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post #1 of 12 Old 09-05-2010, 12:23 AM Thread Starter
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excessive rotor float?

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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post #2 of 12 Old 09-05-2010, 12:05 PM
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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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post #3 of 12 Old 09-05-2010, 05:25 PM
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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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post #4 of 12 Old 09-05-2010, 06:47 PM Thread Starter
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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.
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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I think we are clinging to a great many piano tops in accepting yesterday's fortuitous contrivings as constituting the only means for solving a given problem." - R. Buckminster Fuller
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post #5 of 12 Old 09-05-2010, 07:00 PM
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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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post #6 of 12 Old 09-05-2010, 08:17 PM
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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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post #7 of 12 Old 09-05-2010, 08:34 PM
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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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Last edited by honest bob; 09-05-2010 at 08:35 PM. Reason: changed text
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post #8 of 12 Old 09-05-2010, 09:26 PM Thread Starter
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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.
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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2005 DL650 - dearly departed
"I am enthusiastic over humanity's extraordinary and sometimes very timely ingenuities. If you are in a shipwreck and all the boats are gone, a piano top buoyant enough to keep you afloat that comes along makes a fortuitous life preserver. But this is not to say that the best way to design a life preserver is in the form of a piano top.
I think we are clinging to a great many piano tops in accepting yesterday's fortuitous contrivings as constituting the only means for solving a given problem." - R. Buckminster Fuller

Last edited by The Golden Monkey; 09-05-2010 at 10:05 PM.
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post #9 of 12 Old 09-05-2010, 09:50 PM
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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.

Pat- 2007 DL650A was ridden to all 48 contiguous states. I didn't quite make it to 17,000 miles on the 2012 DL650A.
Nicknames I use to lessen typing, Vee = 2002-2012 DL1000s. Vee2=2014-2016 DL1000s. Wee = 2004-2011 DL650s. Glee = 2012-2016 DL650s
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post #10 of 12 Old 09-05-2010, 10:59 PM
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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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