Advertise on Bikeforums.net



User Tag List

Results 1 to 6 of 6
  1. #1
    Senior Member Briareos's Avatar
    Join Date
    Oct 2006
    Location
    Illinois
    My Bikes
    No bike at the moment; In process of building it.
    Posts
    539
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)

    Question Regarding the Design of Ti Frames

    From what I've read, Ti's advantages are:longevity, nice ride, strong and some others I can't remember.

    But what I read it suffers from, is that it isn't as stiff as alu or carbon.

    My question is...Would a "Triple-Triangle" layout for a Ti frame increase the stiffness enough to make it worth building that way? Maybe add a small triangle wedge between the seat-tube and the down-tube at the bottom...

  2. #2
    Senior Member Nessism's Avatar
    Join Date
    Jun 2004
    Location
    Torrance, CA
    My Bikes
    Homebuilt steel
    Posts
    2,324
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    The stiffness of a tube is controlled by two factors: the metal stiffness, characterized by the Modulus of Elasticity or Young's Modulus, and a shape factor known as the Area Moment of Inertia.

    Ti as a metal is not as stiff as steel (Young's modulus is lower), but stiffer than aluminum. As you know, most aluminum frames are bone rattling stiff. Why? because the shape factor, Moment of Inertia, is high (large diameter tubes and fairly thick)- Aluminum as a material is prone to cracking so the designers make the frame stiff so it doesn't flex which leads to aluminum frame failure.

    So going back to your Ti frame, it's easy to build a stiff Ti frame by increasing the tubing diameters and tube thickness since this increases the shape factor. Look for a frame with a 1-1/2" diameter down tube and 1-3/8" seat tube, this frame will be nice and stiff. No need to monkey with triple triangle mumbo jumbo - just up the tube diameters. Very simple.
    Becareful buying/selling bike parts on-line. I learned the hard way. :(

    Good/Bad Trader Listing

  3. #3
    Industry Maven Thylacine's Avatar
    Join Date
    Oct 2003
    Location
    Wherever good bikes are sold
    My Bikes
    Thylacines...only Thylacines.
    Posts
    2,936
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by Nessism View Post
    Th
    So going back to your Ti frame, it's easy to build a stiff Ti frame by increasing the tubing diameters and tube thickness since this increases the shape factor..... Very simple.
    There's your answer right there.
    Have you earned your stripes? <<click here / Questions about custom frames? Chat me! - warwickg71 (AIM/iChat) ThylacineCycles (Skype)

  4. #4
    Banned
    Join Date
    Aug 2005
    Posts
    5,117
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    " tubing diameters and tube thickness since this increases the shape factor..... Very simple."

    Isn't that just increasing dimensions, how is that a shape factor? Changing wall thickness as a ratio, or changing from round to square is one thing, but just using bigger tubes just sounds like a dimensional change. Often when tubing diameter is increased in bikes the material used retains the same wall thickness so the dimension increases, but the ratio of wall to diameter decreases which is an inverse change in shape. Still stiffer overall because the stiffness increases to the cube of the dimensional increase.

    I have no idea about the specialty terminology, however.

  5. #5
    Senior Member Nessism's Avatar
    Join Date
    Jun 2004
    Location
    Torrance, CA
    My Bikes
    Homebuilt steel
    Posts
    2,324
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by Peterpan1 View Post
    " tubing diameters and tube thickness since this increases the shape factor..... Very simple."

    Isn't that just increasing dimensions, how is that a shape factor? Changing wall thickness as a ratio, or changing from round to square is one thing, but just using bigger tubes just sounds like a dimensional change. Often when tubing diameter is increased in bikes the material used retains the same wall thickness so the dimension increases, but the ratio of wall to diameter decreases which is an inverse change in shape. Still stiffer overall because the stiffness increases to the cube of the dimensional increase.

    I have no idea about the specialty terminology, however.
    The Area Moment of Inertia (shape factor) increases when the tube thickness increases remember when Columbus used to make SL/SLX for the lightweight guys and SP/SPX (which was thicker and stiffer) for the heavyweights?

    Adding thickness is a viable technique to increase stiffness but increasing diameter is a better method if one is attempting to maximize stiffness to weight ratio since the Moment of Inertia increases exponentially as the material moves away from the bending axis (diameter increases).
    Becareful buying/selling bike parts on-line. I learned the hard way. :(

    Good/Bad Trader Listing

  6. #6
    Senior Member
    Join Date
    May 2006
    Location
    Boston
    Posts
    299
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    also, thicker chainstays helps.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •