Topic: 1978k cb750 swithcing from rims to comstars and need help

[vinny9744]

Hey everybody, I just picked up a 78k on the cheap. It needs lots of work, but has a clear title. Among a ton of other things the rims were almost non existent do to rust, having been left outside for a looooong time. I know they're not very popular but I've always liked the look of mag wheels on later bikes. So I picked up a set of Comstars from a 78 supersport. This means switching to a rear disc as well. For the back I obtained the swing arm, rear disc, caliper and rear axle. For the front, all I have so far is the rim.  I was planning on switching the parts over from the existing front wheel but just realized the Comstar uses a five hole disc.

I want to run this Comstar rim for the front but need to know which disc rotor to get.
I think this rim was supposed to go with dual discs, but can I just run one?
As I just want to run one caliper, can I use the one from my 78k?
Does the Comstar need a different axle and spacer?

I'm sure I'm forgetting a bunch of stuff but any help is appreciated.
Thanks

[vinny9744]

Sorry everyone. My computer got stuck I didn't know my first post went through

[Roach Carver]

You will need forks that go with the comstar. As for one vs two disks, you can run one as far as I know.

[Xxthegonzxx]

Check your front axle and let me know if it sits flush with your forks. Mine sticks out about an inch or so. Check it out:

http://i.imgur.com/BEZDou0.jpg

I have dual discs though.

[vinny9744]

I don't have an axle for the front wheel yet. I was hoping I could use the one from my original spoked rim. Is that possible?

Why would I need the forks from a Comstar rim model? I know the Comstars ran the brake caliper behind the fork but I thought I could use my K brake caliper and mount it in the original location. Are the discs a different size?

Thanks again.

[vinny9744]

Thanks Calj, I'll get the original axle out and check that. The comstar is from a 78 Supersport. I checked out the fiche on Bike Barn for both bikes. It looks like the axles are different so I plan to grab one off Ebay. The Comstar uses a 5 bolt pattern disc rotor so my orginal 6 bolt one wont fit. So I guess I need one of those also.

What Im really concerned with is once I obtain the correct axle and disc for the Comstar, will I be able to run it in my 78k forks and use my 78k brake caliper?

Thanks

[Roach Carver]

Anything can be done. Your easiest route is to buy a set of forks. I "think"you can use your trees.

[vinny9744]

Thanks Roach. This is pretty new to me so forgive me if I don't completely understand. If the forks fit in the tree, wouldn't they be the same width at the wheel?. Would I need different forks because of the way the caliper mounts?

[Roach Carver]

I have not done what you are doing so take what I say with a grain of salt. I know the caliper and rotor are different and I would think it is fairly significant. This is a pretty decent sized issue. I am sure with enough fab/ parts swapping/hours and $ you could somehow make anything work. In my experience, the more of the "right parts" that I can use the easier my life is. You already snagged the rear setup that you needed. I think I would scrounge up the whole front setup as well. The comstar setup is not the prefered wheel combo. no offense intended, you like it and thats great. My thought is that you should be able to get the parts you want and not have a lot of competition getting them. Like was previously suggested, look at parts fiche diagrams and see if the part numbers are the same. I doubt that they are. carry on.

[martin99]

As far as I know fork stanchions across the CB750 range are 35mm. The distance between forks (centre line to centre line) on my twin-disc comstar equipped F2 is 7 1/4".

If your forks measure 7 1/4" between centres, and I'm pretty sure they will, you will be able to use your existing triple with the later twin disc forks. Problem you will encounter will be the brakes. The F2 has a different lower triple to take the brake splitter, but you could still use twin discs by running both brake lines from the master - could look messy.

You would really need the later rear-mounted calipers to suit. You can run a single disc if you wish, but this leaves caliper mountings on the redundant lower with nothing attached - also messy.

According to the parts fiche, the F2 comstar and the K8 spoke wheel share the same axle. However the spacers are model specific, so you would need these too.

On balance, the easiest way to achieve what you're after IMO would be to get the whole front end.

[martin99]

Just for inspiration, here's what your bike could look like with a single disc comstar...