Bikes come in a variety of sizes, but there isn't a standardized way to size them. This article takes a closer look at the data to illustrate the problems it can cause.
Welcome to our discussion on bike sizing. Some of the challenges consumers face in selecting a bike, mostly caused by a lack of standardization in how bike sizing is reported, an absence of consideration for how a bike will fit, along with how many consumers shop for bicycles.
Here we are going to look into some of the variances in bike sizes which I hope will help illustrate why being able to confirm how you fit on a given bike is so important, rather than simply selecting a size based on a size label, be it a letter or number.
About Stack and Reach To begin with I want to make sure we are all up to speed on why stack and reach are the primary measure of how a bike will fit. Stack refers to the vertical distance from the centre of the bottom bracket and centre of the top of the headtube.
While Reach is the horizontal distance from the centre of the bottom bracket to the centre of the top of the head tube. Unlike measurements such as top tube, seat tube, and head tube length, stack and reach are the true indicators of how a bike will fit because they focus on the relationship between the two most important fixed points on the bike: The bottom bracket, and the top of the head tube. These points provide reference for both your saddle and handlebar position (your primary points of contact), whereas tube length measurements on their own don’t effectively indicate how a bike will fit. A few examples: A bike can have a shorter head tube but a higher stack. Similarly, a bike can have a shorter top tube but a longer reach. I have had customers come in for a Bike Fit with bikes that have a longer head tube or shorter top tube hoping to be in a less aggressive position but actually wound up with a bike that fits more aggressively. This is why listing bikes by their individual tube length is irrelevant (unless you are looking at the entire geometry chart) and stack. Lastly, while stack and reach are the primary measurements we use to measure a frame, they aren’t the be all and end all to measure or define a rider’s position; handlebar and hood stack and reach do a better job reporting how a rider interacts with their bike. While they are related to frame stack and reach, they take into account component dimensions such as stem length and angle, and hood/shifter dimensions. To elaborate, the same hood stack and reach can be achieved on two different bikes, with different frame stack and reach, by accommodating for the difference in frame stack and reach with a different stem, handlebar, or more/less headset spacers. While this may imply that any two bikes can be adjusted to match the same position, significant changes in cockpit component dimensions can have severe impacts to a bike’s handling characteristics, and there are also physical limitations to the level of adjustment and adaptation possible.
The Data of Bike Sizing: With stack and reach established as our metrics to list and compare sizes, let's look at some of the variance in size across our database of bikes we’ve amassed. In the graphs below we'll plot Stack and Reach coordinates on an X and Y axis. This is a quick and easy way to visualize a bike's size. Currently our database of road and gravel bikes consists of over 30000 models and over 30 of the most popular brands. If we look at the stack and reach of every brand available things get a little muddled by the sheer amount of data, but some outliers do show right away. Bikes such as the Specialized Diverge, Trek Domane, Canyon Grail, Grizl, and Endurace, as well as the Argon-18 Gallium and Sum, all stick out as outliers. These outliers are interesting but they don’t really explain the problem of sizing.
Size Growth
To help understand the root of some of the differences in sizing, we’re going to explore three different philosophies of frame design, specifically in consideration (or lack thereof) to fit. To illustrate this we’ll compare three road “race” bikes. The Cervelo R5, the Trek Emonda SLR, and the Colnago C64 Sloping (Colnago offers two geometries: High which is more endurance oriented and Sloping which they market as a racier bike.) The graph below plots stack and reach coordinates of every size offered in each bike. You can see three very different approaches to bike design.
Cervelo uses a linear stack and reach growth - they design their bikes targeting consistent jumps in stack and reach between sizes. Trek’s growth is slightly curved with more stack gained relative to reach per size. But like Cervelo the growth is relatively consistent. Colnago’s growth is variable (to put it lightly) showing linear growth between sizes 42 - 48 nearly vertical growth from 48 to 54, a massive jump in reach from 54 to 56 and then close to linear growth again. Colnago’s reach variance from a 54cm to 56cm (one size) is almost as large as the Emonda’s variance from 52cm to 60cm (5 sizes). You’ll also notice some strange overlaps with the Cervelo 54, Trek 52 and Colnago 48 (all fitting similarly, yet labelled differently), as well as the Cervelo 58, Emonda 60, and Colnago 56 also being close in size. Interestingly, while on the small end of the scale the Cervelo 54 is closer to the Trek 52, on the big end of the scale the Cervelo 58 is close to the Trek 60. So a blanket statement like “Treks fit smaller than Cervelo” isn’t true.
The Measurements of Bicycle Size
Often we’ll hear someone say something like I need a 54cm top tube. Well, I’ve got news for you - asking for a 54cm top tube is like asking for a t-shirt with a 24cm diameter arm hole. It’s not going to reliably tell you much about how the shirt is actually going to fit, just like a 54cm top tube won’t tell you much about how a bike is going to fit. To illustrate this, the graph below plots a selection of bikes with a 530 effective top tube (aka virtual top tube). The size labels on these bikes run the full gamut - 47, 49, 50, 51, 52, XS, S, and, M. More tellingly the variance in frame reach is 18mm and the variance in frame stack is 50mm. While some of these bikes fit quite similarly, many of them fit quite differently. Consider that within some brands that level of variance could represent 3 or 4 sizes.
Similarly to above, with riders looking for a 54cm top tube, others may go one step broader and look for a “54cm frame.” So, let’s wrap things up with a look at all the reported stack and reach numbers from our database of road and gravel bikes that are listed as a size 54cm - just like above with people asking for a 54cm top tube, others will give the blanket statement that “I’m a 54cm.” Similar to the above argument you can say you are a 54cm and you might be right sometimes, but with the lack of consistency across the industry in how a 54cm actually will fit I’d rather be sure before I spent my hard earned money on a bike.
Some analysis to the graph above: Two bikes (3t Exploro Ultra and Trek Checkpoint) at the extremes of this graph are two “modern” gravel bikes - long reach, short stem - and provide a great illustration of the challenge with sizing by “size”. The variance in reach between the two is a massive 30mm. It would be hard to fault a buyer for expecting these two bikes to fit similarly (given they are both 54cm) but they'll be mistaken. For comparison, a 30mm difference in reach is the difference between the smallest and largest Trek Emonda (8 sizes: 47cm - 62cm). In exploring the stack variance we’ll set aside some of the outliers and attempt an apples to apples comparison of race bikes like the Scott Speedster/Addict and the Specialized Tarmac SL 7 - these two bikes have a similar reach (4mm difference) but have almost 40mm difference in stack, again quite a large difference in the resulting rider position, for two bikes that are reportedly the same size and style.
Conclusion
So there you have it, our look behind the curtain on how much of a mess bike sizing is. Individual frame dimensions have little relation to overall size, stack and reach only tell part of the picture, and manufacturer size labels are inconsistent and uninformative. Currently, as a consumer there are only two reliable ways to confirm the size and fit of a bike:
A Fit First session with Performance BikeFit Studio
To understand how we at the Performance BikeFit Studio are combatting the problem checkout our next post THE SOLUTION TO BIKE SIZING - FIT FIRST SIZING
Have a bike you know really well and compare its geometry to the bikes you are considering.
Some will say test rides should fall in this list but I’m hesitant to agree: Test rides are often short, the initial setup of the bike is likely wrong, and the certain components can need to be swapped to make the bike fit you well.
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