The Full-Moto

Plans for my new bike, which I will be building early in the New Year

Early next year I'm going to build my dream mountain bike frame and it's time for me to start making some tough decisions about the specification. Whatever it ends up looking like, my number one goal is to build a bicycle that maximises my fun in the woods. The bike must give me the confidence to attack a trail and get loose, to be engaging and involving. To achieve this I believe that a bike must do two fundamental things: it must place the rider in a stable position, enabling them to maintain balance while descending; and it must give the rider the best chance of controlling the bike. Both sound pretty obvious, but I believe that these two attributes are compromised on many modern bikes. Firstly, I shall try to explain why before describing what this all means for the bike I will be building.

If the rider is struggling simply to maintain their balance on the bike or their control of its direction and speed then a rider cannot actively and purposefully push the limits, and it is when you can approach these limits with confidence that the fun really starts. This is not a binary situation – it is not simply a case of a bike providing its rider with stability and control or not. Rather, it is a sliding scale, with every combination of bike, rider and trail sitting somewhere along its length. However, I feel that I have always struggled to sufficiently achieve these two things with my previous bikes.
 
Fortunately, both rider stability and control are achieved in similar ways. Stability is achieved when a rider can maintain their balance despite external forces doing their best to disrupt them (such as those generated by riding a technical trail at speed). This has the best chance of happening when a rider is supporting all of their weight through their legs in what is known as an Athletic Stance (please see my previous Blog post for more on this).

For best control, a riders hands must be uncorrupted by the donkey work of having to support any significant proportion of their body weight, with the exception of the odd minor adjustment, leaving them free to focus on the relatively delicate job of steering and braking. So, both good stability and good control start when a rider is able to support their weight fully through their legs rather than also having to rely on their arms.

Importantly, this doesn’t mean that a bike that places a rider in a relatively unstable position cannot be ridden impressively quickly. I have a friend whose mountain bike has a 110mm stem, holding a ‘bar with no rise that’s slammed to the headset, about 150mm lower than his saddle. His bike handling skills are so good that he can compensate for the fact that, on pretty much any downhill, a large part of his effort will be given over to trying to maintain his balance on the bike. I’d love to see what he could do if he were freed from the need to constantly stop himself from falling over the ‘bars. If you need proof of the speed that can be achieved by a good rider, despite riding a bike that lacks inherent stability and control when ridden off-road, then just watch the business end of cyclocross race. What those riders can do is often mind-blowing.

Something that is often overlooked or confused is the difference between a stable bike and a stable rider. What the bulk of the mountain bike industry has been doing for the last few years (and probably longer) is creating ever more stable bikes at the expense of rider stability. Longer wheelbases have been achieved, at least in part, by steeper seat tube angles and a longer reach to the 'bars. This tips the rider forwards, requiring their hands to support an ever-increasing proportion of their body weight more of the time. The resulting shift in weight distribution towards the front of the bike also relies increasingly on the front suspension to absorb feedback from the trail in order for the rider to maintain sufficient levels of control. This has created bikes that can now steam-roller through some pretty rough stuff, meaning that bikes have got faster (good for racing and marketing departments), but also less involving. Essentially, so long as you can hold on to the thing, a modern mountain bike should get you to the bottom of a hill in record time. But the trade-off has been that a good deal of the fun, flow and finesse that comes from skilfully piloting a bike down a trail has been lost.

Interestingly, this runs counter to the geometry of a Motocross bike where, despite around 300mm of front suspension travel, the riders body position is more upright and centred over their feet with the fork doing less, relatively speaking, to isolate the rider. If anything, the closest that mountain bikes have ever got to placing a rider in a similar, stable position were the original Klunkers, repurposed from Schwinn beach cruisers in the 1970s.
 
Based on the thinking described above and covered in my previous Blog posts, the following is an explanation of how I have arrived at the geometry that I will adopt for the frame that I will be building in the New Year.
 
