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Sunday, January 1, 2017

I’m different, you’re different. (Why bottom bracket drop matters for some people. Or an alternative view on the wheel size debate in mountain biking)


Every month or so I have this recurring dream. It goes something like this: I’m sitting in an examination hall with other students. I’m aware that I am about to sit an exam. I flip open the first page and find, to my horror, that I can’t understand the question. I guess that it’s a maths or physics question with complex diagrams and equations but I don’t know what I am looking at much less what I am asked to do. I look up at the other students and see them with their heads down, pens afire; or tapping their chins, pondering. I get a horrible feeling that I am sitting the wrong exam. Then I wake up.

A month later I get the same dream but in a different setting and with a different question. 

It’s vivid and unsettling.

Last night was slightly different. The setting is still an examination hall but this time it starts out with desks and chairs and somehow ends up with coffee tables and sofas. The question appears to be the calculation of the energy level of a string of elementary particles (elementary particles drawn literally like beads on a string). There is a worksheet with the energy levels of other combinations of elementary particles (2x + y + 4z + 3k = 762; x + 3y + 6k = 465; etc). Ignoring the many incongruencies that dreams have with reality I actually, for once, understand the question and so set out to answer it. As I pick up my pen the guy next to me leans right over to the point that I cannot place my hand on the paper to do my calculations. Thinking back on the dream this was the time that the setting changes from chairs and desks to sofas and coffee tables. The guy on the other side of me also leans over pushing me back into the sofa. I push back but other people start leaning over. The perspective changes from my point of view to me looking at myself struggling as even more people pile on top of me. My work sheet, examination paper, and answer booklet go their separate ways. Then I wake up.

WTF?!

I mean seriously, WTF?!

I’ve had these dreams for years - probably the better part of a decade - and, finally, when the question actually makes (some) sense my brain conjures a ridiculous scenario to thwart any attempt to answer it. 

I clearly have some unresolved issues.

Don’t you smirk. I bet you have some too.  

If I could dissect my brain anatomically, biochemically, or metaphorically, I would expect to see something quite different to what I would find if I dissected anyone else’s brain. In particular - for the purpose of an easy comparison - if I dissected the homunculus of my motor and somatosensory cortices it would show a relatively large representation for my hands and a smaller representation for my feet. 


Motor homunculus 


Somatosensory homunculus


My work requires good fine motor skills and, for as long as I can remember, I’ve always had an ability to “see” well using my hands. Nothing otherworldly or such, just a good tactile sense for materials and a good ability to coordinate my motor response accordingly. Regular practice probably enhances these capabilities. 

My feet, on the other hand, are terrible. I literally have no idea where they are at any particular point in time. And without decent sensory input I am unable to modulate an appropriate response. It could be a lack of appropriate sensory end-organs, a problem with the neurological connections, or a problem at the level of the brain. The motor output to my feet does not appear to be any better. As a matter of fact my lack of gross motor skills makes me wonder whether every part of my sensorimotor anatomy - with the exception of my hands - is structurally underrepresented. Sadly, I am no dancer.


There are two types of beings in the universe: those who dance, and those who do not.



Ok, so I’m a little different. Big deal, we are all a little different from each other. That’s the point of this post. But there are a lot of people who don’t, won’t, or can’t dance. It might also explain our preference when it comes to off-road bicycles.
There are a number of geometric variables that predictably affect the way a bicycle handles. Certain things like head tube angle, trail, stem length, reach, stack, wheelbase, front-centre length and chainstay length are well established even if they continue to be tweaked as different materials, shock technology, and the needs of specific disciplines come into play. No metric can be taken in isolation but I think it is fair to say that the big bicycle manufacturers have got their geometries pretty well dialled.

So when I look at getting a bicycle I consider 4 measurements and assume that the other metrics have been optimised for the discipline for which it was made. The measurements are:
- reach (effective top tube length gives me an estimation)
- standover clearance (and seat tube length)
- bottom bracket to mid-point of saddle height
- and, yes, bottom bracket drop

The first three are well-accepted, and any bicycle shop worth its salt would make sure that these points are dutifully attended. Bottom bracket drop (BB drop) has been given less attention as different people have different ideas about it. But it matters. Especially for those that don’t dance.

Road bike geometry - with the predictable environment in which these vehicles operate - have had their geometry well sorted for the past 50 years or so. Bottom bracket drop, in particular, has been standardised at 65-70mm. Off-road geometry, on the other hand, has evolved over the past 40 years of its existence. Starting off as road bikes with fatter tires and flat bars they have morphed into a variety of rather specific machines designed to fulfil the (evolving) requirements of different disciplines. Materials matter (aluminium, carbon fibre) as does technology (suspension, wheel size, dropper posts, and, less obviously, tubeless tyres and clipless pedals) but geometry still remains key.

