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Home   Blog   Issues with Disk Brake Wheels

Issues with Disk Brake Wheels

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.

 

Gravel before disks, Jobst on the Gavia, 1978

 

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.

 

Wheel rolling right to left, clockwise hub torque from disk brake and spoke tension change

 

 

 

front rim braking and spoke tension change

 

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.

 

Rider Center of Gravity

 

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.

 

45mm hub, 350g tubular rim, 100kgf average tension

 

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.

Drawbacks
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

Weight
A set of longer spokes may add 5g to a wheel, a 1/2 to one percent change, insignificant.

Elasticity
Since torque demands small changes in tension, the difference in elongation is less than 0.1mm, microscopic.

Stiffness
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.

Aerodynamics
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.

Nipple aim
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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3 Responses

AJ
AJ

November 02, 2021

I always wonder why force from disc brake is much greater than rim brake. It is relatively smaller ‘area’ to stressed compare to rim brake.

And this is why I don’t really trust mechanical disc brake is far superior than rim brake.

Aaron Goss
Aaron Goss

November 02, 2021

I have been building disc wheels for 20 years now. Absolutely NOT New. Zero difference in strength or durability in the real world. Very geeky article, though.

Alex Hearn
Alex Hearn

November 02, 2021

Would love to see a similar ‘brakedown’ of calculated forces for an avg MTB rider under hard braking on level ground. The calc’s are beyond my skills but surely they’d be much higher than hard pedal forces since you can stop much faster than you can accelerate. My premise is reduced spoke count disc wheels have a certain go/no-go point, but this is gut & not science.
Thanks for all the insight guys, great work in this blog!

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