warped front disk issue

[pShenk]

I noticed a slight pulsing and grabbiness in the front brake when slowing to a stop. My mechanic thought it might be a slight warp in the front disks, and he suggested either replacing them, or changing out the little springs that float the disks in the mount. The stock springs are *really* tight, so much that they are barely even "floating". The theory being that lighter springs would allow the disks to move back and forth more between the calipers, and reduced the grabbiness.

 

Anybody have any experience with this operation?

[Guest ratchethack]

Pshenk, this wouldn't cause "grabbiness", but a typical scenario for pulsing brakes is that the "buttons" that allow semi-floating disks to slightly adjust laterally become seized. These are the nickle-sized, disk-shaped gizmos that separate the disks from the disk carriers. Y'er mechanic may have been referring to the spring washers behind the buttons. They can often be loosened with penetrating oil - I use "Kroil" - and/or heat. I give mine a tiny shot at every tire change for preventive maintenance, followed by a thorough mop-up of the disks with lacquer thinner on a clean shop towel.

 

CAUTION - penetrating oil in contact with brake pads is NOT recommended!

 

Hope this helps.

[pShenk]

Yes, the buttons are definately either *very* stiff, or they're siezed. I'll try the penetrating oil trick.

 

Im thinking that if they are siezed, the disk might be cambered out of true, and this would cause it to hit the pads intermitantly. This might cause pulsing when squeezing to a stop, yes?

 

Seems to make sense to me.

[Guest ratchethack]

Yep. Good way to check is to get the wheel up off the floor and spin it, looking for runout where the disk comes out of the caliper. If you can spin it fast enough, you should be able to detect if it's out of true by eye.

[polebridge]

I had the same problem with my Aprilia. I tried the button thing but they were not the problem. Next I bought new brake rotors. Not the problem. Finally I took it to a new dealer that someone recommended. They discovered that the forks were mounted in the triple clamps slightly cockeyed (one a little linger than the other). They striaghtened it out and the problem has not returned.

 

Have you had your forks serviced, or off of the bike for any reason lately?

[Martin Barrett]

The other thing to consider is the spacer on the spindle, make sure it's fitted to the correct side.

 

from left to right (viewed from riders seat)

nut - left fork leg - spacer over protruding spacer tube - wheel (bearing over space tube against spacer tube shoulder- thick part of spacer tube - bearing on spindle against spindle shoulder) - visable spindle - right fork leg with spindle end pulled flush through.

 

Apparently the spacers internal diameter is the same as the external diameter of the wide part of the spindle, so will fit the wrong side. The whole assembly will clamp down tight against the spacer tube/end of threads but the disks are what centralises it.

 

Don't put it past the factory to fit it wrong. My starter motor cover was fitted with the washers the wrong side so it just pushed on or off over the retaining nuts

[Guest rotorhead]

I have had some rather extensive/frustrating experiences with warped rotors. Wound up replacing both rotors and calipers at a very substantial cost. The long and the short of it was that the rotors got "bumped" whilst being trailered and left an ever so slight mark on the rotor. The rotors warp at this "mark" and continued riding and subsequent heat build up only excacerbated the problem resulting in excessive dust buildup on the rims leading to continued investigation until I discovered the fault.

 

True enough, the "buttons" need to move freely but I would suggest looking for any nicks in the rotor, dust buildup on the rim, etc. Watch them when you ride..........you will see any warpage. Also make sure the pucks in the calipers are clean and they release tension off of the pads.

 

Good luck

 

My

[pShenk]

Polebridge, that's a good call... the previous owner had the forks off to change the fork oil, IIRC. That would be something to check.

 

I will check the spacers also.

 

I had the front off the ground today, to raise the forks in the triple-tree (to try and fix some speed wobble I'm having), and I noticed that the front wheel does *not* spin completely freely. There is always a slight friction of rotor against the brake pads. Is this normal? Maybe it is, but I wasn't expecting it. I suppose I could see why you would always want them to rest very slightly on the disk.

 

Thanks for all the advice! Btw, the rotors do not appear to be warped, at least as far as I can tell by spinning the front wheel.

[pShenk]

Well, maybe I take that back about the rotors... I just went down to the garage and tried this:

1) lift up the front end

2) take a c-clamp and put it around the brake lever/grip. Slowly tighten until the front tire is pretty hard to rotate, like just before the brakes really lock the wheel.

3) Continue rotating the wheel under this consistent braking pressure.

 

So, there is *clearly* a spot where the rotors can barely turn through the calipers. If I force it past this point though, it loosens up and spins much more easily, until it reaches the tight spot again.

 

Oh, and I also measured the length from bottom of each fork tube to the lower triple clamp... they both look exactly the same length.