Modern geometry on a kid’s bike – why it works

To the casual observer a bicycle is a simple machine, and to all intents and purposes it is. But scratch the surface and a bicycle is a complicated combination engineering, design and science… with a bit of art thrown in too. The frame is the heart and soul of any bike, it determines how the bike will feel, who it will fit, its intended use and what the bike will be capable of. The materials and construction methods chosen also effect how the bike rides, but it is the bike’s geometry that is key – traditionally the magic combination of tube angles and lengths. As with anything, trends come and go, and you have to very careful when you start to play around with geometry.

What is geometry?

When it comes to bicycle design geometry refers to the angles, lengths and sizes of all the various parts that make up the main frame and forks of the bike. Most bicycle frames (whether they be road, mountain bike, BMX, etc.) pretty much stick to a standard and recognised set of measurements. All of these measurements are vital, because they all work in combination with one another, but there are some key ones. Head angle is measured in degrees and will affect the handling (steering) of the bike – as a general rule mountain bikers prefer slacker head angles, whereas road riders like it steep for quicker handling. Top Tube measurements are normally in mm and help determine the size of the bike. All of these measurements need to work together to produce a bike that will ride properly, they cannot be seen in isolation.

What do we mean by ‘modern geometry’?

In recent years some forward thinking mountain bike brands have subtly changed the way we look at frame geometry design. Some have gone to extremes, but to put it simply, the tweaks and changes have been made to help centralise the rider on the bike which improves the bike’s handling. Shorter stems, slacker head angles, low bottom brackets, longer reach, and more, all help, but you can’t change one without changing the others. Designers have tried to find the ‘sweet spot’ between a traditional upright sit–up–and–beg bike and a stretched–out road racing bike. These are extremes in the bicycle frame design world, and trying to find that perfect middle ground that suits children has been our goal.

Talking about geometry can get very techy, but let’s look at what we have done to the geometry on our PINTO and SKØG bikes in relation to Modern Geometry. We’ll work from the front of the bike to the rear.

Fork offset

Fork offset is a slightly complicated thing to try and explain, but basically it is how far the front axle is ‘offset’ from the steering axis. We have a comparatively small offset on our fork of 15mm. This (in combination with the list below) makes the steering more stable, which makes the bike easier to balance on, and it also produces less ‘flop” – where the bike can sometimes ‘tuck under’ whilst turning.

Slack head angle

The head angle of a bike is the angle of the front fork against the level horizontal plane. At 67-68º our head angles are slacker than most other kid’s brands. This pushes the wheel slightly further out and forward, which again increases the stability and improves the steering of the bike.

Short stem

This moves the handlebars away from the front axle line, which shifts the weight of the rider back slightly and helps to avoid ‘over the bars’ situations. This works in combination with our longer reach...

Longer reach

The reach on a bike is a horizontal measurement that determines how the cockpit (the area between the seat and the handlebars) feels. You don’t want your child to feel ‘stretched-out’ or too cramped. Lengthening the reach slightly moves the centre of gravity slightly forward and centralises the weight of the rider. The longer reach with a shorter stem is a perfect match.

Wider bars

Our bars are slightly wider than on most kid’s bikes. On the PINTO they are 43cm and on the SKØG 45cm. This gives you improved stability at speed and greater confidence.

Low bottom bracket

Lowering the bottom bracket (the part where the cranks and pedals are attached) also lowers the centre of gravity of the rider, which again improves handling, especially cornering, and helps to make the rider feel part of the bike rather than simply ‘on top of the bike’.

Narrow Q Factor

The Q Factor on a bike is basically the width between the pedals. A narrow (and proportionately small) Q Factor is perfect, and vital, for kids. You don’t want them looking like John Wayne getting off his horse!

Short rear centre

The rear centre is measured from the centre of the bottom bracket to the centre of the rear wheel axle. Many kid’s bikes have massively long rear centres, which can ruin the handling of a good bike. The rear centre on our bikes are nice and compact, meaning that the bikes are quick, agile and stable.

Growing geometry

The unique feature of our PINTO and SKØG bikes is that their frames grow. But how does that affect the geometry as you go through our size changing modes?

Our key aim was to keep the geometry proportionate as the frame grows. As you go from Mode 2 (Small pedal) to Mode 3 (Large pedal) the top tube length grows, so the reach also increases, as does the rear centre – the whole bike gets bigger but the geometry, and therefore the handling of the bike, stays the same. The only significant change to the angles is the seat angle – it gets slightly slacker (leans back), this just gi­ves you a bit more room.