maxcooper

I've heard many good things about the PFC pads, but they were discontinued for the stock FD calipers (at least the 93s were) when I started to hear the good reports. Those were supposed to be relatively streetable track pads, too, which is very attractive if it is true.

Maybe I'll end up with PFCs in the AP calipers. My first sets are the Ferodo track pads they came with and some EBC Greens for the street. It would be great to find some pads that I could use for both street and track, maybe the PFCs will be just such a pad. There's another justification for big brakes: it should be easier to find a dual-purpose pad.

I, too, am concerned about the front/rear bias, but I'll have to try it out and see how it works before I do anything. At least it will be too much front bias if it is off; too much rear would be much worse. I am considering the RS rear brakes. It would be great if I could adapt the stock front calipers for use back there, but I suspect they would give way too much rear bias (though I haven't done the math). I heard something about using C5 brakes on the rear of the FD, too. I don't want to run a proportioning valve if I can avoid it (well, maybe to the rears, but defintely not to the fronts).

If the pedal travel is longer than I like, I'll install a 929 master cylinder. But again, I am going to wait and see how it works with the current setup first.

I don't quite understand the ABS thing, but I'll see how it works out on the track. Unfortunately, I probably won't be running another event until June.

-Max

Ef-Dee

You can never go wrong with Brembo's...

rx7tt95

Once you remove the ABS from the system with larger front brakes, you'll be in a world of hurt that only a dual master cylinder and proportioning valve will solve. Big $$ and development costs. Max, did you get the 4-piston or 6? Can't recall off hand. The 4-pistons offer great feel with the stock master cylinder without switching to the 929. I suspect the 6-piston would benefit from the 929 master cylinder however.
Michel

racedriver

I have been road racing my 93 fd since 1997 and have tried it all with brakes. I would do the 929 M/C even with stock calipers. It will give you far less travel and a firmer pedal. when I installed the brembo F40 kit the brakes were way too hard to modulate and the pedal travelled way too far down, and that was with a 929 M/C. I would apply an extremely small ammount of pressure and they were full on, not what you want when threshhold breaking.
I have always kept the ABS. I would hesitate to install a bias valve. I tried it and the ABS is not as effective at pulsing the caliper. One rain race the pedal went stiff and I flew off the track. And the balance is not ideal with the big calipers. My latest set up has a hole drilled farther down the brake pedal and attached to the mc rod with a block of aluminum. I have supra tt front rotors front and rear. the rears are machined from 1.1 to 1 inch.and are clamped by the c5 rear caliper. custom mounts make the brembo and c5 calipers fit. Caution, Inthe front the rotor offset makes it stick out and your wheel needs a fair bit of clearance. I run the 17' speedlines as once used on pettits car.

wouter

maxcooper

I get it now, the ABS will cover for screwed up brake bias. Squeeze the pedal hard to enough to lock both ends and the ABS will make sure you don't just lock one end.

-Max

Crackers

Hey SleepR1, you are actuly incorect in a sence.

If you take a Hevy vehicle lets say the MKIII Supra and upgrade its Shitty *** single piston callipers with some decent 4pot callipers like the TurboII's or something of that nature It will Decreese Stopping distance.. Now what you are saying is lets say the Fd's stock 4pot callipers and HP's 8pot big break kit you're right you wont stop any sooner only thing it will prevent is when the car gets breakFade..........

jimlab

Well, that's not entirely true...

Let's consider the RX-7, since this is the right forum for it. Let's say the FD3S stops from 60 miles per hour in 115 feet, just for sake of arguement. Upgrading the front brakes with a larger kit won't help the car stop from 60 mph any sooner, and in fact, braking distance will probably increase slightly.

Why? Because the RX-7 is already imbalanced with a bias towards the front brakes that is masked by the power braking system. Eliminate the power brake booster and switch to a manual system, and this becomes immediately apparent. If you increase the front piston caliper area (larger or more pistons per caliper) you're also increasing the amount of brake fluid required to exert the same clamping force. Without an upgraded brake master cylinder to move more fluid, clamping force is actually reduced, probably resulting in reduced brake performance.

