master cylinder for stock front disc + empi rear discs

[mabbo]

hello people,

i was going to upgrade to 944 brakes all around, but the guy i was buying the parts off has let me down so ill be sticking to what i have for the moment.

i have stock vw front discs/calipers with the empi rear disc kit. its a single slider caliper.

i fitted a new duel circuit vw master cylinder but i couldnt get a good pedal. i think the master cylinder may have been faulty. the pedal was firm but went all the way to the bulkhead. (with rear flexis blocked off the pedal felt great)

i fitted my mates 944 master cylinder (which he will need back soon) with the 19mm to the rear calipers and the bigger bore to the front calipers.

now the pedal is really firm, but i have to press the pedal really hard to get the brakes 2 stop quickly. my old stock front caliper/rear drum setup with stock master cylinder worked better :(

should i get another vw master cylinder or try something else?

the others i have considered are: audi 80/audi 100/csp upgraded one (around 20mm bore i belive)/stick with 944

what do you suggest?

cheers guys
mabbo

[evilC]

Firstly what size is the Empi rear piston? The front if a 944 sliding caliper should be 53.5mm piston. The standard 19/19 m/c should be well up to the job if the bias front and rear are within reasonable limits . There should be no need to play around with m/c's and I wouldn't even speculate as to what will work without first assessing the front/rear bias.
Ideally, because of the rear weight bias the starting point is equal braking front and rear. This can then be adjusted to suit preferences and specific car mods.

evilC

[mabbo]

dude
36mm i think. the front bug calipers are 40mm if i remember right?

[flat]

I had the same problem with my stock front disks and empi rears. Just couldn't get things right. So I did the best thing, got rid of them and put on the Boxsters...

But, this should solve most of your probs till you can spend the bread for the Porkers:
http://www.cip1.ca/ProductDetails.asp?ProductCode=C13%2D16%2D9554

The 944 MC on the stock disk/empi setup will be horrible. The pedal effort will be huge! Unless you really need it I keep the master as small as possible to improve line pressure. (I'm using a stock 19mm on my Boxster S brakes with amazing results).

Lanner

[C4 Metal Werks]

Someone please correct me if I am wrong here. The later Bay Window Buses and Vanagons both use a large master cylinder that the SB's and I think you could bolt on up if you had http://www.wolfgangint.com/Parts/Master+Cylinder+Adapter+Kit/

On some of the European parts websites there are pictures of Vanagon looking master cylinder.

[flat]

Yeah, but at 24mm the bus MC is way too big..... You'd need quads like Schwarzenegger to stop the beast.

[C4 Metal Werks]

Even a 71 bus with front discs and a brake booster uses a 26mm master cylinder? I'm pretty sure that is what I used on my 65 bus when I converted it to 4 wheels discs.

Yeah, but at 24mm the bus MC is way too big..... You'd need quads like Schwarzenegger to stop the beast.

[Steve C]

Hi

I'm running early Type 3 front on my daily driver bug, they have 42 mm pistons and on the rear I have some 944 rotors with 2 spot Brembos from some sort of 911, these also have 42 mm pistons, the stock 19 mm MC works fine, the back end is very slightly over braked, I'm going to fit a Tilton proportioning valve to the rear circuit to correct this.

Steve

[evilC]

If you've got 40mm dual pot calipers to the front and single 36mm to the rear then the front/rear bias will be 2.47:1. That is too much front bias. With standard 944 single sliding pot calipers the front bias is 2.2:1 that can be moderated by the 944 24/19 stepped caliper that gives a 1.38:1 bias with the 944 caliper and 1.55:1 with the VW caliper. The underlining problem is that the rear caliper is only a single sliding unit of limited piston diameter. Change that to something better (say a 2 pot caliper) and your problem will be on the way to solution. Don't get hooked up on fluid volumes as the 19mm piston size in the standard dual cylinder should be more than man enough to move sufficient fluid to take up the slack before pad goes solid - assuming of course that there is no air in the system and the discs/rotors are within tolerence?? Also make sure that the front calipers are plumbed up to the rear m/c piston that is closest to the pedal in all cases.
Also, don't worry about over braking the rear as it is easy to install a brake proportioning valve to limit rear line pressure but just make sure you don't drive it without the valve as the vehicle will swap ends in the first panic stop.

evilC

[flat]

Even a 71 bus with front discs and a brake booster uses a 26mm master cylinder? I'm pretty sure that is what I used on my 65 bus when I converted it to 4 wheels discs.

The late bays use a 24mm dual circuit MC. The pedal ratio of an early bus allows the use of a larger MC without drastically increasing the pedal effort. A bug has less pedal ratio and hence you need more pedal effort.

For the record, I've got a 4 corner C2 (1991) setup on my 65 bus with a early 944 stepped MC (23/19), and I'm heavily leaning towards and early 911 (20.6mm) MC to increase line pressure and decrease pedal effort. Although I do admit that my pedal is very reasonable and it's doesn't take much foot to make it stop, but I'm sure it can be better.

