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Author Topic: ‘86 K75 rear drum to disk brake?  (Read 20267 times)

Offline twowheeledgator

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‘86 K75 rear drum to disk brake?
« on: November 09, 2018, 02:31:04 PM »
I imagine it is as simple as finding a rear end with disk and swapping?
Has anyone tried this? Suggestions?
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  • Floridy
  • '86 K75C, '95 R1100GS, '13 Bonnie Scrambler

Offline mw074

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Re: ‘86 K75 rear drum to disk brake?
« Reply #1 on: November 10, 2018, 10:33:09 AM »
 Much more involved than that. You will need: brake pedal, master cylinder, brake fluid reservoir, caliper, brake pads, brake lines, and a rear wheel. This is just off the top of my head. A closer look at the parts fiche is warranted.
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Offline Chaos

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Re: ‘86 K75 rear drum to disk brake?
« Reply #2 on: November 10, 2018, 12:42:45 PM »
I would think if you could find a donor bike it would be a matter of swapping parts.  Ebay has a huge selection or rear ends and such, but I'd hate to sift through all that to find a good one.  The drum is actually pretty effective when set up right, not sure it's worth the trouble to swap everything out. 
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  • sw ohio
1987 K75S    VIN 0231
Original owner, Original litter
200,000 miles (plus or minus) and 5 paint jobs
sold 6/23
2023 Ural 2WD sidecar (BMW's bastard step child)

Offline Martin

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Re: ‘86 K75 rear drum to disk brake?
« Reply #3 on: November 10, 2018, 03:21:40 PM »
In normal riding I very rarely use the rear brake. Mostly I only use the rear for emergency braking and in gravel or sand.

Regards Martin.
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Offline Laitch

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Re: ‘86 K75 rear drum to disk brake?
« Reply #4 on: November 10, 2018, 03:27:55 PM »
Quote from: twowheeledgator on November 09, 2018, 02:31:04 PM
I imagine it is as simple as finding a rear end with disk and swapping? Suggestions?
Imagination lets us down again. :giggles
As Chaos implies, if the drum is within spec, renew the shoes and the cable. Check for worn linkage. Otherwise,  you'll need to be gathering the parts mentioned by Chaos and mw074 for conversion to disc brakes.

I suggest restoring what you have. Parts are still available. As Martin suggests, you'll be ok. Think of your forebears navigating the Alps before the advent of disc brakes. Most of them lived, probably.  :2thumbup:

From the Haynes K75/K100 manual:
 
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  • Along the Ridley in Vermont.
  • 1995 K75 89,000 miles

Offline twowheeledgator

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Re: ‘86 K75 rear drum to disk brake?
« Reply #5 on: November 10, 2018, 03:54:19 PM »
This is my sons bike and he has never ridden with drums.  He is also young and has more energy and time than he knows what to do with.  When we were talking about disk I know some models sold with disk.  So I was wondering if it was even possible to replace the rear shaft housing and back of a disk. I am pretty sure I have enough other things to do than take on a project like this but now I am curious.  Curious if can even bolt up.
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  • Floridy
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Offline Laitch

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Re: ‘86 K75 rear drum to disk brake?
« Reply #6 on: November 10, 2018, 04:40:05 PM »
Quote from: twowheeledgator on November 10, 2018, 03:54:19 PM
I am pretty sure I have enough other things to do than take on a project like this but now I am curious.  Curious if can even bolt up.
Riding with a drum brake isn't going to matter to the average rider, especially in Florida, unless it involves a speed record descent of Broward's Mount Trashmore.

As far as disc assembly bolt up to a drum brake final drive is concerned—not happening. The first image shows a drum brake final drive housing. The second image shows a disc brake final drive housing with caliper mounts as part of the case, indicated by arrows.



