Updated: May 10, 2021
BB height and BB drop. These two terms define the location of the bottom bracket. BB height obviously being the height of the bottom bracket from the ground and BB drop meaning how much lower than the wheel axles the bottom bracket sits. Both measurements are relevant and, while related, are exclusive and both are required to build a full geometric picture of the frame.
Usually my frame design process utilises BB drop as a ‘driving’ dimension and BB height as a ‘driven’ dimension. Despite what others may say I believe it’s the BB drop that really defines how a mountain bike will handle as it denotes where the bottom bracket sits in relation to the other geometric elements. ‘BB height’, as far as mountain bikes are concerned, only really defines how much ground clearance the bike has* and while that is important it doesn’t really tell us how the bike will handle. Not to mention that it makes little sense to talk about the BB height alone when something as simple as changing tyres can alter the number and yet won’t change the BB Drop. And now that some crazy bastards are putting two different wheel sizes on their bikes, more than ever we need both the drop and the height of the BB to fully understand what’s going on.
* Reading such blasphemy, disciples of motorcycle or road bike dynamics and geometry theory will have all manner of alarm bells ringing about ‘roll axis’ and ‘roll moment’, etc. Here BB drop is a concept largely ignored. And those who’ve studied it tend to do the same when analysing mtb geometry which, in my opinion, is a huge oversight. Cornering a mountain bike is a much more dynamic procedure than simply leaning which is what the above terms relate to.
Within the mountain bike world right now one simple line explains the current thinking on BB height and drop - “lower is better”. More specifically what’s often meant is that low bottom brackets (and/ or more drop) corner better. This theory is solid and relates to the aforementioned ‘roll moment’ being closer to the ‘roll axis’ (yawn). But when it comes to the much more dynamic world of mtb the theory is far from absolute and other experimental riders tend to agree. https://nsmb.com/articles/paul-astons-210mm-do-everything-nicolai-g1/
Despite largely escaping road bike based geometry there are still elements and theories that we need to let go of that simply aren’t relevant to MTB.
It’s true in theory, low BBs do create more stability, they create a more ‘in-the-bike’ feel by way of an increase in stack and lower CoG. Riders of all levels will feel more confidence and in certain situations that may also translate to ‘better cornering’. This aligns with lean cornering theory and there we have it - “lower bottom brackets corner better bro”.
So what am I going on about? Well in reality the theory doesn’t apply well to proper mountain biking. A low bottom bracket isn’t helping you corner (Before you rush to the comments - Sam Hill has been riding a bike with one of the highest bottom brackets in its class for the last 5 years or so) At best it’ll aid stability but that’s somewhat unrelated to ‘cornering’ specifically and there are better ways to find stability than putting your pedals and chainring closer to the floor (smashing either one of which into the ground is not very stable, especially during a corner). What’s considered to be a ‘low BB’ is roughly a few cm lower than a ‘high BB’, not enough to have a meaningful affect on cornering even if we consider the ‘lean theory’ but enough to cause clearance issues. And besides that an experienced rider can alter their CoG within a very broad range - much more so than a few cm - and the very best will do so almost subconsciously (somebody mention Sam Hill?) to get them through any corner they encounter, indeed they can even effectively increase their BB drop by pushing through the bike or lowering a pedal. There’s just no need for the bottom bracket to be that low to begin with - the bike can corner just as well with a higher one (maybe even better) and that way you’re not going to hit the ground.
In fact the whole low BB and cornering thing is a red herring. Because as I’ve said - in the real world bottom bracket height alone has a negligible impact on cornering a mountain bike yet the BB drop has a huge bearing on the manoeuvrability of the bike. It’s this oversight that staggers me so and leads me to the conclusion that we should spend more time riding than reading. This concept isn’t mentioned in any of the motorcycle theory I’ve read probably because motorcycles don’t really have any BB drop nor are motorcycle riders required to lift and manoeuvre the bike under their own steam. Put simply a higher BB relative to the axles (less BB drop) makes it easier to pick up the front wheel which is the first step of pretty much every manoeuvre - whether that’s just putting the front wheel where you need it on a technical climb, popping obstacles into back-sides, quick line changes or literally anything else you try to do on the bike...like cornering for example! The oft-touted cornering theory is based on a vehicle upon which our only input is to lean into a corner whereas mountain bikers can literally jump into a corner and regularly do so! Even if our tyres never leave the ground it’s our dynamism that differentiates mtb from road or motorcycle theories. The speed at which you can lean into a corner and into the next and so on is, in my opinion, more to do with how well the rider can pick up, manoeuvre and manipulate the bikes trajectory than anything to do with roll moments and axes.
Despite this I might even concede (but don’t tell anyone) that a few short years ago low bottom brackets were beneficial on the shorter bikes we were riding. Back then a low BB provided a little extra stability where otherwise not a lot was available; today I’d argue the low BB is not only unnecessary but even a hindrance. We now have much more appropriately sized bikes as well as larger wheels and better suspension - our bikes are now quite stable - we don’t need to swing the BB way below the axles into the path of roots and rocks. And by not doing so we not only gain ground clearance, but we also retain an element of agility which is wholly beneficial to cornering - plus your big, modern, stable sled can still handle well, still be manoeuvrable, poppy and even playful.
This is particularly key for a hardtail where the rider must pick their line more carefully and will often need to pick the bike up to go over or around obstacles. If it takes all your strength just to hop and you have to muscle it around quick line changes you’ll be slow and tire quickly - especially if you ride somewhere as rocky my local!
So basically what I’m saying here is that - at least where Dawley bikes are concerned - my recommendation is to utilise a slightly higher BB than has been trendy. I’m not suggesting we go as high as a bmx, just that we should look past the ‘lower is better’ mantra. If stability is required then we can gain it back in other areas that won’t hinder the agility so greatly. Plus you’ll love the feeling of being able to pedal straight through rock gardens!
But like I said, if my argument doesn’t convince you, if you know that a super low bottom bracket works for you - where you ride...Who am I to say otherwise?