Road cycling is a demanding discipline, if only because of the position the rider must adopt for hours at a time. As a result, the fit of the bike is critical to every road rider, however deciding on whether any given frame size, brand, or model will suit an individual can be an overwhelming process.
In this article, Australian tech editor Matt Wikstrom takes a look at the intricacies of road frame geometry as they relate to sizing, and provides a four-step guide for assessing the size of any frame.
The bicycle frame is a fairly simple shape comprising a trapezoid at the front and a triangle at the rear. Eight members are all that are needed to create a frame, and by varying the lengths of each, especially for the trapezoid, it is possible to scale it to fit a wide variety of rider sizes and proportions.
The angles at which each frame member are joined can also be varied, and while this does have some bearing on the fit of the frame, it is far more important to the steering and handling of the bike. With such enormous scope for varying the final product, it is easy to see that there is as much skill in deciding each of these measures as there is in constructing the final product.
Indeed, custom frame builders have long been held in high regard for creating a frame with the perfect fit, especially amongst road cyclists. The sport places the body into a fairly demanding, and arguably unnatural, position for long periods, so a perfect fit can make a big difference to a rider’s comfort and performance.
In the past, it wasn’t uncommon for road cyclists to have a frame made-to-measure so that only minor adjustments to the stem and saddle height were required to achieve the perfect fit. That practice has largely disappeared in the last couple of decades thanks to the convenience and affordability of mass-produced bikes.
The introduction of compact frames at the end of the 1990s brought with it a new view on frame sizing. Where once manufacturers had to create a dozen frame sizes to accommodate the majority of adult cyclists, that number could be halved (or more) with obvious economic benefits. The move also seemed to simplify things for consumers, who only needed to decide on the nearest best fit then adjust/replace the stem and/or seatpost to suit their needs.
As a result, cyclists have come to rely more heavily on making adjustments to stem height and length, and seatpost height and offset, to adapt stock frame sizes to suit their individual needs. In the majority of instances, there are no major drawbacks to this approach unless a rider falls outside the norm.
While there are now fewer frame sizes to choose from for many mass-produced bikes, deciding on a frame size remains a convoluted process. If nothing else, there is no consensus on sizing and the nomenclature used to describe frame sizes has yet to be standardised.
Then there is the matter of the type of fit on offer (e.g. race versus endurance). While this has added to the variety of frame sizes on offer in the marketplace, this suffers from the same lack of standardisation, making it difficult for consumers to distinguish the differences from one brand to the next without carefully studying the geometry of each frame.
There is no avoiding this effort, though. Ultimately, every road cyclist really needs a detailed understanding of their own fit as well as frame geometry to make an informed choice when shopping for a new bike or deciding on the parts for a custom road bike build.
Where to begin?
Every bike provides three contact points for the cyclist: the handlebars, saddle, and pedals. It is the spatial arrangement of these three points that determines the rider’s fit, as determined by his or her body proportions, strength, and flexibility.
Thus, the starting point for any cyclist is to determine the ideal position for each of these contact points. This is something that can be achieved, to a certain extent, by trial-and-error provided the cyclist has the time and patience to systematically explore a range of settings for each contact point.
A far more effective approach is to enlist the services of a professional bike-fitter. This is a field that has been slowly developing over the last couple of decades. Where once it was dominated by opinion and traditional thinking, bike-fitting has evolved into a sophisticated science with tools to match.
While most riders will generally benefit from a professional fitting, some riders are more adaptable (and/or less demanding) than others, and therefore, there is no great need to diligently optimise the position of each contact point. In contrast, this kind of attention to detail is far more important for riders that have suffered overuse injuries and/or chronic discomfort on the bike.
Whatever the process, once a cyclist is confident in the comfort and effectiveness of their position on the bike, it becomes a personalised template for assessing the suitability of any frame size, brand, or model. Unfortunately, making the leap from a rider’s position to the ideal frame size requires a few steps, starting with mapping the position of the saddle and handlebars.
Mapping a rider’s position on the bike
Every map depends upon a meaningful point of reference, and for a rider’s position, it is the centre of the bottom bracket of the bike. At face value, the ground might seem more convenient, however the position of the saddle and bars will be influenced by the size of the tyres and any differences in the height of the bottom bracket (which can vary by 15mm or more from one frame to another).
