Motocross vs Mountain Biking

For some time now I’ve been thinking about fully rigid mountain bikes - how I’d love to create a ‘big BMX’ that drifted, popped and pumped without the fuzz of suspension muffling the connection to the trail. I’ve even acted on these thoughts on more than one occasion, putting together rigid mountain bikes of various wheel sizes and transmission options. But I’ve always been left at least a little disappointed with the relentless hammering that results – what should be fun too often felt like it had turned into simple survival. Rather than forget the idea, I’ve always been inclined to believe that the problem wasn’t necessarily an inherent trait of riding a rigid bike off-road, but was actually down to inappropriate geometry as a result of frames that had been originally conceived to work with a suspension fork.
 
This situation isn’t helped by my rather unusual proportions. At 184cm tall (a fraction over 6 foot), I also have an inseam of 90.3cm (35.5 inches). Whereas the average man has an inseam that is around 46.5-47.5% of their height (see here), mine is a little over 49%. Put another way, my inseam would ordinarily be the average for someone who is around 192cm tall (6 foot 4 inches) and with such comparatively long legs I’m left with a relatively short torso. This makes me rather gangly and probably the reason why I always seem to have cold feet! In terms of my bikes, this means that I have a rather lofty saddle height of 82cm (a little over 32 inches). I can’t just size up my frames as this would leave me with an unworkable reach and so instead I must resort to plenty of headset spacers and either a flipped stem (road bikes) or plenty of rise on my handlebars (mountain bikes). So I suppose that I’ve probably been made more aware than most of the issues associated with low handlebar heights.
 
Linked with the above, I’ve also been wondering for a while why the world of mountain biking doesn’t borrow more from the world of Motocross. Originally called scrambles racing here in the UK, it has certainly been around longer than mountain biking and one can only assume that it has seen a fair amount more investment from companies like Honda, Kawasaki and KTM. While there is an obvious difference between the two vehicles (one has an engine while the other relies on pedal power) there would appear to be enough similarities to at least investigate where the differences are between the two types of machine and why they exist.
 
One thing that stands out is the position that a rider adopts on both types of bike. Fundamentally, this is governed by the relationship between the handlebars and the bottom bracket / footpegs. This is most easily broken down into a vertical and horizontal measurement, similar to the stack and reach often quoted for a bicycle frame, but measured to the handlebars rather than the top of the headtube. It was images of riders from the beach race in Le Touquet, France, that particularly spurred my thoughts. The position that they adopt, even with over 300mm of suspension travel at either end of their bikes, with their weight well back, seemed ideal for a rigid mountain bike (or any mountain bike for that matter). And yet even on something like an Enduro mountain bike the rider tends to be a long way off this position, with their hands both lower and further in front of them.

A rider in one of the world’s largest motorcycle races, the Enduropale du Touquet in France
(www.motorcyclecommunity.co.uk/beach-racing-le-touquet/)

​The differences to mountain bikes are clear. The reach to the handlebars on a motocross bike (which interestingly only come in a single ‘frame’ size) tends to be somewhere between 400 and 425mm, while even many size small mountain bikes exceed this. With the stack measurement, the opposite tends to be true. Motocross bikes have a stack (again to the handlebars) that’s generally a little over 800mm, which is far more than you would find on a regular mountain bike. By way of a comparison a stock Specialized Enduro 650b (not known for having particularly outlandish geometry) in a size medium has a reach to the handlebars of around 450mm and equivalent stack height of just 645mm.
 
The result is that a motocross rider has far more of their weight supported through their feet and far less through their hands when compared to a mountain bike rider. Why is this? Especially when mountain bikes generally have far less suspension travel and I certainly find it much easier to absorb shocks from the trail through my legs rather than through my arms. At least part of the reason must come down to a desire to adopt an efficient pedalling position. But unless you’re racing cross-country, how much should this be allowed to trump good handling?
 
Looking at the history of mountain biking it actually seemed like we were on to a pretty good thing when beach cruisers were first repurposed for use off-road in California when mountain biking began. It’s possibly the closest that bicycle geometry has ever got to that of a Motocross bike. But the rise and subsequent dominance of cross-country racing during the 1990s (with geometry that seemed to increasingly take its inspiration from road bikes) would appear to have been a backwards step in some ways. If anything, I would argue that the use of front suspension on mountain bikes has helped to cover up the poor body position that mountain bikers too frequently have to adopt, allowing riders to go faster for longer whilst still maintaining this “hands low and forward” position. More recently, top tubes and wheelbases may have got longer while stems have got shorter, but the overall stretch to the handlebars hasn’t changed a great deal. One way of thinking about this is to ask “If the genesis of mountain biking had instead been an engineless Motocross frame rather than a cruiser, would the geometry of mountain bikes have ended up back where we currently are today?” I’m not so sure.
 
