BIKE GEOMETRY 101

Mountain Bike Rider|July 2020

More than just fi gures on a spreadsheet, a bike’s angles and dimensions are the key to the way it rides; understanding how they interact will help you choose the right machine and unlock its potential
Alan Muldoon
Bike geometry can be daunting, but it doesn’t need to be. And by looking at it through the lens of weight distribution (bike plus rider), we can simplify geometry dramatically without having to dumb it down. Step by step, we’re going to take a closer look at all the key aspects of bike geometry to see how they impact the rider’s weight distribution. After all, we are looking at a system, and without considering the rider, the numbers make much less sense.

In many ways the individual aspects of bike geometry can be seen as the building blocks on which handling is created. And in the same way that you learn your ABCs before writing complete words, understanding how all the seemingly independent aspects of bike geometry change our weight distribution, will allow you to better understand how and why your bike rides the way it does.

And don’t think for a minute that the purpose of this feature is to transform everyone into bike engineers. It’s more about inspiring you to think about what’s going on beneath you as you ride. And once you start to notice where your weight is on the bike, and the effects it’s having on handling, you can then think about changing it.

So regardless of whether we are talking about frame angles, sizing or fit, understanding how subtle changes to the geometry affect your weight distribution is key to unlocking your full riding potential. And understanding bike geometry isn’t even the ultimate goal here. It’s actually about how geometry helps us achieve a balanced, centred riding position on the bike. And this is of utmost importance because it can dramatically open up your range of movement, which makes everything easier and less fatiguing.

So in the first part of this series on bike geometry, we’re going to walk through all of the key aspects, from head angle to chainstay length, and how small changes in these numbers can have a big impact on the ride quality of your bike. And because we’re looking specifically at frame geometry and how it impacts the position of the rider’s centre of gravity (CoG), it makes sense to start from the ground up. In part two, we’ll look at how we can move that CoG by making adjustments to your bike.

WHEELBASE

The wheelbase measurement is the foundation on which all other aspects of bike geometry are built. In simple terms, it’s the horizontal distance between the contact patches of the front and rear tyres, or the axles, if both wheels are the same size. And to really understand how the different aspects of geometry impact handling, we need to think of the wheelbase as the bottom of a triangle with the rider’s centre of gravity (CoG) at its apex. Using this simple model, it should be clear that, for a given centre of gravity height, having a short wheelbase makes the triangle less stable, while increasing the wheelbase makes the triangle, and subsequently the bike, more stable.

It’s also a great way to visualise how raising or lowering the rider’s CoG is the opposite side of the same coin when it comes to stability. We’ll dive deeper into this when we discuss BB height, but for now, let’s stay focused on the wheelbase.

It should be clear from the geometry chart that the wheelbase measurement is the sum of the chainstay length and front centre measurement. The ratio of both measurements determines, to a large degree, the weight distribution of the bike and rider combined. Now, if we go back to the simple model of the triangle, we can see that shortening the rear centre shifts the rider’s CoG rearward while reducing the front centre moves it forward.

To see how this shift in weight distribution impacts handing, we just need to take a look at early 29ers. To keep the overall wheelbase from increasing – with the associated increase in chainstay length that was needed to accommodate the bigger rear wheel – manufacturers shortened the front centre by reducing the reach and steepening the head angle; both changes moved the rider’s CoG forward. It’s hardly surprising then, that in addition to the improved rollover of bigger wheels, two virtues often extolled by early 29er coverts were the ability to keep the front end down on steep climbs and the improved front-end grip on flat corners. Both traits were a direct result of the geometry shift, rather than the actual wheel size or the length of the tyre’s contact patch. It’s also why shorter stems made the bikes handle better, as they allowed riders to get their weight back more easily for cornering and descending.

Now let’s change gears for a second. If you try and visualise this in terms of an articulated lorry, or even a long-wheelbase van, you’ll understand that changing the wheelbase also affects turning circle. But this analogy is far from ideal, as vans and lorries don’t lean over to turn and if they do, you’d better have your seatbelt buckled. So while it’s often stated that a shorter chainstay length makes a bike corner better, it should be obvious that the same can equally be said of a short front end, as it’s actually the overall wheelbase length that matters most when it comes to more upright steering. It’s the simple reason why longer-wheelbase bikes are harder to navigate around flat switchback turns, as it’s the one time where you do more of your steering with the handlebar, and less by leaning the bike over.

BOTTOM BRACKET HEIGHT

Of all the geometry measurements, BB height is probably the easiest to get your head around. It’s easy to measure too, as it is the vertical distance from the BB centre to the ground. And while it would be easy to assume that BB height is just about optimising pedal clearance on different-travel bikes, to do so would miss the bigger picture. That’s because raising or lowering the BB height directly impacts the single heaviest component part on the bike... you, the rider. Which is why small changes in BB height can have a big impact on handling.

Now, if we return to the simple triangle analogy that we introduced to help understand the interplay between wheelbase and stability, it should be obvious that, if we lower the BB height, and subsequently the rider’s CoG, we can make the bike more stable without altering the wheelbase. In fact, up until recently, that’s been the primary reason why any bike with a geometry-adjust flip-chip, for changing the BB height, would promptly get slammed into the low setting and left there. But with the advent of genuinely long, low, slack bikes, the high geometry position can be really useful, and even desirable, for adjusting the handling.

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