Ok you brake and fork gurus!!

[XS1100 Newbie]

I have a problem that i am having a time figuring out. Let me start at the begining. I noticed over a year ago that my rotors were warped, or so i thought. It has become bad enough that i am getting some rotors from Lee for replacement. I have read a lot about bikes and rotors, and it seems that half the people say that the discs don't warp, (i kinda don't believe that) others say that hard usage building to much heat will warp them, a dragging caliper will warp them, etc. etc.. I have even been told that for it to be warped, that it will warp in more than one place on the rotor. I have read that they may be bent instead. I noticed when re-assembling the front end, ( i cleaned out both calipers, and the master again.) It was hard lining the rotors up. Finally got them, but when i lowered the front down, this changed. I ended up setting them with the weight on the forks. I don't know if this is normal, or could i possibly have bent forks. I also used feeler gauges to measure the warp between one side where the rotor runs through the caliper mounting (because it is stable), i did this on both sides. What i came up with is the right side being out .036, and the left being out .032. Huge i thought, but another strange thing is both sides are in the same spots across from each other, and about 7 inches of the rotor is affected. I don't want to ruin the rotors i am about to get, so i need some ides here on what to check for. I know that someone on here has experienced something like this. So you know again this is a 1980 MNS.

[CaptonZap]

Here again, we are having a translation problem.
What sensory input makes you think the rotors are warped?
What is your definition of warped?
The dictionary says that it is to twist or bend out of a plane.
If the discs are warped, and not overly so, and the calipers are free to slide sideways, as designed, you would not notice anything unusual in the braking action.
If, on the other hand, you noticed a pulsing in the brake lever when applying the brake, this would indicate an uneven wear in the thickness of the disc, which is not warping.
What symptom are you experiencing?
CZ

[BA80]

If you painted the wheel and/or the rotor and there's paint in between where the rotor and wheel make contact that will probably be why your having a problem. even if you didn't paint them and you've never had the rotor loose from the wheel, check it. It could have been done by a PO.

[XS1100 Newbie]

Ok Cap, there is a severe pulse when coming to a stop. It gets worse at slow speeds. Let's just call it warped, because at this point it really doesn't matter whether it got bent somehow, or warped because of some reason. Fact is, it runs closer at one point to a stationary metal part for about 7 inches, than it does the rest of the way around, and it is the same on both rotors, even the same spots across from one another. I am just trying to make sure that when i install the other rotors, that it wasn't something that is still there that caused the problem. (other than my riding style of course)

[JeffH]

there is a severe pulse when coming to a stop. It gets worse at slow speeds.

Symptom described as severe pulse - I'd say its classic Bent Rotor. Likely the rotor took a sucker punch from something at some point in time. Warping from heat or hard use would not be isolated to only one quadrant of the rotor.

I had the exact same problem as you describe. Severe pulse getting worse like a bucking bronco as you slowed down under front braking. It was difficult to locate the problem with the rotor on the bike and measuring from the forks. After removing the rotor bend was obvious. In my case a new rotor solved the problem and immediately restored smooth pulse free front braking.

Jeff

[DGXSER]

I have to wonder about this issue and hwo it relates to the Special brake system. I have had two sets of rotors on my bike and the same pusling problem. It seems to get worse and or better at times after removing and reinstalling the brake calipers.

The fact you can measure the out of flatness of the rotor with a dial indicator mounted to the fork tube is telling that your rotor is out of spec in regards to flatness of the surface. Maximum deflection per the manual is .006".

[TopCatGr58]

Hey DGXSer,

I, too, will add my recent experience with the Special's wonky slanted calipers and a possibly "slightly" bent/warped rotor.

I have had a pulsing sensation in my front calipers/brakes for many years, but since the rotors were $$, and it didn't really bother me that much, I just lived with it, thinking that I had bent/warped rotors, although I've never had it running with excessively dragging pads or an impact event.

