วันจันทร์ที่ 9 มีนาคม พ.ศ. 2552

Bike Frame Materials: Spending Your Gold on Common Metals

Frame construction begins with tube selection. The most common bicycle frame tubes are steel, aluminum and titanium. Carbon fiber reinforced frames can also be considered for lightweight, rigid and expensive frames although I've never shopped for one so I won't comment. Here's a link that might help:

http://sheldonbrown.com/gloss_ca-m.html#carbon

The examples I provide below are for mountain bikes. The principles apply to road bike frames but my experience really stems from over ten years of mountain biking and being on the market for four bikes.

STEEL
Steel used to be the standard in bike frame manufacture. It is cheap, plentiful, easy to weld, roll and cast. Steel is bendable, thus can be repaired easier than more brittle metals. Steel can be manipulated to get a desired hardness by heat treatment, tempering and annealing. Steel has a unique "springy" feel when used in bicycle frames. It absorbs vibrations from shock better than other metals. Steel's ability to absorb vibration energy will also sap a small amount of energy from each pedal stroke. Steel is not as responsive as some of the stiffer metals. Steel will react slowly with the oxygen in the air and corrode, or rust if not coated or painted. Some people coat their frames internally as well as externally to add life to their bike frames. Steel is an alloy of iron and carbon and other metals such as chromium molybdenum (chromo, chromoly, chro-moly) and vanadium.

To maximize steel's characteristics to get the strength and lightness desired for a bicycle, a builder has to buy expensive steel tubes and spend a lot of time manipulating them. Butting, bending and gusseting need to be performed to put material where it's needed most and cut back on material in other areas to maintain lightness. The craftsmanship of quality, steel bike building will always remain an art and will always be expensive. A complete bike with hand crafted steel frame, weighing around 24lbs usually starts around $1500. You may be able to find a new frame (no parts) for under $500.

Bike manufacturers are not buying steel from steel manufacturers, like Columbus, Reynolds, Tange and True Temper in bulk anymore. It is difficult to find a complete bike with a quality steel frame in the $500 range now. Production line steel bikes tend to be very heavy, in the 28lbs, and over range, for a complete bike. Weight is a subjective matter however. Who cares if your frame is heavy if you're bombing down hills or free riding? You may want a steel frame for durability, shock absorption characteristics and peace of mind.

ALUMINUM
Although not long ago it was steel, aluminum is the most common tube used in bike frame manufacture. It is plentiful, recyclable, does not rust or corrode like steel, and has a highest strength to weight ratio compared to titanium and steel at the same price. Aluminum weighs less than one-third as much as steel and is just as strong. Aluminum tubes can be thinner than steel of the same strength, thus lighter, and maintain the same amount of rigidity. To maintain the structural integrity and lateral stiffness of the tube, aluminum tubes are generally oversized. Aluminum has very little of the springiness of steel yet yields a responsive, "pedal to the metal" kind of ride. Newer, expensive aluminum alloys, such as Easton's Scandium tubes, are supposed to have the best of both worlds, the lightness and corrosion resistance of aluminum and the liveliness of steel.

Scandium is definitely a step forward in the world of aluminum construction for bikes. Scandium frames however can cost as much as titanium frames and are not practical for the average rider. For off-road bikes, the use of light, rigid aluminum may be just perfect for use with dual suspension bikes. Although the additional parts involved with suspension add a lot of extra weight, suspension adds shock absorption and comfort missing in aluminum frames.

More highly engineered aluminum frames are butted (similarly to steel) to reduce weight and induce a more springy ride quality. A major manufacturer of performance specification aluminum tubes is Easton, well known in baseball and softball for their bats. Easton and other manipulated tubes and frames are generally more expensive and can be found on new bikes for over $1000. A bike in this range should weigh about 26lb or less. Very high end aluminum frames can weigh as little as 3lbs and yield a complete bike under 22lbs, including racing parts and a lightweight suspension fork. A lot of manufacturers choose to buy generic aluminum tubes because it is usually cheaper and they can stick their own label on it.

