possible bent rotor, new rotor sources?

[misterprofessionality]

MP, assumptions are fine as long as you don't over-rate them, testing various things will point you in the right direction.

From what i read in your explanation there is one friction point on ( one ) of the disks, that does not mean the disk is bent, to test this you can hold a torch behind the brake pad, maybe open the pads up a bit so you can "see the light".
By watching the reflection in the gap and rotating the wheel you are likely to see a change in one place, if the opposite happens on the other side of the disk it is indeed warped and you may have a re-course on the tyre installer.

2nd thought; prise the pads away from the disk, put a sheet of paper in the gap and use the brake lever to close the gap so you can feel the sheet start to lose movement ( like with a feeler gauge ) then repeat the placing at all four pads and see which ones providing the pulsing, you may need some assistance here.

Another simple trick is Kiowa eagle's suggestion, put the old pads in and see if anything changes, here's why;

On my cb900 ( similar system ) i had a buildup of brake pad something, same effect as you describe, pulsing in one place of the wheel, i think this happened when i put the bike away after a wet ride, a little corrosion happened under the pads and because of the change in abrasiveness it picked up some material, i lived with it for some time until the pads needed changing and the problem went away for 90% and improved still more over time, that is all the explanation i can give you and maybe another good suggestion is coming up, no rush, take your time and find out what really happened.

Thanks, this is the kind of stuff that I need. Buying an expensive single-use tool just to get a "yes or no" answer on the condition of the rotors just seemed like a bad idea destined to put me more in the hole. I've already given the rotors a good scrub with brake cleaner, but i haven't used Acetone. I didn't know that was a good option. I actually thought that might have been the problem, just as you describe, because when i tried that i did imagine that it had improved a bit and the oscillation was less noticable, but not totally gone. the next day, however, i realized it was just my imagination and it was operating in the same way. Here's a question though: if I can't see the deposits on the surface, how will I know when i've scrubbed enough?

It was also suggested above that i disassmble everything and put it all back together to ensure that it went together the the way it was in the manual. I've done that before coming here to post, so if i'm making a mistake in that area, i couldn't tell you what it is.

I'll give the light and paper sheet ideas a try. those are free, and the light idea seems like it would provide a pretty definitive answer on the subject of the discs.

To answer another question asked, a member posted that TRW MST278 is a direct replacement for CB1100 front rotors. I don’t know what they cost.

Thanks for that. I'll keep it on file. Would it be safer to buy both discs in the event that I do in fact have a bent disc, or is there no issue with leaving the unmolested original still in place?

[Lord Popgun]

The brake cleaner is most likely good enought. If you go more aggressive, be careful if you try acetone. Do not get it on any paint or plastic or it will mess it up bad.

And you can only scrub so much and just have to figure its done. Ari Henning said you can use 400 grit paper and sand them. I’ve never done that. (Henning is a motorcycle magazine editor/writer)

[KiowaEagle_imp]

It’s hard to find anyone to turn rotors anymore. Just like everything else this seems to be just another part of the “throw away society”.

[peterbaron]

A bent or warped rotor shall not be turned or ground, needs replacement only.
I would never turn a thin rotors, grinding is OK , if permitted.

CB1100 front disc specs:
standard...........4.5 +-0.1 mm (0.18+-0.004 in)
service limit.......3.5 mm (0.14 in)
Do it right.

[GoldOxide_imp]

To answer another question asked, a member posted that TRW MST278 is a direct replacement for CB1100 front rotors. I don’t know what they cost.

(+1)

[misterprofessionality]

welp, it's a fact. to review:

I have cleaned, scrubbed, acetone'd, and even 400 grit sanded the discs to see if cleaning them would have an affect. no luck.

I have swapped sides on the brake pads and disassmbled/re-assembled the whole thing with respect to how the front wheel goes on. no luck.

I have swapped back to the original brake pads, which still have *some* mileage left on them. no luck. still pulsing.

I have attempted the light test suggested above...inconclusive.

I have attempted the paper test suggested above, and discovered that on one of the rotors there's a spot where the paper gets a bit stuck and tries to pull forward with the rotation of the disc. this is the closest thing i have to direct confirmation that the disc is slightly warped.

I think from all this i can conclude that it's not the brake pads, it's not deposits on the disc, and it's not the assembly of the front wheel. That disc is just ever so slightly bent. i am immensely annoyed that it took this much work and time to confirm it. I'll have to just order another one and try it on both sides to see which one is the culprit.

Just to explore something i haven't yet had any way of testing: is there any chance that something could be wrong with the brake calipers themselves that could cause this pulsing-at-low-speeds problem?

[Chuckk_imp]

could you pull one caliper at a time to find out what rotor is perhaps bent..

[misterprofessionality]

well, the only way i have of finding out is conducting rudimentary tests. the best of which was detailed above using a sheet of paper. it determined the right-hand disc had a consistent sticking-point that wanted to pull the paper through the brake pads. this leads me to believe it's likely the right-hand disc. I'll be trying that one first

[Chuckk_imp]

gotcha.. did you find out the cost of a rotor

[max]

MP that is a very detailed description of what you have done and found out, to add a bit more weight to the scale;
If the rotor is bent the opposite effect happens on the other side of the disk, the paper should free up some in the same place ( that you hopefully marked ), if it does not then it's not likely warped and repairable somehow.
( on my cb900 i could not tell the friction spots no matter how hard i looked at the disks and cleaned them, it just went away with new pads, lucky me ).

I think you found out more than the average service shop would, at least you've found the spot and direction of what pulses the brake, you also tried the old pads and found no difference, that's important and overall have established that this pulsing started after the tyre was replaced, also good information.

Would be a bit hesitant to sand the surfaces, you could induce some
'random"abrasiveness that would accumulate more friction in places and make things more complicated, i can't prove this but that is just my hunch.

As for the calipers; they are stationary and thus unable to "choose" a spot in the wheel to grip harder than other spots, what may alleviate things a bit is to check the right hand side pistons for free movement and free of dust and grime, all be it very small amounts if the pistons are not free to move that may play a part, like the rear caliper that's engineered to "float" sideways with brake pad wear and "self align" to the disk, alternatetively loosen the two bolts on the fork leg and give the caliper some wiggle room, don't forget to tighten them back up.

When you lift the front wheel does the disk drag in one place or are the pistons completely free, retracted and make no noise?