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  • Along the Ridley in Vermont.
  • 1995 K75 89,000 miles

Offline twowheeledgator

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Re: ‘86 K75 rear drum to disk brake?
« Reply #7 on: November 10, 2018, 04:55:17 PM »
So it looks like the disk FD will theoretically bolt right up to my shaft housing?
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  • Floridy
  • '86 K75C, '95 R1100GS, '13 Bonnie Scrambler

Offline Chaos

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Re: ‘86 K75 rear drum to disk brake?
« Reply #8 on: November 10, 2018, 04:58:03 PM »
Housing, wheel, calipers, disc, rear master and pedal, switch, I'm sure there's some more miscellaneous parts.  You may be able to recoup some cost by selling the drum wheel, they are rare and getting rarer.  But that money could be better spent on new front brake lines, fuel lines, tires, beer, fishing gear.........
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  • sw ohio
1987 K75S    VIN 0231
Original owner, Original litter
200,000 miles (plus or minus) and 5 paint jobs
sold 6/23
2023 Ural 2WD sidecar (BMW's bastard step child)

Offline twowheeledgator

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Re: ‘86 K75 rear drum to disk brake?
« Reply #9 on: November 10, 2018, 05:09:07 PM »
Did someone say Beer?
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  • Floridy
  • '86 K75C, '95 R1100GS, '13 Bonnie Scrambler

Offline Laitch

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Re: ‘86 K75 rear drum to disk brake?
« Reply #10 on: November 10, 2018, 05:22:36 PM »
Quote from: twowheeledgator on November 10, 2018, 05:09:07 PM
Did someone say Beer?
I heard Succinylcholine.
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  • Along the Ridley in Vermont.
  • 1995 K75 89,000 miles

Offline twowheeledgator

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Re: ‘86 K75 rear drum to disk brake?
« Reply #11 on: November 10, 2018, 05:30:51 PM »
My happy place is rum. Have a bottle of 7 year from Cuba I just need an occasion to crack open.

My son takes after his old man.  When I was his age I had a Plymouth Duster with a slant six and all I wanted to do was turn wrenches on it every night.  Thank heavens a Bronco II crashed into it before I could locate a 340.
He is also at Motorce Mechanics Institute in Orlando so EVERY Motorcycle is a project waiting to happen. 
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  • Floridy
  • '86 K75C, '95 R1100GS, '13 Bonnie Scrambler

Offline twowheeledgator

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Re: ‘86 K75 rear drum to disk brake?
« Reply #12 on: November 10, 2018, 05:36:31 PM »
I tell my wife worse things could happen, he could be on crack. 
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  • Floridy
  • '86 K75C, '95 R1100GS, '13 Bonnie Scrambler

Offline mw074

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Re: ‘86 K75 rear drum to disk brake?
« Reply #13 on: November 10, 2018, 05:48:29 PM »
Quote from: twowheeledgator on November 10, 2018, 05:30:51 PM
My happy place is rum. Have a bottle of 7 year from Cuba I just need an occasion to crack open.

 The fact that you have it is reason enough to crack it open.
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Offline twowheeledgator

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Re: ‘86 K75 rear drum to disk brake?
« Reply #14 on: November 10, 2018, 08:06:59 PM »
I went through two bottles with that reasoning.
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  • Floridy
  • '86 K75C, '95 R1100GS, '13 Bonnie Scrambler

Offline The Mighty Gryphon

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Re: ‘86 K75 rear drum to disk brake?
« Reply #15 on: November 11, 2018, 09:24:28 AM »
To return to the topic.  A few years ago, I was seriously converting my K75RT from rear disc to drum because of the problems I was having with the rear master cylinder.  My personal experience with three bricks is that the rear master cylinder is a piece of crap and if I had a drum brake on the rear I wouldn't think of changing it.

Although I have never ridden a brick with a rear drum, I really can't see where the braking is that much better with a rear disc.
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  • In my garage in Marilla, NY
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Current:
'91 K100RS16V "Moby Brick Too"

Past:
'94 K75RT "Ilsa, She Wolf of the SS"
'92 K100RS16V "Moby Brick" (RIP, deceased in a vehicular assault)
'94 K75S Special Edition Dakar Yellow "Cheetos"
'89 K100RS Special Edition "Special Ed"

Offline mw074

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Re: ‘86 K75 rear drum to disk brake?
« Reply #16 on: November 11, 2018, 10:46:46 AM »
Although the drum brake is adequate, I have found the disc brake to be superior.
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  • Michigan

