pa944red

I guess the only thing left to ask is..... Can I turbo my na?

Tom

morghen

Thats exactly the proof you DONT have enough stopping power.

i can lock up the wheels on my TDI golf too...and it has drums on the rears..does that mean i have enough breaking power? NO..it means i DONT.

More pad surface means you CANT lock the wheels easly and energy will be absorbed trough the pads not trough the tires. What you have is grippy pads that chew trough your disks faster and make the wheel locking easyer..it does NOT mean you upgraded your brakes...unless you had 30 years old crappy pads.

Jfrahm

So... you are making the chain stronger by improving the second-weakest link. Got it.
Er, what?

Van

I supposed it's a matter of semantics on how you define "stopping power".

If you define "stopping power" as the ability to decelerate with the maximum amount of force (measured in Gs), then it's determined by the tires. Or, more specifically, the friction between the moving object (tires) and the stationary object (earth). This is why your ability to decelerate in the snow is less than on pavement - there is a smaller amount of friction.

If, however, you define "stopping power" as the effort required to depress then brake pedal, then it's a simple function of leverage (pivot and fulcrum ratios on the pedal linkage and the diameter of the brake rotors (or drums)) and hydraulic pressure (area of master cylinder and caliper pistons). This is quite similar to the leverage on a wrench - let's say you have a bolt. With a short wrench, regardless of how hard you pull, you might not be able to snap it off. But with a longer wrench (more leverage) you can snap it with ease. However, regardless of the length of your wrench, the bolt has fixed properties that affect it's strength - and the length of the wrench is not something that affects bolt strength.

it is also possible to define "stopping power" as the repeated ability to perform in adverse conditions, or how well the system can handle the heat build-up from repeated stops in a short time. This is a function of heat management and heat transfer. Racing brake pads are designed out of materials that operate at, and withstand, higher temperatures. This includes both the friction material and the "binder" or glue used to hold the friction material together. Other factors related to heat management include pad surface area, vents inside the rotor, the shape of those vents and cooling ducts to direct air to those vents.

Now, to answer the points in your post: you mention drum brakes like they are an evil, dirty thing. However, drum brakes can work quite well - they have large pad surface area (on the shoes) and the pads are pressing on the outer-most radius of the drum for maximum mechanical advantage. But their biggest failings are: they are hard to cool, hard to visually inspect pad wear, and they are more time consuming to service.

You mention "energy absorption of pads and tires" - actually, the kinetic energy can be pretty easily calculated. An object with mass (weight) and velocity (speed) will have a certain amount of energy. If the weight stays the same, but the speed changes, we can know the amount of energy that needs to be added (for acceleration) or removed (for deceleration). The rate of change - or how fast - the acceleration will be proportional to the usage of energy over time. A car with 50 HP can reach 60 MPH, but it will take longer than a car with 500 HP.

When a car slows down, friction is converted into heat - that is how the "energy absorption" you mention works. The brake pads have a certain coefficient of friction - and when they rub against the rotor, heat is generated. Likewise, tires have a coefficient of friction - and they generate heat when they rub against pavement.

The force required on the brake pedal to lock up the wheels is just the point where the brake pad's gripping force is greater than the tire's gripping force. If you have crappy tires and good pads, it will take less effort to lock up the brakes. If you have good tires and crappy pads, you might not be able to lock up the brakes - regardless of how hard you press.

You also mention rotor wear - but this is a function of pad coefficient of friction and duration of time used (which is really "total energy dissipated). Yes, some pads are know for being "gentler" or "harsher" on rotors - but the other part of the equation is pad life. One of the components will lose material from all that friction - there's no magic involved!

Speaking of magic, someone is bound to ask, "What about the turbo? Why does it have bigger brakes?" And the turbo doesn't have bigger brakes because of magic... it has bigger brakes because it weighs more and can achieve higher speeds in the same duration of time (more horse power, right?) - this extra weight and this extra speed means that more heat will be developed during braking. To handle the extra heat, the rotors are thicker (so the vanes can move more air) and are larger in diameter (so the pad can have more mechanical advantage). The pad area is larger to distribute the heat over a greater area. Because the pad area is larger, the caliper incorporates multiple pistons to push more evenly on the larger pad.

And, finally, going back to my car, I have plenty of "stopping power" - in all three definitions I listed above. I pull 1.2 Gs of deceleration force while braking - which is the limit of my tires without aerodynamic downforce. The pedal effort required to lock up the wheels is great enough (800 PSI fluid pressure in the brake circuit) that I have a large "zone" of application pressures that I can utilize to modulate the brakes to control wheel lockup (due to track surface conditions/irregularities) and trailbraking. And, with additional air cooling ducts and good pad selection, my brakes perform at their peak performance for the duration of a 90 minute "enduro" race.

But, to the OP, I can't stress enough how important it is to have a healthy braking system, and to have a braking system that is well matched - from piston diameters to pad selection to tire selection. Trying to mix-and-match other parts in an effort to "save a buck" and not address the actual problem will frustrate you and diminish Porsche's reputation of providing excellent brakes on their cars.

morghen

wow i got you going didnt i ?
Some drums are ok, but drums from a 1996 rabbit TDI are not.

