Dumb-stall double disc forks

Has anyone here by chance had any experience with the Dunstall double disc forks that have the calipers cast into the fork legs. I got this bike with this front end that I thought looked cool and was somewhat rare but I am questioning if that was a good thing or a bad thing. Pistons were stuck in calipers, Brake shoes…..no longer available, chromed discs flaking and rusted. The corrosion of the aluminum pistons in an aluminum bore made them almost impossible to extract. All is restored now with new O-rings polished internal parts, cleared orifices, re-drilled and tapped hub for larger bolts holding the ground discs. I even splurged on the Colorado Norton teflon bushing extended top caps with the double lip seals. Re-lacing the front wheel will make this whole mess an expensive part of the restoration.
Question is……….Are these brakes any good or will this just be good for show and tell? I look with envy at the brembo single disc conversions I see on high end restored bikes.

I have run the Dunstall discs for 56 years. Most recently due to a little corrosion in the aluminum calipers they have been bored oversize, and I have fitted stainless sleeves and I made oversize threaded piston plugs. Previously I had a very good brake, which was far superior to the original Norton brake, however I have used up my stock of Don asbestos pads. So I reduced the dia and thickness of standard Norton pads so that they fit the Dunstall caliper. Now the brake is not so good. I am going to let them bed for a while and unless they improve dramatically I will be looking for different compound pads. So the answer is yes the brake can be excellent with a suitable compound.

With regards to your brake did you use edpm O rings as most other rubber compounds are unsuitable for use with glycol based brake fluid. Any minor corrosion in the caliper cylinder will mean the O rings will not seal. That will mean boring and sleeving as I have done or boring and fitting oversize pistons.

David

David,
Thanks for the thorough reply. It’s good to hear it is actually a descent brake. My Ducati with 2 finger braking has spoiled me and (maybe unfairly) become the basis for comparison.
I did not think of boring the cylinders oversized. I can see how that could be necessary I hope I got the bores clean enough to provide a good seal. I did use EPDM o-rings. A fixture to hold that fork lower square to the boring bar would be interesting, Did you do the work? I was able to find a company that “re-lines” brake material so that the original puck backing plate can be bonded to new compound. I do not know if the master cylinder was part of the “Dunstall Upgrade” so the diameter might not be right for correct feel at the lever but that evaluation will have to wait untill the first road test which is still many months away. I love to hear from owners with years of ownership and direct experience with upgrading these bikes Thank you for taking the time to respond.

The std Norton master cylinder is more suited to twin calipers than single calipers. The magic ratio is 27:1 which you can calc from this site

kommando,
Thanks for the reference, I am on my way out to check the ratio right now. Let’s see……area of a circle is Pi X Radius squared? I better confirm that, memory ain’t what it used to be.

tinkertoy said:

kommando,
Thanks for the reference, I am on my way out to check the ratio right now. Let’s see……area of a circle is Pi X Radius squared? I better confirm that, memory ain’t what it used to be.

Click to expand...

You just need to know pistons diameters and use the table Kommando posted.

But be aware… nothing you do is gonna make this brake like your Ducati… unless you fit a Ducati front end !

I had a friend mill the calipers oversize for the sleeve. Halray Brake, a brake cylinder reconditioning firm in Lismore NSW Australia sold me the stainless sleeve and they provided me with the adhesive for the sleeve, Cyberbond RH99. I cut and fitted the sleeve, tapped the caliper for the oversize piston retaining plugs and I made the oversize plugs to suit.

The master cylinder that was fitted to my bike originally, (as far as I know supplied by Dunstall) is the same as the master cylinder originally fitted to a k1 Honda 750. The piston diameter is 14mm which gives a ratio of 25 to 1 so it is very close to the sweet spot and good feel on the road. In Australia, master cylinder seal kits for the Honda k1 master cylinder are available from Puds for Parts.

David