Brakes...

[andyb]

Unless I'm missing something, a given pressure in the braking system spread over any number of pistons of any number of sizes will end up being the same force per unit of area at the piston.  Otherwise you're violating thermodynamics and creating energy somehow.  Force in = Force out, excluding leaks and shit.

The monobloc calipers actually do increase the amount of force going to the pads, but that's by reducind the loss (the caliper resists expanding away from the rotor better than the older designs do).

Stick new pads in it, spend the money elsewhere for now.  Strong, controllable brakes trying to control bald tires through a worn-out chassis.... etc.

[WestOzXJR]

So, I have a shot at getting a pair of calipers from a friend for super cheap. I just want to make sure I've got this right, and I think there's about a 50% chance I'm completely wrong.

He pulled some gold calipers off of a '91 FZR600. He also says he thinks they were aftermarket; they say "Sumitomo" on them.

Would those fit the 3CV forks and/or the earlier forks with the adapter plate to get them onto the older rotor that comes with the earlier forks?

He's offering them for $30, which seems like a steal--impression is that they're working, but I don't mind rebuilding calipers to save money anyways, and they should probably be rebuilt anyways while they're off.

To buy or not?

According to this list, I might be in luck?

Sumitomo is the manufacturer of  all three types of caliper in discussion here, so no need to think of them as "aftermarket"...

As I understand it, the FZR600 calipers you refer to would probably be of the type that has a SINGLE pin retaining the pad, is that what they are? Whereas the FJ 4 pots have 2 pins...

My belief is, although I've not personally sized it up, the FZR600 (single slider pin type) caliper bolts straight on to the earlier 16" fork lowers with no adaptor plate required and will align with/match up to earlier small discs.

In all the theoretical calculation formulas I've ever read regarding brake design for race cars and bikes, pad area means nothing and is not taken into consideration in the relevant engineering formulas.

Whereas brake rotor diameter, brake to rotor friction coefficient and pounds per square inch force are the only variables in the equation.

To double the pad area is to halve the psi force @ pad to rotor contact. That's not to say it doesn't affect pad or rotor wear rates though.

[WestOzXJR]

oops, duplicate post...

[Dan Filetti]

Pad size increases/ greater surface area against the rotor, are subject to to deflection like any other application of force.   Andy's "leaks and shit" speaks to this.  To exaggerate to the extreme to illustrate the point, imagine a pad that was clamped, as normal, on, let's say a single piston caliper, but the pad surface covered the entire rotor, all the way around.  There would certainly be a significant, measurable difference in clamping force opposite the caliper, as compared to under the piston at the caliper.  Why: deflection, the pad farthest away from the caliper would 'bend' away from the rotor, or more precisely not clamp as much, as compared to the section of pad under the piston.  The larger the pad, the greater the opportunity for deflection -not to mention the additional un-sprung weight trade off.  

There is a reason why motorcycle, (and car for that matter), engineers use smaller pads as compared to the size of the rotors.  Throughout history, large pad 'experiments' have met with failure and ultimately were replaced with more conventional smaller pad brake styles in later versions.

My $0.02

Dan

   

[JCainFJ]

I recently changed over my original and reconditioned 4 piston calipers running EBC HH pads with blue dots and standard pads and felt I had taken a step backwards.

I had to wait a few weeks for my new pads and did 4000 k's ( a couple of trips) with this configuration including a weekend in the mountains so I had plenty of opportunity to use them.

The harder I used them the less I liked them. My discs became glazed and were discolouring from heat.

ribbert,

It sounds like the "standard pads" that you used first in the monoblocks were contaminated. Were those pads the stock Sumitomo G-G. Were they the pads that came with the used monoblocks, or were they fresh and new? I've had one set of monoblocks (out of the  four that I've retrofitted to some of my/our bikes)glaze  discs, and in that case it was because the pads had been soaked in fork oil.
My point in all of this is that its a good idea to use new pads when first mounting a pair of monoblocks.

[andyb]

My belief is, although I've not personally sized it up, the FZR600 (single slider pin type) caliper bolts straight on to the earlier 16" fork lowers with no adaptor plate required and will align with/match up to earlier small discs.

