Gear

Beginner’s guide: how to make sense of a bike geometry chart

A bike geometry chart can be a confusing array of numbers and measurements - here’s what they’re for and how to interpret them

When buying a new road bike, you need to take into account a number of factors – regardless of whether you’re an experienced rider or starting out for the first time.

The type of riding you want to do, your experience, your physical conditioning, your general style preferences and budget all play a key role. Often, therefore, it can be hard to narrow down exactly which bike is right for you.

Two vital considerations are the geometry and the size of the bike – buying the wrong size bike can lead to discomfort and injury, which is a sure-fire way to keep you off the bike.

A bike geometry chart can be just a confusing array of numbers if you don’t know how to interpret it (Pic: Canyon)

Before heading to a bike shop to try out a bike, or buying from direct-to-consumer brands such as Canyon and Ribble, a bike geometry chart is a potentially very informative tool, which can help you first ascertain whether the geometry will suit your goals on the bike, and second what size bike you should ride.

However, a bike geometry chart can also be incredibly confusing, particularly as every manufacturer may have its own way of sizing bikes, and each model may have a different geometry. If you don’t know what to pay special attention to, a geometry chart can also be misleading.

So to help with this, we’ve called in two experts, Ben Coates, road product manager of Trek Bikes, and Ben Spurrier, head designer at Condor, to help point you in the right direction toward being able to make sense of a bike geometry chart.

What is a bike geometry chart?

A bike geometry chart is a table, usually accompanied by a diagram of a bicycle frame for reference, of the key measurements of a particular model in a manufacturer’s range.

The information displayed in a geometry will vary from manufacturer to manufacturer, but it will generally always include the main tubes lengths, and the angle of the headtube and seattube. It will also likely include spatial measurements such as the wheelbase (from axle to axle), chainstay length, and stack and reach, which we’ll come on to.

The chart will also show the various sizes of the frames available, so you can see how the size of the frame affects the critical values which dictate if a bike will fit.

Additionally, a geometry chart may show measurements like rake (the distance between the steering axis and the wheel centre) and trail (the distance between the centre of the contact patch of the tyre and the steering axis of the headtube), as these can assist in determining what type of geometry the bike frame has and, potentially, its intended handling characteristics. We’ll cover that later. However, while all geometry charts will show basic information like toptube length and headtube height, not all will show rake and trail.

A bike geometry chart shows the values which dictate a bike’s fit and handling characteristics (Pic: Kinesis)

How useful is a bike geometry chart when choosing a bike?

A bike geometry chart can be useful in a number of ways, including ascertaining your most suitable frame size, complete bike fit, component compatibility and handling characteristics of the bike – if you know how to read the information, Coates says.

“I say ‘can be’ because, for many people, the geometry chart doesn’t tell them anything other than how sizes or models have different numbers,” says Coates. Learning how to understand those numbers will help you choose the right bike for your needs, he says.

As a result, understanding a geometry chart will help put you in the right ball park to determine whether a particular model of bike is suited to be your needs, whether that’s the fit (do you want an aggressive, racy position or a more relaxed, endurace-style position?) or the handling. However, as Spurrier explains, a geometry chart is only truly useful in determining whether a frame is the right size for you if you know your individual measurements – which he says are best arrived at through a bike fit.

“At the very least, you will learn where your saddle and handlebars should be positioned, as well as understand ideal width of bar and crank length,“ he says.

“A specialist bike fit will often involve further investigation into any biomechanical issues you may have, and any long-term injuries or weaknesses, which will also have an effect on the measurements you’re looking for.”

Buying the right size frame is vital in ensuring you are comfortable on the bike (Pic: Mike Cotty)

What key measurements will indicate a good fit?

In speaking to both Coates and Spurrier, it’s clear there are a few standout measurements you should pay attention to and keep in mind when reading a geometry chart and how the measurements will impact on fit:

Toptube length

“This measurement will give an indication of the length of the frame and allow you to calculate what length stem you might need to pair with it,” says Spurrier. This is a key factor in determining the reach, which you can read about in point three.

Headtube length

“This is a vertical measurement of the tube at the front of the frame, and is one of the key factors in determining whether the frame you are looking at will be able to achieve a suitable handlebar height [to match your bike fit],” Spurrier says. Endurance or touring bikes will generally have a taller headtube, to allow for a more upright position, while race bikes will typically have a shorter headtube, for a more aggressive fit.

Headtube length influences reach (below), though handlebar height can be further adjusted according to the length of the steerer tube, which can have spacers fitted above the headtube to raise the handlebar.

Stack and reach

Stack is the vertical distance from the centre of the bottom bracket to the top (and middle point) of the headtube, while reach refers to the horizontal distance from the centre of the bottom bracket to the same point of the headtube. Take a look at Mason Cycles’ geometry table further down the page.

If you know your fit information, stack and reach will be the most important numbers in getting the right size bike. Coates adds: “This is also the easiest way to compare different frame manufacturers sizing. For example, not all 56cm frames fit the same way, so this needs to be taken into account.” Stack and reach are the most consistent indicators of a bike’s fit, he says, and it’s for this reason that most bike manufacturers now include stack and reach on a geometry table.

As a general rule, a race bike will have a low stack and long reach, while a bike with a more relaxed geometry will have a higher stack height and shorter reach. A 55cm Condor Leggero race bike, for example, has a stack of 560mm and a reach of 395mm, while the Condor Fratello, which is pitched as an all-rounder, has a stack of 570mm and a reach of 386mm.

Seattube angle

“Because saddle height is largely adjustable, seattube length is not as relevant as you would imagine,” claims Spurrier. “The angle, however, will determine the positions you are able to achieve with the saddle and, therefore, will affect the reach.”

