Poll: Brake upgrades..what kind of ratio?, F=big/R= big or F=big/R=little or ??? Options

[fiid]

QUOTE (bondo @ Feb 16 2005, 10:44 AM)

QUOTE (fiid @ Feb 16 2005, 11:13 AM) (IMG:http://www.914world.com/bbs2/html/emoticons/agree.gif) I think you hit the nail on the head.   Pad area would effect wear rate, and I bet it has some thermal effects too - like felxing you mentioned. If you assume that the braking force originates (on average) from the center of the pad, then a bigger pad and larger radius rotor whould also move the braking force further out, giving it a greater mechanical advantage (of course - larger tire radius would lose you this battle from the other end).... Now that I think about it, pad area and heat MUST be very related. If friction is the same for a smaller pad (or nearly the same), the amount of heat energy must also be the same, because dynamic friction is simply the conversion of mechanical energy to heat. So that same amount of heat would be concentrated on a smaller pad area and overheating it sooner. I guess the same thing would apply to clutches. Too bad our bellhousings are so darn small.

(IMG:http://www.914world.com/bbs2/html/emoticons/agree.gif) what you said.

[lapuwali]

QUOTE (fiid @ Feb 16 2005, 11:06 AM)

QUOTE (bondo @ Feb 16 2005, 10:44 AM) QUOTE (fiid @ Feb 16 2005, 11:13 AM) (IMG:http://www.914world.com/bbs2/html/emoticons/agree.gif) I think you hit the nail on the head.   Pad area would effect wear rate, and I bet it has some thermal effects too - like felxing you mentioned. If you assume that the braking force originates (on average) from the center of the pad, then a bigger pad and larger radius rotor whould also move the braking force further out, giving it a greater mechanical advantage (of course - larger tire radius would lose you this battle from the other end).... Now that I think about it, pad area and heat MUST be very related. If friction is the same for a smaller pad (or nearly the same), the amount of heat energy must also be the same, because dynamic friction is simply the conversion of mechanical energy to heat. So that same amount of heat would be concentrated on a smaller pad area and overheating it sooner. I guess the same thing would apply to clutches. Too bad our bellhousings are so darn small. (IMG:http://www.914world.com/bbs2/html/emoticons/agree.gif) what you said.

Thanks. I've been saying this for YEARS, and no one listens, since pad area is "obviously" related to braking power.

Yes, pad area primarily affects pad wear. It can also bring on fade if the pads are small enough that they get so hot their coefficient of friction falls. By itself, pad area has NO effect on clamping force or braking effectiveness.

I'd also hesitate in quoting 911 brake balance figures when talking about 914s. Static weight distribution definitely matters. I'd use Boxster figures, though, with a small grain of salt. Remember that the height of the CG also comes into play, since it affects weight transfer rear to front under braking. A lowered car needs less frontward brake bias than a car with stock ride height, as less weight will transfer forward under braking, so the rear brakes can contribute more to stopping.

[Andy]

QUOTE (fiid @ Feb 16 2005, 09:25 AM)

Friction is a function of normal force, not of surface area.... so it is just the piston size that matters, not the pad area.  ( http://www.school-for-champions.com/scienc...ce/friction.htm )

Then I better hold off on the wide slicks and keep my 55 series for taking the 914 to the track! (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)

Just kidding, friction is independant of surface area using the "standard model" which is a simplified system that is pretty far from covering all examples, it does work for most, but for more extreme examples, like tires, it is no longer accurate. This is kind of a pet peeve of mine as most teachers simply teach the standard model and state that it's just counter-intutive and leave it at that, often not even knowing that they're misleading people. I'm not saying that in case of brake pads the standard model would give inaccurate data. In most cases friction is independant of surface area.

