 0.314961 inches
• 10 mm = 0.393701 inches

Wicks:

• 1/4 in
• 3/8 in
• 1/2 in

Aircraft Spruce is a lot less expensive. Is there a reason?

 

The cords that came with my plane are of unknown age, and they look just like the kind you buy at the hardware store (very stretchy). Aircraft bungees I've seen before seemed much harder to stretch.

 

Wicks has standard inch sizes, and they go up to 1/2". The old ones I have are about 3/8". I'm going to re-do my seat truss to make it strong enough to handle wide gear (bungee type) in case I decide to go that route down the road. I need to make sure it can fit the number of wraps required.

 

I know all this stuff has been hashed out pretty good in old threads, but some of the information isn't specific on a few points. One old thread said 7 wraps for standard gear, and 8 wraps for wide gear. Another says 6 wraps for standard gear. Also, they don't say if they mean 7 wraps over the top and 8 wraps under the bottom, or 6 wraps over the top and 7 under the bottom.

 

I think I also read that the standard gear uses 3/8, and the wide uses 1/2"?

 

Can anyone shed some more light on this, or verify any of the specifics? I don't need official information of what Avid specified or anything. Just hearing what setup has worked for others, so I can replicate it would be great. Hoping to do some welding in the near future...

 

Thanks,

Luke D.

[SuberAvid 617]

Luke,

 

I have been making my own for my extended wide gear.  Be sure to buy the aircraft grade bungee material; it uses natural rubber rather than the cheap synthetic rugger in the hardware store bungee material.  the natural rubber works well in cold temps and does not get hard, plus lasts much longer.

 

I use 1/2" on my Avid+ but on a stock Avid MKIV they used 3/8".  Buying it in bulk lengths from AC Spruce or Wicks is a lot cheaper.  I make a loop on the ends and tie them with stainless safety wire using a safety wire tightener tool (tietite or something like that)  Use double wraps on each tie and do it twice.  They hold great. 

 

I don't remember the standard # of wraps for the MKIV but I think I used 7 or 8 on my wide gear.  On the Avid+ I use 9 wraps; 8 full wraps and a half wrap on each end.  Make the bungee up to length (I think it is 90" for the wide gear but have someone else confirm it for sure.)  Lay the bungee out and divide it up by the # of wraps using 1/2 of a wrap at each end and mark it with a felt marker.  The marks will be the place you want to cross the gear leg tube as you wrap it.  You will need someone inside the plane to pull the end up and a person under the plane to pull it down, stretch it and hold it while feeding the end up to the guy inside.  1/2" is a pretty good workout to wrap but 3/8 is easy.

Edited 25 Apr 2014 by SuberAvid

[Luked 71]

Thanks Randy. That sounds good. I've got safety wire twist pliers. Do you remember how may wraps of the bungees over the truss (top and bottom) you use?

Edited 25 Apr 2014 by Luked

[SuberAvid 617]

Luke,

 

I added to my post above with the wraps I use, but for the MKIV, I think it was 7 or 8.  It is not rocket science though so if you start with 7 and it sags a little when they are new, I would just rewrap them with 8 wraps, divided up equally as I described.  If you are tying the safety wire with standard twist pliers you will really need to be sure to get the wire tight before twisting it.  The tietite tool tightens up the wire wraps as you cinch it down so it really gets the wire wraps tight before bending it over to finish it.  You might want to get one of these tools just for this, they are a bit spendy but not near as much as you save on just one bungee vs ordering them already made up.  You can order them from their web site.

[Luked 71]

Thanks for the extra information. I think that pretty much covers it. I'll check out the tietite tool.

[Luked 71]

I remembered seeing an article linked from this forum about making bungees, so I looked it up again. They show a tool made from a pair of Vise-grips. This must be similar to the tietite tool. I see now why it's important. The article describes how the cord is stretched to that it narrows during the wrapping process. When tension is released, the cord expands again inside the wire wraps and creates increased friction for a firm connection. Here is the article:

 

http://avidflyer.wikia.com/wiki/Shock_Cord_Fabrication_and_Replacement

[IFMT 36]

10 mm from spruce is garbage. Go with the 1/2 inch aircraft grade. 8 wraps even spacing on wide gear. Can't remember the total length eye to eye but 110 inches comes to mind. To tight and you bend stuff, To loose and the suspension sags. Check old posts on here as this has been hashed out before.  Use a set of spark plug boot pliers to pull on the cord. Cheers. B

Edited 26 Apr 2014 by IFMT

[Columbus 30]

Luked- that was THE best article...beers on me should we ever meet