Rear brake judder?

[XTlegend]

My GS650 Kat failed its MOT yesterday on one thing. Rear brake judder/shudder whatever you want to call it. Never come across this before and to be honest I hadn't noticed it. However now that its been pointed out its there if you slow down to a stop using the back brake only. Its very noticeable when moving just a couple of miles an hour as you come to a stop.

The guy who did the test said that it was probably a warped disk. With the bike on its centre stand, engine running and in 1st gear the disk looks fine with no noticeable wobble. Can it really be the disk or could there be another cause? I fitted new brake pads and they must be still bedding in as the bike has only done about 50 miles - could this be an issue? I'm a bit disappointed that the bike has failed on this as its hardly dangerous .....

[TooManyToys]

Man, you hit my magic words, "warped disc".

Runout of a disc does not cause pulsation unless you are dealing with a fixed caliper. A floating caliper will just move side to side with the rotor's runout.

Pulsation is caused either by rotor thickness variation, excessive friction transfer film at one location on the disc, or a hard spot in the rotor that is lower in friction level of the rotor. The reason "warp" gets identified as the issue is all of these get corrected once a rotor is machined, although if it's number three it returns early as the hard crystal structure of the hard spot cannot be machined out.

If the friction material is still green you could be getting the excessive transfer onto the rotor. A green friction material is one that does not have a heat history as yet.

When friction material is made (sintered being the exception) it is first pressed out at abut 350F for a short period of time to harden the phenolic binder. After that if normally goes through a post-bake in an oven to further cure the resin. However some manufacturers do not do a full amount of time and in reality no friction material except sintered need to experience the heat of normal braking to fully cure and turn to carbon, the real friction material at the rubbing surface.

The best way to carbonize and cure too green friction is to do moderate braking and get the friction up to moderate hot temperatures then let it cool down while still riding.

[XTlegend]

Thanks for that! Complicated ehh? :-)

Its a fixed caliipa with opposing pistons on the rear. I've found a cheap disk in good condition on ebay which at least will isolate the problem if it is the disk at fault I suppose. I just feel its all a bit unnecessary as the brake actually works and who uses the rear brake much anyway?!

[tkent02]

Man, you hit my magic words, "warped disc".

Runout of a disc does not cause pulsation unless you are dealing with a fixed caliper. A floating caliper will just move side to side with the rotor's runout.

Pulsation is caused either by rotor thickness variation, excessive friction transfer film at one location on the disc, or a hard spot in the rotor that is lower in friction level of the rotor. The reason "warp" gets identified as the issue is all of these get corrected once a rotor is machined, although if it's number three it returns early as the hard crystal structure of the hard spot cannot be machined out.

If the friction material is still green you could be getting the excessive transfer onto the rotor. A green friction material is one that does not have a heat history as yet.

When friction material is made (sintered being the exception) it is first pressed out at abut 350F for a short period of time to harden the phenolic binder. After that if normally goes through a post-bake in an oven to further cure the resin. However some manufacturers do not do a full amount of time and in reality no friction material except sintered need to experience the heat of normal braking to fully cure and turn to carbon, the real friction material at the rubbing surface.

The best way to carbonize and cure too green friction is to do moderate braking and get the friction up to moderate hot temperatures then let it cool down while still riding.

+1, What he said... try this:

Go about 60 or so, down a long hill is good but it doesn't really matter. Drag the rear brake, get it good and hot, then step own further, say about half of what it takes to skid the wheel. Do not come to a stop, just down to 20mph or so, accelerate right back up to about 60. Do this two or three times. See if your juddering isn't gone. Sometimes it works, sometimes it doesn't.

[TheCafeKid]

+1, What he said... try this:

Go about 60 or so, down a long hill is good but it doesn't really matter. Drag the rear brake, get it good and hot, then step own further, say about half of what it takes to skid the wheel. Do not come to a stop, just down to 20mph or so, accelerate right back up to about 60. Do this two or three times. See if your juddering isn't gone. Sometimes it works, sometimes it doesn't.

+2. What they both said. Just remember, DON'T STOP while doing this or friction material will bond to the disc and youll have it contaminated and be back to square one.
And DON'T use sintered pads on these old discs. They will eat the soft rotors alive.

