JohnM

[JohnM]

Hi All,

My goal is to find a standard product CF tube which could reach the E value and the compressive strength of mild steel.

I can find plenty of tubes that you make, but none have the needed information to make a decision, so I have attached a design document showing the requirement.

Hopefully help is close.

Cheers

John

 

For front push rod              -find Ixx for worst case load, 15660 N cornering,  with SF = 2

 

S.F of 2 to reach YP of MS = 250 MPa t & c, 150 MPa shear. For pure compression τ max = ½ axial stress

A = F/σ = 15660/125        = 125.3 mm²      

Tube dia = 25.4 mm , circ = 79.8 mm find thickness.

A = Pi*r²   t = A/circ = 125.3 /79.8 = 1.57 mm            (1x1/16”  25.4 x 1.57mm)  Ixx = 8350 mm⁴ (stress limit)

                                                                                        

Pcr         = π²EI / l²             (pin – pin ends)

 

Steel      = π^{2 *}207000*8350 /1100²

              = 14100 N (= S.F 0.9)        not ok    2.3 Kg (bow & ball joint eccentricity of less than (0.035”) 0.1%)

 

(1x3/16” 25.4 x 4.78mm)  Ixx = 17300 mm⁴

= π^{2 *}207000*17300 /1100²            

= 29210 N (= S.F 1.86)      just ok   2.67 Kg

 

(1 ¼x1/16” 31.75 x 1.65 mm)  Ixx = 17700 mm⁴

              = π^{2 *}207000*17700 /1100² 

              = 29855 N (= S.F 1.9)        just ok   1.32 Kg  higher drag

 

(1 ¼x0.083” 31.75 x 2.11mm)  Ixx = 21600 mm⁴

= π^{2 *}207000*21600 /1100² 

              = 36470 N (= S.F 2.3)        ok          1.73 Kg higher drag

                                                                       

Ti 1.31x0.065”    E=114 GPa          YP 1100 MPa                             

= π^{2 *}114000*36200 /1100²                     

              = 33660 N (= S.F 2.15)      ok          1.73 Kg higher drag, ($300/tube)    no benifit

 

Al (1.25x0.125” 31.75 x 3.175 mm, max wall) Ixx = 29400 mm⁴      E=69 GPa    YP 275 MPa          

= π^{2 *}69000*29400 /1100²  (6061 T6 tube)

                            = 16540 lbf (= S.F 1.06)    not ok    (0.8 Kg) higher drag

 

CF ???

My goal is to find a standard product CF tube which could reach the E value and the compressive strength of mild steel. Then  the 31.75 x 1.65 mm size would be ok and the weight would be much less.

[John Kimball]

As we don't have specific data for compression on our tubes, and our tubes are all manufactured differently for different performance requirements, we can't give you a perfect answer for you. However, we can tell you that Intermediate modulus tubes will give you the best performance for tensile strength and stiffness, which can come in handy in compression situations as it will deflect less and be less prone to failure. The image attachment below will give you an idea of the different properties that can be obtained from different materials. AS you can see, intermediate modulus carbon fiber has the best overall performance when it comes to stiffness AND tensile strength.

If I read your inputs correctly, you have a 25.4mm tube (ID or OD?) and a 1.57mm wall thickness. Is that correct? 

As a word of advice, the thicker the carbon fiber tube, the better the compression. Even if you double the wall thickness, you will still be much lighter than mild steel.

Here's a couple tubes to consider. All are IM carbon fiber, but nothing over 1.6mm

https://www.rockwestcomposites.com/tubing/round-tubing/round-carbon-fiber-tubing/roll-wrapped-carbon-fiber-tubing/intermediate_modulus_carbon?inside_dimension=0.75-1.25%2C0.750-1.250&pattern=1057

As an option, here are some standard modulus tubes with a few more options for thickness.

https://www.rockwestcomposites.com/tubing/round-tubing/round-carbon-fiber-tubing/roll-wrapped-carbon-fiber-tubing/standard_modulus_carbon?inside_dimension=0.750-1.250