Topic: Master cylinder and Front Caliper Questions.

[Honda?]

Hey guys I have three quick questions.

1) IF I were to convert to dual disc/calipers up front and stick with the stock single caliper master cylinder what would happen? Would the lever just pull back to the grip because it isn't pumping enough fluid to fill the calipers?

2) What is the second hole in the master cylinder reservoir for?

3) I heard of a trick where you put the forks on backwards so the caliper mounts behind the forks rather than in front. Would that work with stock 1975 750F calipers that have the connecting bracket?

Thanks for any help!

-Tyson

[scottly]

Hey guys I have three quick questions.

1) IF I were to convert to dual disc/calipers up front and stick with the stock single caliper master cylinder what would happen? Would the lever just pull back to the grip because it isn't pumping enough fluid to fill the calipers?

My bike was set up this way. I got a rude awakening the first time I grabbed a handful of front brake. It's like having power assist. I learned to only use two fingers on the brake lever.... 

[fastbroshi]

My bike was set up this way. I got a rude awakening the first time I grabbed a handful of front brake. It's like having power assist. I learned to only use two fingers on the brake lever....  

  Please elaborate, because all I've heard is that it turns the lever to mush, which barely improves performance.  I've done a LITTLE research on it and adding that second disc effectively halves your lever ratio from like 13:1 to 26:1, or doubles it however you want to look at it.  You keep the same pressure valve for the fluid and mechanical lever in play, but doubled your work (the pad sweeping the disc), and added some length to the fuid circuit.  This changes the lever ratio.  
  
  Somebody correct me here, but I think the stock master on 750s was a 13mm or 14mm piston.   I believe the solution is to get a master cylinder with a slightly smaller bore.  
  
  I believe that small hole in the master is the return for the brake fluid.  You squeeze the lever and fluid is pushed down to the brake caliper, and comes back through there.

[scottly]

  Please elaborate, because all I've heard is that it turns the lever to mush, which barely improves performance.  I've done a LITTLE research on it and adding that second disc effectively halves your lever ratio from like 13:1 to 26:1, or doubles it however you want to look at it.  You keep the same pressure valve for the fluid and mechanical lever in play, but doubled your work (the pad sweeping the disc), and added some length to the fuid circuit.  This changes the lever ratio.
 
 To put it simply, you get the same braking force, with twice the lever movement, and halve the lever pressure.
 My rude awaking came in the form of damn near doing a 70 mph nose wheely on the test ride before I bought my Seeley. I was catching up a bit too quickly with 45 mph traffic, when I tried the rear brake, it didn't work, so I grabbed the brake hard. Smacked my helmet into the back edge of the windscreen.
I later swapped a K7 fork, but the K1 caliper mounts wouldn't bolt to the K7 forks, so I just used the single K7 setup. I realized then how well the dual disc had worked.   

[MCRider]

That's right. Same master cylinder, same force. MC is the key to force. All other changes simply spread it out, like over two calipers and discs, for less fade, better modulation etc.. Same total force though. And with the two you get more lever travel with less effort to produce the same force.

I would think if you wanted to equalize it back, you'd go for a larger diameter MC piston. If the line dia are stock (who knows with SS lines) the amt of fluid pushed in the MC would be greater witha larger bore, and would match up with the enlarged demand down below. Don't know that for a fact, just fishing.

What I'm curious about is using a larger dia MC with one better disc, like a EBC.

[MCRider]

many of us have swapped fork legs to get the calipers on the back. Should work fine. You have to remount the fender on the center bracket though.

[Honda?]

Thanks for the discussions guys. It sounds like dual disc with single disc type master cylinder would be better suited for racing or the track....perfect. 

I'm planning on flipping the forks around. Seems it helps reduce the "death wobble".

Can anybody tell me off of the top of their heads what the caliper and fork leg differences are? For example I have a 75' 750F. What calipers would match what I already have other than the obvious 75' 750F.

[MCRider]

I'd go to motogrid.com or bikebandit.com and cross reference the parts and models from their parts fishies.

[scottly]

Thanks for the discussions guys. It sounds like dual disc with single disc type master cylinder would be better suited for racing or the track....perfect. 

Hey, it worked great on the street!! I just had to learn how to use it, which wasn't that hard at all..

