Author Topic: Compatible aftermarket front brake master cylinder - K100RS-4V  (Read 513 times)

Offline Quijote_no

  • Curious
  • Posts: 5
Hi all, - I have a case which, to me, seems to be deeper and more difficult to solve than I thought, - maybe due to missing information from different webshops and manufacturers.

I am rebuilding my 1990 K100RS-4V into a kind of a street fighter, with (among a lot more) clip on handlebars and a new front master brake cylinder.

MY PROBLEM IS TO JUDGE WHICH PRODUCTS ARE COMPATIBLE WITH MY BIKE. And I hope to get some qualified feed back from you guys.

I'll take you through what I know, and what I still don't get:

I want to buy a master cylinder like these two:
Magura HC1: https://www.magura.com/en/components/powersports/radialmastercylinders/productdetailpage/?p=1851
or
Brembo 19RCS: http://www.carpimoto.com/en-NO/50719_RCS19PKB-Radial-Brake-Master-Cylinder-Brembo-19RCS-Premium-Kit-Black.htm

My problem is that I don't know if I can use any of these. None of the manufacturers list my bike in their compatibility documents (ABE).
Magura states compatibility between their HC1 model and K75, but not K100. However, their HC3 model is compatible with K100. (HC3 isn't an option because of it's price (about USD 600-800)).

Then, I found something interesting, a German webshop selling the Magura HC1 as an K1/K100 compatible option: https://www.kettenmax.de/detail/index/sArticle/53298/sCategory/0
(Even though the manufacturer doesn't.)

Of course I could just trust Kettenmax, but I want to understand the criteria and what makes a brake cylinder compatible/non-compatible.

Both HC1 and HC3 can be delivered with different piston diameters, but the Magura ABE document says that the 15mm piston variant is correct for the compatible HC3. Kettenmax says that the HC1 with 15mm piston is compatible as well. Who to trust?

Here is what Brembo say about their options (with respect to the piston diameter):

19RCS to be installed on twin disks bikes with axial or radial calipers but NOT on Floating Calipers (My comment: Check! the K100 has floating discs, fixed calipers.)

17RCS to be installed on twin disks bikes with axial or radial calipers whose pistons diameter is 32mm or lower, but NOT on Floating Calipers; for example Ducati Panigale or RSV4, Yamaha R6 etc etc. (My comment: The K100 caliper pistons are 36mm)

Hence, I would guess that the Brembo 19RCS would work, - BUT I AM NOT SURE.


Some specifications about the original front brake system on my bike:

Disc: 2 x 305mm floating
Caliper: 2 x Brembo with 4 cylinders each. Cylinder diameter is 36 mm.
Master cylinder piston diameter is 20mm (but maybe axial)

Another question is why the Brembo model require a (adjustable between 18 and 20) 18-20mm piston (to be compatible) while Magura is fine with 15mm.

I don't know if any of you consider such things this deep, but brakes are the single most important components on a bike, - so I just want to understand and be sure :-)


I would be thankful for qualified points of view!


Regards,
Quijote_no
  • Norway
  • '90 K100RS-4V
K589 (K1, K 100 RS) BMW K 100 RS (0523,0533). Type Code: 0523

Will build a cafe racer, a streetfighter or something in between from my K100.

Offline Laitch

  • ^ Quintessential Motobricker
  • Posts: 6737
Re: Compatible aftermarket front brake master cylinder - K100RS-4V
« Reply #1 on: May 26, 2019, 07:51:17 AM »
I want to buy a master cylinder like these two:
Magura HC1: https://www.magura.com/en/components/powersports/radialmastercylinders/productdetailpage/?p=1851
or
Brembo 19RCS: http://www.carpimoto.com/en-NO/50719_RCS19PKB-Radial-Brake-Master-Cylinder-Brembo-19RCS-Premium-Kit-Black.htm

My problem is that I don't know if I can use any of these. None of the manufacturers list my bike in their compatibility documents (ABE).
On their site, Magura indicates that if your moto is not listed on their compatibility chart, you should contact them for a recommendation and provides a contact link. Have you done that?
  • Along the Ridley in Vermont.
  • 1995 K75T 72,000 miles
I wept because I had no radials until I met a man who had no splines.
http://www.motobrick.com/index.php/topic,296.msg53303.html#msg53303

Offline Quijote_no

  • Curious
  • Posts: 5
Re: Compatible aftermarket front brake master cylinder - K100RS-4V
« Reply #2 on: May 26, 2019, 08:01:16 AM »
Yes, I have contacted Magura and Kettenmax, - but haven't got any answers yet.
I will post here when I have news.
  • Norway
  • '90 K100RS-4V
K589 (K1, K 100 RS) BMW K 100 RS (0523,0533). Type Code: 0523

Will build a cafe racer, a streetfighter or something in between from my K100.