Worse, the rear brakes, which are already prone to locking before the front brakes will lock even sooner, and the front brakes, which do the bulk of the work in stopping the car (about 80-85%, roughly), will be compromised even further. You won't get good pad bite in the front, and the rear brakes will be trying to do more work to stop the car, with a much smaller swept area.

Many people have upgraded their front brakes and left the rear alone because upgrading them is not recommended by many people and the expense is significant. What those people have found, usually in track situations, is that their brake fluid boils. Why? They increased the size of the front brakes and temperatures should be lower, right? Right... but not in the rear of the car, and the systems are connected. Again, this becomes apparent when the power brake booster is removed, as it isolates the driver from the "facts" concerning the RX-7's braking system.

Anyone find it interesting that the RX-7 got three ABS unit changes in only 3 model years, 2 of which were during 1993? Doesn't sound like they had all the bugs worked out of the brake system, does it.

So back to "big brakes not stopping the car any sooner". Forgetting for the moment that there is a front bias inherent in the system, and that a big brake kit in the front increases the problem, let's talk about stopping power.

The tires are the only contact with the road, therefore the tires have a very big impact on braking power. If you let off the gas, you'll notice that the car begins to slow as a direct effect of friction between the tires and the road. The more air you put in the tires, the less friction in the system because they'll have a smaller contact patch. Decrease air in the tires and the contact patch increases in size, increasing friction. Increase the size of the tires and the car stops more quickly, and so on. Elementary, right?

So when the brakes lock, the only thing stopping the car at that point are the tires. However, until the brakes lock, what is stopping the car? The brakes. Friction of the pad against the rotor slows the rotation of the wheels and creates heat. Heat can cause the brake pads to glaze and reduce friction (and therefore "fade"), or boil the brake fluid, reducing line (and pedal) presssure, again causing brake fade.

Increasing the size of the front brakes increases the swept area of the pad, and is usually accompanied by an increase in the thickness of the rotor as well as diameter. This usually increases the mass (depending on design) of the rotor, which will then take proportionately longer to heat, and subsequently lose heat more quickly due to its increased surface area. Elementary again.

So when does a large brake system actually decrease stopping distances? When the stock system is overloaded to the point that brake fade occurs. From 60 mph to 0, the stock braking system on the RX-7 is hardly affected. Stop from 60 a few more times, and it'll heat the system and braking distance may increase slightly, but it's still not a hard workout for the system.

Increase vehicle speed to 100 mph and brake to 0. The brakes do fine, right? Do it again. And again. Eventually they'll overheat, fade, and braking distance will increase.

Now increase vehicle speed to the point at which the braking system begins to fade on the first stop. This happens sooner on some cars, later on others. Stopping from 140 mph as quickly as possible will put a lot of heat into the stock brake system on only one stop. The next stop will definitely be longer.

Now upgrade to a big brake kit. The first stop will be shorter, because the braking system will be able to resist fade to a greater degree. That's when big brakes actually decrease stopping distances.

So back to the point... are big brakes a worthless investment in stopping the car when they don't decrease stopping distances in "normal" use? Probably. An RX-7 with a big brake kit won't stop any faster than one with the stock brakes, from 60 mph. Probably not from 100 mph, either, at least not on the first stop. But if you jam the brakes from 160 mph, you'll definitely stop quicker with an upgraded braking system, which is why *all* of the 170+ mph supercars carry much larger braking systems than the RX-7. They're not just there for looks, and they didn't do it just to increase unsprung weight. When you can break 180 mph, you want to be damn sure the brakes are going to stop the car when you need them.