Measuring the displacement of the handlebars and saddle from the bottom bracket produces a pair of coordinates for each contact point. The centre-point of the handlebar tops and the mid-point of the saddle is normally used for this kind of mapping because it eliminates the impact of variations in the diameter of the bars and the length and shape of the saddle. As for the pedals, the position of this contact point will be determined by crank length, also measured from the centre of the bottom bracket.
While it is possible to complete this mapping with a tape measure and a spirit level, it is difficult to do this with millimetre-perfect accuracy without specialised tools (e.g. Abbey’s Fit Kit). For those riders that are relatively insensitive to small changes (~5mm) in bar and saddle position, this DIY approach should work well, but for those that are affected by small changes, the mapping is best left in the hands of a professional bike-fitter.
From fit coordinates to frame size
Making the leap from a set of fit coordinates to a frame size is not a simple matter. In fact, this is the realm where framebuilders dwell, making use of either scale drawings or BikeCAD to ensure that the final build will satisfy a specific set of contact points. Many bike-fitters are also able to make these calculations, too, and in some instances, will provide the dimensions of the ideal frame as part of their final report.
Designing a frame from first principles provides enormous freedom when it comes to specifying the saddle, seatpost, stem, and bars. Deciding each is largely a matter of preference, though it’s worth considering the impact that stem length can have on the handling of the bike. Once specified, the geometry of the frame can be refined to accommodate each while ensuring accurate positioning of each contact point.
While this kind of detailed approach works well for designing custom-built frames, it is not strictly necessary when sizing up different mass-produced bikes. All that is needed is a detailed geometry chart for a frame that works well for the rider so a side-by-side comparison can be performed to identify where the differences lie for any given candidate.
The key to this kind of comparison is an understanding of the measures that are most relevant to the fit of a road frame, which is what will be addressed in the next section. Some scale drawings or time with BikeCAD may still be required to fully explore the fit of the candidate frame, but for DIYers, this approach will require much less time and effort than working from first principles.
Deciphering frame geometry in terms of bike fit
The geometry of a frame can be represented by a variety of measurements, including the length of various frame members along with the angles that some of them make with the ground. A lot of this information is presented in the geometry chart for a bike, but for the uninitiated, these charts can be overwhelming and impenetrable.
Nevertheless, a geometry chart can be broken into smaller pieces for easy digestion. This is a topic that we have dealt with in some detail already when discussing the measurements that are most relevant to the handling of the bike.
For the fit of a frame, it is the combination of the measures for the front half of the bike that dictate its fit. This includes the effective top tube length, the length of the head tube, the length of the seat tube, and the angle of the seat tube. The height of the bottom bracket can also be important, but not strictly necessary if the stack and reach are provided, while standover height and the front-centre measurement can be considered secondary measures.
That’s a lot of data to digest, however there’s no need to consider it all at once.
Step 1: the height of the frame
The height of the frame is crucial to any rider since it dictates both the minimum, and maximum, height for the handlebars and saddle. As such, this is first measure to compare when sizing up a road frame.
In the past, when all road frames featured a horizontal top tube, the length of the seat tube provided a robust measure for the overall height of the frame. The introduction of compact frame designs changed all of that, and for a time, the length of the head tube served as a reasonable surrogate, but the relatively recent introduction of the stack measurement is far more reliable.
Stack describes the vertical distance from the centre of the bottom bracket to the top of the head tube. It is the modern-era equivalent for what seat tube length once was, a single number that allows riders to quickly determine if a frame is tall (or short) enough for their needs.
It’s important to note that stack does not always include the upper parts of the headset, which can add as much as an extra ~30mm to the stack of the frame. This is especially true for frames that require an external headset, so shoppers should clarify this point before committing to a specific frame size.
By contrast, the stack of a frame always includes a specified fork. For this reason, stack provides a much better representation of the overall height of the frame than the length of the head tube. That’s because there is no standard length for road forks; they can vary from one brand to another, and more importantly, with the amount of tyre clearance on offer.
Stack provides no information about the height of the frame at the seat tube, so it is prudent to consider the length of the seat tube as part of any frame-sizing exercise. This is especially important for those riders that require their handlebar to be more than 100mm below the saddle, in combination with a compact frame design. Subtracting the seat tube length from the rider’s saddle height will yield an approximate length for the seatpost. If the difference exceeds 350mm, then the seat tube will likely be too short, even when coupled with an extra long (450mm) seatpost.
