This post explains the relation between a bike’s stack height and reach.
Definitions
Reach – the horizontal distance between the middle of the head tube and the center of the bottom bracket.
Stack height – the vertical distance between the bottom bracket of the bicycle and the top of the head tube.
How does stack height affect the reach?
Option 1: Stack Increase = Reach Decrease
If we increase the stack height, we are increasing the distance between the bottom bracket and the top of the head tube.
Consequently, the reach will decrease if want to maintain the original head tube angle (HTA) of the bike. (The head tube angle is the angle between the head tube and the ground.)
The graph above illustrates what happens when we increase the stack height and maintain the head tube angle of the frame. The higher the stack gets, the more the reach has to decrease.
The second graph illustrates how the reach increases when we decrease the stack height and maintain the head tube angle (HTA).
Option 2: Increasing the Stack Height and The Reach Simultaneously
It’s also possible to increase the reach and stack height simultaneously by elongating the top tube and the down tube.
The graph above shows how both the stack and the reach can be increased by elongating the top tube and the head tube.
Of course, the reverse process can be executed to reduce the values of the stack and reach.
The Wheel Base Always Changes
It’s crucial to note that in each case the wheelbase of the bicycle changes.
The wheelbase is the distance between the axles and indicates the bike’s length. A longer wheelbase offers more stability whereas a shorter wheelbase makes the bike more responsive.
So, when does the wheelbase change?
If the reach increases so does the distance between the head tube and the bottom bracket. Thus, in this case, the wheelbase gets larger too.
If the reach decreases, we observe the opposite, namely a wheelbase reduction.
So, what is the practical conclusion from the dependencies expressed above?
- If two bikes have the same stack and head tube angle, the one with the greater reach will be longer. The additional length will amount to the difference in reach.
- If two bikes have the same stack, HTA but one has a shorter reach, the one with the smaller reach will be shorter by the difference in reach.
Artificially Affecting The Stack Height
The stack and reach are set in stone once the frame is built.
However, it’s possible to influence their effective values via the stem and the spacers underneath it.
Below is the list of all possibilities:
- Longer Stem + Positive Rise
If a stem has a positive rise, and we increase its length, both the effective stack and the effective reach will grow too.
- Longer Stem + Negative Rise
If the stem has a negative rise, increasing its length will decrease the stack and thus increase the effective reach.
- Adding Spacers Under The Stem
Additional spacers under the stem increase the stack and therefore decrease the reach.
- Removing Spacers Under The Stem
By removing spacers from under the stem we decrease the stack and increase the effective reach.
The Handlebars Can Be Game Changers Too
The handlebars of a bicycle can influence the effective reach and stack height too.
Below is a list of possibilities:
- Riser Bars = Higher Effective Stack For The Same Reach
By elevating the stem, the user is increasing the effective stack and decreasing the effective reach because the stem is attached to the fork’s steerer which has the same angle as the head tube.
Consequently, every upward degree of movement is done at the angle of the head tube and shortens the reach.
However, this effect can be negated up to a point by installing riser handlebars. The bars elevate the position of the hands upward rather than at an angle. Therefore, the reach does not decrease but the stack does.
- Bullhorns = Greater Reach For The Same Stack Height
Bullhorn handlebars offer a multitude of hand placements. When the rider uses the horns, the effective reach is increased tremendously.
If the bullhorn model does not have a drop, the reach will be increased with a minor effect on the stack height. The angle of the bars is important too. If the hornс points slightly up or down, the stack will be affected in the respective direction.
- Sweptback Handlebars = Decreased Reach for The Same Stack Height
If you install sweptback handlebars on your bike, you will reduce the reach because the bars are pointing toward the rear wheel and position the rider in a more upright position.
However, the stack height will not decrease or increase (unless the bars have an additional rise towards the ends).
Stems, Spacers, and Handlebars Are Not a Panacea
Manipulations of the stem, spacers, and handlebars are not a solution when the reach and the stack height are extremely off.
Why?
First, the handling of the bike is affected by the longer/shorter stem. Second, you can’t install random handlebars on a bicycle and preserve its original purpose. For example, you can’t just put sweptback handlebars on a road bike because the reach is too long and then use the road bike as intended.
That said, you don’t have to obsess over getting the perfect reach and stack down to the last millimeter thanks to the available adjustments and the body’s adaptability.
FAQ: Why are people increasingly focusing on the stack and reach instead of the top tube to size a bike?
One of the reasons is that the reach and the stack are technically impossible to change once the frame is built. As explained, they can be influenced by the stem and handlebars, but the final result is not as effective as sizing the bike properly to begin with.
Meanwhile, the effective top tube is affected not only by the stem, spacers, and bars but by the saddle setback too. In other words, the effective top tube is more forgiving.