Can you tell I work in higher education?

http://hyperphysics.phy-astr.gsu.edu/hbase/frict3.html

[lapuwali]

QUOTE (Andy @ Feb 16 2005, 12:11 PM)

QUOTE (fiid @ Feb 16 2005, 09:25 AM) Friction is a function of normal force, not of surface area.... so it is just the piston size that matters, not the pad area.  ( http://www.school-for-champions.com/scienc...ce/friction.htm ) Then I better hold off on the wide slicks and keep my 55 series for taking the 914 to the track! (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif) Just kidding, friction is independant of surface area using the "standard model" which is a simplified system that is pretty far from covering all examples, it does work for most, but for more extreme examples, like tires, it is no longer accurate. This is kind of a pet peeve of mine as most teachers simply teach the standard model and state that it's just counter-intutive and leave it at that, often not even knowing that they're misleading people. I'm not saying that in case of brake pads the standard model would give inaccurate data. In most cases friction is independant of surface area. Can you tell I work in higher education? http://hyperphysics.phy-astr.gsu.edu/hbase/frict3.html

True enough. Tires don't use the "classic" friction model usually taught in school. Brakes, however, do. Indeed, brakes are nearly the ideal thing to use to teach the classic friction model, since all of the effects are present: coefficient of friction, lubrication effects, heat effects. In also somewhat decouples the the mass v. weight issue, since people learning basic physics often confuse overcoming inertia with overcoming friction when taught using the normal "pushing a big stone block" technique.

Tires are so complex it's pretty hard to teach how they really work in a basic physics course.

[fiid]

thanks! I was wondering about the tire case.

[lapuwali]

QUOTE (fiid @ Feb 16 2005, 01:23 PM)

thanks! I was wondering about the tire case.

I've never thought of a good tire analogy. Tires somewhat interlock with the surface, and they seem to employ some qualities of actual adhesives. I've had fresh physics majors insist no one can ever exceed 1.0G in a turn because that would mean the coefficient of friction would have to be higher than 1.0, which is "impossible". You can show them all the data in the world showing that modern slicks top out at about 1.3G on a good surface, and they won't believe it. Of course, bees can't fly, either...

[Mueller]

speaking of pads....from what I have read, if you have 2 different calipers, 1 with 2 pads and the other caliper has 4 smaller pads which equal the sq. inch surface area of the 2 pad caliper, the 4 pad caliper will be more effiecient..it is due to the 4 pad caliper having more "leading edges"

[Andyrew]

I kinda want to put 944 turbo calipers on, then 914 fronts on the back... (or something like that...) but I really want to be able to have a hand brake...

944 turbo's put the same on the front as they did on the back, and those things stop like a mother!!! (especially with some sticky street tires... aka my street race 205's...)

something to keep in mind...

[Mueller]

QUOTE (Andyrew @ Feb 16 2005, 04:35 PM)

I kinda want to put 944 turbo calipers on, then 914 fronts on the back... (or something like that...) but I really want to be able to have a hand brake... 944 turbo's put the same on the front as they did on the back, and those things stop like a mother!!! (especially with some sticky street tires... aka my street race 205's...) something to keep in mind...

simple...get the parts from an early 911...(I'd think the 944 parking brake is very similar)...only downside is you have to pop-off the wheel hub unless you get creative and make it bolt on differently

[Mueller]

are you sure the calipers are the same? I know they use the same size pads, but the pistons might be smaller in the rear.....

[lapuwali]

QUOTE (Mueller @ Feb 16 2005, 03:32 PM)

speaking of pads....from what I have read, if you have 2 different calipers, 1 with 2 pads and the other caliper has 4 smaller pads which equal the sq. inch surface area of the 2 pad caliper, the 4 pad caliper will be more effiecient..it is due to the 4 pad caliper having more "leading edges"

Never heard of that. Can't think of any reason for improved "efficiency" (whatever that's supposed to mean) with more than one pad per side. I've seen such arrangements, and the best story I heard was it allows you to compensate for the unequal wear between the leading and trailing edges of one large pad by simply replacing one of N smaller pads, all of which are the same. On some early 6 or even 8 piston calipers, I've seen setups with a small round pad per piston.

Another way I've seen to attempt to solve this is to stagger the size of the pistons so the leading edge pistons are smaller, and thus exert less force than the trailing edge pistons. I've seen two-piston slider type (pistons on the same side with a slider "claw" on the other), four-piston, and six-piston calipers made this way. One saw this a lot on bikes a few years ago, and may still today for all I know. Still one pad per side, but presumably it didn't taper so much, and wore more evenly.

Perhaps it's "cost" efficiency? Replace only the rapidly wearing leading edge pads, not the whole pair of (presumably tapered) pads?

