A bike’s geometry is perhaps the most important aspect of its design. Below, we define the important measurements that dictate the shape, fit and handling of a mountain bike, and explain how they affect the ride. We’ll start with the basics, including their less obvious aspects, before discussing some rarely mentioned but equally important geometry topics.
Seat tube length
Seat-tube length dictates the maximum and minimum rider height, making it an important reference point when comparing geometry Jack Luke/Matt Orton
The distance from the centre of the bottom bracket to the top of the seat tube.
The seat-tube length defines the size of the bike in a more meaningful way than the ‘Small, Medium or Large’ size structure. This is because it dictates the minimum and maximum height the saddle can be set, and therefore the height range of riders who can comfortably ride the bike, or how low they can drop the saddle for descending.
Two Medium frames, for example, will often have different seat-tube lengths that will fit different riders. Important measurements for handling and fit, such as reach, must be compared to the seat-tube length to define how long the bike is, relative to the height of the rider.
The ratio of reach to seat-tube length is particularly useful.
Effective top tube length
The effective top-tube length relates to how the bike will fit when in the saddle Jack Luke/Matt Orton
The length of a horizontal line drawn from the top of the head tube until it meets the centre of the seatpost.
The effective top tube (ETT) provides a better idea of how roomy a bike will feel when you’re sitting in the saddle, rather than using the basic top-tube measurement (from the top of the head tube to the top of the seat tube). Taken along with the stem length and offset of the saddle, it provides a good approximation of how stretched-out the bike will feel to ride in the saddle.
Stack determines the minimum bar height relative to the bottom bracket. It’s inter-related to the reach Jack Luke/Matt Orton
The vertical distance from the centre of the bottom bracket to the centre-top of the head tube.
This determines how low the bars can sit relative to the bottom bracket. In other words, it determines the minimum bar height. Stack also has an important but rather unintuitive relationship with reach.
Reach is the most useful fit measurement Jack Luke/Matt Orton
The horizontal distance from the bottom bracket to the centre of the top of the head tube.
Of all the commonly available numbers in a bike’s geometry chart, reach provides the best impression of how a bike will fit. It defines how roomy the bike will feel, for a given stem length, when ridden out of the saddle. There is one small caveat to that though, and it’s to do with the stack height.
Take two identical bikes, then make the head tube of one bike taller, so it has a higher stack height. Now if you measured the reach of those two bikes, the one with the extended head tube would measure shorter. That’s because the head angle is not vertical — so, the longer the head tube, the further back the top of it becomes, and so the shorter the reach. But if you used spacers on the original bike, such that the bar height was the same, both bikes would feel identical to ride.
This demonstrates how reach measurements are affected by stack height. When comparing reach between bikes remember the one with the higher stack height will feel longer than its reach figure would suggest.
The easiest way to measure reach is to butt the front wheel against a wall, then measure the distance from the wall to the bottom bracket and to the top of the head tube, then subtract.
Down tube length
Down-tube length is a handy alternative to reach, which doesn’t depend on stack in the same way. It’s easier to measure too Jack Luke/Matt Orton
The distance from the centre of the bottom bracket to the centre of the bottom of the head tube.
Like reach, down-tube length provides an indication of how roomy the bike will feel, but it’s easier to measure.
In much the same way as reach is affected by stack height (the difference in height between the bottom bracket and the top of the head tube), down-tube length is affected by the difference in height between the bottom bracket and the bottom of the head tube.
This means down-tube length is only useful when comparing bikes with a similar wheel size and fork length — such that the bottom of the head tube is at roughly the same height. In this case, down-tube length can be a more useful (and measurable) number than reach.
Front-centre affects how far behind the front axle the rider’s weight is likely to sit Jack Luke/Matt Orton
The horizontal distance from the centre of the bottom bracket to the front axle.
The longer the front-centre, the less prone the bike will be to pitching forwards when faced with large bumps or hard braking. This is because the rider’s weight will naturally sit further behind the front contact patch.
For a given rear-centre length, a longer front-centre reduces the proportion of the rider’s weight supported by the front wheel. This can reduce front-wheel traction unless the rider moves their riding position forwards, or the rear-centre is made longer too.
The rear centre, combined with the front-centre, determines the natural weight balance of the bike Jack Luke/Matt Orton
The horizontal distance from the centre of the bottom bracket to the rear axle (aka chainstay length).
Because the front-centre is usually significantly longer than the rear-centre, mountain bikes tend to have a naturally rearward weight distribution. This can be countered by the rider consciously putting pressure on the bar, but it can be uncomfortable. The ratio of rear centre to front centre length defines the front-to-rear weight distribution before this pressure is applied to the bar.
A short rear-centre relative to the front-centre requires the rider to weight the front wheel through the hands to achieve a balanced weight distribution Immediate Media
Longer rear-centres therefore make it easier (less tiring) to achieve a more balanced weight distribution, which benefits front wheel traction in flat corners. On steep descents, the weight distribution becomes more front-biased anyway due to braking, so a long rear-centre becomes less advantageous.
However, the longer the rear-centre, the more the rider’s weight must be lifted (with the bottom bracket) to lift the front wheel. A shorter rear-centre therefore reduces the effort to manual, but increases the effort required to properly weight the front wheel through the bar.