Step 2: the length of the frame
Before the advent of compact road frames, the length of the top tube (measured from the centre of the head tube to the centre of the seat tube) served as a pretty consistent measure for the length of a frame. It literally provided the horizontal distance between the seatpost and the stem, so it was easy to deduce what kind of stem length would be required to achieve the ideal separation of the two.
The actual length of a sloping top tube will always be shorter than the corresponding horizontal distance. While some geometry charts detail both measurements, most manufacturers provide the horizontal (or effective) top tube length so that the length of the frame can be easily judged in the same way as a traditional frame.
While effective top tube length provides a good idea of the overall length of the frame, it will be influenced by the angle of the seat tube, increasing as the angle gets slacker. This is where reach can provide a more reliable measure for the length of a frame, since it accounts for the horizontal distance from the bottom bracket to the top-centre of the head tube. Since this measure is normalised with respect to the bottom bracket, reach can be compared directly to judge the length of any number of frames. In contrast, the same comparison of effective top tube lengths will only be valid if all of the frames share the same seat tube angle.
It is important to remember that while there is a generous range of stem lengths on offer in the current market, most are limited to 10mm increments (and don’t always measure exactly as stated, either). As a result, riders hoping to achieve a millimetre-perfect fit will need to pay close attention to the length of the frame, and more importantly, understand whether they are more susceptible to a minor excess or deficit in the final reach to the handlebars.
Again, we need to consider the influence that stem length can have on the steering of the bike. In general, long stems (>100mm) will slow down the steering response of the bike, while short stems (<100mm) will speed it up (however neither will dominate the effect of the head angle and trail of the bike). For those trapped in between sizes, the choice between a larger or smaller frame may come down to which stem length will be better suited to the bike.
Step 3: saddle setback
The fore-aft position (or setback) of the saddle is not important for adjusting the reach to the handlebars. Instead, it has a critical impact on recruitment of the leg muscles, and therefore, must be positioned accurately (with respect to the bottom bracket) to get the most out of the rider’s efforts. Fortunately, most seatposts allow the saddle position to be adjusted in very fine (<1mm) increments.
With that said, there is a limit to the range of this adjustment due to the length of the saddle rails. This can be extended, positively or negatively, by swapping to a seatpost with more or less offset, but this may not always be possible, especially when a proprietary seatpost design is involved. Ultimately, the angle of the seat tube dictates the amount of saddle setback, so it is important to consider this measure when sizing up a new frame.
For those riders that require a large amount of setback, they will need a shallow seat tube angle (73° or less) and perhaps an offset seatpost; in contrast, riders that require much less setback will be best served by a steep tube angle (74° or more) and a seatpost without any offset.
Step 4: other measures
The front-centre measurement describes the distance from the centre of the bottom bracket to the centre of the front wheel, and is important for assessing the risk of toe overlap. This measure depends upon the reach of the frame as well as the head tube angle and the rake of the fork, and must accommodate the radius of the front wheel (330mm or more), the length of the cranks (165-180mm), and the portion of the foot that extends beyond the axle of the pedal (at least 60mm).
That makes for a minimum of ~560mm. For large frames, there is generally plenty of reach to avoid toe overlap; in contrast, small frames are much more susceptible, as are those bikes with big tyres (e.g. 700 x 40c) and/or riders with unusually long feet, a preference for longer cranks (>175mm), or cleats positioned well behind the ball of the foot. Fortunately, all can be measured with ease so that shoppers can determine the minimum front-centre for their needs.
Standover height describes the height of the top tube from the ground. Ideally, it should be a little less than the rider’s inseam so they can stand over the frame with their feet flat on the ground. For those frames with a sloping top tube, standover height is normally reported for the middle of the tube, so it is possible for the frame to exceed a rider’s inseam at the head tube junction.
While standover height has some relevance to sizing a frame, it is not nearly as important as the other measures discussed above. For those riders that prefer an upright position, and hence, a frame with extra stack (and a taller head tube), this is likely to increase the standover height of the frame, so it is worth comparing the two measures when sizing up any given frame.
Pulling the pieces together: a quick case-study in frame-sizing
Suppose there is a rider looking at getting a new bike and is having trouble deciding between Giant’s Defy and TCR Advanced Disc.
This rider has already had a bike-fit and knows exactly where he or she needs the saddle and handlebars to be positioned (Figure 1A), so the question that needs to be answered is if one bike is a better fit than the other.
