by Peter Whitford
As a bicycle frame builder and metallurgist, I'm continually asked what is the best metal from which to construct a bicycle frame. For the most part it is an irrelevant question and reflects the efficacy of the marketing hype spewed out by some bicycle manufactures as well as a great deal of hearsay and uninformed opinion. My usual short, glib answer to this question is; it depends.
My first thoughts in writing this article was to delve into a technical discussion about material science covering specific gravities, tensile strength, alloy jargon, torsional stresses and the like. While that type of discussion might be informative, albeit somewhat dry for the reader, it doesn't yield a practical insight when contemplating a folding bike purchase. So I'll just cut to the chase.
Most everyone is aware that aluminum is lighter than steel, roughly 1/3 lighter. This leads many people to conclude that an aluminum bike should be considerably lighter than a bike make of steel. That would be true if the frame tubing used for an aluminum bike and a steel bike was of the exact same dimensions, but that is never the case. Why? Because aluminum just doesn't have the stiffness of steel and therefore one can't make a bike with enough structural integrity that uses the same dimensional tubing typically employed to make a steel bike. To compensate for this lack of stiffness, aluminum bikes almost always tend to use larger diameter and thicker walled tubing than steel bikes. By the time you dimension up an aluminum bike to gain the required stiffness and structural integrity, you really start to approach the weight of a well made steel bike. In fact, a really well designed steel bike can actually weigh less than a marginally designed aluminum bike. Now these statements apply to bikes in general. In the case of folding bikes, many of them are indeed made of aluminum because they are typically smaller than a standard size bike and therefore can't use off the shelf, high quality, lightweight double-butted steel tubing. To keep costs down and to produce an acceptably lightweight bike, manufacturers use aluminum in their construction rather than steel. The exception to this statement are some cheaply constructed folding bikes that use low cost straight gauge steel but these are almost always unacceptably heavy.
As a quick aside, "double-butted" means that a frame tube has been constructed with variable wall dimensions whereby the ends, where the welds are placed, are thicker than the middle. "Straight gauge" tubes have the same thickness for the length of the tube. Producing double-butted tubes adds considerably to their cost over straight gauge tubes.
If really pressed for an answer on the best bicycle frame material I would have to concede that it would be titanium. The strength, corrosion resistance, weight and fatigue life of titanium is quite ideal. However, as one can readily see from the price of titanium bikes, they are typically quite expensive. It's not so much that titanium is an overly expensive and rare metal itself, it's just that titanium is a more difficult metal to work with and the bulk of the added expense results from the cost of the equipment used to cut, prepare, and weld a bike frame. In the case of folding bikes, there are very few available made from titanium. This may change in the future but you can be assured that they will be considerably more expensive than an aluminum or steel folding bike. You'll have to decide if the relatively slight saving in bike weight is really worth the price.
What about carbon fiber you say? Well there is no disputing that carbon fiber is widely used on many racing bikes and some mountain bikes where weight is a prime issue. There are very few folding bikes made from carbon fiber. As in the case of titanium, you'll have to decide if the weight savings plus perhaps the "cool" factor is worth the price. However, I would advise against a carbon fiber folding bike since they typically sustain more banging around from transporting. Carbon fiber can easily be damaged by impact blows.
Ride characteristics...A lot of the discussion about the merits of one bike frame material over another many times is focused on the bike's ride characteristics. Terms like springy, whippy, harsh, and soft are bandied about, usually capriciously without a basis in fact. Some people don't like aluminum bikes because they claim they have an unacceptable ride quality. Well, that may be the case for that particular bike to which they riding but such a statement can't be extended to another aluminum bike or generalized to all aluminum bikes or indeed attributed to the fact that their bike is made from aluminum.
A few years ago, a particularly finicky customer of mine would make all kinds of unsubstantiated claims about the differing ride qualities he attributed to the frame material. So I made a wager with him. I bet him that he couldn't tell the difference between a steel, aluminum, or titanium made bike. I selected three bikes of the same design and put masking tape on the frame to disguise them and fitted each with the exact same components (wheels, tires, groupset, handlebars etc.). I had him ride each bike twice. While he was convinced he picked out each bike correctly, he didn't even pick one correctly. After his rides, I didn't tell him the results just yet though. I told him I was going to make a minor change on each bike and have him test again. This time I put different tires on each bike. A firm one, a soft one and a medium one. I had him test the bikes again. This time his comments on ride quality were exactly in line with the characteristics of the tires. After I told him the results he was quite chagrined. The point is that the tire and wheel set up so overwhelms any supposed frame material induced ride quality factors that may or may not be present.
The bottom line is that the type of material used to construct a bike is a trivial issue when compared to the design of the bike and the quality of the components. To be sure, when selecting a folding bike, since you will be transporting it in some fashion fairly often, its weight should be taken into consideration. Therefore, focus on the overall weight of the bike and not so much on the material that is used in its construction.
So to conclude, when I say "it depends" when asked the question about frame material; it depends on how light a bike you want and how much money you have to spend and not so much on the material itself.
Ride safe! - Peter