Topic: GL1100 VS CB750 Front End

[brycegp]

Hey guys! 

I have a set of 1980 GL1100 Interstate Front Forks with perfect chrome.  So I was thinking of using them on my CB750/836 project.

I don't have the triple tree or the front wheel / axel so I don't really know what differences there are between the triple tree, front wheel hub and the axels....and what parts are swappable between the two.

1.  Will the triple tree bolt up or will I have to modify the stem?
2.  Will the CB750 front hub and axel (OD and length) work with the GL1100 triple tree and forks?
3.  If #2 is yes, will the CB750 brake discs line up to the GL1100 brake calipers?

Any insight to doing this swap would be appreciated.

Thanks!!

[brandEn]

Usually people use the 1975-78 GL1000 front ends. I have never heard of a GL1100 being used. Maybe you could cross reference the tree part numbers. The 75-78 GL1000 trees bolt right on, if they are the same then your in luck. The GL1100 uses mag wheels, Comstars maybe? I am not sure if the CB750 hubs will fit but I am guessing that if they do you will need to use the axle, spacers, rotors and calipers for the GL1100 to get them to line up. Interested to see what you come up with.

[brycegp]

after reading more search results...its seems that I need to use an earlier front end.  but I am still trying to piece it all together since I am basically able to put it together ala carte. 

I'm thinking I can use the GL1100 forks with an early GL1000 triple (both 37mm) to get the correct spacing between the fork lowers.

So I just need to nail down the year and model of each individual part.  See below.   Looking to build a simple parts list....based on the fact that I already own the 1980 forks...and of course the CB750 front end that is on the bike.

I already own...
Forks: '80 GL1100
Entire Front End: Early CB750K

I need to get...
Triple Tree:  '75-77 GL1000
Calipers:  '80 GL1100?
Spoked Hub:  CB750K
Rotors: 2 x CB750K (driilled)
Speedo Drive:  ??
Axel: ??
Spacers:  ??

[brandEn]

The early GL hubs and CB750 hubs are the same FYI. You will need GL rotors to get the correct offset needed for the calipers. Something to keep in mind the GL trees are about 1' wider than a stock CB,

[dusterdude]

Arent the gl1000's 35 mm same as the 750?

[brandEn]

Nope. 37mm

[dusterdude]

Cool,thanks

[Geeto67]

Brycegp.

You are doing it wrong.

 #1 you can't use gl1100 calipers with cb750 rotors for two reasons: 1) the spacing is different, 2) The rotor thickness is different.

#2 you can't use gl1100 rotors with a cb750 hub because the bolt pattern is different. There may be some Honda spoked wheels with a 5 lug pattern like the comstars use, but I don't know what they are, maybe 1981-82 750k?

#3 you can't use earlier calipers or 750 calipers on the 80 fork legs because the caliper mounting spacing is different.

Let's start over:

To use your gl1100 forks you need the gl1100 calipers, rotors, axle, spacers, speedo drive, and a 5bolt pattern wheel (which will probably be a comstar). You will also need the gl1000 trees but be forewarned you need to find out if your fork diameter is 37mm. I am guessing it is not since it shares a part number on the fork seals with the cb1100f (39mm forks).

The better way to do this is to buy a 75-77 Gl1000 front end complete. It will cost you less and bolts right up. If you want to save on shipping you can reuse your cb750 wheel only.

Don't buy it piecemeal buys complete front end. You will have to rebuild and service th thing anyway, don't turn a $250 front end into a$500 one because the bolts and hardware nickel and dimed you.


I know you don't want to hear this but for what you want to do those 1980 gl1100 forks are useless to you. Sell them and buy the right parts.

[Rigid]

^ +1 your wasting time and money over an early GW setup to gain nothing.  You won't be the first here to do that however.

[brycegp]

I know you don't want to hear this but for what you want to do those 1980 gl1100 forks are useless to you. Sell them and buy the right parts.

Actually Geeto, that's exactly what I want to hear.  Thank you for the break down.  I knew it was long shot using those forks and that is why I threw out the question.  I did a front end swap ( CB550F onto a CB750K) on my last bike and know all the variables.  I just didn't know how those variables would play out on this particular swap...the GL1100 forks...they're compatibility...or lack thereof.

Honestly, I was so happy with the CB550F swap...I think I'll go that route again.  Basically, its a standard CB750 dual disc conversion.  BUT the CB550F forks / triple bolt right up using the 750 calipers, mounts, axel, spacers, speedo drive, rotors and hub.    If I can find a set of 550 dual disc forks with perfect chrome again....

[Geeto67]

aren't all 550 forks dual disc forks?

What's the advantage of a 550 swap on a 750? the 550 forks are slightly shorter so if you keep the rear the same height that's about the only advantage I can see. The 750 caliper is a better caliper so I don't know why you would want the 550 one.

The advantage to a GL1000 swap is 37mm forks and a more rigid front end. the GL rotors aren't as thick as the cb750 rotors and the GL uses an alloy rim (same one on the 76 cb750A hondamatic). because of the lighter wheel and rotors, you don't suffer a weight penalty for the bigger forks over just adding a disc to a stock cb750 setup. The downside is the GL forks are longer so you do have to make spacers or slide the tubes up the trees.

FWIW the 750 big fin calipers used to be sought after for racers in the 1970s. IF you look at Cal Rayborn's XR750TT here, you will notice that the caliper and rotor are cb750 even though the fork is ceriani and the wheel is morris:

[brycegp]

aren't all 550 forks dual disc forks?