Offline rbm

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Re: ‘86 K75 rear drum to disk brake?
« Reply #17 on: November 11, 2018, 12:50:37 PM »
No big difference.  The rear brake shouldn't need be used below 20 MPH (30 KPH), IMHO anyway.  Maybe some will expound about trail-braking in corners but that's a whole different soapbox discussion. :popcorm
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  • Regards, Robert
Toronto, Ontario

1987 K75 - Build Blog @http://k75retro.blogspot.ca/

Offline johnny

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Re: ‘86 K75 rear drum to disk brake?
« Reply #18 on: November 11, 2018, 01:31:46 PM »
greetings...

gotts to trailbrake for maximum yeeeeehaaaaa... gotts to enter hawt... trailbrake to the apex... whack the throttle on exit...

the brain is engaged when on the throttle.. and the moto will go where you gotts your nose pointing... thats the reason you gotts to ride eyes open nose up looking far ahead and on the throttle... ease on the back brake and ease it off to navigate the twistys...

and breathe mang... breathe...

maximum yeeeehaaaaa...

j o
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Offline Laitch

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Re: ‘86 K75 rear drum to disk brake?
« Reply #19 on: November 11, 2018, 02:11:25 PM »
Quote from: rbm on November 11, 2018, 12:50:37 PM
No big difference.  The rear brake shouldn't need be used below 20 MPH (30 KPH), IMHO anyway. 
I use the rear brake when maneuvering at very low speed, when heading downhill on loose gravel and sometimes to supplement threshold braking but it wouldn't matter to me whether the rear was a drum or disc. Some riders don't use the rear brake at all and assert they get along just fine. The braking technique umbrella is a large one. :giggles
 :popcorm
If twowheelgator's son undertakes this project, maybe photos of it will be posted as it progresses.

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  • Along the Ridley in Vermont.
  • 1995 K75 89,000 miles

Offline rbm

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Re: ‘86 K75 rear drum to disk brake?
« Reply #20 on: November 11, 2018, 02:37:54 PM »
That's the way I see it too, Laitch.
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  • Regards, Robert
Toronto, Ontario

1987 K75 - Build Blog @http://k75retro.blogspot.ca/

Offline Chaos

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Re: ‘86 K75 rear drum to disk brake?
« Reply #21 on: November 11, 2018, 02:56:30 PM »
But if you REALLY want to stop suddenly, some of our comrades insist "layin 'er down" is the most pragmatic approach. The rear brake works quite well for this trick.  This belief may stem from their insistence that using the front brake will toss you over the handlebars. 
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  • sw ohio
1987 K75S    VIN 0231
Original owner, Original litter
200,000 miles (plus or minus) and 5 paint jobs
sold 6/23
2023 Ural 2WD sidecar (BMW's bastard step child)

Offline twowheeledgator

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Re: ‘86 K75 rear drum to disk brake?
« Reply #22 on: November 11, 2018, 05:40:46 PM »
Johnny: you're my kind of dork and I mean that in the best possible way.
Laitch: I do the same, and living in a heavily congested area there is a lot of low speed maneuvering between lights.  A lot of time I try to not put a foot down until the last minute so I end up playing the "go slow" game waiting for the light to change. 
I normally use front and rear during normal braking and that is how I taught my son.

His biggest reason for thinking it does not have enough stopping power is that it will never lock up no matter how much pressure you apply to the peddle.   His experience with rear disc is easily locking them up.   
The shoes have a lot of pad left.  The pads are not glazed.  The drums could use a little sanding but are not too bad. It is in proper adjustment.
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  • Floridy
  • '86 K75C, '95 R1100GS, '13 Bonnie Scrambler

Offline beemuker

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Re: ‘86 K75 rear drum to disk brake?
« Reply #23 on: November 12, 2018, 07:48:31 AM »
Why would you want to lock it up? that;s what they invented ABS to avoid.
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  • Panama City, FL
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Offline twowheeledgator

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Re: ‘86 K75 rear drum to disk brake?
« Reply #24 on: November 12, 2018, 08:36:02 AM »
I think his thinking is That The fact that it can’t be locked up with full force indicates to him that it does not have the same stopping power as a disk.
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  • Floridy
  • '86 K75C, '95 R1100GS, '13 Bonnie Scrambler
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