I was re-reading my last Blog post the other night and it got me thinking. In describing the geometry of my ultimate rigid mountain bike I only really gave the bottom bracket location a rather fleeting acknowledgement. So I thought that I would expand a bit on what I think I know and what more I’d like to find out. From what I can tell, there appears to be a commonly held, but incorrect belief that a low bottom bracket on a bicycle will lead to greater stability when cornering. I suppose the theory must go that by moving the rider closer to the ground and therefore lowering the overall centre of gravity a bike will be more stable in the corners. This belief has most likely come about because, given some cursory thought, it seems to make perfect sense.
Think of something like a double decker bus. In fact, scrap that. Think of a quadruple decker bus with all the passengers on board squeezed on to the top deck. In other words, something relatively tall, short and narrow, with a high centre of gravity. Drive quickly around a corner in such a vehicle and there’s a decent chance it will roll over. Similarly, brake hard and it will most likely topple forwards. Even a child would have an inherent understanding of this affect.
So why wouldn’t the same theory apply to a bicycle going around a corner? Surely, something with a high bottom bracket that raises the rider up would be less stable when cornering? Well, the short answer is no, this simply isn’t the case.
Two-wheeled vehicles go around corners very differently to their four-wheeled cousins. On a four-wheeled vehicle that’s cornering at speed the forces that are generated (centrifugal) can cause it to tip over. Lowering the vehicles centre of gravity will help to combat this affect. But because a two-wheeled vehicle leans into a corner these same centrifugal forces actually help to push it down on to the ground, increasing grip.
In fact it’s a good job that bicycles aren’t affected in the same way as cars as they tend to have a pretty high centre of gravity. The bicycle itself may only weigh around 10 kilograms while the rider sat on top of it may be ten times this. In other words, very top-heavy. The combined centre of gravity is going to be way above the bottom bracket, so moving its position up or down a few millimetres relative to the ground can only have a fairly marginal effect. Either way, the centre of gravity is still very high.
So what is happening when a cyclists goes around a corner and what affect, if any, does the bottom bracket height play? The most common analogy that gets used to help describe this situation is one where a person is trying to balance a broomstick on the palm of their hand, with the head of the broom up in the air. As the broom begins to fall in one direction the person must move their hand in the same direction, back under the broom’s own centre of gravity, to stop it from falling. Riding a bicycle is much the same. Even if you don’t realise that you are doing it, when riding along in a straight line you are making minor adjustments to ensure that the tyres (the palm of your hand) remain underneath you (the broom).
One of the most counter-intuitive things that I’ve ever heard was the first time I was told that this relationship also means that to turn right on a bicycle you first have to steer left. I had to try it myself to be sure. To go back to our analogy, what you are doing is purposefully moving the palm of your hand one way to make the broom fall the other. Only once the bike starts to fall do you then start steering in the same direction to stabilise the turn. It’s one of the main reasons why learning to ride a bike can be so difficult and a great example of how our brains can subconsciously solve a problem that we may never become consciously aware of.
Anyway, I digress. So, what does all of this mean for bottom bracket height? Well, to go back to the broom analogy one last time, a longer broom will fall more slowly than a shorter one as it must move through a larger arc. As a result, the taller the broom is the easier it will be to keep it balanced, but, equally, it will require more effort to make it fall. The same is true of a bicycle, with a higher bottom bracket resulting in a higher centre of gravity that will react more slowly to inputs, giving the rider more time to respond, but also requiring more effort to change direction. Wikipedia (Bicycle and Motorcycle Dynamics) has the following, slightly more scientific explanation:
“A bike is an example of an inverted pendulum. Just as a broomstick is easier to balance than a pencil, a tall bike (with a high centre of mass) can be easier to balance when ridden than a low one because its lean rate will be slower. However, a rider can have the opposite impression of a bike when it is stationary. A top-heavy bike can require more effort to keep upright, when stopped in traffic for example, than a bike which is just as tall but with a lower centre of mass.”
This last point is an important one, and may also help to explain why low bottom brackets are often equated with greater stability in the corners.
So far all of the above only deals with cornering. But what about braking in a straight line? In this scenario we are back to dealing with something that is much more closely related to the behaviour of a four-wheeled vehicle. Now it is a lower, not higher, centre of gravity that will make the bike more stable by making it harder for the bike to flip over forwards (or endo). But the really big changes in stability come about when the bottom bracket height is adjusted relative to the wheel axles. Getting the bottom bracket below them will result in a much more stable ride than having the bottom bracket above them, making the rider feel more ‘in’ than ‘on’ the bike.
Many riders have commented on this phenomenon when going from a 26 inch to a 29 inch wheeled bike. The bottom bracket height above the ground may not have changed, but its height relative to the wheel axles will have dropped. In fact Chris Porter of Mojo Suspension comments in this article on how he feels that 29er wheels place the bottom bracket so far below the wheel axles that it makes it difficult to get enough weight over (rather than behind) the front wheel when braking.
Going back to my previous Blog post that compared the geometry of a mountain bike with that of a Motocross bike, it’s interesting to note that once the latter includes some static sag in the suspension (the weight of a rider on board), which is normally around 70mm at the front and 100mm at the back, the height of the foot pegs above the ground is around 320mm. This also happens to be almost exactly the same height as a Motocross bikes wheel axles, i.e. 0mm 'bottom bracket' drop at static sag. This is also surprisingly close to the bottom bracket height of a lot of mountain bikes, despite the requirement of the latter to accommodate cranks and pedals. When determining the geometry of my own frame I went with a relatively large bottom bracket drop of 70mm, resulting in a bottom bracket height with 650b wheels and 2.6 inch tyres of just 290mm. The thinking being that I want the stability under braking combined with the agility in the corners that this would afford.
Something that I’m still struggling with is working out how these dynamics are affected once a bike gets into a slide. How does bottom bracket height affect a rider’s ability to control a two-wheeled drift? Does the pendulum analogy breakdown at this point or does it still hold true or maybe even invert? Unfortunately, I can’t currently find anything on this particular issue, but I’ll continue searching. I’d be very interested to know your thoughts if you can shed any light on this topic.