Rob Penn on Frame Geometry
(excerpted from "It's all about the Bike" (the Pursuit of Happiness on Two Wheels)
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. Pew cyclists ever think about frame geometry. If you buy a mass mauufactured 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 flt 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 (STLO in the diagram), they can vary from 65o 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 roadracing 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 (HTLO 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, my experience and the type of riding I plan to do. The result will be a sportive-style bike: the handling Will feel sharp, but the bike will be comfortable enough to sit on all clay, and stable when I’m steaming down a mountain in the Dolomites at 70 kph.
Unless you are a very experienced rider, you’ll struggle to distinguish between two sportive-style bikes with a one-degree difference in the head-tube angle, but ride a triathlon bike and then jump on a touring bike and you get the message. Be warned though; the more you learn about geometry, the faster your. beard will grow.
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. Pew cyclists ever think about frame geometry. If you buy a mass mauufactured 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 flt 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 (STLO in the diagram), they can vary from 65o 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 roadracing 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 (HTLO 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, my experience and the type of riding I plan to do. The result will be a sportive-style bike: the handling Will feel sharp, but the bike will be comfortable enough to sit on all clay, and stable when I’m steaming down a mountain in the Dolomites at 70 kph.
Unless you are a very experienced rider, you’ll struggle to distinguish between two sportive-style bikes with a one-degree difference in the head-tube angle, but ride a triathlon bike and then jump on a touring bike and you get the message. Be warned though; the more you learn about geometry, the faster your. beard will grow.
