Dual master cylinder sizes with 4 psiton calipers

[Guest iskone]

I'm going to be doing a brake upgrade once the body work is done. I'm getting AZC 12.2/4 pistons in the front for sure but may go with ross' kit in the rear for the p-brake

 

I have dual MC's that are 3/4" in the front and 5/8" in the rear. I was thinking about it and if the common upgrade is a 15/16 I don't think I'll have to upgrade my MC's.

 

Makes sense to me.

 

Isk

[JMortensen]

http://sth2.com/Z-car/Brakemath.xls

 

It's a little confusing figuring out all the variables, but if you look on Wilwood's site they have the brake pad sizes there. I used .4 as my coefficient of friction based on Cary's advice when I was figuring mine out.

 

What this doesn't tell you is how mushy the pedal will be. I expect it will be on the mushy side with cylinders that small. Single masters are cheap though (assuming your holes are a standard size), so if it was wrong you wouldn't have to spend a whole lot to fix it. I wonder about the 5/8" on the rear though. Might need to switch the masters front to rear.

[Guest iskone]

Thanks for the spread sheet. I like how it's already setup with AZC's in the front and 240sx in the rear!!

 

From the chart it seems like AZC's in the front and 240sx in the rear is a good way to go. I don't know what the CG height is on my car though. If I want to find it I'll have to dig up my books (for the math) and take measurements, which I should do.

 

I could be wrong about the way the MC's are mounted I just assumed the larger one would be for the front because I assumed they would need more force. Why would I want the smaller MC in the front?

 

The pedal is very firm now. If I do need to change one or borth tehy are only about $50 IIRC.

 

Isk

[blueovalz]

I had to think on this one a bit, but in changing to dual MCs, one would assume the following:

The same sized MC (15/16" for this discussion) would be needed to keep the same MC/SC size ratio for a proper leverage. But with split (one MC for each half of the braking system) cylinders pushing a certain volume of fluid at this same leverage, you'd only need about 1/2 of the pedal movement to obtain clamped calipers (two 15/16" cylinders pushing fluid instead of a single one)

 

Thus, I would surmize that the pressure on the pedal now doubles (assuming for a moment that front and rear calipers and MCs are identical) because you've got 2 MCs to push instead of a single one (basically doubling the MC bore cross section).

Now, reducing the size of the MCs so that the volume moved is enough to return the pedal to its normal range of motion will in turn increase the leverage to the calipers (when both MC are compressed).

So, if all this is correct, then I would assume that a reduction in bore size is required to maintain fluid flow and leverage at the OEM characteristics.

 

Going further, the assumption is that 2/3 of the braking done on our cars is by the front brakes, and that 1/3 is done by the rear brakes. This then is used to modify our assumptions above by reducing the size of the front MC bore (when converting from a 15/16" single to 2 smaller dual MCs) to 2/3 its original (15/16") single MC size, and reducing the rear MC to 1/3 of its original (15/16") size. This then leaves the sizes as being 3/4" and 9/16", which is very close to your planned bore sizes. My guess on this set-up then would be a midpoint setting on your balance bar.

[JMortensen]

It's been a while since I've actually seen the inside of a master cylinder, but aren't there 2 primary cups in there, so basically there are two cylinders inline with each other? I'm pretty sure that's the way it works.

 

If you look at the sheet, if you bias the bar all the way to the front, you're still only capable of 1.03g decel. With the masters reversed and the 5/8" in front, the nominal (with the bias adjuster dead center) braking g is 1.18.

[preith]

I have dual Tilton .75†masters with Arizona Zcar’s 12.25†brakes, front and rear, on Hawk Blue pads if it matters. Heh, not a lot of thinking went it to it on my part; I went off an Eprod Zcar build featured in SCCA's Sports Car magazine back in '99 when it was being reintroduced to the Procuction ranks.

 

Anyways, I’ve run the car successfully with this setup on about 20 hours of actual wheel to wheel road racing time. Now, I’m not quite old enough to have any experience with a non brake boosted car, but initially the pedal felt HARD, to say the least. When I first drove it on the street to “shake it down†a bit, I thought “how in the heck am I going to have enough leg muscle to stop this thing on the track?†My first inclination was to step down to 5/8†bores, but I decided to stick with the 3/4's and it’s to the point now where they feel pretty good. Maybe it’s more of a comfort thing? With that said, I still have to treat them like I hate them.

[JMortensen]

Good point preith. There's two different things that happen with the pedal effort. One is the leverage ratio of the pedal, and the other is the size of the master. Unless you know what the ratios are beforehand it's hard to say how soft the pedal will be.

 

One thing I think we can all agree on is that iskone has the duals already. If the pedal is stiff now, it's going to be less stiff with the bigger calipers on. Exactly how much has yet to be seen.

[Guest iskone]

I can't enter any values into the spread sheet from home, so I'll have to wait untill I'm at work.

 

As far as the pedal feel I sometimes feel the same way as preith with my current setup. I'll make the swap with the brakes and just see how it does. New MC's are cheap and can be swapped in less than a day if need be.

 

Something I've seen on other duals is a bar that connects them both. I have no bar to connect the MC's.

 

isk

[JMortensen]

If you have no balance bar then you need to use the middle g rating on the spreadsheet. IMO you should plumb in an adjustable proportioning valve into the rear brakes so that you can get it dialed in a little closer.

[preith]

Iskone, I’m surprised you don’t have a balance bar. If you have Tilton master cylinders, it’s fairly easy to do with the kit they offer. A sleeve needs to be welded to the brake arm. If you, or anyone else is interested, I can try to snap some pics of my setup.

[Guest iskone]

I'll have to look into setting up a balnce bar. Here's a quick pic of my setup. I'm not the one who set it up.

 

 

 

I can't weld aluminum. I'll have to get in there and take a close look at everything and start putting together a parts/to do list.

 

 

Isk

[Guest iskone]

On a side note I called powerstop today and got a qoyte for slotted 300zx rotors. They will have to custom make them for $80 a peice.

 

Isk

[preith]

I can't weld aluminum. I'll have to get in there and take a close look at everything and start putting together a parts/to do list.

Isk

 

You shouldn't have to weld any aluminum. The brake arm is steel, and that's the only thing you need to weld. Girlings I see. I don't have any clue wether the Tilton setup would work, but it's worth looking into. You may have to fab up a new plate depending on how far apart the master cylinders are. The Tilton setup comes standard for 2.5" on center, but you can special order 2.75 and 3".

 

I wonder if you could get away with a proportioning valve instead?

[tube80z]

If you're brakes are pushed on equally by a rod that goes through the brake pedal you might see if you can get a copy of Race and Rally car sourcebook. In it they have a low buck bias adjuster that would be easy to build.

 

Cary