Engineering Principles of Butting

Butting is a process that varies the wall thickness of a tube to provide local reinforcement. It was first applied to steel tubing in the 1890s, and was patented by Alfred Reynolds and J.T. Hewitt in 1897.

When properly applied, butting can significantly enhance the fatigue endurance, and thus the service life, of a frame tube. Fatigue endurance is improved because the thicker tube wall in the butted area is stronger.

Butting can reduce weight, too, since the unbutted areas of the tube are lighter than the butted areas. And it can improve ride quality if the thinner center sections of the tube are allowed to flex somewhat.

Butting always makes a tube stiffer locally, at the butt, but only locally. Contrary to common opinion, any local stiffness increase gained through butting does not have much effect on overall tube stiffness. That is, frames with butted tubing are not automatically stiffer than frames with straight-gauge tubing.

Tubing can be butted at one end (single butted), at both ends (double butted), or can have any number of wall thicknesses to solve specific problems (leading to triple butting, quadruple butting etc.). Generally, true butted tubing is considered to be seamless and cold-worked to shape. Other externally or internally applied reinforcement methods, such as gussets or sleeves, are sometimes referred to as butts, but this is a misnomer. In this discussion, butting will only refer to tubing made with seamless starter stock, and without gussets, sleeves, or other secondary reinforcements.

Double Butting

The mechanical properties in the welded or brazed joints of any steel or titanium frame are always lower than in the unheated areas. This loss in strength is an important consideration because the joints are usually the most highly stressed areas on the frame, and most frame failures occur at the joints. Fortunately, titanium retains a greater percentage of its raw yield strength after welding than steel, so the drop in strength is not severe.

Nevertheless, it is desirable to minimize stress levels at the joints whenever possible. Butting the tube-making it thicker at the ends and thinner in the middle-is an efficient way to strengthen the heat-affected zone (HAZ) at the joints without adding appreciable weight. Put another way, applying proper butting techniques to a thinwall non-tapered tube allows a significant weight reduction without sacrificing fatigue life.

This is not to say that butted tubing is always necessary. Since, under a given load, a stiffer tube has lower stress and, therefore, improved local fatigue life, there are some areas of the frame in which a tube can deliver the desired ride characteristics and also have more than enough bending stiffness at the joints. That is, the tube's geometry (its inside and outside diameter) can be adequate to keep joint stresses reasonable.

For example, the performance requirements for road bikes and mountain bikes are very different. A mountain frame built from a butted road tube set could have adequate fatigue life, but it would not be stiff enough in bending or torsion. Adding stiffness to this frame in any optimal way would also increase its ability to resist bending stresses, which in turn would help improve its fatigue life. In this case, the need for butted tubing would be greatly reduced.

However, when a tube is designed for a given application, there is usually more than one goal, and the goals often conflict: weight vs. stiffness, weight vs. strength, stiffness vs. resiliency, and so on. In these cases, butted tubing can be an excellent solution.

Internal and External Butting

Tubing can be butted internally, which is the traditional method patented by Reynolds and Hewitt, or externally, which is a more recent approach. Internal butting is useful for lugged construction where the reinforcing lug slips over the outside of the tube. Internal butting is also cosmetically appealing, since wall thickness variations are not apparent to the eye. And the forming mandrels for internal butting are less expensive than external rolling dies.

However, external butting offers certain advantages, and is a superior method for tube reinforcement. If two tubes of identical bending stiffness and which offer equal fatigue endurance at the joint are butted, one internally and one externally, the externally butted tube will be lighter.

If these same tubes are modified slightly to offer identical weights, the externally butted tube will be stronger, and will also exhibit lower stress at the joint.

To see why this is so, it is important to consider all of the variables that affect fatigue strength, stiffness, and weight. The most efficient way to improve the specific fatigue strength of a tubular joint is to make it stronger. A stronger tube handles loading better, and is generally more resistant to fatigue failure.

Strength can be gained by increasing the thickness of the tube, and indeed, an internal butt performs just that function. This is not an ideal strategy, however, because making a tube thicker adds strength and stiffness rather grudgingly. When wall thickness is doubled, for example, the stress level in the tube per given load is cut roughly in half. The most efficient way to improve strength without a significant weight penalty is to increase the tube+s diameter, which improves the picture rapidly at a ratio of about 1.6:1, strength to weight.

If all things were equal, then, it would seem that the best way to butt a tube would be to simply flare the tube ends. Though this might be the case in a lower grade tube that has not been optimally designed, any tube that has been properly engineered for minimum weight and maximum fatigue endurance will already be at its maximum diameter limit. At this point, if the diameter is increased by flaring without a corresponding increase in wall thickness, the tube will surpass its buckling limit, and will collapse like an aluminum can when heavily loaded.

Thus, the optimum strategy is to simultaneously increase the tube wall thickness and the tube diameter in an ideal proportion-which is to say, to externally butt the tube. The external butt provides maximum strength with minimum weight. It cannot be surpassed.

External butting also offers the greatest flexibility in choosing optimum wall thickness differentials between the butted and unbutted sections. To see why this is so, it is important to understand that internally butted tubes are manufactured not by adding material to the ends of the tube, but by displacing material from the center of the tube to make the tube thinner in that area. When this process is complete, the internal mandrel that is used to thin the center sections must be withdrawn past the thicker ends. Typically, internally butted tubes are limited to a 40 percent thickness differential to allow the mandrel to be pulled out.

Externally butted tubes suffer from no such differential limitations. Indeed, only external butting allows every possible permutation of tube diameters and wall thicknesses, and an optimum strength-to-weight ratio.

