Stem Clamp Diameters: A Brief History + Survey Results

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Ever wonder why there are so many handlebar and stem clamp diameters? Or which are the most popular? In this piece, we look at how the main five stem sizes came to be and dig into each’s history before revealing our survey results, in which close to 1,300 readers responded with their preferred stem clamp diameter…

It’s becoming increasingly more common to hear readers expressing the need for (and absence of) 35mm diameter handlebar attachments and clamps for bike bags and accessories. This recently piqued our interest. How many people are using newer 35mm handlebars? And how does that compare to the number of riders using the more prevalent 31.8mm diameter, or other sizes such as 22.2 or 25.4? We recently published a survey to find out. Read on to learn about the five most common stem clamp diameters and find the survey results indicating the most popular sizes from nearly 1,300 submissions.

Stem clamp diameter, handlebar clamp area

Five Stem Clamp Diameters: How we got here

One of the most significant advancements in modern bicycle technology came in the early 1990s when American engineer John Rader created the threadless stem and headset most of us use today. At the time, it was a significant departure from the traditional threaded systems. Rader’s design eliminated the threaded steerer tube, replacing it with a clamp-on stem system that employed stackable spacers and a top cap for bearing preload.

Aheadset Patent Drawing, John Rader

This innovation modularized the stem, simplified installation, enhanced adjustability, could reduce bike weight, and spawned a variety of different stems and stem clamp diameters. Nowadays, we have handlebars made with clamp areas ranging from 22.2mm to 35mm. It’s a pretty interesting progression. Find details on each of the five main diameters below.

22.2mm (78”)

The 22.2mm (or 7/8″) clamp area is by far the simplest of the five. Since the clamp diameter and the grip area are both 22.2mm, bars are made from straight-gauge tubing, which is much easier to fabricate. Bar tubing is simply cut and bent to achieve rise and sweep. Tapered handlebar designs are more complex and have to undergo a shaping process prior to bending that typically involves butting and swaging: butting alters the tube’s wall thickness, and swaging modifies its diameter.

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Of course, 22.2mm bars aren’t as stiff as tapered bars. Even so, they’re commonly used on BMX-style handlebars today, integrating a cross-bar to increase stiffness. That being said, a number of smaller builders using straight 7/8” tubing for mountain bike handlebars, such as the Doom Bars shown above.

25.4mm (1”)

In the early 20th century, efforts were made to establish some degree of standardization for handlebar clamp area diameter. However, it wasn’t until the mid-20th century that the industry settled on a few key sizes. The prevailing size became the 25.4mm (1”) clamp diameter which was set as an ISO (International Organization for Standardization) standard for road and city bikes and later became the standard for mountain bikes.

26.0 inch handlebar clamp diameter

25.4mm bars and stems remain a popular option among bike restorers, vintage tinkerers, folks who breathe new life into 90s mountain bikes, and those who appreciate retro vibes. As shown above, Lucas runs a 25.4mm MAP + Ahearne bar on his commuter. And if you peruse the alt-bar options at websites such as Rivendell Bicycle Works or Velo Orange, you’ll find plenty of 25.4 options.

26.0mm (1 132”)

As technology evolved and bicycle disciplines diversified, so did stem clamp diameters. These larger diameters offer increased stiffness and strength, making them suitable for riding on rough surfaces or increasing power dynamics for sprinting and racing. Road bikes, for instance, saw a shift towards larger clamp diameters, such as 26mm (1 1⁄32”), which became the Italian standard and eventually the road standard for many years. During this time period, there were also intermediate sizes introduced, such as 25.8mm, to try and achieve compatibility with either the ISO or Italian standards.

31.8mm (1 14”)

As legend has it, the 31.8mm (1.25”) stem-to-handlebar interface was a happy accident. It was originally designed by Funn Components in the late 1990s for a bar called the Fatboy Slim. As the story goes, it was supposed to be 32mm, but due to materials contraction in the manufacturing process (and perhaps a stroke of fate), it ended up at 31.8mm. The Fatboy Slim’s popularity is largely responsible for the rise of the 31.8 stem clamp diameter, and it somehow stuck to become the prevailing industry standard still today.1

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While it can be argued that the move from 25.4mm to 31.8 was developed to promote consumer uptake, many would insist that as handlebars as they trended wider, this was a much-needed shift and that the larger diameters further increased stiffness and durability, making them better suited for rugged trails. Ultimately, the wide adoption of the 31.8mm interface spurred a massive range of stem and handlebar options from various manufacturers throughout the 2000s. At the time of publishing, 31.8 is the most popular size among our readers.

35.0mm (1 38”)

As mountain bike handlebars grew wider, some companies saw the need to increase the diameter yet again. In 2012, as handlebars were pushing past 800mm wide, component manufacturer Easton launched the 35mm (1 ⅜”) bar clamp diameter with a new handlebar and stem. This was specifically developed for high-load applications such as downhill mountain biking. Several companies followed suit, and over the last three or four years, it’s started to become the new norm on modern trail bikes.

Bar Clam Size

While some of us still see the 35mm handlebar as a little excessive, it can easily be argued that it’s a good move for carbon bars, which are a popular choice for modern mountain bikes. OneUp’s Carbon Handlebar is an impressive example of how these opposing thicknesses can result in a super comfortable carbon bar that’s proven to be durable too.

1 As reported by Travis Engel, BETA Mag, January 2022 (link)

Survey Says…

As we were discussing this topic internally, I jokingly suggested, “If PAUL doesn’t make a Boxcar for it, it’s a dead standard.” The fact that the Boxcar comes in 31.8, 35.0, and 22.2 almost accurately predicted the results of the survey we published in last week’s Friday Debrief. We had 1,233 responses, and you can find the results below. It’s also worth noting that the most accessory clamps and parts—such as bells, GPS attachments, light mounts, etc—are sized for 22.2 and 31.8mm diameters, though many include various shims.

Stem Clamp Diameters Survey

If you have any insight into the history of handlebar/stem clamp diameters, please leave us a note in the conversation below, as we’d like to add to this resource for reference in the future.

Further Reading

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