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Bottom Bracket Height: The most misunderstood dimension?

1/2/2017

17 Comments

 
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.
17 Comments
John Anning
21/12/2017 14:04:15

This is helpful, thanks. One thing that puzzles me: it would seem that a higher bottom bracket (everything else being the same) would change the relationship of the seat to the handlebars. I’m considering a road frame with a high BB, but I’m concerned I won’t be able to get the bars high enough. Thanks, John

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Oli link
21/12/2017 14:33:01

Hi John and thanks for the question. You're correct - all else being equal, a higher bottom bracket will raise a bikes saddle height relative to the handlebars. However, you would like to think that most manufacturers would compensate for this with a taller fork and/or longer headtube. The best way to check is by looking at a bikes stack height (the vertical distance between the bottom bracket and the top of the headtube) as this is essentially a proxy for handlebar height.

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R
22/1/2018 23:49:34

One point concerns me: BB location relative to hub axles. If the wheels and frame are assumed to be rigid, for this purpose, the bike rotates or "falls" the same about the roll axis. The centre of mass interacts with the ground contact points via the same vectors. I've long felt it's a misconception to attribute significance to BB vs. axle position, but I'm open to an explanation of how these interact.

Chris Porter's experiments represent some of the best work of its sort in the industry and I agree with the vast majority of his opinions. This, however, bears further scrutiny!

Thanks!

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Oli link
23/1/2018 09:51:51

Hi R and thanks for the comment.

Whilst braking hard with the front brake most bikes will want to start to endo (but build a bike long and low enough and it will want to just slide the front wheel instead). This 'rolling-endo' means that the bike will start to pivot around the front axle (rather than the front tyres contact patch). The situation is then similar to the action of a single pivot rear suspension set-up. If the front hub axle (pivot point) is above the bottom bracket (rear hub axle) then the bottom bracket will initially have to move backwards until it has risen far enough to be level with the front axle (at which point it will start moving forwards). The lower the bottom bracket gets relative to the front hub axle, the harder it becomes to lift the bike and rider around it. I don't believe that wheel size per se is the main issue. Rather, it is the relative height of the bottom bracket to the front hub axle. Maintaining the same bottom bracket height while using larger diameter wheels is just one way of achieving this. I'll happily admit that this is a complex issue that I don't fully understand, but that's where my thinking is currently at.

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Ryan
23/1/2018 17:55:28

Hi Oli,

I have a few concerns about this argument:

1. The centre of mass of the rider-bike system is not at the BB, so while the BB may move a few millimeters rearward if rotated about the front hub, that doesn't mean the centre of mass is doing likewise. If we rotate the centre of mass about the front hub, the difference in height of the front hubs for various wheel sizes is small - not trivial, though - but there is no particular significance of the BB position relative to the hub.

2. I'm not convinced of this "rolling endo" effect. If it was true, then an extremely large wheel with a hub above the combined centre of mass would be impossible to endo from braking forces. If the acceleration vector on the centre of mass is at a shallower angle than a line from the front contact patch to the centre of mass, there exists no support to resist an endo and the system will rotate at the contact patch (a point which moves forward slightly more rapidly with a large wheel, though it's a minor effect). As such, hub height would not matter.

Your thoughts?

Ryan
23/1/2018 18:49:01

I wanted to check the amount of rearward motion at the BB for various wheel sizes, assuming the system rotates around the hub (which I question). Even if this is the case, the rearward motion is minuscule. Assuming modern enduro geometry with a front-centre of 780 mm and keeping BB height constant across wheel sizes:

29er: ~0.6 mm
650b: ~0 mm
26": ~0 mm

The difference in the angle between the BB and the hub, which is only relevant if the centre of mass is at the BB, is comparably small, ranging from about 2° to 0°.

Larger wheels certainly do affect handling, but I'm not convinced BB drop is a part of it.

Thanks for continuing to explore the issue!

Oli link
24/1/2018 20:34:17

Hi Ryan

I had another look through a couple of books on motorcycle dynamics last night to try to help answer this question as I'll freely admit that I took Chris Porters comments together with my own experience on the bike and didn't question it much further. A lesson learnt! Unfortunately, the books that I have appear to avoid the issue of axle height almost entirely.

I suspect that a significant part of the problem with trying to understand what is happening relates to the dynamic relationship between the bike and rider, as they do not form a single, rigid object.