So, in my spare moments I often ponder what a mountain bike might look like if I was left alone with a brazing torch and some steel tubes. What if my starting point was to emulate the position of a Motocross rider? So, a reach to the handlebars of around 420mm and a stack of around 800mm. This essentially fixes the front end of the bike with just a single frame size (just like on a motocross bike).
 
To achieve the correct seated position on a mountain bike there are normally two main ways that its length gets adjusted. The first is through changes to the reach, with larger frame sizes getting longer reach measurements. The other is through having a seat angle that isn’t dead vertical, so as the seat gets higher it also moves rearwards. As I’ve already mentioned, on a motocross bike (and as I am suggesting for my frame) the first value is fixed, which also means that the bikes reach value is fixed.
 
So, this just leaves the seat angle to play with. One benefit of this is that it side-steps the often-murky world of the ratio between stack and reach. This may not sound that important, but as the high-end road bike company Rolo point out, this is far from the norm. It’s as though frame manufacturers think that people of different heights have different physical proportions. Going back to the Specialized Enduro 650b, its stack to reach ratio for each frame size is as follows: S – 1.43; M – 1.37; L – 1.34; and XL – 1.30. Surely bike frames should just get proportionately larger, with the ratio of stack to reach remaining constant regardless of frame size? After all, why shouldn’t someone who is the correct size to ride a size small bike be able to adopt the same relative position as someone who fits a size large?

Mountain bikes, like these road bikes, commonly fail to maintain a fixed ratio of stack to reach
(http://rolobikes.com/geometry/)

​One challenge for mountain bikes if they were to adopt a fixed reach is allowing riders of various heights to get into a comfortable pedalling position. This could be achieved by having a slacker seat tube angle of 70-degrees (compared with something more conventional of around 74-degrees), which would result in the saddle moving more horizontally for a given change in height. This seat tube angle teamed with a 420mm handlebar reach would mean that for someone around 173cm tall (5’8’’) to achieve the same stretch to the handlebars as on a size medium Specialized FSR 650b, they would only need to slide the saddle forwards by 18mm. Similarly,  someone around 189cm tall (6’2’’) on a size extra-large would only be required to slide the saddle backwards by 27mm. These are obviously just averages, but these numbers are well within the scope of most saddle / seat post combinations.
 
The main issue with a slacker seat angle is that it then places a rider’s weight too far over the back wheel if ‘normal’ chainstay lengths are used. This would be likely to result in a lot of wheelies when climbing. Compared to a 74-degree seat angle on a size large frame, an average rider would be sat around 55mm further back. So, it seems reasonable to add this to a pretty normal chainstay length of around 430mm, growing to 485mm. This keeps the relationship between the rider’s backside and the rear wheel the same when seated and also helps with stability when the rider is standing as they will now be intentionally well back over the rear wheel, aided by the shorter reach and higher stack height. A relatively large bottom bracket drop of 70mm would help to keep the riders weight ‘in’ the bike rather than ‘on top’ of it, while shorter 165mm cranks would help to minimise pedal strikes with relatively little impact on power output (see here for some crank length theory).
 
Another interesting fact about modern mountain bikes is how little they actually tend to vary between sizes. For example, the stack measurement of a size medium Specialized FSR 650b is only 18mm lower than you’ll find on an extra-large frame. This is despite the fact that the former is designed for someone around 173cm tall (5’8’’) while the latter for someone around 189cm tall (6’2’’). The difference between the average saddle heights of these two riders is around 70mm, so why does a taller rider have to have their hands so much lower relative to their saddle height than a shorter rider, or have to resort to spacers and a high rise handlebar to achieve the same relative position?
 
What the above does show is that the fact Motocross bikes only come in a single size is not necessarily such a big challenge for most riders. Relatively minor tweaks to a Motocross bike in terms of handlebar rise, bar mount position and seat height (extra padding) can accommodate a large range of rider heights. Achieving the best possible handling through sorted bike geometry on a Motocross bikes would appear to be an aim that’s placed above all else, fixing things like the wheelbase and front centre. And the fact that bikes from companies like Honda, Kawasaki, Suzuki, Yamaha and KTM all share such similar geometry would suggest that they’ve hit upon a sweet spot. The rider is then able to tweak their position through adjustments to contact points. Why can’t mountain bikes be like this?
 