[Andyrew]

Dad doesnt think theres any difference in the front or the rear..

I might look into that rear parking brake system...

hmm..

[bondo]

QUOTE (lapuwali @ Feb 16 2005, 04:59 PM)

QUOTE (Mueller @ Feb 16 2005, 03:32 PM) speaking of pads....from what I have read, if you have 2 different calipers, 1 with 2 pads and the other caliper has 4 smaller pads which equal the sq. inch surface area of the 2 pad caliper, the 4 pad caliper will be more effiecient..it is due to the 4 pad caliper having more "leading edges" Never heard of that. Can't think of any reason for improved "efficiency" (whatever that's supposed to mean) with more than one pad per side. I've seen such arrangements, and the best story I heard was it allows you to compensate for the unequal wear between the leading and trailing edges of one large pad by simply replacing one of N smaller pads, all of which are the same. On some early 6 or even 8 piston calipers, I've seen setups with a small round pad per piston. Another way I've seen to attempt to solve this is to stagger the size of the pistons so the leading edge pistons are smaller, and thus exert less force than the trailing edge pistons. I've seen two-piston slider type (pistons on the same side with a slider "claw" on the other), four-piston, and six-piston calipers made this way. One saw this a lot on bikes a few years ago, and may still today for all I know. Still one pad per side, but presumably it didn't taper so much, and wore more evenly. Perhaps it's "cost" efficiency? Replace only the rapidly wearing leading edge pads, not the whole pair of (presumably tapered) pads?

Hmm, interesting.. I wonder if this could have anything to do with why some pads have a slot in them (probably not) Maybe it's just a wear indicator.. but that wouldn't make much sense either unless you remembered there was a slot there when you installed the pads.

[fiid]

QUOTE (lapuwali @ Feb 16 2005, 03:05 PM)

QUOTE (fiid @ Feb 16 2005, 01:23 PM) thanks!  I was wondering about the tire case. I've never thought of a good tire analogy. Tires somewhat interlock with the surface, and they seem to employ some qualities of actual adhesives. I've had fresh physics majors insist no one can ever exceed 1.0G in a turn because that would mean the coefficient of friction would have to be higher than 1.0, which is "impossible". You can show them all the data in the world showing that modern slicks top out at about 1.3G on a good surface, and they won't believe it. Of course, bees can't fly, either...

They must be adhesive since slicks will actually pull gravel out of a poorly bonded surface.

The other assumption that might be wrong is that there is only gravity acting downwards - a well setup car will also be generating downforce which is a direct mulitplier in the friction equation.

[xitspd]

Mike,

I am using a 993 Twin Turbo front on my Teeners front

[xitspd]

QUOTE (xitspd @ Feb 16 2005, 08:04 PM)

Mike, I am using a 993 Twin Turbo front on my Teeners front

And a 993 Twin Turbo rear on the rear with good results.

[Mueller]

QUOTE (xitspd @ Feb 16 2005, 09:04 PM)

Mike, I am using a 993 Twin Turbo front on my Teeners front

nice.... (IMG:http://www.914world.com/bbs2/html/emoticons/drooley.gif)

according to a chart I've read....the piston area of your front calipers are 2X larger than the rears (IMG:http://www.914world.com/bbs2/html/emoticons/smile.gif) ...they have the most amount of front bias than any other P-car (production)..... 2:1

love your car (IMG:http://www.914world.com/bbs2/html/emoticons/pray.gif)

[neo914-6]

Bill ran this on his track car, now Neo914...

911T Front Struts / Vented Discs / Calipers with Porterfield R4 brake pads
23mm Front Torsion Bars
Front Brake Ducting

944S Rear Vented Discs / Calipers with Porterfield R4 brake pads

19mm Master Cylinder

Cockpit Adjustable Brake Bias Valve

[seanery]

my upgraded car has 951 calipers front boxster calipers in the rear with (I think I remember) Carrera front rotors and SC rears.

[seanery]

here's the scoop on the 951 calipers front vs rear:

The physical size of the caliper is the same and they use the same pad.
They are both 4 piston calipers, but the rears have slightly smaller pistons.
It took a while to figure it out.

the 951 fronts are great....and I think the rears would be fine on a 914...on the front