As a starting point, I suspect that a rider’s hands should be placed no lower than their hips if they want to have good stability and control while riding off-road on challenging terrain (a position that I have always struggled to achieve with stock frames). If a rider's hands get much lower than this, then these hands will have to start regularly supporting a significant amount of the rider's body weight. Because of my particularly long legs it just-so-happens that, for me, this results in a handlebar stack height that is the same as many Motocross bikes. It then seems reasonable to also look to Motocross bikes for the reach measurement. This then fixes my standing position on the bike, one which places me in a far more upright position than a standard mountain bike with my weight supported fully through my legs.
 
Having fixed the reach and stack measurements, I have simply used a seat tube angle that results in a stretch to the ‘bars when seated that is slightly shorter than my current mountain bike. My thinking is that a more upright riding position won’t require the same stretch to the ‘bars as something where your hands are lower and therefore required to support much more of your body weight more of the time. Rather reassuringly, this seat tube angle also happens to be similar to that used on the original Klunkers.
 
I’ve chosen a relatively slack head angle based, pretty much entirely, on my own experience, having had mountain bikes with everything from 64 to 70 degrees. I have tried to balance the need to keep things stable, helping to get my hands more ‘behind’ than ‘on-top-of’ the front wheel, whilst retaining some agility. This, combined with the rest of the frame geometry described above, gives me the bikes front-centre.
 
The chainstay length has been derived as a proportion of the overall wheelbase. Again, I have looked to the world of Motocross, adopting a rear-centre measurement that is the same proportion of the front-centre measurement as a Motocross bike. This should then provide a similar weight distribution. If you ask me, the marketing departments of the big mountain bike manufacturers have done a fantastic job of convincing us all that we need the same, short chainstays regardless of rider height. Quite why someone who is 5’ tall should be riding around with the same chainstay length as someone who is 6’6’’ is beyond me. The front centres on these two bikes will be massively different and so the weight distribution will also be completely different. Bonkers. But I digress.
 
For the sort of trails that I ride (primarily the Surrey Hills, Swinley Forest and the odd Welsh trail centre) I believe that a bike that provides me with good stability and control can be created without the need for suspension so long as the geometry is right. I’ve chosen 27.5+ wheels as the plus-sized tyres will provide some extra comfort over standard ones (important given the lack of suspension). And while they might still be lacking in this department when compared to 29+ wheels, I think that this is a worthwhile trade-off for the extra zip that they should bring. After all, this is a bike built for fun rather than mile munching. This is complimented by a relatively large bottom bracket drop that will keep the handling nimble at the expense of some ground clearance. But without the need to account for suspension travel constantly changing the effective bottom bracket height, this should be easier to accommodate.

A comparison of modern bike geometry (black - my XL Last Fast Forward) against my new bike (grey)

​My current mountain bike is an extra-large Last Fast Forward 29er hardtail, with geometry numbers that are pretty typical of many modern mountain bikes (the black silhouette above). With a seat tube angle of 73.8 degrees and a reach of 475mm the rider is pushed further forwards than on bikes with more traditional geometry, increasing the frequency with which the rider will have to support their weight through their hands. As I've already discussed, while this might reduce the stability of the rider, it increases the stability of the bike as a result of a lengthened wheelbase (particularly when combined with this bikes slack, 64 degree head angle). But while outright speed may be easier to achieve with modern bike geometry, the confidence of the rider to control the bike from a stable body position will be diminished. So, I may not be setting any land speed records on my new bike (the grey silhouette above), but the inherent stability that the bikes geometry should give the rider (by enabling them to support more of their weight through their feet more of the time) should provide the confidence to really attack a trail and therefore have more fun.
 
The image at the beginning of this post has been pulled, poked and tweaked for more than a year, and is my current best guess at what the finished bike will look like. This will be an experiment in trying to build a bike that handles in a way that I have always wanted, but never found. Time will tell whether I am right or not. As always, I’d love to know what you think.