“As part of a bike fit, you will get a measurement from the centre of the bars or stem, back to the nose of the saddle, which will tell you how far forward or backward you need your saddle to be positioned atop the seat post.”

The angle of the seat tube obviously has a knock-on effect on this, because it dictates how far back the seatpost will naturally sit.

Seattube angle is one of the key measurements on a bike geometry chart, according to Condor’s Ben Spurrier

Should you just focus on these measurements, then?

While basic measurements like toptube and headtube length give an indication of how a bike will fit you, they can’t be taken in isolation as an absolute indicator of whether a certain frame in a certain size will actually work for you. Confused? Spurrier explains with a clearer analogy.

“It’s rare that any two manufacturers will design and build their frames in line with each other’s’ geometries,” he says. “For example, take three pairs of jeans from different brands into a changing room and it is likely that one pair will suit you better than the other two.

“When looking for a bike, it may be that you find a frame you like, with an appropriate length toptube, but the brand favours a taller headtube and thus your handlebars would be positioned too high.”

Again, this is why stack and reach have become a popular way of sizing bike. Coates also points out that while geometry charts can indicate how a bike will fit, it doesn’t necessarily follow that it’ll ride as you expect. It’s important to look at the overall picture, he says.

“A great geometry [that fits you] with either too little or too much stiffness might not ride very well for you,” he adds. “This means there is no hard and fast rule for what makes a great riding bike other than expertise, and trial and error.

“That is why it is so important to make sure you are working with a reputable bike shop and manufacturer,” Coates concludes. “Not all bikes are created equal.”

The information displayed in a geometry table varies significantly from brand to brand - Mason Cycles' is among the most comprehensive and contains all the figures you'll need to determine fit and gauge handling characteristics (Pic: Mason Cycles)

Which measurements on a geometry chart help indicate the handling characteristics of a bike?

As Coates says, the ride quality of a bike is a sum of more than the measurements a bike geometry chart can offer. However, the following numbers can provide an indication of what to expect from a bike’s handling.

Front-centre length

This is the measurement from the front axle to the bottom bracket, and helps to indicate how direct your inputs to the handlebar will feel. It therefore dictates how responsive the front end of the bike will be both while steering and when leaning on the ‘bar when out of the saddle. It can also have an impact on how direct road vibrations can feel through the bars.

Rear-centre length

This is the measurement from the rear axle to the bottom bracket, and has a similar effect as the front-centre measurement. The longer it is, the longer and slower the bike will generally feel to respond from power inputs and with regards to handling.

Trail

“A lot of people talk about headtube angle but in fact, it’s the headtube angle and the fork rake (the distance between the steering axis and the wheel centre) combined that influence the way the front end of a bike handles,” says Coates. This combined measurement is called the ‘trail’. In essence, more trail will be more stable while less trail will usually result in quicker handling at the front end of the bike. Trail figures will typically range from 50mm to 70mm.

Chainstay length

Just as more trail leads to more stable handling, more length at the chainstay does likewise. “Typical ‘race’ bikes have a chainstay length of between 405-415mm, while ‘endurance’ bikes are usually greater than this at around than 420mm,” explains Coates. To illustrate this, a 56cm Trek Madone race bike has a chainstay length of 410cm, while a 56cm Trek Domane endurance bike has a chainstay length of 420mm. The wheelbase also tells a similar story – an endurance bike will have a longer wheelbase than a race bike.

“As a rule of thumb, longer is more stable and less nimble,” Coates says, “and it’s usually an indicator of the frame’s tyre clearance too.”

Bottom bracket height

The lower the bottom bracket, the lower centre of gravity the bike has. This results in more stable handling as you steer and lean into corners. There are limits to this on the road because of the need for clearance to pedal while cornering. Bottom bracket height is usually designed in tandem with the trail to build a balanced bike.

Bottom bracket height is normally measured as bottom bracket drop – the vertical distance the centre of the bottom bracket lies below the rear hub. Again, using the Trek Madone and Domane as an example: the racier Madone has a BB drop of seven centimetres (on a 56cm frame), while the equivalent Domane has a BB drop of 7.8cm, contributing to the more stable handling expected of an endurance bike. The Madone’s higher bottom bracket will help racers pedal through corners.

Bottom bracket height and chainstay length will have an impact on how a bike handles (Pic: Factory Media)

Handling characteristics are generally defined by the sum of all these parts and they will all have the same impact, be it applied to an aero, climbing or endurance bike.

However, as Spurrier explains, the overriding key, when it comes to handling characteristics, is that most of us need to initially look past the numbers, and just get on bikes and ride them. It’s why having the opportunity to test ride a bike (or a number of potential choices) can be important in choosing the right machine.

“Only once you have tried and tested a few, can you go back and compare how the various angles affected how they handled,” Spurrier says.

The best bet when it comes to getting a feel whether a bike worth for you, is to take it for a test ride (Pic: Factory Media)

Conclusion

Bike geometry charts are important reference points for both the dimensions of bikes and their inherent handling characteristics. However, this depends on your experience level as a road cyclist, your knowledge level of your own requirements, and how you balance other factors as well, such as your desired ride type.

Coates remains clear, though, that while a geometry chart can show how a bike is intended to handle, the numbers which relate to fit are the most important.

“Those are the most important because if the bike does not fit right, it doesn’t matter how it handles,” he says. “A great fitting bike with less-than-optimal handling beats a poor fitting bike with optimal handling, any day.”

As a result, while a bike geometry chart can show you the intended potential handling characteristics of a bike, it’s the way the bike fits you and your individual needs that’s the most important thing – and there’s no substitute for trying out the bike to see how those different measurements and angles influence your riding.