Shoptalkforums.com

I hope I can contribute a bit to the topic of master cylinder diameter and caliper piston size. I've read a lot during the past days, and a very interesting topic I found is on germanlook.net, well-known people providing knowledge and real-life feedback: https://www.germanlook.net/forums/showthread.php?t=4595.
Other sources of (useful) information were:
https://www.pelicanparts.com/techarticl ... _calc2.htm
https://www.germanlook.net/forums/showt ... hp?t=11287
https://www.germanlook.net/forums/showthread.php?t=2183

So my conclusion, starting with the current setup of 944NA brakes and stock 19.05mm pump and going to 944T four pot calipers and at first trying with same stock 19.05mm pump, is that master cylinder actually does affect only pedal hardness "feel" and volume of brake fluid displaced, but not the brake bias. I plugged in some data based on the formulas in the topics above, and results seem to confirm the "feeling" part.
1. 944NA brakes (54mm and 36mm slave cylinders) with 19.05mm pump.
(54/19.05)sq=8.0352.
(36/19.05)sq=3.5712
Resulting ratio is 2.25. So huge front bias. Same result if using 23.8mm pump or stepped 23.8/19.05mm pump (explanation why stepped pump is not affecting bias is here: https://www.germanlook.net/forums/showthread.php?t=2183). The stepped pump just increases the volume of brake fluid going to the front slave cylinders and makes the pedal harder. This explains what I saw during "MOT", a high efficiency of front brakes (80%) compared to rear (40-50%), this being correlated to front and rear weight. So I do not see any other option except bias bars (or dual pumps) to overcome this issue.
Pedal feel is great from my point of view, soft and long travel. I hate hard and short pedal.
2. Considering 944T brakes (40/36 front and 30/28 rear) with 19.05mm pump (same would be with any other pump):
((40+36)/19.05)sq=15.9161
((30+28)/19.05)sq=9.2697
Ratio is 1.717, so still front biased, but much less. Now, considering the same piston sizes were used in 930 with a 23.8mm pump (no step), and that the 930 is has a closer static/dynamic weight ratio to Bug (both rear engined), compared to 944 (front engined), it seems these brakes will be much better suited for a Bug, ratio-wise not power-wise. I'll try with the stock 19.05 pump first, I have read a lot of positive feedback on this setup.

This would explain Wallys point of view (feel) that the best combination was KG or Type3 front calipers (42mm) with 944NA rear calipers (36mm). Brake bias would be 1.36, a good number for front biasing based on some people driving similar setups.

So Wally, am I correct, in your opinion, a 17mm MC would give me pedal movement with 4 wheel "kit" disc brakes? Like I stated before (maybe on GL.net), and like TZ, I do not like a stiff pedal with no apparent movement. As I also stated before, I have no issue with brake modulation, I just don't like the pedal feel.

H2OSB

TZepeSH wrote: ↑Mon Aug 27, 2018 10:31 am I'll try with the stock 19.05 pump first,

Is this MC aluminum? Do you have a pic you can post?

No, I'm referring to the stock steel Bug pump.
H2OSB, I don't know if Wally is following this thread, but he is following the thread on Germanlook.net and he responds fast there (first link I inserted).

TZepeSH wrote: ↑Tue Aug 28, 2018 12:26 am No, I'm referring to the stock steel Bug pump.
H2OSB, I don't know if Wally is following this thread, but he is following the thread on Germanlook.net and he responds fast there (first link I inserted).

You're probably right. I meant to post that over there.

H2OSB

EDIT: I am wrong. Ignore everything below here. Im just leaving it for posterity



Ok, sorry about the res yall, but I see this thread linked all of the time, and I need to correct something here.

A dual-circuit master cylinder outputs two different pressures. The entire point of them is to change break bias

A dual MC is set up with a floating piston between the two separate cylinders.



Just to show the different pressure outputs lets assume we are at steady state (piston no longer moving, just generating more or less force) and that the brake line ports are plugged. Assuming this piston is not connected by a bar and the spring force is minimal when breaking (this is the case for all but a few exceptions in small package track cars), the pressure in the primary circuit (P1) is going to be the input force (Fin) divided by the plunger area (A1). Without any solid connection or added force between the two channels, Fin must be equal to the force the fluid in the first circuit applies to the floating piston (F1) must be equal to Fin. Secondly, the force the floating piston applies to the second circuit (F2) must be equal to the force applied by the fluid in the primary circuit. This force (F2) is equal to the pressure (P2) in the piston multiplied by the area applying the pressure (A2). Therefor, if the area of the two circuits are not identical, the pressure within them will be different as well

I ended up getting some tight 90 degree master fluid feeds and bushing from a Ducati dealership, as a inner tie rod zerk fitting had cracked my stock front feed fitting over the years. They offer two sizes of bushings with the plastic 90s, there are probably cheaper options but they were the first I found. (brembo part)

With the exception of the 23/19 split 944 masters, all the cylinders discussed so far have straight bores, so the primary ways to change bias are:
(in no real order)
a)wheel cylinder/caliper piston size
b)different friction material
c)tire diameter (huge knob)
d)rotor effective diameter
e)proportioning valve (pressure limiting)

I have been looking into adapting a rear ABS (rabs) setup off a late 80s-90s truck, but the valves used apparently had issues, and age and being rebuilt endlessly haven't helped matters...

The 1st gen Ford units apparently worked by dumping pressure to a spring loaded secondary reservoir/piston, which returned the fluid when the pressure was released at the master.

Using a MS3 or such with custom firmware and a more modern ABS pump setup would likely be a better idea.

hmmm... haldex makes pretty decent gear... MBI AQ960514 ...its for trucks, but it might be a good start.

eskamobob1, I agree partly to your argumentation. But what you have drawn is exactly what it is NOT in the stepped pump (the pistons of different areas are NOT hard linked together). The forces and pressures you computed are correct however if the secondary piston would look like that. But the inside of the pump looks like in the picture from the link above. Meaning that the force applied to the second piston is NOT the input pedal force, but it is derived from the pressure of the first: pressure in first chamber (P1) is Fin/d1 (say 400N and 23mm as in the picture), and the force on the second piston is P1*D2 (19mm for second piston). Thus the pressure in the second circuit is the same as in the first circuit. Your idea would work if the pump would look like below inside, but as far as I know, it dies not...

Im sorry it took me so long to get back to you. Upon further research you are dead right. The diagrams I was looking at that I thought was as jointed step was actualy a quick uptake system. It does look like Farrari and Fiat have done dual pressure output MCs in the past, but thats realy about it and they are very far from the norm. I edited my OP to reflect my poor assumption as well. tyvm for the detailed response btw