Big brakes are useful in track driving, especially tracks with many turns and short straights, and in stopping from very high speeds because they resist fade. If you plan on seeing 140+ mph in your car on a regular basis, a big brake kit is a very wise decision. If you never go over 100-120 mph, you're probably wasting money unless you're doing it for looks. The stock system will stop you just fine in a panic situation.

In any event, if you increase the size of the front brakes, do two more things. First, upgrade and increase the size of the rear brakes, and switch to a 4-piston caliper. Second, balance the caliper piston area (simply calculate the total area for the diameters of all pistons in the caliper) from front to rear. The stock system is above 4.0:1, front to rear. Upgrading only the front brakes increases the imbalance. Ideally, it should be around 2.0:1. If you don't have a manual brake proportioning valve, your best course of action is to balance caliper piston area. If you do have one or install one, find the point at which the front and rear brakes are locking at the same time through trial and error to balance the system.

And that's probably more than you wanted to know about the RX-7's brakes. Hopefully there was some new information in there that you can make use of.

maxcooper

Bigger pistons means more clamping force, but at the expense of longer pedal travel. The ABS does the masking, not the power.

Were you out partying tonight, Jim?

Bigger front brakes will mean the fronts will lock even sooner than they did with the stock setup, which was already front-biased (meaning that the fronts lock first).

I didn't know that. Do you have any details of what the changes were?

I agree with the rest.

Except the 4:1 ratio of front to rear piston areas. The rear calipers have one big piston, that is bigger than each of the four front pistons (as I recall from looking at them), so the ratio must be less than 4:1. Perhaps something like 3:1. If I wasn't so lazy, I'd whip out my Workshop Manual and calculate it.

-Max

maxcooper

I got the 4-piston kit. SE37K wheels have tons of room for brakes. A 14" rotor would fit in there no problem from the looks of it. That would be overkill for my purposes, though, and would only increase rotational inertia and unsprung weight. 13" is good.

I can't "officially" comment on feel yet because I haven't bleed them too well yet. Right now the pedal is softer than I would like, but theer is probably a lot of air in them so it is too early to tell. I'll probably get the 929 master anyway. I hate brakes that work on pedal position. I'd rather have the pedal not move at all and just work on the amount of pressure I apply. That would be the ideal brake pedal.

-Max

jimlab

Bigger pistons don't necessarily mean more clamping force. Line pressure is reduced if the rest of the system remains the same, because of the increased fluid capacity of the front calipers, resulting in reduced clamping force.

It's also my understanding that it's not the ABS that does the masking, it's the power brake booster, or at least a combination of the two. Talk to Brad Barber if you want the lowdown from someone who's been to hell and back with the RX-7's braking system. He'll fill you in on what he and others discovered after switching to the dual manual master cylinder setup.

I believe that's incorrect. Increased fluid capacity in the front calipers means that increased fluid flow is required to increase line pressure to the point of locking them, while the rear caliper size remains the same. I believe that the rear brakes will lock even sooner in such a situation.

"Front biased" means that the caliper area was already proportionately much larger than the rear, not that the fronts lock sooner. Talk to Brad.

If the brake master cylinder can only push so much fluid, and you increase the capacity (by increasing piston number or diameter) of the front caliper, line pressure (and therefore clamping pressure) will be reduced. The rear calipers, however, remain static. Press down farther on the pedal (because of the increased capacity in the system) and the rears will lock even sooner because they require less fluid and line pressure (than the fronts) to do so.

At least that's the way it was explained to me in talking to several people, and no, I haven't been drinking.

Of the three major recalls, two were fuel system related and the third was the ABS unit. There was a mid-year change in 1993, and then a new ABS unit for '94-up.

I said "above" and meant to say "about" 4:1. The rear caliper piston is not that large, I've just had my brakes apart recently, and the rear calipers are laying on the floor down in the garage. The ratio is nearly 4:1, and a big brake kit in the front will raise the ratio over 4:1. Brad Barber had the Porsche big brake kit in front and calculated his piston caliper ratio as 4.7:1.

