Just How Critical is Brake Rotor Direction?
 

Regardless of the reason - be it technical or esthetic - just how dangerous is it to mount a brake rotor the "wrong" way around? There are rotors from Grimeca with the arrow seemingly pointing in the wrong direction...

https://cimg3.ibsrv.net/gimg/bikefor...d03403c713.jpg


https://cimg0.ibsrv.net/gimg/bikefor...7ef4557fd2.jpg

And there are some Hope Tech rotors that don't have an arrow at all...

https://cimg5.ibsrv.net/gimg/bikefor...8690baa62c.jpg

https://cimg1.ibsrv.net/gimg/bikefor...c5dc68037f.jpg

I mean, in both cases, it would probably make more sense to mount the rotors with the "spokes/arms/whatever" pointing forward so that they compress rather than stretch out under braking, but, from a non-competitive, everyday-use kind of perspective, is the direction the rotor is mounted in really that critical?

I'd assume it would the other way around.
A thin strip of steel would be more stable to stretch than compress.

Barry

So, you're saying that, other than that Grimeca rotor above, all arrows on all brake rotors in all the world point in the wrong direction?

Never said that!
But if were handed a rotor with no arrow markings, I'd go for tension over compression.

But what do I know, I'm and electronic engineer turned IT guy.
Zero credentials in the ways of physical properties.

Barry

I don’t think “competitive” vs “non-competitive” makes any difference.

You're right, maybe it doesn't. But has a flipped rotor ever caused anyone to lose a race or be late to work (or worse?)

FWIW, the Shimano rotors on my bike are opposite of the ones with the arrow in the OP. I'd wage though that this thread is going to turn into a big pissing contest going 20 plus pages.

:popcorn

Yeah, I'm sure it will. I was sure that it would even before I started it.

Except for this and another type of Grimeca rotor (and a couple of Hope models that don't have arrows at all,) every rotor I ever see has its spokes pointing in the same direction as the arrow, just like the Shimano ones you mentioned. Which makes me wonder if Grimeca made their rotors this way by design or someone goofed up at the factory and they didn't think it was critical enough to initiate a recall. It also makes me wonder if Hope's rotors would be fine mounted either way.

I heard a statement once I’ll attempt to paraphrase.

The narcissism of petty differences.

As to the rotor direction. I would bet it doesn’t matter as long as it lines up with the caliper.

Good question. I’ve used at least a dozen different rotors in my time, and every one indicated a direction opposite the Grimea. In your first pic.

I assume they have there reasons,

And that's not the only Grimeca rotor that has its arrow pointing in the "wrong" direction:

https://cimg1.ibsrv.net/gimg/bikefor...4b7c680ca6.jpg

I've seen remarks being made online about this being/not being an issue dating at least as far back as 2005, but I've never seen any incidents being reported of people losing life and limb because of a backward-mounted rotor. Im fact, I've seen a good number of people - with some comments dating at least as far back as 2005 - stating that they never noticed any difference in braking performance or reliability with the rotor mounted either way. One rider quoted an email he got from Hayes in response to their query about this saying that the direction marking is just for better cooling.

The arrow points in the direction that would put the "spokes" in tension during braking. That said, my rotors all mount in the other direction.

The wrong way it can buckle, the right way it can’t. Follow the directions. Eeesh

FWIW, brakes are way more powerful than legs. Don’t screw around.

Thanks for this! I've been searching for the right description for the ceaseless, stupid, trivial, and mean-spirited arguments that have been going on. I was thinking of words like "no life", "juvenile", "insecure", "grandiosity", "manic" (i.e. the phase of bipolar often showing irritability and grandiosity at once), and many synonyms thereof.

Narcissism is a great descriptor.

So, you're saying that anyone who mounts their brake rotors the wrong way around is possibly a sociopath? :D

If you mount them backwards the brakes will make the bike speed up instead of slow down.

Yeah but that's better than what happens when you mount a chain backwards... :backpedal:

At least one Japanese crankset manufacturer made at least one model with arms that were not tangent to the center of the crank. Every example I ever came across had arms that were in tension rather than compression. My guess about why discs with swept-forward or swept-back arms appeared on the market is the same as it was for those cranks---that someone thought it looked racier that way.

The one benefit to directional arrows on discs is probably the same as that for directional tread pattern arrows on skinny bike tires: consumers asking fewer questions. You have to feel sorry for the crank manufacturer, since the crank can only be installed one way.

The use of tangential arms for discs to offer slightly improved cooling might make sense (depending on the designs being compared), since there could slightly more surface area for a given cross-section. Can't see how the choice of rotation direction would make an iota of difference, though.

Just follow what the manufacturer recommends and mount them the way the arrow is pointing. The ones without the arrow should be mounted the same direction as the ones with the arrow...It's that simple.

From earlier in the thread:

Megalomaniacal?

Anyway, i was thinking of starting a thread about the tendency for people (really bad people---like me :D ) to get tendentious and downright nasty in debates on this forum (as in almost every other part of life ... but ... )

I am on forums where no one would ever use the derision, the disrespect, that I and others sometimes use here .... and how can we all change?

Of course, we all know exactly how such a thread would turn out ..... but it would be popular until it was closed.

The problem here is that the OP's disc has the direction arrow opposing the normal mounting convention for disc rotors. It's not made out of some magic material, so it's obviously a production error. I would mount it like any other disc, i.e. ignore the arrow in this case.

Is it actually dangerous to mount it the wrong way round? I wouldn't like to answer that 100%, but as a qualified mechanical engineer I wouldn't expect it to assplode.

Convention being the operative word here...

FWIW, I have one of those Grimeca rotors in the OP on my bike and I have mounted it the "wrong" way around, i.e. with the spokes pointing forward (and the markings facing inward, just like in the second picture in the OP.)

I'm pretty sure that's the way Grimeca intended. They are using a very common spoke pattern, which I've never seen mounted the way their arrow points. But this has made me curious as to why this kind of directional spoke pattern is commonly used, as opposed to a radial pattern. A quick Google didn't help, so obviously not a common question. Neither are there any horror stories of assploding rotors mounted the wrong way.

I would think it is all about putting the spokes in tension, which is why they are invariably mounted with the spokes pointing forward. If you mount them that way then the spokes will tend to straighten out under load. If you mount them as per the arrow, the spokes will tend to collapse under load.

If you mount the rotor backwards, does it make it easier to ride the bike backwards?