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Re: The BUM Shocks spring rate ?

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  • Frank
    Would it possible or easier to adapt strut/shocks from something else with a known spring rate instead of building them from scratch? It seems what you built
    Message 1 of 8 , Nov 1, 2006
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      Would it possible or easier to adapt strut/shocks from something else
      with a known spring rate instead of building them from scratch? It
      seems what you built was pretty much the same as front forks from a
      small motorcycle. Of course there is always weight to consider, so
      using struts from a small car is probably out of the question, but
      the unishocks used on mountain bikes might give you what you need,
      and they are very lightweight.


      --- In Airchairgroup@yahoogroups.com, rexnstudio@... wrote:
      >
      > Scott,
      > To answer all three questions ...
      > 1. probably not enough.
      > 2. probably not enough.
      > 3. probably not enough.
      >
      > I'll probably end up re-doing the whole she-bang or going with
      exposed
      > bungees. Maybe I can find some heavy valve lifter springs to
      stack up in
      > there.
      > ...oh well it's a good excercise
      >
      > Rex
      >
    • Kenneth Cook
      Hi I was cleaning up a friends yard this weekend and there under the leaves an idea struck me for shocks....... Pogo Sticks......Don t know if they would work
      Message 2 of 8 , Nov 6, 2006
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        Hi
        I was cleaning up a friends yard this weekend and there under the leaves an idea struck me for shocks....... Pogo Sticks......Don't know if they would work or not. Just a thought.
         
        Ken Cook
         
         
        ----- Original Message -----
        Sent: Wednesday, November 01, 2006 2:06 PM
        Subject: Re: [Airchairgroup] Re: The BUM Shocks spring rate ?

        In a message dated 11/1/2006 12:11:54 P.M. Eastern Standard Time, cormac_maccormac@ yahoo.com writes:
        Would it possible or easier to adapt strut/shocks from something else
        with a known spring rate instead of building them from scratch? It
        seems what you built was pretty much the same as front forks from a
        small motorcycle. Of course there is always weight to consider, so
        using struts from a small car is probably out of the question, but
        the unishocks used on mountain bikes might give you what you need,
        and they are very lightweight.
             I came up with some scooter forks at the scrapyard, but they really dont have more than about 2" of travel and the resistance is light. I wanted to get a buttload of travel, so I decided to build from scratch. I wanted to stay light & low profile so I planned around 1" tubes. That limited me to a 7/8" spring diameter which I located in a 70 pound 6" spring (6 for $12) There are 3 of these in each strut, compressed about 2". That leaves about 7" of travel. Unfortunately there will be more takeoff weight on each of the main wheels than this current rigging can support. I think I got tunnel vision & dropped the ball there. I have held onto the idea of motorcycle forks for a long time, but all of the ones I have seen are pretty heavy in order to carry the forward & torsional loads on a bike. In the past few hours I have been looking into robbing the springs from motorcycle forks and adapting them to my design. I can re-use the lower plunger tubes, upsize the upper tubes to what I need & machine plugs to make up the difference. Spring weight and size is a concern, more travel, more tension gets me more payload .... proper tension, shorter springs, lighter, gets me less travel, but that may be the trade off.
             As far as the scratch building, it keeps me out of trouble.  I am in the middle of a cover job so Ive been bouncing back & forth to the BUM while dope is drying. I am an "extreme hobbiest" when it comes to machine shop stuff ... got me some used tools to play with.    still learning.
        Thanks for the input ... more pics to come.
        Rex

      • Scott Perkins
        I once found a lot of info doing google from companies making all kinds of springs and I think they are called die springs. It seems steel springs are linear
        Message 3 of 8 , Nov 7, 2006
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          I once found a lot of info doing google from companies making
          all kinds of springs and I think they are called "die" springs.

          It seems steel springs are linear in that if the spring rate is 100
          pounds
          per inch, if you put 200 pounds pressure, it will deflect 2 inches and
          so
          on until it is collapsed.

          It turns out that ultimately, rubber offers a much better performance
          in terms of total weight of the shock absorber for any given amount of
          force to be dissipated over a large distance.

          Whether steel springs or rubber bungies I always thought they should be
          so stiff that zero deflection would occur during 1 G operations.
          In other words sitting on ground fully loaded ready to fly.

          Maybe not even show any deflection until 1.5 or 1-8 G's as
          I wouldnt want them bottoming out too easily on a hard landing.

          It seems simple enough although maybe not easy, to measure the
          total weight to reach full deflection ( bottoming out ) and
          be able to calculate the number of G's required to reach
          the limit before bottoming out.

          The next question is how to build adequate friction into the shock
          system to minimize rebounds.
          Scott
        • red
          Rex, Have you considered an air shock from a car or bike? Shock absorbing, light weight, progressive rates (progressively harder to bottom-out as it is
          Message 4 of 8 , Nov 7, 2006
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            Rex,
            Have you considered an air shock from a car or bike? Shock
            absorbing, light weight, progressive rates (progressively harder to
            "bottom-out" as it is compressed), and you can inflate it to any
            starting compression-rate.
            --
            If there is an "A" in my email address,
            please delete the A to respond.
            Cheers,
            Red


            -----Original Message-----
            From: Airchairgroup@yahoogroups.com
            [mailto:Airchairgroup@yahoogroups.com] On Behalf Of rexnstudio@...
            Sent: Tuesday, November 07, 2006 6:37 AM
            To: Airchairgroup@yahoogroups.com
            Subject: Re: [Airchairgroup] Re: The BUM Shocks spring rate ?

