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Updated: Jan 9, 2023

Bike sizing isn't quite as simple as it seems. This post will discuss some of the problems I’ve encountered as a professional bike fitter with the state of bike sizing over the past 10 years.

One of the most challenging things I experience is fitting people on the wrong size bike. The reason riders end up with the wrong size can vary; from receiving a hand-me-down, to mistakenly selecting the wrong size, or the more unfortunate cases where they have been advised by a retailer. As the bike industry moves increasingly towards a direct-to-consumer model, it is also getting harder for consumers to confirm the ideal size of a given model. This post represents part of a series of posts on why bike sizing is broken and how it can be fixed. In this post I’ll outline what I feel are the key issues with sizing bikes in today’s market. the next one will take a look into the data collected on the current state of bike frame geometry and sizing. Finally, in the last part I'll present our contribution to solving the problem. Before we dig in, let's start by clearing up the difference between bike sizing and bike fitting.

Bike Sizing is the selection of the appropriate bike (size) to accommodate a rider's proportions and position. Bike Fitting is the adjustment of the bike's touch points (saddle, handlebar/hoods, cleats/pedals) to the optimal position for the rider.

Logically there is an element of both the rider’s proportions and their target position when selecting the optimal size of bike. Bike sizing is one of those things that may seem quite simple, yet is so often done poorly. Perhaps the key issue often overlooked when sizing a bike is that there is an element of position. A bike can in theory be the right "size" but not accommodate the target position for that rider. So, what is wrong with bike sizing? There are five main ways people currently select a size of bike:

  • Manufacturer Size Charts

  • Universal Sizing Calculators

  • Geometry Charts

  • Store Test rides

  • Fit First Service

Each of these methods have their own pros and cons, but only one of them really solves the problem of sizing on its own. Let’s explore how each both contributes and falls short of providing a good solution to selecting the right size bike.

Manufacturer Size Charts Sizing charts are the most basic way to size a bike. Typically, for each bike model, a manufacturer will provide a recommended range of rider heights, and sometimes inseam lengths, for every size they offer. In this case a bike size is selected because the rider falls into the recommended height range provided by the manufacturer. If you are Xcm tall you should fit a size Y. While sizing charts often provide a good starting point, this approach doesn't take into account either the individual body proportions of the rider, or their target position on the bike. Though many manufacturers put significant time and energy into designing bikes to fit varying sizes of people, the nature of sizing charts is that they assume the target riding position for a rider, rather than recommending a bike based on the position the rider would like to be in. Given that every rider has a height, the result of relying on sizing charts is that one size of every bike on the market should “fit”, when this is not the case in reality. The more humbling truth is that not every bike has a size for every rider. Check back in for our next post of this series where I’ll dive into some examples of how these sizing charts and the bikes they represent can cause some confusion when selecting a size.

Universal Size Calculators Size calculators provide a tool that adds on the idea of a sizing chart by taking into account a few more body metrics than simply height. Usually after a fairly lengthy process of measuring each of your limbs, the calculator recommends a size expressed as either a dimension (e.g. top tube length, seat tube length, etc) or a numerical frame size (e.g. 54cm, 56cm, etc) or sometimes as generic "T-shirt" size (e.g. S, M, L). While these systems are generally more accurate than strictly height based recommendations, the problem is that each of these outputs have limited value.

Frame Dimensions:

  • A recommended frame dimension such as top tube has limited usefulness on its own. Top tube length, or any other single dimension, provides only one part of the equation of how a bike will fit a rider. Labelling a bike size by top tube length is nearly as useful as labelling a trouser size by leg hole diameter. Sure, there is probably some correlation to overall size but it doesn't really indicate how/who the trousers will fit.

Numerical Frame Size:

  • Though there was a day where a 54cm frame had some correlation to the physical dimensions of the frame, there is currently no industry standardization in labelling a bike size. Two bikes labelled as 54cm (even of the same brand) can vary drastically in their dimensions and how they will fit. This provides limited value or confidence to consumers. I'll go deeper into how similarly labelled bikes fit differently, and how differently labelled bikes can fit the same in the next post.

T-Shirt Sizes:

  • Similar to numerical sizing, “T-Shirt” size recommendations lack consistency between models and brands. At best this lets a rider identify roughly where in the range of bike sizes they might fall, but provides little to no information upon which a rider should actually use when purchasing a bike.

Though some more advanced calculators do provide more informative outputs, such as stack and reach ranges, or allow you to compare measurements against a database of bikes, they often still fall short of helping to find a bike that matches the position the rider is looking for.

Geometry Charts

Looking at geometry charts involves undertaking a thorough review of the geometry chart for the bikes you might be considering. Some online platforms exist to help with this process, but it is fundamentally one that requires significant research and each manufacturer doesn’t necessarily publish the same measurements and details in their geometry charts which can make comparisons challenging. While this is often a reliable way to size a new bike, it requires the rider to know the geometry and set-up of an existing bike that fits them well. Given this baseline to work from, it is much easier to compare the geometry of a current bike and the bikes you are considering to see how (dis)similar they are. Of course this also relies on shopping for a similar bike to the one you have. This process is completely unhelpful for riders that have a road bike they know fits well, but are shopping for a time trial or a mountain bike. So, this method can be convenient if you have both a bike you are comfortable on, and are knowledgeable enough to understand how to compare different frame geometries and component set-up. However, there is much to be desired for all the riders for whom this does not apply. Newer riders, those without a bike they like, those shopping for a different type of bike than what they have, and anyone not comfortable in comparing bike geometries are still left without any meaningful way to choose.

Bike Shop Test Rides

Most consumers still buy their bikes through brick and mortar bike shops. When shopping, consumers are typically sized by a salesperson and are hopefully able to take the bike on a test ride (though this is increasingly rare with the supply chain issues of the past two years). In many cases sales people are only able to rely on the manufacturer sizing charts and their own experience to inform the recommendations they provide. Good shops will offer customers a rudimentary "floor fit" before sending the rider out for a test ride. In some cases, given a knowledgeable sales person, this approach can yield good results. In other cases riders are sent out almost blindly on a test ride, often with only the most minimal of saddle height adjustments. Perhaps unsurprisingly, results in these cases largely come down to chance. A rider that happens to be put on a bike that works for them may have a great experience, alternatively the bike could be set up improperly, and while it could be the perfect bike and size, the consumer will have a bad experience on this bike. While this approach often has better results than blindly picking a bike based on a size chart or calculator, it still leaves a lot of room for improvement to reliably select the right size.

Fit First Services

This is the Gold Standard approach to Sizing & Fitting a New Bike, just in the same way as determining the correct size and Fit for a Fully Bespoke Frame build. These services will require booking a multi-hour appointment with a professional bike fitter. Using an adjustable fit bike, the fitter will determine a rider’s optimal riding position, then identify which bikes work best with that position and what touchpoint adjustments might be necessary. In general, this is the correct route to take to accurately determine the correct Off the Peg Sized Bike, though it can have some downsides. Location and availability can make fit-first services inaccessible or at least inconvenient to some of the riding population. So while a fit first service ticks all of the boxes needed to correctly size a new bike, they can remain out of reach to some riders shopping for their next bike.

I hope this overview of the current methods for sizing bikes has helped to start shedding light on the challenges facing consumers looking for their next bike. Stay tuned for the next part of our Bike Sizing series which will dig into some data of the current bike sizing landscape.

Many Athletes have used Performance BikeFit to improve their cycling position. Whether you are a veteran or new to the sport, Performance BikeFit can help you improve your position and improve your enjoyment of cycling indoors and Out.

© 2022 Performance BikeFit

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