Ever ridden a wobbly bike; a bike that shook throughout the frame, especially at higher speeds? What about a clumsy one that was slow to steer?
These two problems can be caused by geometry problems throughout the entire frame. The major culprit though, is improper trail. It is one of the chief factors that should be considered when putting a bike together. Sadly, few consumers, DIYers included, understand trail. What’s really terrible is that trail is a concept that many engineers at assembly plants out here either don’t truly understand or choose to ignore. This isn’t as big a problem in America, but these are still ideas any cyclist should understand, especially those hoping to build their own bike.
I’ll discuss two important concepts. Photo reference below.
Trail: Trail is the distance between two points located on the riding surface (ie the ground). The first point comes from the extension of the steering axis (head tube). The second point is the contact point of the wheel. This point is also directly under the front hub. Any vehicle with wheels has trail measurements: cars, motorcycles, shopping carts. It’s just a measurement. Here’s a link to a deeper discussion on Josh Putnam’s site.
I won’t turn this into a physics lesson. Practically, trail is a major influence on the handling of the bike. A larger trail measurement increases the tendency to steer straight ahead. Bikes that are easily ridden no-hands have bigger trail measurements (cruisers). Smaller trail leads to more maneuverability and responsive handling (track bikes). Big problems occur when trail is too big or too small.
It’s a precise game. The idea trail of a bicycle falls between 45mm-60mm. A trail less than 45mm causes shaking and an increased danger of falling. A trail greater than 60mm makes for a very clumsy and hard to control bike. As you can see, this is only a 15mm window of effective bike function. Trail is a very precise combination of frame and fork. Folding bikes more often have bike shake issues. Mountain bikes and city bikes more often have cumbersome handling issues. Many think keeping top-shape tires can solve a clumsy bike, but tires are just one piece of the puzzle.
Offset, or rake: Once again, imagine the line extending through the middle of the headset (the steering axis). The distance between this line and the axle (exact center) of the front wheel, when both are at the same level, is the offset. “Both are at the same level” describes the plane parallel to the ground. A very important aspect of offset is shock absorption, because roads have all kinds of unevenness and little potholes. Greater offset=greater shock absorption. As we know, shock to the front wheel runs right up through the forks, headset, stem, handlebars, and to the hands. No one wants handlebar palsy. Furthermore, shock limits the lifespan of the bike’s parts, especially the ball bearings in the headset.
So where do the problems come from? They occur at various points throughout the manufacturing and distribution process, even at the end-user level.
First, design itself. Often, factories will copy frame designs from foreign companies who come East for their manufacturing. They will either try to improve on designs, or just copy them directly. But, out of either lack of understanding or carelessness, they will ignore the trail measurements. We so often see designs that have the measurements of the parts, but nothing in the way of trail or offset. This causes serious follow-up problems, like, once you have your frame design, how do you even pick a fork? They don’t know how to choose the correct measurements. Even worse, sometimes designers just treat the offset as the trail, which causes some pretty ridiculous design errors.
Second, come problems at assembly plants. Few brand names make all the parts for their bikes. It’s a question of economics. Factories are very specialized, because it is most efficient to focus on making one part, and making it very well. Assembly plants then buy the parts, brand them, and put them together. If assembly plant technicians/engineers don’t rigorously check the manufacturer’s accuracy, then it’s really hard to find the error after the fact, when the completed bike starts experiencing problems.
Finally , enthusiasts who put own bikes together often select frames and forks only based on qualities such as weight, looks, etc., when a major focus should be engineering: trail and offset. Encountering bike shaking problems, riders will wrongly increase offset to solve the problem, but this will only reduce trail and make the bike shake even more.