            These are 70 pound springs that I have. Specs say 70# to fully compress.
            Stacking up 3 of 'em is still 70 pounds although I get the 7 - 8 " of
            travel that I want. I agree about the 1.5 g load at takeoff rest. For
            that I think I will need 125# springs (more size, weight, blah blah)
            That seems to be what the Cub types have. Bungees would be better
            although ugly .. ( this whole plane is ugly anyway) If I go bungee, I
            will pull the springs out. I havent checked the weight of the gear,
            shocks, rims, tubes & tires. But it seems like a big weight penalty for
            an assembly that isnt used but 1% of the time.

            (might go back to a skid & drop-away dolly before its all over)

            Deck lid lifts (a la hatchback door) would be a good dampener. My
            salvage man has them by the tons in all different sizes.
            REX
          • LES WARD
            I would start out, at a motorcycle wrecking yard. Front forks, rear swing arm. with the new style unishock, which has a preload adjustment. You could easily
            Message 5 of 8 , Nov 7, 2006
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              I  would start out, at a motorcycle wrecking yard. Front forks, rear swing arm. with the new style unishock,  which has a preload adjustment.  You could easily find a setup with 4 to 8 inches of travel and a wide variety of weight capacitys.  I am thinking around 125 cc honda/yamaha
              Aloha, Les
              ----- Original Message -----
              Sent: Monday, November 06, 2006 4:37 PM
              Subject: Re: [Airchairgroup] Re: The BUM Shocks spring rate ?

              Hi
              I was cleaning up a friends yard this weekend and there under the leaves an idea struck me for shocks...... . Pogo Sticks...... Don't know if they would work or not. Just a thought.
               
              Ken Cook
               
               
              ----- Original Message -----
              Sent: Wednesday, November 01, 2006 2:06 PM
              Subject: Re: [Airchairgroup] Re: The BUM Shocks spring rate ?

              In a message dated 11/1/2006 12:11:54 P.M. Eastern Standard Time, cormac_maccormac@ yahoo.com writes:
              Would it possible or easier to adapt strut/shocks from something else
              with a known spring rate instead of building them from scratch? It
              seems what you built was pretty much the same as front forks from a
              small motorcycle. Of course there is always weight to consider, so
              using struts from a small car is probably out of the question, but
              the unishocks used on mountain bikes might give you what you need,
              and they are very lightweight.
                   I came up with some scooter forks at the scrapyard, but they really dont have more than about 2" of travel and the resistance is light. I wanted to get a buttload of travel, so I decided to build from scratch. I wanted to stay light & low profile so I planned around 1" tubes. That limited me to a 7/8" spring diameter which I located in a 70 pound 6" spring (6 for $12) There are 3 of these in each strut, compressed about 2". That leaves about 7" of travel. Unfortunately there will be more takeoff weight on each of the main wheels than this current rigging can support. I think I got tunnel vision & dropped the ball there. I have held onto the idea of motorcycle forks for a long time, but all of the ones I have seen are pretty heavy in order to carry the forward & torsional loads on a bike. In the past few hours I have been looking into robbing the springs from motorcycle forks and adapting them to my design. I can re-use the lower plunger tubes, upsize the upper tubes to what I need & machine plugs to make up the difference. Spring weight and size is a concern, more travel, more tension gets me more payload .... proper tension, shorter springs, lighter, gets me less travel, but that may be the trade off.
                   As far as the scratch building, it keeps me out of trouble.  I am in the middle of a cover job so Ive been bouncing back & forth to the BUM while dope is drying. I am an "extreme hobbiest" when it comes to machine shop stuff ... got me some used tools to play with.    still learning.
              Thanks for the input ... more pics to come.
              Rex

            • Scott Perkins
              The problem with autos is they nearly all weigh at least 750 pounds per wheel and hitting a pothole at 40 a few hundred times over its lifetime is a lot worse
              Message 6 of 8 , Nov 7, 2006
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                The problem with autos is they nearly all weigh at least 750 pounds per
                wheel and hitting a pothole at 40 a few hundred times over its lifetime
                is a lot worse than a few hard landings in an ultralight. Hence the
                heavy weights of the auto coil over shocks.

                I've looked hard and found some promising alloy air shocks ( air is
                lighter
                even than rubber ) in midget race cars. Larger than go karts but not
                as big as a Cadilac.

                Equally interesting were the lightweight alloy A-Frames and suspension
                setups. Just take the whole thing off the midget race car and
                transplant.

                Also, rubber donuts in a compression tube instead of steel springs
                are lighter also and have been used. You should call ahead before
                you visit your Krispy Kreme as they are pretty scarce.

                attached are pics of motorcycle front fork shocks setup for 10 or 12
                inches
                travel by Roger Mann on his Storch prototype.
              • tjwardx
                ... Rex: Have you taken a look at the suspension on the Schreder HP series of homebuilt sailplanes? It s sprung by an air strut, pressurized with nitrogen or
                Message 7 of 8 , Nov 9, 2006
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                  --- In Airchairgroup@yahoogroups.com, rexnstudio@... wrote:
                  Rex:

                  Have you taken a look at the suspension on the Schreder HP series of
                  homebuilt sailplanes?
                  It's "sprung" by an air strut, pressurized with nitrogen or air.
                  It uses a high-pressure Schrader valve, so by varying the pressure,
                  you can get whatever "spring rate" you want.
                  see: http://www.soaridaho.com/Schreder/Construction/Cnstrctn_tips.html

                  Tim Ward
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