Brembo 4 piston calipers on a K75

[DRxBMW]

Todd Lampone did this conversion/initial write up a couple of years back.

Interesting, decided I would give it a shot on Heath's 1992 K 75 C project bike.

I just happened to have a spare pair of EBC rotors and a 4 pot R 1200 front caliper gathering dust in my garage. FWIW, the OEM K 75 master cylinder works fine. 

Brembo 4 piston calipers on a K75:

First off I have never had any issues with the stock 2 piston calipers that came on my 94 K75, but I always like the look of the 4-piston calipers on later models and wanted to see if I could get them to work on my bike. My main quest was to get them mounted and working with no custom modifications on my part. Short answer is YES they fit with no major issues.

My curiosity began shortly after obtaining my K75 in ~August of 2008. But what really got me going was the R100R that I saw on the floor of Bob's BMW. At that point I knew nothing about their compatibility but it was a 94 R100R, and hey... my K75 is a 94 too so they MUST fit, haha.

I started to do a little online part number research and figured out that the R100R shared the same fork part number as the K75 which led me to believe that the caliper mounting bolts had to be the same spacing on my K75 as there were on the R100R. A rough measurement helped only increase my interest.

I was lucky enough to hit up an eBay auction for the calipers off of a 1999 K1200RS (#34112333111 and #34112333112) for $52 ($20 was for shipping alone!). As soon as they were delivered I ventured down to the garage for a test fit. This is what I saw:



As hoped, a perfect for!!!





Or was it?



As you can see from the last picture there is a terrible offset issue with the rotor. At that point I thought that the only possible fix was to grind down the mounting tab of the caliper to "pull" it away from the rotor. My estimate was ~3mm. I was really hoping to avoid that.

This called for another trip to Bob's.

My next thought was to see if there were any other BMW bikes that had a rotor that would bolt up to the K75 hub. I tried to see if the R100R rotor would work, but it uses a spoked wheel and thus I couldn't confirm that the rotor would work. It was, however, the only BMW bike that I came across that used a 4 bolt mounted rotor. Checking with EBC's online parts list, I saw that there was a different part number for the R100R and K75 front rotors. This meant that there was something different between them. I was hoping that Bob's had an R100R rotor and a K75 rotor in stock so that I could sit them on the counter and measure the differences but that wasn't the case.

So I made a call to EBC and their customer support was very helpful and gave me the break down on their (BMW equivalent) rotors. This is what they told me:

EBC MD605LS (R100R):
Outter Diameter = 285mm
Bore = 62mm
Thickness 4.7mm
Offset = 28mm
4 bolts mounting

EBC MD604LS (K75):
Outter Diameter = 285mm
Bore = 62mm
Thickness 4.7mm
Offset = 31.5mm
4 bolts mounting

Highlighted in bold is the only difference between the rotors; a 3.5mm offset, toward the hub!!! This was exactly what I needed.

At this point I was confident that the setup would work. The only thing the EBC CS could not confirm was the actual bolt pattern of the rotor. So back to Bob's for me (luckily i'm only about 3 miles from the dealer).

I did a quick measurement of the mounting bolts and it looked right on. This was enough evidence for me to put in an order for a set of rotors and a set of pads (EBC Rotors MD605LS/MD605RS, EBC Sintered Pads FA407HH).

Finally the rotors, well one of them at least, was delivered. Of course I had to put it on immediately. Here's how it went:

First off the rotor bolts up just fine. Here is a pic of the rotor and caliper on but no pads installed:



As you can see it's not a perfect fit I had hoped for but it was good enough to continue. So off the calipers came and a set of brand new EBC pads went in. I remounted the caliper and this is what I saw:



While still not a perfect fit there is clearance on both sides and the wheel spins freely!!!!

I still have to wait for the other rotor to come in and actually bleed the 4-pots to see how the K75 master cylinder will work.



At this point I was confident that the setup would work. The only thing the EBC CS could not confirm was the actual bolt pattern of the rotor. So back to Bob's for me (luckily i'm only about 3 miles from the dealer).

I did a quick measurement of the mounting bolts and it looked right on. This was enough evidence for me to put in an order for a set of rotors and a set of pads (EBC Rotors MD605LS/MD605RS, EBC Sintered Pads FA407HH).

Finally the rotors, well one of them at least, was delivered. Of course I had to put it on immediately. Here's how it went:

First off the rotor bolts up just fine. Here is a pic of the rotor and caliper on but no pads installed:

As you can see it's not a perfect fit I had hoped for but it was good enough to continue. So off the calipers came and a set of brand new EBC pads went in. I remounted the caliper and this is what I saw:

While still not a perfect fit there is clearance on both sides and the wheel spins freely!!!!

I still have to wait for the other rotor to come in and actually bleed the 4-pots to see how the K75 master cylinder will work.

The left side rotor has already been delivered and I will mount it up, bleed the brakes, and see what happens. The last concern is the master cylinder. The R100R and K75 both share the same 13mm master cylinder, but later MC's used with 4 piston calipers has a 15, 16, and even 20mm MC pistons.

More updates tonight or tomorrow.

