Why mountain bikes are getting longer and longer: We explain why ‘long is the way to go’

Back when 26-inch wheels were the norm, mountain bikes rarely had wheelbases longer than 1,100 millimetres. Today, an extra 100 millimetres – or ten centimetres – is no longer uncommon across all MTB disciplines. Naturally, larger 29er or even 32-inch tyres require more space within the frame. However, the tyres alone do not explain the increase in length. Longer main frames – and in some cases rear triangles too – are a deliberate choice made by modern geometry designers. But why exactly are mountain bikes getting longer and longer? Let’s take a look at the facts.

Conclusion

“Rides smoothly” – we often use this phrase in our test reports. However, stability and agility are not opposites that are determined solely by the wheelbase. A long bike with modern geometry and components simply offers greater directional stability. Steering inputs and reactions to disturbances are slowed down. This is why, subjectively speaking, a long bike feels smoother at high speeds. – Jan Timmermann, BIKE editor

Mountain bikes are getting faster and faster

To understand why mountain bikes are getting longer and longer, you need to look at why they’re getting faster and faster off-road. Anyone who hurtles down a slope at 50 today on a modern trail bike is unlikely to feel terrified straight away. On a bike from the 90s, things might be quite different at the same speed. It would probably feel unstable or ‘jittery’.

The difference is, on the one hand, down to ever-improving technologies and materials. Frames, components and, above all, brakes, tyres and suspension components have become significantly more effective over the last few decades. Larger wheels reduce the roll angle and are less likely to lose momentum on rough terrain. With sturdy components and good suspension, mountain bikes can ride over obstacles without losing much speed. Secondly, there have been huge leaps in development in the area of geometry – which brings us back to wheel size.

The secret behind steering response

If a mountain bike is made longer, the most significant effect is on the steering geometry. With a longer wheelbase, the front and rear wheels are further apart. A flatter steering angle increases the wheelbase if no other changes are made to the geometry. A small change in the steering angle results in a comparatively smaller change in direction on a long bike. A longer bike tends to require more space to achieve the same change in direction and reacts less spontaneously. Put simply: a long bike responds less to a small steering input. With a short wheelbase, even a small movement of the handlebars is enough to change the direction of travel more significantly. This is utilised, for example, on dirt bikes, which are specialised for rotational tricks and the like.

Off-road, there are constant minor disturbances: roots, stones, steps, wind resistance, potholes and the rider’s movements. A longer bike has greater geometric leverage between the front and rear wheels. At low speeds, this inertia can be something of a disadvantage – you have to steer more forcefully to get the bike round a bend quickly. At high speeds, however, it is perceived as a smooth ride. It becomes particularly interesting at high speeds, because the forces acting on the tyre change with speed.

The interplay between trail and offset

​However, the wheelbase alone does not determine a bicycle’s stability. A key parameter is trail. Put simply, it describes the distance between the point where the extended steering axis meets the ground and the actual contact point of the front tyre. Trail creates a self-centring effect: if the front wheel is moved off a straight line, the forces acting on the tyre generate a torque that can pull the wheel back towards a straight line. Greater trail therefore normally results in a stronger ‘self-steering effect’. Combined with a longer wheelbase, this often produces a bicycle that is less sensitive to minor disturbances at high speeds.

Whilst the trail on mountain bikes from the 1990s was usually around 50 millimetres, it has now increased to around 65 millimetres or more. However, the fork offset has also been adjusted at the same time. Some modern suspension forks have relatively large offsets, which allow the trail to be controlled despite the flatter steering angle. Over the last 30 years, bike geometries as a whole have changed significantly. When comparing modern mountain bikes with their predecessors, they are characterised by greater reach, longer wheelbases, flatter steering angles and greater trail. All of this results in significantly smoother handling at high speeds.

What do you think: do you find a long bike more useful on the trail, or do you miss the short bikes of the good old days? Let us know in the comments!