Handlebar height is something that I believe is critical to handling, but which gets relatively little attention (or at least the attention that it does get doesn’t always seem to be helpful). One man that knows a thing or two about going fast and, crucially, someone who understands a great deal about bike setup, is Fabien Barel. He has had some interesting things to say about handlebar height, suggesting that (perhaps counter-intuitively for us mountain bikers) higher handlebars make it easier to effectively weight the front wheel when descending than lower handlebars. The thinking being that your body position first needs to be stable, and not falling forwards due to too low handlebars, before you can move your weight around with purpose and accuracy. While I’m almost certain that he wasn’t thinking of handlebar heights in the same realm as a Motocross bike, it does at least buck the current trend for lower handlebars. This approach certainly doesn't seem to be slowing Motocross riders down.
 
Given the above, and my earlier comments on stack height, it does strike me as odd that on a size small Specialized FSR 650b the bikes handlebars are around 18mm above the saddle (for an average height rider designed to fit this frame size), while as the frames sizes get bigger the handlebars begin to drop relative to the saddle until you get to a size extra-large where the handlebars are actually about 56mm below the saddle. I have no idea why the taller you are the more you should have to bend over to reach the handlebars, but this appears to have become convention? Again, why bother to have different frame sizes if they don’t change in proportion to people’s physiology (or change very much at all for that matter)?
 
While I can find very little research on how to calculate the optimum handlebar height for a mountain bike, I would suggest that for good handling (if not necessarily the most efficient pedalling position or the most aero dynamic) the handlebars should be no lower than the saddle height and preferably a bit higher than this. Essentially, I would argue that the handlebars should be just about as high as you can get them with the limiting factor being the ability to maintain a comfortable pedalling position. This helps to keep the riders weight centred between the wheels and reduces impacts through the hands coming from the front wheel. After all, if this approach is good enough for Honda’s professional Motocross riders then I would argue that it will probably work for the rest of us.
 
While I think we should be more willing to experiment with different handlebar heights, something that I believe should be sacrosanct is stem length. This is critical to a mountain bikes handling, and shouldn’t be used as a means to ‘fine tube’ bike fit. Instead, it should be viewed as a fundamental component of a frames geometry, just like head angle or bottom bracket height. My preference is for something as short as is practical, not just to keep the handling quick, but to keep the riders weight further behind the front axle. Kirk Pacenti is someone who has some great stuff to say on this subject (here for example).
 
In terms of head angle, I’d be inclined to push the front wheel out to something relatively slack to both help counter the longer chainstays (helping to keep the riders weight centred) and to add stability (requiring less steering input through the hands to keep the bike tracking straight). I think that Chris Porter of Mojo has some interesting things to say on this subject (here for example) and a 66-degree head angle doesn’t sound unreasonable (and, once more, not far off an original beach cruiser). Again, partly because of Chris Porters well-reasoned arguments, I’ve chosen a relatively small fork offset using a stock fork resulting in a trail figure of around 120mm (a similar figure to that found on a Specialized Enduro 650b as well as many Motocross bikes).
 
In terms of wheel size I would forgo the bump-eating abilities of a 29er wheelset for the added agility of 650b with the latest 2.6 inch tyres that are now appearing. As Fabien Barel once said in an interview (here at 09:58), larger wheels are good for carrying speed, but smaller wheels are good for generating speed. The former is possibly more suited to flat-out racing against the clock, while the latter is about being immersed in the challenge of controlling a bike at speed across demanding terrain. It’s the latter that I’m most interested in.

​​​Something that I found quite surprising was that to get my seat height down from my standard pedalling height (an admittedly rather lofty 820mm) to that of a motocross bike (around 590mm) I would need a 230mm dropper post – something that doesn’t yet exist. Rockshox currently have one with 170mm of drop while 9point8 apparently have something with 200mm in the pipeline. Let’s see what the future brings.
 
One day I hope to have the funds and time to find out how a frame like the one described above rides, but a few already come close. Jeff Jones seems to have got nearest with his latest Plus bike. From what I can work out, he hasn’t looked to the world of motocross for inspiration, rather plenty of trial and error and an experience of riding a vast array of different bicycles including tandems and town bikes. However, this bike uses a 29+ wheelset and has gone in the opposite direction when it comes to fork offset. Perhaps unsurprisingly, the Transition Klunker (last available in 2015 I think), which looked for inspiration from those first mountain bikes, also arrives at something similar in terms of geometry, but lacks the stack height.
 
So now all that remains is for me to find enough money down the back of the sofa and (most importantly) to convince my (non-cycling) wife that what I really need is yet another bike. I’m sure that she’ll appreciate my nuanced justification based on the need for a fresh look at fork offsets and handlebar stack heights!