Fabien Barel is always a rider to watch in terms of position on the bike - as one of the early proponents of longer bikes with bigger reaches, his position is the stuff of textbooks - if you look here you can see that he's centred within the bike and in a strong, relaxed position, ready for whatever the trail throws at him next.

This was the first observation. When the arm and torso are in the strong, ready position, the hip is pushed backwards, and apart from any weight distribution implications, you can see clearly that the angle is closing - this puts the hip in a weaker, less comfortable position.

On the same bike, if you come forwards to put the hip in a more natural position for aggressive riding, you can see that this then pushes the torso forwards over the stem - a very precarious position to hold.

On a proportionally-sized bike, you can see that the torso and arms are in the strong, ready position and the hip has space to take a strong, ready stance position also.

When the bike is too long, you can see that although the arms and torso are still in strong position, the hips are brought forwards on the bike - stretching the body out to a weak position where your control of the bike is reduced.

"Longer, lower, slacker." Those three words have become something of a cliché in how we talk about new bikes. Before we go any further, let me be clear, all three aspects are good progress. I don't think anybody would argue that even in the recent past bikes were too short, too high and too steep. So as product managers and their teams work to try and make their new bikes better than the ones they made before they tend to follow those three things.Those product teams work in relative isolation from other brands - you don't think the engineers at Specialized want to show their hands to their counterparts are Trek, do you? It's a competition and it is pretty undeniable that if you make a bad bike or at least one significantly worse than your competitors, then your business is heading down the drain (you know who you are). So the result is that when the new bike gets passed to the marketing team, the first question is (or at least should be), "Why is this better than the old bike we were making?" You can picture the conversation, "Oh, well, we made it longer..." You know how the rest goes.If we pause and consider the three elements, I want to make the argument that one of these adjectives is not fit for purpose. There is plenty of mileage in discussing how slack a head angle should be, or how low a bottom bracket can be, they are both important in determining how a bike behaves on the trail. Where I take issue with the triumvirate is "Longer." Answer me one question - why is a longer bike better? If you have been keeping up on bike launches, I would expect you to mention something about being more stable, faster maybe. Yet these descriptors are a nonsense - it's an empty argument. Why would you need a more stable bike when your current one feels pretty good already? And how do you measure stability? What does it mean?This is where mountain biking's road-based roots show through. The first mountain bikes were basically whatever was lying around with a big handlebar and some fat tyres. Then came the adapted road bikes, more or less the same frame with a flat handlebar. During mountain biking's first epoch it is fair to say that geometry wasn't a huge consideration, things like not snapping, braking, and having suspension that worked, quite rightly, came to the fore. It is only more recently, when buying a new bike is far less of a lottery than it has ever been, that people started looking at the layout of the mountain bike and asking, "How could this be better?" Enter the race to "longer, lower, slacker."While we stay talking in superlatives, we are staying firmly within the realm of marketing fluff. I believe that it is only when we get past this that we can start to really understand how a mountain bike is supposed to fit its rider. And for me, that is the most important word of all: fit. If you look to the road, those guys know precisely how a bike should fit the rider. If Chris Froome turns up to ride for your brand, you can bet he and his management will come with a precise specification of how he needs to be placed on the bike to perform at his optimum. The same goes for XC. For those disciplines the rider is in the saddle for the critical part of the racing and it is well-understood where the rider needs to be in relation to the bicycle and it needs to be millimeter perfect. When you start looking at more gravity-fed parts of cycling the simple truth is that there isn't the same level of detail or understanding available. To call it guesswork is probably a bit harsh, but going by the number of times "longer, lower, slacker" was repeated in 2017, it may not be too far from the mark. Until we are talking about fit as opposed to marketing vagueries, then we are still pissing in the shallow end.So, how should your bike fit you? I spoke to a few people about this and there seems to be little consensus, so what I am about to lay out is an idea, a theory. It is based on riding a bunch of different bikes and a lot of time thinking about their different qualities, the positives and negatives and what they mean out on the trail. What it certainly is not is definitive. All I am sure of right now is that I have a clear idea of a range of reach values I want my mountain bikes to fall within, and I can put together at least a semi-coherent argument as to why. Well, maybe not even semi-coherent, that is for you to judge...This all began for me with the two bikes I had in 2015. One was a Mondraker Foxy, the other an Ibis Ripley. If you are the type to sit and study geometry charts (I am, I'll admit that), then these are probably not two bikes that many people would choose in combination. The Mondraker was a medium, yet sported a full 473mm reach, paired to a 30mm stem. The Ibis was a large, the biggest bike in Ibis' range I could physically swing a leg (and a dropper post) over, but the reach was a very old-school 409mm with a 50mm stem bolted on. The rough calculation puts these two bikes at about 45mm apart in terms of reach, which in some ways is not much, but out on the trail, it became very apparent and significant.The first