Lanner

[mabbo]

thanks for the replys guys :)

well, im not 100% sure that the bug caliper has 2x 40mm pistons, im sure i read that online somewhere.

im quite suprised how front biased the brakes are with the stock front vw disc/caliper setup and a empi rear kit. when i couldnt get a firm pedal with my stock duel circuit master cylinder i thought it must be the problem because empi wouldnt sell so many rear disc kits if they havent been designed to work on a near to stock bug... guess i was wrong.

so, to improve the bias, im going to have to improve my rear calipers... again?

i thought the early 944 rear calipers also had 36mm sliding calipers on the back, with big (54mm?) sliding calipers on the front? so the rear 944 caliper and rear empi caliper are v.similar? what if i upgraded my front calipers to something bigger? maybe a 4 piston wilwood or something then keep the 944 master cylinder?

sorry if that goes against what you have just said - im really not all that clued up on brakes or braking systems - hence my posts :)

mabbo

[evilC]

Yes, the Empi 36mm single pot rear is very similar to the 944 rear that is also 36mm single pot. Simply transposing the 944 set up into a bug is not the way to go as the 944 is a heavy front engined car that needs a strong front brake bias. Even though it is reasonably balanced front to rear ~50/50 with the rear transaxle, under braking there is a substantial weight transfer, needing all the effort to the front. The bug however, is tail heavy ~37/63 and requires a 50/50 braking bias. Originally the 944 had a 19/19 m/c that gave a front bias of 68.75/31.25 but this was later moderated back to 59/41 with the stepped 23/19 m/c.
To make life simple for you to select your brake package lets assume that the ideal brake bias for the bug is still 50/50 then:
Total Front Piston Area/master cylinder piston area = Total Rear Piston Area/master cylinder piston area.
As previously said, if the rear is larger than the front then that is easy to deal with by incorporating a pressure limiting valve in the rear line.

evilC

[mabbo]

hey dude.

thanks for that. how does that calculation work with twin or 4 piston calipers compared to single sliders?

i have just worked this out but i dunno if its right as the front calipers are 2 piston, and the rears single piston:

944 master cylinder:
40/23 = 36/19
1.74 = 1.89

bug master cylinder:
40/19 = 36/19
2.11 = 1.89

my maths is rubbish, i dunno how to change that to a ratio :S

mabbo

[mabbo]

i have just been playing around with the excel calculator Angelo Amato posted on another thread:

http://myweb.tiscali.co.uk/angeloamato/BIAS.xls

with 40mm front, 36mm rear and 19.06 master cylinder = ratio 1.235 to front
with 40mm front, 36mm rear and 23.81/19.06 master cylinder = ratio 0.791 to front
with 53.5mm front, 36mm rear and 23.81/19.06 master cylinder = ratio 1.415 to front
with 53.5mm front, 36mm rear and 19.06 master cylinder = ratio 2.209 to front.

what sort of ratio is acceptable? i assume from the list above 1.235 is the best, so maybe a stock 19.06/19.06 master cylinder would be best, or, the 20.64/20.64 would give the same ratio with a firmer pedal.

am i on the right track?
mabbo

[evilC]

hey dude.

thanks for that. how does that calculation work with twin or 4 piston calipers compared to single sliders?

i have just worked this out but i dunno if its right as the front calipers are 2 piston, and the rears single piston:

944 master cylinder:
40/23 = 36/19
1.74 = 1.89

bug master cylinder:
40/19 = 36/19
2.11 = 1.89

my maths is rubbish, i dunno how to change that to a ratio :S

mabbo

The calculation requires the TOTAL piston area for the caliper(s) it will therefore take care of the single pot, 2, 4, 6 or even 8 pot calipers, even those with different sized pistons in the same caliper.

Your maths is rubbish:) You have factored in the diameter not the area . you can discount pi in the calculation as it is a comparison and features on both sides of the equation therefore your comparison should read:

944 master cylinder
4 (2 pistons each side) x 1600 (40 squared)/529 (23 squared) = 12.10
2 (1 piston each side) x 1296 (36 squared)/361 (19 squared) = 7.18
Therefore the front rear ratio is 12.1/7.18:1
i.e 1.69:1

bug m/c
4 x 1600/361 = 17.7
12 x 296/361 = 7.18
Therefore the ratio is 17.7/7.18:1
i.e. 2.47:1

Joe Amato's figures are wrong insofar as they don't take into account the total piston area of the caliper. The hydraulic fluid is acting on both pistons simultaneously of a 2 pot caliper

evilC

[mabbo]

dude

cheers for that :)

so, what sort of ratio should i be aiming for?

would you just stick with the stock bug master cylinder?

its a street car, there are loadsa people using the empi kit with stock master cylinder id imagine - the bias must be acceptable?
if the reason my pedal was 2 soft wasnt because of the 19mm bore (im sure it wasnt), it must have been because of a faulty master cylinder...

mabbo

[mabbo]

just a thought... eventually i was considering upgrading to some wilwood 4 piston calipers front and rear. i could do this sooner rather than later if i could use a bug master cylinder with them successfully, as it would save buying the uprated CSP master cylinder and fitting kit.

