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Deep wheels: An engineering feat that goes beyond aero

What's so deep about fast wheels? Numerous engineering complexities make deep-section rims hard to perfect.

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The sound of a deep-section wheel hurtling through a high-speed corner is the sound of modern racing. The wheels are ubiquitous at local crits and time trials because they’re fast. They also have unique idiosyncrasies.

Deep-section wheels are vertically rigid. This is a function of their geometry: A deep profile, created to reduce aerodynamic drag, makes for a vertically stiff structure requiring few spokes. However, most people don’t realize that deep-section wheels are laterally flexible. When sprinting, the rim tends to bend back and forth, often rubbing the brake pads. And due to the carbon layup, most also generally become slightly out of round when the tire is inflated.

Why are deep-section wheels so laterally flexible? Again, it’s due to geometry. The tall rim section, when pushed sideways against the road, acts like a lever that hinges where it meets the spokes — the deeper the rim, the greater the leverage on it. (See illustration below.)

Hinged at the spoke attachment, the rim leans further over than the plane of the wheel when pedaling the bike pushes the side of the tire against the road. There is no mitigating this because the rim is hinging beyond where the spokes can laterally control it. Changing spoke tension or spoke patterns will have no effect. Bill Mould, author of “The Bicycle Wheel: Physics & Engineering,” created an excellent online video illustrating this effect.

Deep-section rims are built from carbon fiber because aluminum rims of equivalent height would be very heavy and could not be rolled into a circle without wrinkling. Most deep carbon rims are molded into a single piece with the spoke attachments being at the bottom of the rim section (as with most rims). Thus, all of the carbon in the wheel is structural.

Some deep rims, however, consist of a shallow-section rim with a thin, non-structural, deep-section carbon “skin” fairing bonded to its inner diameter. Rather than attaching at the bottom of the deep section, the spokes pass through holes in the skin and attach to the shallow rim section, which dictates how the rim flexes. (Don’t hang a bike on a hook by one of these wheels, because the thin fairing would collapse.) Devoid of a standard deep-section wheel’s long lever, these wheels hinge less at the spoke attachment, resulting in reduced lateral flex when sprinting.

The deeper the rim, the greater the leverage the road has on it while leaned over. The road’s leverage on a shallow-section rim (left) is relatively less, so it is pushed sideways less than the deep section rim (center). This is especially true when the bike is heavily loaded, for example, when sprinting out of the saddle. A deep-section wheel designed with a shallow-section rim and a thin, non-structural carbon “skin” fairing (right) is also laterally hinged at the spoked attachment point; however, since this attachment is far inside, and not at the inner diameter of the deep section, it does not flex as much as the traditional deep-section wheel.

The carbon in both types of rims is “pre-preg” unidirectional carbon fiber fabric; the fibers are parallel and pre-impregnated with resin that bond layers of the fabric together. Fabric is freezer-stored to prevent the resin from curing, and it is cut and placed into molds in cool rooms to prevent the resin from sticking to cutting blades or work gloves. Fabric pieces are machine-cut and laid by hand in a prescribed orientation in the mold. After the material is heated, outward pressure from the bladder within the carbon compacts its layers against the inside of the mold, squeezing out excess resin.

In a shallow-depth rim, a long strip of fabric can be laid into the mold while folded lengthwise into a U-shape, wrapping the entire way around until the ends meet. This technique is generally not used with deep-section rims since the carbon fibers are so rigid that the strip’s outer diameter won’t stretch, and the inner diameter won’t contract; thus, the walls would wrinkle.

Instead, deep-rim sidewalls are generally made of arc-shaped pieces of straight fibers laid in a patchwork fashion in the mold. Conversely, the rim bed is made of continuous strips wrapped around the rim. Envision a wooden wagon wheel made of arc-shaped pieces of straight wood constrained by a flat, steel band wrapped around them.

As the pressure in a tire on a wheel increases, the rim width also increases (if it’s a clincher rim) while the spoke tension decreases because the rim’s diameter compresses due to the force of the air pressure. If the carbon layup were uniform throughout the wheel, the decrease in spoke tension would also be uniform. But on most deep wheels, the tension drop is not uniform, because the layup of arc-shaped fabric pieces is not the same at every spoke. Radial fibers compress less than fibers at low angles, so the spokes around radial fibers maintain the most tension. Spokes where the fibers are at the greatest angle to the wheel diameter lose the most tension. Consequently, the higher the tire pressure, the more the rim slightly loses its roundness.

Now you understand there’s a lot more to know about deep-section wheels than aerodynamics.

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