Smaller rear disk alternatives for stock 89

[RobbieKNobbie]

Like the title says...

The rear disk on my (new to me) '89 is warped and needs to be replaced, and I wouldn't mind shedding a little unsprung mass in the process. Is anyone aware of a smaller disk that would bolt directly to the rear rim? I understand that would require changing out the caliper and caliper bracket as well, the new bracket's no problem, I just don't want to modify the rim or the new disk.

Thanks guys!

[Pat Conlon]

...I just don't want to modify the rim or the new disk.

Couple of options from easiest to hardest :
1) The easiest is to replace heavy dual faced (vented) iron rotor with a single faced stainless rotor (eBay and RPM) plug and play
2) Realize that there are better tire selections out there when you install a 17" rear rim. Along with the wider and lighter 17" rim will be a smaller diameter rotor. The proper caliper bracket and caliper will be included as part of the swap. For example, look in the Files for the GSXR rim swap.
3) Keep the narrow and heavy oem rear rim and match up the bolt pattern on the rim to a smaller diameter rotor and fab up a custom caliper arm and brake stay rod.

Here is a web site that lists the bolt patterns and offsets for the various rotors used on several different makes/models of motorcycles.
http://www.fjowners.com/index.php?topic=8623.0

[racerrad8]

If sticking with the OEM rear wheel

The BRAKING Brake Rotor is only 2 lbs 11 oz compared to the EBC Brake Rotor coming in at 4 lbs 9.2 oz. I don't have a new OEM rotor to weigh so I don't have the weight on a new OEM rotor. However, the people that have switched to the BRAKING Brake Rotor over the EBC have reported that it does not warp as badly as the EBC Brake Rotor or the "cheap" eBay rotors. Both of these rotors are plug and play requiring no modification to the wheel, brake stay, disc, or having to change the brake caliper.

Robert - RPM

[CutterBill]

Just a few data points for anyone replacing the rear disc:

1. The stock rear disc thickness is 7.5mm.  Wear limit is 7.0mm.
2. Many of the aftermarket discs are 5mm thick.
3. The pistons in the brake caliper are short and squat. They don't have a lot of length to keep them straight in the bore when a side load is applied (braking.) The whole caliper, piston and pad assembly was designed to work with a minimum disc thickness of 7mm.  And now you want to use a 5mm thick disc? A full 2mm thinner than the minimum specified?  Proceed at your own risk.

One event does not constitute "proof" but here's my story:
After receiving my '92, I discovered that the rear brake was useless. Removal and disassembly of the caliper revealed that one piston was frozen solid and the other was almost so. The caliper was thoroughly cleaned and carefully reassembled with new seals, rubber grease, etc. I'm pretty good at this and I did the best job I knew how. The pistons moved freely in their bores with no sign of sticking.  Caliper was re-installed onto swingarm with used, but only slightly worn, pads. Shop testing showed that depressing the brake pedal would cause the brake pads to extend properly but they would not retract enough to release the disc.  Hmmmm....

Something didn't look right. There was too much gap between the disc and caliper. Which led to my discovery of the 7.5mm vs. 5mm thick discs. Ordered and installed a new EBC disc with the proper 7.5mm thickness. Reinstalled caliper with new pads. Caliper then worked as designed. No problems during 2300 miles to Ouray and back home.  YMMV...

Bill

[Pat Conlon]

Good point Bill.
You can use the YZF600 rear caliper made for the single face disks.

[Firehawk068]

Bill, I think your instance of a problem might have been an isolated issue?

I have been running a Galfer Wave-Rotor on the rear of my FJ for a bunch of years now, and have never had any instance of the caliper not releasing the pads from the rotor.
The Galfer Rotor measures 5mm thick. I am using the stock FJ rear caliper still.
Even with extremely worn down pads, I've never had an issue. I know of others that are also using this rotor, and other brands that are 5mm thick, without issue.

I also have EBC Pro-lite rotors on the front of my FJ. These also measure 5mm thick. They were on the bike when I purchased it, with the stock calipers, and I never had an issue with the fronts either.
Both the Galfer, and the EBC rotors have been designed to work on the FJ with factory calipers.

With the depth that the pistons are placed inside the caliper on either side, I doubt that 1mm of extra movement on each piston is going to cause an issue. In fact, I am certain it won't, as I have ridden tens of thousands of miles on it that way.
I would bet that the minimum thickness of 7mm on the stock rotor is so that there would remain at least "some" material left on either side of the venting ribs.

Anyway, not to discount your experience, as you obviously had an issue of some sort, but mine has been "different".

[aj52]

I had put an Yamaha V-star 1100 rear disk on when the vented disk become worn.
It is 5mm thick,but I made 2 stainless shims(1.25mm each) and installed it between the.piston and brake pads.
I made it the same shape as the back of the pads,even the 2 holes of the pins that ran through pads,so there is no way it can come out.

I never had any issues with it.
I have since done the GSXR mod.

Hannes

[aj52]

The rear disk and one of the shims

[CutterBill]

AJ... that is actually a good idea. I like it... 

[RobbieKNobbie]

Wow, lots of good information here. Thanks guys. For those of you who have had their caliper apart, is there anything special about the pistons? I mean, any reason it wouldnt be just as easy to make new pistons .040 or .080 longer than stock?

[CutterBill]

Hmmm... possibly hard-chromed, definitely center-less ground to specific dimension, roundness and surface finish...

Would be vastly cheaper to buy the correct 7.5mm disc. Or cheaper yet, to make a set of AJ's shims...

[Pat Conlon]

....or spend $16 and buy a YZF600 caliper
http://www.ebay.com/itm/Yamaha-2001-YZF600-YZF-600-3-7-REAR-BRAKE-CALIPER-/401098589936?hash=item5d6356c6f0:g:u~wAAOSwoudW8z5W&vxp=mtr

[Firehawk068]

....or spend $16 and buy a YZF600 caliper
http://www.ebay.com/itm/Yamaha-2001-YZF600-YZF-600-3-7-REAR-BRAKE-CALIPER-/401098589936?hash=item5d6356c6f0:g:u~wAAOSwoudW8z5W&vxp=mtr

Yamaha Parts Fiche shows the same part number for the YZF600, and the '89-up FJ1200 rear calipers. Even the Pistons and the Seal Kits have the same part number.

I would just use your stock rear caliper. This has never been an issue in the past?

[racerrad8]

Yamaha Parts Fiche shows the same part number for the YZF600, and the '89-up FJ1200 rear calipers. Even the Pistons and the Seal Kits have the same part number.

I would just use your stock rear caliper. This has never been an issue in the past?

Agreed, we have sold close to 100 of these rotors and have not had one reported issue.

Robert - RPM

[FJ_Hooligan]

My YZF caliper had a bunch of small scratches on the side facing out.  

I used the inside half of the YZF caliper and the outside half of my FJ caliper because it said YAMAHA on it.