Aftermarket Rear Discs

[koko64]

Noticed that some aftermarket wave discs are about 1mm thicker than oem (5.5mm vs 4.5mm).  I dont know if this applies to aftermarket round discs, but maybe the wave shape needs more material to resist warping.  If you thought rear brake pedal freeplay, fluid level, clean caliper pistons, etc were important it becomes critical when the disc is a fatty back there.

My M900 came with a thick wave disc and I know someone who just cooked the fat wave disc on their M750. When I checked pedal freeplay, reservoir fluid level, etc it was pretty good, but with new pads and a fat disc, regular pedal freeplay was not enough. I tend to run the pedal on the sloppy side just to be sure and my rear wave disc gets hot enough with little use just from pad drag. That little caliper gets a fair bit of heat build up in its small piston volume.

Rear brake pedal freeplay can be annoying to some people who run the minimum with little margin, so I have to add pedal freeplay to customer bikes on a regular basis.  Extra brake drag back there is evidenced by all the posts here about cooked discs and calipers. There's not much room in that little caliper with new pads and no give with solidly mounted disc and caliper, so when you jam a fat disc in there everything's gotta be spot on. I guess heavy users can get a special floating rear disc. Also organic pads shouldn't run as hot as sintered and be easier on the disc but wear themselves more quickly.

No wonder rear pad wear is rapid despite how little work the rear brake gets with most riders. Two customers with dirt bike backgrounds chew through rear pads. One mate just bought my Hyper and he needs to carry spare rear pads in his pockets . I'm fitting a used old oem disc to my Monster as I don't like the look of the wave disc that came with it. I use the rear brake to stabilize the rear suspension rather than back it in.

Anyway just a heads up for people running fat rear discs.

[stopintime]

I wonder if there is a build up of deposits just outside the piston seals, making a thicker disc and/or new pads problematic due to a non returning piston...  Makes me think I'll clean my pistons this winter. Still on my original pads. Thanks.

[koko64]

  The pistons might just hang a little, so out with the old toothbrush and brake cleaner

[Speeddog]

~~~SNIP~~~
Still on my original pads.
~~~SNIP~~~

143k km on them?

[ducpainter]

143k km on them?

Who uses the rear brake?

[stopintime]

143k km on them?

154,000 as of today 

Who uses the rear brake?

Obviously not I  I wish I knew how, but I never started and I'm afraid to since there is very little feel. Maybe a thumb brake is next....

[ducpainter]

Every time I try, I lock the thing up. I'll use it if I go off track, or off road, out of necessity, but otherwise it's a no-no for me.

[Speeddog]

I didn't think they'd even turn that many kms with no use at all!  

I use mine a lot, but nothing too strenuous.
Grew up on a dirt bike.

I've not encountered a fat disc issue, all of the BBQ jobs I've had in were freeplay or maintenance related.

If you want a rear brake that actually does some useful work, there's later model Ducs (749/999, Multis, etc) that come with 34mm piston calipers that use the big pads.
I'm using that setup and it's a real rear brake.

[koko64]

Hadn't thought of that as an upgrade. Easy bolt on?

I didn't think they'd even turn that many kms with no use at all!

True I reckon.

[ducpainter]

<snip>

I'm using that setup and it's a real rear brake.

I can't handle a fake rear brake.

[koko64]

 
Actually my Hypers rear brake was quite useful, e.g. downhill on a dirt road and settling the rear end (or unsettling it)

[Howie]

I would go with the dirty caliper bore theory.  Free play doesn't change from the fat rotor. 

I do try to use the rear brake.  The stock brake will lock if bled and you stand on it.  Doesn't slow the bike much though, just sends it sideways.

[koko64]

In theory the freeplay shouldnt be affected but I reckon it is. 

[ducpainter]

I'm with howie. Free play isn't affected by rotor thickness.

[koko64]

I reckon the fatter rotor results in more drag or exacerbates any condition that results in drag which heats up the fluid and robs free play. After a ride and the disc is hot, freeplay has been halved. Once it coools off, the freeplay returns. That is what I observe even if there is generous freeplay to begin with.
So when cold, you are correct, I just think the thicker disc exaggerates the known issues which eats up freeplay as the fluid heats up.