3/4" o.d. tubing

Discussion in 'Motorized Bicycle General Discussion' started by Finfan, May 17, 2009.

  1. Finfan

    Finfan New Member

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    Well I have run afoul of another oddity in my current project. I have discovered that the seat tube on the frame I am using has an inside diameter of 0.79" which is smaller than any standard seat post currently available. At least that I can find. Does anybody have a source for some steel tubing with a 3/4" outside diameter that is strong enough to act as a seat post? I am sure there must be something out there but I am not having any luck finding it. Thanks!
     
  2. datz510

    datz510 New Member

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    Fin.. If you make it up to Phoenix sometime, check out Industrial Metal Supply up on 48th St, south of Broadway. They have a great showroom that is half full of remnants dirt cheap. They have very nice 3' long DOM tubing remnants that they sell by the lb (scrap steel price). They should have plenty of thick wall 3/4" OD DOM tubing for you in various wall thicknesses.
     
  3. Finfan

    Finfan New Member

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    Hi! I'm bumping this thread to get some input on some possibilities I found.

    Candidate #1: 3/4" x 1/16" aluminum tubing. Feels too light probably not a good idea. How strong is this stuff?

    Candidate #2: 3/4" x 1/16" steel welding tube. I do not know how strong/brittle this type of steel is. Anybody familiar with it?

    Candidate #3: Solid 3/4" steel rod. A bit heavy but I have no doubt it will take the load.

    So if anybody with a little practical metallurgy experience could give me some feedback here I would appreciate it!

    :confused:
     
  4. captainrichhill

    captainrichhill New Member

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    Go on E-Bay and look up seat posts. Some of the listings are from bicycle parts dealers that sell sleeves that convert your seat post to something bigger. The measurements are all in mm's though so you're gonna have to convert 3/4" to mm. If you don't want to go through the hastle, just get some thin wall tubing (maybe an exhaust coupler)and make your own sleeve.slit it down the side and cut off the extra after wrapping it around your 3/4" tube.
    Just an idea....
     
  5. Finfan

    Finfan New Member

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    The problem is that all standard seat posts are larger than 3/4". The converters won't work. The inside diameter of my seat riser is almost dead on 20 mm so I would be looking for a 19mm post. Believe me I have been checking Ebay and various online vintage bike shops. Nobody seems to have such a thing. I am guessing at this point it must have been an odd design that Murray used for 1 or 2 years and nobody has it. So I am looking for stock material to replace it with.
     
  6. RedB66

    RedB66 New Member

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    Have you researched a "Lucky 7" seat post. They are smaller than standard post.
     
  7. Norman

    Norman LORD VADER Moderator
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    you could machine it down to size you need.
     
  8. Cabinfever1977

    Cabinfever1977 New Member

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    #8 Cabinfever1977, May 23, 2009
    Last edited: May 23, 2009
  9. Creative Engineering

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    Fin,

    Picture please...I would like to see the seat tube and clamp setup before I whip up a solution. :D

    I have plenty of tubing here so it's no problem. If you decide to go it alone...use a minimum of 3/32" wall steel tube. Check your seat post I.D. down the tube a bit. The top may be splayed which would give you a false reading.

    Jim
     
  10. lennyharp

    lennyharp New Member

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    Ream the seat tube out to fit a standard size as an option. .065 wall thickness 4130 CroMoly tubing is the choice for seat post
     
  11. Finfan

    Finfan New Member

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    Well, here are pictures not that I see it doing much good. Sorry about how dark the last one is but I had to shade it to avoid glare on the scale. I have a regular bicycle post clamp that I am going to try using. The only id on the seat riser is a sticker that says: " Murrey Manufacturing Company"/"Lawrenceburg Tenn U.S.A" with a vertical "03-78" (date code?). I've remeasured the inside diameter a couple times now and I keep getting 0.784". I'm thinking at this point I may just take the weight increase and go with the solid post. At least I know it won't break under me.

    Edit: Meh! That last one is useless! In the original you could barely read the scale.
     

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    #11 Finfan, May 23, 2009
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  12. Retmachinist

    Retmachinist New Member

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    There must be a million different sizes when it comes to seat posts. Half standard and half metric. I have six bikes out in the shop right now and all six are different, with the last one being 29.2 MM.
    Sounds like all you need is a piece of 20 MM tubing or rod.

    John
     
  13. lennyharp

    lennyharp New Member

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    "03-78" (date code?) Made in America and it was an odd size. The CroMoly tube is what I have used as a frame builder a piece of it with .065 wall thickness is not going to bend and works for most BMX bikes. The Murray and Huffy from that time frame were not worth much and the steel was junk. I may be jaded from working in bike shops a lot back then.
     
  14. Finfan

    Finfan New Member

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    It may have been junk back then, but I bet it would compare favorably with what is coming out of China now! It seems everything gets more cheaply made as time goes on.
     
  15. Creative Engineering

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    I'm figuring that the top of the tube is splayed a bit, and that the I.D. down lower is .755-.760 which would be normal by U.S. standards.

    What size is needed for the reciever end at the seat? Is it also 3/4"?

    The newer ones are swaged to a smaller diameter at the end that the seat connects to.

    Lenny is right .065" wall Chromoly is not going to bend! If you are not sure what type of tubing you are getting, and you want a safety margin, use .090" wall. 1/8" wall Aluminum will be fine also.

    There is no need to use solid rod, but if that's all you can find it will certainly work!

    Jim
     
  16. jimraysr

    jimraysr New Member

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    The only standard tubing you will find at Lowe's or Home Depot at 3/4" or below would be 1/2" EMT, (.706" OD) light weight conduit or 1/2" (.625: OD) copper tubing. In general pipe 12" and under is reference by the ID, not OD, So 1/2" carbon steel pipe is .84" OD. Iron pipe is ductile, so it would bend anyway.

    Jim
     

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