You occasionally hear bike manufacturers claiming they've placed the shock low down in the frame to give it a lower center of gravity, and it's often assumed that a lower center of gravity (COG) makes the bike more stable.
Probably the most striking example of this was when Specialized went to a bottom bracket concentric main pivot with their 2015 Demo
, citing a lower COG as one of the main reasons. More recently, Cannondale
(pictured above) nestled the shock inside the down tube to get it as low as possible.
It might sound reasonable that putting heavier components lower in the bike would lower the COG and therefore make the bike more stable. After all, a car with its weight lower down rolls less and corners better, and if you were a sumo wrestler trying to avoid being knocked over, getting lower is a good idea. But with a bicycle, things are a little different.
Most obviously, the effect of lowering the shock on the center of gravity is pretty negligible. The heaviest coil shocks weigh about a kilogram. The total system weight - that's the bike plus the rider - weighs about a hundred kilograms, and the center of gravity of all that weight is typically about 1 m from the ground (we only need rough numbers here). The difference in height between a high shock position, just under the top tube, and a low shock position, just above the down tube, is about 20 centimeters.
The difference between a high and low shock location is about 20 cm.
Moving that 1 kg shock 20 cm lower in the frame lowers the center of gravity of the entire 100 kg system by about 2 mm, or 0.2%. You could literally have a bigger effect on your center of gravity by wearing thinner socks.
Okay, but maybe it isn't only the centre of gravity of the system
that matters. Maybe it's the centre of gravity of the bike itself. If you're trying to manoeuvre the bike left and right relative to your body, then it's the centre of gravity of the bike which determines how much effort that takes. A typical bike weighs about 15 kilograms, so if you are lowering the shock by 20 centimetres, the COG of the bike will be lowered by one-fifteenth of 20 cm, which is about 13 mm. The Center of Gravity location of a bike is in the region of 50 centimetres from the ground. So you're lowering its centre of gravity by about 2%.
But bikes are all about marginal gains at this point, so assuming you could
lower the center of gravity by a noticeable amount, perhaps by putting an ebike motor right by the bottom bracket or by lowering the bottom bracket height (and therefore the mass of the rider) by a couple of centimetres, wouldn't that improve the stability of the bike, at least a tiny bit?
It depends on what you mean by stability.
In terms of pitching - that's how much the frame tilts forward when you apply the brakes, or squats backwards when you stamp on the pedals - lowering the COG will reduce this pitching, which could be described as an improvement to stability. Just like increasing the wheelbase, a lower center of gravity makes the bike less prone to "trip up" or pitch you over the bars - this is why taller riders need a longer wheelbase.
No doubt a lower COG makes it (slightly) easier to avoid going over the bars.
But here's the counter-intuitive part: when it comes to cornering and balancing from side to side, lowering the center of gravity doesn't improve stability.
That's because a bicycle is essentially an inverted pendulum, like balancing a baseball bat upright on your hand. In order to balance the bat, you continually move your hand so it remains under its center of gravity. Similarly, on a bicycle you're constantly steering so the wheels are directly underneath your COG. By the way, within a certain range of speeds, gyroscopic and caster forces will do this automatically (this is known as self-stability
) but at lower or higher speeds, the rider has to make regular corrections to the steering to remain balanced.
Here's a video demonstration of a higher COG being easier to balance.
A baseball bat is easier to balance on your hand than a pencil because its higher COG means it takes longer to fall over and so you have more time to move your hand to correct the lean. Also, if your hand is knocked to the side by a given distance, the change in angle of the bat is less than it is for the pencil, so it is easier to regain balance. Similarly, if your bike wheels are knocked to the side - perhaps by a loose rock or sliding in a turn - with a taller COG the angle by which the bike will become off-balance will be less, and you'll have more time to correct for this by steering into the direction of lean.
So does this mean that we should all be riding bikes with the highest possible center of gravity to make them more stable? No, partly because of the pitching reason we discussed earlier, but partly because there's a trade-off between stability and maneuverability in the corners.
The higher the COG, the longer it takes to deliberately
change the lean angle when initiating a turn or going from a left to a right turn (or visa-versa). Before you can turn left, you need to lean your weight to the left of your tires; in order to turn right, you first have to lean to the right. As you change the lean angle, the bike and rider pivot around the roll axis, or the line connecting the two contact patches. The distance between the roll axis and the COG is called the roll moment of inertia, and the longer the distance, the longer it takes to change the angle of lean, and so the longer it takes to change from turning left to turning right (or visa-versa). For this reason, you might want a low center of gravity for a series of tight turns, but a high center of gravity for a fast straight full of pinball rocks.
There's even an equation called the control authority
which links COG height, along with wheelbase, handlebar width and head angle, to "twitchiness" - how much the bike responds to a given input. A lower COG height has the same effect on this measure of twitchiness as a shorter wheelbase, steeper head angle or narrower handlebar.
So, just like any other geometry measurement, the center of gravity height is a tradeoff between twitchy and slow handling responses. But counter-intuitively, a lower COG makes the bike more twitchy, not more stable.