Offline rbm

  • ^ Quintessential Motobricker
  • Posts: 1941
Re: Compatible aftermarket front brake master cylinder - K100RS-4V
« Reply #3 on: May 26, 2019, 04:03:45 PM »
The K1100 master cylinder has a 20mm actuation piston similar to the K100-4V.  The minimum size necessary to actuate 4-piston calliper set used on the K100-4V and K1100 is 16mm.  The reason is the volume of fluid that must be moved to actuate the calliper pistons. If you use a smaller master cylinder, the feel will be soft, requiring more lever movement to gain the same stopping power. If you use a larger MC piston than 20mm, the braking will feel wooden and you'll lack the proper feedback.

You now will have to solve the electrical switch problem you'll be facing and, more critically, the throttle problem.
  • Regards, Robert
Toronto, Ontario

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

  • Curious
  • Posts: 5
Re: Compatible aftermarket front brake master cylinder - K100RS-4V
« Reply #4 on: May 27, 2019, 03:23:15 AM »
Thanks for your reply Robert

I understand. But still Magura have certified their HC3-15mm for the K100-4V. Maybe this cylinder in longer, - hence the piston will still move the same amount of fluid? (I'm awaiting answer from Magura.)

You mentioned the throttle. Is there some significant challenge other than getting a suitable twist grip and wire?

(I haven't mounted the circuit card yet... ;-))

/Espen
  • Norway
  • '90 K100RS-4V
K589 (K1, K 100 RS) BMW K 100 RS (0523,0533). Type Code: 0523

Will build a cafe racer, a streetfighter or something in between from my K100.

Offline rbm

  • ^ Quintessential Motobricker
  • Posts: 1941
Re: Compatible aftermarket front brake master cylinder - K100RS-4V
« Reply #5 on: May 27, 2019, 07:49:50 PM »
The connection between the throttle and throttle bodies needs to be figured out so that it operates properly.  That means thinking through the problem before acting on buying parts.  Choose a proper length cable, around 94cm long.  It has to terminate in a proper cable lock end that will fit the cam actuator.  Read my How-To on fitting K1100 front end to a K75 to see how to adapt a throttle cable.
  • Regards, Robert
Toronto, Ontario

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

Offline Quijote_no

  • Curious
  • Posts: 5
Re: Compatible aftermarket front brake master cylinder - K100RS-4V
« Reply #6 on: May 28, 2019, 11:40:27 AM »
Thank you, Robert, - I really appreciate your input! Will have a look at it.
  • Norway
  • '90 K100RS-4V
K589 (K1, K 100 RS) BMW K 100 RS (0523,0533). Type Code: 0523

Will build a cafe racer, a streetfighter or something in between from my K100.

Offline Quijote_no

  • Curious
  • Posts: 5
Update with new info from thread starter
« Reply #7 on: May 28, 2019, 11:49:01 AM »
Hi all,

I have had a dialogue with Magura in Germany, and the short story (about compatibility between Magura HC1/HC3 and K100RS-4V or any bike with 20mm oiston in the front brake master cylinder) is:
- I got a new ABE document from Magura (which is not yet on their web site) stating that their master cylinders (HC1/HC3) with 15mm piston are compatible with the K100RS-4V.
- Then I challenged them, and asked them to explain why they would recommend this product, HC1-15mm. (I thought maybe it could have something to do with a special Magura technology or something.)
- The case was escalated internally within Magura.
- Magura wrote me back, and thought my master cylinder piston was 13mm. I told them it was 20mm (standard).
- Their last mail gave me an apology, and they acknowledged that they were mistaken, and they would not recommend any of their master cylinder variants, - neither the 15mm nor the 18mm.

SO, I switched my focus to the Brembo 19RCS. Brembo is not that specific regarding compatibility with a certain make/model, but since they recommend their 19mm model to any bike with dual, floating discs and 4 caliper pistons larger than 32mm, AND the difference between the stock piston and their model is only 1mm, I chose to go with it.


Notice: No one say that the Magura HC1 (or any other product with similar specifications) won't work. But the brake feeling and the brake effect will be affected, and the manufacturers are of course very careful with taking any kind of responsibility for the security of their customers. (Magura should have been even more careful.)

/Espen
  • Norway
  • '90 K100RS-4V
K589 (K1, K 100 RS) BMW K 100 RS (0523,0533). Type Code: 0523

Will build a cafe racer, a streetfighter or something in between from my K100.

Offline GaryFrank

  • Motobrick Curious
  • Posts: 25
Re: Compatible aftermarket front brake master cylinder - K100RS-4V
« Reply #8 on: August 10, 2019, 02:08:53 PM »
A bit late, but Iím busy fitting a Vfr800 master cylinder to my K100

ID is 12,7mm.... I will post some feedback once Iíve tested her
  • Cape Town
  • K100 RS

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