Scott

[Laminar]

I would think if you wanted to equalize it back, you'd go for a larger diameter MC piston. If the line dia are stock (who knows with SS lines) the amt of fluid pushed in the MC would be greater witha larger bore, and would match up with the enlarged demand down below.

Line diameter shouldn't matter, as at these pressures, brake fluid can be considered incompressible. You could have a 100 gallon brake reservoir somewhere in the brake lines and you'd get the same pressure out as you put in.

[scottly]

100% correct, Laminar; the volume of the system has absolutely nothing to due with pressure. I will add that stainless-steel, braided lines do not expand as much as rubber types under pressure, and will give a "firmer feel".

[MCRider]

100% correct, Laminar; the volume of the system has absolutely nothing to due with pressure. I will add that stainless-steel, braided lines do not expand as much as rubber types under pressure, and will give a "firmer feel".

Thanks for that, I wasn't really sure so I thought I'd keep the variable out. But as you say, no difference.

Better parts will optimize a system otherwise restricted by the MC. Braided lines that don't expand, discs and pads with higher friction, but all from the same force in.

[camelman]

100% correct, Laminar; the volume of the system has absolutely nothing to due with pressure. I will add that stainless-steel, braided lines do not expand as much as rubber types under pressure, and will give a "firmer feel".

I have to agree.  If you do the dual disc conversion and don't want that mushy feeling, then definitely go with the stainless line.  I swapped in a stainless line set on my 350F to replace the aging rubber hoses and it cut my lever travel by 65%.  The lever barely moves now, and I have much better feel.

Camelman

[Kevin400F]

Another thing that affects overall brake lever travel is internal geometry (axial position) of the master cylinder plunger relative to that "second hole" in the MC reservior.  I just installed a new K&L kit in my track bike master cylinder, and the K&L plunger is just slightly shorter than the old one.  The result is that the lever has to travel a little more before the primary cup masks the recouperation hole and starts building line pressure.  Although the lever travel is still acceptable and everything works fine, I've got about twice as much overall lever travel at max brake pressure as I had before, everything esle being equal.  (and yes, the brakes are bled thoroughly).   

[Yoshi823]

On my double disc conversion on my 'Summer' commuter 550F1 I used the standard master cylinder with Aeroquip lines & Ferodo pads. With careful bleeding of everything I got a good firm feel at the brake lever...enough to lock the front wheel when I was targeted by a speed cop with a radar gun one morning.

I thought about putting the calipers to the rear of the forks but I wasn't sure because the caliper mount brackets would then be in compression rather than in extension if on the front. The second caliper came from a CB250G5 because as far as I could make out the calipers are all the same except for some of the calipers on the 750K bikes (the offset brake pipe location/bleed nipple).

[MCRider]

On my double disc conversion on my 'Summer' commuter 550F1 I used the standard master cylinder with Aeroquip lines & Ferodo pads. With careful bleeding of everything I got a good firm feel at the brake lever...enough to lock the front wheel when I was targeted by a speed cop with a radar gun one morning.

I thought about putting the calipers to the rear of the forks but I wasn't sure because the caliper mount brackets would then be in compression rather than in extension if on the front. The second caliper came from a CB250G5 because as far as I could make out the calipers are all the same except for some of the calipers on the 750K bikes (the offset brake pipe location/bleed nipple).

The compression/extension argument was around when we first started flipping them to the back. Its simply a non-issue. The parts are so overbuilt for the job it doesn't matter. Witness thousands of people who have done the flip, not a single problem.

If you think about it, what difference does it make to that hinge pin? The pulling force would be equal to the pushing force and can the pin tell the difference?

[Laminar]

I thought about putting the calipers to the rear of the forks but I wasn't sure because the caliper mount brackets would then be in compression rather than in extension if on the front.

That thought went through my head as well - when the calipers are in front of the forks, the braking force pulls the calipers away from the forks, tending to straighten them out. If the calipers are behind the forks, the braking force pushes the calipers towards the forks, trying to cause them to deflect one way or the other.

The compression/extension argument was around when we first started flipping them to the back. Its simply a non-issue. The parts are so overbuilt for the job it doesn't matter. Witness thousands of people who have done the flip, not a single problem.

If you think about it, what difference does it make to that hinge pin? The pulling force would be equal to the pushing force and can the pin tell the difference?

The issue isn't the shear force on the hinge pin, but the direction of the force and the force moment it creates about the hinge pin.

Whether or not there have been problems, it's still something to take into consideration.