# Thread: Rotor performance - is there a difference?

1. ## Rotor performance - is there a difference?

Looking at some of the lighter replacement rotors vs. typical stock rotors the biggest visual difference is the number & size of the knockouts in the brake track.

As the knockouts get larger, like that last one, I wonder if it's having a cheese-grater effect on the pad - anyone experience this?

Also it seems like the more material removed from the brake track the less actual friction you can create, meaning lower braking power. *Does this bear out in practice?

Any other concerns?

2. I could swear the fancy Avid rotors - G3? - are better than Roundagons.

The knockouts are supposed to help the pads stay clean and the whole system manage heat. While I doubt that heat management is important for me personally, big dudes on long descents report problems at times.

As far as friction is concerned, it's typically calculated as a coefficient based on force or pressure. There "should" be no change. In practice, my Roundagons worked like ass when they got covered in mud, and I've had more consistent behavior with fancier systems.

There's some sizing funniness between Shimano and everybody else. That may or may not effect you.

3. Originally Posted by AndrwSwitch
As far as friction is concerned, it's typically calculated as a coefficient based on force or pressure. There "should" be no change.
This is kinda what I'm driving at - I think it's something like [total friction = coefficient x force x surface area], so what happens when surface area decreases? It'd follow that force would have to increase to maintain total friction. I'm at the limits of my physics abilities here

Also assuming that pads are incompressible the knockouts shouldn't affect the pad life, but that's a big assumption I think. But maybe it's true.

4. No, friction is calculated as either coefficient times force OR coefficient times pressure times surface area. Which is the same thing. Of course, it's a mathematical model. The only guarantee is that it's at least a little bit wrong.

I would expect the knockouts to cause faster pad wear. But not by much, and that's just a gut reaction.

5. I run Ashima Airotors (you can look those up) on my FS geared bike and on my rigid SS. It has no affected braking performance for me on either bike (Avid XO's on the FS, BB7's on the SS) nor has it really caused any accelerated pad wear, it seems. Now - I know a bit about physics, so let me say this: I'm CERTAIN they do increase pad wear. It's just a question of whether they increase it significantly or not, and I would say no, they don't. Now, for the big thing. There are no long/big descents around here. I do a lot of cross country riding, and I'm a pretty good sized guy. With my pack on, I weigh about 220. Even at that, and even having some places where I brake pretty hard, my rotors are not discolored. But if you switch, you need to keep an eye on that because with these rotors that have less material, heat management will be your biggest concern. Because they have less material, they will heat up faster, and not dissipate the heat as well. So, your current rotors not being discolored, then switching to something with more cut outs, and having those become discolored would indicate you are, perhaps, pushing your luck, and may experience some serious brake fade.

6. The first two rotor pictures you have are the Magura Storm and the Storm SL rotors, both of which I am using...

Yes, there is a difference between the two...the Storm handles heat better for prolonged braking especially during long descents avoiding brake fade compared to the Storm SLs. But for feathering when riding the trail, the Storm SLs are better performers....

7. For a given RPM friction will be

(coefficient of friction)*(applied force)*(pad contact area)

In the simplest case a solid rotor will give greatest friction. Cut-outs will reduce pad contact area (per revolution). However heat degrades coefficient of friction, so there is a benefit to cut-outs to the extent they dissipate heat.

A good approach would be to optimize two competing processes: (maximum surface area) + (maximum heat dissipation)

My guess is that manufacturers over do the cut-outs because it looks trick and saves a few grams.

Supposedly XT ICE rotors optimize heat dissipation

8. ^^ This makes good sense and I think you're right. But ultimately if the rotor grabs sufficiently & real-world performance is intact I say cut them up & make 'em look cool.

The Ice-tech's I believe are a sandwich of aluminum & steel and aluminum supposedly dissipates heat 4x better than steel so it seems like a sound idea.

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