What size bike do I need?

Expert bike mechanic Toby Hockley explains how to pick the right bike frame for your size and what adjustments you can make at home for a smooth ride.

BARCELONA, SPAIN - OCTOBER 03: An athlete competes in the bike leg of  during the Ironman Barcelona on October 03, 2021 in Barcelona, Spain. (Photo by Eric Alonso/Getty Images for IRONMAN )

What size bike do I need? It’s a question that’s often asked in bike shops up and down the country. Finding the answer is one of the most important things a cyclist can do, as it will improve comfort, endurance, efficiency and speed.

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This applies to cyclists of all abilities and ages, whether a first timer on two wheels or an experienced rider.

It’s also one of the hardest things to do without some advice and guidance from someone who understands both bicycles and the human body. Evans Cycles Technical Training Manager Toby Hockley is on hand to advise.

Is bike size important?

The consequences of riding the wrong size bike can be serious. If the bike is too big for you, it can cause aches and pains in your shoulders and back as you lean too far forward.

This will also cause discomfort and potentially serious problems where you come into contact with the saddle, as your body and sit bones rotate too far forward and cause pressure on the delicate soft tissue in that area.

On the other hand, if a bike is too short or small, you’ll struggle to get the saddle height high enough and this may result in knee pain and a less effective pedal stroke.

Your body will also be compacted by the shorter reach of the bike and this will result in discomfort and poor bike handling.

What bike set-up do I need? 

Cycling on a damp road
Credit: Pexels

When we think of road bikes they generally fall into two categories of geometry, aggressive racing or endurance. It’s important to know your own cycling goals and buy accordingly.

Your own body will also dictate which bike is for you. The long and low geometry of a racing bike will require a great level of flexibility and core strength from the rider, whereas the more relaxed and upright position of an endurance bike is more suitable to the newer or less flexible rider.

As a rough rule of thumb, if you can bend over and touch your toes with ease, you may be able to ride a more aggressive bike. If you can’t do this, then an endurance bike, or some yoga/flexibility exercises, would benefit you in the long term.

How are bikes measured for size? 

Once we have established which bike is the right type for us, we then need to look at which frame size might be correct.

Traditionally, in the days before compact frame design, the humble bicycle was sized as the measurement from the centre of the bottom bracket, along the seat tube, to the centre point where it met the top tube.

The advent of the compact frame did change the world of bicycle frames for the better, but as a result, this way of sizing a bike no longer applies. Sizing is now calculated based on the theoretical connection point if the bike did have a horizontal top tube. In technical terms, this is now known as the ‘effective top tube’.

What bike size do I need? 

Now we understand roughly how bikes are measured, we then need to go to the trusted internet and look at one of the hundreds of sizing charts readily available. This will give us a rough starting size for a bike we might need.

For example, I’m 175cm tall and most size guides suggest I should ride a 54cm bike. I’m also pretty flexible, so have a wide choice of bikes available to me, but it’s worth remembering that some manufacturers will measure bikes slightly differently.

As geometry has changed over the years, so have the start and end points for where the bikes get measured for size. In reality, I have ridden bikes ‘sized’ as 51-56cm. So how do we know the difference from one manufacturer sizing to the next?

The easiest way is to look at their bike geometry charts and see what length the effective top tube length is. If it is close to the size you expect, in my case 54cm, then that is the bike you should try first.

How to adjust my bike to fit me 

Adjusting a road bike
Credit: Alexander Dummer/Pexels

My next step is to find the bike I like in the right size and go and sit on it, as there is no better way to see if it is correct than actually getting on it. The human body comes in all different shapes and sizes, and a good bike sales person will be able to advise on whether a bike is the right size for your unique fit.

First, we want to set the saddle height correctly and this is the easiest adjustment to make. Adjust the height so when the heel of your foot is on the centre of the pedal, your leg is locked out at the knee.

When you place the ball of your foot in this position, you should now have a slight bend at the knee, but if this is not the case then adjust the saddle accordingly.

Our next step is to look at the upper body, with your hands on the shifter hoods there should be an angle of 80-90 degrees. This will be slightly less for an endurance bike and there should also be a slight bend in the elbow.

This particular measurement is harder to adjust as saddles can slide forwards and back on their rails, but with only an inch or two for adjustability. Stems can also be swapped for longer or shorter ones, again only an inch or two.

If these adjustments cannot get you in the right position, then it’s time to look at the next frame size, either up or down depending on your situation.

If you struggle to find the right size bike with this guide, then the next step is to speak to a professional bike fitter. Many of them offer bike sizing services and can help ensure you find the perfect bike.

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To find out some more useful tips on bike fitting, visit Evans Cycles’ bike sizing guide