993 rear calipers to 74 911 front struts??

[ALXinNZ]

Hi

Can anyone help with this one?

Will this actually work????!!

I have just fitted 993 rear calipers to the front of my '74 911 project. I haven't used them yet, it was just a crazy idea I had and so far it has been too easy!!! Am I missing something?

i.e.

I can run my 15 inch wheels no problems as they clear the calipers.
I didn't need to change rotors or machine them.
They line up correctly (centre) with the disc rotor.
And the location of earlier front calipers (in front of strut) is perfect!

I do have brake bias adjustment fitted.

All that I had to do was machine the mounting holes with a 2.5mm off set and make some spacers to suit and fit new brake hose fittings.

Like I said, TOO EASY!!

Any ideas? All comments welcome...

Cheers
Alx

[KTL]

You'll need that bias adjustment. What you've effectively done by using the 993 rear calipers on the front is a reduction of the net piston area at the front.

Front '74 S calipers with 48mm pistons = ~14,476 sq. mm
993 rear calipers with 30/34 mm pistons = ~12,918 sq. mm

Pad area for the calipers:

Front '74 S calipers = 78 sq. cm
993 rears = 86 sq. cm

You've increased the pad area, but it has little effect on the bias (friction is a function force and coefficient of friction, not the surface area).

So your brake setup as it sits right now is going to be more rear biased than original. You should also notice an increase in pedal effort for high speed, heavy braking since you've reduced the net piston area which reduces the amount of force applied by the caliper to the pad & rotor, given the same pedal force. In order to stop the car in the same distance as the original calipers, you have to press the pedal harder.

[DW SD]

Remember that one major issue of a brake upgrade is the ability to absorb additional heat. This is accomlished through thermal mass. Most thermal mass can be attributed to the rotors themselves. If you are not upgrading to larger rotors then you are likely not offering yourself an upgrade. Assuming Kevin's #s are right, you may be downgrading. At the least, you are throwing off your bias.
If the car locks up its wheels then you have enough braking torque. Not to oversimplify, but from there, it is mostly a matter of heat management (rotor and caliper cooling) and thermal mass itself.

Doug

[ALXinNZ]

Thanks for you time and effort
I will have to come up with something else for the front end.
Excellent to know whats going on before I get out on the street...

Regards

alx

[911pcars]

Originally Posted by KTL

You'll need that bias adjustment. What you've effectively done by using the 993 rear calipers on the front is a reduction of the net piston area at the front.

Front '74 S calipers with 48mm pistons = ~14,476 sq. mm
993 rear calipers with 30/34 mm pistons = ~12,918 sq. mm

Pad area for the calipers:

Front '74 S calipers = 78 sq. cm
993 rears = 86 sq. cm

You've increased the pad area, but it has little effect on the bias (friction is a function force and coefficient of friction, not the surface area).

So your brake setup as it sits right now is going to be more rear biased than original. You should also notice an increase in pedal effort for high speed, heavy braking since you've reduced the net piston area which reduces the amount of force applied by the caliper to the pad & rotor, given the same pedal force. In order to stop the car in the same distance as the original calipers, you have to press the pedal harder.

Hmm. These are my calculations of piston area using: A = Pi x (RxR)

pre-'84 front caliper with 2 x 48mm pistons
Area = pi x r2 = 3.1415 x (24x24) = 1809.56 x 2 pistons = 3,619.12 sq. mm

