Z Car Rear Brakes

[Ron Earp]

FWIW... I've never really turned down pressure to the rear drums...Have you? I rode around with a regulayer for years, til I relized ....never really use it. Never had problem locking rear drums....unless combined with sloppy downshift.

Hey David,

Don't worry about the drums. Once the FWD weight situation gets resolved we'll write in for allowances for the drum braked cars in IT because we now see that we're at a disadvantage. Heck, when selecting our cars to race who would have thought that drum brakes would be a disadvantage in IT???

Anyhow, no, I don't turn down the pressure on the rears. I've got a proportioning valve but it stays open all the time. I can lock the rears in some uncommon situations but for the most part a set of shoes lasts indefinitely. I ran the same set all of last year and I don't plan on changing them for this year. I do use Carobotech shoes.

However Parrish goes through rear shoes on his car. Drums turn blue and he's cracked them before. I've never come close to doing that so I must be doing something wrong. I adjust mine so that the drum is hard to get on. They are newish drums and do not have a lot of signs of wear - no "cavity" on the inside where the shoe can't touch the drum. I have new wheel cylinders, new lines back to them, and use the parking brake to adjust the brakes too. But I can't seem to get the braking that Steve gets with his setup.

Any other comments from Z guys on their rear brakes?

[JeffYoung]

I have the same damn problem. I use the rears, but nearly impossible to lock them up.......and that is with high dollar shoes and full pressure to the rears. Weird.

[betamotorsports]

A lot depends on shoe material. You want a soft bite but a lot of friction. Experiment with different shoe materials and make a big effort to get the rears to do as much braking as possible. That takes some load off the fronts and helps keep the brakes under the car the whole race. Bleed often and adjust the rear shoes after every session.

[spawpoet]

Hey David,

Don't worry about the drums. Once the FWD weight situation gets resolved we'll write in for allowances for the drum braked cars in IT because we now see that we're at a disadvantage. Heck, when selecting our cars to race who would have thought that drum brakes would be a disadvantage in IT???

Anyhow, no, I don't turn down the pressure on the rears. I've got a proportioning valve but it stays open all the time. I can lock the rears in some uncommon situations but for the most part a set of shoes lasts indefinitely. I ran the same set all of last year and I don't plan on changing them for this year. I do use Carobotech shoes.

However Parrish goes through rear shoes on his car. Drums turn blue and he's cracked them before. I've never come close to doing that so I must be doing something wrong. I adjust mine so that the drum is hard to get on. They are newish drums and do not have a lot of signs of wear - no "cavity" on the inside where the shoe can't touch the drum. I have new wheel cylinders, new lines back to them, and use the parking brake to adjust the brakes too. But I can't seem to get the braking that Steve gets with his setup.

Any other comments from Z guys on their rear brakes?


Z's have rear brakes????????????????:eek: Funny, I never knew. Our experience is similar to yours Ron. We have had very little wear on the rears, though our car hasn't yet been run as hard or long as everybody elses car on here has. I believe we are on Carbotechs as well.

[pballance]

Ron, do you have the OEM distribution valve in the rear?

[dspillrat]

Yeah, tell me about it.....Brakes and Zs.... first couple of laps....we're in good shape.... But I love my Z.

Me thinks the proper-rationing valve, even Wide open, restricts more then straight line.... I have the splitter in the back, .... I wear out the front shoe in a weekend at road atlanta...... I took out my park brake assembly....it would help at the end of race I guess. Just too stubborn to puttem back in.

Man!!! I'm ready to go :eclipsee_steering::eclipsee_steering::eclipsee_st eering::eclipsee_steering:

[Ron Earp]

Hey Paul,

No, I don't have the stock valve in place.

David,

I think you might be right on the proportioning valve. What I'd like to do is use a pressure gauge to do a test on it and see.

High time to get it back on fellows!

Ron

[pballance]

Reason I asked Ron was that I recently re-plumbed the rear of my car and the rear braking improved. I also opened the proportioning valve wide open but that did not do as much as the plumbing.

