Re: Ranger's 1980 CB650C Scrambler build
Postby Ranger » Thu Apr 23, 2020 3:59 pm
I've already started fitting up the rear wheel from a 1992 CB750 Nighthawk. It's a 17x4 drum mag, so the 150/70/17 Kenda Big Block tire fits it perfectly.
However, it has a 17mm axle instead of a 20 mm axle and is, of course, wider, so all of that has to be taken into account. If I center this wheel, then the sprocket mounting surface is actually about .3725" closer to the left axle plate than factory.
So basically, here's what I did to make the axle/wheel work:
1. I'm using a 17mm axle (not the one that came on the 1992 nighthawk, as it was too long, but I had one that was the right length in my stash... no idea what it came off of).
2. I'm using the CB650 axle adjusters to keep things simple.
3. I had 2 flanged bushings made out of 1.25" stainless steel round stock. They are 1" long, with a .25" shoulder, and a 17mm bore all the way through. .75" of the overall length has been turned down to 20 mm. These will slip through the axle plates/adjusters on each side of the swingarm.
4. Then I had the factory wheel spacers from the CB750 cut down to the proper length, and cut a .25" deep 20 mm counterbore on once side of each.
5. The Nighthawk drum looks like it will work fine with the CB650 brake rod and stay. Once I get everything bolted up, I might end up swapping the arm on the brake plate with the one of the CB650, but just eyeballing it, it looks like the nighthawk one will work.
6. I'm going to need to trim the sprocket studs a bit. At the moment, they hit the lower shock bolt. But I'll wait until my bushings/spacers arrive to do that. Absolute worst case, I'll rework the shock mounts.
I'll post a picture of the setup once that parts are back from the machinist... probably next week.
Then all that was left to do was to figure out how to offset the front sprocket. I looked, and I couldn't find anybody that makes an offset front sprocket for the CB 650... That said, there are a couple of options out there for the CB750. Since the CB650 and CB750 share the same sprocket shaft splines, I took a chance and ordered a 17-tooth 520 3/8" offset sprocket from Cognito Moto.
https://cognitomoto.com/collections/all-products/products/17-tooth-3-8-9-5mm-offset-sprocket-for-cb750-cb550-cb500tThe sprocket did fit on the splines, but it was too thick to allow the retaining ring to rotate into place in the notch on the sprocket shaft.
It got a little confusing, since the sprocket is actually indexed off of the retaining ring (and thus the notch), rather than being indexed off the bearing surface on the side of the motor. But I figured out that if I had a little material machined off of each face of the sprocket, I could have it line up perfectly with my new rear wheel, and trim enough off of the back that there was clearance between the sprocket and the bearing. So I sent it to my machinist with instructions on how much material to remove.
It's kind of a shame to outsource this stuff since I just bought a lathe and mill last summer... but unfortunately I haven't had the time to go through them and sort out a couple of issues so I can run them. Guess I've been spending too much time working on bikes...
The last thing I needed to do on the drive line was figure out the gearing and purchase a sprocket for the rear. I used the gearing calculator at
https://gearingcommander.com/ and figured out, that with a 17-tooth front sprocket, and a shorter rear tire, a 41-tooth sprocket would get me close to factory gearing. So I ordered a custom sprocket from Rebel Gears.
http://www.rebelgears.com I've ordered sprockets from them on several occasions for strange applications. They're always super fast and very reasonably priced.
I'm assuming that once I get the bike going I'll need to tweak the gearing a bit, but I figured I'd start out at factory gearing as a baseline.