Honestly, I've never owned a 550 so I don't know.  My cb500 was single disc.  I was thinking the 550F has dual and maybe the 550K didn't...?

What's the advantage of a 550 swap on a 750? the 550 forks are slightly shorter so if you keep the rear the same height that's about the only advantage I can see. The 750 caliper is a better caliper so I don't know why you would want the 550 one.

Just shorter by about 1.5".   I used all other CB750 parts...calipers, rotors, hub, etc...

The advantage to a GL1000 swap is 37mm forks and a more rigid front end. the GL rotors aren't as thick as the cb750 rotors and the GL uses an alloy rim (same one on the 76 cb750A hondamatic). because of the lighter wheel and rotors, you don't suffer a weight penalty for the bigger forks over just adding a disc to a stock cb750 setup. The downside is the GL forks are longer so you do have to make spacers or slide the tubes up the trees.

If I go this route, I would lathe (and rethread) the tubes down to a 28.5" overall length...about the same length as the CB550 forks.

FWIW the 750 big fin calipers used to be sought after for racers in the 1970s. IF you look at Cal Rayborn's XR750TT here, you will notice that the caliper and rotor are cb750 even though the fork is ceriani and the wheel is morris:

That bike is rad...

[Geeto67]

Honestly, I've never owned a 550 so I don't know.  My cb500 was single disc.  I was thinking the 550F has dual and maybe the 550K didn't...?

none of the SOHCs actually came with dual disc. They all came with the mounts on the fork legs for dual disc and for a brief period honda did sell the conversion pieces to add a second disc to the 750 both as a separate piece and as part of the 970 kit.

getting back to your GL1100 forks for a second. If they are anything like the 1981-82 900F forks they are pretty good (US 900fs got 39mm forks) once setup, the internals on the 900f forks are supposed to be way better than the SOHC 750 forks.

I saw this today and thought of this thread:
http://www.ebay.com/itm/1981-Honda-CB750-Front-Wheel-STRAIGHT-SPOKE-/360548839550?fits=Year:1981|Model:CB&hash=item53f262b07e&item=360548839550&pt=Motorcycles_Parts_Accessories&vxp=mtr

this is the 5 bolt spoke hub the 1981 cb750K used. Notice the hub is only drilled on one side which precludes the use of dual disc.

[Retro Rocket]

none of the SOHCs actually came with dual disc.

CB750 F2-F3 did....

[trueblue]

none of the SOHCs actually came with dual disc.

CB750 F2-F3 did....

So did the Aussie 650Z

[Retro Rocket]

none of the SOHCs actually came with dual disc.

CB750 F2-F3 did....

So did the Aussie 650Z

True Blue.....

[754]

 They used 750 finned calipers back in the day, because they were common.......Lockheeds Or Ap style were maybe sought after..

[Geeto67]

They used 750 finned calipers back in the day, because they were common.......Lockheeds Or Ap style were maybe sought after..

Harley's race shop preferred them. I don't know why but they used them on almost all the disc brake race bikes. I can guess it has something to do with the cooling fins as they only used the big fin calipers. They had the money for the Lockheed stuff but they chose those instead.


RR - touché, I meant spoke wheel single cams, but the words failed me.

[Retro Rocket]

They used 750 finned calipers back in the day, because they were common.......Lockheeds Or Ap style were maybe sought after..

Harley's race shop preferred them. I don't know why but they used them on almost all the disc brake race bikes. I can guess it has something to do with the cooling fins as they only used the big fin calipers. They had the money for the Lockheed stuff but they chose those instead.


RR - touché, I meant spoke wheel single cams, but the words failed me.

 

[Geeto67]

just went back and looked at the pics I have on my phone and the Team Obsolete 909 XRTT uses a set of grimeca or lockheed calipers (I don't remember which) but HD's race shop didn't build that bike - it was backdoored by Dick O'Brien in parts for a privateer effort. I know the Rayborn bike TO has uses big fin calipers because some of the racing spares we took to mid-ohio  came out of my spares, and two or three other factory XRTTs we have use the calipers as well.  What's interesting is that they are fixed in position, not floating. Somewhere in my spares I have a conversion bracket to use a cb caliper on a street ironhead (I also have a set of neck cups to run a street ironhead with a 75-77 GL1000 front end) so it was a popular mod in the 70's in all genres of motorcycling.

Anyway, enough harley talk.

[Ricky_Racer]

Moved to Performance Section. RR

[Parkwood60]

You can use a CB750 DOHC front end, which is 37mm for certain models, and the CB750K DOHC spoked rim, but you only get 1 disc (there is no way to mount a 2nd to the K hub the way it is cast). You can use a 1982 CB900F 39mm front end with 2 piston calipers, dual discs and a Comstar rim as well. You need to use the triple tree that goes with it. I am using the 900F set up, and working on machining some Suzuki GS1000 discs to bolt to the original 6 bolt spoked hub. I have yet to find a local machine shop that can do it, but I only asked the one I normally go to. He just had no way to hold it in his lathe.

[754]

What part of the disc are you machining?. Can probably tell you how to hold it..

[Henkies]

aren't all 550 forks dual disc forks?
FWIW the 750 big fin calipers used to be sought after for racers in the 1970s. IF you look at Cal Rayborn's XR750TT here, you will notice that the caliper and rotor are cb750 even though the fork is ceriani and the wheel is morris:

Are these calipers working in fixed position? Honda mounted them floating...

[754]

I have seen where the mount to caliper is rigid. Then the mounts to fork are shoulder bolts or a bolt and bushing.
Then the holes in the calper  bracket are slightly oversize. This allows it to float or mofe side to side as needed.