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brake shudder update

991 Views 3 Replies 2 Participants Last post by  Retro
Ref my earlier post on this topic.
After installing new pads, bouncing out the front, checking steering head bearings, etc.
The shudder still occurs occasionally. Only right before you stop at moderately hard braking levels. At this point I am confident the issue is NOT warpage. I've tested enough to see the shudder/vibration occurs in less than a wheel revolution, right before you stop. A simple warped disc (think potato chip) would not do this. You would feel that with each wheel revolution at higher speeds. This is a fast vibration, that happens at a high rate compared to the wheel revolution. Actually, I can see bluing marks between the holes in the disc. To make a long story short, I now suspect the vibration is at a frequency closer to that of the spacing of the holes in the disc than a wheel revolution. I'm NOT saying it's the holes in the disc! Just that potato chip style disc warping is very unlikely to be the issue.
At any rate, on my bike, this happens just as I stop, and is thus not a real safety issue. And it happens only occasionally. I have not had it happen enough to characterize exactly when it happens.
My plan is to fully scrub in the discs and pads, and re-evaluate then. Currently, you can still see honing marks on the disc. After I wear it solidly in, we will see if it still happens.
If I had to guess/speculate wildly: I'd bet on a new disc or new bike, the drilling may raise the edges of the holes. Or maybe the holes heat up differently than the surrounding material. And thus sometimes cause this vibration when you are going slow enough to have the individual rows of holes upset the pad as they pass. My only evidence for this the slight bluing of the disk in spots between the holes. Like I said, wild speculation. YMMV. FWIW.
I will report again in a couple hundred miles.

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I think you've about nailed it.

Just to geek out for a bit, I think the shudder happens when frequency of the blued areas (non-holes) passing the pads corresponds with the natural vibration frequency of the forks.
Like the forks on a large H-D idling at a stoplight; I've seen 'em vibrating about an inch at the axle.

BTW, that Carroll Smith article was very educational. As far as applying some of it to our Monsters, we need to take into account that the OEM rotors are Stainless Steel. Virtually every automobile on earth has cast-iron rotors, and that's where Carroll is coming from.
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