[WetShrub]

A friend of mine has an F4 track bike and needs help with his front vibrating under braking. Here is his info:

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OK, so this is what I know.
I have replaced my pads recently and hit a deer (not on the track but that is why my bike is a track bike only now). I am getting serious vibrations on any type of braking. I initially thought that the rotors were warped but when I spin them I see almost no movement.

My question is this, what else could cause these vibrations? I thought maybe the deer impact tweeked by forks or something else but the only time I get the vibrations is on braking, the rest of the time the bike feels as good as the day I got it. Can I get my rotors tested? Is there anything I can do to test them myself?

My test was to remove the calipers, line a lazer on the edge of the rotor and spin the wheel. Like I said I saw very little movement (like under a mm).

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I suggested that even 1mm of discrepency would cause some vibration. Also, his wheel could be tweaked and since his rotors are mounted to the wheel, that could be causing the vibration? His axel could be tweaked with the same result? Forks twisted inside the triples? I'm just brain storming here. Anyone have any ideas or suggestions? He wants to avoid buying new rotors if it's not needed.

-Shrub

[WetShrub]

Bump...

[BDA116]

Where is the vibration felt? In the brake lever, the handlebars, the pegs/seat?
What was hit/damaged when he said hello to Bambi?
Could be the tire, rim, rotors, pads, forks, even coming from the rear.

[dB]

All the stuff mentioned, plus the rotors may need a good bead blasting to clean old brake material off (it builds up unevenly). Though obviously if the symptoms only started after the crash, this is unlikely.

[nedro]

Is it a vibration or chatter?
Vibration = side to side oscillation
Chatter = up and down oscillation

[WetShrub]

Originally Posted by nedro

Is it a vibration or chatter?
Vibration = side to side oscillation
Chatter = up and down oscillation

"Chatter - well at least this is what it feels like"

Originally Posted by BDA116

Where is the vibration felt? In the brake lever, the handlebars, the pegs/seat?
What was hit/damaged when he said hello to Bambi?
Could be the tire, rim, rotors, pads, forks, even coming from the rear.

"Handlebars. It started before I hit the deer but has progressively gotten
worse."

[BDA116]

From your first post, it sounds like he changed the pads before hitting the deer also. Assuming it started after the pad swap and just prior to the deer meeting, I would start with the most simple: have him take the pads out and scuff them a bit, then do the same to the rotors. He could have the rotors blasted, or take a green or red scotch brite pad to them. clean the calipers, replace the pads and go.
Could be the wheel/tire is out of balance, could also be worn out forks. Those two depend on how many miles and amount of abuse the bike has taken, but start with the pads and rotors.
It doesn't sound like anything is bad or broken, just needs some attention.

[ND4SPD]

Could it be the stem bearing?

[BDA116]

Originally Posted by ND4SPD

Could it be the stem bearing?

Could be, but you'd typically feel an initial 'clunk' through the bars.

[nedro]

From the description of everything here...You have heat spots on your rotors.
Just to give you an idea of how heat spots will effect your feel and brake performance as opposed to simple warping of the rotors.

Rotor warpage max tollerance is 0.010"
Rotor out of parrallelism max tollerance is 0.0005"
That's right! 5 ten thousands of an inch or 1/2 thousands of an inch.
So a couple of hard brake aplications after heat spots started to form will multiply this feeling exponentially.

If they have heat spots, chuck them and get new ones.
I know, I know!

[Trackho]

Originally Posted by nedro

From the description of everything here...You have heat spots on your rotors.
Just to give you an idea of how heat spots will effect your feel and brake performance as opposed to simple warping of the rotors.

Rotor warpage max tollerance is 0.010"
Rotor out of parrallelism max tollerance is 0.0005"
That's right! 5 ten thousands of an inch or 1/2 thousands of an inch.
So a couple of hard brake aplications after heat spots started to form will multiply this feeling exponentially.

If they have heat spots, chuck them and get new ones.
I know, I know!

THis sounds exactly like what my brakes were doing w/warped rotors---I kind of figured t out by changing wheels(rotors)--and it was gone-was worse under really hard, ass-in-the air braking

[WetShrub]

Results from my buddy, we are going to T-hill on June 5th. I told him it was probably carbonized deer blood.

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Here's the thing...

I sanded down the pads last night, took about 0.5 to 1 mm off each,
scrubbed the rotors with some 1000 grit sand paper and put everything
back together. Took my bike out and got smooth braking. When I was
sanding the pads I saw pretty large lumps of what I can only guess was
carbon (looked like charcoal) and some rusty looking stuff that Mr.
Adams had pointed out the night before. Hopefully that was the problem
and it is resolved. I will find out on the 5th though . I'll be taking
my sandpaper with me.

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-Shrub

[nedro]

Originally Posted by WetShrub

Results from my buddy, we are going to T-hill on June 5th. I told him it was probably carbonized deer blood.

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Here's the thing...

I sanded down the pads last night, took about 0.5 to 1 mm off each,
scrubbed the rotors with some 1000 grit sand paper and put everything
back together. Took my bike out and got smooth braking. When I was
sanding the pads I saw pretty large lumps of what I can only guess was
carbon (looked like charcoal) and some rusty looking stuff that Mr.
Adams had pointed out the night before. Hopefully that was the problem
and it is resolved. I will find out on the 5th though . I'll be taking
my sandpaper with me.

---------------------------------------------------

-Shrub

Nasty!

[bubba]

Originally Posted by WetShrub

Results from my buddy, we are going to T-hill on June 5th. I told him it was probably carbonized deer blood.

-Shrub

that is good eating