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  #1  
Old 22 Mar 2011
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Cast Iron or stainless

My BMW GSA1150 has a set of cast iron disk brakes.
Why?
Are they better in some way, lighter, cheaper.

I still have the original stainless and so I suppose I can put them back on but why would someone want to have rusty cast iron brake disks?
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Old 22 Mar 2011
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Were the original stainless disks damaged or bent in anyway?

Can't see why you'd want inferior disks from my point of view, but maybe I'm missing some key piece of information... I'll put this out to any 1150 gurus that care to shed some light.
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Old 22 Mar 2011
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The only thing wrong with rusty cast iron discs is they don't look very good, until you put the brake on and clean them, I am not sure about actual braking performance compared to the originals on an 1150 but my R80GS stops better particularly in the wet with cast iron. They are cheaper than the originals and last longer.
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Old 23 Mar 2011
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Cast brake disks are superior to SS in stopping ability, but are softer and do not last as long as SS. Cast discs have a higher coefficient of friction, therefor improved stopping ability and better feel. Race bikes use cast. I personally have replaced the SS disk on my old GS with a cast replacement from EBC, much better braking in stock form.
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Old 23 Mar 2011
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Cast iron discs presents no surprices. They act the same way dry, wet and cold. All my bikes have Motorworks cast iron-discs - BMW factory/original discs on /7 are crap; they warp, crack and bend: this does not happen to the cast iron discs.

Downside - cast iron disc do rust... ss tents not to rust...

I choose the better braking over the better look, since I -use- my bikes.
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Old 23 Mar 2011
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Stainless is purely cosmetic, it is in most other ways inferior to cast iron.

You would want an inferior stainless disk if your bike was sat in a display case, kept in the garage 50 weeks a year or was sat in a dealers showroom. The only way to make it more inferior is the Honda approach of coating a steel disk in chrome (a lubricant!).

If you ride you want cast iron. Don't forget to change the pads at the same time, while it doesn't make a huge difference off the race track, pads designed for stainless disks loose some of the CI's performance.

Andy
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Old 28 Mar 2011
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Another vote here for cast iron. I replaced all three on my R100RS about five years ago and find them far better, though not as pretty of course.

I do sometimes have a problem when one of the discs gets a patch of a thin layer of rust, it's more noticeable when I'm moving slowly. I'ts not an issue really, it comes and goes and I suspect it's hard braking that clears it.

Anyone else found this to be a problem?
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Old 29 Mar 2011
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Hi John.

I do have the same "problem" with my cast iron discs. One hard braking, and the problem is gone

Motorworks fully floating cast iron discs on all my summer rims. s/s is not an option to me.
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Old 13 Apr 2011
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Cast iron discs can be zinc plated, this will reduce rusting especially in those areas that are not swept by the pads.

They are much heavier than stainless and do seem to distort.

I now prefer the grimace replacement discs for airheads.
Charles
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Old 13 Apr 2011
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Quote:
Originally Posted by chasbmw View Post
Cast iron discs can be zinc plated, this will reduce rusting especially in those areas that are not swept by the pads.

They are much heavier than stainless and do seem to distort.

I now prefer the grimace replacement discs for airheads.
Charles

Zinc plated?
What's wrong with paint?
You'd have to remove the zinc from the contact area before use or I'd imagine it would seriously contaminate the pads!


I have had 4 pairs of BMW's "rustless steel" discs on my GS12 due to warpage.
Thumbs down from me.

Cast iron, oh yeah!
Anyone got any contacts for cast 1200gs discs?

Dave.
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Old 13 Apr 2011
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I was not aware of any problems with the zinc contaminating the pads, seemed to work fine for me, norton used to do this on their original disc brake bikes, but in those days the discs were cad plated.

Motorworks are the usual source of cast iron discs for airheads, not sure if they do them for oil heads
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Old 13 Apr 2011
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Motorworks BMW Motorcycle Specialists - Shop - Spares and Accessories

Motorworks BMW Motorcycle Specialists - Shop - Spares and Accessories

Looks like MW have an option, but not cast-iron (yet) -
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Old 14 Apr 2011
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Zinc will simply flake off rubbing area. The trouble with the sort of plating Elf-n-Safety has forced on the world is that a lot of it won't like the expansion rate of the disk when hot so the whole lot will flake off. Those old Norton discs without holes, slots and other fashion accessories are probably about the peak of practical development. The rubbing surface will still be too cosmetically poor for your local Uber showroom/Cruiser boutique, hence the stainless, drilled, slotted, flower shaped **** you can buy.

All pads quickly become contaminated, its how they deal with it that's important. One of the greatest leap forwards during my spell in the industry was the switch to improved mixes on sintered pads. The porous structure not only gives a rougher surface and therefore improved braking, it also lets various "dusts" pass through to some extent before clogging. Zinc dust should come and go in the first week.

As I think someone has already said, change the pads with the disk, it's vital enough not to penny pinch on.

Andy
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Old 14 Apr 2011
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Andy,

I don't know if there is anything special about my zinc plating, but after 5000 miles or so so on my rear cast iron disc it is still working to keep the unswept areas of the discs clean and reduces rust produced on the swept areas.

It had the same effect on the front discs, until I junked them after they warped. No apparent detrimental effects on the padsm, ferodos suitable for cast discs.
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Old 15 Apr 2011
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ive got cast iron on my 100gs. the problem i find is that they do not rust evenly, and you get patches where the pads sit. if left for a while ive found you get pitting in the shape of your pads which doesnt go so easily. i think this could be why my forks now judder under braking? (any thoughts on that one?)
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