I wonder if changing line sizes into the rear "T" can gain us some hydraulic advantage. IE larger diameter line into the t than what is normally run to the wheel cylinders. Of course, that means new plumbing all the way from the master back to the T. Hmmmmm?

[JeffYoung]

Isn't line pressure ultimately determined by master cylinder bore size and wheel cylinder diameter, and not line diameter?

[pballance]

Probably, but unless you have reached max pressure exiting the master increased volume of a larger diameter line ?might? give a slight pressure increase if reduced to smaller diameter. I am not sure if the pressure increase would equate into farther piston travel in the wheel cylnder though.

Crap, we need to be on track instead of doing this ......... This makes my head hurt trying to remember my physics

When is the next race?

[Greg Amy]

Isn't line pressure ultimately determined by master cylinder bore size and wheel cylinder diameter, and not line diameter?
Hydraulic pressure is wholly dependent on master cylinder bore size and piston rod force (your foot.) Wheel cylinder size does not affect pressure.


...unless you have reached max pressure exiting the master increased volume of a larger diameter line ?might? give a slight pressure increase if reduced to smaller diameter.
No. Line diameter only affects flow rate, the speed at which the system becomes static; think of the difference between sucking through a coffee stirrer versus a Mickey D's soda straw.


I am not sure if the pressure increase would equate into farther piston travel in the wheel cylnder though.
It won't. Slave cylinder travel is wholly dependent on volume displaced by the master cylinder.

The most likely answer for why your rear braking improved when you replumbed is because you removed the flow restriction of the stock prop/balance valve... It's also possible the line size was too small, also restricting flow rate. But, unless you changed the master cylinder to a larger one then you got no more volume flow, you just increased the rate of flow with the other mods... - GA

[pfcs]

Changing wheel cylinder bore doesn't change the line pressure but it sure changes the force acting against the brake shoe-the larger the bore, the harder it pushes against the shoes. VWs have had (interchangeable) wheel cylinders of 14, 15, 17, &19mm bores. (legality issue I won't go into) If you lack rear brake, the only options are to change the coefficients of friction F/R, or the hydraulic ratios: smaller caliper pistons/larger wheel cyl bores. One could put a bias valve on the front brake circuit, but In my experience that would be asking for trouble.

[JeffYoung]

Phil, that's where I was going with my post, although I phrased it wrong. A standard "fix" to no rear brake on the TR7/8 is to use MGB wheel cyliders with different bores; obviously illegal in our class.

So, given that I've run all new lines and still lack rear brake, and run the Carbotechs, I think I've done all I can do.....yay baby.

[pfcs]

Jeff-I saw that you understood what you were talking about. If you have no legal option regarding F/R bore ratio, you still can do a lot with friction materials.

[Tom Donnelly]

Different year z's have different proportioning valves in the back. 69-72 have one type, 73 on another. And they're different on the rest too. I found out while trying to decide on the master cylinder choice. If you can get the parts listings for the different models, you'll see the difference. The parts manager at Nissan called John Williams while I was there and John stated that if you switch masters, you need to switch the valves that are back by the differential. A '73 master cylinder with the '70 model valve can cause too much pressure in the rear.

The early models have two t-fittings and one valve by the diff, the later models one t with a valve. I think, I'll have to get my pictures from the parts manual back out.
But you get the idea. The valves in the z brake lines are model year range specific and a matched set. Mis-matched leads to questionable results.

Using an e-brake was what Keith always told me to do, and I never did. And I never wore out the rear brakes like Chet did either.

I wasn't as fast as Chet either.

[510rcr]

I have a 510 which is extremely similar. It took years to figure out why I didn't wear the rears. I read a thread somewhere that said prior to every event you must pull the drum off and tighten the screw wheel till you can bearly put the drum on and rotate it. Then back it off a couple clicks.

AMAZING RESULTS !!! The first time out after doing that I hit the brakes and the whole car squatted and STOPPED. Shoe wear increased dramatically. I could go way deeper, virtually stopped front lockup issues. You have to do this every session to dump the dust out and re-tighten. Try it, it really makes a difference. You will never not do it again.

Matt A.