If a person who is simply squatting on the ground stands up, then clearly their CoG would also rise with them. But when a rider stands on the pedals of a bike things aren't quite so simple as they are not a fixed object. Essentially, they are partially suspended above the bike. They therefore have the opportunity to influence the CoG in a different way.

Having said that, I don't disagree with anything that you've said. My problem is that it doesn't seem to tally with what I experience on the bike. I clearly need to do some more digging. Thank you for challenging my thinking and if you get to the bottom of this then I would love to hear the answer.

Oli

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Ryan
24/1/2018 20:45:44

Oli,

Your willingness to keep an open mind is a rare - and enormously appreciated - trait in an internet discussion!

The "in" vs. "on" sensation is certainly present, but the source is difficult to isolate. It's probably due to a combination of factors. My suspicion is it's the rate at which front-end dive increases or, closely related, the rate of load transfer. A short front-centre, high BB, and soft suspension will all contribute.

Email me and I'll let you in on a secret ...

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Mike P
8/6/2018 15:51:03

Very informative and intelligent discussion. Thank you!

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Mike
12/7/2018 23:33:22

Sounds like you have been following the thoughts Geoff Apps?

At 71, I want my hands higher; above the seat to relive neck and hand issues. So a shorter reach is implied.

All of my thinking revolves around a full rigid bike. I am also thinking about not having any BB drop. It seems to me, that such a bike should have a slack HTA - (65?) or at least a long front center to counter going over the bars. How much is unknown. It would seem that such a bike would respond to steering inputs better via the hands and not ones backside. Therefore, not built for speed; which may be fine for me.

Thinking that a dropper post would resolve any mounting and dismounting concerns.

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Oli Davey link
13/7/2018 09:37:45

Hi Mike and thanks for your comment. Yes, Geoff has provided me with some very helpful feedback on my bike. He's already done so much thinking about this kind of stuff.

I don't know what size wheels that you are thinking of, but having zero BB drop on a rigid mountain bike seems like a bold move? Potentially very good in the slow, technical stuff, but may come unstuck as speeds rise.

I'd love to see what you come up with.

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Jay
29/12/2018 01:18:11

When you talk about endos (and conversely looping out) BB height or drop is effectively irrelevant. Imagine the BB at near enough ground level and CG at a saddle height of say 1m (legs straight). Now imagine the BB is at 0.5m but the CG remains at 1m (legs bent). The resistance to endo or looping is exactly the same. It's a vector from front or rear contact patch to CG that resists rotation induced from braking or acceleration. If you move that CG forwards while all else remains the same, the angle from CG to front CP is steepened reducing resistance to brake induced rotation, whilst the angle to the rear CP becomes shallower increasing resistance to acceleration induced rotation. Lowering the CG whilst keeping it Central improves the vectors at both ends. Having a lower BB gets your CG lower for the same BB to saddle length, improving braking and the feel of the handling. Stability is a combined product of CG placement between CPs, wheelbase and trail. Lowering BB makes it easier to lower CG which improves everything. The only downside to lowering BB is ground clearance. 330mm seems to be about as low as you can go on MTBs before pedal strikes become unbearable, regardless or wheel size and axle height. If you think about it, you naturally drop your outside foot and weight it when cornering to get your CG lower (assuming saddle isn't keeping it higher as on road bikes). That makes your effective BB height through the turn something like 160mm, but it's getting your CG down that's making the difference not how low you get your foot.

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Lawrence Bishop link
8/1/2021 09:41:29

Great read, thanks

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Steve Hantman link
24/5/2022 17:21:40

I think too high a bottom bracket makes the optimal seat height adjustment "out of range" for a short person who still wishes to be able to touch tippy toes to the ground while sitting on the bike. - Crank length also factors in to help with this adjustment of fit. This aspect of fit may out weigh the other pros and cons of BB height and must be considered.

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Oli link
2/8/2022 09:23:46

Unsurprisingly, Ryan at FortNine does a far better job of explaining the affect of centre of gravity on two wheels that I do!
https://www.youtube.com/watch?v=GoK4hPtW-rg

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Josh W
21/8/2022 02:17:51

So then what does a trail bike benefit from a lower BB height?

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Oli link
22/8/2022 09:47:21

Hi Josh. All else being equal, a lower bottom bracket will help to (marginally) lower the combined bike and rider's centre of gravity, making them slightly more stable under braking and a bit more agile whilst cornering. Ultimately, it's all trade-offs when it comes to bike geometry in terms of both handling and fit, and more often than not I think its simply about trying to find the best compromise.

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