1500 fnt discs & 944 rear discs

[bug-er]

Hey
Im gonna go for the following setup on my bug, 1500 cross drilled front disks with the stock vw calipers and early 944 rear brakes, the ones with the slide calipers. Mainly because the rear 944 brakes have twice the power and are a third of the price of the kits on the market, and the parts are more readily available.
What i want to know and am concerned with is will this setup be ok with the stock master cylinder?? Ive spoken to Michael ghia and he says he knows of several people running this setup, so is anyone on here using this setup? He also mentioned that i may have to use some sort of bias control so as the rears dont over brake the fronts, i really dont wanna do this if i can help it. Any hints from someone who has already done this setup would be great.
any info is appreciated guys
Adam

[Shad Laws]

Hello-

I wouldn't go that route. I'd either use CB Rotohub rear discs (solid discs, 1-pot 34mm calipers), which are dirt cheap (~US$250 a set) with the 1500 fronts, or go for the full 944 (or Kerscher/CSP equivalent) setup. Brake bias will be much better, and spare parts will be cheaper.

The 944 rears have 36mm calipers. With a symmetric master cylinder, this makes the F/R hydraulic leverage ratio with 1500 fronts (40mm calipers) a scant 1.23:1 - way too rear biased. The 34mm CB setup will give you a F/R hydraulic leverage ratio with 1500 fronts of 1.38:1 - much better. Lots of people have done this (including myself) and have confirmed that no proportioning valve is needed.

If you want the cheapest route, I'd really get CB rotohubs and be done. It's less money and less headache than new 944 rotors, pads, and caliper seals (but used hubs/calipers) and an adjustable proportioning valve and the time of fiddling with it. If you want the nice route, go 944 all the way.

Oh, and if you are looking to get by even cheaper, I have a slightly used CB rotohub setup sitting on my shelf. When I unbolted it, I put grease all over the rotor surfaces, so there's no rust... make me an offer :-).

Take care,

[ricola]

I had the set-up you described with the stock master cylinder on my speedster replica while I was finishing the 944 front brakes. It worked fine, probably even better than now as with the light front end of the speedster I need a good rear bias.
The CB kit is much more expensive in the UK, or if you order it from the US you get stuck with import taxes, shipping etc so the 944 works out easier. Although you would have to get new redrilled front discs to match...

Rich

[bug-er]

Originally posted by Shad Laws

The 944 rears have 36mm calipers. With a symmetric master cylinder, this makes the F/R hydraulic leverage ratio with 1500 fronts (40mm calipers) a scant 1.23:1 - way too rear biased. The 34mm CB setup will give you a F/R hydraulic leverage ratio with 1500 fronts of 1.38:1 - much better. Lots of people have done this (including myself) and have confirmed that no proportioning valve is needed.

Hey,
Im not sure how you worked out those ratios but i would like to know, so as i can have a play and find the best master cylinder for the job, i dunno if i could use one of the following mastercylinders Porsches 924S/944 - 23/19mm front/rear, it could even be installed backwards if it had to be. If not it looks like the proprotioning valve is the way to go.
I cannot install the front 944 disks at the moment, as they will push the wheels into the arches, and in order to get them to fit ill have to narrow the front beam.
As rich says above the CB kit soon becomes unviable here in the ik after all the taxes, postage etc have been added to it.
It probably would have been alright on a kit car as they have barely any weight over the front wheels, but on a bug it may cause troubles, dunno what you think rich? But master cylinder wise it would push enough fluid.
cheers guys
Adam

[dgluyas]

I can attest to the stock master cylinder being quite adequate when it comes to running Porsche 944 or 944turbo brakes. I was running the non turbo brakes and they worked great! . Now I have 944Turbo 4 - pots and the stock master still runs it fine. The only difference is with the Turbo brakes you have to bleed them very well,tapping the caliper each time with a rubber mallet to get all the small bubbles off the inside walls of the caliper, otherwise you will end up with a bit too much pedal play. It took a few "bleedings" and now its great !. Remember to start bleeding the caliper furthest away from the master cylinder and work foward.Remember to check the reservoir often,especially with the turbo calipers. They draw so much fluid you can run out quickly and have to start alll over again!So how does it feel? Well, the pedal goes down a couple of inches more but its quite acceptable. The only problem you have as others who have done this conversion will also note , is that the brake lights dont come on as quickly when you are at the traffic lights. Its wierd but you have to press the pedal a little harder than normal to get it happening compared to stock. Having had this setup and tested it extensively I can assure you it brakes very , very well! Its like having progressive power brakes. The bias feels fine to me.

[Shad Laws]

Hello-

Hey,
Im not sure how you worked out those ratios but i would like to know, so as i can have a play and find the best master cylinder for the job,

Hydraulic leverage ratios are simply the ratio of the slave to master cylinder area, which is the ratio of the diameters squared. So, for a T1 front disc w/ T1 master cylinder, that's (40/19.06)^2 = 4.404:1. For a 944 NA rear disc w/ T1 master cylinder, that's (36/19.06)^2 = 3.567:1. For the F/R ratio, just divide those two numbers.

Note that technically-speaking, brake bias is determined by braking _power_, not force (as the above relates to). The difference is the multiplication by the geometric average rotor radius and the wheel speed. But, since these figures are more-or-less the same for front and rear on the cases you described, we can neglect these terms.

i dunno if i could use one of the following mastercylinders Porsches 924S/944 - 23/19mm front/rear, it could even be installed backwards if it had to be.

As-intended (with 23.86mm side on front and 19.06mm side on rear), the 944 MC makes the rear favored more. You definately need the opposite.

