October 27, 2017 3 Comments
As gravel beckons, where are we with disk road wheels?
Disk brakes have come late to road bikes and, until recently, have met a cool reception. Two questions arise:
1/ Will the feel and response match the best rim brakes?
2/ How well will road wheels support disk brake loads so fundamentally different than rim braking?
Question 1 is still open but not slowing disk brake adoption. Most riders feel net benefits and makers have responded quickly with many designs. For some applications disk brakes are clear winners and in others, the outcome is closer, perhaps more subjective and aesthetic.
Question 2 is solidly answered. Wheel forces generated by disk brakes are not significantly more challenging than with rim brakes. No surprise, as slowing down dissipates energy that wire wheels have managed for 150 years.
Disk brakes operate from the hub and the wheel sees torsional force rather than the distributed radial force of rim brakes.
Good news about all these forces:
• they are relatively small
• torsion from braking is the same, but reverse, force as pedaling
• tangentially spoke wheels (better for torsion) have no significant drawbacks
Relatively small forces
The bicycle is unique in our transport world because it has such a high center of gravity compared to wheelbase.
Deceleration begins with friction at the front tire ground contact. The bike’s mass wants to topple forward well before any serious G’s develop. Result is less work for brakes and wheels. A bike crashes before torque from braking becomes significant. Wheels that can take pedal force have no trouble with disk brakes.
Low brake force is even more the case in wet or unpaved situations. Both reduce traction which reduces deceleration potential and demand on the wheel.
Pedal force = disk brake force
Tangential spoking is a great solution for torque force. All spokes contribute at once. Half lose and half gain small amounts of tension. The wheel is barely stressed. Below are results calculated (and empirically tested) for hub windup, a good indicator of the work a wheel does with torque load.
Calculations based on a small flange hub laced to a lightweight tubular rim with various spoke patterns. The test torque of 50 kg-m at the crank is equivalent to full body weight on the pedal in a middle gear. You can see that hub windup is fractions of a degree, for example — only 1/4 degree for a X3, 32 wheel.
Tangential spoking has few costs:
• slightly longer spokes weigh more
• longer spokes are more elastic
• lateral stiffness might be reduced
• crossed spokes are less aerodynamic
• rims may not allow nipples to aim correctly
A set of longer spokes may add 5g to a wheel, a 1/2 to one percent change, insignificant.
Since torque demands small changes in tension, the difference in elongation is less than 0.1mm, microscopic.
Lateral stiffness is a function of hub geometry and rim diameter. Best to visualize a cross section triangle, radial to the wheel. The triangle's base connects the hub flanges and the sides connect to the rim. This shape does not change when spokes change their cross number. Lateral stiffness is unaffected.
Wind resistance is primarily a function of rim shape, tire-rim match, spoke number, and spoke shape. Cross pattern, like nipple exposure, is way down a list of aerodynamic factors.
A spoke should not need to bend as it reaches the nipple. This begins, for many rims, when the entry angle exceeds 10° (below 90). The calculator at GRIN gives an angle for any theoretical wheel. For most non-motor hubs and full size (26, 27, 700, 29) rims, these angles are easily managed.
So go forth and design, build, and (best) ride disk brake wheels in demanding environments. Wash, eat, sleep, repeat!
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