Braking - More then Big Rotors and Lots of Pistons

[johnc]

From Mark Ortiz's latest chassis newsletter:

 

BRAKING DISTANCE, BRAKE TORQUE, AND TIRE TRACTION

 

I recently saw a quote by John Barnard about the early days of carbon fiber, where he said that Niki Lauda was braking for a certain corner at 100m but when carbon brakes were tried he could brake from 60m.

 

I have always been under the impression that if the brakes on a car were powerful enough to lock up the wheels, then the shortest braking distance would be dictated by the load on the tires and the coefficient of friction between the tires on the road. Given the same tires and road surface, how can one type of brake stop a car faster than another one?

 

Perhaps John Barnard reads Racecar Engineering, and perhaps he will see this when it appears in my column, and perhaps he will favor us with his own explanation. Absent this, I will speculate as best I can.

 

In theory at least, braking is primarily limited by tire traction, provided the brakes can lock the wheels. The brakes can only stop the tires. The tires then have to stop the car. However, real world braking at the end of a straightaway is constrained by some additional factors besides sheer braking power.

 

First of all, the driver is not locking the wheels. The driver must avoid locking the wheels, in order to maintain directional control and not flat-spot the tires. That means it is crucial that the brakes exert a predictable and consistent torque, and that the front-rear balance be appropriate.

 

It is important for the brakes to come up to desired torque promptly: they must have good initial "bite". They must apply equally on both sides of the car throughout the braking event, so that no large yaw moments result. Any problems in these areas will require the driver to brake earlier to compensate.

 

Brake release is more important than many people realize. If the brakes continue to drag after the pedal is released, not only does that heat the brakes unnecessarily, it saps cornering power from the tires. That lowers cornering speed, again requiring earlier braking.

 

Finally, brakes relate to driving technique, particularly as regards the ability to trail brake. It is commonly believed that trail braking was adopted primarily because certain drivers, such as Mark Donohue, recognized that it could improve lap time by allowing braking to be delayed. This is true, but it is also true that trail braking as we know it was not possible until the advent of brakes with good directional stability and consistency, and controllable release properties. In the days of drum brakes, drivers had to do their braking in a straight line because brakes were not controllable enough to allow the driver to turn and brake at the same time, or to controllably reduce braking while feeding in steering.

 

Any combination of these factors could allow one set of brakes to outperform another approaching a particular turn, even if both designs can lock the wheels.

 

The highlighted section above is why I keep harping about how a properly setup stock 240Z braking system can work better then a slapped together 13", 6 piston caliper setup. Its all about balance and control.

[cygnusx1]

Since I've upgraded to the AZC brakes on all four corners, I have begun to experiment with trail braking on quiet backroads up here in the sticks. I'll tell you that with the good stock brakes on my 280Z the car was squirrily under braking which did not inspire confidence. I think the confidence level that comes from good brakes has alot to do with how late and how you brake. A lot of it is mental.

 

At Watkins Glen last Summer (with the AZC brakes), the instructor commented that he liked the way I was using the brakes. I can tell you that with the old brakes, he would not have said that. Maybe it's due to the hardware or maybe it's just my brain (software) trusts the AZC brakes more.

[primadonna z]

I would concur with all above. I am running the Arizona Z Car 13" - 6 piston setup on one of my cars. And I love them.

A sucessfull system should create a harmony between the componets, create balance, and offer predictable function. This end product is a result of proper engineering, material knowledge, and testing. When buying a system do your homework and go with a proven design.

When mixing and matching on your own be prepared for alot of testing and changing.

If your performance, safety, and cost concience, a proven system like the AZC system maybe a bigger bargain than you thought.

Did I mention, They look great too.

[Challenger]

Doesnt braking also depend heavily on your suspension? Im not sure of the term but doesnt the angle of the front a arms related to the ground effect how the car squats and how it brakes?

[260DET]

With the S30 I found that adding a brace across the body underneath connecting the two tension rod pickup points on the rails helped stability under heavy braking at high speed. That was with an upgraded brake system, guess it would benefit the stock system too.

[Dat260]

260DET, do you mean something like this.

[Sparky]

260DET, do you mean something like this.

 

Thats interesting..hmmm....is that the white Japanese super Z?

[thehelix112]

It appears to me that there are perfectly valid reasons to increase both the size of rotors, the size of the calipers, and the number of pistons in the calipers. Whether they are the reasons why most people run such a setup is open for debate.

