Choosing the Materials
Written By Rickey M. Horwitz
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In the quest of building the ultimate HPV frame, the frame material becomes an important issue. For building the Thunderbolt, I have narrowed the materials down to three metals; Carbon steel, CroMo and Aluminum. Each of these materials have their strong and weak points as I shall attempt to point out in this chapter. Before we proceed, I want to warn you that I did not double check my technical sources. Consequently, some numbers or data may be incorrect, but should be close enough for the home-builder enthusiast.
Aluminum is quickly gaining ground as the material of choice for bicycles such as Trek, Cannondale, Mongoose, Klien and other top brands. New technologies have emerged recently that have lowered the cost of aluminum fabrication making it a strong contender to CroMo. The proverbial 6061-T6 grade aluminum is now the industry standard for aluminum frame composition. This is in part because this type of aluminum can be easily welded and is relatively strong.
Home builders may find aluminum fabrication challenging as it requires a TIG or MIG process and is more difficult to weld than CroMo or carbon steel using the same process. Additionally, 6061 T6 losses half its total strength when welded. Consequently, a post heat treatment process is required to regain it's full T6 strength.
The 6061 series of aluminum is alloyed with magnesium & silicon. However, these are only the majority of alloying elements as other elements are included such as copper, zinc, maganese and titanium. A total of 4% of alloying elements comprise the 6061 specification. The 6061 series is divided into grades or designations that refer to the post temper process. The most common grades are T0, T4, and T6. The grades, processes and strengths are depicted below:
|T0||fully annealed aluminum||18,000||8,000|
|T4||Solution heat treated and naturally aged||35,000||21,000|
|T6||Solution heat treated and artificially aged||45,000||40,000|
Aluminum is extremely easy to work and machine making frame fabrication painless. Depending on the diameter and wall thickness, aluminum tubing can be easily formed using a standard conduit bender. Since aluminum is relatively soft, it can be filed, drilled and sanded with ease.
Just for conversation, I'll also mention the 7075 series aluminum. This type of aluminum is heavily alloyed with zinc making it incredibly tough and strong. However, this aluminum is very difficult to weld making it's use limited to machined components or glue bonded frames.
Chromium Mollybenduim (CroMo)
I do not have extensive knowledge or a lot of experience frame building with CroMo. I do understand that it is an extremely hard and strong material making it an ideal choice for frame building. The strength to weight ratio of 4130 (most common) CroMo exceeds that of 6061 T6 aluminum. This true statement makes you wonder why most aluminum frames are lighter than their CroMo counterpart. The best attribute of CroMo is its inexpensive cost relative to other performance materials.
Because of the alloys used for 4130 CroMo, the material must be welded using a TIG or MIG process, although it can also be brazed. The major alloys used for making CroMo are Chrome and Mollybenduim. Surprisingly, only 2.3% of alloy material is added to give this material its super strength.
The designation '4130' refers to the alloy and carbon content. The '41' represents the alloy type and quantity and the '30' represents the carbon content as a 10th of a percent.
As with Aluminum, 4130 CroMo has different Temper conditions. However, these temper conditions are not readily identified making the 4130 specification almost meaningless. For example, I may have a bicycle frame advertised as made from 100% 4130 CroMo. The yield and tensile strengths can vary drastically depending if the material was 'normalized' or 'water quenched and tempered'. Although many people think that the difference are conjectural, there is a large variance as the example below depicts:
|Temper||Tensile strength(PSI)||Yield Strength (PSI)|
The temper condition is a post process, meaning that a post-weld, heat treatment is applied. If a CroMo frame is not heat treated after welding, the joints subjected to heat during the welding process are annealed or normalized. Remember that a chain is only as strong as the weakest link. Therefore, if a CroMo frame has not undergone a post heat treating process, the frame is inherently weak. This shortcoming is prevalent in the recumbent industry as most frame builders (who know nothing of metalurgy) ignore this important issue.
The most popular material for bicycle frames is carbon steel as it is inexpensive and can be welded or bonded by a number of processes making it ideal for mass production and for the home frame builder. Several grades of carbon steel exist. The grades I am familiar with are 1010, 1018, 1020, 1028. The last two digit represent the carbon content of the steel as a 10th of a percent. Therefore, 1028 has .28% carbon which is slightly less than 4130 CroMo. The more carbon, the harder and stronger the steel. However, too much carbon causes the steel to become brittle. Therefore, .5% carbon is about the maximum you shall see for frame building.
For frame building, I would prefer to use 1028 if available. It offers a tensile strength of 87,000 PSI and a yield strength of 72,000 PSI. However, plain old 2 inch muffler pipe might work just as well providing that the rider is under 180 lbs.
CroMo Vs. Aluminum
In the bicycling community a lot of debate is being stirred up between the use of Aluminum and 4130 CroMo. Each material does have its good and bad points. However, I wish to dispel some unfounded myths between the two.
