Rear brake judder?

[XTlegend]

OK doesn’t seem to have made any difference. The brake itself works very well! The problem is only evident when applying the rear brake at very slow speed. But who does that? I’m a bit pi@sed off that I’m not legally allowed to use the bike because of this L

Anyway, there is a second hand disc and caliper on ebay so I might try this and hope they are usable and don’t give me the same problem. Thanks for all the advice.

[XTlegend]

The saga continues …..

I bought and fitted a second hand rear disk and I thought things were better. Now I’m not sure if I imagined this but by the time I had ridden the 15 miles to the test centre it seemed as bad as ever. Sure enough it failed again. No improvement the guy said. I feel gutted by all this. So a bike I had high hopes for is turning out to be a real lemon. I’m going to take one of the discs to work tomorrow in the hope I can get it skimmed or if there’s a surface grinder I might try that. The only thing left to try is a new disc which will cost around 120 GPB which is half what I payed for the bike in the 1st place. Nightmare.

[hampshirehog]

I bet it's not the disc. Have you completely stripped and cleaned the caliper / hose / MC? That's where the problem is I reckon.

[TooManyToys]

Sorry, been away from this thread.

A second used disc may have the same issues that your original had. If you have the ability to grind the disc at work that would be great. Hopefully it would be set up so all the grinding that is done is indexed off the mounting hub, ending up with little or no runout.

The GS1100E likes to have front disc pulsation issues, too. Years ago I had access to a Blanchard grinder and was able to grind both the hub surface and one rubbing disc side with the disc mounted to the magnetic chuck so they were parallel, then flipped them over and did the remaining side. Worked better then getting new discs under warranty.

[TooManyToys]

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

In an ideal world both pistons will move evenly. But even the passageways through the caliper can cause a higher resistance for the piston on the opposite side of the hose inlet to not move as far, if that is the side that is a little lethargic. The bigger issue would be less retraction of the piston on brake release if the seals or corrosion were hanging up the piston movement. This would allow the brake pad on that side to slightly rub against the rotor, and if the rotor has some runout (which almost every rotor does), the point of high runout would wear slightly and that area would then be thinner then the rest of the disc. That will lead to pulsation when braking. As the rotor transitions between thin and thicker sections the wedging effect results in high and low brake pressure events, and you get high and low deceleration, pulsing.

[XTlegend]

An update and its good news – another 2nd hand rear disc off e-bay (originally of a GS1100G but it’s the same part number) has cured the problem! Very smooth now. So it seems it was the dreaded warped discs!!

[TooManyToys]

Please, please, please don't use the term 'warped disc". It continues the belief that is is disc runout when in fact it's thickness variation, or less possible, friction material transfer. You can have runout and not have pulsation, and you can have 'almost' no runout and have pulsation. All you need is 0.0008" thickness variation to feel the pulsation.

Glad you got it fixed.