Topic: Thinning CB 750 discs

[754]

I had one that had runout, after riding the bike today & seeing what happened when I applied brakes, I finally decided to true it up..

 Made an arbor on the lathe to fit the disc and  mounted it, useing the mounting face as reeference surface.

 I cant for the life of me figure this out, maybe it got hit, but the disc had .005 or .006 thou runout on the backside.. , and looked like it was not cut.. so I had to true both side..

 Disc was around .210 thick, so I am thinking a mm was already cutoff. anyway I trued up both side, got a bit of chatter on one side but turned out pretty good.. as they are not easy to cut.

 Anyone try thinning them?.. not as easy as it looks..

[markjenn]

I've always heard you can grind them, but not cut them.

- Mark

[fishhead]

 Anyone try thinning them?.. not as easy as it looks..

Yes, I tried to resurface discs on the lathe and I got chatter also. They probably need to be ground down like a flywheel.

[HondaMan]

Yep, they were ground: you could definitely see the grinding pattern on the originals. On low-mileage ones, you can still see it on the edges where the pads have not worn it off, yet.
The pattern suggested they were ground, then mounted to the spoked hub, so I imagine a long line conveyor, passing under a grinder like a Blanchard, then someone flipping them over for the other side, then to the next grinder. Thicknesses varied from 5.55mm to 6.55mm on new ones. Honda says 5mm is the minimum, but mine is below that with no issues. I think they were just being conservative as it was the first disc brake out there.

[fishhead]

Its a crappy pic, but here is a disc that was thinned to about 1/2 thickness. The stock rivets were removed and the discs were held to the carriers with bolts (Yamaha style).

[babyfood1217]

If your thinning this much, have you put any thought towards an actual disc replacement?  If you want stock carrier and are ok with bolting it all together, I wonder if either...

-custom cut SS disc (water jet or laser would be quickest with cleanest edges
-graft on a disc from another pre-existing rotor (might need additional trimming)

Also, weren't some of the early rotors cast iron?  I'm always heard that a person should never really turn down cast iron rotors.  Hell, the stuff is brittle enough as it is.

-Chris

sidenote:  Anyone has a clue as to what alloy/grade the stainless rotors are?

[754]

I do have access to a flywheel grinder, and had planned to true it up on there...

 but.. after watching what it was doing under braking, I wanted to fix it NOW..

 so I pulled it off.

 Now the disc was already thinned, plus it is drilled to 4 bolt pattern (I am running it on a Kawi 4 bolt Morris Mag)

 It trued up now..

 i actually built a tool, a while back for thinning, but did not use it on this. It cuts with 2 tool, 1 per side. a bit tricky to set up, but I bet it wont chatter!

 I ended up at around 200 thou..
on the disc carrier it says 6mm min thickness.

[MCRider]

Mine are .174" or 4.5mm thick, which is pretty thin I think.  

I think the earlier ones (mine) were thicker to begin with than later models?

In any event I had mine thinned in 1974, and was told at that time it couldn't and shouldn't be done. I found a brake shop that did it, and they were cut not ground. I remember the machinist said it was a #$%*, but he did it. I've run them as a road racer and high mileage street bike ever since.

For upcoming use I had them drilled and surfaced as shown.



They are definitley not cast iron, never a touch of rust.

My FJR has cast iron rotors, at least an alloy, as they will rust if left unridden for a while.  I had to clean them recently.

[bwaller]

To remove much material they should be turned, but best to finish grind so Speedracer here claims. He did a great job on a couple sets for me. Both times we went well under Hondas minimum suggested 6mm.

[MRieck]

To remove much material they should be turned, but best to finish grind so Speedracer here claims. He did a great job on a couple sets for me. Both times we went well under Hondas minimum suggested 6mm.

That's exactly how mine were done.

[HondaMan]

These "6mm" numbers are kinda new to me. 
All of the discs I have here measure 5.8 to 5.5 (except y high-mileage one). I recently measured some on a Shadow, at 3mm thick. They are cast iron, though, not 18-8 stainless like our SOHC discs (at least, that's as close as I've been able to figure their metal...). If left alone for many years outside, a surface rust will appear on 18-8, which resembles the occasional rust I've seen on these over the years. 

[bucky katt]

the best rotors to use for stopping power are iron as i understand it. theyre nice and soft and theres plenty of braking power with them.

[Clyde]

I have machined discs from standard (6.7mm) down to 5.5mm on my lathe.
These were done to fit on a race bike using a Brembo 08 caliper.
It took a long time and was not easy to do. I found that the hardest part was at the end taking small cuts, due to the flexing of the disc.
To hold the disc I made up a mandrel from aluminium.

I have since had a look at a special brake lathe which cuts(not grinds) both sides at the same time and hence does not allow the disc to flex. My understnding is that, this is the most common way of machining car discs

Modern discs go down to 3.2mm with no problem.

[HondaMan]

I have machined discs from standard (6.7mm) down to 5.5mm on my lathe.
These were done to fit on a race bike using a Brembo 08 caliper.
It took a long time and was not easy to do. I found that the hardest part was at the end taking small cuts, due to the flexing of the disc.
To hold the disc I made up a mandrel from aluminium.

I have since had a look at a special brake lathe which cuts(not grinds) both sides at the same time and hence does not allow the disc to flex. My understnding is that, this is the most common way of machining car discs

Modern discs go down to 3.2mm with no problem.

I suspect that double-grinding them like that also destresses the metal, which makes perfect sense: they would not be prone to warpage then, even with heat.

[754]

I may try the double tool while I still have the setup in the lathe, well have to insert a pin or bolt to stop disc from turning..

 If i can dig out the double cut tool, and have time I will give it a try.. should only take 20 minutes, once the tools are set.. the only problem I foresee, is not being able to control the chips..

 I am thinking to end up around 180 thou... about 4.5mm

 If it works well, I will be able to do a bunch of them..