Choosing the right frame size is one of the most important decisions you make when buying a new bike. Frame size affects the comfort, agility, stability and all-round handling of the bike – arguably more than the differences between two comparable models from competing brands. Here’s advice that’s been repeated a thousand times: don’t be tempted by a bargain on a bike that’s not the ideal size.
Check the chart
So how do you choose the right size? Luckily, these days it’s usually incredibly easy. Go to the website of the bike you want to buy, click on the Geometry/Size section and see what size the manufacturer recommends for your height. That’s it. It may sound too simple, but these days manufacturers have gotten to a point where sizing recommendations are a good guide for most people.
However, there are two potential problems with this: what if you are on the border between two size recommendations from the manufacturers chart, and what about older (used) bikes?
What if I’m between sizes?
If you’re between sizes based on your height, you have a decision to make. You should be able to ride both without problems, but generally with a smaller size you get a more lively ride that’s suited to tighter terrain, lower speeds and a more playful riding style; A larger size offers more stability with less likelihood of tipping forward under braking or riding over bumps, which suits faster, rougher terrain or a more “on point” riding style.
In the video above I compare two sizes of the same bike in terms of ride feel and against the clock. For me there wasn’t really a difference in how fast I could go and it’s surprisingly easy to adjust to both, but one thing I noticed is that the larger size made more sense once I knew the track and was up to date. So if you typically ride trails that you know well, that may tip the scales for a larger size, while if you frequently explore new trails, the agility of a smaller size might make more sense.
Ultimately, it’s best to try both sizes and see which you prefer. If that’s not possible, trying a number of similar bikes can help. When you find a bike that suits you well, make a note of it to reach Number – this is the horizontal distance between the top of the head tube and an imaginary vertical line drawn through the bottom bracket – which can usually be found on the manufacturer’s website. While it’s not perfect, it’s the single best number to gauge how long a bike feels when ridden. Once you get a feel for how much range you want, you can apply that knowledge to the bike of your choice.
Note that the reach is only a measure of the frame – it does not take into account the length of the stem, the height of the handlebars or the number of spacers under the stem. All of this affects how big the bike feels to ride. So make sure the bikes you ride have the same stem length as the bike you plan to buy and a handlebar height that works for you.
Also, the reach alone doesn’t tell you how far the handlebars are in front of the saddle; that also depends effective seat tube angle, which is the angle of a line drawn from the bottom bracket to the top of the seatpost. The seat tubes have become significantly steeper in recent years, which moves the saddle forward towards the handlebars. This means that a modern, long-reach bike doesn’t feel very long or stretched when seated. The effective top tube length – that’s the length of a horizontal line drawn from the top of the head tube all the way back to the seatpost – is arguably a better measure of how roomy the bike feels when seated. However, because the saddle can be slid back and forth on the saddle rails to adjust the distance between the saddle and the handlebars (with a fixed reach), it’s usually better to focus on reach when choosing a bike.
What about older bikes?
If you are buying an older bike – by that I mean one made before about 2015 – the above may not apply.
In the early days of mountain biking, sizing was based almost entirely on the seat tube length, or the distance from the bottom bracket to the top of the seat post clamp (usually measured in inches). Larger riders needed a greater Frame to get their saddle high enough and smaller riders needed a shorter frame to get the seat low enough but horizontal length of the bike (which is crucial for stability and handling) was almost an afterthought and changed little between the smallest and largest sizes.
Throughout mountain biking history, this has gradually shifted to the point where long and travel-adjustable dropper seatposts allow most riders to fit a range of sizes without even having to worry about seat tube length . But if you’re considering buying an older bike, most experts would now agree that the reach and wheelbase will be on the short side (especially for taller riders), making for handling that’s more likely to be described as “patchy”. would become. or “terrifying” instead of “agile” or “funny”.
So in this case, it’s usually worth increasing the size if you can. For example at 191cm or 6’3″ I choose an XL or sometimes a Large on modern bikes, but on bikes from a couple of years ago I would look for an XL or XXL. The limiting factor for the size above is usually the seat tube length – this is because older bikes have long seat tubes and short reach numbers.
To find out if the seat tube is too big, use a bike where the saddle height is the correct height for pedaling, and then measure the distance from the seat rails to the center of the bottom bracket. Now subtract the seat tube length of the bike you want to buy. The number you are left with is the “collar to saddle rail distance”, i.e. the space left for a dropper post. You can then enter that number into OneUp’s calculator to find out which is the longest seatpost that will fit.