In the $300-500 range you can find aluminum bikes with straight unadulterated tubes. You will get a lighter bike in this price range, in comparison to steel, although heavy, less expensive parts and the overbuilt aluminum frame will keep you from getting the most out of aluminum characteristics for your money. Any buyer should feel secure that any bike in this price range is a much better value today, than was available 10 or even, 5 years ago. A mountain bike in this price range will be 28lbs or under.

Generally speaking, aluminum frames cannot be repaired. Aluminum tends to shear upon breakage and can not be re-welded or patched due to the initial heat-treatment of the frame. Oversized, stiff, thin wall tubing will dent easier than smaller diameter tubes. It is good to know that most frames are warranted to the original owner, for up to 5 years or for the life of the frame under "normal" use.

TITANIUM
Titanium is the least available metal and is the generally the most expensive. It has a very high strength to weight ratio and has the natural springiness of steel. Titanium, in its usable alloy form, does not corrode. Titanium, also used in the aerospace industry, could easily be the perfect tube for bike building. Unfortunately, titanium tubes are expensive and they are expensive to work with. Titanium need to be welded in a special gas environment (argon, I believe it is) so that when heated, it does not react with gases in the air. Prohibitive costs of titanium have seemed to stifle the use and development of titanium in the cycling world.

Sandvick is one supplier of performance specific titanium tubes. Strength, rigidity and lightness is engineered into a variety of tubes. Before Sandvick, and even now, manufacturers have been buying generic tubes that meet the bike builders' specifications.

Although impossibly difficult to break, titanium is also difficult to repair. As with aluminum frames, there are some parts on the frame, such as the derailleur hanger that are made to be replaceable via small bolts. Titanium frames can last several lifetimes if the minimum maintenance precautions are observed. Titanium is tough and highly scratch resistant. Frames are usually left raw, and unpainted, as they are not very porous and do not hold paint well.

Complete titanium frame bikes with suspension fork usually weigh-in similarly to aluminum, around 22lbs. Titanium, of course, will provide a supple, lively ride, unlike aluminum. Titanium frames alone cost around $1000. A complete bike starts at $2000.

Some builders are using rear suspension shocks with no mechanical parts on titanium frames. The rear triangle is designed to flex to provide up to 2" of travel. Titanium is springy and does not fatigue like steel. This results in a very comfortable frame with little added weight from suspension parts.

CONCLUSION
Most frames, no matter what material, especially production line frames, are overbuilt and heavier than they need to be. This is good because they will tend to last under heavy use but it is bad because the full advantage of using performance tubing diminishes as layers of durability pile-on. It is cheaper for the manufacturer to overbuild the bike frames the first time around rather than provide replacements for frail frames in the long run.

Frame material is definitely an important factor in selecting a mountain bike. Unfortunately, the bicycle industry has already decided in favor of aluminum by providing the largest selection of aluminum frames. It is very difficult to find an intermediate level steel frame on which one can grow and develop riding skills. Fortunately, aluminum quality is now fairly diverse and getting more comfortable. It is also good that there are new ways of controlling shock and vibration via suspension parts. If you can afford titanium, it is a good investment, considering that they last two lifetimes. Oddly enough, I have noticed that titanium resale value is only marginally better than some steel or most aluminum. Used bikes (check Ebay) go for about 50% off their sticker price after 1 year and depreciate in value about 15% every year after that. Titanium may only depreciate 40% after the first year and only 10% every year after that.

You will want to start with a good frame and replace parts with upgrades as you go. Most manufacturers will outfit one or two models in their line with a premium frame with cheaper parts. That's the one you should get as a new rider. For example, Gary Fisher makes one frame for their high end "professional" series bikes. There are however, 4 models or so, that use this same frame. If you are on a budget, the one to get is the least expensive of the 4 because you will be getting the same frame that is used on the top model, costing almost twice as much. The parts are replaceable. The frame is the heart and soul of the bike. Of course, you want to figure out your budget and then you want to squeeze as much value as you can in there. You'll be better off riding now and inevitably replacing parts as you wear them out rather than sitting on your money. It's probably best to stop worrying about frame material and focus on frame fit and feel. Ride your friend's bike, ride my bike, ride the bikes on the showroom floor and get out there and ride.