Seat tube angles. A few years ago they were barely worth a mention in a bike review, and now entire paragraphs are being dedicated to a seemingly minor geometry figure. What gives? Are bikes so good now that reviewers are obsessing over details that barely matter? Not exactly. To set the stage, let's go over the changes that have happened over the last decade regarding bike fit.
There was a time when bike sizing was almost solely dictated by seat tube length, similar to how road bikes are measured. You'd figure out what length seat tube allowed you to show just the right amount of post and call it good, maybe going with a longer or shorter stem to fine tune the fit. It was the arrival of dropper posts that really started to shake things up – seat tubes no longer needed to be as long, and riders could easily fit on multiple bike sizes, especially in more recent years as posts with 170 and even 200 millimeters of drop have hit the market.
With seat tube length no longer as relevant, and shorter stems paired with wider handlebars becoming the norm, a new number started appearing in geometry charts: reach. If you draw a vertical line upwards from the bottom bracket, and then measure from that line to the center of a bike's head tube, that's your reach number. It's a way to get an idea of how a bike will feel when you're standing up out of the saddle – your typical descending position.
I've heard people say something along the lines of, “Reach is the only measurement that matters,” but that's not really the case, especially if you're planning on spending any time sitting down while you're out on a ride. Reach is a useful number, but a bike's effective top tube length is still worth paying attention to when trying to figure out what size bike to go with. Due to the range of seat tube angle measurements, it's entirely possible to have two bikes with the exact same reach number, but with very different top tube lengths. That means that while the bikes may feel similar while standing, your seated climbing position could be extra stretched out on one, and almost cramped on the other.
The Whyte S-120 and Pole Machine have the same reach number, but the seat angle gives them very different seated pedaling positions.
For instance, take the Whyte S-120 and the Pole Machine. Yes, one's a trail bike and the other's an enduro monster, but put that fact aside for a moment. A large S-120 has a reach of 480mm, the same as a medium Pole Machine. But when you look at top tube lengths, the Whyte measures 640mm, while the Machine measures 607mm. That's a big difference, one that's immediately noticeable when you're sitting down and grinding up a hill, and it's created by the fact that the Whyte has a 75-degree seat angle versus the Machine's extra-steep 79-degree effective seat angle. There's also the fact the Machine's actual
seat angle is quite steep as well, at 78-degrees.
Before we go too deep down this rabbit hole, it's worth pausing for a minute to go over the difference between actual and effective seat tube angle. Effective seat tube angle is what you'll typically see in a bike's geometry chart, and it's calculated by figuring out the angle of a line running from the center of the bottom bracket to a certain saddle position, typically one that's parallel with a line drawn horizontally from the head tube. Of course, that saddle position isn't going to be the same for all riders, and the effective seat angle is going to get slacker the higher the seat gets, but it's a point of reference, a way to compare apples to apples when looking a multiple bikes' geometry numbers.
Actual seat angle is just that – the angle of the seat tube on the frame. It's useful because it makes it possible to get an idea of how much the seat angle will change as the post is extended. On the aforementioned Pole, the actual seat angle is 78-degrees, which means that the starting vs final angle of the seat doesn't change all that much, while on the Whyte it's much slacker. So why not skip using effective seat angle altogether? Because not all seat tubes start from the same point in relation to the bottom bracket, which means the actual seat tube angle isn't as useful in figuring out what your pedaling position will feel like.
That brings us back to the original question – why is everyone obsessed with seat angles all of a sudden? It's pretty simple, really. Bikes have gotten significantly longer over the last few years, and pedaling around a long, slack bike is much more challenging if you also have a slack seat angle. Slack head angles = good. Slack seat tube angles = not that good. A steeper seat angle puts you in a more centered position for climbing, and it's easier to shift your weight forward and backward as needed, compared to feeling like you're going to loop out any time you start climbing. Of course, you can have too much of a good thing, and really steep seat angles can make a bike's cockpit feel too cramped, even if the reach number seems long. Sliding a seat forward or back on its rails is the obvious solution for fine-tuning a bike's fit, or going with a slightly longer or shorter stem, but there are limits to both of those methods.
It can all seem overwhelming, but it's not worth losing sleep over trying to decide which size bike you should buy. Studying geometry charts and brushing up on the relevant terms will only take you so far - the next step is to attend demo events and test ride whenever possible, ideally trying both sizes if you're not sure which one will be the better fit. In the end it all comes down to personal preference; just remember to make your decision based on what works for you and your riding style, not what someone in a bike shop or on the internet decides is best.