The left side rotor was delivered and I installed it. The next step was to install the calipers and bleed them.

When I initially test fit the caliper it was done dry, and therefore none of the lines were disconnected. My 94 K75 has a single flexible line from the MC to the right caliper and a hard line that spans the front fender to the left caliper. I started my install by installing the left side caliper. With the caliper unbolted from the fork tube, I noticed that the mounting bolts were not lining up. Turns out that the threaded hard line hole on the 4-piston caliper was in a slightly different spot than on the 2 piston calipers. Luckily it wasn't very far and only required a little bending of the hard line. I did not what to do this by hand so what I did was screwed only a couple threads of the hard line into the caliper and the bolt down the caliper. From there I tightened the hard line allowing it to bend itself only as much as needed. I needed to do this for the right side as well.

I proceeded to bleed the brakes in the traditional, painful, repetitive manner. For some reason the DOT 4 fluid I was using was brown even though it came from a sealed container. So I'll have to pick some more up tonight are redo it.

As impatient as I am I immediately went for a short ride around the neighborhood, getting up to, at most, 30mph. The lever felt very soft as I expected it too. The brake has a great feel to them but I was a little uneasy about the overall ability. I attribute a lot of that to the fact that both pads and rotors are brand new and need time to bed themselves. Also, the questionable fluid is a concern. I will do a bleed with fresh fluid and give the pads and rotors some time and then decide if a large master cylinder is necessary.

The brake look fantastic

[Flick]

Have you gotten some miles in yet? how do they feel?

BTW I hate you for adding another project onto my list   :2thumbup:

[Rick G]

I tried running the four pot calipers with the standard K100 MC and it just didn't quite do the job. The 20mm MC is OME with the K1100 and gives a good hard feel but I would like to try a 16mm MC as I reckon it would gove a softer feel which I like

[DRxBMW]

Have you gotten some miles in yet? how do they feel?

BTW I hate you for adding another project onto my list   :2thumbup:

Too early to tell about the final outcome,roughly 275 miles so far.

Modulation is fine for my taste.

Practiced some "full on" panic stops,NO issues.

[frankenduck]

The 20mm master cylinder feels fine to me on my K75 with 4 pot brakes.  (Then again it's got K1100 forks and I've owned numerous 4V Ks so it's something I'm used to.)  I usually just two finger brake on it.  I've braked pretty hard on it numerous times and the ABS has never kicked in.

[toddlamp]

Hi.  I'm Todd (the guy that did this initially)

For what it's worth I have put many many thousands of miles on my K since doing this "upgrade" without any issue.  As my postings suggest I found the stock 13mm MC to be inadequate for my tastes.  The lever was much too soft for me and I decided to look into changing it over to a 20mm one from the later 4V K's.  Ultimately I was successful in doing so with great results but not without some hang ups.

The primary issue was while the MC fit just fine the throttle mechanisms are different and thus the throttle cab;e is different.  When the throttle cable that is meant to be used with the 20mm MC I found that the outer covering was too long.  In order to combat this I had to trim the plastic bit at the throttle end of the cable.  In retrospect what I should have (and will do; eventually) was to grind away the lip of the throttle cable stay that is attached near the throttle bodies.  This should allow me to use OEM, or OEM replacement cables, without modification.

Secondly were the mirrors.  I do not recall exactly what the issue was but I think it had something to do with how the right hand side mirror attached and that the ones I had for my K75 MC would not work.  At the moment the part number escapes me but there is a matching set of mirrors that will work with both the right side and left side of the K75 w/20mm MC (yes you will have to buy new mirrors).  The info might be listed in my BMWMOA post where the above info and pictures came from.

Other than that it has worked like a charm.  It's been so long since I have ridden with the original 2-piston calipers but I have no interest in ever going back.

[toddlamp]

Found the info on the mirrors (and this only applies to handlebar mounted mirrors as with my K75 standard):

Another issue I have come across is that the K75 RH mirror is not compatible with the 20mm setup. What is compatible is the mirror from a R1100RS/GS (BMW #51162313530). If you do get it make sure you pick up the bolt (BMW #51162308645 and cap #51162313670). The bolt has a reverse thread.

Unfortunately this is a different style than my K75 LH mirror, and because I am picky, I will likely purchase the matching left hand mirror (BMW #51162313529).

[mystic red]

Thanks for the report Todd.  :2thumbup:

[pugsley]

The 4 piston brake upgrade in this thread seems to have been done by fair few people over the years on several different forums.

The two front brake master cylinder options I have seen are; leaving the standard 13mm K75/K100 one in place and accepting the long lever travel or installing the 20mm K1100 one for finger tip braking, albeit requiring some other minor changes to the throttle assembly and mirrors.

I would be interested to know if anyone has done this upgrade using a front master cylinder sized between 13 and 20mm, like the 15mm R100R twin disc p/n 32722314022 or any other combinations of cylinder, calipers & disc rotors and how did they work out?

[Gibson]

I have the 4 pot Brembo calipers on my 94' mystic R100R. There is a very noticeable difference in brake performance. The Mystic has a far stronger (and more sensitive) front brake. This is a good mod.