993 rear caliper with 4 x 30/34mm pistons
Area = pi x r2 = 3.1415 x (15x15) = 706.86 x 2 = 1413.72 sq. mm
Area = pi x r2 = 3.1415 x (17x17) = 907.92 x 2 = 1815.84 sq. mm
1413.72 +1815.84 = 3229.56 sq. mm
----------
944T front caliper with 4 x 30/36 pistons
Area = pi x r2 = 3.1415 x (15x15) = 706.86 x 2 = 1413.72 sq. mm
Area = pi x r2 = 3.1415 x (18x18) = 1017.88 x 2 = 2035.76 sq. mm
1413.72 + 2035.76 = 3449.48 sq. mm
------------
944T front caliper with 4 x 36/38mm pistons
Area = pi x r2 = 3.1415 x (18x18) = 1017.88 x 2 = 2035.76 sq. mm
Area = pi x r2 = 3.1415 x (19x19) = 1134.11 x 2 = 2268.22 sq. mm
2035.76 + 2268.22 = 4303.98 sq. mm
----------
944T front caliper with 4 x 36/40mm pistons
Area = pi x r2 = 3.1417 x (18x18) = 1017.91 x 2 = 2035.82 sq. mm
Area = pi x r2 = 3.1417 x (20x20) = 1256.68 x 2 = 2513.36 sq. mm
2035.82 + 2513.36 = 4549.18 sq. mm
--------
944T front caliper with 4 x 36/42mm pistons
Area = pi x r2 = 3.1415 x (18x18) = 1017.88 x 2 = 2035.76 sq. mm
Area = pi x r2 = 3.1415 x (21x21) = 1385.40 x 2 = 2770.80 sq. mm
2035.76 + 2770.80 = 4806.56 sq. mm
-------
There's another 944T version with 36/44mm pistons.
Area = pi x r2 = 3.1415 x (18x18) = 1017.88 x 2 = 2035.76 sq. mm
Area = pi x r2 = 3.1415 x (22x22) = 1520.49 x 2 = 3040.97 sq. mm
2035.82 + 3040.97 sq. mm = 5076.79 sq. mm

The 993 rears w/30/34 pistons, when used in the front will produce a rear brake bias.

The 944T caliper w/30/36 pistons is fairly close to the factory 2-piston caliper. The other 944T versions will create a slight front bias and may need a larger master cylinder for the increased fluid volume. I believe all will fit under 15 or 16" factory Fuchs wheels. All of the above are, of course, relative to the rear caliper pistons. As most suggest, try to keep the same front/rear bias when upgrading either or both ends of the car brakes.

Hope this helps,
Sherwood

(36/42mm caliper calc added)

[KTL]

Thanks for the correction. I used ΠD^2 instead of r^2. Despite my math error, the result is still the same- the piston area of the stock front calipers is 12% greater than the 993 rears. These 993 calipers used on the front do not make the car rear biased, but do give up frontal clamping force which therefore lowers the hydraulic bias ratio (HBR). Stock 1974 HBR front-to-rear is 1.596 (front piston area/rear piston area). The "fronted" 993 rear calipers still have more piston area (3230 sq. mm) than the stock rears (2x38mm pistons = 2268 sq. mm) and therefore the HBR = 1.424 which is still front biased.

Thanks again for the correction- I hate making math errors. Please note there is no such caliper as a 30/36 basic Brembo front that was produced for the 944T. There is however a 30/36 "S4" big red type Brembo caliper that was used on the 993 RS for the rear. These calipers are very rare and desirable. Bill Verburg would have a fit if someone wanted to use them on the front!

As always, must give credit to Bill V. for his always handy charting of 911 brake applications and technical data

http://www.pelicanparts.com/MotorCity/wmv/brakes.htm

[911pcars]

Hi Kevin,
Thanks for the corrections and add'l info. The caliper information I included was from previously contributed tech data.

VCI uses a 944T caliper with 36/42mm pistons as part of their front/rear upgrade kit. Thus, to add to the various 944T front caliper specs:

944T front caliper with 4 x 36/42mm pistons
Area = pi x r2 = 3.1415 x (18x18) = 1017.88 x 2 = 2035.76 sq. mm
Area = pi x r2 = 3.1415 x (21x21) = 1385.40 x 2 = 2770.80 sq. mm
2035.76 + 2770.80 = 4806.56 sq. mm

While there appears to be quite a few variations of the 944T front caliper, I'm not sure this is a definitive list.

Sherwood

[Bill Verburg]

the only calipers used on 944t front were
small body 36/38 and 36/40
mid/large body 36/44 used on TS

the 36/42 is a 928S4 mid/large body w/ the old style scrapers
later 928S4 use the newer style 36/44 mid/large body.

they all used the same small body 28/30 rears

and using any of them in an axial mount front configuration by hogging the holes is a bad idea no matter what