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

[Wally]

Jeremy,
That old post of Shad is indeed very informative and gives us the possibility to put some ratio numbers to our 'feel'.

There is one thing that doesn't seem to add up with Shad's calculations though:
Using his front/rear bias ratio calculation, a bigger front cup diameter in your m/c, would yield a more rear biased set-up!?
To stay with the title of this thread: 'that doesn't seem right'

I can't figure this out yet as to why or what I am missing here...?

BTW, I'am running now the N/A 944 rear disks on my square (has stock 42mm front cup caliper diameter), which would give a F/R ratio of 1,36.
It brakes great, but feels just not as powerfull as my former 1303 with the stock 40mm front cup caliper diameter disk and rear 944 N/A, which gave a 1,23 F/R ratio, but that 1.23 ratio may be on the edge in wet conditions.
Because of the square d², a little variation in diameter gives quite a different result in the ratio!

All this leads me to believe that between 1.30 and 1.35 should be about 'right'.

[Jeza]

Originally Posted by Wally

There is one thing that doesn't seem to add up with Shad's calculations though:
Using his front/rear bias ratio calculation, a bigger front cup diameter in your m/c, would yield a more rear biased set-up!?
To stay with the title of this thread: 'that doesn't seem right'

I can't figure this out yet as to why or what I am missing here...?

It a leverage thing, if you increase the diameter of the slave cylinder in relation to the mc you increase the power to the brake. Increasing the MC diameter will increase the hardness of the pedal but not the power to the brake. Jeff Hibbard has a section in his book Baja Bugs and Buggies that explains it well without getting too technical. The most common example is those that increase the size of the slave cylinder in the rear drums / fit T3 drums to increase the braking.

Originally Posted by Wally

BTW, I'am running now the N/A 944 rear disks on my square (has stock 42mm front cup caliper diameter), which would give a F/R ratio of 1,36.
It brakes great, but feels just not as powerfull as my former 1303 with the stock 40mm front cup caliper diameter disk and rear 944 N/A, which gave a 1,23 F/R ratio, but that 1.23 ratio may be on the edge in wet conditions.
Because of the square d², a little variation in diameter gives quite a different result in the ratio!

All this leads me to believe that between 1.30 and 1.35 should be about 'right'.

What you have now is probably a very good ratio for a rear engined car.

I also ran my beetle with the stock fronts and 944 rears (stock mc) and it brake quite unusually being so rear biased. I imagine you would get away with this most of the time, but my main concern is the differing way they would react when pushed, ie the rears would remain very cool and responsive while the fronts would get very hot and possibly fade (non vented rotor and small pad area in comparison to the rear).

The 944 brakes with the 944 MC gives a ratio of 1.41, which I reckon will probably work quite nicely.

Cheers
Jeremy

PS have you done brake performance comparisons or is it just a feel thing when you mention your 1303's powerful brakes?

[Wally]

Originally Posted by Jeza

It a leverage thing, if you increase the diameter of the slave cylinder in relation to the mc you increase the power to the brake.

I agree; it also complies with the formula Shad used.

Increasing the MC diameter will increase the hardness of the pedal but not the power to the brake.

Now that doesn't comply with the leverage formule from Shad!. Tho I think you are very right about the hardness aspect of the pedal thingy.

What you have now is probably a very good ratio for a rear engined car.

Yeah, I agree. Probably safer also in extreme situations than an even lower ratio.

The 944 brakes with the 944 MC gives a ratio of 1.41, which I reckon will probably work quite nicely.

Cheers
Jeremy

Yes, I agree, but the mean reason for the calculated relatively low ratio (for a front engined 944) is that the 23mm (IIRC its 21mm, not 23 mm like in the thread where Shad chimes in, but lets use 23mm for calculations sake) front m/c cup diameter is in the lower part of the equation, thus decreasing the slave/master ratio of the front brakes!!!!!!!
i.e. : 944 fronts:
(54mm caliper cup diameter / 23 mm front m/c cup diameter)²= 5.51
944 rears:
(36mm rear caliper cup / 19.06 mm rear m/c cup diameter)²= 3.57
F/R ratio would then be 1.54
With a 21/19 m/c the ratio would be even higher at (54/21)² / (36/19)² = 1.85.
THATS the point I a trying to make in this equation:
The bigger the front cup of the m/c, the more REAR biased the set-up gets!!!!

Therefore, 180 degrees from my beliefs before, to get a more rear biased set-up from the stock front setten-up 944 brake system, you DO NOT install the stepped m/c the other way around: i.e. 19mm side to the front and the 21/23mm side to the back. That would worsen the problem.
The above holds only true of course IF Shads formula is correct (and 'knewing' Shad, he is quite an educated guy (even then), so I must really believe him)

Jeremy, I am realy curious what your or others thoughts are about my above 'conclusion'. True or false?

This would also explain the enormous front biased'ness' of my current 944 turbo Cup brake set-up on my 1303 SB with the stock 19/19 m/c ! I think I need the biggest step m/c (23/19?) I can find and maybe additionally a bit smaller front calipers or a front propotioning valve (if possible).

PS have you done brake performance comparisons or is it just a feel thing when you mention your 1303's powerful brakes?

No, just a feel thing, but when I drove that car every day back then, I was very aware of any change, even the very subtle ones and this wasn't subtle, but a very, very strong brake action. You can only draw the conclusion that stock bugs are set-up very very conservative with rear break bias.

