Under tank dual disc M/C size

[jagarra]

After I pulled the M/C to check to see why my brakes would not hold I discovered that my M/C pitting was the cause of the issue. The M/C is an early 16mm unit, they went to 17 later in the production of 90S models. I am going to send it to an outfit called Apple Hydraulics in New York, for $95.00 they will sleeve the unit. Not bad compared to the $350.00 price from BMW. Now I have 2 40mm calipers on the bike, according to the ratio charts that should put me in the area of 12.49. The charts says 12.1-14.1 is optimum for single piston calipers. Now comes the dilemma, Snobum says to try the old 14mm when you add dual calipers, (problem is the 14mm M/C is single output) he listed the ratio site as a source for making this decision, but he had some disagreements with the data's results. Have any of you that still has an under tank M/C used a different size than the 16 or 17mm offered from BMW with good dual caliper results? If I have it sleeved to 14mm then my ratio will be 16.31 with the 40s, if I do that I have another set of 38mm calipers then the ratio would be 14.86.

[Seth]

I've got a 77RS with 40mm calipers. Plus I caretake a 75 R90S with 38mm calipers. And the guy who owns the R90S also has a 77 R100S with 40mm.
On all three of these bikes, we put single disk masters. A machinist friend of mine machined in the second port on the boss. Both single and dual disk masters use the same casting.
I also used braided lines to improve the feel.
The lever is much softer than with the dual master, but much more powerful.

Some people put a splitter on the single disk master, but machining it with the second port makes it look original.

[jagarra]

Good info Seth. Since I have a 14mm master I decided to mock it up using a splitter. Used an steel line down to the splitter and hooked everything up. I bled the system, now I am burping it over night. First impression was it felt OK, travel was longer and if I really squeezed hard I could bottom the lever on the handlebar. Will see how it works tomorrow, putting the bike together enough so I can ride it round the block to see how it feels. I am thinking if I used the 38mm calipers it would be pretty darn good.

[Airbear]

I hope ChasBMW chimes in. I'm pretty sure he swears by a 13mm MC for the twin 38mm ATE calipers on his R90. If that is so, a 14mm should work with the 40s (in my mind, but not with real-world experience). Best of luck with it.

[Seth]

You may find the lever is more firm if you replace the factory flex lines with braided steel lines.
With the single disk master, you are delivering higher line pressure for a given lever pressure due to leverage.

I find the single disk master to be a very good upgrade. Not as good as a handlebar mounted master, but good if you're looking for original style. And if you already have a single disk master (they are available cheap on ebay), it's a reasonably priced upgrade.

[jagarra]

Seth,

I have new lines from Spiegler on the system. I have the lever pumped up and tied back, so it should be pretty well bled tomorrow morning. looking forward to trying it out.

On drilling the opposite side to accept the hose. I can drill and tap it but I was wondering about the cone at the bottom of the hole. Any idea how you friend did that part of the conversion?

[Mal S7]

I have a 14mm handlebar mounted M/C with twin 40mm ATEs.
It works good.

Mal