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A 3-2.5 Tubing Comparison
In the U.S., the three most common grades of 3-2.5 titanium used in bikes are:
- 3-2.5 AMS grade 105, the same stuff you would find under the hood of a 747. This material
must meet all AMS specifications (Aerospace Material Specifications, as issued by the Society
of Automotive Engineers) for hydraulic tubing. Theoretically, buying AMS 105 tubing directly
from the mill allows the designer an unlimited choice of diameters and wall thicknesses. In
reality, there are large minimum order requirements and long lead times involved, and only
the largest titanium fabricators, such as Merlin, can afford this luxury. Buyers sometimes
add to or modify the standard specifications for AMS tubing. Merlin's MTS325 tubing varies
from AMS grade 105 in that it has more stringent tolerances for straightness and surface
texture. Merlin's tubing also exceeds AMS specs for minimum ultimate tensile strength and
minimum yield strength.
- 3-2.5 -sports grade. Sports grade tubing is marginally less expensive because it is
subjected to fewer processing steps, which is supposed to cut costs. However, the cost
savings to date have had a detrimental effect on material formability and surface texture,
both inside and out.
- Scrap 3-2.5. This is material which has not met aerospace and/or sports grade
specifications, or is simply a small amount of overrun. One of the problems in using scrap
tubing is that there are no certifications or specifications, and thus no means for the
buyer to determine whether any structural anomalies exist.
3-2.5 Tubing Processing Variables
Although AMS standards prevail for all certified aerospace tubing, there is a window
of acceptable performance, and processing plays a large role in the quality of the final
product. There are three manufacturers of 3-2.5 tubing in the U.S., and each makes its
tubing in a different way. These processing differences create a wide range of 3-2.5
tube quality.
There are three main processing variables in U.S.-manufactured 3-2.5 titanium tubing:
- The crystal grain orientation of titanium, sometimes referred to as its texture,
affects some of its properties, and can be controlled by processing. Crystal orientation
is measured by testing the material+s contractile strain ratio (CSR), which is a numerical
index of crystallographic texture determined by the ratio of diametral strain to radial
strain. A small value, such as 0.3, denotes tangential crystalline grain texture, while
values above 1.8 can be considered radially textured. A CSR from 1.7 to 1.9 promotes the
highest fatigue strength possible while maintaining excellent bending ductility. Additional
radial texturing can push the CSR past 2.0, which improves bending ductility even further,
but only at the expense of fatigue life; fatigue endurance drops dramatically at CSR levels
above 2.0. For best results, CSR should be controlled and determined at the mill when the tubing
is made. Tubing diameter and wall thickness are always reduced at the same time, but not
always at the same rate, and it is the difference between diameter reduction and wall
reduction that determines the direction of grain texture. Larger reductions in wall generate
a radial grain texture, while larger reductions in diameter offer greater circumferential
grain texture. Tube texture can be detrimentally affected by cold working after the tubing has run
through its final cold-worked, stress-relieved (CWSR) cycle at the mill. For example,
forcibly reducing a tube (as by swaging or tapering) after it has completed its final
CWSR cycle rotates the crystals out of their radial orientation and lowers CSR. Reduction
processes like these, often used to taper main tubes and chainstays, diminish the endurance
limit of the tube.
- Surface finish, both inside and outside, is directly affected by processing. Titanium
is more notch-sensitive than steel. A defect-free surface makes a significant contribution
to longer fatigue life. The inside diameter of most titanium bicycle tubing also plays an
important role in promoting fatigue endurance; typically, the tube wall is so thin that
both the outside and inside diameters undergo a cycle of relative compression and tension.
The tension, or pulling, causes micro-cracking, which in turn can cause the tube or joint
to fail. If the inside surface texture is much rougher than the outside, crack growth can
begin on the inside.
- Any surface or chemical defect will affect the tubing. The only way to avoid this is
through rigorous quality-control procedures throughout manufacturing.
These factors, individually or in combination, greatly affect the longevity of a 3-2.5
seamless tube, and, in turn, the quality of the finished product.
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