Fitting a rear disc brake on a drum brake Commando

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Hello All!

I am working on a 1969 Commando
This motorcycle is massively modified and most of these modifications moved this motorcycle to a dirt tracker. I am tasked with unwinding these modifications to make this a viable road bike in Dunstall styling.

One feature this bike has is disc brakes made by Airhart - I have decided it is not feasible to rebuild these pieces as they were preserved in water - destroyed by the elements.

The bike has speedway racing type "Rocky" brand wheel hubs. I am looking to see of there is any advice on fitting a rear brake master cylinder and a caliper on this bike that is a straight forward as possible. I am prepared to fabricate if needed, but I am hoping there is a vendor out there that might make something close to bolt on.
CNW has bits, but we are avoiding them at the moment because pricing is a bit high compared to making completely random parts fit from a modern bike.

Thank you in advance! Much appreciated
 
Best start with some pics of the hubs, something like a CBR600 setup would be cheap. But first you need to get the current disc hole diameter, number of hole and spacing from the hub and then start looking for current discs using the same hole pattern. I use the Galfer catalogue track down matching discs. Once you have the disc then you are left with choosing the caliper and master cylinder.

 
I think a stock 75 disc brake setup would mostly bolt up using a 75 swingarm/wheel combo, a left side brake pedal/cylinder combo would need to be rigged up.
 
I think a stock 75 disc brake setup would mostly bolt up using a 75 swingarm/wheel combo, a left side brake pedal/cylinder combo would need to be rigged up.
That's true, but it's such a heavy pig. Unless you're looking to use that really cool looking "Norton-Lockheed" brake caliper (which I really do like) , or if you were wanting to use the two piece rear axle, otherwise I would look for an alternative source. I'm also not much of a fan of the huge offset built into the Mark 3's, rear wheel cush drive/hub. I would think adapting one from a donor bike would be the way to go. but that's just my 2 cents worth.
 
I think that converting from drum to disc would be a large project unless you are a machinist. I am sure someone has done this but I haven’t seen it
Mike
 
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I think that converting from drum to disc would be a large project unless you are a machinist. I am sure someone has done this but I haven’t seen it
Mike
I fitted a disc back wheel in my 750 it's really not hard to do
But if you don't possess a lathe I'd imagine it'd be quite difficult to do
 
The bike already has disc brakes, but the calipers are no longer supported with seals etc hence the need for new calipers and master cylinders. The Rocky branded hubs are an unknown type to me hence the need for pics.
 
I've never been a fan of the MK3 rear disc brake...it is not much better than the drum brake. Except for appearance sake there is little gain except for added weight and bulk.
 
Do you want to maintain the cush in the hub?

I've had luck converting several Suzuki GT and GS cush hubs, both drum and disc to bikes. You'd certainly need a lathe.

If you don't have access to a lathe for a DIY scenario, the flat track guys have this all figured out:

Cheney Engineering
Banke Performance
 
Just a couple of questions to clarify your post. As I read it, you are planning to keep the rear wheel and disc, but are looking for a compatible choice of caliper and master cylinder. Is that correct, or are you also looking for a complete system, including new rear wheel and disc, as some of the replies seem to indicate?

Is this the caliper design you currently have?

Early Caliper.jpg


150X.jpg


Or is it the later Tolomatic style, as in the attached PDF?

Ken
 

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I just noticed that Speedway still lists the 150X1 caliper as available for $159.99. Would that work for you? There are also other companies, like Wilwood, that make similar go-cart calipers with the same 2.25" mount hole spacing and for similar disc thickness for around the same price, but with much higher quality. I've used the small Brembo caliper and matching master cylinder (from an Aprilia) on a Commando rear, similar to what the flat track suppliers mentioned above use. Lots of options out there.

Ken
 
One way to go is to find a wheel from an MX bike, but it will be 18"
Relatively cheap, with or without cush drive.
Mine is from some Honda 500.
Weight saving is massive, and you have a choice of sprockets.
Just make sure it fits in the swingarm, and that you can line out the sprockets.
But if you don't have access to a lathe, even a simple spacer becomes an obstacle.

Fitting a rear disc brake on a drum brake Commando
 
One way to go is to find a wheel from an MX bike, but it will be 18"
Relatively cheap, with or without cush drive.
Mine is from some Honda 500.
Weight saving is massive, and you have a choice of sprockets.
Just make sure it fits in the swingarm, and that you can line out the sprockets.
But if you don't have access to a lathe, even a simple spacer becomes an obstacle.

Fitting a rear disc brake on a drum brake Commando
Looks good Ludwig, can you please post a picture of the master cylinder and brake pedal on the LH side ?
I
 
This is a shot of the Aprilia master cylinder mounted on my MK3 project bike. I'm converting to right side shift to accommodate the Quaife gearabox, which is right shift only. It's not very close to normal Commando configuration, but I'm throwing it out as an example of one way to do the conversion. Plenty of other ways to mount a similar master cylinder.

Fitting a rear disc brake on a drum brake Commando


This is the Aprilia caliper mount. Again, not a normal Commando wheel and swingarm, but a good example of ways to fab a mount.

Fitting a rear disc brake on a drum brake Commando


Ken
 
I don’t know how you can keep looking at that Ken and not be inspired to get it rideable !
 
I don’t know how you can keep looking at that Ken and not be inspired to get it rideable !
Actually, Nigel, I have been working on it lately, just nothing much worth posting yet. I just sold three bikes, two race bikes (Gilera and Rotax) and one street bike (Rotax) and a huge quantity of spares, tools, and literature, to get completely out of the Rotax powered and Gilera projects. Planning to sell off a large batch of Yamaha singles stuff. And a Ford Model A hot rod. All so I can focus on the Norton projects.

Ken
 
One way to go is to find a wheel from an MX bike, but it will be 18"
Relatively cheap, with or without cush drive.
Mine is from some Honda 500.
Weight saving is massive, and you have a choice of sprockets.
Just make sure it fits in the swingarm, and that you can line out the sprockets.
But if you don't have access to a lathe, even a simple spacer becomes an obstacle.

Fitting a rear disc brake on a drum brake Commando
Grandpaul i think went with an early cb750 honda wheel setup tho drum brake. Pretty sure the cb750 line then switched over to a disc setup, good bet some mag wheel deals in there.
 
.. Ludwig, can you please post a picture of the master cylinder and brake pedal on the LH side ?
Fitting a rear disc brake on a drum brake Commando



The mc is bolted on 2 tabs, welded on the frame, like the footrest.
I have no Z plates, everything is more inboard.
No reservoir. 20 cm of clear tube holds plenty of fluid.
DOT 4 'sweats' through crystal tube, so I use DOT 5 here.
First I used a banjo bolt with integrated brake switch, but when 2 failed in short time, I made a custom switch( white thingy)
 
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