Butting Considerations In Titanium

With the possible exception of mercury, no metal likes to be pushed around too much, but titanium is especially sensitive to manipulation. In fact, its properties are radically altered by cold working. This is both good and bad. It's good in the sense that strength increases can be achieved through simple cold working. But it's bad in that any cold work after final anneal and stress relief will change the tube properties, often for the worse.

At the root of this behavior is titanium's crystallographic texture (CT), which is determined when the tubing is made. The measure of crystallographic texture is called "contractile strain ratio" (CSR), which compares the tubing's diametral strain to its radial strain.

The tubing's CSR, and thus its CT, is optimized by controlling the rate of size reduction. During the manufacturing process, a reducing die is rolled over the outside of the tube while the inside of the tube is supported by a mandrel. The titanium is squeezed between the die and mandrel like cookie dough under a rolling pin. As deformation occurs, the titanium molecules are forced to rotate and realign.

Only so much of this manipulation (called rocking, because the die rocks back and forth along the tube), can take place at one time. For Merlin MTS325 tubing, the process starts with titanium tube hollows roughly 2.375 inches in diameter, with a wall roughly 0.8 inches thick-a long way from the thinwall small-diameter tubing used in bicycle frames. Getting to the final dimension takes many reducing, or pilgering, steps, each step followed by a trip through an annealing oven to eliminate excessive hardness and loss of ductility due to the cold working of the tube. The rate of pilgering is the primary way in which CSR is controlled.

Pilgering control of CSR can be accomplished through either wall ironing or diameter sinking. Wall ironing takes place when the reduction in wall thickness is proportionally greater than the reduction in diameter. Diameter sinking results when the reduction in diameter is proportionally greater than the reduction in wall. Ironing pushes CSR up. Sinking forces CSR down.

Cold working is, therefore, a good way to fine-tune the tubing+s bending characteristics and fatigue strength. But too much cold working at the wrong rates can destroy those properties, weakening and embrittling the tubing significantly-even radically. The useful window for CSR in bicycle tubing is narrow, and tubing that falls outside a CSR of 1.6 to 1.9 suffers from poor fatigue endurance.

The only way to obtain a consistent CSR of 1.6 to 1.9 throughout the tube is to create a constant wall thickness and a constant diameter. It is not possible to change the dimensions of the tube through material manipulation without affecting molecular structure, and thus CSR. Both wall ironing and diameter sinking destroy the ideal CSR of the starter stock and thereby shorten the service life of the tube. The effect can be dramatic, with the drop in fatigue endurance alone exceeding 10 percent.

Internally butted tubes are created at the mill through wall thinning, or ironing. Tapered tubes are created by diameter sinking. Even though the tube may have had ideal properties before pilgering, the ironed or sunken sections of the internally butted or tapered product will exhibit significantly poorer properties.

Merlin MTS325 butted tubing is created through proprietary processes that do not alter the ideal CSR range. Because CSR remains constant, there is no loss of fatigue strength or ductility.

What of the claim that CSR should be altered for different parts of the frame? Under this argument, chainstays that need to be bent would use tubing with a different CSR, or radial texture, than, say, the down tube, which does not require bending. Although this argument may sound plausible, further examination reveals a fundamental problem: the CSR that offers the highest fatigue strength also offers excellent ductility. High ductility supplies the best bending characteristics. Thus, while enhanced bending is sometimes touted in higher CSR tubes, ductility actually falls as CSR rises.

From where, then, did an argument for using a range of CSR values arise? Most bicycle frames are built with tubing obtained from more than one mill, and the range of CSRs is an inevitable byproduct of this multiple sourcing. To make the best of a bad situation, some manufacturers have touted these varying CSRs as a virtue. In reality, however, there is no advantage to using tubing with any CSR outside the optimum range.

Tubing production speed, and thus final cost, also plays a role. Tubing costs can be reduced through faster pilgering. Unfortunately, though, running the tubing through the mill faster also leads to higher CSR values and greater radial texture. To keep costs down, most "sports grade" tubing is produced in this way, and the high radial texture that results is sometimes proclaimed a benefit. However, slower pilgering and lower CSRs create a stronger, more durable frame.

Comparison of Butted Properties

There are three common types of butted titanium tubing. Two are butted internally and one, Merlin MTS325, is butted externally. To distinguish the internally butted methods, we have designated the configurations type 5I and type 3I.

Type 5I tubing: This internally butted tube is made with high-strength starter stock (125 ksi UTS). The tube is butted by wall ironing. As noted above, wall ironing disturbs the titanium's molecular grain structure; thus, only the thick, unironed ends of the tube retain the starter stock's original properties.

The tubing will also be subject to internal scratching, gouging, or notching, due to the action of the supporting mandrel. Notched surfaces create stress nodes in the tubing, leading to premature failure. Unfortunately for the consumer, once the frame is built there is no nondestructive way to determine whether the tubing has a poor internal finish.

Notches, gouges and scratches are of less concern in the thin center sections of the tube than in the transition zone, or butt taper, between the thin center and the butted tube ends. This area is highly stressed and extremely sensitive to surface degradation. Notching here will lead to almost certain tube failure.

Type 3I tubing: Another internally butted tube, but made with annealed or low-strength starter stock. Butting is also performed by wall ironing. The thinned section of the tubing has slightly better properties than 5I tubing, but the thicker end sections suffer from extremely low strength.

Type 3I tubing is less expensive than 5I tubing, because the low-strength starter stock is easier to manipulate. Aside from price, it offers no real advantages. Like 5I tubing, 3I tubing is subject to notch failure from damage caused by the supporting mandrel.

Merlin MTS325 tubing: Merlin tubing is externally butted without mechanically altering material properties or CSR. No internal notches or stress nodes are created during or after butting, so full fatigue strength and CSR are maintained.