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Discussion Starter #1
While I had the front wheel off today (replacing fork oil), I noticed one of the brake rotors is showing a fair amount of rotational float. I know they're supposed to, but I can't seem to find any info about how much is too much.

 

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While I had the front wheel off today (replacing fork oil), I noticed one of the brake rotors is showing a fair amount of rotational float. I know they're supposed to, but I can't seem to find any info about how much is too much.
I'm not an engineer, but I'm having trouble understanding why floating this way would be desirable. I know the calipers have to move laterally, but I don't see why the rotor would need to rotate forward or back relative to the wheel. Is this "play" simply to allow for expansion due to heat?
 

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Since the rotor is not flat a solid attachment to a differing metal especially would cause the rotor to dish rather than stay flat. There isn't a wear limit that I've seen. As long as the movement is axial, it shouldn't be a problem.
 

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Discussion Starter #4
Since the rotor is not flat a solid attachment to a differing metal especially would cause the rotor to dish rather than stay flat. There isn't a wear limit that I've seen. As long as the movement is axial, it shouldn't be a problem.
As I understand it, axial float would be movement along the axis -- perpendicular to the rotation of the wheel -- which of course is the main point of floating disks. I'm concerned about the rotation of the disk in relation to its hub.
 

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The braking surface is steel and the hub is aluminum. The aluminum expands more with heat than the steel. The play is necessary to allow the aluminum room to expand, not increasing the inside radius of the steel more than the increase of the outside radius, causing dishing. Axial float is rotational movement around the axis, not along it. Movement along the axis is handled by the floating caliper.
 

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Axis, a line around which an object rotates. Like an axle. Axial float as I know it is movement along the axis. Traveling along the axle so to speak. Also known as end float. Axial Float - Dictionary. If this rings true for the disc then axial float of the disc refers to side to side travel along it's rotational axis. The disc buttons are then used to allow this motion as required, without damage to the disc mounting. BrakeTech
 

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Golden Monkey; After a second look at your picture I think I see what your referring to. The disc is actually rotated around the mount a little. I would assume it will rotate back a similar amount. This would be a product of the free play in the disc buttons, but I don't see any mention of a limit for that in the manual either.
 

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Discussion Starter #8 (Edited)
The braking surface is steel and the hub is aluminum. The aluminum expands more with heat than the steel. The play is necessary to allow the aluminum room to expand, not increasing the inside radius of the steel more than the increase of the outside radius, causing dishing. Axial float is rotational movement around the axis, not along it. Movement along the axis is handled by the floating caliper.
I do understand about play being necessary to accommodate different rates of heat expansion and so forth. And normally I would absolutely defer to whatever you say, but your definition of axial float doesn't agree with various other sources I found while researching this question.
Axial Float - DiracDelta Science & Engineering Encyclopedia
Axial Float - Dictionary

I think you mean radial float? That would be movement along the radius, i.e. perpendicular to the axle. This link is in reference to cutting tools, not brake rotors, but the concept is the same: Tapping Holes - Basics of Cutting Tools

However, neither of these is the kind of movement I'm talking about, which is rotational. Honest bob got it -- my concern is the bobbins being worn or compressed, thus allowing too much radial float and/or rotation of the rotor in relation to the hub. It seems like you're saying don't worry about it, and I'm not that worried. Just wondering how much is too much, and would indicate a need to replace the bobbins. Sorry if my original post wasn't clear enough.
 

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It could very well be I've been using the wrong terminology. Still, I've never heard of a single problem in that area.
 

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Question - can you grip the rotor and move it around?

Floating rotors are always a bit weird when cold....
 

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Golden Monkey; I looked at my front discs on an 07 with 16000km on it and I can't get the discs to rotate around the mount at all by hand. Do you feel any vibration when applying the brake? I dug this info up from the renowned EBC co. Information on EBC Brakes' S-Drive Brakes They have come up with a solution for the very problem you have encountered. It must be common and not desirable. There is a short video, a written explanation and two good pictures showing what our disc buttons look like, and what their new square drive buttons look like. I think you could ask them about your situation, by e-mail, if you click on "contact us". If you can, let us in on their response. I think we need to coin a new word to describe this type of float. How about circumferential float? Radial float would be travel/expansion along the radius.
 

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Discussion Starter #12 (Edited)
I tried moving the rotor by hand before I put the wheel back on earlier today, but it wouldn't budge -- not really surprised. I'm just about to do a quick test ride; I'll report if anything feels weird. The only brake strangeness I've noticed before is an occasional bump at the lever if I'm on the brakes when I hit a sharpish bump. It feels almost like an ABS pulse, but more violent.

Honest bob: Brembo has a new float system too: Brembo - New T Drive Disc - 1

While "circumferencial" is probably more accurate, I like "rotational float," mainly because that's what I've been calling it. :fineprint:
 
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