Are my Disks Warped?

[Luckypants]

Took the Viffer in for a service yesterday and they reported that the front brake effort has more than a 25KG deflection in brake force which is an MOT fail. They reckon this is caused by a warped disk. They have not measured the run out on the disks because the mechanic noticed it on the road test. They put the bike on the brake testing machine and showed me and yes I could see it.

Now I have never noticed any vibration or pulsing from my front brakes ever, the bike has covered 24K miles with these disks and is still on the original front pads (I don't use my brakes much), so to my mind the disks should be fine. On the ride home I played a lot with the brakes and found no pulsing at the lever or vibration under light/normal/heavy braking. However, when free wheeling at low (walking) speed and I apply the front brake gently I can feel a definite 'drag' of more braking force for half a wheel rotation, but still nothing at the lever or vibration in the bars etc. The 'drag' effect coincides with a bit of a squeak from the front brakes. Now the bike has stood since October, so I'm wondering if this is just a 'sticky spot' on the disk caused by a pad being in contact or similar.

What I want to know is:-

1. Is this likely to be a sticky spot in the disk, rather than a warped disk?
2. Will this wear off with use?
3. How do you measure run-out on brake disks without special tools?
4. what else could cause this?
5. Is 24K miles reasonable mileage for front disks?

Useful posts only please about brakes and possible problems.

Thanks

[fastdruid]

Originally Posted by Luckypants

1. Is this likely to be a sticky spot in the disk, rather than a warped disk?
2. Will this wear off with use?
3. How do you measure run-out on brake disks without special tools?
4. what else could cause this?
5. Is 24K miles reasonable mileage for front disks?

Useful posts only please about brakes and possible problems.

Thanks

1) Possible, I've seen quite a few discs where the pad has rusted to the disk and left a 'high' spot
2) Yes but you may need *hard* use to do it. You may find getting them skimmed easiest and will verify they are flat.
3) Lift the front wheel, attach a marker pen with a rubber band so that it does not quite touch the disc (as close as you can get without it touching), spin the wheel, see if it marks, move it so it *just* touches, spin the wheel and see if you get an unmarked section. Not sure about VFR but bear in mind that the Suzuki SV OEM spec is 0.3mm and I've had brand discs with that, they show *no* signs of pulsing with 0.3mm, so I'd expect it to be noticable.
4) Had your tyres replaced since last MOT? Dropping a wheel with discs attached on the disc can bend them.
5) No, my VFR discs have 70+k on them.

Apart from that, some issues I have seen are part seized calipers push floating discs to one side, this wears the springs under the rivets and when one (eventually) breaks the disc is unbalanced and gives pulsing under light braking (but evens up under hard braking). Eventually this also wears the spider/centre (and to a partial extent the disc) allowing it to move front to back as well as side to side and then when all the springs go the discs *really* rattle and make a horrible sound at slow speed (plus push the pads back making braking for the first corner interesting).

Druid

[dyzio]

Hmm.. I think there has been something said on here about Honda's discs when krhall had problems with his.

Originally Posted by Luckypants

what else could cause this?

Don't know if this will be any help to you, but Falco's discs are also prone to warping. However this is not always the case, sometimes the rotor "buttons" get dirty and don't allow the disc to float, hence the pulsing on the brake lever.
When I got mine it had an advisory note from the previous MOT about "slight fluctuation on brake" and you could feel the lever pulse.

Once I've cleaned the buttons, bike went past the next MOT with no problems. Not sure if they tested it, but the lever doesn't pulse any more.

As you have pointed out, the disk surface can also affect the stability under breaking. When I got my 7R, the disks on that (which were after market) were buggered. Not warped, but they resulted in a very jerky ride when braking hard. The lever did not pulse (as with warped brakes) but instead the braking force varied depending on the position of the disk. the bike only had 18k on it so the disks must have been pretty new.

It's a bit of a pain, but I'd just change em. I have used disks from breakers in the past and had no issues. Others will say go for new but that's just my experience.

[Luckypants]

Aha, thanks guys one thing to check up on is the buttons. I'll also be checking the actuator on the front left calliper for the linked brakes system, these can seize but I have been meticulous looking after mine. Will check the run out with pens as suggested.

Keep the information coming.

[yorkie_chris]

See normally I'd advise cleaning the calipers out as per the druids possible cause, but on yours maybe you will have died of old age by the time you get all the million different circuits bled. New bike?

Check the runout with a dial gauge after you have cleaned the buttons and taken it for a spin. Do this with the calipers removed from the discs.


If they are warped, you can usually cover for A BIT by making sure the calipers are well slick with rubber grease, remove the dust seals if needed.

Had to replace my cbr disks for the same reason the last time it was mot'd before I sold it... not cheap

[punyXpress]

[QUOTE=yorkie_chris;2162421]See normally I'd advise take it for a spin. Do this with the calipers removed from the discs.

SLIGHTLY edited above, but did you REALLY mean to say that, Chris?

[fastdruid]

[QUOTE=punyXpress;2162747]

Originally Posted by yorkie_chris

See normally I'd advise take it for a spin. Do this with the calipers removed from the discs.

SLIGHTLY edited above, but did you REALLY mean to say that, Chris?

Yes he did, there are two actions there.

1) Clean buttons and take for a spin.
2) Remove calipers and check run-out.

Of course taking it for a spin without the calipers would fix the problem too but not in a way I'd advise!

Druid

[embee]

Brake discs get pad material transferred to the surface, and this can be patchy and give this sort of effect.

Obviously check calipers as the others say.

If you can't see or measure true run-out on the discs, and they aren't varying thickness around, then take the wheel out and thoroughly clean the disc surfaces first with a very coarse abrasive paper (aluminium oxide or similar, maybe 40grit) to get through any glaze or deposits, then use a slightly finer grade, maybe 120, and thoroughly wash the discs to ensure all traces of abrasive are removed from the drillings. Finally a spray over with brake cleaner. A soft sanding block is advisable.

Ensure pads are clean. Don't use abrasives on pads, the grit can get embedded in the pad material.

Refit and carry out a brake bedding procedure, brake repeatedly from slow then progressively faster speeds reasonably firmly to get the discs and pads gradually up to temperature (and they do need to get properly hot). Then see if it's any different.