Landing Gear Bungee Load Ratings - Recommendations?

[Toro]

I just completed the installation of 3" extended gear on my 1956 PA-18A including the installation of new hydrosorbs and bungees. I use 8.50 x 6 wheels in the non-snow season and AeroSki M2000's in the winter.

While researching the project, I was advised to install one 1280HD and one 1380HD bungee per side by a couple of different folks with experience in the matter. According to a number of suppliers, two 1280HD's are apparently standard on a PA-18 so the recommendation seemed reasonable. I placed my order but signals must have been crossed because I received two 1280's and two 1380's instead of the HD versions. We went ahead and installed those because the combination was virtually identical in test load to two 1280HD's (see data below).

Once we had the gear together, we noticed right away that it was very stiff - when rocking a wingtip in the shop, only the tires flexed. I was advised not to worry about it and that things would soften up over time. I put a few hours on the wheels and switched to skis a couple of weekends ago. All I can say is that takeoffs and landings are bone jarring when on any packed snow and all of the energy seems to go right up into the airframe. The actual gear is not absorbing anything and I'm worried about the gear attach fittings and longerons. I've done at least thirty take-offs and landings and this gear is not softening up.

I had a look at the bungee manufacturer's web site and here is what I found for bungee size and test loads for the different bungees:

1080 5/8" x 8" 750 lbs
1080HD 5/8" x 8" 900 lbs
1280 3/4" x 8" 750 lbs
1280HD 3/4" x 8" 950 lbs
1380 13/16" x 8" 1160 lbs
1380HD 13/16" x 8" 1460 lbs

According to this manufacturer, 1080HD's are standard for the PA-18. Two per side would equal an 1800 lb test load. In my case, with a 1280 and a 1380, my test load is 1910 lbs. I'm surprised that this minor difference in test load would make such a large difference in actual feel on the plane.

I need to make a change as this bungee combination is simply too harsh on my plane (while on skis anyway). Unfortunately, I don't know what the bungee spec was on the stock gear before we did this mod but the energy absorption was very reasonable (although the hydrosorbs were completely worn out).

According to various suppliers I talked to, it seems that a lot of PA-18 owners will tend to install the heavier bungees for longevity but they often have larger tires installed to - so they're not relying on the gear itself to absorb much shock. I need to set my bungees up for ski use. What do you guys recommend for a bungee combination? I really appreciate your comments and experiences.


Regards;

Chris

 

[cubpilot2]

Chris:

For what it is worth.... The "smaller the tire the lighter the cords".

If I recall correctly the original 150 hp cubs with the 800 x 4 tires came with 1280 cords. These were OK when used in standard category and worked well to soften the "crash and go" student landings.

When we converted them to sprayers we would install the 1280HD cords; otherwise the gear would sag under load.

When you start putting the larger tundra tires on, then it starts to depend upon the tire size.

For example:
I used the old 25 x 11 x 4 tires with 1280HD cords and did fair.

When installing the larger 30 or 31 inch tundra tires the gear would start to "sag" or "spread" especially when you did any sort of tight turn when parking, like with one brake set. It appears that the increased leverage of the "longer arm" created by the tire was enough to spread the gear. The 1280HD cords could be fairly new and still do this. It was even worse in colder weather.

10 or 15 years ago it started to become common in Alaska to use one 1280HD cord with one 1380HD cord. This all but eliminated the sag on the ground but does stiffen the ride some. Not a problem with the softer tires.

If you are not going to run large tires and only skis then perhaps the 1280 or 1280HD would be better.

Also you would want to look for cord part numbers with a CW suffix. 1280HDCW (for cold weather). They seem to just be better quality cords and last a lot longer.