[Scott]

I have the 4 pot Brembo calipers on my 94' mystic R100R. There is a very noticeable difference in brake performance. The Mystic has a far stronger (and more sensitive) front brake. This is a good mod.

Yep, agree, my '95 r100r Mystic's brakes are wonderful, my '93 k75 brakes are good, but nowhere near the Mystic in stopping power.  The Mystic handels a bit better too, IMO. I think the k75 is a much better looking bike though. The Mystic is kinda homely. cheers.

[pugsley]

From what I read and from both of your comments the 4 piston Brembos with the 15mm master cylinder are a well matched combination. The only downside is the availabilty of the R80R and R100R master cylinders secondhand. Are there any other options anyone has tried apart from the 13mm K75/100 standard and the 20mm K1100 master cylinders?

[pugsley]

Many thanks to Todd Lampone who did the donkey work on this and to the others who followed and refined it.

Last year I completed the Brembo 4 piston brake caliper upgrade on my 1986 K75S (non-ABS) and I have now put plenty of road miles in on them. What finally prompted me to do it, was, I came to the end of a long saga trying, unsuccessfully, to get rid of the common K75 front brake pulsing problem. I know when I was thinking about doing it I had a few unanswered questions so here are my comments, for what they are worth:-

I used a pair of 1998 BMW K1200RS Brembo 4 piston front calipers and a lucky find pair of very low mileage second-hand floating EBC MD605 RS & LS brake discs (off a BMW R100R).

At first, I tried using the standard K75 Magura 13mm front master cylinder, but I gave up with it in the end as there was too much lever travel for me, even with a well bled hydraulic system with braided brake lines. Obviously a matter of personal preference, others have been happy with the 13mm version whilst others weren’t and went for the 20mm version from the K1100. I went for the middle option at 15mm.

I was lucky to find a Magura 15mm front master cylinder off the dual disc option of the R100R (BMW Part No: 32722314022). Since the R100R is fitted with the same 4 piston Brembo calipers I shouldn’t have been surprised to find that the master cylinder - caliper match is excellent. Plenty of brake feel, similar lever travel to the original set-up, great stopping power and no brake pulsing.

Initially I tried Brembo 07BB24SA sintered pads but found there was too much initial bite/grab for me so I replaced them with EBC FA407 organic pads which have a more progressive initial feel which I like.

The only thing I regret is why I didn’t do this upgrade a long time ago, instead of wasting time, money and riding enjoyment, trying, unsuccessfully, to sort out the original brake pulsing problem.     

[Laitch]

... to get rid of the common K75 front brake pulsing problem.   

Considering that there have been complaints here of 4-pot brakes pulsating, too, it might be more likely that few riders take seriously the need to bed-in new pads on deglazed rotors. It's an inconvenient procedure, for certain but it's grounded in reality.

[Martin]

I've had a pulsating problem twice in 20 years. I have successfully straightened warped discs. We are talking minor warping not major buckles. If you don't have patience and restraint read no further and get another disc. Be warned it is a slow process requiring lot's and lot's patience and a delicate touch, and only works on solid rotors not floating ones. Set the bike up so the front wheel is off the ground. You need to remove the calipers and attach a dial gauge indicator (DGI) with whatever you can rig up solidly to the top caliper retaining bolt. It needs to be set up so it runs on the outer most edge of the disc, and can be rotated away from the disc when adjusting the warp. This will only work if the disc has a smooth track to run on, if it doesn't forget it. With the DGI set up to run on this track rotate the disc a determine where the warp is and how big it is and mark with a pencil this part will be repeated after each adjustment. To remove the warp you will need a piece of  broom handle 10" - 12" long and a plastic hammer. The next bit requires patience and a delicate touch. Rotate the DGI so it is clear of the disc and place the end of the broom handle on the outer most edge of the disc carrier (not on the disc) where you marked the position of the warp, and tap gently with the plastic hammer.  If the warp is bulging out from the centre you need to tap it in. If it is bulging in need to tap it out. When tapping it out, the broom handle will be running at an angle this is the reason for it's length. You need to realign the DGI after every tap and check that you have moved it and by how much. Under no circumstances belt it or hit it hard, you will eventually get a feel as to how hard to tap it believe me it won't be a lot. Hitting it hard will only push it too far and will require you to tap it back the other way. You need to avoid tapping it to and fro, this will happen occasionally but needs to be reduced to the minimum. You are aiming for a figure under .007", well under this can be achieved with patience.
Regards Martin.

BMW rotor alignment tool.jpg (60.99 kB . 768x576 - viewed 1755 times)

[pugsley]

After trying the commonly believed reasons and solutions for the K75 brake pulsing/shudder (referring to the links below), over a number of years and with varying degrees of success (pulsing/shudder became a vibration), the problem was never universally cured. That and the very lucky availability of the parts at low cost was a major factor going down the 4 piston caliper route. More time & miles will tell if the brake pulsing/shudder will be a problem with the Brembo 4 piston caliper and EBC floating disc set-up but for now it is something that has been pushed to the end of the list of pleasures of K75 ownership.

http://www.eilenberger.net/K75S/K75S.html

https://www.motobrick.com/index.php?topic=1062.0