Recently, some bicycle manufacturers have positioned the rear shock lower on the frame to reduce the center of gravity (COG), claiming that a lower COG enhances bicycle stability. Certain well-known brands have moved the rear shock downward or even integrated it into the down tube to place it as low as possible.

It seems logical that placing heavier components lower on the bicycle would lower the COG, thereby improving stability. However, there’s a differing perspective suggesting that the impact of lowering the rear shock on the COG is negligible.

The heaviest rear shock weighs approximately 1 kg, while the total weight of the bicycle and rider is around 100 kg. The COG of the entire system is typically about 1 meter above the ground. The height difference between a high shock position (directly below the top tube) and a low shock position (directly above the down tube) is roughly 20 cm. Lowering a 1 kg shock by 20 cm reduces the overall COG by about 2 mm, or 0.2%. This raises the question: could wearing thinner socks have a comparable effect on the COG?

However, many aspects of bicycle design chase marginal gains, so let’s assume the COG can be significantly lowered. When braking causes the frame to pitch forward or pedaling causes it to squat backward, a lower COG reduces this pitching motion, which can be considered an improvement in stability. Similarly, a longer wheelbase reduces the likelihood of the bicycle “tripping” over obstacles, which is why taller riders often require a longer wheelbase.

When it comes to cornering and lateral balance, lowering the COG does not necessarily improve stability. A bicycle behaves like an inverted pendulum, similar to balancing a baseball bat in your hand. To keep the bat balanced, you constantly move your hand to keep it under the bat’s COG. Likewise, when riding a bicycle, you continuously steer to keep the wheels directly beneath the COG. Incidentally, within a certain speed range, gyroscopic forces and trail effects automatically stabilize the bike (known as self-stability). However, at lower or higher speeds, the rider must periodically adjust steering to maintain balance.

A baseball bat is easier to balance than a pencil because its higher COG means it takes longer to fall, giving you more time to move your hand to correct any tilt. Similarly, if a bicycle’s wheels are knocked sideways—due to a loose rock or sliding during a turn—a higher COG results in a smaller angle of imbalance, giving the rider more time to correct by steering into the tilt.

Does this mean we should all ride with the highest possible COG for better stability? Not quite. This is partly due to the pitching principle discussed earlier, but also because there’s a trade-off between cornering stability and maneuverability. A higher COG means it takes longer to initiate a turn or change the bike’s lean angle. Before making a left turn, the COG must shift to the left side of the tires, and vice versa for a right turn. When changing the lean angle, the bicycle and rider rotate around the roll axis (the line connecting the two tire contact patches). The distance between the roll axis and the COG is called the roll moment of inertia. The greater this distance, the longer it takes to change the lean angle, and thus the longer it takes to switch from a left turn to a right turn, or vice versa. For this reason, a lower COG may be preferable for navigating a series of tight turns, while a higher COG could be better for fast, straight sections with loose rocks.

A lower COG has a similar effect to a shorter wheelbase, a steeper head tube angle, or narrower handlebars. Therefore, like any other geometric parameter, COG height involves a trade-off in handling response. Contrary to intuition, a lower COG does not inherently make a bicycle more stable.