Load Bearing Shock Absorbers

[gyropilot]

Happy New Year everyone!

I'm in evaluating changing the main landing gear suspension on my gyro and am looking for a pair of load bearing shock absorbers.

I've been searching the web trying to find something, but unfortunately I haven't been able to find much in the way of technical data online... only automotive application guides. Since the shock I'm looking for won't be used on a vehicle, so this doesn't help me. I'm hoping someone out there might be able to suggest something.

Here's what I'm looking for:

The shock needs to be load supporting... possibly a coil type spring/shock combo or maybe an air adjustable shock. The static load the shock will have to support is about 350 pounds *each*.

Minimum *usable* stroke length (to full compression) of at least 3" after the 350 pound preload is taken into account. What I mean is that I need to subtract the length the shock will be pre-compressed with that static 350 pound static load sitting on it. I've looked at some 3" max. stroke go-cart spring/shocks online mfg. by Azusa, and most of these would already be compressed 2/3 (or more) with a 350 pound load applied. That would leave only 1" of remaining stroke length, which isn't enough. I've attached a page from an old 1997 Azusa catalog with a compression/load chart which illustrates this problem. Their latest catalog omits this chart.

Overall shock length of approx. 12" (not critical). Smaller overall size is better (both in diameter and length).

Round bushed ends (smaller = better).

As light as possible.

The shock dampening ability is secondary to having the proper stroke length and load supporting ability. In other words, no fancy dampening performance features are needed or desired.

I will need one pair of these shocks.

Any pointers are most appreciated.

[donshoebridge]

John,

For reference, there is a good book on landing gear design available. I don't know the exact name of the book.

I'm using a polyurethane tube material as a spring. The big benefit to this material is that it acts as a spring and a hydraulic dampener at the same time. You can change the length, diameter, and durometer to get the required dynamics you need.

Here is what I'm going to use on my Hornet... 2" OD X 1" ID X 9" long, 50 Shore A durometer polyurethane hollow rod (visit McMaster-Carr). This may be a little light and I may need to go to 70 Shore A or more. I'm allowing for about 4 inches of travel at max gross times 3. At about 500+ pounds, this gives a spring rate of 187 pounds per inch. This should give me about 1/2" pre-load when sitting on the ground.

I just received the steel tube today to make my set, and I already the polyurethane. I post some pictures when I get the parts finished. You can also see the drawings for this assembly in my Hornet documentation package.

Hope this helps, at least for a starting point.

Don

[ToddP]

John,

I just saw these go on e-bay. They are a strut custom designed for an ultralight. If these would work for you, it will probably make you sick that they just sold a few days ago for $50 ???

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&category=26438&item=2449040 647

[gyropilot]

Todd,

It appears those eBay struts have nothing more than 3" stroke Azusa spring/shocks mounted inside a sleeve on the top. This was almost exactly the type of landing gear arrangement I'm thinking of converting my GyroBee to... until I started crunching some numbers and realized it might not be adequate! The problem is that these shocks are rated at something like 650-700 pounds for full compression. They have a progressive spring rate which apparently allows more compression travel at first.

Because of the geometry of my Bee's landing gear, I calculate each strut has about 350 pounds of compression on it at takeoff weight. At this weight those Azusa spring/shocks would be compressed at around 2/3 of their travel leaving me only 1" left. I'd like to have more travel than that to absorb higher g-forces.

Thanks for the pointer though.

[gyropilot]

Thanks for the idea Don.

I know using urethane, rubber, and neoprene is a popular choice for gyroplanes and I'm now starting to understand why.

It seems that by the time you find a spring/shock (or an air shock) big enough and with enough travel to work properly, the thing will weigh a ton and be too big!

I'll study your idea and I'll probably have more questions for you.

Thanks again,

John Landry

[MattPearson]

John, I use those exact shocks on my Super Fly and I love them. They do not bottom out and they work great!
My AUW is 580 lbs.

[MattPearson]

John, I have a brand new red set that I will sell to you for $50. Let me know

[Heron]

Matt:
If John does not take it I will!
Heron

[gyropilot]

Matt said:

John, I have a brand new red set that I will sell to you for $50. Let me know.

Matt,

Assuming those are Azusa part number 1700-136, I most *definitely* want them! I'll send you a private email to arrange things.

Thank you very much,

John Landry

[rfi]

John,
The red spring/shocks are 1700-236 which are rated at 730 pounds each at full compression. They are almost the same shock as the 1700-136. $50/pair is a good price.

[rehler]

John,

Be careful when you calculate the compression needed. Some people assume that a load (upward force) of 300 puonds on each wheel will mean that the shock will compress by 300 pounds. This is far from correct, because the struts are not vertical - they are at an angle. The greater the angle the greater the compression force on the shock. Also the angle of the lower support affects the load a lot.

The load on the shock is entirely up to the geometry of the landing gear system. For example on my gyro the shock load is in excess of 750 pounds with 350 pound of weight on each wheel.

There is a good program on the internet that will calculate the compression load for you:

http://www.ma.psu.edu/~herzog/em11/truss/truss-1.1-example.html

[gyropilot]

Ken Rehler said:

There is a good program on the internet that will calculate the compression load for you:

http://www.ma.psu.edu/~herzog/em11/truss/truss-1.1-example.html

Thanks for the link Ken... a very interesting and helpful little progam. Running the numbers on the main landing gear of my Bee was an eye opener.

Regards,

John