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:

• The diameter of the tire vs the diameter of the swept area of the brake disc. A larger disc has greater mechanical advantage than a smaller (diameter) disc, and the larger the disc compared to the tire diameter, the greater the mechanical advantage.
• The handlebar lever, which advantage is the ratio between the length of the lever where you apply force, and the length of the lever between where the cable attaches (both measured to the lever pivot).
• The lever on the master cylinder, which advantage is the ratio between the length to the cable attachment and the length to the steel bar that pushes the master cylinder piston (both measured to the lever pivot). I measure this to be 52mm vs 30mm, or a ratio of 1.73. I think that the length of this lever changed between different versions of the under tank master cylinder.
• The hydraulic system, which advantage is the ratio between the total area of all the "active" pistons in the slave cylinders to the area of the master cylinder piston.

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).

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).

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:

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.

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:

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%.

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

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:

• Switch to a handlebar master cylinder with a smaller diameter piston. Tom Cutter at Rubber Chicken Racing Garage is one who can set you up with the right stuff. This may involve changing the handlebar switchgear, and it will change the look of the bike.
• Install the second disc, caliper, etc. This will raise the hydraulic mechanical advantage by increasing the area of the slave cylinders as opposed to reducing the area of the master cylinder. You'll have to either drill a second brake hose hole in your master cylinder, or rig up a splitter in the hose, to accomodate driving the second caliper from your master cylinder.
• Have your under tank master cylinder sleeved down to a smaller diameter. People on the Airhead list on micapeak.com have suggested several places that can sleeve a master cylinder. Of course, then you will have to find a compatible piston and seal kit for your smaller master cylinder, so this will be a research project.

Contact info for brake sleeving services:

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