Topic: '78 CB750F2 front end swap for spoked wheels

[ionizer]

I am slightly confused with the information I've been seeing in relation to swapping out the comstar dual disc wheels for spoked wheels...(it hardly seems possible)

I can see my options are limited to basically either going with an earlier model CB750 front end and losing a rotor or use a GL1000 front end and keep the dual discs. (correct??)

If I were to locate a GL1000 front end is there a specific year that i need to look for to make an easy fit. I dont want to do much fabrication if possible so I want to find a direct fit. Are there any other modifications I should be aware of with the GL1000 swap? (ie. speedo/steering stops)

Last what would I do for the rear wheel? Is there a direct swap for it or would i also need to locate a part from another bike?

Any help is appreciated this is my 1st cb major tear down and i want to make it handle and look perfect.

Thanks from the noob

[Geeto67]

the problem is the brake parts not the wheels itself.

The comstars use a 5 lug brake pattern, the spoke wheels use a 6. The rotors on the comstar are thinner meaning you can't use the thicker earlier disc with the 77-78F calipers. Plus the caliper mounts are different between the forks. This is why you have to swap the forks, calipers, and wheel when you want to convert to spokes.

The GL front end you want is 1975-1977 spoke wheel only (some 77s are comstar - you can't use those).

You can convert an earlier spoke wheel brake setup on a cb750 to dual disc, but honestly the swing caliper bracket setup is inferior to the calipers on your 78F. Plus you can upgrade to the 1981-82 dual piston calipers really easily with the setup you have and better brakes are always a good thing.

I have heard that the DOHC 1979-1982 use a 5 lug rotor pattern and might work. I have never had one of these wheels off a bike so you are on your own for research there. my suggestion? buy a 1979-1982 cb750K wheel and see if it fits. Worst case scenario is you can relist it on ebay if it doesn't

I know you are all gay for spokes and such, but just because you have a 77-78F doesn't mean you are married to the traditional comstar wheel. There are boomerang comstars, reverse comstars, cb1100F wheels, honda dymag look alikes, etc....if you do some research you might just find that wheels from other hondas of the later era work. I have seen someone's bike here with VF1000R snowflake comstars - it looks hot.

for the rear, the same rule applies - you need to swap the brakes with the wheels. You can use a 75-76 rear disc wheel and brake setup, your cush drive (or the one from a 1978K) and mix and match the spacers. that should be bolt on.

Honestly, this is one of those situations where handle perfect and look perfect may run opposite. If you chose handle better it may mean no spoke wheels but you can really soup up the brakes. If you chose look better (and again this is subjective because I don't think spokes look cooler than the honda dymag copies or the boomerang comstars) and put spokes on it means a compromise in braking just for style - which means your bike is a no-go showboat. Like I said before there are options out there beside spokes, you just have to let go of the notion that spokes are "more authentic".

[ionizer]

Thanks for the info i appeciate it.

Haha I really am not "gay" for the spokes i just want bigger tires and prefer the classic look to the modern mags since there is a pretty limited selection for my bike. I didnt realize that the rear brake set up would also have to be changed. I got a response on another forum that the 70-76 rear was a direct bolt on fit.

I am leaning towards buying the GL1000 front end because that way i can keep the dual disc , get the spokes, and have a fatter tire up front. Just finding a decent fork with all the other parts is some what challenging. I assumed that this front end would give me a little better handling and stopping since its made for a bigger bike.

Ive got a lot of other work to do so this is kind of a back burner project i just needed to find out exactly whats necessary so thanks again for the info

[Geeto67]

Sorry but I am about to ruin your day again.....

Stock for stock there is no size difference between a 77-78 comstar rim and the stock spoke rims on eariler cb750s. You can lace wider rims to the earlier hubs but that requires a custom rim and custom spokes (from a place like buchannans). Last I checked wider alloy rims plus spokes plus lacing and truing will run you close to $700 for the pair.

If you want to run "bigger" tires for less the way to go would be later mags off a cb1100F since they are 2.75 front and 3.00 rear. If you look around in europe you can find 3.50 rear boomerang comstars. Don't be a fraid of buying parts from europe, there is a guy there from holland who routinely sells european markets wheels to the US for $300 a set shipped.

The GL front end does not use a wider rim stock than the cb750. In fact the rim itself is the same as the spoke one that comes on the cb750A in 76. The advantage of the GL front end is 1) a cheaper alloy rim, 2) 37mm forks instead of 35mm, 3) dual disc with a speedo drive. Out of the box the GL is marginally better (it is noticable) but the "right" way to do it would be to get springs setup for your weight, a proper fork brace, and if possible cartirdge emulators. you will have to push the tubes up through the triple clamp a couple of inches to restore ride height. To do the GL swap you need the triple clamps, the forks, the brake hardware, and the wheel. You will also need to fab a gauge mount or adapt yours to fit. It seems like a lot of work to go backward in brake technology.

 I still say you try to find a spoke wheel off the 1979-1982 cb750K as those look to have the correct brake bolt pattern. and you may be able to keep your dual disc setup.

