Tech Talk: How frame geometry affects a bike’s handling

Beta on different BB heights, headtube angles, chainstay lengths, and more

Components Tech
The same size bike can have very different designs and dimensions depending on its intended use. Photo courtesy of Art's Cyclery

The same size bike can have very different designs and dimensions depending on its intended use (click to enlarge). Photo courtesy of Art’s Cyclery

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.

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. Photo courtesy of Art's Cyclery

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 (click to enlarge). Photo courtesy of Art’s Cyclery

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.

Continue to page 2 for a geometry comparison of Specialized cross-country, all mountain, and downhill models »

About the author: Arts Cyclery

This article was originally published on the Art's Cyclery Blog. Art's Cyclery is dedicated to offering free expert advice, how-to videos, and in-depth product reviews on ArtsCyclery.com to help riders make an educated decision when selecting cycling gear.


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  • Myths says:

    “Pretty soon, after riding enough bikes, you’ll know exactly what a bike will feel like, just by looking at the geometry chart.”

    That’s the idea theoretically yet somehow it rarely works that way. Possibly because factors like frame material, weight and component spec aren’t taken into consideration when looking at cold geometry charts, part of the magic that is the modern mountain bike stems from how two bikes of identical geometry can manage to feel so completely different from one another on the trails.

  • Eznitram says:

    Exactly what I thought Myths.
    … plus tire type, size, and pressure can make for a totally different bike feeling, not to mention spring and damping setup of the suspension. Geometry is without doubt the starting point to choose a bike, but test riding a friend’s bike will most likely not give you a good impression of the differences. It is just too complex.

  • oldmtb says:

    One thing you’ll notice with slacker head angles is that your front fork will absorb more of your forward momentum when you hit obstacles. Also small bump sensitivity at slower speeds is worse with slack head angles

  • john says:

    On Page 1, the author goes into some detail regarding steering characteristics associated with trail, which leads one to think that surely trail must impact bike handling in some key way. And yet there is nothing about trail or offset listed in the tables! Why is that?

    • Duane says:

      Trail is good to know to understand the physics of how a bike works, If the fork offset and the wheel diameter is the same for all the bikes you are shopping for then the head tube angle will give you the trail. So, the head tube angle will give you a general idea of how fast or slow a bike will handle.

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