 

I would think it useful to increase the size of the rotor if you wanted more braking torque for a given brake system pressure. It would seem logical to do this when you are at the practical limits of acceptable system pressure; whether it be because the master cylinder(s) can't generate any more, or your brake fluid isn't rated to the heat involved in going higher.

 

Increasing the size of the calipers would seem to have two effects, one almost certain, and the other only potential.

 

The certain effect is that you are going to increase the size of the brake pad the caliper takes. I think you might want this because you want more life out of a given set of brake pads. A larger caliper means larger brake pads, which means not only higher thermal mass, but more longevity. This would probably only be important on endurance racing cars?

 

The possible effect is that you are getting more piston area. The more piston area, the more force is applied to the pads (and rotor) for a given brake system pressure, and hence, more braking torque. Obviously with an increase in piston area you will require additional volume in master cylinder(s) to maintain the same pressure/pedal-travel relationship (read: pedal feel).

 

And lastly, increasing the number of pistons in the calipers: I only mention this as this is what people are doing when they purchase the 6-piston wilwood superlite versus the 4-piston (as sold by AZC). They are not getting any advantage in terms of brake pad thermal mass as both calipers accept the same pad. The only advantage aside from increased piston area (as discussed above) is in terms of pad wear. The pressure from the 6 piston caliper will be applied more evenly along the pad surface, thus it is more likely to wear evenly.

 

 

Ok I got distracted with work and can't remember if I was going to say anything else. So i'll quite while I'm behind.

 

Dave

[thehelix112]

Oh thats what I was going to say, I completely acknowledge the points johnc refers to: that brake feel, balance, initial bite, and quick release are very important.

 

Balance can be tuned on the fly with an appropriate dual master/pedal box arrangement.

 

Initial bite and quick release are things I hadn't really thought about to be honest. At first glance I would think that this is all a big juggling act with brake feel.

 

If you want quick bite you want the pressure to come up ASAP, but that likely means a high pedal ratio and sensitive brakes which means more driver control is required and locking/flatspotting is easier.

 

How quickly the pads release from the rotor will (barring any jamming in the caliper) depend on any deformation of the pad (hopefully minimal?) caused by the pistons, and how quickly the pressure in the line drops to normal. The speed of this drop (ie, the system reaching equilibrium with the increased volume caused by the piston(s) in the master cylinder(s) retracting) will depend on how high the pressure is. The higher the pressure the faster the drop. I think, been a while since I learnt about equilibrium.

 

So from that, I guess you would want to operate your brakes at approximately the highest pressure the brake fluid can stand while not overheating the fluid during consistent hard braking.

 

Everything in automotion is a compromise. Very sad really.

 

Dave

[Dat260]

Thats interesting..hmmm....is that the white Japanese super Z?

 

Yes, it is from a white Japanese Z. I think the owner's name is Sato.

Cheers,

[cygnusx1]

What needs to be underscored is that a brake system need only be sized large enough to dissipate heat generated without exceeding it's maximum capabilities during it's intended use.

 

Not of any lesser importance, is the proper setup, balance, smooth-firm function, and proper friction material choice.

 

Engagement needs to be smooth and linear. Release needs to be smooth and linear. Pedal feel should translate to the driver exactly where the tires are in their "grip". The driver should be able to "find" the right pedal pressure and adjust his foot pressure to where it needs to be at every point during the braking manouver; consistently.

 

Performance braking is NOT pushing down on the pedal as hard as you can and then releasing it all at once. You need to be able to "drive" the brakes....modulation. Performance driving is more like a Ballet than a game of Football.

[MrFancypants]

It's a pet peeve of mine when people say brake improvements are required for high hp applications. Braking is determined by weight, not horsepower. Yes you can hit turns hotter with more horsepower, but the power isnt the real reason for the poor brake performance.

 

I agree 100% with all the other comments on modulation, bite, heat dissipation, etc. The stock Z brakes have terrible feel -- mine will be replaced soon!

 

 

- Greg -

[JMortensen]

A sucessfull system should create a harmony between the componets, create balance, and offer predictable function. This end product is a result of proper engineering, material knowledge, and testing. When buying a system do your homework and go with a proven design.

When mixing and matching on your own be prepared for alot of testing and changing.

And here I was thinking Dave came out with that setup because Ross had something bigger and he was trying to cater to the bigger is better crowd.

Did I mention, They look great too.

This is the key to selling parts in my opinion. Nobody does that better than Dave.

[Pop N Wood]

I would concur with all above. I am running the Arizona Z Car 13" - 6 piston setup on one of my cars. And I love them.

