Another disc brake thread... problems?

I recently fitted an Avid Juicy5 front brake with a 160mm Avid rotor to a set of Pace RC38 forks, both the brake and disc were bought used some time ago but look to be in good condition.

Problem is that when braking hard I seem to get a "shuddering" effect, almost as if the brake is biting and then releasing. On more gentle braking this doesn't occur.
I've checked the headset and that's nicely tight, no slop there.
I've checked the forks and there is a little movement there so probably time to get it serviced sometime soon, but the forks seem a bit flexy under hard braking anyway.

My thoughts are that there may be some contaminant on the disc or pads?
How can I tell if this is the case?
How can I clean up the disc and/or pads?


I'm still a bit of a disc brake noob only had one bike with discs on it before and they were problem free.

Same problem with my elixir 5's. I have put it down to them getting very hot in the heat which can cause similar symptoms, but would be interested to hear if other people have fixed this.

Could be that you have a contaminant on part of the rotor, which is causing a decrease in power whenever it passes through the pads. This would be noticeable as a juddering effect when you're braking harder or from a higher speed. Or perhaps your rotor is slightly out of true?

Try giving the rotor a proper clean first, using a proper disc brake cleaner (available from any half decent bike shop, or good old eBay!). Make sure you wipe all the residue off when you're done though or you'll be back to square one!

You could also use Methylated Spirits to clean the rotors. I used to use wire wool and meths to clean my rotors whenever I fitted a new set of brakes as the wire wool roughs up the surface of the disc rotor a little bit too

BBB.

New pads? Maybe they havent bedded in yet.
Or maybe theyre glazed over?


Could be contaminated as above also.

hmmm... no the pads are whatever came with the brake when I got it 2nd hand, so their history is unknown.

I'll try cleaning the rotor, will probably invest in some brake cleaner as it's sure to be used again in the future.

I'll see how things go....

Sorry,cant advise on the juddering,really need to feel that type of issue.

I find just in general use on the road that rotors pick up road grease and thats partly the problem with them
Disc brake cleaner is a must and a new cloth each time[good quality kitchen roll doesnt disintegrate too badly]
Personal choice for the rotors is motorex[Switzerland] 750ml for £11.Big can lots of dosh but worth it in the long run for the amount you get over the smaller bottles.
To clean the pads i use a blowlamp to burn off the solvent and then a rub against a bit of wet and dry laid flat on a piece of glass.*
When using the heat,dont bring them up to cherry red,youre not trying to reforge them just burn off the solvent.You'll know as it will start smoking.
ALLOW TO COOL dont run them under the tap

* I hear horror stories about pad separating from backing from heating and cooling, but in the 10 years ive been using this method ive yet to have that problem.
Pads arent just stuck on,theyre mechanically bonded through the backing plate,thats what those holes are for