Master cylinders and calipers... Confusion and questions

[85RZwade]

I'm trying to match up the front master cylinders I have to the front calipers/rotors of the project bikes languishing in the barn, and I've been referring to the magic Vintage Brake ratio chart. http://www.vintagebrake.com/mastercylinder.htm
I started out looking for a suitable master for my RD350 with 6-piston TL1000R caliper and 320mm disc (I know): four 27mm pistons and two 24mm pistons is 3193.38 square mm. I have on hand a 1/2" master (CBR600F2) that the chart says will yield a 25.2:1 ratio, which is right in the sweet spot. But...that F2's four 25.335mm pistons are 2015.45 square mm, which the chart puts at 15.9:1, well into the "wooden" category. I can't imagine Honda getting it wrong, so I checked the Ol Faithful RZ for comparison. It has two stock 38.18 pistons (2288.6 square mm) acted upon by a 5/8" master, which the chart says is an 11.46:1 ratio; oughtta be awful. It isn't, and hasn't been for 20 years. I must be doing something wrong, and it's surely my math but I can't find my mistake(s). Anybody feel like crunching numbers?

[MK]

I'd try a 19 mm radial hand pump, maybe one with adjustable mechanical ratio to fine-tune the lever force / travel characteristics.

If you need something cheap, try the Adelin 17.5 or 19 mm pump.
https://m.de.aliexpress.com/i/32922517315.html

[85RZwade]

Thank you Martin

[(F5)]

Most people with 6 pot go to 4. Tokikos are not as good as Nissin generally.

With a change to braided you can often go down a size in mc. Old yams did a lot of work expanding the brake lines but could feel wooden when braided put on as a result.

I did the typical blue spot on another yam losing 6 pot sumitos. Lost about a kg in total. Huge. But needed 14mm. Then neat.

[MK]

Next to the hydraulic ratio the hand pump also needs to deliver a volume of brake fluid.
This is caused by brake lines expanding under pressure and from calipers bending up and requiring a piston travel to compensate.

Old stuff is less stiff than new, so saying "my 4 pot blue spot calipers are ace" is not wrong, but also not helpful, if you need a hand pump for a pair of "old" 6 pots.
(except the message is "dump the old and heavy stuff" which I'd also agree to, if used on race track)

For this reason the big bikes developed smaller & radial hand pumps over the decades. They only work in combination with the appropriate stiff lines & calipers.

In the 90's the bikes had like 16mm for 4 pots and 19 mm for 6 pots.
Nowadays that's more like a 14mm standard or 16 mm radial for dual 4 pots.

[85RZwade]

So my particular situation is a single 6-piston caliper with a braided line. 320mm disc on an 18" wheel. If I'm looking at radial master cylinders, what diameter is suggested?

[MK]

One single disc?
So far I understood we're talking about standard dual disc setups.

If it's a single, you can make profit of my experience as I'm running a single 320 disc with a Yamaha YZF750 6 pot caliper on my road EFI RD350.
I've tried several pumps and ended up with a 16 mm TRW/Lucas radial pump plus Lucas SRQ brake pads. These are kind of important as they offer around double the friction coefficient of standard pads that even increases when heated up.
The downside: They wear pretty fast. 1500-5000 km.

As a cheap variant you could use the 5/8" (aka 16mm) radial pump of the Honda CBR 1000 SC57/59.
Also the Supermoto market offers lots of 16mm radial stuff. Some have 15mm which should work, too, but don't go lower.


What didn't work was the stock Mito pump (too small diameter and thus too much lever travel).
A 16 mm standard pump from the RGV offered decent lever travel but had a too high lever force.

Thus the solution was to sacrifice a bit of feeling for lower force by increasing the mechanical ratio (by going from standard to radial). And then further lower the force by using high-end brake pads.

[85RZwade]

Excellent, a wealth of information! I have something to work with now, Danke schoen!

I also found this tonight:

https://www.wristtwisters.com/threads/c ... der.48345/

Great information!

[85RZwade]

CBR1000RR master cylinder ordered and on the way

[MK]

That's BTW what I use on my VTR to actuate the two 4 pot calipers.

If it's not already ordered: Get some of these nice adjustable CNC levers. Even the Alibaba stuff is usually of good quality and has the same origin than the expensive road approved stuff.

[85RZwade]

Hopefully, this is the solution:

[crogthomas]

If it helps your calculations, for a sliding type caliper like the 1983 RZ/YPVS, the sliding part counts as an extra piston. So your calculations should count four 38mm pistons, which with the standard 5/8" master cylinder makes the ratio 22.92 according to that table.

Of course that table simplifies things a bit. You should really take into account the lever ratio itself, the size of the disc and even the size of the wheel. The table might suggest that the CBR600 would feel wooden, but it doesn't account for the larger disc and smaller wheel than would be fitted to most vintage bikes.

Did the CBR1000 master work out okay?

[85RZwade]

Wow, thanks for the information! Since I posted this, a great deal of life has happened: a divorce, a heart attack, two moves, a reunion with an old love, a wedding and a house purchase. The master cylinder is still packed away with the rest of the project, waiting to be moved into the garage of the aforementioned recently-purchased house! I hope to have an update soon.