Geometry lesson — it’s not just about the angles

Geometry lesson — it’s not just about the angles

Often we’re asked, “what’s special about a SwiftCarbon bike?” Our standard and simple answer is “the ride quality”. Explaining why is another thing altogether though, with the infinite number of variables in designing a frame. There is no magic formula, but the geometry charts of our bikes go a long way to explain how they behave – the balanced and neutral handling that makes the machines a pleasure to ride.

We could never pretend to have discovered a loophole in the laws of physics that we’ve been exploiting to get the magic balance between responsiveness and stability (that the Ultravox is known for). Modern bike frame geometry is a result of a long evolution – a century of research and experimentation that would need a 3000-word footnote to acknowledge. Still, when we started out there was still room for improvement on established ideas and relatively small changes could have a significant effect. Our ‘tweaks’ were made to our tastes, after combined several decades of racing and working on bikes. Taking into account all the factors below, we’ve aimed to crack that perfect combination so a SwiftCarbon bike feels like an extension of its rider.

It’s important to note there are two conversations to be had around the topic of geometry. One is about handling – in this description – and the other about fit (we mention this briefly in the end).

Steering response, wheelbase and bottom bracket height: these are the three major factors that have the greatest influence on the ride quality of a bike and are based, in no small way, on a rider’s perceptions of handling and mindset after throwing a leg over a race bike. You’ll see how the human element keeps coming up more and more.

Steering response

Trail, wheel flop and tyre width and their interdependent relationships gives us our first clues of the handling. Note the ‘response’ part, because the word ‘steering’ alone seems only to refer to the moving of the handlebars. It’s more complex than that (another recurring theme here). It’s also about the rider leaning and steering with their body with the saddle (so it’s actually your butt doing it too) and how the bike responds to it.

Trail is a combination of head angle and fork rake, calculated by drawing a line down through the centre of the head tube to the ground, then measuring backwards to the tyre’s contact patch (with wheel diameter and tyre volume also influencing this value). Check out the diagram for a clearer picture. The greater the trail value, the more stable the bike. A lesser trail translates to faster, more direct steering. More direct steering may feel fast on a quick test ride outside the bike shop, but the faster you go, the more you’ll appreciate the balance.

Wheel flop refers to the front axle dropping when turning the bars. With a word like ‘flop’, you’d think it’d be something to avoid at all costs. However the greater the wheel flop value, at low speed, the faster the bike turns. It’s possible to alter the wheel flop and still maintain the same trail value by changing the head tube angle and fork rake. We aim for a balance – not too much that the steering is overreactive to input, not to little so the bike still feels responsive.

Tyre width also has an effect on trail and wheel flop. Today we see higher volume tyres specced on bikes and with their typically lower pressures and greater contact patch, there’s a greater perceived effort required to turn the bars. So when we are designing a frame that’ll use a higher volume tyre, we’ll need to bear this in mind. Of course, on rougher, unpaved surfaces, we like a little extra stability, but whether it all evens out is up to some good old real world testing to decide.


Segwaying neatly from the topic of weight distribution, we tackle wheelbase, which is all interlinked with the ‘front centre’, chain stay length and the highly dynamic system of rider and bike. A shorter rear centre places the rear wheel more ‘underneath’ the rider, which translates to responsiveness, but possible skittishness on rough corners and a feeling of imbalance. A longer wheelbase (found more on touring bikes) means a larger turning circle and more stability. Just the right amount? That’s up to the testers…

Bottom bracket height

We usually call this BB drop, simply because BB height is influenced by the tyre volume, and it’s easier to work with constants. Let’s leave the title as ‘height’ though because it best describes the key point here: how high the rider sits on the bike. Closer to the ground (not too low or the rider will strike a pedal while applying power in a corner), a rider gets the sensation of more stability. In practical terms, it’s a nuance, and it’s just as important is how a rider is positioned between the front and rear wheels — balance and weight distribution is critical. While this is more up to the rider and their position on the bike, it’s up to us to pay close attention to fit (balancing the seat tube angle, front centre and stack).

With all this in mind when designing, we aim for a neutral and gradual turn-in as the rider leans deeper into a corner, with no sudden ramp ups in the steering. For most, leaning the bike is a subconscious action, so the bike should inspire confidence, in turn allowing more intuitive control. To a point, this characteristic can be predicted in the geometry, but the rest is up to the field testing. Again, combining some basic science with instinct. And of course we haven’t yet discussed how the ride quality is influenced by carbon layup and tube shape design…

Still, all said and done, probably the most important element when it comes to how well a bike handles is fit — it’s critical to be on the right sized frame. Most experienced riders know enough to select correctly, but if there’s any doubt, here’s a shoutout to our growing network of Partner Shops – now in the UK, Portugal, South Africa, Chile, Brazil, Korea and Thailand. They’ll be happy to help. There’s also our handy chart, and if in doubt, give us a call.