'81 CB750C w/ GSXR front end - rotor help

Joseph New York

Active Member
Hi, long story short, wrecked my DOHC CB into a Bens, bent the front end up, but you should see the other guy.

The unroll-able bike is at a shop waiting for parts, and while I have the insurance company's dime, my fantasy of having a GSXR front end is under fruition.

I ordered to the shop a complete '08 GSXR front end, currently in route along with a conversion stem and bearings. This doesn't include a wheel and rotors, which leads my question to you fine folks:

What disc rotors should I order? I plan on using the CB mag wheel and need to place an order for a set of rotors that I know will fit the rim's lug pattern as well as the GSXR calipers. Will I need spacers as well?

I could wait for the shop to receive the GSXR parts and tell me what they need, but from experience with them that will add at least a month delay on this project.

I normally do all the wrenching myself, but she's too crippled for my sidewalk-garage, and I have to project manage this remotely.

Cheers!
-Joe


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Re: '81 CB750C w/ GXSR front end - rotor help

OD is the deciding factor, so if you know where the caliper will be in that direction you should be able to find a rotor that matches that and the bolt pattern and be good to go.
 
omgwtfbbq! said:
OD is the deciding factor, so if you know where the caliper will be in that direction you should be able to find a rotor that matches that and the bolt pattern and be good to go.

Sorry, what's OD? Outer diameter?


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In my research, the stock OD seems to be 310mm and ID 102mm. I found some decent used GSXR OEM rotors on eBay, but they have a 6 hole bolt pattern, where the CB750C OEM rotors have 6. Would I have to find rotors with 6 somehow?


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Yeah it might be easier to match the wheel bolt pattern and then just adjust the caliper mount to fit the rotor. Or you could do what I did and just run a gsxr front wheel. Way easier to keep everything in one family :D
 
I just edited my last comment BTW. The GSXR rotors have 6 hole bolt pattern where the CB750C has 5. Would a disc with 5 exist for GSXR or do I just look for a 5 hole pattern rotor with the same thickness, ID, and OD of a stock GSXR?


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omgwtfbbq! said:
5 hole with the same dimensions would be the easiest. Dunno if that exists though.

Yeah, I found one set that happens to be 310mm OD and with 5 holes, and no offset* What I can't seem to find are the PCD* of the mounting holes and the hole diameters: EBC XC Series https://www.ebay.com/itm/231440036393 I also don't know the PDC* of a CB750C rotor...

I contacted a company in UK called Bike HPS that makes custom rotors and hopefully they can help. Your right, would be much easier and cheaper to just grab a GSXR rim and rotors and be done with it, but I just can't get myself to slap on a 17" wheel, when the CB is blessed with a stock "19.

* https://www.bikehps.com/isr/ISR_Custom_Disc_Order_Form.pdf


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You can search on the MetalGear site by dimensions, offset, number of bolt holes, etc.....


http://www.metalgear.com.au/search_by_disc_measurements.php
 
hillsy said:
You can search on the MetalGear site by dimensions, offset, number of bolt holes, etc.....


http://www.metalgear.com.au/search_by_disc_measurements.php

Thx, that was worth the try. Searched and nothing came up that would work.

My last attempt at this will be to call HPS in Britain about building me a set of custom ISR discs. But I'm guessing these rotors will set me back around $600, which was more than the replacement front end... Regardless, I'm gathering the CB's hole specs now, and once I put the bike together I'll be able to measure the needed offset to align to the calipers. At that point I'll have all the specs I need to make the custom rotors.

Thanks!


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don't want to throw a spanner in the works but if you are using the stock wheel, i'm guessing you're using the speedo drive too? make sure that the line that the cable takes will clear the offset/spaced-out discs as the new disc spacing may well be a fair bit further out than the stock disc placement and the speedo drive takes the cable fairly close to the discs in stock form
 
spotty said:
don't want to throw a spanner in the works but if you are using the stock wheel, i'm guessing you're using the speedo drive too? make sure that the line that the cable takes will clear the offset/spaced-out discs as the new disc spacing may well be a fair bit further out than the stock disc placement and the speedo drive takes the cable fairly close to the discs in stock form

Thanks for the advice. My new GSXR triple clamp doesn't have anything for gauges so my minis were destined to eBay. I'm glad you brought up the speedo because it made me lookup the law and I missed the cutoff by a year for my state's law to be speedoless. I now need to have to plan for a gauge. Perhaps I'll mount it inside my headlight bucket! Always liked that.


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other options for speedo if the cable is a problem is the magnetic (bicycle type), requires wire running up the forks which can be attached to brake line
or GPS type which can be a bit pricey but is by far the neatest solution
 
spotty said:
other options for speedo if the cable is a problem is the magnetic (bicycle type), requires wire running up the forks which can be attached to brake line
or GPS type which can be a bit pricey but is by far the neatest solution


Yep. I'm digging the GPS setups myself. Fewest visible wires and many say more accurate than the wired wheel speedos, at least while moving a bit.
 
spotty said:
other options for speedo if the cable is a problem is the magnetic (bicycle type), requires wire running up the forks which can be attached to brake line
or GPS type which can be a bit pricey but is by far the neatest solution

That's a great idea, I never would have thought a bicycle speedo could work!? I'm def looking into this.

Can anyone get me the diameter of the mounting holes as well as the distance between the center of two opposite mounting holes (PCD) for a 81 CB750C brake rotor? Can't find it online, and don't have the rotors.


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Bicycle speedos "work" but they are rubbish next to an Acewell or Trailtech gauge.


You pretty much get what you pay for in this area.
 
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