While there isn't one single "exact" number of spacers universally required under an MTB stem, as their thickness varies (commonly 5mm, 10mm, etc.), there is a widely accepted recommended limit based on the total height of the spacers used.
According to recommendations often provided by bike manufacturers, you should have no more than 30mm of total spacer height below your stem on a mountain bike. This limit is crucial for maintaining structural integrity and predictable handling.
Why Is There a Limit on Spacer Height?
The number and height of spacers under your stem directly affect your handlebar height. More spacers raise the stem, while fewer spacers lower it. While adjusting stem height allows for tuning rider position and comfort, exceeding recommended limits can compromise safety and bike performance.
Key reasons for the height limit include:
- Steerer Tube Clamp Area: The stem needs to clamp securely onto the straight, reinforced section of the fork's steerer tube. Exceeding the maximum spacer height can cause the stem to clamp too high, potentially onto the tapered or weaker section of the steerer tube, increasing the risk of failure.
- Handling Characteristics: A very high stack of spacers significantly raises the handlebars, shifting your weight balance rearward and upward. While this might feel more upright and comfortable for some, it can negatively impact front-end traction, steering precision, and stability, especially on technical terrain.
- Stem Length and Rise: The effect of spacers is compounded by the stem's length and angle (rise). A very tall stack of spacers combined with a high-rise stem can create leverage issues.
Recommended Spacer Setup
The common recommendation, based on insights often shared by bike manufacturers, is focused on total height:
- Below the Stem: No more than 30mm of total spacer height.
- Above the Stem: Typically no more than 10mm of total spacer height. Spacers above the stem are often used during bike fitting to allow for future stem height adjustments.
Reference Note: Most bike manufacturers recommend having no more than 30mm of spacers below the stem and no more than 10mm of spacers above the stem. (Information based on reference dated 28-Sept-2022).
Practical Examples
Let's look at how the 30mm limit translates to the number of spacers, depending on their size:
| Spacer Thickness | Maximum Number of Spacers (to stay at or below 30mm) | Total Height (mm) |
|---|---|---|
| 5mm | 6 | 30 |
| 10mm | 3 | 30 |
| 15mm | 2 | 30 |
| (Mixed sizes) | e.g., Two 10mm + One 5mm | 25 |
| (Mixed sizes) | e.g., Three 10mm | 30 |
Note: You can mix spacer sizes as long as the total height does not exceed the 30mm maximum.
Adjusting Stem Height
If you need to significantly raise or lower your handlebars and cannot achieve the desired height within the 30mm spacer limit below the stem, it is recommended to consider changing your stem instead. A stem with a different angle (rise) or length can alter your handlebar position more effectively and safely than using excessive spacers.
- To raise handlebars further: Consider a stem with a higher rise angle.
- To lower handlebars further: Consider a stem with a lower rise angle or flipping your current stem (if it has a positive/negative rise capability).
Always ensure that your fork's steerer tube extends high enough above the top headset bearing to allow for the stem and any spacers, with enough steerer tube inside the stem's clamping area for a secure fit, usually with a few millimeters of steerer tube below the top of the stem body. Consult your bike or fork manufacturer's guidelines for specific requirements.
