If your two-wheeled roots go back a bit, you're probably familiar with the 1/2- to 3/4-inch wide serrations in the bar clamp area on some stock and many aftermarket handlebars. These small metal ridges are called "knurls" or "knurling," and serve several purposes. When done right and assembled in the correct context—more on these variables in a minute—knurling can reduce the slipping of the bars inside the risers on your motorcycle. Before getting too far into the weeds on this edition of WTF?, let us explain how knurling is applied during the manufacturing process, and some of the technology's pros and cons in different applications.
Don't let the file-like surface serrations on knurled handlebars lull you into a false sense of security. Truth is, there's a science behind why smooth, non-knurled handlebars clamp tighter in steel risers, and this story explains why.
HOW KNURLING IS APPLIED
After cutting a section of raw handlebar tubing to the appropriate length for a specific bar design, this tube section is clamped into a lathe. Once he has identified the center of the tube section, the machinist uses a knurling bit mounted on the lathe's tool post to engrave a crosshatched pattern into the surface of the tube. Depending on the tool design, this pattern can look like the surface of a metal file, or simply as straight lines cut into the surface. Most knurled handlebars feature two strips of knurling spaced 3.5–4 inches apart in the middle of the bar clamp area.
KNURLING PROS AND CONS
Knurling can and does reduce handlebar slippage inside the risers, but only reliably in the following conditions:
- Knurled STEEL bars in ALUMINUM risers
- Knurled ALUMINUM bars in ALUMINUM risers
We emphasize ALUMINUM risers because this material is soft enough to let the sharp teeth of the knurled surface bite into the clamp material. Riser clamps constructed with steel (all Biltwell risers, for instance) are too hard to allow the knurling to bite into the clamping surface. As the accompanying illustration shows, knurling REDUCES the surface area between the handlebars and the riser, significantly reducing the riser's ability to clamp the bars tightly.
Maximum contact area inside the risers is crucial to ensure reliable clamping power around the handlebars. Knurling (left) reduces clamping surface area by creating peaks and valleys on the surface of the handlebar. Smooth handlebar tubing (right) provides full contact inside the risers to maximize both the surface area and clamping power of the riser.
STEEL IS REAL
As everyone who owns a set can tell you, Biltwell riser clamps are made of investment-cast stainless steel (Slimline and Thunder risers) or high-tensile steel (Murdock and Gordo risers.) We favor steel over most commonly used 6061 T6 aluminum to build handlebar risers, for the following reasons:
- Approximately 1.5x higher ultimate tensile strength
- Approximately 0.5x greater sheer strength
- Approximately 4x greater density
That last specification—steel's 4x greater density compared to aluminum—is particularly telling, as it explains why knurling doesn't cut into steel risers like many home customizers assume it does. Bottom line: we feel steel is a superior material for manufacturing handlebars and risers, especially ones engineered for spirited riding on a 700-pound motorcycle.
NOW YOU KNOW
Knurled steel handlebars certainly have their place, just not inside steel risers, in our opinion. As we've attempted to illustrate, the serrated surfacing can and does bite into aluminum risers. For reasons related to its superior density, tensile- and sheer-strength compared to aluminum, however, we prefer steel for manufacturing Biltwell risers. It's heavier and stronger than aluminum, and that's the point. Especially when you're installing them on a 600-pound Harley.
Handlebars and risers are critical parts that set the tone for the look and feel of your custom motorcycle. We make a wide range of both components, and have strong opinions about the materials and specifications we integrate into each one.