Editors Note: This article was written by Art’s Cyclery web content editor Brett Murphy, who uses his mechanical engineering background to explain the latest industry advances and breakdown component design. The original post can be found here.
Bicycle geometry affects more than just fit. It drastically changes how the bike rides. Different manufacturers sell the same fit size frame with significantly different ride characteristics based on headtube angles, chainstay lengths, bottom bracket heights, and numerous other dimensions.
Here’s a breakdown of a few of the most important dimensions and how they affect ride quality, plus a comparison of some different industry leading frame designs. The intent here is not to show you how to size a bicycle for personal use, but to reveal how the same size bike can have very different designs and dimensions depending on its intended use.
Headtube angle is the angle formed between the horizontal plane and the fork steerer/headtube. Commonly you’ll hear people talk about slacker angles, which are seen on all-mountain and downhill bikes. Smaller headtube angles push the front wheel further out in front of the bike, providing less snappy steering, but a more stable feel at high speeds while descending. Cross-country bikes have larger angles, providing tighter steering response, good for slower speeds and climbing.
When discussing headtube angles, it’s always prudent to mention fork offset. The fork offset is best described as the distance from the axis through the center of the steerer tube to the center of the axle. It is a positive value that, when increased, will lengthen the wheelbase of the bike, pushing the front wheel further out, resulting in decreased trail and snappier steering.
The combination of headtube angle and fork offset can be measured by trail. The steering torque that must be reacted to by the rider is equivalent to the trail multiplied by the lateral forces being exerted on the wheel. So by changing headtube angles, offset, and even the crown to axle length of the fork, trail is changed, ultimately changing the amount of torque required at the handlebars to turn the wheel and tire. The diagram below illustrates these angles.
Changing these steering dimensions to increase trail increases the torque input required by the rider to change directions, resulting in a more self-centering, stable feeling bike. Or if a nimble climbing bike is desired, altering geometries to reduce trail will decrease required steering torque, making the steering quicker and snappy but requiring more frequent rider input to keep the bike going in a straight line.
Bottom bracket height, the distance from the ground to the center of the bottom bracket, greatly affects a bike’s corning ability. Higher bottom brackets generally suggest a higher center of gravity. Ideally, we want the center of gravity as low to the ground as possible, to keep the bike planted as we lean it over through a corner. This is the same as comparing a low-riding sports car to a truck or SUV when going through a corner. The higher center of gravity becomes further off-axis at higher speeds in a truck than the sports car, requiring more driver input to correct it, and slower speeds to prevent rollover.
When designing a bike frame, there also needs to be enough clearance so that the pedals and frame aren’t constantly striking obstacles on the trail. Another element is introduced when compressing suspension, especially in full suspension bikes, as the suspension is compressed, the bottom bracket gets closer to the ground.
Wheelbase is the distance from axle to axle. Longer wheelbases produce a more stable feeling and help keep the bicycle traveling in a straight line. This is why longer wheelbases are preferred in downhill riding, but aren’t ideal in tight, technical turns when it is necessary to “throw” the bicycle around a corner. Shorter wheelbases make the bicycle feel more nimble and agile in these situations. Chainstay lengths also affect wheelbase and can change ride characteristics similarly. There is a delicate balance between altering overall wheelbase and just chainstay lengths though, as the rear triangle design also determines suspension characteristics and behavior.