A sucessfull system should create a harmony between the componets, create balance, and offer predictable function. This end product is a result of proper engineering, material knowledge, and testing. When buying a system do your homework and go with a proven design.

When mixing and matching on your own be prepared for alot of testing and changing.

 

Aren't the bigger 6 piston calipers and 13 inch rotors a $650 option with the AZC front brake kit?

 

Guess they don't offer any additional braking power since AZC only offers a single brake kit for the rear.

[primadonna z]

Aren't the bigger 6 piston calipers and 13 inch rotors a $650 option with the AZC front brake kit?

 

Yes, they are a $650.00 option

 

Guess they don't offer any additional braking power since AZC only offers a single brake kit for the rear.

 

Are you sure about that? I would give it some thought.

[strotter]

OK, there have been a lot of interesting facts about brakes and braking going on here, but nobody has really answered the original question:

 

I have always been under the impression that if the brakes on a car were powerful enough to lock up the wheels, then the shortest braking distance would be dictated by the load on the tires and the coefficient of friction between the tires on the road. Given the same tires and road surface, how can one type of brake stop a car faster than another one?

 

Ultimately, everything else being equal, doesn't it come down to the brakes being able to bring the rubber/road interface to maximum static friction, and keeping it there long enough to bring the vehicle to a halt?

 

EDIT: reading the quote above, I'm getting the impression that the writer was under the delusion that "locked up" was the quickest way to stop; I know that it is not, so please pretend he said "the brakes were powerful enough to cause maximum static friction braking". Thank you.

[Mikelly]

There are a couple of things to consider here guys...

 

Weight is a factor, in rims, brake setups, and suspensions. Anytime you add those big brakes, you need to make sure to size them accordingly. In most cases the 12.2 inch setup sold by Ross or Mike Gibson (SCCA Here) are as efficient as you need. The mass is less, unsprung weight lowered, and with the appropriate pad and fluid, performance can be GREAT!

 

If you're not pushing mega HP, Sometimes bigger isn't always better... It can be OVERKILL and unwanted weight in the wrong areas!

Mike

[JMortensen]

Strotter, everything else being equal I think you are right. So the question then is really how big you need to go to get to that point, and then how small can you get them beyond that point and not have problems with boiling fluid and chunking pad material, etc. Basically on brakes (and wheels) you want the smallest thing that works, because it's all rotating weight. The bigger it is the larger a lever it is, meaning it is harder to accelerate and decelerate a 50 lb hub/brake/wheel/tire combo that is 26" in diameter than one that is 22" in diameter. That's not the most popular position, but it is mine.

 

Every time this subject comes up I think back to about 10 or 12 years ago... anyone else remember when AZC advertised the 12" brakes "For Z cars that regularly see over 200 mph"? I wonder if the 13" are for Z's that regularly see 250 mph... Maybe there will be a 14" set for 300 mph Z's.

[tube80z]

Ultimately, everything else being equal, doesn't it come down to the brakes being able to bring the rubber/road interface to maximum static friction, and keeping it there long enough to bring the vehicle to a halt?

 

The simple answer is yes. But everything is always a lot more complex. I asked a similar question in the motec seminar and got a really long answer that was very interesting.

 

The short version is the system that you need to get to that peak friction as quickly as possible and need a really controllable system to hold it there and ride down the back side as you slow down.

 

Now we get into the gritty details of everything that contributes to doing this from caliper/mounting stiffness, disc growth at temp, friction materials, and break pressure timing, pedal box design, and on and on.

 

In the case of carbon brakes Claude told me that there are three unique things to them that help them be a lot better. They weigh a lot less, they hardly grow at all with temperature, and Cf of the braking material is very linear once up to temp.

 

He went on to explain that on a modern F1 car a driver isn't capable of locking the wheels at speed and a number of top drivers actually pull their foot back to allow them to literally kick the pedal to get the initial pressure spike up. He had data traces showing pedal pressure and stopping distance. He also showed us a number of tricks to get the most from the brakes.

 

Cary

[Pop N Wood]

Are you sure about that? I would give it some thought.

 

Well, maybe I was being too subtle. You were stating how well engineered and matched the AZC set up was. If it is so well balanced with the 4 pistion, 12.2 inch front brakes, how can it possibly be balanced with the larger stuff up front and the same rear brakes? Unless, of course, the bigger front brakes have no more stopping power than the smaller ones. So either one of those two systems is an unbalanced, mixed bag of catalog parts or "the option" just adds $650 of bling.