Bike size tips: How to find the right size for you

Bike size can be hugely influential on how a particular bike feels to ride. It can be relatively easy to get in the right ballpark when it comes to size, but with many bike sizes having a degree of overlap you could find you’re able to fit on two or even three different-sized frames.

So GCN’s James Lowsley-Williams has compiled his top tips to help you know what to consider before buying your next bike.

What is frame size?

Unfortunately, there is no one-size-fits-all approach to frame sizes as brands often use different scales to display frame sizes. Some brands use an XS-XL or XXL scale whereas others use a scale rated in centimetres which typically denotes the length of the seat tube.

Bikes that use an XS-XL type scale can easily be converted into the centimetres scale by looking at the bike's geometry chart, usually displayed on the bike brand's website, and finding the quoted seat tube length for any given size.

It isn’t always quite that simple though as the size of a bike is made up of more than just the seat tube length. The reach, stack and effective top tube length all have a big role to play in how a bike feels. It is quite possible for two bikes with the same quoted frame size to feel different based on other determining factors.

© GCN

The reach, stack and effective top tube length of a bike all have a part to play when it comes to bike sizing

What happens if I ride the wrong size bike?

Bikes are very adaptable with plenty of scope to adjust the contact points to dial in the perfect fit. With this in mind, it is important to understand that bikes are designed around an optimum range.

If you find yourself riding a bike with a frame that is too small you’ll need to have a lot of seat post exposed and fit a longer stem to the bike. This will not only affect the handling of the bike but also put you in a more aggressive position that might not be sustainable out on a ride.

Likewise, with a frame that is too big, you will end up having your saddle very low relative to the bars which will put you in an upright position. Fitting a shorter stem can also make the steering feel very twitchy, especially at higher speeds.

It is hypothetically possible to make a frame that is the wrong size fit but it will come at the detriment of some of the other characteristics of the bike.

Read more: Five of the most common bike fit mistakes you should avoid

How do I find the right frame size for me?

© GCN

Checking in on saddle height, saddle position and stack are all important factors in getting your bike size right

There are a few simple areas to pay attention to that should help in finding your perfect bike size.

Saddle height

The first place to start is to ensure you can achieve your ideal saddle height with the frame and seat post provided. While it is possible to fit a longer seat post in the frame, as a general rule replacing the seat post to achieve your optimal saddle height usually means the frame is too small. Looking at the manufacturer's geometry chart should give you an idea of the maximum saddle height each frame is designed around.

Saddle position

To compensate for a range of riders the seat post of a bike is at an angle relative to the horizontal plane. Typically this is in the range of 70-75 degrees. This angle means that as you raise your saddle higher it has the added effect of increasing the reach of the bike – the measurement from the saddle to the head tube. If you find yourself at the upper limit of the saddle height range you might find that the reach grows too far or that your saddle is positioned too far behind the bottom bracket for you to get in an optimal position.

The stack of a bike

Something else to consider is the stack of a bike. This is the name given to the vertical measurement between the bottom bracket and the top of the head tube. The stack of a bike will dictate how aggressive the bike will feel. A smaller stack measurement will put the front of the bike in a lower position whereas a bigger stack measurement will provide a more relaxed and upright position. You can use different angle stems and stem spacers to compensate to some extent. However, much like with saddle height, if you are having to adjust to this extent it could be a sign that the bike’s not the right size for you.

© GCN

The stack is the vertical measurement between the bottom bracket and the top of the head tube

Use an online geometry database

If you have a bike you know fits you or if you want to check how one bike compares to another, a good resource to use is an online geometry database such as Geometry Geeks. This will display a bike's complete geometry that can be compared to any other bike in any size. This can be particularly useful if you’ve had a professional bike fit and you want to find a bike that you can replicate this position on.

Should I size up or size down?

If you find yourself perfectly between bike sizes and have the option of two frame sizes, the one you decide to go for is a matter of preference. Going for the smaller bike will provide a racier, more aggressive fit with a slightly shorter wheelbase for a snappier ride. Alternatively, the larger frame size will be more relaxed and stable.

Finding the right size bike is crucial for your enjoyment on the bike but also to prevent any long-term injuries from developing due to a poor setup. If you are in any doubt about the right bike size for you try attending a demo day, visit your local bike shop or ask to try out a friend’s bike to get a better feel for what you need.

For more on how to get the right gear for you head over to our dedicated buying advice section of the GCN website.