Please do. Knowing precisely what the ratio is would be nice. Unfortunately I sold my front calipers and didn't think to measure them prior to sending them.

jimlab

Max, if you want verification of my (and other's) theories on piston caliper area balancing, you only need to look as far as the revised rear rotors and calipers on the latest Japanese RX-7s.

They increased the size of the rear rotor to 11.1 x 0.81" and upgraded the caliper, and left the front brakes unchanged.

Brad and others are switching to the same rotor, with a Wilwood Dynalite 4-piston caliper with an even larger piston area using a mounting bracket made by David Breslau to balance large aftermarket front brake kits with their dual master cylinder setups.

SleepR1

Jim,

I have the upgraded '99 FD Type RS brakes (fr/rr). The front brakes are considerably larger than the US spec fronts.

After reading yours and Max's exchange, I'm glad I went with the Type RSes rather than fooling with the other kits

ajmacdon

A couple points to remember as I think this has gotten too complicated:

1. Bigger piston area always means more clamping force.

The key point here is that pressure is equal everywhere in the system so a bigger caliper piston will always produce more force.

2.Proper front to rear bias is a funcion of three basic elements: F/R wieght distribution, CG hieght, and wheel base.

On a car like the RX-7 with a low CG and 50/50 distribution the front brakes will need to do 65 to 70% of the work.

3.Piston area is not the only factor that changes bias, rotor diameter, swept area, pad material, and of course a prop valve or ABS system also have an affect.

The total front brake system needs to be 70% *more effective* than the rear, there are many ways to get there, one of them happens to be piston area.

4. Remember, when calculating piston area ratios that on a single piston caliper you must double the area of the piston due to the geometry of a sliding caliper.

Again, these ratios are *meanignless* if other factors effecting bias also change (rotor diameter, etc.) The whole system needs to be considered.

5.Boosters and master cylinders only generate pressure, they can not affect bias (not counting dual master systems where master ratio is the best way to change bias)

6. The ABS system electronically biases the system. Even when you don't feel ABS it is working to limit pressure to the rear.

To keep the flames down, I am totally open to everyones thoughts, although the points above are basic princples with a lot of data to support them. For referance, I am an engineer for a major ABS/brake manufacture and work closely with the author of the article that spawned this great disscussion (this is what lists are for!) There is more to say about what is the best bias, we'll see, now I need lunch

maxcooper

New stuff starts with ###...


quote:
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Originally posted by maxcooper
Bigger pistons means more clamping force, but at the expense of longer pedal travel. The ABS does the masking, not the power.
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Bigger pistons don't necessarily mean more clamping force. Line pressure is reduced if the rest of the system remains the same, because of the increased fluid capacity of the front calipers, resulting in reduced clamping force.

### Bigger pistons mean you'll get the same clamping force with less line pressure (PSI x Area = Force), which means the big new brakes will have lots of clamping force. Since you don't have to push as hard to stop, the old stock rear brakes will not be grabbing as hard.

quote:
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Were you out partying tonight, Jim?

Bigger front brakes will mean the fronts will lock even sooner than they did with the stock setup, which was already front-biased (meaning that the fronts lock first).
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I believe that's incorrect. Increased fluid capacity in the front calipers means that increased fluid flow is required to increase line pressure to the point of locking them, while the rear caliper size remains the same. I believe that the rear brakes will lock even sooner in such a situation.

"Front biased" means that the caliper area was already proportionately much larger than the rear, not that the fronts lock sooner. Talk to Brad.

### "Front-biased" means the fronts lock first by definition. This is most likely because they have more piston area and thus exert more clamping force for a given line pressure, or have bigger rotors, or better pads, or any number of things. Bias simply refers to which end reaches the limits of the tires first.

If the brake master cylinder can only push so much fluid, and you increase the capacity (by increasing piston number or diameter) of the front caliper, line pressure (and therefore clamping pressure) will be reduced. The rear calipers, however, remain static. Press down farther on the pedal (because of the increased capacity in the system) and the rears will lock even sooner because they require less fluid and line pressure (than the fronts) to do so.

