Rear brake issues

Yup. Rotor is cupped beyond saving. I'm having it machined down a bit anyway so it will at least stop rubbing on the caliper and I can move the bike. 

My theory is that the outside piston seized, causing the inside piston to push on it while it heated up, causing the cupping. 
 

If you're going to have the disk resurfaced, I would consider sending it to truedisk.

truedisk.net/

He's all set up for this, his prices are very reasonable, and the quality is excellent.

Also, it might be worth measuring the disk to try to guess if you'll be under the minimum thickness.  There isn't a lot of extra, and yours looks pretty bowed.

Also, PM me if you're interested in a new EBC rear rotor.  I have an extra.

denniskirk.com/1909390.sku

Had it machined locally already. Says he has to take about 100 thousandths off. Looks like it in spec in the middle, but a little under near the edge. I've already ordered new EBC rotors anyway. I really just wanted to be able to move the bike for now without it running in the caliper. 

Wow, that is one warped disc...!!!

Just thinking out loud here.  I'm wondering what purpose is served by having a minimum thickness for the disc?  All I can think of is due to heat buildup from braking.  Therefore, if you are careful with your rear brake you should be OK to run with an undersized disc.

The rotor would get too hot and warp or crack.
Brake pads will fade due to the high heat.

Pistons would get pushed back in the cylinders.
The first time you push the pedal, it goes to the floor(or pedal stop).
You have to pump to get your brakes back.

In racing, it's the risk they take to reduce rotating mass.
Carbon/Carbon pads and rotors allowed them to go thin and light without the warping. You just need deep deep pockets.

You don't really want to think about managing your brakes going down a mountain.
It's better to keep the rotor above minimum thickness to dissipate the heat.

 

So even with the rotor machined, it was still rubbing on the caliper slightly. 

EBC brake rotors arrived and installed today. These fit perfectly centered in the caliper as they should. (No pictures of the clearance, sorry). That leads me to believe that the old rotor must have warped from closer to the hub too. 

Interesting that the EBC rotors are so much thinner than factory. 4.5mm min instead of 6mm minimum. I assume this is for heat dissipation? Having a drilled rotor, it will dissipate heat more efficiently and can thus be thinner. It did mean I had to adjust my rear brake pedal a bit. 

I'm thinking I may tape off and paint the center of these rotors gold to match the tank stripes. 
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wrote:

Interesting that the EBC rotors are so much thinner than factory. 4.5mm min instead of 6mm minimum. 

Are both the front and rear EBC rotors 4.5mm, or just the front?  I know with the OE rotors, the rotors used on the single-disk front setup are thicker than the rotors used on the dual-disk bikes.  Any chance you ordered a front rotor intended for the dual-disk bikes?

wrote:

So even with the rotor machined, it was still rubbing on the caliper slightly. 

EBC brake rotors arrived and installed today. These fit perfectly centered in the caliper as they should. (No pictures of the clearance, sorry). That leads me to believe that the old rotor must have warped from closer to the hub too. 

Interesting that the EBC rotors are so much thinner than factory. 4.5mm min instead of 6mm minimum. I assume this is for heat dissipation? Having a drilled rotor, it will dissipate heat more efficiently and can thus be thinner. It did mean I had to adjust my rear brake pedal a bit. 

I'm thinking I may tape off and paint the center of these rotors gold to match the tank stripes. 
​​​​

 

Sorry for the thread revive, but do you have a link to these rotors? I cannot find them anywhere. Trying to get some for me 750 fou.

Thank You!

If you just need resurfacing and drilling go to truedisk.net

wrote:

Sorry for the thread revive, but do you have a link to these rotors? I cannot find them anywhere. Trying to get some for me 750 fou.


 

The application model search is here: ebcbrakes.com/ebc-brake-pad-selector-tool/#lets-go

But note that outside of North America these bikes were called "Z750" as opposed to "KZ750".

I don't know what year and model bike you have, so I can't tell you a specific part number. But I found a part number for an '81 KZ750 and put it into google and they are in stock at amazon...

wrote:

wrote:

Interesting that the EBC rotors are so much thinner than factory. 4.5mm min instead of 6mm minimum. 

Are both the front and rear EBC rotors 4.5mm, or just the front?  I know with the OE rotors, the rotors used on the single-disk front setup are thicker than the rotors used on the dual-disk bikes.  Any chance you ordered a front rotor intended for the dual-disk bikes?

Kaylinator, you never responded to my comment. I still think you're using the skinny dual-disk part in a single-disk application.