Hi all,

The bike in question is a 1991 k100rs 4v abs.

I've been through 3 sets of bobbins on my bike in the last year or so. Each time it's around $120 USD in parts and I'm getting a bit tired of it.  They last about 3 trips around town until the discs get really (*really*) rattly again, and I just let it stay that way till it feels unsafe because the bobbins are just completely destroyed.  The discs themselves have plenty of meat left on them. It's likely that the holes themselves,where the bobbins go, are very worn out.  This seems unnecessary due to the following:

I see that the older K100 / k75 models do not have floating discs on the front.  The calipers don't seem to be of the floating variety either from what I could tell in pictures.  This seems like a bit of a sturdier design and I'm interested in altering my front end for that purpose.

-Are there non-floating discs that I could put on my current front end with no ill-effects?  Why do the discs have to float anyways?
-If something's gotta float, are there floating calipers I could get that would bolt on correctly?
-If neither of those is an option, then I'd be looking at changing the entire front end from triple tree down, which would be kind of silly.

The rear brake doesn't have anything floating.

Thanks!

If you are going through bobbins at that rate then there is something wrong somewhere, perhaps with the bobbins that you are being supplied with. Floating discs were an update and you would go to loads of hassle to downgrade your bike. All 4 valves including the K100 and K1100 had floating discs and the bobbins last for years. do you cover your front brake lever with your fingers while you are riding? Could that be dragging the discs without you realising it?
I fitted new EBC discs to mine and they came fitted to the new carriers with bobbins as a complete unit.
The rear brake is not a floating disc because it does very little work.

The bobbins on EBC discs are not circular; they have flat sides, giving a much larger surface area to take the load. As such, they should last the life of the disc.

Here's a pic. 
https://i.servimg.com/u/f37/17/23/83/39/downlo10.jpg

Paul.

Are the bobbins showing a lot of wear?  On my K100RS they sometimes rattle slightly even though they aren't significantly worn.  They do need to have some play in them in order to work as intended.

I would suggest that if the rattle really bothers you, you might want to reassemble the bobbins onto the disc assembly with a tiny dab of silicone gasket material.  Once it hardens, it should dampen the rattle, but still allow enough movement to allow the disc to float in the caliper.

No no, I mean *destroyed*:

https://imgur.com/gallery/dWci8

These bobbins are maybe around 1k miles old...

Where are you buying those bobbins from? They look like alloy ones!

From Max BMW motorcycles, via the parts fiche.

I had the exact same discs on my Cobas bike before I changed the wheels but the bobbins (I think) are stainless steel. They stood up to some long hard braking at circuits like Spa and Donnington and never gave any problems or showed any signs of wear. The parts you were supplied are surely not fit for purpose and look positively dangerous to me, please don't use them again!
I have a saying I use "Buy cheap, buy twice!"

Here it is on those discs at Spa.

Wow, cool build, Mike.  That looks great.
Yeah, I certainly believe that.  There's no way these things are up to the task.  I'm baffled by these types of things sometimes.  Things that shouldn't be allowed to fail in the way they are.  My handle bar mounts just snapped recently, about a week apart from each other.  The U shaped part just popped right off of the bolt part.  I had to take them out and weld them back together.  

Anyways,
Might this be the rotor set I'm looking for? http://www.beemerboneyard.com/abm305kitwc.html
With the square / flat style bobbins?

Ouch! Expensive.

Try this page on Motorworks website.
https://www.motorworks.co.uk/vlive/Shop/Parts.php?T=5&NU=15&M=40&Ct=IA&SbCt=BA_15_40_IA_50&spPage=1

I know it's in the UK but all their kit is very good quality and may save you a few $

Mike.

I have stainless bobbins on my K1100 discs and they are very firm but after a bit of hard braking they become slightly loose as in they don't rattle but move easily on the bobbins from side to side but immediately centre when moving again.
Don't grab the disc with bare fingers to find out if it is loose.
The discs don't rattle but at low speed I sometimes hear the pads rattle.
If the bobbins are loose when cold the carrier and maybe the disc will need replacing. I have over 100K on the same stainless bobbins. The disc should be very firm and not rattle but be able to expand when it gets hot and not warp.