### Making the fronts bigger means you need less line pressure to lock them. That means you won't push as hard on the pedal (but you will push further, as more fluid volume is required) which means the rear brakes won't grab as hard as they once did.

quote:
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Perhaps something like 3:1. If I wasn't so lazy, I'd whip out my Workshop Manual and calculate it.
--------------------------------------------------------------------------------

Please do. Knowing precisely what the ratio is would be nice. Unfortunately I sold my front calipers and didn't think to measure them prior to sending them.

### Oops, you're right. The front piston bore is 1.42" and the rear is 1.37". So the front:rear ratio is (4 x 1.42) / 1.37 = 4.15:1.

### There are a lot of weird things that can happen dynamically, but the basic relationship of how hard (not how far) you push on the pedal (adjusted for the size of the lever arm = 4.1:1 on the FD) divided by the area of the master cylinder (0.94 on FD) will give you the line pressure. So, with 200 lbs of pressure on the pedal, you get (200 x 4.1) / 0.94 = 872 psi of line pressure. The clamping force will be 872 times the area of the caliper's pistons, so clearly bigger pistons give more clamping pressure for a given line pressure. There are limits to the stroke of the master clyinder, so it is possible that you run out of pedal stroke before you can start exerting a real force, but that would be a pretty F'ed up setup.

### There are some funky things in the braking system, like a proportioning valve (that reduces pressure to the rear brakes above 570 psi, according to the manual that I am looking at right now) and maxpesce reported some device that is supposed to retain some braking power in the event that you spring a leak. This last item may lead to weird behavior -- imagine that the device thinks something is wrong based on the amount of fluid that is flowing to your new, giant brake calipers. It might close off the line to the front brakes, which would reduce the pressure. This would allow you to build pressure to operate the rear brakes and stop the car (however slowly). Maybe something like the scenario I describe here leads to some unexpected behavior when you upgrade the fronts.

### From my own experience of driving my car tonight with the AP 4-piston calipers on the front, the bias is more forward. The fronts lock up even sooner than rears than they did before and it takes more pedal travel and less pedal force. The increased rotor diameter will be partially responsible for the increased front bias, but the AP calipers also have a larger piston area, which explains the difference in pedal feel. Of course, I still have tons of air in them and need to bleed them properly before I make too many claims about the pedal action, but the basic change was unmistakable. It was wet out tonight, so I did a few ABS stops to check things out.

-Max

maxcooper

On the RX-7, the front caliper is 4x1.42" and the rear is 1.37". So, you are saying that because of the caliper design, the F:R ratio is really (4x1.42) / (2x1.37) = 2.07:1. That makes sense to me -- two pistons opposing each other aren't going to pinch any harder than one piston pushing against a stationary object.

Quote:

Again, these ratios are *meanignless* if other factors effecting bias also change (rotor diameter, etc.) The whole system needs to be considered.

Understood. I think we're just trying to deepen our understanding of the piston size part so far.

Quote:

To keep the flames down, I am totally open to everyones thoughts, although the points above are basic princples with a lot of data to support them. For referance, I am an engineer for a major ABS/brake manufacture and work closely with the author of the article that spawned this great disscussion (this is what lists are for!) There is more to say about what is the best bias, we'll see, now I need lunch

Thanks for joining the discussion!

-Max

BOOSTD 7

I've basically decided that I don't have the time or money to try numerous caliper/rotor combinations. So instead I'm going to build in plenty of adjustability to my choice, so that I can tune it to work properly.

A non-power dual brake master cylinder setup with a balance bar and proportioning valve should give all the adjustability you need. And adjustability is especially important on a car with significant suspension changes. Since the way weight transfers from rear to front under braking is greatly affected by your suspension setup. Also camber changes can affect the way tires interact with pavement under braking, thereby affecting the demands on your brakes.

