Bicycles have a nearly 200 year history starting with German inventor Karl von Drais. This early design was developed from 1817 to 1819. Despite having two wheels and handlebars the design is missing key features when compared to the modern bicycle. The front end steering geometry did not feature any caster effect such that pivot point of the steering was placed directly above the front wheel contact point with the ground. In addition it was powered by scooting along via foot rather than a chain drive. It wasn’t until 1885 when John Kemp Starley invented the first modern bicycle with equally sized front and rear wheels, front steering geometry with caster effect, and a chain drive powered by pedals. This has become known as the “safety bicycle” because it replaced the high wheel bikes ridden by daring young men willing to take a spin that might end up in a nasty spill.
The front end caster effect is called trail in the bicycle and motorcycle design world. To complicate matters there are two types of trail normally used to describe handling, ground trail and mechanical trail. Mechanical trail is the more significant term from a scientific point of view, however, ground trail is often used in discussions because more people seem to be familiar with this measurement. Steering trail is a product of the head angle (the bicycle steering axis) and the rake the the front fork. Mechanical trail and ground trail do not have a linear relationship. As the head angle and fork rake change they will not have the change together by the same amount. However, the difference is on the order of 10% or so.
In the picture below a 45c tire is mounted to a 700c spoked wheel for a diameter of 712 mm. The fork has a rake of 50mm. The ground trail is 63.1 mm and the mechanical trail is 60.0 mm. The head angle of the bike is 72 degrees. As trail is increased it leads to a bike with greater self stability at speed. In basic terms this means the rider can go “no hands” with less effort. Stability is a double edged sword. Adding trail improves straight ahead path stability at the expense of maneuverability. Military fighter jets and F1 race cars are designed to perform at the limit of what is considered stable, and in many cases they are inherently unstable like the flying wing Stealth Bomber. With active input from a rider/pilot a machine designed at the limit of stability is often superior in the hands of an expert user. However, it would be a poor choice to design a Boeing 747 at the limit of stability because it would cause a hair raising ride for the passengers. In the same respect, a bike designed for beach cruising or a senior citizen need not perform like a race machine in the hands of a Tour de France rider.
Ground trail is often referenced by frame builders such as the Paterek Manual which recommends frame and fork construction to feature between 50mm and 65mm of ground trail. In automotive terms of over, under, and neutral steer Spectrum Cycles notes in this link that 56 mm of ground trail leads to a neutral steering condition. Calfee Design also has this article which recommends 57 mm of trail as an ideal combination of agility and stability. Mountain bikes tend to have much more trail than a road bike. PVD uses around 70 mm of mechanical trail for a mountain bike and 55 mm for a road bike as noted on his www.pvdwiki.com website. In conversations with Black Sheep Cycles in Colorado they like to ride bikes with about 60 mm of trail, on an off road. This corresponds well to Rivendell Bikes where Grant indicates he likes to ride design in the high 50s to low 60s range at this link.
Regardless of the trail chosen by the user it is important to understand that the head angle has additional meaning. A specific amount of trail can be designed for a given head angle by choosing the appropriate fork rake. However, head angle also determines how fast the input from the handle bars is translated into a change in the wheel. Slacker angles tend to be more forgiving than steeper angles. Its normal for a mountain bike to have between 68-71 degree head angles while road bikes have 72-74 degree head angles.
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