I have been poking at a CB750 dual disc front disc set up for a little while now. I wanted to have (1) a spoked wheel (2) a solid fork brace, (3) better suspension (4) a more solid (less flexible) front suspension.
I put one together tonight. I basically swapped out a 1980 CB750 SuperSport's [Comstar] front forks and brakes. (I am pretty sure these are nearly identical to a 78-79 SOHC supersport). This one also has a beefy alloy fork brace, but it's not in the pics. I'm sorry it is late/dark so the pics suck. You can click the pics for a larger view:
I built the front end on the 1980's bike, just because it was easier for me. I'll swap it tomorrow and install the Gaffer steel braided lines and new master at that same time -- I have a big ride planned tomorrow night, so it needs to be ready by then. It is hard to tell, but the rim, hub, spokes, rotors are all perfect. The tire still has the nubs along the middle. If you click and zoom in, you can see the red disc brake quiet material, and then between the caliper mounting bolts you can see the little spacer bracket I had to make to align the calipers to the rotors. It is kind of wavy shaped.
That blue thing is my Most used tool -- my "little helper." It is my kneeling pad.
That is the black box of my micrometer. I used that a lot today. On the other side of the wheel is a speedo ring gear. I will be modding that in the morning so I can have a speedo with this setup. Also, you can see the oil cooler that I probably am going to rob and stick on my bike.
Here is the stuff I encountered, it you want to reproduce this:
1. First thing is I had to get the spoked wheel centered in the forks. I had to machine a spacer/washer down to 2.4mm, and that did the job perfectly. I also fount it easiest if I swap the speedo outlet from the left to the right side.
2. CB550 rotors need to be used -- they are 275mm diameter. They are 20mm smaller in diameter than a CB750, but the same diameter as the comstar dual rotors; except the Comstar rotors are 4mm thick, and the Cb750 and CB550's rotors are 6mm thick.
3. Because the rotors are thicker, the caliper brackets had to be machined out about 3mm so the CB550 rotors would fit. This was the most difficult part of this job. I used an angle grinder and a hand file, and it took a few hours to get them right.
4. Once the calipers will fit over the thicker rotors, they have to be centered. Miraculously, I had some bar stock sitting around that was more or less EXACTLY the correct thickness to use as spacers for the calipers. They really had to be perfect because there is so little clearance for the rotor inside the cast aluminum caliper bracket. Each caliper's spacer needed to be 4.8mm thick; the calipers needed to be moved in (toward the middle of the wheel) 4.8mm on each side.
5. Then there was some finish work, like trimming the calipers' stainless pad sliders and stuff like that.
While I've been thinking about it for a long time now, this project only took up about 1/3 of my day, maybe about 6 hours. I had all the parts sitting around the workshop.
So that's that. Once again, the hardest part was machining/opening up the caliper brackets a little for the thicker rotors. The rest was just me moving slow, drinking beer, and taking my time trying to figure out how to fit stuff together.
Peace and grease.
-fang