Let’s take the trail/ xc bike that most of us ride. My 2002 On-One Inbred has a steel frame, 26” wheels, a wheelbase of 1070mm (give or take as it has sliding dropouts), with a bottom bracket drop of 40mm. The frame is optimised for rigid forks giving it massive clearance along the entire top tube.


Perfect for a being that don’t dance.








There are, of course, a couple of problems with a low bottom bracket that limit how far this metric can be taken in isolation. The most obvious is clearance at the chainring (note the Inbred wears a chainring guard) and toe/ pedal strike. The second is handling. A lower bottom bracket, combined with a wider wheelbase, makes a bicycle more stable (i.e. more forgiving). It also makes it less manoeuvrable. The perfect balance between these two competing qualities is what makes an off-road bike good fun to ride. Brant Richards’ design with the On-One Inbred seems to have hit the sweet spot for someone with my sensorimotor capabilities (or lack thereof).

In 1991 Cannondale’s marketed the M800 as the “Beast of the East”. According to Mombat  (http://mombat.org/MOMBAT/BikeHistoryPages/Cannondale.html) this wasn’t the first off-road Cannondale with a high BB (specifically intended to achieve better clearance) but it was the first bicycle I remember riding and knowing that it had great clearance. To be honest it was a pretty awful experience. The high bottom bracket (ie low BB drop) and my rubbish gross motor skills made for a jittery, nervous ride. Good clearance is purely theoretical if you can’t actually ride the bicycle in its natural setting. Yes, there are many, many riders who can. And they love this bike. I just ain’t one of them.


The original “Beast of the East”


1991 was a long time ago. A couple of years back I bought a 2012 tic-tac orange Santa Cruz Chameleon (http://campagnolodelta.blogspot.com.au/2014/10/candy.html) which is, by all reports, a very good general purpose mountain bike. Surely if kids can shred this thing over some seriously gnarly terrain then a middle aged man can pedal it over relatively sedate singletrack. Not so. The big clearances needed for the kids means a low BB drop to accommodate the quick-handling but small (for the terrain it is made to clear) 26” wheels. The Chameleon is simply not stable enough for me. I’m not a dancer.


You stay right there. I’m walking home.
(It might have been different if I had spent some time on a BMX bike or a trials bike - two genres that necessarily have a BB rise)


The move to increased wheel size for xc and trail riding over the past few years has almost made BB drop a mute point. Bigger wheels, among other things, allow a larger BB drop while negating the problem of clearance. But if you are a smaller rider with a correspondingly smaller engine then it also makes the bike less nimble. Not necessarily slower. Just less fun to ride. 

So what’s the deal? 

If you are over 6 foot tall (1.83m) then 29ers are probably the way to go for trail and xc riding regardless of terrain and riding style. It doesn’t matter whether you dance or not. If you are this tall then 27.5” should be reserved for enduro style riding in which case you are a dancer (and a good one at that) and this post doesn’t apply to you.

If you are shorter than 6 foot then choose between 26”, 27.5” or 29” wheels for trail and xc depending on the terrain you ride and your particular riding style. If you can’t dance then BB drop (including the recommended sag of a suspension system if present) and wheelbase length should also be taken into account.

For me - 5’8”, 64kg (67kg with gear), non-dancer - a 40mm BB drop works well for a hardtail. I can get away with a bit less when factoring the longer wheelbase of a dual-suspension bicycle. Increasingly technical trails and today's market leans towards a 27.5” platform as I prefer the manoeuvrability and acceleration of smaller diameter wheels. 

Well then.

One problem sorted.




Sunday, December 4, 2016

Percutaneous reduction and fixation of the flexed fractured scaphoid


Here’s a neat technique..


17yo male with scaphoid waist fracture


CT reveals comminution










And a humpback deformity


You get the impression there is a pre-existing bone cyst through which the fracture occurred






Lunate extension secondary to scaphoid flexion





This is not the typical case for percutaneous fixation but:
1. a Matti-Russe approach and fixation with screws or a scaphoid plate is not without its problems
2. there is good evidence that stable fixation can obviate the need for bone grafting
3. ask yourself “Can this be done better?”

Well, maybe..



Traction applied


K wire is passed through the lunate


Lunate flexed relative to RCJ (extending scaphoid at the fracture site)




Two guide-wires passed across the fracture site


Scapholunate angle corrected on lateral view


Percutaneous screws




Lateral view to confirm acceptable SLA