I don't have all the answers, but by giving myself options I can avoid having to spend more money in the future. Build adjustability into your braking system, and you should be able to save some headaches. It will take lots of track time to get it dialed in, but should be worth the effort.

racedriver

ajmacdon is right. You must also remember that if keeping the abs that it is designed to pump a certain ammount of fluid. Big changes to piston size or # of pistons means a larger fluid displacement occurs and the abs will be less effective.

DamonB

I want to clarify my stance and what I take from reading the article:

Once the car can stop without fade to the brakes, any other improvement is just extra weight to carry around.

I therefore completely agree with Jimlab:

"So when does a large brake system actually decrease stopping distances? When the stock system is overloaded to the point that brake fade occurs."

The tire stops the car in every instance, but if the brakes can't lock the tire because they are overheating then big brakes are a sure improvement. The last couple paragraphs in the article make the point: if your racecar can already stop all you want without overheating the brakes, then bigger rotors, pads, pistons etc present no performance gains and actually are just adding weight. To make matters worse it's the worst kind of weight: unsprung.

I still need to post pics of my iron CART rotor. It's a beautiful piece, but what struck me most (as with my RX-7 rotors) was how HEAVY the thing is. This from a lightweight racecar that makes every effort for brake cooling and lightweight suspension components. Fact of life is you have to have enough iron in there to absorb and dissipate the heat.

DamonB

I never considered that. I guess than if going to big brakes the stock ABS should just go, it wouldn't be able to modulate correctly anyway if this is true and stopping distance would go up since the line pressure would drop.

jimlab

Not arguing, just trying to assimilate.

If a single piston pushing against a stationary object is equal to the force of two opposing pistons of the same size, then why are there 4, 6, and 8 piston calipers, when you could simply use a caliper (and more cheaply, at that) which had 4 on one side and pushed against a pad or pair of pads on the opposite side? Just curious.

And on a completely unrelated note, since BOOSTD 7 is in the hizz-aouse, anyone know why when quoting an article, sometimes only the last section (like in Max's post I responded to) shows up and not the first two "paragraphs"? I've noticed this problem a couple times now, and had to cut and paste what I wanted to respond to.

jimlab

Adjustability, yes, but at a price; reduced line pressure, requiring a brake pad with a higher coefficient of friction, if what I've heard is correct.

Talking with Mark Valskis and Brad Barber about their braking systems, both said that if I chose to go with a "street" pad like the Hawk HPS or EBC green pads, that the coefficient of friction was not high enough to provide ample stopping power with a manual master cylinder kit. Mark recommends Pagid brake pads, which are definitely not a street friendly solution.

Anyone know if this is true? Or does the power brake system simply reduce pedal effort for the same line pressure?

Now we're getting into some good, quality technical discussion...

jimlab

Except when you increase brake size and reduce unsprung weight by going with a system like the AP Racing "Champ" rotor setup.

jimlab

And while I'm on the subject of reducing unsprung weight, anyone have an opinion on moving the braking system inboard on the IRS (a custom IRS, of course) to eliminate their unsprung weight in the rear of the car?
The Ford 9" IRS systems that I'm considering can go both ways.

Cooling is reduced, of course, but a larger brake system could be used without adversely affecting unsprung weight which would provide improved heat handling.

BOOSTD 7

The proverbial house ...

I don't know why it does that. Donate a few g's and I'll get a team of developers on it asap.

BOOSTD 7

This is true. I have driven a manual master cylinder car before, and it did take some getting used to. Best start doing some squats if you want be able to mash the pedal enough ... espcially if you're mating a manual setup to stock pedals. Aftermarket pedals assemblies can give you a longer lever arm, and more torque to push the pedal. But done right, the net result is more stopping force.

Although I don't think I'd reccomend doing it on a car that's going to see much street duty. Mine's going to be a trailer queen, so I don't care about that