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Author Topic: 1975 CB750 Rear brake hub help  (Read 612 times)

Offline texmish

  • Posts: 12
1975 CB750 Rear brake hub help
« on: Sep 14, 2017, 16:32:03 »
Hello, I'm finishing up a custom cb750 build but am running into issues with the rear brake hub. The issue is that the outside of the hub rubs on the outside of the wheel like there is a spacer missing. Looking at the parts fiche it doesn't appear to have a spacer in there but if that's the case then does that mean i have the wrong brake hub for the wheel?

https://photos.app.goo.gl/unY1F3mqXIuGBVcD2

https://photos.app.goo.gl/KY0do9GHiQbCYqfJ3

https://photos.app.goo.gl/Wc4zPi7V3bPCP9rA2


Offline Redbird

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Re: 1975 CB750 Rear brake hub help
« Reply #1 on: Sep 15, 2017, 11:39:31 »
Are both original to the bike?
I have a '75 750K rear hub/brake in the shop. I can get you some pics/measurements this evening.
When you are Dead, you don't know that you are Dead. It is difficult only for Others.

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Offline texmish

  • Posts: 12
Re: 1975 CB750 Rear brake hub help
« Reply #2 on: Sep 15, 2017, 14:35:13 »
I am told they are original to the bike and the brake hub seems to match the fiche and what I see for sale on that year. Measurements would be awesome! Thanks!

Offline Redbird

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Re: 1975 CB750 Rear brake hub help
« Reply #3 on: Sep 15, 2017, 19:10:10 »
Ok, this is what I got...
Measurements taken from the axle shaft opening, from inside to outside edges of the bearing/spacer surfaces.
Brake hub- 43mm
Drum hub- 180mm (with sprocket side wheel spacer installed)
              - 168mm (without wheel spacer installed)

What I have looks just like what you have.


I did notice though, mine will rub together if not seated properly. Which leads me to ask, have you checked the fitment/rubbing issue with the axle installed?
It's a close tolerance and the axle keeps it aligned properly.
When you are Dead, you don't know that you are Dead. It is difficult only for Others.

It is the same when you are Stupid.

Offline texmish

  • Posts: 12
Re: 1975 CB750 Rear brake hub help
« Reply #4 on: Sep 16, 2017, 21:54:05 »
So I have some measurements to compare to yours. I feel like the drum hub measurement was way off so I included an image to make sure we were measuring the same thing. If we were measuring the same thing then I'm assuming this is not the stock hub for this bike based on the results.

1. Drum hub inside the axle width - 166mm with spacer on sprocket side (From end of number 8 to end of number 32 image below to make sure we measured the same)
https://photos.app.goo.gl/Q6DwzMLFXKf6VqTo1

2. Brake panel - 43.7mm (I assume this is the same as what you got since we are so close)



Now I took some measurements to see why the brake panel was scratching and how much space was needed to correct:

3. Difference between brake panel surface being scratched 15.7mm and brake panel axle spacer 9.75: 15.7-9.75 =  5.95mm
https://photos.app.goo.gl/ko3CJzrpckULJ7It2

4. Difference between drum hub and bearing surface 7.6mm (needs to be less than the above 5.95mm measurement or it will rub)
https://photos.app.goo.gl/2xnaIFOL9OzDa6tf1

7.6-5.95 = 1.65mm.

So basically I'm 1.65mm short of hitting the bearing surface and preventing the rubbing issue. I have to get some things machined anyways and was wondering if there is any issue with machining a 3mm spacer to fit between drum bearing and brake panel to fix my problem? I'm helping my friend finish his build he started a year ago and everything is powder coated and re-laced on the wheels so I'd hate to have to start over.

My plan for spacer here (take a look at those scratches this has been rubbing for a while!): https://photos.app.goo.gl/iNhgg8bXawnebcd83

Input/advice/thoughts appreciated??


« Last Edit: Sep 16, 2017, 22:27:20 by texmish »

Offline Redbird

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Re: 1975 CB750 Rear brake hub help
« Reply #5 on: Sep 17, 2017, 12:10:31 »
I was thinkin the same about simply adding a washer/spacer to move the brake panel out just a tad. You'd have to figure a way to hold it in place with the drum and brake assembled until you could insert the axle though. Not too difficult, just a thought though.
Or maybe machine the outer lip on the drum down to achieve the same clearance?
When you are Dead, you don't know that you are Dead. It is difficult only for Others.

It is the same when you are Stupid.

Offline texmish

  • Posts: 12
Re: 1975 CB750 Rear brake hub help
« Reply #6 on: Sep 28, 2017, 20:19:07 »
Got my 3 spacers installed today and all issues are fixed. 1 spacer inside the brake hub to keep the brake panel from rubbing and the other 2 to center the wheel and align the chain sprocket.

Figured out along the way I'm working with a 75-77 cb550f swingarm, late model cb750 hub, and (correct for the bike) 1975 cb750 brake panel all working together now!




Offline Redbird

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Re: 1975 CB750 Rear brake hub help
« Reply #7 on: Sep 29, 2017, 19:37:38 »
Glad to hear you got it sorted ;)
When you are Dead, you don't know that you are Dead. It is difficult only for Others.

It is the same when you are Stupid.