Take care,

[Shad Laws]

Hello-

I can attest to the stock master cylinder being quite adequate when it comes to running Porsche 944 or 944turbo brakes. I was running the non turbo brakes and they worked great!

If you work out the leverage ratio there (54mm front caliper size, 36mm rear, 19.06mm MC), you get 2.25:1. That's WAAAY front biased. I'd imagine that the fronts lock quite a bit before the rears are doing much if anything at all.

Take care,

[bug-er]

hey Shad cheers for all the help
got a few more questions for you if you dont mind
-what is the optimum in terms of braking bias ratio???
-theres a guy i found running the following setup
a second hand (23mm/19mm) 944 master cylinder,
At the front: VW T1 calipers - 40mm diameter
At the back: Sierra calipers - 43mm diameter
I'll be running the master cylinder 'backwards' with the 23mm section running the rear calipers.
now ive had a go at the maths and ive got a ratio of 1.26:1
back 43mm dia/23 mc (43/23)^2 = 3.495
front 40mm dia/19 mc (40/19)^2 = 4.432
therefore 4.432/3.495 = 1.26:1
he reports that this setup is really good with no problems at all and he well chuffed with the results
Is i try the same setup i get a ratio of 1.91:1
i dont know the optimum ratio so i could really know what is the best to get as im a little confused
cheers
Adam

[Shad Laws]

Hello-

hey Shad cheers for all the help
got a few more questions for you if you dont mind

No problem!


-what is the optimum in terms of braking bias ratio???

Good question. I dunno! :-)

As you can imagine, it depends on a billion things. And, depending on how accurate you want to be, it'll eventually include a contribution due to caliper rigidity and whatnot, too (1-pot calipers are behind on this one).

The way I estimated what would be correct is by looking at comparable, proven setups. A good example is an early 911, which has 48mm front calipers, 38mm rears, and a 19.06mm MC. So, the overall F/R ratio is 1.60:1. The T1 front disc and CB rear is also another proven combo, with 1.38:1. So, as a good starting point, I'd try to get somewhere in between here. Too high of a ratio and the rears become useless. Too low and your rears lock first and your car is out of control. Too low of a ratio can be helped by a proportioning valve; too high can't. Just some considerations.

The 944 NA setup, as intended with the correct MC, gives 1.44:1.


-theres a guy i found running the following setup
a second hand (23mm/19mm) 944 master cylinder,
At the front: VW T1 calipers - 40mm diameter
At the back: Sierra calipers - 43mm diameter
I'll be running the master cylinder 'backwards' with the 23mm section running the rear calipers.
now ive had a go at the maths and ive got a ratio of 1.26:1
back 43mm dia/23 mc (43/23)^2 = 3.495
front 40mm dia/19 mc (40/19)^2 = 4.432
therefore 4.432/3.495 = 1.26:1

Note that the MC sizes are actually in 1/16"s of an inch, hence the use of sizes like 17.46mm, 19.06mm, 20.64mm, 22.23mm, and 23.81mm. That "23" is actually closer to 24... that makes the ratio seem a little higher :-).


i dont know the optimum ratio

There isn't one :-).

The perfect braking system is the one that just *barely* locks the front wheels before the rear ones in every single situation possible.

One danger with really low ratios (high rear bias) is that it may work for *most* situations, but when you really push it in a funny situation (rain, turning, over a bump, heat-soaked pads, cold pads, etc.), it won't.

Take care,

[bug-er]

cheers again
it looks like im gonna have to run a proportining valve then in order to run the setup i can afford
just out of curiosity i tried to work out the braking bias on a stock bug, i got 3:1 now surely this has got to be way off??
bug drum rear cylinders are stock 19mm cyls and front disks at 40mm, what you make of this shad?? is it a different calculation for drums???
thanks
Adam

[Shad Laws]

Hello-

cheers again
it looks like im gonna have to run a proportining valve then in order to run the setup i can afford

Sounds good!

just out of curiosity i tried to work out the braking bias on a stock bug, i got 3:1 now surely this has got to be way off??
bug drum rear cylinders are stock 19mm cyls and front disks at 40mm, what you make of this shad?? is it a different calculation for drums???

The physics is the same, but the parts we could previously neglect can no longer be neglected...

When you have two disc brake setups with similarly-sized rotors, similar pad materials, similar heat/water/etc. problems, and similar wheel speed (i.e. not a 30" rear wheel and a 10" front one :-), then you can just think about the area ratios. But, when you change one of these substantially, you can no longer neglect the effects of these things.

Drum brakes have a LOT of very big differences compared to discs, so the simple approximations used above are no longer very good approximations.

Take care,

[proudbugowner]

Holy thread revival!!!

I recently came to the conclusion that the 944 stepped MC (23mm/19mm) does NOT affect brake bias, because the secondary piston is moved by the primary piston by pressure and not by force (they are not attached).

I demonstrate this with this schematic of the 944NA system.

[H2OSB]

Hehe, old thread.

I installed N/A 944 brakes front and rear (stock M/C). I simply used Porsche M calipers on the front. With their 48mm pistons and the rear 36mm pistons, I have a bias ratio of 1.7:1. AND, the M calipers bolt on.

[proudbugowner]

Hi John! They have the same PCD with the 54mm sliding units? They also align with the stock 282mm rotor?

Thank you for your input!

[Steve C]

Hi

See if you can find some early Type 3 callipers, they use a bigger piston than Beetles and bolt straight on, you can identify them as they only have single bleed nipple. It might help with your bias.

Steve