Front brake discs

Rohan · Sep 16, 2013

chasbmw said:
with a revised mastercylinder ratio and modern pads the brakes on my 75 R90 are fine, wet and dry. I will keep the original disc on my Commando, but get it bead blasted and zinc plated.

Apart from the rusting, the other big downside with cast iron discs is that they weigh a lot more than equivalent stainless.

I had an 83 BM G/S, and its stainless brakes needed a bit of time to wipe dry before they had any bite.
(So do drum brakes after they've been through the river Matty - read ze handbooken)

Cast iron disks are usually heavier because they have the intregral central hub in iron too.
Stainless is usually heavier per volume than castiron, depending on the grades.
But the ss rotors can be thinner for equivalent strength.
Its often that lightweight alloy centre that makes them (SS) lighter...

What is the coefficient of friction of zinc plating ?
You may lose a lot of the braking performance like that.
Cast iron rotors are good braking precisely because iron has a higher coefficient of friction.
And the rust dust adds a bit !?
Keep em well used, and iron won't rust...

JimC · Sep 17, 2013

acotrel said:
My Seeley has two Suzuki discs on the front. They don't look like they are made of stainless steel - perhaps a grade of high speed steel (about 3% chromium). I tried carbon pads, however now I have four of the old asbestos pads and the braking is superb - predictable, but stops like hitting a wall from high velocity. I've ridden the bike in the rain and won a race under those conditions - at times like that I tend to stroke the brake, not grab it.
When I first bought the rolling chassis, it was fitted with a stainless steel disc and one Lockheed caliper with the asbestos pads - when I'd built the bike, it would not stop. I managed to get the extra caliper by swapping a medium sized GP carburettor for it.

Did you change the master cylinder when you went with a two disc setup?

Biscuit · Sep 17, 2013

The original rotors on my MK3 were chrome plated cast. In the old days before I new better I used (over used?) the rear brake a lot. The chrome gradually wore away evenly and that same rotor is still in use. The front was a totally different story. Only some of the chrome wore off on either side and in sections, resulting in the worse front end juddering probably anyone ever felt. About 15 yrs ago I replaced that rotor with a new Andover part that even had the correct color grey painted hub, but it was not chrome plated thank God. I just did a brake job on my pick up truck with four new zinc plated cast rotors. The plating wore off almost immediately as the manufacturer said it would. The plating was only to keep the hub, edge, and vents from rusting. I don't think zinc plating is nearly hard enough to expect it to last on a brake rotor surface nor do I think it takes heat well.

donmeek · Sep 18, 2013

I know from experience that here in Florida it is difficult to find someone with a blanchard grinder. Then, if you do find one, it is difficult for them to grind the Norton rotor because of the raised part on one side. Also, they don't like to re-set a machine for small jobs like that, so the answer would be to get a bunch of discs together and send them to a shop as 1 job lot.

beng · Sep 18, 2013

Wow, the usual load of usless irrelevant and jibberish answers.

Getting back to the thread start: Norton supplied both bare cast iron and chrome plated front brake rotors on the Commando!
My old black 850 roadster had a bare iron disk on it, and later 850s I had all had the plated disk. I went over this with an old Norton dealer once and he said that there was even a different pad part number to use on each type, makes sense.

This same old and smart Norton dealer offered the service of grinding and cleaning up the surface of Norton brake disks because so many of the plated ones wore unevenly and looked and worked like holy hell.

Your job will be to look at period photos and parts books for the facts concerning this, I got rid of my Commandos years ago and really don't give a shit anymore, but in my experience with the four disk-braked Commandos I owned starting over 25 years ago it looked to me like the disc brake Commandos made through 1973 mostly had the bare iron disks, which would include some of the first 74' bikes. Later 74's and onward seemed to have the plated disks.

OF course the bare iron disks are better. I rode the living shit out of my bare iron disked Commando, when I was young and had some adrenalyn and hand strength I could do stoppies with it. Through all the abuse the front disk never changed it's appearance and never showed any excessive wear. On the other hand the later Commandos I owned with the plated disk all looked like crap, and surely once they wore through in spots the braking quality they had to offer with their unevenly worn surfaces was sub=par.

Biscuit · Sep 18, 2013

^ You need to go to the toilet more often.

donmeek · Sep 18, 2013

the original post asks if they can be re-ground. the answer is yes, with a blanchard grinder, which is hard to find these days.

chasbmw · Sep 18, 2013

beng said:
Wow, the usual load of usless irrelevant and jibberish answers.

