Adding rear disc brake on 73 Roadster

I have a 1973 850 Roadster. I am thinking about upgrading the drum brake to a disc set-up.

My plan is to use a MkIII wheel and disc mounting plate, with an AP-Lockheed Pull Type Rear Brake Master Cylinder p/n CP3756-4. I'll need to fabricate a braket for it to fix to the left hand footrest hanger.

Are there any problems putting the MkIII wheel straight into the MkII swinging arm? Any chain alignment difficulties?

I am sure a number of people here have done somethng similar. Is it worth the trouble and expense. i.e. is the braking improved enough to bother doing it?

Thanks.

I have done this to my 850 Interstate. You need at least the entire Mk3 axle, sprocket, spacers, hub, stub axle, and chain adjuster. And I believe the Mk3 spindle is larger so the slots in the swingarm may need to be enlarged. (Don't quote me on this!)

I used the entire Mk3 swingarm assembly. It is much more substantial than the 750 and even stronger than the earlier 850 styles. I mounted my master cylinder on the frame plate behind the battery and in front of the rear fender, then fabricated a mount and adjuster and used the stock Commando brake cable to operate it. I don't know if the AP-Lockheed M/C will fit in front of the fender, but there are ones that will.

I went one step further and replaced the steel caliper mount with and alloy one from a Ducati 900SS CR and mounted a Brembo caliper and adapted the Ducati rear rotor. It probably took 6# of weight off the rear wheel.

My opinion is replacing the Norton drum with the Norton disc is really not an upgrade. A lot of added weight and little improvement in braking. Not to mention a lot of fiddling and fab work. The Brembo is definitely an improvement.

Ron L said:

I have done this to my 850 Interstate.

My opinion is replacing the Norton drum with the Norton disc is really not an upgrade. A lot of added weight and little improvement in braking. Not to mention a lot of fiddling and fab work. The Brembo is definitely an improvement.

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Thanks, Ron.

You might just have talked me out of it! I figure it's going to cost about $1000. If it doesn't improve the braking, I'd be better off putting the money into the front end.

You say Brembo is an improvement. Do you have Brembo caliper(s) on the front, too? If so, which model. The ColoradoNortonWorks kit is pretty expensive, and I'd have to ditch the standard Lucas switches, which I don't want to do (perverse, I know, but I like 'em). Brembo make a caliper with the same mounting holes as the Lockheed racing type that the Norvil adapters fit. But, is that Brembo (108) so much better than a Lockheed?

I agree with Ron, as the MkIII rear disc isn't really any better than a drum (in my opinion) so it would be a lot of work for nothing, unless the disc brake itself was also upgraded?

I have posted these photos before, so I apologize to the regular forum readers. To Corona850, I used a 30/34 Brembo 4 piston front caliper (40mm mount) with the Al Miles sleeved Norton master cylinder to retain the stock switchgear. The front disc is a full floater adapted from a Japanese crotchrocket. The rear is a Brembo P32 two piston. I am using an Airheart mastercylinder from a dirt track set up.

I am currently revamping my Mk3 cafe racer using a similar set up but will use a Brembo 13mm coffin style or a Magura 13mm on the front and will try using the stock Norton rear master cylinder. I have converted the switchgear to Ducati items and will use a Magura clutch pivot and handle.