Commando disc brake

Take a sander to it.

I have had mine refaced by a local engineering company for £20. Until I get out and ride it I don’t know if I’m keen on their job tbh, and I was considering buying new, they aren’t all that expensive and I’m keen on stopping. 

Answering your question

A. is it safe?  To recondition it yes,

B. Yes the disc can be restored by machining. Usually i grind them.  

The commando was one of the first bikes to have a disc brake and to keep them looking pretty they hard chromed the disc. This wears off unevenly after prolonged use and can cause numerous problems like pulsing lever and rapid wear of pads etc

Just using a sander on the disc carries the danger that it will be easy to go too far and create a hollow on the disc, doesn't have to be deep, a thou or two and you will have a pulsing front brake.

Better to scrape the rust off if grinding isn't required. Less chance of digging a hole.  

Even if the Brake appears to work fine with pitted discs at some time  when under really heavy continuos use (decending a mountain)  the brake pad will overheat and deteriorate or vapourise any moisture in the cylinder and cause brake failure. The pitting slows heat transfer from pad to disc and the cooling ability of the disc is lost. Learnt this the hard way in the Swiss Alps.

Change your brake fluid more regularly! There shouldn't be any moisture in it to vapourise!

Probably a sucking egg thing, but the lesson is one of never being on the brakes continuously but letting engine braking do most of the job. Brake to a slower than you need to, then let engine braking take over and allow your brakes to cool as your speed picks up again. Then a short hard braking to bring speed down again. Repeat this until you are at the bottom.

That is what I was taught and it has worked for me on all vehicles I've driven in the mountains.

George. 

This thread raises the question "what is the minimum allowable thickness of the disc?"

The workshop manual is silent on both the original disc thickness and the permitted wear.

George,My brake fluid was changed every year after the first issue,and I was always the slowest vehicle going down due to using low gears ,Often being passed by HGV,s with their brakes on fire. on very dangerous roads . Only left the problem behind when new Discs fitted. I drove over the High Alps every year ,for decades. Drivers are now  tought NOT to use gears to slow down. I have always used the gears ,that's why the discs get rusty!!.

Hi,

Strange this thing about not using engine braking?!

I always use the engine, particularly if the roads are the slightest bit slippery - its more controlled.

I guess it might be the thing to do with ABS? I don't know! Riding my old Ariel with drum brakes it stops well as long as I'm in the right gear for the speed and further more with the clutch engaged; otherwise relying on brake alone is not good idea especially since they fade if stopping from speed.

Just my thoughts and if I stand to be corrected that's fine as long as the argument is logical.

Keith

Getting back to the restoration of the disc, if it's  not pitted in any way you could try electrolysis. I'm  sure there is advise within the forums on how to carry out this method of de-rusting, if not it'll  be on U tube.                                   Chrome discs were pretty useless anyway and they were lethal in wet weather.

I was two up, with a friend, fully loaded with camping gear on a GT500 two stroke Suzuki descending a mountain road in central France, heading for the Bol'Dor at the Paul Ricard circuit. Halfway down I realised that the brake lever was slowly getting closer to the handle bars! Deciding to stop at the first opportunity, i braked harder and juddered to a stop at the next layby. My friend who was admiring the scenery immediately asked why have we stopped? My reply was ' if we don't stop now we wont stop at the bottom, Look!' and i pointed to the brake lever that was a quarter of an inch from the handle bar. I still had the brake applied as we had stopped on the still inclined roadside. 'Jesus' was his reply!

Half an hour later when the thing had cooled down (making cracking and tinkling noises) we set off again. Along with other escapades that holiday was ace!        

I had to use bottles of cold beer to cool the calipers down. Sacre bleu!!

What a waste of cold beer.....

Next time drink the beer and then piss on the calipers.........

By the side of the road, in France ,no problem,but them guntotin Swiss Cops are not so cool about it.

No Swiss cops near the Bol D’or. But watch out for the ones that nicked me on the way down in ‘99 on my Blackbird. 

The original question on this thread concerned the feasibility of restoring a rusty disc. On the basis of some of the answers, I imagine William Hayley is wondering why he bothered.

 If the disc isn't pitted there is no reason to use electrolysis and to advise someone to try this without actually sounding like you have done it yourself is not very responsible at all. Electrolysis will de rust steel but with the likes of a disc its the soundness of the underlying metal that is paramount and electrolyised steel can have loads of stress raisers. (microscopic holes for want of a better word)

How electrolysis works on a cast iron disc i have no idea either and i have no desire to try    

You could end up with at best a disc that has a question mark over its integrity and at worst a disc that fails catastrophically 

buy a new one!

Do you want originality or modern braking? if the latter  - upgrade it!

My Commando has been fitted with an AP Lockheed master cylinder and AP Lockheed caliper. It still retains the original disc. I am thinking that a larger disc would be an improvement. Recommendations please! Sorry if I have hi-jacked the thread but it is the same subject.

I have AN's 13mm m/c, Venhill braided hoses, AP CP2696 caliper & RGM's 12" cast iron floating disc; some don't like the RGM mounting bracket required to raise the caliper (I don't find it an issue) & feel safer with this braking set up (it's not as good as my Guzzi, but way better than the original set up)  

Other options are available, & I'm sure other posts will follow  ;-)

Hi, Excuse me asking, please, but who are AP?

Thanks,

Keith

Hi Roger 

RGM offer larger disc and a mounting plate for your caliper links below  what bore size is your master cylinder? 

https://www.rgmnorton.co.uk/buy/adaptor-plate-to-fit-lockheed-grimeca-racing-caliper-onto-fork-leg-adjustable_2558.htm

https://www.rgmnorton.co.uk/buy/fully-floating-close-grained-cast-iron-12-racing-disc-for-commandos-r-h-side_2623.htm

Best regards 

Katherine Scott

Keith,

AP are Automotive Products who make Lockheed brake systems

Thank you Katherine and others who have taken the trouble to reply to my query, I have ordered an RGM disc, but not the adaptor plate. I am looking to source that from elsewhere.

Regards

Roger

I am now up and running with the RGM 12" disc, but using the Norvil adaptor plate. I have only done a shake down run up to now, so no real evaluation of this set-up as yet. 

William Hayley

As I understand it the preferred method to recondition a disk is to have it 'Blanchard' ground, aka rotary surface grinding.

Page 12 of the workshop manual (065146) has the following:

So I assume 0.25" is the minimum disc thickness.

Regards, Andy