Question Regarding the Design of Ti Frames
08-03-08, 04:48 PM
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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...
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...
08-03-08, 07:21 PM
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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.
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.
08-04-08, 02:42 AM
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08-04-08, 10:58 AM
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" 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.
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.
08-04-08, 04:15 PM
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" 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.
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.
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).
