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Posted by: Mueller May 6 2004, 12:12 PM

With everything being equal (piston size, pad size, rotor...etc....)


Would there be a performance difference between the two setups besides the obvious weight difference?


I'm thinking at normal temps, both would be equal.

I'm also guessing that once one gets more aggressive with the brakes, the aluminum calipers would be better and would be less susceptible to brake fade due to the better heat dissipation of the aluminum.

Posted by: Dave_Darling May 6 2004, 12:26 PM

The aluminum is lighter, but it flexes more. I haven't read of the heat dissipation of aluminum being better for braking than iron or steel--the two points everyone seems to focus on are the weight and the flexing.

--DD

Posted by: Mueller May 6 2004, 12:32 PM

I knew somebody would mention the damn flexing

Let's say we are not using 30 year old technology/manufacturing, but modern CAD/CAM designed parts

Hey Dave, are you in on the WB02? Sounds like we have a total of 4 if you say yes.

Posted by: banderson May 6 2004, 12:53 PM

Does anyone use an aluminum caliper?

If so, then forget what I am about to say.

GO STAINLESS STEEL OR TITANIUM

Heat is generated between the pad and the rotor. The rotor dissipates most of the heat to the air. Getting the heat to the air is the key.

Getting heat to the fluid = fade. Cast iron is an insulator compared to aluminum. the last thing you want is hot calipers.


Thermal Conductivity of
Al~100
cast iron~25
Stainless steel~10
Titanium~5

Posted by: lapuwali May 6 2004, 12:54 PM

I suppose the increase heat transfer of Al v. Fe could rub both ways. Most of the heat will be in the rotor and pads, and the iron caliper wouldn't transfer it to the fluid as well. However, once the iron caliper did get hot, it would take longer for it to cool off.

Iron has 3x the stiffness of aluminum, so theoretically 1/3 of the section could be used with equal stiffness (and equal weight) with iron. However, resistance to bending loads also varies with geometry (given a rod and a tube of equal weight and materials, the tube will be stiffer, with the stiffness increasing with the diameter of the tube). It's possible to play games with that, varying section thicknesses to get more stiffness for equal weight with Al than with Fe.

The weight thing can be really significant, too. Wilwood four-pot aluminum calipers weigh next to nothing.

Posted by: davep May 6 2004, 01:05 PM

Good heavens, don't tell the factory that aluminum is bad! They have used it for almost all the race cars and most of the modern production. I believe all the 4 pot on up are aluminum. The first 930 used the aluminum "S" that was first used in 1969. The 908 and 917 also used aluminum. Go figure.

Posted by: banderson May 6 2004, 01:20 PM

Ok, OK, I take it all back

Posted by: Ira Ramin May 6 2004, 02:03 PM

Mike,
Are you trying to choose between the aluminum 911S and iron SC calipers? I was planning on upgrading to SC front calipers due to the “flexing” that I thought I felt from my S brakes. Luckily I first checked out a friends car with freshly rebuilt calipers and new pads. They were as hard as a rock, no flexing that I could feel. I’m sticking with my S brakes. They are lighter and dissipate the heat from the pads better.

Ira

Posted by: phantom914 May 6 2004, 02:19 PM

Heat is transferred from the pads to the fluid through the pistons. Good place for stainless is there.

Actually 1/3 the section would result in 1/27 the stiffness if you keep the material the same. If you use Iron, you can get away with a thickness of about 70% of the thickness you would need in aluminum for the same stiffness.



Andrew

Posted by: Mueller May 6 2004, 02:29 PM

Hi Ira,

I have all your parts...I'll ship them off tommorow via UPS slow boat, Adams parts shipped last week, he should have them by now.


No "S" caliper, I'm thinking of going opposite of most peoples thinking and instead of going to 4 pot calipers up front with a big vented rotor, I'd be using an ultra-lightweight 2 pot aluminum caliper, aluminum hat and solid rotor (with cooling air ducting) (I'm also working on a lightweight rear trailing arm for my 914, I'm not too sure if I'll go all aluminum or a combination of Chromoly and aluminum)

Car will be auto-x mostly with just a few time trials and fun runs.

I helped put 300 miles on a Bp 914 up at Thunderhill in one day and between both of us drivers, we never had brake issues on a car running stock solid rotors. My setup would be lighter...I see no reason why it shouldn't work as good or even better than a stock system which does work if properly maintained.

Speaking of the use of aluminum calipers to help dissipate the heat from the pads, I've wondered if the use of Ti or SS "insulators" between the pads and the pistons are a bad thing since it could cause the pads to overheat???

Posted by: phantom914 May 6 2004, 02:44 PM

There isn't a whole lot of contact surface area between the pad and piston anyway, and techniques are commonly used to minimize heat transfer through the piston to avoid boiling the brake fluid, so I don't think overheating the pads is a huge concern.

Andrew

Posted by: Dave_Darling May 6 2004, 03:30 PM

Hey, it's what I've read. People, particularly those with 30-year-old "S" calipers, claim that they are getting noticeable flex from them. Ludvigsen also mentions the flex in some aluminum calipers somewhere in the 2nd volume of his work.

Me? I have no clue... I have stock steel calipers on my 914.



Sorry, incoming email was hosed all day yesterday and also early this AM. Don't know if it's OK yet or not. My situation with the WBO2 is a little complicated...

(You gonna be in Fremont this Saturday? If so, I can try to remember the plenum.)

--DD

Posted by: Mueller May 6 2004, 03:50 PM

not too sure if i'll be in fremont or not, I might help a friend move in San Meteo on Sat or Sun. I'll give you a call.

I wouldn't doubt the S calipers flex a little, but to what point does it start to affect the feel and operation of the braking system??

Posted by: MattR May 6 2004, 04:24 PM

Here is something straight from my physics textbook:

Thermal conductivities (W/m*C') at 25 C'

Aluminum - 238
Copper - 397
Gold - 314
Iron - 79.5
Lead - 34.7
Silver - 427

We need some silver calipers!

Posted by: drew365 May 6 2004, 05:32 PM

I ran S calipers for two years until one started sticking. We changed them out to M calipers. I truly havn't felt any difference.

Posted by: Brad Roberts May 6 2004, 07:56 PM

Bleed the brakes some time on a car with S calipers. You can see the caliper flex (much more than a iron caliper)

MM,

The new calipers you saw at a local shop are made from stainless steel.


B

Posted by: TimT May 6 2004, 08:10 PM

Thats easy...when the caliper starts to flex, any additional pedal effort you apply only causes the caliper to spread more...but you dont get any braking for the effort you suppied

Posted by: TimT May 6 2004, 08:18 PM

Oh a very seriously cool alum caliper is the Wilwood Forged Superlight, they also have a 6 pot caliper that fits a 3.5" spacing...

In some porsche circles wilwoods are sneered at... they do however stop cars that weigh a heck of alot more than ours do...

http://www.wilwood.com/products/calipers/sl6lug/index.asp