Wheel brakes

Jean Claude

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All bicycle brake discs are mounted in the direction shown on this picture.
That is to say with "spokes" working in compression rather than in traction (white arrows)
This seems to me to take the unnecessary risk of buckling.
Does anyone know the reason for this surprising direction?

Sans titre.png
 

Acamacho

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I just think they have put it together backwards without realizing it.
 

Jean Claude

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No, this is the direction requested by the manufacturer
Sans titre.png
 

wolfy

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Jc I totally agree with you the manufacturer want the spokes in compression.
I would never put those tiny 2 mm thick spokes in compression.
They are in tension. I ignored the arrow, the wheel spins the other way to your red arrow. The front of the gyro is to the right.

wolfy
 

Vance

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I would follow the manufactures recommendations.

There may be more going on there than is readily apparent.
 

bugflyer

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I found this on an Australian bike forum for what it's worth:

The reason for the spoke design is that there are two sources of stresses in the rotor. The first is mechanical stresses due to torque and the second is thermal stresses within the rotor. As the braking surface heats up, it expands. The inner portion of the rotor near the hub is comparatively much cooler. With the outer braking surface expanding with higher temperature and the temperature of the center remaining largely unchanged a thermal stress is imparted on the spokes. The spoke design is specified such that the mechanical stresses and the thermal stresses occur in opposite orientations, attempting to cancel each other out and lowering the total stress in spokes as opposed to adding together. The result is the “sweeping forward” spoke pattern.

smiles,
Charles
 

Tyger

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I believe that heat distortion – and heat dissipation – is indeed of primary concern in these designs.
 

bugflyer

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Makes sense to me.

smiles,
Charles
 

Jean Claude

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Thank you, Mike
A link ?
 

bugflyer

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Jean Claude

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About 500 h (FW + ultra light)
After thinking about it, I don't believe that the direction of the effort in the spokes is chosen for reasons of thermal expansion of the disk.
This only produces a bending stress in plane for the spokes, whatever the direction of the stress.
(See top picture)
Rather, I believe that the spoke compression helps push the disc against the fixed caliper pad during braking and correspondingly reduces the required clamping force .
While the extension force of the spokes tends to move the disc away from the fixed caliped pad and increases the required clamping force.
(see bottom picture)

Sans titre.png
This effect does not exist for motorcycles since they have sliding calipers instead of fleching discs

Sans titre.png
 
Last edited:

wolfy

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Jc the bicycle calipers have a piston/pistons either side of the disk. There is no fixed pad, both sides of the caliper compress against the rotor.

wolfy
 

XXavier

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Jc the bicycle calipers have a piston/pistons either side of the disk. There is no fixed pad, both sides of the caliper compress against the rotor.

wolfy


Not all... In my bicycle, at least, each brake has just one piston. The disk warps a little when braking...
 

Jean Claude

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About 500 h (FW + ultra light)
Yes, when they are hydraulic, Woolfy.
Disc mechanical brakes have a single moving pad pushed by a ball cam. This pad pushes the disc against the fixed pad.
This requires easily flexible spokes
 

Vance

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Yes, when they are hydraulic, Woolfy.
Disc mechanical brakes have a single moving pad pushed by a ball cam. This pad pushes the disc against the fixed pad.
This requires easily flexible spokes
All of the single acting calipers I have seen allow the caliper to slide so the disk doesn’t have to flex.

The Predator uses single acting hydraulic calipers and the disk does not flex.

I am not familiar with all bicycle brakes.
 
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