74 R90 – bar mounted master cylinder – how to

[bbelk]

I used a 13mm master when I made the conversation on a 1975 R90/6. I think 12mm might work a little better, but I was told at the time, the 12mm could not be converted to duel disks. The break is much better than original, but still short of the ATE on my 1979 R65 and way worse than a 1999 Honda.

[SteveD]

I used a 13mm master when I made the conversation on a 1975 R90/6. I think 12mm might work a little better, but I was told at the time, the 12mm could not be converted to duel disks. The break is much better than original, but still short of the ATE on my 1979 R65 and way worse than a 1999 Honda.

I reckon that says a lot how how much that brake talks to ya bbelk!

[Rob]

I believe the size of the master cylinder is relative to the size of the caliper(s).
And I may be mistaken (the most I know about fluid dynamics is the name), but the two may be inverse proportional? That would not make sense to my personal logic, though.

[Ken in Oklahoma]

I believe the size of the master cylinder is relative to the size of the caliper(s).
And I may be mistaken (the most I know about fluid dynamics is the name), but the two may be inverse proportional? That would not make sense to my personal logic, though.

Close but no cigar I think, Rob. Mostly it's about the relative areas of the master cylinder piston compared to the slave cylinder piston or pistons total area. Generally speaking the surface area of the brake pad(s) themselves does not make any appreciable difference in retarding the wheel rotation. That sounds counter intuitive I know, but by and large it works out in the real world. I remember doing a lab experiment in my freshman college physics course demonstrating what I"m talking about.

Of course there are if's, and's, and but's. Take a tire for example being pulled across some concrete or asphalt. There the grip of the compliant rubber against an irregular surface becomes significant. (That's why fuel dragsters can accellerate faster than the theoretical limit of 1G.) Similarly, a hot tire will stick to the surface better than a cold one. (Which is why dragsters do burn outs.) But pull a brick with a string across a concrete surface and with the same weight of brick placing the brick flat, or on it's side, or even on it's end would make little difference in the amount of pull required to move the brick.

But I must confess that I haven't done a lot of (make that any) tests pulling a brick across concrete. (Nor have I piloted any top fuel dragsters.)

Ken

[Major Softie]

It's about ratios, but not just one ratio. First, the ratio of Master Cylinder piston area to Slave piston/pistons area, but also, and this is often forgotten in these discussions, the leverage ratio designed into the lever at the master cylinder. Because of this, two master cylinders of the same piston diameter can actually have different leverage/feel.

[jagarra]

I guess it's time to post this again, concerning the ratios needed for proper feel.

http://www.vintagebrake.com/mastercylinder.htm

[Ken in Oklahoma]

It's about ratios, but not just one ratio. First, the ratio of Master Cylinder piston area to Slave piston/pistons area, but also, and this is often forgotten in these discussions, the leverage ratio designed into the lever at the master cylinder. Because of this, two master cylinders of the same piston diameter can actually have different leverage/feel.

Very good point, Major. Thanks for picking it up. I knew this and have even posted about it, but it didn't enter my mind this time around. And I noticed that your careful wording encompassed both a handlebar mounted master cylinder lever and the much reviled under the tank master cylinder (with both the handlebar lever lever on the master cylinder itself operating in conjunction.
Ken

[Major Softie]

I guess it's time to post this again, concerning the ratios needed for proper feel.

http://www.vintagebrake.com/mastercylinder.htm

Thx, Jagarra. I considered Googling that up to include in my post, but I figured someone else would already have it close at hand.

The text of that brings up the other important ratio: disc diameter to wheel diameter, but, while that's something that racers and custom/cafe guys may play with, most of us live with the ratio the factory has given us on that factor.

[jjwithers]

Well done!

[Ken in Oklahoma]

Jagerra and Major:

I had not seen (remembered?) that web page before. It was very interesting to me. But even that list didn't mention what the Major mentioned, which is the leverage ratio of the hand lever to the master cylinder piston size. The list makes me wonder if, when in uncharted territory, if a person doesn't have plan on trying two or three different master cylinder piston sizes (within a given brand/style) to zero in on the preferred action.

Ken