This post explains the difference between the effective top tube and the actual top tube.
Actual Top Tube. The actual top tube is essentially the length of the top tube itself.
It can also be referred to as the distance between the middle of the head tube and the middle of the seat tube from the level of the top tube. (image below)
Effective Top Tube. The effective top tube measures 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)
The Differences Between The Actual Top Tube and The Effective Top Tube
- The Effective Top Tube Is Almost Always Longer
The vast majority of modern bikes have a sloping top tube that isn’t horizontal to the ground. Consequently, the actual top tube is shorter than the effective top tube in most cases.
The only exception would be frames with a perfectly horizontal top tube. The models that fit the criteria are retro road and track bikes. In this situation, the actual and the effective top tube are the same (image below).
- The Effective Top Tube Provides More Useful Information
The effective top tube indicates the space that the rider will have when pedaling in a seated position and is therefore a critical measurement for long-distance bicycles.
Meanwhile, the actual top tube reveals very little, especially when it has an aggressive slope and shape.
- The Saddle’s Position Affects The Effective Top Tube
By sliding the saddle forward, the rider can artificially shorten the effective top tube. Sliding it backward does the opposite and is often done when the frame is too small/short for the anthropometry of the rider.
In other words, the effective top tube is dynamic whereas the actual top tube doesn’t change.
- The Effective Top Tube Doesn’t Care About The Top Tube’s Shape
Some bikes (e.g., urban models) have a low and curved actual top tube. In that case, the measurement of the top tube is even less useful because one has to take into account too many variables (e.g., the degree of the curve, the attachment points of the top tube…etc.).
In similar cases, the effective top tube saves the day as it doesn’t really care about the shape or even the presence of an actual top tube (some bikes don’t even have a top tube).
The same applies to full suspension MTB as they have short and curved top tubes attaching low at the seat tube.
- Strong Relationship With The Seat Tube Angle
Both, the actual top tube and the effective top tube are highly dependent on the seat tube angle. (The angle between the seat tube and a horizontal line passing through the bottom bracket).
A steeper seat tube angle shortens both the actual top tube and the effective top tube. A slacker seat tube angle, on the other hand, requires a longer actual and effective top tube.
FAQ: What’s better – a slightly longer or shorter effective top tube?
If the effective top tube is very close to the needed number either situation will work because you can make up for the imperfect measurement by using a longer or shorter stem and positioning the saddle at a different location.
However, if the top tube is excessively short or long, a good fit will be very difficult to achieve.
That being said, most people would rather have a frame that’s slightly smaller than needed. A smaller frame is lighter and easier to control and almost always could be made more comfortable via the saddle and stem.
A larger frame, however, is a lot more difficult to repurpose. In many cases, it’s not even possible because you can shorten the stem only so much before completely changing the handling of the bicycle.
FAQ: What about reach?
The reach is a measurement used to indicate how the bike will feel when pedaling outside of the saddle. If you want to know the difference between the effective top tube and reach, I recommend reading the dedicated article.
Summary: What You Need To Know
- The actual top tube doesn’t reveal enough useful information when it comes to bike fitting because its value isn’t dynamic and can be manipulative due to the common slopping shape.
- The effective top tube is independent of the top tube’s shape and can be affected by the position of the saddle.
- Consequently, the effective top tube offers more useful data when analyzing a bike fit.
- The value of the effective top tube matters the most when we analyze how the bike will feel when the rider is pedaling in a seated position.
- If we want to know how the bike will perform when the rider stands up, we have to look at the frame’s reach.