comparing different m/c bore size versus caliper size and showing what is
optimal.
http://vintagebrake.com/
It's a ratio thing, m/c bore size versus caliper bore size. You will trade
more lever travel for more braking and vice versa.
So think first pads (Ferrodo Platinum), then new ss hose, then different
m/c, in priority order, easy to more involved plus $$ considerations, to
improve your brakes. Follow the instructions in the above link for
installing new pads for best braking results.
As has been said, pre 9/75 production bikes (I.e., '74 models) came with 16
mm m/c and 38 mm calipers, then production after 9/75 (i.e. '75 & '76
bikes) came with 17 mm m/c and 40 mm calipers.
The R90/6, with its' one disc came with a m/c bore of 14 mm compared to the
R90S m/c bore of 16 then 17 mm. Back then BMW recognized the benefit of a
smaller m/c bore size since the R90/6 had only one disc and thus needed
more braking power.
SO if you sleeve your R90S m/c with stainless by Mark Frappier (see below,
about $70 last time I did it), and send Mark a new R90/6 14 mm piston he
will bore your m/c and fit a stainless sleeve to suit.
If you have a '74 caliper it will NOT be stamped with a 38; if it is a '75
or '76 model it will be stamped with a 40, so you can tell the difference
between the two.
So for an R90S m/c, ideally, go to a 14 mm bore and a 40 mm caliper (if you
don't already have that size caliper) for maximum braking power, unless you
go to a handle bar mounted m/c, which is more money and not original
looking. Many do that too since it eliminates the flex of the cable from
the under tank m/c to the handlebar.
Moto-Bins in the UK might carry an even smaller diameter piston size.
Google them and see, after looking at the chart vintagebrake.com
Some go to a more modern front end entirely, again much more $$, but one of
the very best options. I think I was told a K bike front end can be fit,
but verify that prior to going that route!
And you can replace your original rubber brake hose with a new stainless
one as it will flex less and give you better feel, but not better braking.
If your hose is old, well it's time
https://spieglerusa.com/
Spiegler will also make you a stainless hose covered with a black cover, so
it looks most like OEM, if you want it that way. BUT I think you can also
specify pink!!
And ALL m/c cylinders have ferrous metal in them whether they are bored for
a new sleeve or not, so rust is always an issue, whatever you do, even if
you do nothing. But having a stainless bore, where the brake fluid sits and
is working, is a plus.
I and others have done most all the above with good results, and combined
with Ferrodo Platinum pads, my brakes work acceptably well and are a very
good improvement over stock, which IMO the R90S needs.
But remember, when BMW came out with disc brakes in '73 with the R90S, they
were the best around and folks raved about them. Things change, huh? It's
pretty amazing.
I probably should find a set of 40 mm calipers for my bike, just to
complete the job. But too many projects...
And DO use Acetone on all new metal brake parts as it will not leave a
residue, and even new discs have a manufacturing residue on them. Goo for
cleaning brake parts even leaves a slight residue that will reduce breaking
results.
Here are my notes re Mark Frappier. I just spoke to him just the other day.
Re-sleeves master cylinders:
Still sleeves in stainless only, minimum size hone he has is .50 inches.
The R80/G/S is 12 mm or .472 inches, so he can’t do that size.
Mark Frappier
82 Mountainview Street
Agawam, MA 01001
413-789-8665
800 528 5235
Good luck.
Mac Kirkpatrick
Glenmoore, PA
After all, what is adventure, but inconvenience, properly regarded?"
C. Donahue