The 69-76 Drum rear is no more a bolt on in the rear than a cbr600F2 rear rim would be. sure they all use the same axle but that is about it. You will have to mix and match the axle spacers but keep in mind you are then converting your rear disc to drum, adding weight and decreasing brake performance. Plus you will need to figure out how to control the rear brake as your current setup is hydro and the drum is mechanical. Your best bet is the 75-76 cb750F rear spoke. Again you will have to mix and match the axle spacers and cush drives but at least you can keep your foot controls. The 75-76 rear caliper is a better caliper (in fact it is overkill) but it is heavy.

you need to do more research and more measuring. judging by your responses this bike doesn't sound really well thought out or researched. It also doesn't sound like you have a lot of experence working on bikes. If you are the standard newbie novice I run into this may be punching above your weight, but then again the standard newbie novice I know blows the bike completely apart and never puts it back together again. I would refocus and try to solve one problem at a time. Identify your goals and then seek out how to accomplish them. Here is an example - let's look at your rear and see what you will need to figure out to do the swap:

1978 comstar to spoke conversion
- goals:
        - wider tire
        - better performance
        - Classic look

option 1: stock honda 1975-76 cb750F rear rim (for the sake of brevity I am going to fill in stuff I know)
 - Need to know:
     - Rim size (not any wider than a comstar)
     - axle diameter
     - chain offset
     - Brake part compatibility (In this case No)
     - cust drive compatibility
- Advantages:
    - moderatly easy to adapt 
    - easy to modify for a wider tire
    - no real custom pieces
    - you like the look
- Disadvantages
    - not a performance upgrade (heavier)
    - expensive to modify (wider alloy rim plus spokes $$$)
    - Requires a lot more parts (spacers, rotor, caliper, caliper mount, torque arm, etc...)
Option 2: Later comstar or other wheel off 1979-1982 CB750F/900F/1100F
- Need to know:
   - Axle sixe
   - cush drive compatibility
   - brake parts compatability (yes)
 - Advantages
   - Easiest to install
   - relativley cheap
   - works with your existing brake setup
   - no custom work
   - easy to upgrade with 81-82 calipers
- Disadvantages
   - hard to modify (Kosman gets around $700 to widen a comstar per wheel)
   - married to existing tire size or one size bigger depending on the wheel you choose (you can push all the way out to 140 with the 3.50 rim)
   - you hate the look

Option 3: something custom
Advantage : you can find something to fit and look right and work right.
Disadvantage: it will cost a boat load of money, require skills above your ability, and you will need special parts.

some examples of option 3 are: kosman rear racing hub for a cb750 (I have one and it is not for sale, I have never seen another), GS750 rear wheel which requires machining to get the chain line to match up and also modifying a gs1100 arm to fit the cb750 frame (more machine work).


Remember when I said you may find out pretty quick that you are going to have to make some compromises to get a fully bolt on deal, well here is where those compromises come to a head. And we are just talking about the rear wheel here - that's the easy one.

an old friend use to say bikes can:

- look great
- perform great
- be cheap

pick two.

[brandEn]

Having done the GL1000 and spoked wheel conversion on my 78 CB750 F3 I can say Geeto has hit the nail on the head. He has just posted all the info I could have hoped for when I was doing my conversion. I thought I would be able to do it for pretty cheap but that's just not the case. It all ads up and I chose to replace all bearings/seals, brake lines, brake pads, caliper rebuilds, tires, chain, ect.

Finding a rear hub from a 75-75 F model is not as easy at it sounds! You will have to adjust spacers to get your chain aligned correctly.

75-77 GL1000 front ends are a dime a dozen on Craigs and Ebay. The tree is wider and you will have to either weld a couple blobs on the stops or install bolts (what I did) in the stops to prevent the stops hitting the tank at full lock.

[Old Scrambler]

GREAT time for this thread.......I just rolled my F2 project out of storage because my motor is nearly finished.....mild cam upgrade in a '76 F1 motor because it has a title.....I have the early GL1000 front end and appreciate the comment on the lock-tabs......I am using the front aluminum wheel with spokes from an auto......and lacing a fat rear aluminim from a GL1000 to a 76 F1 rear hub with disk.  The finished bike will have mild-placement set-backs, euro-bars, and a nock-off Dunstall fairing/tank/seat combo that need A LOT of material to be removed. On the rear wheel alignment problem for the 77-78 motors, the sprocket carrier should exchange to give you the correct offset. Because I am using the standard motor-sprocket width I will use the extended sprocket from Ken at CYCLE-X to clear my fat rear tire.  I anticipate the 17-inch rear will require the front forks to be adjusted in the trees by about 1/2-inch mostly depending on the length of the rear shocks. 

[brandEn]

The rear carrier will fit but the spacers will most likely need to adjusted. I used the carrier from my Comstars and still needed to make changes. Just speaking from my conversion/setup.

[Geeto67]

I have done it before, I usually mix and match spacers from both the spoke wheel and comstar wheel. I think once I had to make a spacer for a friend's K8 using a 76F rear disc setup so I used a spare and cut it in a miter box with a hack saw. Which leads me to my next point.......


