# Thread: Rotor Tech - Shape

1. ## Rotor Tech - Shape

Can someone tell me why the arms of most (not all) brake rotors are, for all practical purposes, oriented to operate in compression rather than in tension?
TIA

KLF

2. It's about "load" stress versus "sheering" stress. Note that the quoted terms might not be what a structural engineer would use, but the gist of it is that a structure can be built to handle a load predictably, while the oposite, which would be sheering if sideways (which your "tension" type would be, effectively) tends to be less predictable in behavior.

3. I appreciate the answer and question but don't think that is what the OP was after.

I think he he is talking about push/pull forces parallel(same axis) to rotor, while you compare forces in different axises.

Motorcycle brake rotors don't usually have this design.

Estethics is what I am going with, hopefully someone else has a better answer and explanation.

I don't know the answer to the question

4. Originally Posted by Learux
I appreciate the answer and question but don't think that is what the OP was after.

I think he he is talking about push/pull forces parallel(same axis) to rotor, while you compare forces in different axises.

Motorcycle brake rotors don't usually have this design.

Estethics is what I am going with, hopefully someone else has a better answer and explanation.

I don't know the answer to the question
Actually, Cotharyus' answer is basically the reason I have heard.

"Compressing" the rotor arms makes them more stable.

5. I see what he is saying, with sheering I think he ment compressing.(with the chances of the material buckling)

That must be it, with the help of the two of you, I understand it now too

6. With the arms pointing forward.....the arms try to bend outwards this puts the braking surface of the rotor in tension...all the way around.

When the arms point backwards the arms try to bend inwards this puts the braking surface of the rotor in compression.....but of course the pads also try to put it in tension...

That means at some amount of braking force the braking surface will actually alternate between compression and tension as the rotor rotates....not a good idea...leads to wierd vibration patterns.

7. Originally Posted by jeffscott
With the arms pointing forward.....the arms try to bend outwards this puts the braking surface of the rotor in tension...all the way around.

When the arms point backwards the arms try to bend inwards this puts the braking surface of the rotor in compression.....but of course the pads also try to put it in tension...

That means at some amount of braking force the braking surface will actually alternate between compression and tension as the rotor rotates....not a good idea...leads to wierd vibration patterns.
I understand what you are describing but I'm not sure the alternating forces can be avoided.
But it does help to recognize that the braking force is applied to the rotor braking surface only at the caliper.
Regardless of what the arms are doing, the rotor/disc surface just "ahead"of the caliper has to be in tension to create a braking force in the proper direction, but the portion "behind" the calipers, similarly, has to be experiencing a compressive force in the direction of braking. OK, so maybe that is only a compressive force in the sense of being a reduction in the tensive force.

8. Hope has rotors that have straight arms, but they also have more supporting arms: Photo

Formula has a couple of straightish armed rotors: Photo 1 and Photo 2

Rotor on the wrong way around: Photo 1, Photo 2 and Photo 3.
Those last 3 photos were from this discussion.

9. Originally Posted by KLF
I understand what you are describing but I'm not sure the alternating forces can be avoided.
But it does help to recognize that the braking force is applied to the rotor braking surface only at the caliper.
Regardless of what the arms are doing, the rotor/disc surface just "ahead"of the caliper has to be in tension to create a braking force in the proper direction, but the portion "behind" the calipers, similarly, has to be experiencing a compressive force in the direction of braking. OK, so maybe that is only a compressive force in the sense of being a reduction in the tensive force.
The key point being "at some braking force the rotor can alternate between tension and compression as the caliper passes the arms.

With the arms forward is is tension and yes tension does vary, but it can never pass into compression at any braking force.

10. I believe that since the only major forces involved are tangential, you will end up with half the arms in tension and the other half in compression during braking (think of the forces in the braking surface of the rotor itself - half the rotor is moving futher from the caliper and is therefore in tension, the other half is moving toward the caliper and thus is in compression).

The likely reason for the arms being angled the way they are is to reduce shear stress. If the arms were tangential, all of them would be subject to pure shear stress along their narrowest dimension - the angle directs the shear stress into a wider cross section while converting some of the shear stress to tension or compression depending on the arm location relative to the caliper.

-Pete

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