Z1 OEM dual discs

DiamondSkyBlue1000 wrote: Read a great article on this subject just recently where they compared brakes on a Z to brakes on a Ducati back in the 70's. Can't find it here at work. They found that the brakes out front of the fork leg made for poor handling because of their mass location in relation to the rake angle. That's fairly intuitive but they did the experiments to prove it. The bikes were fairly evenly matched (believe it or not!) when the fork legs were turned around so the Z calipers were behind the legs.
As to why they put the calipers out front, there was no real reason given, they figured the Japanese just thought it looked better. Some discussions I've had with guys who were active in racing back then raised a few theories. The calipers stayed cooler, there was less chance of interference with other parts of the bike (?) they were easier to get to, it showed off the new, simple designed calipers to poke the finger at HD, and finally, they didn't think it would matter.

Gentleman's Express: Some good reading about the reason we do some of the mods that we do...

Great article DSB! I like how it's compared to the Z1 at the end. Haha

mtbspeedfreak wrote: Using a 1/2" bore master cylinder on a dual front disc setup is NOT safe. The 1/2" piston doesn't push enough fluid through the system.

I'm curious as to how Kawasaki fixed this problem. Did they ship out a new 5/8" bore MC?

Ive ran many dual disc bikes with the small master cyl for many years without issue

wireman wrote:

mtbspeedfreak wrote: Using a 1/2" bore master cylinder on a dual front disc setup is NOT safe. The 1/2" piston doesn't push enough fluid through the system.

I'm curious as to how Kawasaki fixed this problem. Did they ship out a new 5/8" bore MC?

Ive ran many dual disc bikes with the small master cyl for many years without issue

Where does it say it's safe (and legal) in Kawasaki factory bulletin writing? Every place I've looked has stated that, while it is possible to use a 1/2" bore MC with dual discs, it leads to greater lever travel with the possibility of pulling the lever all the way back to the bar without full braking power, which is why they made the 5/8" bore MC. Just going by what I've read.

Patton from way back in 2007 :

Same linear movement of larger piston pushes more fluid. When Kawaski first offered the optional second front disc on 1976 KZ900 USA models, the factory kit included the larger 5/8" master cylinder along with the other necessary parts (as came standard on the KZ900 European models).

Believe 1/2" inch bore m/c on single disc KZ900 was increased to 5/8" for dual discs to allow additional volume for operating second caliper. Probably 5/8" would be okay for single disc (with less lever movement needed to brake) but 1/2" may not push enough volume to safely and consistently operate two calipers. Lever may possibly pull against the handlebar grip before pads sufficiently grip the discs. Might be okay with an otherwise perfect brake system with fresh fluid and completely bled, so long as there's a good hard (not mushy) lever feel with a margin of unused extra movement space available before hitting the handlebar grip.

With that said, am guessing there are many KZ900's pushing dual front calipers with 1/2" front master cylinders. Best wishes and good luck to them.

IMO brakes are too important to take even small chances with or second guess Ma Kaw engineers, so in abundance of caution I would recommend not using the 1/2" master cylinder in place of a 5/8" m/c for use with dual calipers on KZ1000.

If the OEM '81 KZ550 pushes dual calipers with a 1/2" m/c, good for it, but the OEM KZ1000 pushes dual calipers with 5/8" m/c, and I'm unfamiliar with the different engineering involved between them.

Happy Trails.

I've not run twin discs with the 1/2" myself, but I was asking about related stuff at my local Race Engineering shop when they were doing the rebuild on my old Z900. Short story is they said it was fine to run the smaller M/C and even added they prefer it "as provides more feel"

mtbspeedfreak wrote:

wireman wrote:

mtbspeedfreak wrote: Using a 1/2" bore master cylinder on a dual front disc setup is NOT safe. The 1/2" piston doesn't push enough fluid through the system.

I'm curious as to how Kawasaki fixed this problem. Did they ship out a new 5/8" bore MC?

Ive ran many dual disc bikes with the small master cyl for many years without issue

Where does it say it's safe (and legal) in Kawasaki factory bulletin writing? Every place I've looked has stated that, while it is possible to use a 1/2" bore MC with dual discs, it leads to greater lever travel with the possibility of pulling the lever all the way back to the bar without full braking power, which is why they made the 5/8" bore MC. Just going by what I've read.

Patton from way back in 2007 :

Same linear movement of larger piston pushes more fluid. When Kawaski first offered the optional second front disc on 1976 KZ900 USA models, the factory kit included the larger 5/8" master cylinder along with the other necessary parts (as came standard on the KZ900 European models).

