Upgrade your /6 and R90S front brake

  Notes for 2018 Supertech disc brake seminar

  presented by William F. Dudley Jr.

The mechanical advantage of the braking system is what lets you stop your motorcycle with mere hand pressure. The kinetic energy of your moving motorcycle (and you) is converted to heat by the friction of the pads against the disc. The better the mechanical advantage, the easier it is to clamp the pads down on the disc.

In my opinion, dual discs are called for if the bike is to be ridden in a "sporty" fashion, or ridden two up at high speeds, when there is more heat to get rid of. For "normal" solo riding, I think the single disc is adequate.

On the /6 and /7 bikes with under tank master cylinders, there are four different contributors to mechanical advantage:

On a stock R90/6, the slave cylinder (ATE caliper) diameter is 38mmm (to 9/75) or 40mm (from 9/75) and the master cylinder diameter is 14mm (for all years).

Early R90/6 (to 9/75)
Part diameter area
caliper 38mm 1134.0 mm2
master 14mm 153.9 mm2
ratio 7.37

Late R90/6 (post 9/75)
Part diameter area
caliper 40mm 1257.0 mm2
master 14mm 153.9 mm2
ratio 8.16

On an R90S, the two slave cylinders (ATE calipers) are 38mm (to 9/75) or 40mm (from 9/75) and the master cylinder is either 16mm (to 9/75) or 17mm (from 9/75).

Early R90S (pre 9/75)
Part diameter area
caliper 38mm 2268.0 mm2
master 16mm 201.0 mm2
ratio 11.28

Late R90S (post 9/75), early R100RS
Part diameter area
caliper 40mm 2514.0 mm2
master 17mm 227.0 mm2
ratio 11.07

The improvement that I make is to switch to the larger ATE calipers and the smallest master cylinder. On my 77 R100RS, all I had to do was replace the 17mm under tank master cylinder with a modified 14mm master cylinder from an early R90/6. The modification is to drill and tap a second hole in the master cylinder to accomodate the second hydraulic hose.

The details on the drilling and tapping are as follows: the thread is an M10x1, and the tap drill is 9.1mm, which is 0.358 inches ('T' drill). The depth should be .46 inches. In the bottom of the tapped hole, drill a #29 hole .71" deep (measured from outer face of master cylinder). In the bottom of that hole, drill a #53 hole through into the brake fluid chamber.

If you replace the undertank master cylinder on a 1976 or later bike (last year R90S, early /7's), then the front brake cable will also no longer fit properly, as the nipple size at the end of the cable changed. This from Brook Reams:

The front brake cable changed in the 1976 model year.

--> 32 73 1 234 400 is the cable THROUGH 09/1975 (the end of the 1975
model year)
--> 32 73 1 234 857 is the cable AFTER 09/1975 (the start of the 1976
model year)

Bud Provin suggests switching the arm from the old master cylinder to the new one. To quote Bud: The pivot pin is simply driven out. When you get the lever off, you'll see that it should be lubed from time to time-but no one ever does.

Improved 77 R100RS
Part diameter area
caliper 40mm 2514.0 mm2
master 14mm 153.9 mm2
ratio 16.33

There's a widely used (and cited) table of combinations of hydraulic brake components (calipers vs master cylinders) hosted here: http://www.vintagebrake.com/mastercylinder.htm The problem with this web page is that the numbers don't directly apply to our under tank master cylinder bikes because of the extra lever at the master cylinder. However, it's still a useful tool to compute areas and ratios.

I sampled the bikes in my garage and measured the approximate mechanical advantage of the lever. Here are the results:

bike pivot to cable pivot to index finger ratio
75 R90S 33mm 72mm 2.18
72 Commando 27mm 52mm 1.93
87 Rebel 23mm 60mm 2.61
93 Guzzi SP3 23mm 68mm 2.96

You'll notice that the R90S has the a ratio at the low end of the range shown, along with the Norton Commando.

However, note that the R90S has the additional lever on the under tank master cylinder, and so I believe that changes the lever mechanical advantage to 2.18 x 1.73 = 3.77. So, even though the hydraulic mechanical advantage looks quite low compared to the suggestions on vintagebrake.com, the high mechanical advantage of the combined levers makes up for this.

I think that the best approach when trying to improve the feel/braking power of a motorcycle is to make your changes based on percentage change in ratio (from before the mod to after the mod). For example, my R90S, before the mod, had a hydraulic ratio of 11.28. After the mod, the hydraulic ratio is 16.33. So from this I calculate a ratio improvement of about 145%.

Be aware that as you improve the mechanical advantage, you do at the expense of lever travel and perceived "mushiness". You can alleviate the mushiness -- to a point -- by switching the rubber hydraulic hoses to braided nylon lines. If you have too much mechanical advantage, the lever will hit the handlebar grip before you've achieved maximum braking.

Another example of a brake ratio improvement is my Norton Commando, which, like the R90S, had a very "wooden" feel to the stock disc brake. The Norton's specs are similar to the R90S: 19" wheel, 272mm disc (vs 260mm for the R90S), caliper diameter of 44.5mm and master cylinder diameter of 5/8 inch, which is 15.88mm.

In the stock configuration, the Commando brake gives new meaning to the word "wooden" -- I used to joke that it took both hands squeezing the brake lever to stop the thing.

I tried various "fixes" -- I replaced the stock rubber hydraulic line with a braided line, I had the disc "skimmed" to remove the chrome plating and reveal the bare cast iron, and I replaced the pucks with "sticky" pucks. All these things helped, but the bike was still hard to stop.

Eventually, I upgraded my master cylinder by having it sleeved down to 13mm. The result was a dramatic improvement. Now, it's a two finger brake. The ratio improvement is about 149%.

Stock Norton Commando
Part diameter area
caliper 44mm 1521.0 mm2
master 16mm 198.0 mm2
ratio 7.68

Improved Norton Commando
Part diameter area
caliper 44mm 1521.0 mm2
master 13mm 132.7 mm2
ratio 11.46

Parts needed for converting to early R90/6 14mm master cylinder

Name BMW Number BMW List
14mm master cylinder 34 31 1 234 381 $363.73
the following needed if master cylinder not drilled for brake switch
stop light switch 61 31 1 244 070 $13.22

If you have a single disc /6 or /7 airhead with undertank master cylinder, you have three choices to increase the brake "power", shown in order of increasing difficulty:

Contact info for brake sleeving services:

Apple Hydraulics
1610 Middle Rd, Calverton, NY 11933
(631) 369-9515
Apple actually shows /6 master cylinders on their web site.

White Post Restorations
1 Old Car Drive, PO Box D
White Post, VA 22663
(540) 837 1140

Bob Fleischer (a.k.a. Snowbum) also lists companies that sleeve master cylinders on his web page, here: