I think roll wrapped tubes are best for bicycle building. True?
But what is equivalent to the 22.2x0.9 chromoly steel tube?
Not sure how to calculate. Some carbon tube makers give a little info, but most have none.
This is the info I can not find anywhere.
Stiffness of an object is determined by its geometry and the material.
âSecond moment of areaâ is the geometry factor to the stiffness equation. There are also handy calculators to calculate that second moment of area like this one: Area Moment of Inertia & Section Modulus: Calculator & Formulas
The material factor is called âYoungâs modulusâ aka âmodulus of elasticityâ. You can look up that Youngâs modulus for steel and the youngs modulus for âcarbon fiber compositesâ (this is different than pure carbon fiber).
The stiffness of a tube in the âsecond moment of areaâ times the âYoungâs modulusâ. That way you can calculate the stiffness of two tubes made from different materials and of different size.
That is stiffness. Strength is different againâŚ
Agreed! I have a sheet that I use to do rough comparisons between materials. Feel free to take a look at it here. I make no claims that it is 100% accurate, but on first blush it seems correct for estimating relative stiffness.
I have made two frames with CF top-tubes and down-tubes glued into steel lugs. I used the 30mm woven-finish roll-wrapped tubes from easycomposites with OD 30mm and 1.5mm wall thickness. I would basically double the thickness that you would use in chromoly.
The easycomposites website gives some info about the tubes. If we look at these 25mm ones:
It says the tensile modulus is 64GPa. Thatâs about 3x less than steel, which is why you need a bigger diameter to get that stiffness back. You can find calculators and formulas online for the flexural rigidity of a tube, which is probably what youâre interested in.
I make it about 4.3 kN/m^2 for those tubes, vs 5.7 kN/m^2 for 22mm/0.9 steel. Those 25mm CF tubes work out to about equivalent to 20mm/0.9 steel tubes. Thatâs probably OK and the size to go for. But are these for chainstays? Itâs likely to be very hard to fit the tyres and chainrings in if thatâs the case (and you canât put dimples in CF!)
I got 22.2 steel tubes right now (chainstays). Got a little wiggle room. So I can probably fit 25mm with no bends/dimples.
I think youâd struggle to lose that much without an all-carbon monocoque construction of that part. There are so many joints there that if you went for pre-made tubes glued into lugs you wouldnât save much!
