This post aims to determine whether it’s better to use the stack and reach values rather than the seat and the effective top tube lengths as a way to size a frame.
Let’s start by defining each of the terms.
Reach – the horizontal distance between the middle of the head tube and the center of the bottom bracket (image below).
Stack – the vertical distance between the bottom bracket of the bicycle and the top of the head tube.
Effective Top Tube (ETT) – a line horizontal to the ground connecting the middle of the head tube and a line extended from the center of the seat tube (image below).
Actual Top Tube (TT) – the length of the top tube measured from the center of the head tube to the center of the seat tube.
The Old School Way
In the past, people were sizing a bike primarily by analyzing the length of the actual top tube and the seat tube.
But that practice is no longer as common because most top tubes are now “sloping“.
Consequently, the length of the top tube does not always indicate the distance between the head tube and the seat tube with the needed precision.
For that reason, people began focusing on the effective top tube (definition above) as it’s technically independent of the top tube’s shape and connection points.
The ETT is a useful measurement when we want to determine how the bike would feel when pedaling in a seated position.
However, the ETT is subject to manipulation via the saddle setback and the stem’s length.
In other words, even if you don’t get the perfect ETT for your height and anthropometry, you will still be able to come up with an acceptable fit as long as the ETT is sized within a reasonable range.
A Note On Seat Tube Sizing
In the past, people were also focused on the length of the seat tube. Naturally, riders with a longer inseam (distance between the bottom of the foot and the crotch) were looking for longer seat tubes.
However, with the popularity of sloping top tubes, the focus on the seat tube decreased as people began to rely on a long seat post to make up for the height that the seat tube might lack.
Of course, this doesn’t mean that the length of the seat tube shouldn’t be taken into consideration at all, but it’s rarely the emphasis anymore.
The New Way
There has been a tendency to concentrate more on the stack height and reach (definitions above).
First, the stack height and the reach are close to set in stone.
The only adjustment that would help with poor reach and stack height is the stem. A longer stem with a higher rise will make the effective stack height and reach larger.
However, a stem shouldn’t be used to compensate for reach and stack height that are totally off as it will negatively impact the handling of the bike.
In other words, the stack and height offer more stable and concrete data points for the overall size of the bike and are less forgiving than the top tube and the seat tube. If you don’t get them right, a decent bike fit cannot be obtained.
The second reason would be the ability to size different bikes quickly. If you know the stack height and reach that your body requires, then you can translate the data to various models rapidly.
Once you have selected a frame/bike with the required stack height and reach, you can begin evaluating the saddle setback and stem length that you need.
Also, the reach and stack height don’t care about the lengths and shapes of the top tube and the seat tube. Thus, it’s easier to evaluate and size frames with unorthodox designs.
The Reach and Stack Height Are “Downtube Dependent”
The reach and the stack height are dependent primarily on the length of the downtube. If the downtube gets shorter, the reach and the stack decrease too as the bottom bracket gets closer to the head tube.
How Can I Calculate My Stack Height and Reach?
The ideal scenario is to get a bike fit done by experts who know what they’re doing.
What is a bike fit?
During a bike fit, a professional evaluates your anthropometry, fitness level, and goals and optimizes your position for a specific cycling discipline.
Truth be told, the bike fit isn’t a series of numbers that just “open the gate to success”. It’s a process.
For example, initially, a bike fit may feel fine, but after accumulating a certain amount of miles under your belt, pain caused by repetitive stress could come out of nowhere and demand a change.
In short, your ideal fit isn’t set in stone. It’s a variable operating within a certain range.
Once you have approximate reach and stack height data from your fitter, you can use it to evaluate frames.
FAQ: I don’t have the funds to pay for a bike fit. What’s my other best option?
Option 1: Select a Bike Online and Test It Locally
Numbers don’t matter when a bike doesn’t feel right in practice. For that reason, it’s always recommended to test a bike before buying it.
A bike doesn’t have to feel extremely comfortable immediately, but it shouldn’t be uncomfortable either. Don’t forget that minor adjustments are expected to be made for maximal comfort to be achieved.
Option 2: Select a Brand and Follow Their Sizing Protocols
If you plan on buying online, select the bike brand that you want and read the sizing guidelines that it offers.
The list below contains a link to the sizing guides of popular manufacturers.
This method has one advantage – it takes into consideration the specificity of the brand.
Very often two frames that are technically the same size but from separate manufacturers have different dimensions.
Summary: What You Need To Know
- With the popularity of sloping top tubes, people stopped relying on the lengths of the top tube and seat tube as a way to size a bike.
- The main focus is now on the reach and the stack height as their value is more “stable” and cannot be affected by the position of the saddle. (You can, however, influence the reach via the stem length.)
- The current method for sizing a frame is to get your stack height and reach “right” and don’t worry about the effective top tube too much as you can change it via the saddle and the stem.
- If you know the stack height and reach for your body type, you will have an easier time selecting a bicycle regardless of the manufacturer.