Believe 1/2" inch bore m/c on single disc KZ900 was increased to 5/8" for dual discs to allow additional volume for operating second caliper. Probably 5/8" would be okay for single disc (with less lever movement needed to brake) but 1/2" may not push enough volume to safely and consistently operate two calipers. Lever may possibly pull against the handlebar grip before pads sufficiently grip the discs. Might be okay with an otherwise perfect brake system with fresh fluid and completely bled, so long as there's a good hard (not mushy) lever feel with a margin of unused extra movement space available before hitting the handlebar grip.

With that said, am guessing there are many KZ900's pushing dual front calipers with 1/2" front master cylinders. Best wishes and good luck to them.

IMO brakes are too important to take even small chances with or second guess Ma Kaw engineers, so in abundance of caution I would recommend not using the 1/2" master cylinder in place of a 5/8" m/c for use with dual calipers on KZ1000.

If the OEM '81 KZ550 pushes dual calipers with a 1/2" m/c, good for it, but the OEM KZ1000 pushes dual calipers with 5/8" m/c, and I'm unfamiliar with the different engineering involved between them.

Happy Trails.

Of course ive never read anything good about modifying a bike period in a kawasaki buletin period other than it voids the warranty.
I wonder how kawasakis engineers from back in 1973-80 would feel about usd forks and monoshock rear suspensions/180 rear tires/big motors/turbos......................... :woohoo:

The squeezing pressure needed on the brake lever with a stock 14 mm master cylinder and stock single front disc is approximately the same squeezing pressure needed for equivalent braking with a stock 5/8" master cylinder and stock dual front discs. This is the designed "sensitivity" of the brake lever. If too sensitive, the front brake will be overly prone to locking the front wheel when applying the brake, especially under adverse conditions. If not sensitive enough, "too much" squeezing pressure is required.

Had Kawasaki so desired, front braking could have been designed with toggle switch sensitivity to lock the front wheel. The degree of sensitivity is a balance of comfort and safety for the majority of riders.

Some riders may prefer a more sensitive front brake where lighter pull on the lever applies relatively stronger braking, which is what happens when a second front disc is added while retaining the same 14 mm master cylinder. An unusually sensitive front brake is imo more appropriate for a veteran rider who may better cope with and grow accustomed to it.

A rider who is accustomed to the "stock" brake sensitivity, and hasn't yet acclimated to -- or is uninformed about -- a more sensitive front brake, may unintentionally and unexpectedly lock the front wheel when braking on slippery pavement (water, oil, sand, damp leaves, etc.). This has reportedly happened and caused accidents in real life.

At minimum, anyone converting from single to dual front disc while retaining the same smaller 14 mm master cylinder should at least realize, be aware of, and know to expect the additional sensitivity.

And for goodness sakes, go out there and practice emergency braking, so that it will be instinctive and second nature when it really matters.

Good Fortune!

Ahh alright. Guess that clears things up. Thanks wireman and Patton!

I'll be mounting a set of new AP Racing two-piston calipers - I'm sure they're a significant improvement over the stock single caliper. Wondering if I should bore out my round MC to 5/8" so I don't lock up the front?

mtbspeedfreak wrote: ...mounting a set of new AP Racing two-piston calipers - I'm sure they're a significant improvement over the stock single caliper. Wondering if I should bore out my round MC to 5/8" so I don't lock up the front?

Would suspect the caliper design also influences the strength of grip applied to the disc with any given squeeze applied to the brake lever using the same master cylinder.

Other things equal, a larger diameter caliper piston would exert different pressure than a smaller diameter caliper piston. Factoring in more than one piston per caliper would also change the pressure exerted on each piston.

It gets into hydraulics computations (the explanations of which are gladly referred to more competent others).

The new AP Racing two-piston calipers might or might not have the desired sensitivity when used with the existing master cylinder. Would test the braking power in progressive steps, beginning slow and easy, until becoming intimately familiar with the resulting new feel at the brake lever. And practice emergency braking with the new system until it becomes instinctive.

Modern braking systems are still designed with a given degree of sensitivity, (which is less than toggle switch sensitivity). Many are capable of producing impressive stoppies, and so powerful that anti-skid systems are becoming more common.

Good Fortune!

mtbspeedfreak wrote: Ahh alright. Guess that clears things up. Thanks wireman and Patton!

I'll be mounting a set of new AP Racing two-piston calipers - I'm sure they're a significant improvement over the stock single caliper. Wondering if I should bore out my round MC to 5/8" so I don't lock up the front?

If your 1/2" mc is worn out and you have access to good machinist who can bore it to 5/5" I see no problem with doing that.
That would be a good idea with any of those oddball trapzoid mc's on the 78a2's since Ive never seen a rebuild kit for one of them.