Bike is a 2002 998 that I purchased a few years back. It was a pile, so I did lots of work before ever riding it. Part of this work was swapping out the forks with some FGRT803s made for an 848/1098. Because the caliper spacing on the 848 is slightly different to that on the 998, I used spacers to space the brake discs 5mm to fit properly. The spacers however, do not have hubcentric rings for the discs to be centered on, they're just held in place by the fasteners. I also replaced the brake pads and swapped the calipers for some radial units from a BMW. First time riding the bike I noticed a braking shudder under hard braking. No pulsing in the lever, just a disconcerting feeling under hard braking. So when I got home I drilled out and replaced all the buttons with some Brembo full floating buttons. Next time out, same issue still persists. I understand that most people aren't in the position of having to use spacers and such, but has anyone had a brake shudder/vibration and alleviated it by replacing the buttons as I've done? My initial thought is if the spacers were causing an issue, I'd have a vibration all the time, not only under braking. But also if the discs had some slightly high/low spots, wouldn't the movement offered by floating buttons eliminate that problem? I've got 3 possible fixes 1: Replace discs with new discs with correct offset eliminating need for spacers. But then I wouldn't know if the discs or the spacers were the culprit. 2. Replace discs with new discs of same offset and keep the spacers. Obviously if the shudder persists I'l be stuck with having to change rotors yet again. 3. Have some hubcentric spacers made to eliminate that from being the potential problem.
Contaminated discs or incompatible brake pad compound can cause a shudder. I’d take some emery paper to the discs and clean with isopropyl alcohol just to make sure before spending money on parts.
I don't see how spacers have anything to do with it. I had once carrozzeria rotors make shudder/vibration due to aluminum spokes leading to buttons been too thin and would set weird frequency under braking causing shudder/vibration. Don't think brembo would have that problem so most like disc surface.
What's the material being used for the spacers? Couple thoughts that crossed my mind: If it's a soft material such as Delrin, it's possible heat is a factor. Another thought is that perhaps the effects of compression from the hardware has deformed the material. Also, it appears the rotors have a countersink and the bolts are cap style...
I am assuming you have eliminated suspension settings as the cause of the shudder? If you are too close to bottom under hard braking and the fork is riding the bump stops you can induce shudder as well. Did you bed the rotors/pads in properly when new?
I’ve used a rotor hone to clean them up and cleaned them off with alcohol a couple of times now. The pads are also new (installed when I put the bike together)
6061 Aluminum, no deformation. At least not relative to the rotor hat. And yes the disc hats are countersunk and the bolts are cap style- same as stock bolts.
I’ve not changed springs but have adjusted damping quite a bit to get the bike in a good handling zone. I don’t use full travel even under hard braking. That being said I haven’t raced this bike yet, so maybe I could under those circumstances.
For clarification, the cap bolts that are shown in the pic of the center of the wheel are the factory bolts and the discs are aftermarket? Being that this is a front end swap, have the head bearings come into consideration?
The discs are stock, the forks are not. Because the calipers on these forks are wider spacing, 5mm spacers are used on the discs. The fasteners are the same as stock, only 5mm longer so they can be used with the disc spacer. Headstock bearings have been replaced during the build. And also have been rechecked.
I'm bettin' the drilled rotors are the issue. This could be a classic example of them having been overheated cuz previous operator ran the pads down too thin. A rotor hone will not restore the distorted surface to a flat surface, it just cleans up whatever it rides over. The problem persists. Get some new discs (non-drilled preferred if you're a heavy braker) or send the used ones out for a dual-orbiting surface grind.
That’s odd as almost all modern motorcycles have drilled discs. Even racey aftermarket Brembo discs are drilled. I’ll seek some replacements and see what results I get.
If those drilled brakes are pushed at World SuperBike levels, they'll be replaced between 1500-2000km. Street bikes don't see that level of use and the average club racer isn't pushing those levels. If someone happens to be good enough on the brakes or rides a track that requires lots of extended, high speed braking or just neglects to know what kills rotors, they'll be buying discs regularly as an average joe.
I would lift the front of the bike and spin the wheels and see if the rotors are warped. If there not then I would see maybe you have a loose triple clamp, that would give you a loose feeling and vibrate on braking
It's impossible to check runout on a floating disc. The disc has a huge amount of runout if you apply lateral force on the disc. I service this bike very regularly and have double triple checked all the front end bits - even changed the steering bearings because I didn't like the tapered rollers I installed. None of it has made any difference in the brakes. I reached out to ChrisBoy at MotoCorse and he had a used set of 15mm discs from a 748R that he so graciously parted with. These have the 15mm offset 6 hole design that's so hard to come by. They also have been upgraded with floating brembo buttons. Next time I get this turd out on track I'll see if my issue is solved. I'll check them for flatness before hand and see what's what.
What is the proper way to check for runout on full floating rotors? Remove the rotor from the carrier and check on a flat surface?
I suppose you could lay them on a flat surface, check them with feeler gauges, and peen them flat. It’s a common practice in grind shops.
