Condensed Answer: The vast majority of tapered forks have a steerer tube with a 1 ⅛” diameter at the top where the stem clamps and a 1.5″ diameter at the bottom near the fork crown.
The goal of this sizing is to keep the forks compatible with all sorts of tapered head tubes.
That said, the length of the tapered section could vary between models. This measurement, however, has little effect on the fork’s compatibility with different frames.
1 ⅛” Top
Threadless stems are designed for 1 ⅛” steerer tubes. Tapered forks have to respect that measurement or else they won’t be compatible with standard stems.
The steerer of threadless forks comes longer than necessary so that the fork can accommodate as many setups as possible. It’s up to the user to cut the steerer to the desired length. Consequently, the 1 ⅛” part of the steerer has to be much longer than the tapered section.
The bottom part of tapered forks spreads. At its widest, the tapered steerer has a 1.5″ diameter.
The entire tapered section is usually 86mm/3.38in. That said, the length of the taper isn’t a strict number because it doesn’t affect the compatibility of the fork as long as it ends far before the top cup of the headset. Hence why this measurement varies across brands.
In rare cases, a tapered fork may end up being too big when installed on a very small frame with a short head tube. In that scenario, the fork is unusable with that particular frame.
To avoid this problem, one has the following options:
- Measure the head tube and compare its length to the tapered-to-non-tapered distance of the fork. If the head tube is longer than the distance, the frame can accommodate the fork.
- Contact the manufacturer of the frame to learn what type of forks it supports.
1 1/8″ x 1 1/4″ Tapered Forks
Another far less common tapered size is 1 1/8″ x 1 1/4″. Those dimensions are found mainly on carbon road forks. Since the number of 1 1/8″ x 1 1/4″ forks is quite scarce, some people consider the associated frames a less than ideal long-term investment.
Other Fork Sizes
Below you will find a list of the most common fork sizes:
- 1″ Threaded Forks
1-inch threaded forks are found on older bikes (e.g., retro MTBs). As the name suggests, the fork steerer is 1 inch in diameter and is secured via a headset that threads on it.
- 1 1/8″ Threaded Forks
Just like the 1″ threaded standard, this is an old size. The only difference is that the steerer has a larger diameter and is therefore considered “oversized” compared to 1″ models.
- 1 1/8″ Threadless Forks
Currently, this is the most common fork steerer on the market. Unlike the threaded models, it comes without threads and is secured to the frame by the stem.
- 1 1/8″ to 1.5″ Tapered
The most common tapered fork size.
- 1.5″ Threadless Forks
Forks with 1.5″ steerer from top to bottom. The benefit is that the fork is extra stiff. That said, such forks are rare and limit the rider’s choices.
FAQ: Why are tapered head tubes better?
The advantages of tapered head tubes are:
- Stronger headtube and downtube (potentially)
- Stiffer and more responsive front end
- Lighter fork steerer (potentially)
- Compatibility with modern high-end suspension forks.
For more information, consider reading the full article on the topic.
FAQ: Can you install a tapered fork on a non-tapered frame?
A tapered fork can be installed on a non-tapered frame with the help of an adapter that positions the low headset cup outside of the headtube.
The advantage of this method is that the user can combine a newer tapered fork of higher quality with an older frame. The downside is that the geometry of the bicycle is altered by approximately 1-degree due to the raised front end.
FAQ: Can you install a non-tapered fork on a tapered frame?
A non-tapered fork with a 1 1/8″ steerer from top to bottom can be installed on a tapered frame with the help of a reducer headset adapter which fills the gap that such a fork would otherwise leave.
The advantage of this method is that the user can buy a tapered frame and thus secure an upgrade path to a higher-end fork while running a cheaper straight fork in the meantime.
Summary: What You Need To Know
- The size of most tapered forks is 1 1/8″ x 1.5″. There’s also a less common 1 1/8″ x 1.4″ size.
- The length of the tapered steerer section is about 86mm/3.38in but may vary ever so slightly across different brands and models.
- In some cases, a tapered fork may be too long for a small frame. This scenario is fairly rare.
- Tapered forks come with a long threadless steerer cut according to the bike and the user’s needs.
- The main advantages of tapered forks are extra stiffness as well as stronger head and down tubes.
- A tapered fork can be installed on a non-tapered head tube via an external headset cup.
- A non-tapered fork can be installed on a tapered frame via a reducer cup.