Overlaying this map onto the geometry for a M/L Defy reveals that the bike will be a good fit. The stack is ideal for the handlebar position, and there will be no need to change the stock 110mm stem (Figure 1B). At the same time, no special effort will be required to achieve the ideal setback for the saddle with this frame.
The TCR Advanced Disc in the same size (M/L) will also work quite well, however this frame has less stack (562mm vs. 586mm) and a longer top tube (570mm vs. 560mm). As a result, a swap to a shorter stem (100mm) will be required along with some spacers under the stem (27mm) to provide the rider’s ideal handlebar position (Figure 1C). As for the saddle, the seat tube may be shorter for the TCR Advanced (500mm vs. 515mm) but the seat tube angle is identical for the two bikes, so there won’t be any trouble positioning the saddle according to the rider’s needs.
Clearly, both bikes will work for the rider, however the combination of a shorter stem (100mm vs. 110mm) and a steeper head angle (73° vs. 72.5°) for the TCR Advanced Disc promises that it will offer quicker steering than the Defy. A test ride may be require to help decide the matter, but armed with the information on his or her fit, the rider will be able to request a stem swap before leaving the store if the choice is made to buy the TCR Advanced Disc.
The difference between the right frame size and the wrong one
Since the introduction of compact frames, it has been possible to fit a wider range of riders to a single frame size thanks to the extra adjustability afforded by the shorter seat tube, lengthy seatposts with various amounts of offset, and a growing number of stem lengths and angles. As a result, a rider can make use of a bike that is either a little too big, or too small, without any practical consequences.
Indeed, the remarkable adjustability of the modern road bike has had a profound effect on the relevance, and appeal, of made-to-measure frames because the majority of adult shoppers can achieve a satisfactory fit on a mass-produced bike with only minor modifications. The only real shortcoming concerns the aesthetics of the bike in the form of extra spacers under the stem and/or a saddle positioned at one end of the rails or the other.
Generally speaking, those extra spacers are no more significant than a styling faux pas, but as the modifications become more extreme — for example, an extra-short stem with 50mm of spacers below — they will have an effect on the balance, poise, and handling of the bike. In short, the geometry of a rider’s fit cannot be divorced from the geometry of the bike’s handling.
This is where a custom-built frameset will always shine because the geometry of the final product can be fashioned to suit the rider’s precise fit as well as their position on the bike.
There is a second concern regarding these extreme modifications, and that revolves around the structural capabilities of the bike. As the saddle is raised and set further behind the bottom bracket, the rider’s weight will create more leverage on the post and the seat tube of the frame. Likewise, leverage on the fork steerer will increase as the stem is raised and lengthened.
In fact, fork manufacturers often also have guidelines for the maximum (and, sometimes, minimum) amount of spacers that can be safely installed.
For small riders, there probably isn’t enough weight involved to increase the rate of fatigue, but for taller, heavier riders, the risk is much harder to dismiss.
Of course, there are other aspects that bear upon this issue, such as the materials involved and the way in which the bike is being used. Unfortunately, there is no way to easily judge the impact of any of these variables, but by opting for a frame size that is as close to ideal as possible, buyers can avoid the issue altogether.
When viewed from a distance, the road market appears populated by well-defined products for discrete categories of cyclists. However, when it comes to fit, the distinctions are not nearly as well defined. Indeed, the range of rider sizes, body proportions, and flexibility occurs on a continuous spectrum that defies categorisation.
The development of compact frame sizes with an amount of built-in redundancy has to be counted as an elegant solution to this challenge. While this strategy cannot match the accuracy of a personalised product, the last two decades has proven just how well it can serve the vast majority of adult cyclists.
Be that as it may, the recent emergence of endurance-oriented frame geometry featuring taller head tubes and shorter top tubes is a clear acknowledgement from the industry that there was a need for more variety in frame geometry. Interestingly, that geometry has become something of a commodity in today’s marketplace and some manufacturers go so far as to promise a better fit for its customers.
However, for those riders that have a good understanding of their fit, such assertions won’t carry much weight. After all, the dimensions of any given frame will either suit an individual, or they won’t, and no amount of marketing will ever change that fact. Having a wider range of frame sizes from which to choose can only be regarded as a good thing, but it’s still critical to know how to correctly make that decision.
Thus, for those cyclists hoping to negotiate all that the market has to offer with greater confidence, it all starts with an understanding of their fit and the ideal position for each contact point on the bike.