early 944 brakes, stock MC....dosent feel right

[Vdubjim]

i wonder what car that is from?

[MattKab]

First of all Happy new year to all!

Shad's formula is standard textbook mechanical advantage formula. As Shad mentioned in his post, we are fortunate, in that we can use the formula in it's stripped down, simplified version. We can ignore factors like friction, radial differences front to rear, pad area, etc. as these can be considered to be constants

Like Wally, I still have the 3/4" T1 m/c and 40/36mm & 30/28mm Brembos (~15,000miles)

I got to a value of 1.72 to the front and I had 1.23 as the bias of NA 944 rears/T1 front and a T1 m/c.


If we had the 1986 only 944T Front calipers with their 36/34mm pots, we would have a bias of 1.45

This compares very well to the stock NA 944 with the Ate calipers bias of 1.44

Now compare this to the very earliest 911 which had a bias of 1.88 for the first year. This was then changed to 1.6

By lengthy mathematic (plus assumptions) I get the T1 front/CB roto hub and stock m/c to have a bias of 1.38

I put Al B's gold custom Bug (with the cool 'flame' wheels and ALL new parts) on our Arex computer test lane and I recorded stock T1discs/drums and m/c to have a bias of 1.57

I have 'invented' an m/c that can be 'made' for our Brembo 4pot systems that will yield a bias of 1.47...

If I fitted the Kerscher split bore m/c my bias would be down to 1.3

m/c bore has no affect on bias, just the leverage ratio.

Feel free to challenge my values, I still have the pages of arithmetic I used. Plus many possible configurations, feel free to ask.



Matt

[Wally]

Originally Posted by MattKab

m/c bore has no affect on bias, just the leverage ratio.

Feel free to challenge my values, I still have the pages of arithmetic I used. Plus many possible configurations, feel free to ask.

Matt

Hi Matt,
I assume your above statement is a 'misprint' ? Anyway, it doesn't seem to match up with Shad's mathematics, who I value greetly.

I just fitted a 944 23/19mm mastercylinder and hope to inform you all the difference on brake bias as opposed to the stock 19/19mm m/c I had before.
Reason for the switch was my terrible brake bias: too little back-side braking.

Oh, I have now fitted the 23mm to the front, as it is stock in the 944, in order to lessen (!) the brake pressure to the front.

Regards,
Walter

[MattKab]

Wally, your talking about changing to a split bore m/c from a m/c with a straight bore. This is how bias is changed

Ok, edit: m/c bore (when F = R, regardless of dia.) has no affect on bias..)

I too have a lot of faith in Shad's calculations and written word, I bet he too would approximate your new bias to be 1.1..

Is there differences in bore sizes between NA 944 and 944T/951? Can anyone confirm? I've asked before..

My idea is to integrate the rear 15/16" piston from a 16v Golf m/c into a 'newly split bored' 13/16' m/c from the 8v Golf. The larger rear piston will push the front calipers and give a front/rear bias of 1.3:1 Similar to the Kerscher m/c :agree:

Wally, my front tyres are only 95% the diameter of my rears, this will have a significant affect on my bias, bringing it close to the 1.6 of a 1980's 911. Which is a little too high for a Bug perhaps. I can't imagine anymore braking power in my Beetle. But I know there is some more there. My eyes already go

May I suggest you experiment with an adjustable proportioning valve in your rear line. I plan to.

Matt

[Pillow]

>stock 19/19mm m/c I had before.
Reason for the switch was my terrible brake bias: too little back-side braking.<

Could be because that VW MC is designed for drum brakes instead of the discs.

When I converted the '66 Westi to discs all around Russell at Old Speed said they normally had to rebuild a drum MC to run the discs because there is some "feature" in there that pulls the fluid back to the MC instead of keeping a semi constant pressure residual like a disc MC will. Pardon my lack of technical experience here, just paraphrasing someone elses story

I still contend that the 911 19/19 disc/disc MC is the way to go on a Beetle as it is setup very similar in weigh charactaristics and still similar in the disc brake setup (do not quote me on specifics though, but having rebuilt the 911 brake calipers before they are not way different fluid volume wize than 944 units).

Seems to a degree as with anything that certain combos still need tested to determine the perfect conversion MC.

For my IRS build-up (shhh, the pan should be here in two weeks, but do not tell the wife yet) I just bought a 19/19 911 MC to try out with the full 944 single pot setup.... Give me a year or two to post results

[MattKab]

May I apolagise to Wally, I muddled big-time, you have different (44/36) Brembo's to myself (40/36). I never questioned your math. I see exactly the same ratio's.

Matt

[Wally]

Haha, no worries Matt I seem to have the rather rare 944 turbo Cup brakes, which have more resemblence to 928 units (i.e. radially mounted up front).
With your addition (F=R), it makes perfectly sense now. You see how easily we can misunderstand each other when we write things down, while thinking ahead already...

Sorry Pillow, but the fact that the bug m/c was designed for rear drums has nothing to do with my bias problems with the huge Brembos I have now.
I will report when the 03 is on the road again with the 23/19 alu 944 m/c.

Regards,
Walter

[paul_f]

Originally Posted by Pillow

>stock 19/19mm m/c I had before.

Could be because that VW MC is designed for drum brakes instead of the discs.

When I converted the '66 Westi to discs all around Russell at Old Speed said they normally had to rebuild a drum MC to run the discs because there is some "feature" in there that pulls the fluid back to the MC instead of keeping a semi constant pressure residual like a disc MC will. Pardon my lack of technical experience here, just paraphrasing someone elses story

You remembered it the wrong way round!

A drum mc has a residual pressure valve in, of around 10psi, this prevents the springs on the shoes retracting the wheel cylinders all the way in.

On a disc car, if the MC is lower than the calipers, a 2psi residual pressure valve is sold to prevent the brake fluid draining back to the MC and retracting the pads from the discs.

Both of these will cause the first press of the brakes to have excessive travel, if you were to pump the brakes the pedal would rise back up though

Paul

[Pillow]

I stand corrected after doing the math. The standard 944 N/A setup seems to be a good bias as compared with the early 911s 1.60 bias.

Just for the record 911 SCs (20.64 master with 48/42 calipers) had a 1.31 bias BUT used a rear preasure regulator that cut in at 33 bar. From "Porsche 911 Performance Handbook" by Bruce Anderson page 216.

... More to come, but I am thinking a 944 MC (23/20) with 911 "A" caliper front (48mm) and 944 rears (36mm) might work well with a 1.34 bias. Very rear heavy for a streeter though I think..?

Thanks,

[MattKab]

Originally Posted by Vdubjim

i wonder what car that is from?

8v Golf

[Vdubjim]

here is the vw one that looks to be a bolt in.
there is another but the spacing on the mount holes is too far apart
http://germanautoparts.com/Volkswage...it/Brake/137/5

these guys are usually the cheapest around and BEST quality. Normal price is 90-110.00 from what ive seen.