[XTlegend]

OK thanks - I'm going to try this tomorrow and will let you know if it makes any difference. I have 10 days to sort it out or will have to pay for another test which is around $50.

[tkent02]

I have to do this every six months or so on my wife's Lexus.

[hampshirehog]

If you got oil on the pads that can give you the same effect. Plus did you rebuild the caliper - if one piston is stuck the same thing can happen. And finally, dodgy wheel bearings can cause a similar sensation.

Don't bust the MOT guy up about it - I have heard a lot of tales locally of the VOSA 'mystery shoppers' calling the test stations out with BIG fines over stupid stuff so unless you know the guy well they aren't cutting any slack. £2k fine plus 6 month ban for a bloke down the road who missed a tyre on back to front on 1000 mile bike (never thought to check the tyre had been turned round on such a low mileage bike) and rear bulb re-wired the wrong way round (bright tail lght, dim brake light).

[TooManyToys]

XTlegend,

With a fixed opposing piston caliper you could have an issue with a high runout rotor if one of the caliper pistons is rusted frozen. When the high runout area of the rotor comes against the frozen piston the pressure against the pad increases while the opposing piston moves further towards the rotor. This essentially develops into a higher clamping load like you are trying to decelerate more so, but only during that part of the wheel rotation. So you go into a cyclical high-low deceleration rate.

However, with an opposing piston caliper if there is rotor runout eventually you develop a thin area in the rotor as the high runout area touches against the brake pad as you are riding but not stopping. This thick-thin cyclic action also developing into the deceleration variation or pulsation.

So it would be a good idea to check that both pistons have free movement.

[XTlegend]

Thanks again TooManyToys.

I took the caliper off yesterday to give it a good clean and there is some corrosion on the pistons. Before I disconnected the hydraulic line I applied the brake to push the pistons out. Neither piston is sized though one came out a lot more than the other – is that normal? I also seem to remember that when I took the old pads out one was worn more than the other.

After cleaning as well as I could (I didn’t remove the pistons fully) I pushed them back in and replaced everything, replenished fluid and bled the system. It didn’t seem to make any difference to the ‘judder’.

I’m thinking that today I will fully dismantle the caliper to remover the pistons completely and give them a good clean?

I’m going to become an expert on old GS brakes at this rate ….. J

[tkent02]

I’m thinking that today I will fully dismantle the caliper to remover the pistons completely and give them a good clean?

Or you could just try this...

Go about 60 or so, down a long hill is good but it doesn't really matter. Drag the rear brake, get it good and hot, then step own further, say about half of what it takes to skid the wheel. Do not come to a stop, just down to 20mph or so, accelerate right back up to about 60. Do this two or three times. See if your juddering isn't gone. Sometimes it works, sometimes it doesn't.

[XTlegend]

Or you could just try this...

Go about 60 or so, down a long hill is good but it doesn't really matter. Drag the rear brake, get it good and hot, then step own further, say about half of what it takes to skid the wheel. Do not come to a stop, just down to 20mph or so, accelerate right back up to about 60. Do this two or three times. See if your juddering isn't gone. Sometimes it works, sometimes it doesn't.

OK I will - is there any reason why I cant do this with the bike stationary on its centre stand?

[tkent02]

Yes. Beside being extremely dangerous, it won't work.
The brake needs to make a **** Ton of heat. It takes a LOT of speed and energy.

[XTlegend]

Yes. Beside being extremely dangerous, it won't work.
The brake needs to make a **** Ton of heat. It takes a LOT of speed and energy.

OK - I'll take it out in a bit - you'll pick up the fine if I get stopped by the cops for riding without a valid MOT and tax right? :-)

[hampshirehog]

If you have got pitted pistons replace them - you would be amazed how little pitting is needed to make them fail. Best replacement is with GSXR stainless pistons from

Cheapest (and best place) is from Nick Chambers via Ebay. You have a choice of booted or plain pistons (plain are a tad cheaper). As they are stainless you don't need the boots. These will fit your bike:

http://www.ebay.co.uk/itm/390367523674?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649