When ever you are doing custom work like this it pays to buy the whole setup, not just parts and piecemeal. Too many noobs try to be cheap up front and buy just the wheel and then have to spend money on the back end to get other things like the axle, axle spacers, etc.....when they could have just bought the whole thing from swingarm pivot to axle adjusters for like $150-$200. It is always best to find someone parting out a bike and just buy as much as you can rather than scouring ebay for individual pieces.

And by the way can I rant a little? what is it with sellers on ebay wanting a nickel and dime for every single little thing? I was recently shopping for a wheel for another bike and over half listed did not come with rotors. I am using it for a custom application so I needed the wheel the rotors and the speedo drive. I emailed 4 sellers of wheels with rotors and only one would include the speedo drive - the other 3 told me I would have to bid on another auction and pay separately. the one guy who did have the speedo understood what I was trying to do and was so nice about it (included it for free) that I marked him as a favorite seller and plan on buying other stuff from him in the future. The other 3 I probably won't buy stuff from. I understand the bike dismantler mentaility that it is a business and every pound of a bike is worth something, but honestly when there are like 50 other guys selling the same wheel and some of them are willing to include the rotors and drive how do you think you can compete with that. I actually paid a $15 premium to the seller willing to include the speed drive because his customer service was that good.

Anyway, the other piece of advice I can give is don't make assumptions about what stuff does and how it fits. Hard data beats a guess everytime. Yes the GL1000 was a bigger bike but really maybe only 100-150lbs heavier than a stock cb750 and it wasn't anything more than a marginal stopper either so while it will be an improvement in the braking department how much of one is anyone's guess. Honestly if you take 100 lbs off a cb750 the brakes get instantly better.

[Old Scrambler]

Especially if the lbs are on the seat.......................

[Imago]

One of the best threads ever on this subject... very handy for my 1978 CB750F Comstar to Spoked conversion contemplation.

Particular thanks to Geeto67.

[tweakin]

Great info here Geeto...

I did this last year on my F2.  It wasn't too difficult but wasn't cheap either.  I went with a 1977 GL front end, brakes, rotors, speedo drive and hub.  The GL and 76 cb hub are the same.  The GL tubes are also longer than the CB so I ordered 1 inch under tubes from forks by Frank, you can also slide the tubes up in the trees if you want to.  All balls bearings work great to mate the GL front end to the CB frame (can get conversion bearing sets from them) but I believe the GL and CB fork stems are the same.  After racetech springs, emulators and fork brace I could not be happier with the GL front end.  I did go with Buchanan's aluminum Sun rims and stainless spokes, and if was a treat learning to lace wheels the first time.  IMO the brakes on the GL perform much better than the stock supersport brakes and the larger GL front feels much more stable at speed.  This could be due to the fact I completely rebuilt the brakes and went with stainless lines and upgraded master cylindar.  One thing to keep in mind is that the turning radius changes with this mod and the wider stance allows the fork tubes/headlight ears to touch the tank in sharp turns if you do not adjust the steering stop.  To fix this I just drilled a hole in the stock steering stop, threaded it and put a stainless bolt and nut in there to stop the forks before they contact the tank.

For the rear I picked up a 76CBf rear disc hub including cush drive from forum member MCrider.  I used my stock rear axle and axle adjusters and got the spacing to fit right by using a different combination of CB rear spacers.  Didn't keep track of exactly which ones I used and sorry.

I did weigh the comstars and the new spoked wheels with aluminum rims and the spokes were lighter but that was some time ago and I am not sure by how much.  Since then I added aluminum rear sprocket.

I love the spoked conversion on my 77 but love the comstars on my 78 just as much.

[MRieck]

I have an F2 front end (forks and calipers) with a 75f spoked wheel on my 75F. It can be done but the rotors have to be thinned and the OD reduced. You also need a spacer to offset the right rotor a bit. I helicoiled the hub to eliminate the long rotor bolts and used countersunk Allen flat heads to clear the speedo drive on the right side. Left rotor is held with stainless hex bolts. It works fine but machining the rotors is a pain.

[Geeto67]

since the rotors are stainless I can't imagine the time and expense to someone who isn't you mike. It isn't the softest stuff on earth.....

[MRieck]

since the rotors are stainless I can't imagine the time and expense to someone who isn't you mike. It isn't the softest stuff on earth.....

I totally agree. Some of the machining was done CNC but most was done on a regular (but large) lathe. The rotors had to be surface ground after which is a pain in itself as it is difficult to keep both sides parallel. The other stuff was pretty straight forward.

[Geeto67]

Someone was telling me recently that there is a modern sportbike rotor that has the same 6 lug pattern as the earlier wheel but I have forgotten which bike he said. I think it is like cbr929 or cbr600 f4

[Greg405]

I found a pair of 76 CB750F model rims/spokes with rotors and it was a straight up swap with my 78 CB750F2 Comstars. I kept the original front end and all 3 calipers and only had to use a couple of spacers between the brake caliper mounts on the forks in the front. I had them respoked by a pro here in Minneapolis because and all worked out well. Here's the finished bike...

[Geeto67]

Ok, so how did you account for the difference in thickness in the rotors?