the calipers i want have the choice between piston diameters of 1.00" (25.4mm), 1.25" (31.8mm) or 1.38" (35.1mm) for the fronts, and the rear handbrake caliper has 1.00" (25.4mm) pistons.

if i used a stock bug 19/19 master cylinder, to keep the bias right, would you suggest something like the 1.25 for the fronts, and 1.00s for the rear, or the same front and rear then make adjustments using a regulator valve?

mabbo

[evilC]

dude

cheers for that :)

so, what sort of ratio should i be aiming for?

would you just stick with the stock bug master cylinder?

its a street car, there are loadsa people using the empi kit with stock master cylinder id imagine - the bias must be acceptable?
if the reason my pedal was 2 soft wasnt because of the 19mm bore (im sure it wasnt), it must have been because of a faulty master cylinder...

mabbo

I would aim for a caliper/mc ratio of around 8:1 for both front and rear although as previously said the rear can be larger because the pressure can be adjusted with a proportioning valve.

I would stick with a standard bug m/c but make sure its all working 100%.

In a panic situation locking up the front brakes is infinitely preferable to locking up the rear so the Empi kit is intrinsically safe. However, you can't get maximum redardation because the rear never comes up to full braking force 'cos the front locked up long ago. Ideally, maximum stopping power will be acheived with both wheels on the verge of locking up. Using a balance bar arrangement is the best way to dial in the front/rear proportions.

just a thought... eventually i was considering upgrading to some wilwood 4 piston calipers front and rear. i could do this sooner rather than later if i could use a bug master cylinder with them successfully, as it would save buying the uprated CSP master cylinder and fitting kit.

the calipers i want have the choice between piston diameters of 1.00" (25.4mm), 1.25" (31.8mm) or 1.38" (35.1mm) for the fronts, and the rear handbrake caliper has 1.00" (25.4mm) pistons.

if i used a stock bug 19/19 master cylinder, to keep the bias right, would you suggest something like the 1.25 for the fronts, and 1.00s for the rear, or the same front and rear then make adjustments using a regulator valve?

mabbo

Choosing the 4 pot wilwood calipers, I would go for equal sized units front and rear assuming that your discs (rotors) are of similar diameter? However, I would get the best out of your present system unless there's a fashion statement being made here. With your set up you could purchase/fabricate a balance bar unit that would allow you to adjust the braking effort front and rear to maximise the effectiveness of your current system. If after that you must have such braking force as provided by the Wilwoods then you will also have the best system to dial the Wilwoods in. A standard brake proportioning valve is a relatively crude unit that just limits the rear line pressure but doesn't apprortion the line pressure front to rear as will a balance bar set up.

I often see brake systems in bugs that are hugely over spec'd and more appropriate to very fast and heavy track racing cars. Most of these installations have poured money into just one end of the braking system without considering the whole (mainly I think that 'cos most of it is not visible).

evilC

[mabbo]

cheers for that mate.

good news that the stock master cylinder should be fine for what i have, and work ok with the wilwoods should i choose to upgrade at a later date.

i have had a quick look at the balance bar setups and to be honest - im not sure if id fancy undertaking that on my bug. its a street car and im struggling with the brake theory as it is, never mind with a balance bar - they look pretty hardcore!

the reason for considering the wilwoods was that they fit the discs i already have (stock bug fronts, empi kit rears... both similar size) and i thought they would be relitevely easy to make brackets for, and i could keep my 15" 4 stud wheels... with the added bonus of improved braking should i ever need it over what i have at the min. once my setup is working as it should with a better suited master cylinder i may be happy with them.

thanks for your help
mabbo

[Angelo Amato]

interesting read,

my maths too are bad :rolleyes: maybe i should stay away from formulas !

Angelo

[Wally]

interesting read,

my maths too are bad :rolleyes: maybe i should stay away from formulas !

Angelo

Which is agood idea, as there are too many variables!

I tried reasoning with it too and it is totally useless trying to calculate it, although I agree with most of EvilC's post ;-)
Just try out one combo and change according to your experience. Works much better for me.

I now have 993 rear brakes (30/34 cups) at the rear and GT2 rear brakes (330x28 with 28/30 cups) at the front with stock MC.
While the cups are much BIGGER at the rear, I still have quite a noticeble front bias as the front still locks up (too) easy and I have really the most front weight beetle of most here...
How you explain that?
So, just try and adjust, try and adjust ;)

[flat]

I tried reasoning with it too and it is totally useless trying to calculate it,

Good point Wally, I've come to the same conclusion. On paper it shouldn't work, or the bias should be way off but in practice it works great!

Also, when working out a brake system, don't get tied up on the hydraulic bias calculations because overall you are concerned with brake torque. There is a hydraulic component to the brake toque calculation, but there are additional variables like moment arm (disk diameter) and the friction coefficient of the pad.

I've got the Boxster S brakes (318x30, 40/36 F and 298x24, 30/28R...IIRC) with a stock 19mm dual circuit and I do have a slight front bias. Works great, I don't think I could improve this setup with aftermarket MC's...

Lanner