Getting back to the thread start: Norton supplied both bare cast iron and chrome plated front brake rotors on the Commando!
My old black 850 roadster had a bare iron disk on it, and later 850s I had all had the plated disk. I went over this with an old Norton dealer once and he said that there was even a different pad part number to use on each type, makes sense.

This same old and smart Norton dealer offered the service of grinding and cleaning up the surface of Norton brake disks because so many of the plated ones wore unevenly and looked and worked like holy hell.

Your job will be to look at period photos and parts books for the facts concerning this, I got rid of my Commandos years ago and really don't give a shit anymore, but in my experience with the four disk-braked Commandos I owned starting over 25 years ago it looked to me like the disc brake Commandos made through 1973 mostly had the bare iron disks, which would include some of the first 74' bikes. Later 74's and onward seemed to have the plated disks.

OF course the bare iron disks are better. I rode the living shit out of my bare iron disked Commando, when I was young and had some adrenalyn and hand strength I could do stoppies with it. Through all the abuse the front disk never changed it's appearance and never showed any excessive wear. On the other hand the later Commandos I owned with the plated disk all looked like crap, and surely once they wore through in spots the braking quality they had to offer with their unevenly worn surfaces was sub=par.

My 72 combat engined interstate had plated discs. I had a feeling that it was cadmium plated, but I could be wrong.

Ron L · Sep 18, 2013

My '73 MkV 750 (11/72 dated tag) came with bright chrome plated disc. Which I had ground off somewhere around 1975.

comnoz · Sep 18, 2013

The cast iron disks can also be cut on a lathe if you are careful to prevent chatter. The spiral groove problem does exist although it is temporary -especially if the final cut is done very fine.
Finishing the rotor with a fresh scotchbrite pad on a hand grinder after it has been fine cut will take care of the problem also. I have done many iron disks this way.

A blanchard grinder is a necessity when working with a stainless rotor. Cutting a stainless disk without chatter is nearly impossible. Jim

pierodn · Sep 19, 2013

comnoz said:
The cast iron disks can also be cut on a lathe if you are careful to prevent chatter. The spiral groove problem does exist although it is temporary -especially if the final cut is done very fine.
Finishing the rotor with a fresh scotchbrite pad on a hand grinder after it has been fine cut will take care of the problem also. I have done many iron disks this way.

A blanchard grinder is a necessity when working with a stainless rotor. Cutting a stainless disk without chatter is nearly impossible. Jim

Hi Sirs,
I cannot translate and understand what means "blanchard".
Someone could post a picture, please.
Ciao.
Piero

pete.v · Sep 19, 2013

Go here.

https://www.google.com/search?q=blancha ... =368&dpr=1

pierodn said:
comnoz said:

The cast iron disks can also be cut on a lathe if you are careful to prevent chatter. The spiral groove problem does exist although it is temporary -especially if the final cut is done very fine.
Finishing the rotor with a fresh scotchbrite pad on a hand grinder after it has been fine cut will take care of the problem also. I have done many iron disks this way.

A blanchard grinder is a necessity when working with a stainless rotor. Cutting a stainless disk without chatter is nearly impossible. Jim

Click to expand...

Hi Sirs,
I cannot translate and understand what means "blanchard".
Someone could post a picture, please.
Ciao.
Piero

rvich · Sep 19, 2013

My rotor off my '74 has intact chrome. I tried to turn it on the lathe to take the chrome off and couldn't get the tool to bite. Any tricks for getting the chrome off?

As for blanchard grinding...

http://www.instantweb.com/k/kwikcut/images/bg4.jpg

Rohan · Sep 19, 2013

You may be able to sandblast it off ?

I've removed the last traces of the chrome with an orbital sander and coarse paper.
If its still thick and continuous, this probably won't work.
Cleans up any rust quite well though, if its been standing a while.

The local taxi crew here have a little machine that crawls around the disk and grinds it back smooth and flat.
May be worth enquiring ?
There don't seem to be any specs for the minimum thickness of a Commando disk, don't get it too thin...

donmeek · Sep 19, 2013

thanx Jim for clearing this up. I was hoping you would jump in here. I have seen cast discs that have been done by a lathe and they were all very unsatisfactory. Obviously done by someone that does not possess the skills or knowledge that you have. I hope Pierodn' s questions have been answered.

Front brake discs

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