The term road bike is usually used to refer to racing style bikes with dropped handlebars and multiple derailleur gears. It can also include touring and audax bikes (so called 'fast tourers', 'sport touring' or 'credit card' touring bikes) but these are dealt with in another section. The basic design of the road bike frame has not changed in over a century, though the materials and technologies have advanced considerably, particularly in recent years.
A top class racing bike now weighs less than 8kg and indexed gears that change from the brake levers with a click
For many years bicycle frames were made from steel tubes brazed into lugs. The quality (strength to weight ratio) of tubing gradually improved over the years, with the British-made 'Reynolds 531' setting the global standard for over 50 years from the late 1930s. However, in the last 10 years aluminium alloy has become the material of choice for racing bikes. This is due to the improvement in joining technology and the development of new aluminium alloys (5000, 6000 and 7000 series) and other materials like metal matrix alloys and carbon fibre.
The result has been frames that weigh less than 1kg - compared to 1.5kg for the lightest steel frames - and carbon fibre forks that weigh around 300g. A top class racing bike now weighs less than 8kg, has 20 indexed gears that change from the brake levers with a click, and it will usually have a miniature computer sitting on the handlebar. Compare this to 15 years ago when the average steel framed racing bike weighed 9-10kg, had only 12 or 14 gears - with no indexing.
One of the most obvious visual changes is that the properties of aluminium alloys require larger tube cross sections to make the best of the material. Oval and teardrop section tubes are now commonplace and the conventional steel frame looks 'anorexic' by comparison.
1950s bike
There have been numerous attempts to 'improve' the design of the bicycle frame over the years, but a glance at any top professional road race bunch reveals that the bikes use a similar 'diamond' style frame, usually with horizontal top tubes. However, the 'compact' style of frame is becoming popular where the top tube is sloped downwards from the head-tube towards the rear.
The theory is that the shorter seat tube makes the frame torsionally stiffer and the long seat pin, usually of titanium or carbon fibre, acts as a shock absorber. From a manufacturer's point of view this design reduces the number of sizes that have to be produced, as much more adjustment is available on the seat pin.
The advent of the mountain bike in the early 1980s, and the previously mentioned material and technical improvements, has brought a number of design concepts from mountain bikes into many road designs.
Time trial bikes are a specialist subsection of road bikes. Designed for use in unpaced, individual races against the clock, their design variations and possibilities are rather wider than conventional road race bikes. The rider still sits in a very similar position to a road bike albeit slightly further forward with arms tucked in, out of the air stream on the tri-bar rests.
Time trial bikes are a specialist subsection of road bikes
These bikes are only suitable for riding very fast in a fairly straight line at an almost constant power output. Chris Boardman's Lotus bike designed by Mike Burrows and ridden to a gold medal in the 1992 Barcelona Olympics fuelled the trend.
There have been several other successful monocoque (one-piece) frames made from carbon fibre but these are expensive and of limited commercial success. In a time trial a handful of seconds can mean the difference between winning and losing so anything that maximizes the rider's power or provides him with any small advantage is worthwhile - hence the great emphasis on the aerodynamics of the bike and rider.
Carbon fibre composite is an excellent material for such machines as it can be moulded into appropriate shapes but unfortunately this is an expensive process, especially for 'one-offs'. Composite, deep section or disc wheels, a monocoque frame and tri-bars (to emulate a downhill skier's aero-tuck position and first adopted by triathletes before famously being used by Greg LeMond to beat Laurent Fignon by 8 seconds in the 1989 Tour de France) are all design features to enable smooth passage through the air.
The design of any road bike frame is usually constrained by certain dimensions that have become 'standard' - either through convention or regulation. To try to deviate from what is conventional is a path littered with good ideas that just did not manage to defy the standards of the day.
Despite all of the advances of the last 20 years, if you take a modern bike and stand it next to a Rover Safety bicycle of pre 1900 they are remarkably similar in the basic design
Large companies like Shimano and Campagnolo however have forced the issue by introducing new equipment. The obvious example is the increase in the number of gears available to riders. The rear spacing of road frames has gradually increased from 120mm to accommodate 5 sprockets, to 130mm to accommodate 9, or in the case of Campagnolo, 10 sprockets. For years the steering column of the forks has been 1" diameter but 1 1/8" columns, copying mountain bikes, are gradually becoming more common in the threadless 'Aheadset' type, where the handlebar stem clamps onto the steerer and also positions the headset (steering bearing).
Despite all of the advances of the last 20 years, if you take a modern bike and stand it next to a Rover Safety bicycle of pre 1900 they are remarkably similar in the basic design. The improvements are all down to technology.