Wouldn't be real hard to compare the two by measuring the distance between the mounting bolts.  Early forks require 83mm between the centers of the bolts that mount the calipers, iirc.

I think the FZR600 calipers are on the same 83mm centers as the early FZR400.  From what I was looking at, the FZR600 calipers should fit the 84-87 FJ if they're the earlier ones, so pre-91 models should fit.

Which means that they're not really an upgrade, or they won't fit without faking it a bunch.  Meh.  Do it properly, upgrading the forks, wheel, and brakes all together, or just slap good pads in and change the fluid, job done.

[1tinindian]

   Do it properly, upgrading the forks, wheel, and brakes all together, or just slap good pads in and change the fluid, job done.

+1

[Dan Filetti]

   Do it properly, upgrading the forks, wheel, and brakes all together, or just slap good pads in and change the fluid, job done.

I'm thinking if you are going the cheap route, just pads and fluid, and have no intention to do the other 'proper' modification steps anytime soon, then new braided brake lines is in order as part of a bare-bones, minimum stuff to do list.  The old ones are at least, depending on your model year, 20 years old! 

[SlowOldGuy]

Unless I'm missing something, a given pressure in the braking system spread over any number of pistons of any number of sizes will end up being the same force per unit of area at the piston.  Otherwise you're violating thermodynamics and creating energy somehow.  Force in = Force out, excluding leaks and shit.

The monobloc calipers actually do increase the amount of force going to the pads, but that's by reducing the loss (the caliper resists expanding away from the rotor better than the older designs do).

Disclaimer:  I'm NOT a brake system designer; however, the mechaincs and physics of the system are straightforward.

Andy,
Your first statement is correct.  The pressure (force per unit area) is the same on ALL piston surfaces.  However, the force generated as a result of that pressure is a function of the TOTAL piston surface area that's exposed to the pressure.  As the previous calculations show, the caliper with 2 3" pistons has more area than the caliper with 4 2" pistons.  Therefore the "old style" 2-piston caliper , in this example, has more braking force at the brake pad.

Whether that higher force results in "better" braking performance is dependent on pad coefficient of friction.

For Pat,
First, be careful mixing Force and Stress in the same context, they are related but are NOT the same thing.

A larger pad is desireable for several reasons.  It spreads that clamping force over more area thus reducing the total mechanical pressure (stress) on the rotor.  Thus the rotor "should" wear less with larger pads.

Larger pads also provide more surface area to dissipate heat to the rotor instead of the caliper.  That's also why the hollow side of the piston is facing the brake pad; less piston area exposed to the braking heat thus less heat conducted into the brake fluid.

So, how do you get bigger pads?  Larger piston seems reasonable, and it is to a certain extent.  Grow the piston too much and you have to start growing the diameter of the rotor (more unsprung and reciprocating weight).  
Solution: Add more smaller pistons along the swept surface of the disc.  This touches on Dan's warping pad.  If you could provide pistons all the way around the disc, you could have a pad that's also the size of the disc.  But that's not very practical, or even required.

With multiple smaller pistons, the rotor can actually be decreased.  The lower stress from spreading out the clamping force combined with better heat transfer means smaller rotors can be used without affecting performance.

Comparing the old 2-piston caliper to the newer 4-piston and even the monoblocs, the 2-piston area is greater than either of the 4-piston calipers.  Therefore, when keeping the stock 5/8" size master cylinder, braking force will DECREASE when "upgrading" to the newer calipers.

The force will decrease, but overall performance can actually increase due to better (modern) pad composition.  Which gets me back to my original point.  The pad composition is far more important than the number of pistons pushing on it.

The monobloc calipers HAD to have a smaller m/c in order to retain braking performance.  It is a better system and, as Arnie points out, has better modulation.

But, has anyone noticed that for the last few years Yamaha has been putting 15mm master cylinders on most (if not all) of their front brakes?  Maybe someone at Yamaha "thinks" they've been giving riders too much front brake power?  Just a thought.

DavidR.

[fj11.5]

fzr400 rr twin spot calipers will fit,  hard to find and wreckers want $85 each for them, have to be the rr model as normal fzr400 ones wont fit, , these are for the 16" front wheel fj,s