What i was saying is simple and i'll repeat.
The fact that i can easly lock the wheels does not mean i have stopping power on my TDI
The fact that you can easly lock the wheels on your 944 is also NOT a sign you have stopping power.

To use a judgement similar to yours:
I had early 944 brakes on my 924 and i could lock the wheels under hard braking.
Now i have the same master cyl, same tires but i have M030 calipers and disks and i cant lock them anymore but on hard braking i'm hanging suspended in the seatbelt with my *** off the seat.

Now thats stopping power.

If i had grippier pads that would have not given me much more stopping power, if any. What it would have done is it would have moved the locking point closer and the brakes would have had a more on/off behaviour that i whould have had to correct with tires and/or bias valve.

Van

You have crappy tires.


You have changed the hydraulic force ratio between the master cylinder and the calipers, and physically can not press the pedal hard enough to lock up the wheels.


I bet you have better tires on the 924 than on the Rabbit TDI.

Turb-OH Brad

Step 1: Upgrade tires until you cannot break traction under heavy braking.

Step 2: Upgrade brake components until you can again.

Step 3: Repeat.

Morghen has a flawed view of how a braking system works.

morghen

please elaborate, i'm open to learning.


i dont have crappy tires, i have changed the relation between the master cyl and the calipers and the TDI is just a side comparison.

is there is a problem with the same master cyl operating bigger calipers since you dont add mass or "speed" to the vehicle ?

will951

Wow. Instead of ganging up on the guy.....how about we offer him some other options or constructive advice??

Way to make the Rennlist community look like a bunch of jerks.....

Turb-OH Brad

This is exactly correct. A master cylinder can only move a set volume of fluid with a fully extended pedal. If that volume of fluid has to move a larger surface area of piston, the line fluid pressure will be much lower.

As Van said, you have effectively lowered the maximum pressure exertion possible on your calipers, and can no longer provide enough line pressure to push the pistons with enough force to lock up your brakes. Brakes that physically cannot lock are relatively unsafe.

You have decreased the amount of brake force to a point that the traction of your tires is now stronger. You should probably install a larger master cylinder as soon as possible.

I am not an engineer, but I played one on television...

morghen

My brakes therefore my stopping power improved, what do you make of that?
I bet you cant lock these calipers and wheels that easly even on the 928 S4 that they came on.
The reality is that the late 924 master cyl is probably internally identical to the later models and has alot of volume to push...and even if its not internally identical it still does the job.

The fact is that my setup can be locked but there is SO MUCH stopping power that even going near to the locking point is NEVER required.

In a nutshell: i've been using this setup for about one year, drove in different conditions (including 260km/h and then hard braking and the car was on par with a later model 911)
To chip in for the topic, i'd say do an efort and buy 951 brakes and you'll be just fine !

xschop

As a rule of thumb, I try to never go over 20% increase in caliper piston area when using the factory MC. After that it's pads/ bigger rotors that will fit under whatever wheel, then sticky tires.

I am building an LS1-928 now and am having to revisit the LS430 calipers because the 6-pots have too much piston area for it's MC.

I have found that the 318mm dia. Boxster-S rotors fit under 993 16" wheels

Turb-OH Brad

Again, this viewpoint is flawed. You 100% DO NOT have MORE braking power with bigger calipers. The fact that you could lock your brakes before, and cannot now, proves this.

Think of it like this... Disassemble a bic pen and shove a pea in there, then try to blow it out. Works well, right?

Using your breath again, try to blow a tennis ball out of a PVC pipe... Much more difficult, right? Same volume of air being moved, pushing on a larger surface area...

morghen

i see your point, not sure it scales right but thanks for the input.

Van

Ultimately, if you're happy with your car - that should be good enough. It is easy, of course to get into "analysis paralysis" - there's no reason why a certain caliper upgrade, even if the master cylinder isn't changed, that happens to have a pad coefficient that compensates for the change isn't going to feel great.

Looking back at my long post, the point was to explain some of the physics behind the braking system and to debunk some common myths. I spend a lot of time with actual vehicle telemetry - brake line pressures, actual G forces, suspension movement, etc. - so I see changes that have measurable results. But for the average person, they only have the "seat of their pants" or the "butt dyno" to make a decision.

I've also helped people work on a lot of cars - and I've been surprised by how people are impressed with their braking system after just new flexible lines and a fluid flush. I'd hazard a guess that 80% of all cars driven on the street have never had a brake fluid change. I'd even guess that half of the 944s out there haven't had this done.

Brake fluid is hydroscopic - meaning it attracts moisture. Not only does water not withstand the heat of the braking system well, but it can also cause corrosion inside calipers... which can bind up movement.

There are a few things which I think shouldn't be skimped on - and those are tire selection (tires are your only connection with the road - which counts for a lot while driving) and brake selection. (Kind of like the old saying for pilots - takeoffs are optional; landings are mandatory. In a car, going is option; stopping is mandatory!)

So I'm glad that people are trying to upgrade their brakes and improve their stopping performance, I just hope the rest of their braking systems are up-to-snuff before they go jousting windmills. Nothing turns 944 ownership (or any vehicle ownership) sour than spending money and only getting frustration in return.