• Bicycle overview

• Weight

• The Bicycle Frame

  • Frame Overview

  • Steel Frames

  • Aluminum Frames

  • Titanium Frames

  • Carbon Fiber Frames

Bicycle overview

A bicycle is a pretty simple and elegant device. The basic bicycle has been around for around 100 years. Most ideas for bicycle parts have withstood the test of time. Nonetheless, there are always new ideas. Some of these are good, some are bad, and some are ugly (like the Campagnoly Delta brakes). The best overall source of advice is a gentleman by the name of Jobst Brandt, a mechanical engineer. His thoughts can be found on rec.bicycles.tech . There are a few places where his advice and mine differ, but not many.

Weight

There are some people who are obsessed with the weight of a bicycle and/or the parts on a bicycle. Unless you are a professional (one who makes money riding a bicycle), or have really minimal bodyfat (like 5% for men, and 12% for women), I think it is silly to worry about a few ounces or even a pound of weight on a bicycle. After all, a full water bottle weighs over a pound. You can likely stand to lose a pound. Many bicycle designs to shave weight do so at the expense of safety, reliablity, or cost. I have seen really silly attemtps to save weight, including a frame with a series of 1 inch diameter holes drilled in it. That being said, there is no reason to have a frame like the original Schwinn Varsity which likely weighed 10 pounds.

Frame Overview

The frame is the heart of the bicycle. There are many designs, but the most common (and overall best) is the so called 'diamond' frame. It is rigid and light. For offroad bicycles with a rear suspension, there are many modifications to this basic design. These modifications allow suspension travel which is good, at the expense of weight, rigidity, or durability.

There are many different frame materials. The most common ones are steel, aluminum, titanium, and carbon fiber. All of these materials can make a good bike frame, as well as a bad bike frame. The quality depends on the material, the design, the quality of the construction. Most frames are made out of tubes. These tubes need to be joined to each other at odd angles in order to form a frame. This requires reasonably high precision and good techniques. One interesting feature about most things constructed of tubes is the stress is highest near the joints. One clever thing to do is to make the tube thicker near the joints and thinner near the middle. This is known as 'butting'. Quality tubes are 'double butted' which means they are thin in the middle, and thicker at both ends.

Steel Frames

Steel is the most proven material, and can make a light, durable, reasonably inexpensive frame. There are two common families of steel used to make frames. One is chromium-molybdenum (Columbus) and one is manganese-molybdenum (Reynolds 531). These steel tubes generally come from the factory already heat treated. Therefore, when joining them it is a good idea to minimize the temperature. Welding is generally a bad idea, as it involves melting and fusing of the steel, destroying the heat treatment. Brazing is a lower temperature method of joining tubes. Silver soldering is brazing using a silver brazing material, and is done at a lower temperature than conventional brazing. Although it is possible to build the frame, and then heat treat the finished assembly, I have never heard of it being done for steel frames (it is common for aluminum frames).

Aluminum Frames

Aluminum is available in a wide variety of alloys for making bike frames. Aluminum is lower in density than steel. Early Aluminum frames such as the Alan and Viscount, has similar diameter tubes as steel frames. They weren't much lighter, and they were quite flexible. (This is because the stiffness of a tube is proportional to its radius to the 4th power. That means that a 2 inch diameter tube is 16 times stiffer than a 1 inch diameter tube.) Since aluminum is less stiff than steel, an aluminum tube with the diameter of a steel tube will be less stiff. (The modulus of elasticity for steel is 190-210, aluminum is 69, titanium is 105-120, carbon fiber reinforced plastic 125-150.) So the obvious solution, is to make the aluminum tubes bigger in diameter. This will produce a frame that is as stiff or stiffer than a steel frame, and almost always lighter. The downside is many parts used to be made to fit the standard steel frame. This isn't an issue any more, due to the popularity of 'oversized' tube aluminum frames. Cannondale uses 6061 aluminum, and heat treats the entire frame after welding. This can produce slightly warped or bent tubes. However, all the mounting points are perfectly aligned, so it is a minor cosmetic issue. Well known brands are Klein (who popularized oversized aluminum frames) and Cannondale (who made aluminum frames popular.)

Titanium Frames

Titanium frames are available in two common alloys 3AL-2.5V and 6AL-4V. 6AL-4V is stronger, though most tubes are formed from flat sheets and have seams. This is heavy and weakens the tube. It is also more expensive than 3AL-2.5V. Many titanium frames don't have butted tubes. The first titanium frame was the Teledyne Titan. I remember seeing a road and track bike in 1974. The problem with the frame was the welding was not done in an inert atmosphere, and titanium when hot is quite reactive. The welds were contanimated, and failed. In order to weld titanium, you have to keep oxygen away from it when it is hot. The simple way to do this is to use vaccuum or an inert atmosphere. It isn't cheap. Other methods have been tried with variable success. Commercially, people use MIG welding and lots of inert gas. It may work. Some well known frame makers are Litespeed (expensive) and Habanero (less expensive).

Carbon Fiber Frames

Carbon fiber frames have been available for a while. They are really carbon fiber in an epoxy matric. There are basically two ways to make a carbon fiber frame. The first is to make carbon fiber tubes and then join them. The second is to make the whole frame (or large parts of the frame) as one piece. Using tubes is more flexible, but there is the problem of joining the tubes. Making big pieces is simpler, but harder to make different frame dimensions. Carbon can be lighter than steel, aluminum or titanium. It will likely be more expensive. The material is easier to damage than metals. If you bike is likely to get banged around, carbon isn't for you. Well known brands are Trek and Klein (who popularized oversized aluminum frames.)