Bike Geometry the Basics
Bicycle frames are built with specific uses in mind. This becomes most obvious when you compare different styles of bikes. A cruiser frame looks substantially different than a mountain bike frame, and a road bike frame looks different still.
The geometry of a bike frame is defined by the angles and distances between the basic parts of frame, such as the top tube, down tube, the seat and chain stays, the seat tube, the head tube, and the fork. The precise angles in which these parts are arranged determines much for a bicycle: how it handles, how it rides, how it feels, and what type of terrain it can deal with.
This article will go over the basics of bike frame geometry so that you can make sense of the numbers and get the perfect bike for your style.
Uncommon frame types such as those used on step-through bikes, recumbents, folding bikes, and cantilever-style cruisers have their own unique geometry. This article will focus on the most common frame type in use today: the triangle frame (so named because of the two triangles formed by the shape of the frame when viewed from the side).
Why does frame geometry matter?
As we mentioned already, frame geometry determines a lot of the characteristics of your bike. It affects turning speed, stability, body position, and a host of other critical aspects of riding. Also, if you spend a lot of time on a bike frame that is too big or too small, you will be uncomfortable and may experience back or leg problems as a result.
In the old days, the length of the top tube of a bicycle was the standard measurement defining the "size" of a bike, and it worked fine when all top tubes were straight as a whistle. But those days are over: sloped top tubes are now more common on bikes than not.
These days, top tube length isn't reliable, but luckily there is a replacement measurement called "effective top tube length". This measures the distance that the top tube would cover if it ran completely horizontally from seat tube to head tube.
In general, road bikes designed for speed feature downward-sloping top tubes and upright seat tube angles (in the realm of 72 to 75 degrees). This positions the rider in a more aggressive position to maximize aerodynamics and pedal power without impinging on breathing. However, this position is less comfortable, especially over long rides. Touring bikes and road frames optimized for long-distance comfort tend to feature flat top tubes.
Cruisers and utility bikes, which are built for comfort, generally use a more relaxed seat-tube angle around 70 degrees, which puts the rider in an upright position. This is more comfortable for long rides, but less efficient -- something that can quickly become apparent when riding a bike of this type up hills or over any substantial distance.
In general, the lower the angle of your head tube, the more your bike will track in a straight line (and the more difficulty it will have with agile maneuvers at low speeds). Steeper angled head tubes are the opposite; they will be easy to turn even at low speeds, but are less likely to track straight and less stable at high speed. In general, most people can adapt to riding with just about any head tube angle, as long as the geometry of the rest of the bike is correct.
Mountain bikes, especially downhill bikes, tend to have the lowest head tube angles around (some kid bikes have these angles as well, to provide sufficient space for pedaling without banging against the front wheel). Road bikes designed for racing -- especially shorter or crowded races -- generally have the highest head tube angles.
Another important angle to consider when looking at a bicycle is the "fork rake," which is sometimes called offset. This measurement describes how far the hub of the front wheel is from the steering axis (the steering axis is defined by a straight line drawn through the interior of the head tube down towards the wheel).
The fork rake is defined by the offset of the front fork. Both curved and straight-blade forks can offset the wheel. A bigger offset can provide more stability when steering at high speeds, but makes a bike difficult to steer at low speeds. Low offset does the opposite. A standard fork rake distance for road bikes is around 47mm, which provides a good balance for maneuverability at high and low speeds.
Wheelbase is defined as the length between the front and rear wheel axles. A shorter wheelbase, which is generally accomplished through steeper head tube and seat tube angles (as well as chain stay length), provides a more lively feel for agility. A longer wheelbase is more stable, more comfortable, and provides more room for panniers at the rear of the bike.
What should I look for in a bike frame?
There are many other angles and measurements which may affect some aspect of performance to a greater or lesser degree, but once you understand the basics of bicycle geometry, you can purchase a bicycle with much greater confidence that you'll get the correct one for your style or riding. As always, the bottom line with choosing a bike is feel. Don't overthink geometry; if a bike feels good when you ride it, it's probably the right one for you.