GEOMETRY LESSONS
MBUK tester and selfprofessed tech geek Seb Stott unravels the complexities of frame geometry, and explains how you can ensure your next bike fits you perfectly
Get your geek on and learn how frame angles and tube lengths affect the way your bike handles – and, ultimately, how fast and confidently you can hit the trails
Abike’s geometry – its relative tube lengths and angles – defines how it handles far more than its suspension or spec. Over the past few years mountain bikes have transformed in shape, in a bid to improve their stability through technical terrain, making them easier and safer to ride downhill fast, and better at climbing too. But what exactly are these changes, and just how do they affect the ride?
Mountain bike geometry has evolved in two key ways since the early days of the sport: bikes have got longer and steering has become more stable.
The increase in length is partly down to an increase in reach. Longer reach in the frame puts you further behind the front wheel when standing on the pedals and allows use of a correspondingly shorter stem without the cockpit becoming cramped. Shorter stems make the steering more predictable and quicker, while making it easier to get your weight back on steep descents.
Mondraker pioneered this back in 2012 with their Forward Geometry concept, which employed stems as short as 10mm. Since then, most brands have settled on 30-50mm stems for all but their cross-country race bikes, yet reach has continued to grow as riders adopt a more forward riding position.
longer, lower, slacker – faster
At the same time, head angles on trail and enduro bikes have become slacker. This pushes the front wheel further in front of the head tube. In combination with a shorter stem, this puts your hands much further behind the front wheel, which makes it even easier to keep your weight behind the wheel on steep descents, for added stability and confidence.
With a longer reach and a slacker head angle, the front-centre becomes longer, which again puts your weight further behind the front wheel. To a certain extent, bottom brackets have become lower too, and
this also helps with stability. Imagine a line connecting the front contact patch with the rider’s centre of mass. The shallower the gradient of that line, the less the bike will pitch and dive when the front wheel is pushed backwards as it comes up against a bump. So with a modern bike, the rider’s mass is generally further back and lower down, making it easier to ride rough, steep terrain without fear of going over the bars, and without having to push against the handlebar or use a firm fork to stop the front end diving.
pay attention at the back!
But all things being equal, a longer front-centre means less of the rider’s weight is loaded through the front wheel, meaning reduced front-end grip. To cancel this out, the rider must adopt a more forward stance on the bike, applying more pressure to the handlebar. Happily, the longer front end makes it possible to ride the bike in this forward position without worrying about going over the bars or bracing against bumps.
Extending the rear-centre can help compensate for growth at the front, but although rear-centre lengths have increased slightly they haven’t kept pace with the front-centre in percentage terms. The reason for a longer rear-centre is that it provides a more even weight distribution between the wheels to offset any loss of front-end traction. A few bikes even have longer back ends in their larger frame sizes for this purpose.
However, a longer rear end makes a bike harder to manual – for the same reason that more length at the front makes you less likely to get pitched over the bars. Namely, with longer chainstays, the rider’s centre of mass is further in front of the rear axle, so the rider has to move further back to reach the balance point. Bikes with short rear-centres are often described as more playful because it’s easier to get the front wheel aloft, but longer rear-centres make it easier to keep the front tyre gripping in flat turns. On both counts, longer front and rear-centres make more sense for taller riders.
angle of dangle
The wheelbase is simply the combined length of the front-centre and rear-centre. Having a longer wheelbase has disadvantages in tight turns, because the rear wheel takes a tighter arc than the front wheel. This is why a long-wheelbase van is harder to get around a tight corner without the rear wheels clipping the kerb on the inside of the turn. Longer-wheelbase bikes therefore require more proactive riding to get around tight turns, which could include sliding or lifting the rear wheel round the corner (by applying the front brake).
Increased reach creates more room for bikes to offer steeper (a higher number) effective seat angles without the handlebar being too close to your hips. A steeper seat angle compensates for uphill gradients and the fact that the rear suspension sags when climbing, putting you in a more efficient and comfortable position for winching up steep ascents. Plus, a steeper seat angle combined with a longer rear-centre puts your weight further forward when seated, making the front wheel less inclined to lift. A more central seated position also isolates you from bumps that cause the bike to pitch, just like sitting nearer the middle of a seesaw results in less up and down motion.
making the numbers work
In recent years we’ve seen bigger wheels, slacker head angles and, recently, shorter fork offsets. These three factors all increase the amount of ‘trail’ – the distance between the front tyre contact patch and the steering axis. This ‘trail lever’ is comparable to the link that connects a wheel to a shopping trolley and allows it to follow (or ‘trail’) the direction of the trolley.
On modern aggressive bikes, the trail lever is much longer. For example, a traditional 29er trail bike with a 68-degree head angle and 51mm fork offset would have a trail of about 90mm, whereas a modern 29er enduro bike with a 64-degree head angle and 44mm fork offset would have about 122mm of trail. That equates to much weightier steering that’s harder to knock off line in rocky sections and is more inclined to self-centre (return to pointing straight ahead) when riding hectic terrain, especially when braking.
Shorter trail makes the steering lighter but twitchier, particularly when tackling steep, bumpy turns, where the steering can feel like it’s trying to tuck underneath you. You can go too far the other way with trail though – too much will make the steering feel ‘floppy’ and wander from side to side when riding flatter ground.