Greetings,
Walter

[Jeza]

Originally Posted by Wally

The bigger the front cup of the m/c, the more REAR biased the set-up gets!!!!

Therefore, 180 degrees from my beliefs before, to get a more rear biased set-up from the stock front setten-up 944 brake system, you DO NOT install the stepped m/c the other way around: i.e. 19mm side to the front and the 21/23mm side to the back. That would worsen the problem.
The above holds only true of course IF Shads formula is correct (and 'knewing' Shad, he is quite an educated guy (even then), so I must really believe him)

Jeremy, I am realy curious what your or others thoughts are about my above 'conclusion'. True or false?

Walter
You have hit the nail on the head. Now all we need to do is to spread the word, as I think this is where a lot of people get confused and go wrong.

My 944 MC however is definately a 23mm one as it is cast into the body. Actually 23.86 as they are imperial measures which should give me a good front to rear bias (although I don't know where I got the figure above?)

Originally Posted by Wally

This would also explain the enormous front biased'ness' of my current 944 turbo Cup brake set-up on my 1303 SB with the stock 19/19 m/c ! I think I need the biggest step m/c (23/19?) I can find and maybe additionally a bit smaller front calipers or a front propotioning valve (if possible).

Steve C was chatting to someone on the shoptalkforums a while ago and the comment that came up was that sometimes the science can be taken too far. Sometimes feel / and a few test emergancy stops is the best way to go. To get things perfect you may need one of these
http://www.wilwood.com/products/Peda...fmbp/index.asp

Originally Posted by Wally

No, just a feel thing, but when I drove that car every day back then, I was very aware of any change, even the very subtle ones and this wasn't subtle, but a very, very strong brake action.

It would be good to compare a few test emergancy stops to see how the different setups compare.

Cheers
Jeremy

[Wally]

Originally Posted by Jeza

It would be good to compare a few test emergancy stops to see how the different setups compare.

Cheers
Jeremy

Glad you agree. It all makes more sense now! Need to get a 23/19 m/c for the 03

Yeah, I did several emerency stops with my car then, both dry and in the wet and the fronts always (just) locked up first!
A friend of mine with exactly the same set-up (stock VW front disks, 944 n/a rear and stock m/c) but in a 1300 ('68) model and a 2,7 ltr type 4 engine took it to the circuit and reported that on braking hard and steering into a corner, the rear did lock up first.
Now that is a very extreme situation, but shows it really is depending what you do with the car that dictates how you want to set it up!

Thanks Jeremy,
Walter

[Wally]

Originally Posted by Wally

Need to get a 23/19 m/c for the 03

Just got out and measured the cup sizes of my 944 turbo Cup brakes:
Front: 44,0 mm and 36,0 mm (radial mounted 928S alike)
Back: 30,0 and 28,0 mm

With a 944 n/a 23,81/19,05 (which is even more accurate if the bores are indeed 15/16" and 12/16"), that would give a ratio of 1,23 which is low, but similar to my old 03 which had 1,26 and braked like a boat achor
Already saw one on ebay

Greetings,
Walter

[LOAF]

As I had mentioned earlier, I am leaning on using the Wilwood product line, instead of the 944 MC.

They have a 1-1/16" Tandem Master Cylinder Part # 260-4893

Dimensions and specifications for this master cylinder are as follows:
Piston Diameter 1-1/16 inch, which is equal to 26.98 or 27mm

Fluid Displacement 1.20 Cubic Inches
Stroke Movement 1.35 inch

Bias Split 69% Front, 31% Rear

Given these dimensions, can anyone “guesstimate” if it would be in the ballpark. I would imagine using a proportioning valve for the front. But I am still learning..
Based on earlier readings I would think that the stroke and amount of fluid would be enough to move the 944 N/A single calipers in the front and back.

Thoughts, or comments appreciated?
But based on the front to rear ratio, it would seem like to high 2.2:1.
Alex

[Jeza]

Originally Posted by LOAF

As I had mentioned earlier, I am leaning on using the Wilwood product line, instead of the 944 MC.

Good idea.

Originally Posted by LOAF

Based on earlier readings I would think that the stroke and amount of fluid would be enough to move the 944 N/A single calipers in the front and back.

It should be plenty.

Originally Posted by LOAF

Thoughts, or comments appreciated?
But based on the front to rear ratio, it would seem like to high 2.2:1.
Alex

Sorry I can follow Shads calculations and use them on my car but thats where my knowledge stops. As you say the ration front to rear comes out as very front biased, but I don't know how the % bias affects that ratio. I suspect you may be better choosing an option with an adjustable bias bar. This would be a better option than fitting a proportioning valve.

Walter did you do the calculations of your current cup set up (with the 19mm Bug MC) as a comparison?

Cheers
Jeremy

[LOAF]

Jeremy..

What is an adjustable Bias Bar..

I assume that this is something built into the MC..

Why is this better than a proportioning valve.. Please explain..

I think I may know why, might have an idea, but in the interest of not sounding to silly please explain..

Thanks
ALex

[Wally]

Originally Posted by Jeza

Walter did you do the calculations of your current cup set up (with the 19mm Bug MC) as a comparison?

Cheers
Jeremy

Yes, I did: that is now a whopping 1.92 F/R ratio !!
As said before, this does explain my current enormous front bias. I hardly wear off the light rust on the rear disk!

Greets,
Walter