I probably should have posted all of this before my blog ‘Motocross vs Mountain Biking’, as it explains much of my thinking behind that earlier post. Better late than never I suppose. Anyhow, many of the recent trends in mountain biking – longer front centres, larger wheels and more sophisticated suspension damping, to name a few – have helped to provide greater stability when riding off-road. A primary aim of these developments has been to increase the speed at which a trail can be ridden. While this all sounds great, there’s a potential pitfall. What if these advancements are covering up a bike that is fundamentally flawed? What if longer front centres, larger wheels and more sophisticated suspension damping are helping to conceal the fact that the geometry numbers behind these bikes aren’t as good as they could be? In evolutionary terms, has the mountain bike travelled down a dead-end in which we are now just tweaking something that, while still being improved upon, needs to start over completely to fully realise its potential? Have we all been focusing, heads down, on the details, when what we really need to do is pause and take a step back?
Firstly, I should just clarify that bicycle geometry can be split into two distinct, but interrelated groups. The first are those angles and measurements that primarily influence how a bike will handle – think head angle or chainstay length. While the second group has more to do with the position a rider adopts on a bike - things like head tube and downtube lengths that are commonly reduced to a stack and reach measurement (a vertical and horizontal measurement from the bottom bracket to the top of the head tube). It is this latter set of geometry figures that I’m most interested in here and specifically what impact these have when a rider chooses to stand on the pedals.
Mountain bikes in the early 90s looked to road bikes for inspiration when it came to geometry and rider position. The industries almost obligatory 71/73 head/seat angles were just a small massage of the classic 73/73 found on so many road bikes. We even kept the long stems. So, with this in mind, how far has mountain bike and road bike geometry really developed over the intervening 20-odd years? How has the position adopted by a rider changed to better deal with the unique set of challenges that each discipline presents? For many people, particularly those who only ride either road or mountain bikes rather than both, I suspect the assumption is that there is now a great deal of difference between the position a rider adopts on these two types of bicycle.
Let’s look at two ends of the cycling spectrum to see how much diversification there has really been. To do this, first consider someone who’s about the same height as me, so around 183cm (6 foot) tall. The chances are that such a person, with average proportions, will fit a size large mountain bike frame and a size 58cm road bike frame.
Starting on the road, a Specialized Tarmac road bike is about as racy as they come. Used by two of the UCI World Tour teams (think Tour de France) and ridden by some of the worlds fittest athletes. This is a no-holds-barred, scalpel of a bike designed to convert every watt of energy into forward propulsion in order to ride up the side of a mountain quicker than anything else. In a size 58cm it has a stack to the handlebars of around 630mm and an equivalent reach of about 490mm.
Turning our attention to riding off-road, if you’ve been following the World Enduro Series you’ll be aware that the tracks that are used aren’t far off full-on World Cup Downhill levels of brutal. Steep, rocky and fast, the bikes and their riders take a serious beating. The Specialized Enduro 650b is raced at this highest level. In a size large it has a stack to the handlebars of around 660mm, just 30mm higher than the Tarmac. And with an equivalent reach of about 460mm, that’s also just 30mm off a World Tour road bike. The difference between these two bikes would almost fit on a postage stamp.
And these dimensions become even more similar if you compare a slightly less racy road bike (like a Roubaix) with a less gravity orientated mountain bike (like a Stumpjumper FSR) – just 5mm and 15mm between their respective stack and reach measurements to the handlebars. How can two bikes designed for such completely different purposes end up placing the rider in such similar positions when they stand on the pedals? Surely the answer to the question “What is the optimal body position for a mountain biker when descending a technical trail?” can’t be almost exactly the same as “what is the optimal body position for a road cyclists in a Grand Tour?”
This isn’t a situation unique to Specialized, quite the opposite. I have simply chosen Specialized because they are such a well-known company that are representative of what is currently happening in the world of mountain bikes. The rise of gravel racing / adventure road bikes is a stark reminder of just how close the geometry of road and mountain bikes have remained. Something like Cannondale’s Slate, with a suspension fork and 650b wheels wrapped in knobbly tyres, but retaining dropped handlebars, is an illustration of how easy it is to blur the lines between road bikes and mountain bikes.
Just for fun, let’s compare a couple of motorbikes designed with similarly diverse purposes as the Tarmac and the Enduro. The first is a Yamaha YZF-R6, one of the fastest things that you can ride on the road or a race track. It has a stack to the handlebars of around 485mm and an equivalent reach of about 695mm. Second is a Yamaha YZ250F, designed to be ridden and raced off-road. This has a stack to the handlebars of around 805mm and an equivalent reach of about 425mm. The difference is clear: the off-road bike is 300mm taller and 270mm shorter, placing the rider in a completely different position in order to handle a completely different set of challenges. The Motocross bike puts more of the rider’s weight over their feet rather than their hands, allowing them to better absorb impacts in order to retain greater control over the bikes steering. The idea that you could so easily convert either of these bikes so that a rider might adopt an appropriate position for the other discipline seems slightly ridiculous.
Some of the explanation of why road and mountain bike geometries are so similar to one another is almost certainly down to the need to allow them to be pedalled with some degree of efficiency. However, it is hard to believe that the geometry of a road bike (which is so similar to that of a mountain bike) also happens to place a rider in an optimal position for riding off-road. So, where does this leave mountain bikes? Potentially they are in a very exciting place. For the last two decades it would appear that the position a rider adopts on a mountain bike has been gifted to them from the world of road bikes and this has remained almost completely unchallenged. Only then has anyone tried to increase stability through changes such as lengthening the wheelbase and slackening angles. Just image what all this effort and ingenuity could achieve if it were applied to a bike who’s starting point was a geometry that was designed from the outset to be ridden off-road. I don't know for sure whether a fundamental shift in mountain bike geometry will work, but I at least want to try. I will hopefully be in a position to create such a bike soon…