It's All About the Bike by Robert Penn Read Free Book Online Page A

week.
    When the jig was set up and the tubes clamped in place, he checked everything over, reverting one last time to the piece of paper pinned to the wall with the measurements for my frame. ‘Head 73°; Seat 74°,’ he said to himself, like an incantation.
    Jason was referring to the two angles that are fundamental to the geometry of the frame: the angles of the head tube and the seat tube. The geometry of a frame — that is the angles between the tubes of a frame — is largely determined by the intended application of the bicycle. Criterium, triathlon, time trial, touringand sportive bikes are all variations of the road bike, for different purposes. They may look roughly the same shape, but in fact they each have a different geometry, giving them different ride characteristics. Mountain and commuter bikes have a different geometry again.
    Frame geometry is an important factor in how a bicycle rides, how comfortable it is, how it responds to a rider’s manoeuvres, how it corners, descends and even climbs. Many other factors also affect ride quality — from the frame and fork materials to tyre pressure — but the geometry of a frame sets the parameters. Few cyclists ever think about frame geometry. If you buy a mass-manufactured bike, it’s scarcely a consideration. When I brought up the subject of frame geometry with a friend, he said: ‘Rob, just how long is your beard going to be when you’ve finished this book?’ And he’s a cyclist.

    Along with the immaculate fit and the right tubing material, geometry is an intrinsic part of buying a bespoke bicycle. Get the geometry of the frame wrong and you could end up with a bike that is at best uncomfortable, and at worst, dangerous to ride. Get it right, and the bike will have the handling characteristics you desire.
    Seat tube angle: measured in degrees relative to the horizontal plane (ST∠° in the diagram), they can vary from 65° to 80°. Steeper angles (75°—80°) push the rider’s weight forward on to the handlebars and are less comfortable over long distances, but more aerodynamic; they are common on dedicated time-trial bikes, track bikes and triathlon bikes with aero bars. Slack angles (65°), which place more weight on the saddle, belong on commuter or other bikes for short trips. Conventional road-racing bikes with drop handlebars tend to be between 72° and 75°. The angle is partly determined by ergonomics — that is, the saddle being in the best position for efficient pedalling. The seat tube angle on my bike is 74°.
    Head tube angle: again, measured in degrees relative to the horizontal plane (HT∠° in the diagram), it has a marked effect on steering characteristics and shock absorption and can vary from 71° to 75°. Steeper angles mean a bike handles more quickly — turn your head and the bike turns too (such bikes are often described as ‘twitchy’ or ‘Italian style’ and are favoured by pro racers for criteriums — short road races round city centres, with many tight corners and a densely packed peloton). Slack angles make a bike more stable, notably on descents, and generally more comfortable over long distances. Touring bikes have slack angles. The head tube angle on my bike is 73° — bang in the middle and accepted for at least seventy years as the optimum angle for a road bike. Tour de France style bikes, sometimes known as ‘stage racing’ bikes, commonly have a head angle of around 73°: it’s sporty but sensible.
    Other geometric measurements that contribute significantly to the ride characteristics of a bicycle are the wheelbase — the distance between the front and rear hubs — and the height of the bottom bracket. Both, again, affect the handling. Brian determined the geometry of my bike taking account of my physique, myexperience and the type of riding I plan to do. The result will be a