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  #1  
Old 11 Feb 2005
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Replacement Cast Iron discs (2)

I've just received a new set of replacement discs £79 each. See other thread in BMW Tech.

I can't help wondering if these could be made at infinitely smaller cost by simply reusing the carrier and bolting on the disc from a suitable car item after first machining it to suit. I'm not just being cheap here, it's just that as I happen to have the facilities available then it does seem an obvious and interesting solution.

Anyone out there tried it? How about you, Steve (Pickford) you seem to have done a bit of engineering on your brakes. I must say I do have some misgivings about the ten little 6mm(?) screws used to hold the discs on to the carriers, but, if that's what they do now then it must be ok. BMW rivet their discs to the carrier, but their discs are stainless and so a lot less likely to crack while being riveted.

** DISCLAIMER** In case somebody tries this and comes to grief as a result of reading the above, then I am NOT suggesting you go and try it. There, that's covered that.

John
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Old 17 Apr 2005
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Yes it can, a mate of mine made me some using the original centre and using disc from a Citroen BX, discs cost a fiver from a scrapies, he machined them up for me when on nights so all I paid was a few pints, work well on my G/S and he made a pair for my old RS. He is a very very good machinist though and the quality was outstanding. He used high tensile nuts and bolts with spring washers and loctite so very secure, his own R100 has covered 50k miles with his disc conversion with no probs. Sorry but he only does them for mates but if you have access to a machine shop and are suitable qualified then Citoren BX discs are what you want.
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  #3  
Old 18 Apr 2005
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Hi John,

Bit beyond my engineering skills but I know a friend who did something similar on his Dominator in the early 90's. Fed up with the rapid wear of the stock discs, he scoured a scrap yard until he found a suitable disc ( Jaguar in this case). He then machined it to be a direct replacement for the Honda disc. Breaking was as good if not better whilst having a much improved wear rate.
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Old 19 Apr 2005
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I am certainly not going to disagree with good results, but I find the car disk conversion hard to imagine. All of the car disks that I have seen are 2-3 times as thick as a motorcycle disk and extremely heavy. They are also designed for a different caliper (heavy, wide pistons, wider friction surface). While the mechanics are certainly possible, it seems that you would pay a weight penalty in doing this. Have the conversions also involved machining the thickness of the disk to a motorcycle width of 5-8 mm? Just curious....
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Old 19 Apr 2005
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Thanks, everybody

So it can, and has, been done then. Cast iron, mehanite in this case, surprisingly is not difficult to machine normally, though when it is the form of a relatively thin disc it might be a little trickier.

In a way what Timo says about car ones being a lot thicker might make it easier to machine, at least you could get a decent depth of cut to clean it up initially. If I remember the final thickness on mine were 6mm, so there's certainly enough meat on to play with. The finish on my Motor Works replacement Cast Iron discs was a fine turned finish whereas the original BMW stainless ones were ground.

Motor Works made no mention of checking the tightness of the little bolts securing the discs to the carriers, it does worry me rather that they really do seem rather small, they are 6mm diameter and there are ten or twelve of them per disc. I'll try and find out the torque setting used for them and let you know if they tell me. Mind it might be a problem getting a torque wrench into the space between the spokes and the disc. No, I haven't looked.

John

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Old 19 Apr 2005
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John,

If I recall correctly, the G/S front disk is not bolted, but riveted to the carrier. I believe the conversion involves grinding these off and replacing with bolts of a similar diameter. I am not sure why BMW choose to rivet, perhaps because then there is no chance of backing off, and they didn't forsee the need to ever replace this stellar piece of braking equipment (ha ha).

If you are looking for car disks keep in mind that many modern disks are ventilated internally (air space sandwiched between two thinner surfaces) and will not handle significant machining. These are quite obvious, but just thought I'd mention it.

Good luck and let us know how it goes if you follow through with the conversion.

Don't rule out finding a suitable bike disk that could be adapted in the same manner. This would likely give you a larger (diameter) disk, which will increase leverage (good) but will require an adapter to change the position of the caliper (more work). I think the major issue with the stock disk is diameter, not construction, so in my opionion a larger disk or second disk is the ultimate solution if you are seeking better brakes.

[This message has been edited by Timo (edited 19 April 2005).]
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Old 19 Apr 2005
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Hi, Timo,

No, what I had in mind was to start off with a larger disc and then machine it down to the same dimensions as the original. As for riveting the disc to the carrier, I'm not keen on that but only because cast iron, unlike stainless, is a lot more likely to crack while actually being riveted- sod that!

Cheers,

J

PS Stellar: good one!

[This message has been edited by John Roberts (edited 19 April 2005).]
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Old 20 Apr 2005
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John,

I hadn't meant to suggest re-riveting the new disk to the carrier - I agree this would be foolish in the extreme. I'm not even sure if what BMW does could be called a 'rivet' but I don't know what else to call it!

I don't see why a good quality bolt will not do the job just fine. Good luck and send pictures. Hey, maybe there is even a small market for this. Of course the liability insurence might get a bit costly...
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Old 26 Apr 2005
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As promised, have just spoken to Motor Works, re torque settings for the carrier-to-disc bolts, they can't tell me what the torques should be. They did say that they have had no complaints in ten years of selling them.

For my part, as I check the tightness of other bolts at the appropriate service intervals I will be including these in the schedule from now on.

J

[This message has been edited by John Roberts (edited 26 April 2005).]
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Old 26 Apr 2005
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Hi John -
Just take care with this conversion, take small cuts and lots of coolant - very easy to overheat and distort the disc whilst machining when they're at or near bike thicknesses (6mmish) That's why it's very difficult to get bike discs reground.

Hwyl fawr
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Old 26 Apr 2005
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Sut ma'i, Neil?

This is what I had in mind in my mail above (my second one in this thread) and I quote:

Cast iron, mehanite in this case, surprisingly is not difficult to machine normally, though when it is the form of a relatively thin disc it might be a little trickier.

Cast iron does form a hard skin on its wearing surface, as on the sliding surfaces of machine tools; I've found a skin on a brake drum when skimming one clean although I can't be certain that the same mechanism was in play. Hence my comment that it would be handy to have enough meat on a used disc to allow such a skin to be penetrated at one pass. I would imagine that chatter might also be a problem with a relatively thin disc such as this.

By the way, did Lars ask you about coming to Istanbul- the thing is, it's got to be soon, I'm hoping to set off in next two or three weeks.

Cofion,

John
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Old 29 Apr 2005
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John
sent mail to your wanadoo account re Istanbul.

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