CroMo is stronger then Aluminum.
Yes and No. The strength to weight ratio of CroMo is slightly more than 6061-T6. The strength difference is about even when compared to 7005-T6 aluminum with aluminum taking the advantage. However, the 7075-T6 alloy aluminum is far superior to CroMo in strength to weight.
Aluminum frames tend to crack more easily than Steel.
Absolutely false. First of all, aluminum frames are generally manufactured as a medium to high end market. Therefore, the fabrication is of higher quality. Up until recently, the same was true with CroMo, but since new manufacturing technologies have evolved, a CroMo frame is now found on many low quality bicycles. Obviously, a low quality frame with marginal workmanship has more of a chance of failure than one of higher quality. Secondly, aluminum frames are built lighter than CroMo making them more susceptible to breakage. This gives the impression that aluminum breaks easily. Lastly, many CroMo frame builders do not post heat treat their frames making them weak. In the last 20 years, many aluminum frame designers (such as myself) have learned to used oversized tubing and web gussets to reduce tube flexing. The major cause of failure with aluminum is fatigue caused by overflexing. Unlike CroMo, aluminum is not resilient and shall eventually fail if allowed to overflex. Using a rigid frame design, aluminum becomes a very reliable material.
Aluminum is used extensively in aircraft. If the physical properties were inferior to steel, we would expect to see aircraft made of materials other than aluminum.
Aluminum frames do not last as long as CroMo frames.
In theory, this has some truth, but has not been proven in the bike industry. The use of aluminum in mass production bicycles is only recent. Therefore, not enough data exist to compare aluminum to CroMo. It is theorized that a 6061-T6 aluminum frame shall eventually fail as the aluminum ages over time and becomes brittle. This aging can be reversed by re-heat treating the frame (a costly undertaking). It should also be noted that CroMo frames will eventually rust leading to premature failure. However, this condition can be prevented by performing periodic maintenance.
Earlier aluminum frames used a bonding process that glued the tube into a lug. These frames were infamous for their tendencies to fatigue and crack. Additionally, they were fabricated using small diameter tubing causing the frame to flex easily. Today, a small number of aluminum frames are still bonded. However, the processes have been refined and the tubing diameters have been increased to provide a rigid more reliable frame. Additionally, these frames use the stronger 7075-T6 aluminum. Some of the lightest frames available (with the exception of Composites) are made from bonded 7075-T6 aluminum. Although the 7075-T6 aluminum has a much higher strength to weight ratio, it does have a higher tendency of cracking than 6061 T6.
These facts only prove that the reliability of a bicycle frame is more dependent on the quality of the design and manufacturing process rather than the type of material it is made of.
Yes it does. The 6061 series of aluminum has good corrosion resistance. However, if left partially submerged in a salt water it will eventually corrode. Once aluminum is placed in an oxygen environment it builds a protective layer of oxidation. Although this layer plays havoc during any welding process, it does protect the aluminum from many outside elements. As for bicycle frames, this corrosion is benign as it possesses no threat to the structural integrity. The same cannot be said about steel. It can and will rust. If left unchecked, a rusting steel frame shall fail.
Aluminum frames are lighter than CroMo frames
This is not always true. As pointed out, CroMo has a strength to weight ratio advantage over 6061-T6 aluminum. However, very few bicycles are built out of 4130 CroMo. Most cases mild or low carbon steel is used for lugs, crowns, dropouts, and various tubes on the frame. Secondly, most 4130 CroMo frames are not heat treated. In order to retain ample strength at the welded seam, the tube wall thickness is increased making the overall frame heavier. The end results is a steel frame that is heavier than aluminum. Only the finest steel frames rival aluminum frames in lightness.
Ultimately, it is the frame designer who decides how much the frame weighs.
The facts about Aluminum and CroMo
|Aluminum cost more than CroMo|
|CroMo is relatively inexpensive (cost a fraction more than carbon steel)|
|Aluminum welding requires a high degree of skill than CroMo, aluminum cannot be brazed.|
|CroMo can be brazed, TIG or MIG welded, but cannot be welded with an oxyacetylene torch.|
|Aluminum frames do not rust, CroMo and Carbon steel do rust.|
|CroMo is hard to machine|
|Aluminum tubing comes in a much wider range of sizes and thickness than CroMo|
|Aluminum is a reliable metal, after all, most aircraft use aluminum, go figure.|
From the arguments covered, you must obviously think I am biased towards aluminum. This is because I am one of the few HPV frame builders that has mastered the technique of aluminum welding and manufacturing. Most HPV or recumbent manufacturers rely on working with CroMo as it takes very little skill to TIG or MIG weld this material. In my opinion, an aluminum frame properly welded is a work of art that has no equal. I am quick to add that aluminum is not always the best material to use in all applications.