part of my observation is very easy for you to repeat the next time you head out riding. When you reach the fun part, pay attention to where your sternum is in relation to the stem. Good riding position is fairly straight forward, your spine should be flat-ish, arms relaxed, not too straight or too bent, so they are ready to react to whatever the trail throws at you. When you're in this position you should notice that your sternum is just behind the stem, with a modern trail bike, this will leave you nicely behind the front axle. This is a good place to be, strong, relaxed enough to hold for extended period and with full control of the front wheel. Take time to think about this a few times and, if you're lucky enough, try it on a couple of bikes and you should find that this part of your riding stance is a constant, regardless of the bike you are on (this doesn't apply to dirt jump/pumptrack bikes in the same way).Once I realised that this torso/arm position was a constant between both of my bikes, the next question is to look at how I was compensating between the two very different bikes. What I observed is that it was my hip angle was closing to keep my torso position constant on the shorter bike, although I also think my torso became slightly more upright on the shorter bike. If you look at racing shots of Aaron Gwin or Loic Bruni, it's clear to see that they keep their torsos relatively flat, and there are very few riders with such perfect form as that pair. Switching back to the Mondraker I realised that my hip angle on that bike was more open, putting me in a stronger position to react to the trail and creating less tension in my hip, which made the bike more comfortable on long descents. Back on the smaller bike, I tried focusing on the hip angle and ignoring my torso. Relaxing my hip angle to the same position I was riding on the Mondraker pushed my torso forwards, my sternum coming ahead of the stem, putting me in a very precarious position with all my weight over the front axle.The next step was to take these observations and try them on several different bikes. The first and one of the most interesting bikes was a friend's large Mondraker Dune. That had a reach of over 500mm and for me, it was far too much. If I put my torso in my ideal position, the bike was so long that I could feel my hips coming over and ahead of the BB, in other words, it was stretching me too far out. Over the following year, I rode quite a few different bikes of different sizes, tried some different bikes and kept coming back to my arm and hip angles. After a while it became apparent that on a bike shorter than 450mm I could feel the additional stress it put through my hips and I was stuck choosing between compromising one part of my stance or the other.So where does this leave me? Well, I'm a 5'9" rider with something like 29/30" inseam - in other words, I have short legs, hence a long torso for someone of my size. By focusing on the two angles I have arrived at the point where I consider myself to need a bike with a reach between 450mm and 475mm. I tried shorter and I tried longer - I spent a few days on a 480mm Orbea Rallon this summer on a job and it was close to being ok but didn't quite work. On more open, straighter terrain it felt ok, but as soon as we got too tight, natural switchbacks, it was simply too much bike for me to get through easily. Which is fine, at 5'9" the idea that I would be on an XL is ridiculous. After a couple of years on the Mondraker, adapting to the different demands of riding longer bikes, going back onto a bike with a 450mm reach, which is still relatively long by current standards, felt short, lively, playful, you can recalibrate your perceptions and I am still to find a true downside to these bigger bikes.Taking a look at current bikes on the market, my preferences are clearly towards the more progressive end of things, although brands like Transition and Giant have nailed their sizing close to where I now believe a modern bike needs to be. On the flipside, it is here that I don't necessarily agree with what is being pushed by companies like Geometron and Pole. While I would like most companies to build bigger bikes, I think there is also a limit to how big we can and should go. My caveat here is that I do think Geometrons and the suchlike do have their place, I am certain that taller riders will need to ride bikes that smash past the 500mm reach barrier that many brands seem wary of, and they are leading the charge in that respect. I am also 100% certain that there are benefits to bigger bikes, but only to the point where the bike still fits the rider so he or she is able to enjoy that benefit. The 480mm Orbea is certainly more stable than the 450mm one I ordered, but it's not a usable benefit for me.As I have already stressed, this is my theory. I have tested it on quite few different bikes and it seems to work for me. I hope at least this piece is a start in opening a discussion into how we talk about bike sizing. I now have a range of reaches that I consider proportional to my body size - I think the fact that it's a range is important, that within that range there is some scope for personal preference, but those preferences are based on the idea of how the bike fits me, not on marketing bullshit. There are still questions, Chris Kilmurray (Tahnee Seagrave and Greg Callaghan's trainer) stressed that, "Joint mobility, muscular strength and elasticity are very individual specific qualities", so he was not sure how generally applicable this is, that it may not be possible to derive a conclusive model of bike fit.Even if I am right, I don't exactly know how this can be adapted into something useable for consumers or bike companies, at the end of the day I am a photographer/journalist, I don't have the time or resources to conduct a meaningful study into bike fit and I need to focus on things that will pay my rent. Maybe this is not the right way to be looking at this problem, and actually, I'm ok with that. What I hope is that we can move to a point where how a bike fits the rider is what we talk about. A wise, or at least grumpy, man once told me that bike companies need to move past "better" and talk about performance criteria for their bikes. I believe that fit should be at the forefront of that list.