<P> Roadster bicycles traditionally have a fairly slack seat - tube and head - tube angle of about 66 or 67 degrees, which produces a very comfortable and upright "sit - up - and - beg" riding position . Other characteristics include a long wheelbase, upwards of 40 inches (often between 43 and 47 inches, or 57 inches for a longbike), and a long fork rake, often of about 3 inches (76mm compared to 40mm for most road bicycles). This style of frame has had a resurgence in popularity in recent years due to its greater comfort compared to Mountain bicycles or Road bicycles . A variation on this type of bicycle is the "sports roadster" (also known as the "light roadster"), which typically has a lighter frame, and a slightly steeper seat - tube and head - tube angle of about 70 to 72 degrees . </P> <P> Historically, the most common material for the tubes of a bicycle frame has been steel . Steel frames can be made of varying grades of steel, from very inexpensive carbon steel to more costly and higher quality chromium molybdenum steel alloys . Frames can also be made from aluminum alloys, titanium, carbon fiber, and even bamboo and cardboard . Occasionally, diamond (shaped) frames have been formed from sections other than tubes . These include I - beams and monocoque . Materials that have been used in these frames include wood (solid or laminate), magnesium (cast I - beams), and thermoplastic . Several properties of a material help decide whether it is appropriate in the construction of a bicycle frame: </P> <Ul> <Li> Density (or specific gravity) is a measure of how light or heavy the material per unit volume . </Li> <Li> Stiffness (or elastic modulus) can in theory affect the ride comfort and power transmission efficiency . In practice, because even a very flexible frame is much more stiff than the tires and saddle, ride comfort is ultimately more a factor of saddle choice, frame geometry, tire choice, and bicycle fit . Lateral stiffness is far more difficult to achieve because of the narrow profile of a frame, and too much flexibility can affect power transmission, primarily through tire scrub on the road due to rear triangle distortion, brakes rubbing on the rims and the chain rubbing on gear mechanisms . In extreme cases gears can change themselves when the rider applies high torque out of the saddle . </Li> <Li> Yield strength determines how much force is needed to permanently deform the material (for crashworthiness). </Li> <Li> Elongation determines how much deformity the material allows before cracking (for crash - worthiness). </Li> <Li> Fatigue limit and Endurance limit determines the durability of the frame when subjected to cyclical stress from pedaling or ride bumps . </Li> </Ul> <Li> Density (or specific gravity) is a measure of how light or heavy the material per unit volume . </Li>

When was carbon fiber first used in bikes