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Interbike Tech: Mad Fiber wheels

The $2,600 Mad Fiber wheelset from Ric Hjertberg is light, stiff and boasts low rotational inertia.

Day 1, Outdoor Demo West, LZ bike with Mad Fiber wheels
Lennard Zinn's bike equipped with Mad Fiber wheels.

Before I go into the details of why the $2,600 Mad Fiber wheelset is so different from others, you should know that the pair weighs 1,080 grams (!) and is very stiff. Being so thin and flat everywhere, they are probably quite aerodynamic as well.

I rode the Mad Fiber wheels on the first day of Interbike’s Outdoor Demo and was very impressed with their stiffness when sprinting hard, as well as their steering precision and low weight. The braking, with the supplied cork brake pads, was quite effective, and the superlight quick releases work nicely.

Ric Hjertberg, who, along with his brother founded Wheelsmith, later became the developer of FSA’s wheels and is the guy who brought wooden rims back to the U.S. market. Whatever the material, after all of these decades of working with bicycle wheels he understands what is required for performance and durability. And he recognized that his adopted hometown of Seattle — thanks to the presence of Boeing and high-end boat builders who cross-pollinate ideas with each other — is a hotbed of composite manufacturing expertise and infrastructure.

So when he decided to make his own all-carbon wheel, he consulted this brain trust and combined it with his bicycle wheel know-how rather than follow in the well-worn paths of other wheel makers. One notable difference, for example: The Mad Fiber rim and hub flanges behave as spokes.

Virtually all deep-section carbon rims are made by molding the rim under pressure and heat while pulling a vacuum on the carbon layers and providing pressure against the carbon layers from the inside by either inflating a bladder inside the rim or by inserting a silicone mandrel inside that is the shape of the inside of the rim. After the carbon cures, the flexible bladder or silicone mandrel is pulled out through a hole in the rim opposite the valve hole; the hole is patched over afterward. Molding may be done in an autoclave to increase the pressure on the layers.

A Mad Fiber rim, by contrast, is formed by molding two thin, flat rings (rim sides) and a rim cap out of carbon fiber, and then bonding them all together to form the hollow rim. The rim parts are molded under high heat and mechanical pressure while the layers within the mold are “vacuum bagged,” a process shared with the production of many carbon bike parts in which a suction is pulled on a sealed plastic “bag” surrounding the carbon layers.

When the resin in the fabric reaches curing temperature, it becomes as liquid as water and can wet all of the fibers in the layup as well as allow the vacuum to pull out almost all of the air trapped in the structure. This is a type of out-of-autoclave manufacturing, which is a hot topic among composites engineers, mainly because it saves the expense of autoclaves.

Day 1, Outdoor Demo West, hub flanges, rim section
The Mad Fiber's hub flanges and rim section.

Mad Fiber’s rim walls are very smooth, and the compaction of the layers and elimination of excess resin is high. The flat spokes and cylindrical hub shell parts are molded the same way. The outer layers of the rims, spokes and hubs are in a very broad weave of large bundles of fibers, giving them a distinctive look. A rim cutaway shows how thin and light these rims are, and Hjertberg chuckles, “People tell me I shouldn’t show this cutaway rim section to people, because it will scare them (because it’s so thin and light).”

The Mad Fiber spokes are bonded to the hub flanges and enter the rim through slots in the rim walls, where they are bonded to the inner walls of the rim. Hjertberg says that the gluing surfaces are seven times as big in area as the load limit of the adhesive, which he says means that you’ll “never see fatigue” in the bond. The section of the end of the spoke where it is glued against the inner wall of the rim is probably 4cm long and is covered with a carbon fiber patch to further reinforce the bond with the rim.

In a method reminiscent of the original Spinergy wheels, the hub flanges are pulled apart from each other to pull tension on the spokes. Each flange is molded along with a cylindrical hub shell section, and those sections are sized to slide one over the other. So once one section of hub shell along with its flange slides in place, it is bonded to the inner cylinder that is integral with the opposite flange.

Besides the manufacturing process, another unique feature of the Mad Fiber wheels is in how they carry load. The spokes, as they are bonded over a large area of the rim, out of which emanates bundles of fibers dispersing out to the rim walls, pull tension on the rim walls. This makes them act as spokes crow-footing outward to the top of the rim from the central spoke attached to the hub, rather than as static objects being pulled down in spots by spoke nipples.

Hjertberg says that fiber orientation is critical to the wheel’s characteristics.

“Anywhere there is a load, we want a fiber there and nothing extra,” he says. “We don’t want any fibers just along for the ride.” At their broad bases, the spokes also spread their tension out over the rim flanges as well. And on the drive side of the rear wheel, the spokes lock around the six sides of the large, hexagonal aluminum flange backed by a carbon flange.

The hub internals, which are made by White Industries, have oversized bearings and oversized 15mm cromoly steel axles, which are so big and stiff that White Industries only uses them in mountain-bike hubs.

Obviously, Hjertberg could get his wheels under a kilogram for the pair if he were to make his hub guts as minimal as those found in most lightweight wheels. But he is looking for loftier goals than mere wheel weight.

Hjertberg claims that the rotational inertia of his wheels is lower than any other, including the Lew-designed Reynolds RZR wheels, since the weight of his rims and spokes is so low.

And he claims that superlight bikes benefit from the stiffness provided by his large, stiff axles and bearings, both in the fork and rear triangle, the same way that closing the bottom of the “box” of a mountain bike fork or rear end with a stiff axle stiffens up a mountain bike and improves steering precision.