VERUS: STRESS RELIEVED 4130 STEEL TUBING

With all the hype in the bike industry about high strength this and higher strength that, why would True Temper even offer a 110,000 psi steel? Well, caviar is a lot fancier than pasta, but if you're going on a century ride, pasta is the better choice. True Temper's RC2 Series tubing has twice the strength of a basic steel, so it is definitely a high strength steel. It has been proven plenty strong for bicycle frames when used in wall thicknesses of .6mm or greater.

True Temper's VERUS Stress Relieved 4130 steel is designed for reliability that is even better than commercial grade 4130. This heat treatment removes internal stresses left over from the manufacturing process. The end result of removing internal stresses is that the tube (and your frame) lasts much longer. [Why: click here]

The trade off for increased strength in ANY material is decreased ductility. The real world result is that the failure mode gradually changes from controlled gradual bending to instant catastrophic fracture. Heat treatment can vary the strength of 4130 steel from a very ductile 70,000 psi up to a hard and brittle 225,000 psi. At 100,00 psi, Stress Relieved 4130 is a very reliable material with optimized properties for bicycle use.

But, you say, having absorbed all the hype, "Doesn't low strength mean high weight?" Not necessarily. It turns out that, by the time you make a steel bike stiff enough that it feels great, you have used enough steel in the tube walls to lower the stresses to where you don't need a high strength steel to withstand the loads and still have a good safety factor.

For these reasons, True Temper uses Stress Relieved 4130 steel for the general purpose VERUS main triangle tubing, seat stays and chain stays, and its fork blades.

[What Stress Relief Does for VERUS 4130 Steel]

Stress relieving doesn't increase the tensile strength of the steel. What it does is to reduce the internal stresses created inside the steel as it is drawn over mandrels and through the dies to create the thicker and thinner walls found in quality butted bicycle tubing.

Why is it important to reduce or remove these internal stresses? It has to do with a key characteristic of steels. This is a phenomenon called the Fatigue Threshold. The fatigue threshold is a stress level about equal to one half of the yield strength of the steel. As long as any elastic flexing cycle does not create a stress in the steel that exceeds the fatigue threshold, the flexing cycle will not be a [fatigue cycle]. So, what does this have to do with your bicycle? Your pedalling efforts will not create stresses high enough to exceed the fatigue threshold of 4130 steels. If all we were talking about were pedalling loads, your 4130 steel bike frame would last forever. Loads above the fatigue threshold occur when you ride fast over rough terrain or rough roads, and when you crash, etc. But these occurrences are relatively infrequent.

[So, what is a fatigue cycle?]

Fatigue failures are insidious. Failure can occur even though the part was never highly stressed. So why does it fail? Metals have a crystalline microstructure, and the crystals are not perfect because no metal can be perfectly manufactured. Under repeated loads the stresses cause micro cracks to form in the crystal structure of the metal. Over time enough of these cracks form that as they gradually extend, they begin to connect to form larger cracks. As these micro cracks begin to appear, the flow of stresses through the part is changed. The stresses must flow around and between these disruptions. The stress concentrations at the ends of the cracks act to multiply the increased local internal stresses, leading to faster crack propagation. As more cracks form and more cracks join, the conditions get worse and worse, and the fatigue effects accelerate. Failure becomes inevitable.

Steel is a very fatigue resistant metal. It has what is called a "Fatigue Threshold". Research has found that as long as the stresses are below about one half of the yield strength, there is little or no tendency for the micro cracks to grow and connect, and so the fatigue life approaches infinity. Bicycles are designed for optimum rigidity for the intended use. By the time a steel frame is made stiff enough, it is usually strong that the fatigue threshold is seldom reached. This is why steel frames last so long.

Welded construction does reduce the fatigue life at steel joints. This is because TIG welds cause localized metallurgical and structural based stress concentrations that locally magnify stresses about 4 to 6 times. Higher strength alloys reduce this tendency. The air hardening of True Temper OX PLATINUM Series increases the strength about 10-15,000 psi, which helps. An important advantage of OX PLATINUM is that it is less sensitive to metallurgical damage from welding, so the stress risers are reduced.

Henry James Bicycles makes lugs allowing the use of brazed construction which does not melt the steel the way welding must. Lugged construction physically supports the joint, thus reducing stresses. Brazing causes a minimum of metallurgical change. The OX PLATINUM alloy is ideal for silver brazed lugged construction. The alloy is not taken into the air hardening temperature range (which is not necessary, thanks to the support of the lug) and it loses only a small amount of strength at the low silver brazing temperature. Competing air hardening alloys generally tend to lose a lot more strength at silver brazing temperatures than OX PLATINUM. So True Temper has a real winner here: An alloy that works great for both brazed and welded construction.

Aluminum, on the other hand, does not have a "Fatigue Threshold". Every complete revolution of the cranks causes 2 fatigue cycles. Ride 2,000 miles a year at 15 mph and 60 rpm cadence, and every year, you will add about a million fatigue cycles. To stave off the inevitable, designers of aluminum frames reduce the stresses as much as possible in order to slow crack formation and propagation. They achieve low stresses by increasing the tube diameters. The Hype is that "Fat is Better"; that aluminum frames are more stiff; and therefore more efficient. The problem is that you, the rider, gets a rough, harsh ride, which leads to rider fatigue! And the aluminum is still more sensitive than steel is to scratches and dents which cause stress concentrations leading to accelerated fatigue effects in the aluminum. Remember that the end result of fatigue is not bending or buckling, it is catastrophic failure: The tube breaks in two.

Me, I'll stick with steel. It rides better, and lasts longer.