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Lew PRO VC-1 bead-seat clincher (BSC)™
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Shimano or Campagnolo cassette
Lew PRO VC-1 bead-seat clincher (BSC)™ Technical Data |
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Figure 3
Bead-Seat Clincher (BSC)™ |
A traditional clincher rim attaches the tire by lodging the tire bead under a hook in the most outer diameter of the rim. This style of clincher rim is typically referred to as a hook-bead or crochet style rim. As the tire pressure increases, the force on the outer diameter hook portion of the rim increases. At 145 psi (10 bar), the hook must retain approximately 7975 pounds of total force from the air pressure in the tire. Figures 1 and 2 show a properly attached tire comparing a hook-bead clincher and a Lew PRO VC-1 Bead-Seat Clincher (BSC)™. |
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The VC-1 BSC™ design relies on the inner diameter of the tire chamber to register the bead of the tire (Refer to Figure 2, Figure 3, & Figure 4). The means of attachment of the tire to the rim is accomplished by the bead registering or seating on a perch located on the inner diameter of the rim chamber (Figure 4). This is a commonly accepted and understood wheel/ tire interlock for motorized vehicle wheels and tires, and it directs the 7975 pounds of force inward, toward a more secure position. |
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It is partially true that high pressure tires require a hook-style rim for retention. Metal wire bead tires tend to stretch, and particularly high pressures can grow the bead diameter a significant amount to dislodge the tire from the rim without the hook for retention. With the wide availability of folding, Kevlar™ bead tires, there is no concern for the diameter of the bead to increase with pressure. Kevlar™ does not elongate, and Kevlar™ beaded tires do not stretch. A Kevlar™ bead is the first step required to eliminate a hook bead design. A hook shape in the outer edge of the rim creates a stiffer, stronger rim than a straight sidewall of the same wall thickness without the hook. The problem begins when carbon composite materials are applied to a traditional alloy design. The factor of safety of a carbon fiber hook in a carbon fiber hook-bead rim design is dangerously small as compared to the hook of an alloy hook-bead design. |
Tire bead register on inner diameter
of the rim chamber |
Compression strength for aluminum alloy is approximately 42,000 psi as compared to just 28,000 psi for carbon fiber. If off-axis fibers and epoxy resin are taken into account the resistance to failure from compression in the hook bead region of a carbon fiber clincher drops to an amazingly low 4,608 psi, approximately one-tenth the strength of aluminum. The clincher tire is generating 7975 total pounds of outward force, so it’s no wonder hook-bead carbon clinchers’ durability suffers at higher tire pressures.
Fortunately, there are several remedies:
1) Change the nature of low compression strength carbon fiber by reinforcing it with Boron fiber, which is roughly equal to aluminum in compression.
2) Modify the tire-rim interface to a more composite friendly design-- one that utilizes the outstanding tensile properties of carbon composite, while minimizing the effect of the poor compression strength.
3) Add material to the critical high-stress regions where the tire loads the rim with the most stress.
The VC-1 rim contains three times the amount of material in the sidewall, and tire interface region than a traditional hook rim clincher, thereby improving the strength over a typical carbon hook-bead clincher design by over 300% without factoring in the added compression strength in these areas from the boron fiber structure. As a result of the BSC design, the composite material in the sidewall is not subject to compression as a significant stress, but rather tension, where the composite material performs the best. |
There are additional benefits to the VC-1 that prevent rim deformation under high-heat braking conditions. First, VC-1 rims use a reinforcement quality (boron and high-modulus carbon) that far exceeds any other rim manufacturer’s standards modulus reinforcement. Second, high-temperature resin resists temperatures exceeding 400 deg F. Third, and most significantly, the high-pressure tire focuses the bead of the tire toward the bottom of the rim chamber opposite the point of highest load as opposed to upward and outward toward the hook of a traditional rim. |
The VC-1 design eliminates the possibility of the tire detaching from the rim unless the tire itself fails. Tire failure in this sense does not refer to punctures, but it refers to complete loss of annular tire structure, so that the tire would no longer remain in an annular form but instead have two disconnected ends.
The strength of the VC-1 mechanical attachment increases as tire pressure increases, forcing the tire bead against the ID of the rim to a more secure position as opposed to forcing the bead outward toward the point detachment from the rim. |
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A traditional clincher rim-tire interface has a relationship similar to a hook pulling on a rope, specifically the radius of the rim hook which is 1.5 mm (Figure 13). The VC-1 design has a rim-tire interface has a relationship similar to a rope pulled over a large diameter barrel, more specifically the diameter of a bicycle wheel bead seat which is 622 mm, over 414 times larger than the 1.5 mm traditional clincher rim hook bead (Figure 14). The Lew Racing Pro VC-1 BSC design creates a more stable, less contorted shape between the rim interface and tire interface by exiting the rim in a direction parallel to the side of the rim as opposed to hooking inward, and then outward. This parallel exit reduces the stress applied to the tire, and creates a more favorable tire shape to present to the road, and significantly reduces the possibility of pinching the inner tube, minimizing the possibility of a flat tire. |
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Below are links to the entire White Paper and Tire Charts from Paul Lew |
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