This spring I did the very involved dual piston modern caliper conversion for my bike/special, getting rid of the slanted calipers and swivelly bolt, and now have solidly mounted to the forks calipers. Once I got the new used pads reseated/worn in to the old rotors...I did some test runs and brake sessions, expecting to feel the same PULSE, but to my surprise there was NO pulsing whatsoever!!!! I did not do anything to the rotors, just removed the front wheel when I pulled the forks to peform the machining/mod on the mounting brackets and such.

SO....I, too, feel that the wonky calipers can exaggerate a very mild out of round/flat rotor and produce an otherwise annoying pulse that might not occur or be felt with the standard calipers with FLAT pads!

I also noted that the swivel mount bolts on both of my calipers were slightly bent...not sure if this would have contributed to the pulsing sensation??

T.C.

[XS1100 Newbie]

That is interesting. I purchased new caliper bolts, and will install tomorrow. I am going to test the rotor with the calipers off, and examine how things go back together. I may put the calipers on with no wheel just to see if i can see anything. Won't be working on it long as it is supposed to be near 100 tomorrow. Also going to check the pads and see if i might have bent backing plates on the pads. Just a thought on that though.

[TADracer]

The new bolts might solve your problem.

Long ago I noted that on specials the first time you set the brakes after reinstalling the calipers the new bolt MIGHT slightly bend. If it does then you can notice that when removing the calipers. If they wobble as you unscrew the bolt you must change the bolts as getting the old ones back in the exact same position is impossible. They can cause pulseing, squeeling, uneven pad wear and a whole host of things.

Also, what BA80 said should be checked. Even a bit of rust or paint between the rotor seat and the wheel can cause exactly the runnout readings you are getting.

[jetmechmarty]

Ok Cap, there is a severe pulse when coming to a stop. It gets worse at slow speeds. Let's just call it warped, because at this point it really doesn't matter whether it got bent somehow, or warped because of some reason. Fact is, it runs closer at one point to a stationary metal part for about 7 inches, than it does the rest of the way around, and it is the same on both rotors, even the same spots across from one another. I am just trying to make sure that when i install the other rotors, that it wasn't something that is still there that caused the problem. (other than my riding style of course)

I think the first thing to do is clean the rotors. Source some 100 grit garnet sandpaper. Using a block, thoroughly sand both sides of the rotors. It is possible that this simple activity could solve the problem. It worked for me. (the black paper will make it worse)

[CaptonZap]

I think the first thing to do is clean the rotors. Source some 100 grit garnet sandpaper. Using a block, thoroughly sand both sides of the rotors. It is possible that this simple activity could solve the problem. It worked for me. (the black paper will make it worse)

Yep, that's a start.
Then, when that doesn't help, whats the next step?
A 0 to1 inch micrometer, applied to various spots around the disc, all the same distance in fro the rim, disclosed a .0035 inch difference in thickness, presumably from wear.
Will sandpaper work with that?

And the slots in the rotors make it an interupted cut job on a lathe, which plays hell with the carbide cutters needed for the stainless steel, so that is a dead end.
Ohhh, woe is me. CZ

[BA80]

As I said before, it's probably the mounting surfaces and not the rotor(s) itself.

[jetmechmarty]

DIY - Disk Brake Removal and Sanding with Garnet Paper to resolve Brake Problem.

Garnet Sanding for Rotors

His advice, given my rotors didn't appear worn at all, was to take 120 grit garnet paper "NOT sandpaper"

This is often misdiagnosed as a warped rotor. If caught early enough, this can be fixed by cleaning the rotor surfaces with 200 grit garnet paper followed by spray brake cleaner to remove any compound buildup on the rotors.

And many more say the same thing. Why not give it a try?

[XS1100 Newbie]

I know for a fact that the rotors have never been off this bike, and the problem just started a little over a year ago.

[BA80]

OK, so you know when it started. Do you know what preceeded?

Tire change, new pads........ect. ect.