sjanzeir

Not that I'm academically qualified to disagree with you, but I think should've been stated the other way around: If the spokes are pointing forward, they would be "pressed" towards the hub under braking, as opposed to being "stretched outward" if they were pointing backward - which is also in opposition to what Darth Lefty said earlier about the rotor buckling if the spokes were pointing to the back.

DaveSSS 

If center lock rotors are used, they only go on one way.

https://carsoncitybikeshop.com/brake...xoCwgoQAvD_BwE

Trakhak

For those who think the rotational direction of the rotor arms represents anything other than a marketing decision, perhaps you'd be interested in acquiring these cranks:

sjanzeir

I think those are designed to be mounted with the drive side on the left. If you install the chainring side on the right, that right angle in the middle of the crank arm is a weak point of failure that'll definitely fatigue and break off in the middle of a steep climb

Darth Lefty 

You're right. I've been had.

tyrion

Those "backwards" discs are specifically designed for use in the Southern Hemisphere.

PeteHski

I'm sticking with my first answer. If in the top photo on your first post, the wheel is rotating anti-clockwise as per the arrow, then if you clamp the disc the wheel will try to press the spokes toward the hub. The wheel is applying an anti-clockwise torque against the clockwise braking torque. Even with a degree in mech eng I still find forces and reactions tricky to visuslise,but I think this is correct. Otherwise the conventional direction of disc rotation would not make sense.

PeteHski

While those crank arms are complete bs, it's not really applicable to the stress distribution on disc spokes. The latter is not merely a marketing decision.

sjanzeir

Well, now that you put it that way, I see where you're coming from; we were talking about two different things, you and I. What you meant to say was that, if the rotor has been mounting with the spokes pointing to backwards, the braking force will try to fold the spokes down into the hub, What I was saying is that the braking force will try to stretch them outwards along their length. Still, given what you said, wouldn't the spokes actually be stronger and stiffer that way?

tomato coupe

Are you saying the "spokes" (of the disc in the first photo) are in tension or compression during braking?

PeteHski

In tension, but with a bending moment collapsing them toward the hub.

Mounted correctly that same bending moment would be straightening the spokes away from the hub

Whether or not it really matters is less clear. But the latter makes more sense to me.

tomato coupe

Agreed. So, the next question is why are most rotors mounted to put the spokes in compression during braking? I'm guessing it's because they want to utilize higher compressive versus tensile strength, and aren't worried about buckling. Maybe?

PeteHski

Yeah I would think buckling is a non-issue in this application.

Another thought is that the asymmetrical profile is probably less prone to resonance than a radial design, regardless of mounting direction.

It would be interesting to find out, but I can’t find anything obvious without digging further.

wolfchild

The arrow in the very first pic is correct and that's the way all other rotors without the arrow should be installed.

sjanzeir

Care to corroborate those statements?

wolfchild

I just looked at my AVID rotors which I have on my bikes and that's the direction that the arrow and rotor spokes are pointing. My SHIMANO rotors which I have on another wheelset are the same as AVID.

Trakhak

You and others in this thread doubtlessly know far more than I do about disc brakes (I don't own any bikes with them) and engineering, so I'm doubting myself regarding rotor arm orientation and whether (or how) it pertains to spokes and braking forces.

So my first thought was that if arm orientation really makes a difference in some tangible way, then by now, all disk rotors would be designed with the arms in the same orientation. Are they? Did a search . . .

Looked on Amazon for 180-mm disc brake rotors. Found many. The majority have the center portion of the disc rotated in one orientation with regard to the braking surface. A minority have the opposite orientation. (I checked the photos on the pages for a couple of those rotors, and they do show them installed with that orientation on a fork, so it's not that the main photo was inadvertently reversed.) A small number have the arms arranged tangentially.

By the way, a lot of the rotors I saw in my Amazon search had what looked to me like implausibly thin arms connecting the inner part of the rotor to the outer. I was picturing far more metal in the connecting material. So it definitely looks as if I was wrong and there might be some windup going on.

That said, I don't see how any such windup can somehow serve to protect the spokes from seeing greater forces than they would if the rotor were solid. I would think that the braking force transmitted through the hub and transferred to the tire is going to be what it is, regardless of what the rotor looks like.

So I'm far less sure than I was that rotor arm orientation is strictly a matter of marketing, but I'm hoping that someone will show up who has either designed bike disc brakes or at least done some of the pertinent math.

PeteHski

The braking forces don't change. It's the stress/strain distribution in the spokes that varies with their profile. So in a very simple case if you compare very thin spokes with very thick ones, the thin ones will be more highly stressed and deflect more for a given braking force.

PeteHski

Can you post a picture of your bike?

Here's a random Avid rotor

wolfchild

That's exactly where the arrow is pointing on my AVID rotor. and I have them installed with the arrow outwardly.. You can see the arrow when looking at the bike from the left side.

sjanzeir

Please, do look at these two rotors and tell me which one you think is "wrong:"



By the way, I'm running both of these rotors on my bike - a folding Dahon that I ride mostly on flat ground - and while I've got the Avid mounted in the orientation the arrow points to...



I mounted the Grimeca the "wrong" way around (according to Grimeca)...

PeteHski

So you need to look more closely at the arrow in the OP's first pic. It's the opposite to yours and that is the question raised in this thread.

wolfchild

It looks like some disc rotors are designed to be installed with the arrow facing inside in order for the rotor spokes to have proper orientation. The second pic of a rotor in OPs post is the outward side and the arrow is meant to be inside.

wolfchild

The first rotor needs to be installed with the arrow inside ( facing towards the fork leg ) in order for the spokes to have proper orientation. The manufacturer should really be printing that arrow on the other side of the rotor.

sjanzeir

If I put the rotor on with the arrow on the inside (which I did - see earlier post) then the arrow would be pointing in reverse!