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Steel
Although the ultimate tensile strength of many premium steels is greater than 3-2.5 titanium,
this raw strength is meaningless in the final bicycle frame because:
- The strength advantage is lost in welding.
- Steel's strength-to-weight ratio is lower than that of titanium, both before and after welding.
When comparing materials, strength after welding, or heat-affected strength, must be considered
first, because the highest stresses in a frame are at the joints or heat-affected zones. For
example, Columbus SL steel tubing has a cold-worked (as received) ultimate tensile strength of
roughly 135 ksi, making it equal to Merlin 3-2.5. Ignoring for a moment that Merlin's
strength-to-weight ratio is almost double that of the Columbus SL, we find that SL's yield strength
drops to 70-78 ksi after welding. Merlin 3-2.5 has a post-weld yield of 97-100 ksi. In addition,
for a given weight 3-2.5 titanium has roughly twice the post-weld fatigue strength of 4130
chrome-moly steel.
External and internal reinforcements, such as gussets, butts and lugs, can improve steel's
fatigue strength somewhat. Internal butts move the weakest points away from the areas of
highest stress. In some cases, however, it is not possible with current manufacturing
equipment to create a butt of optimum thickness. The maximum differential between the butted
and unbutted sections of a production premium steel tube is about 40%; any further improvement
must be achieved in some other way-with gussets, lugs, or some variant of these.
An optimally butted steel tube will outperform a gusseted or lugged tube because:
- A gusset or lug does not reduce the heat-affected zone (HAZ) at the sides and end of the
reinforcement. An ideally butted tube provides equal strength and equal or lower weight with no HAZ.
- Gussets and lugs create stress raisers at their endpoints, with a further reduction in
fatigue life due to the HAZ. An ideal butt with a properly designed taper eliminates the stress
raisers and also saves weight.
Whether gussets and butts are employed or not, there is still a wide gap between the fatigue
strength-to-weight ratio of 4130 steel and 3-2.5 titanium. Claims that it is possible to create
a steel frame of comparable weight and strength as a titanium equivalent are unsupportable, as
proven by raw objective data, and by the fact that no such frames exist.
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