On the subject of big brakes, lets try to collect some facts which we can all agree on. Lately there has been a lot of debate on this issue, and it would be nice to get facts straight on pros and cons. not least it would be nice to get this very complicated issue explained in a way we can all understand.
In this first post of the thread it is my idea that we build a knowledge base of questions and (correct) answers which we can agree on. As the thread grows with Q&A's from members, I will Cut & Paste them to this first post. To the best of my ability, I will also try and post the Q&A's in some form of logical order.
I've started out by writing a few questions and answered them to the best of my knowledge.
All answers in this thread is subject to revision at any time if necessary. Feel free to raise an issue if you think an answer needs to changed or rephrased.
Hope you will all contribute with both Q&A's
How can you reduce brake fade.
1)increase the size of the brake disk. More surface means the disc can absorb more heat
2)Improve heat dissipation. Drilled holes can help, but in general discs are cooled by surrorunding air.
3)Ensure gasses are not trapped between pad and disc. Modern Performace Brake Pad compounds almost eliminated this problem. Grooves in the disc will also help.
4)Use Brake fluid with a higher boiling point. It will only help marginally in terms of absorbing heat. As the discs can no longer acept any more heat they transfer heat to pads and pads in turn transfer heat to the brake fluid. Boiling the brake fluid is dangerous and means you have to change it. Using high temp brake fluid is at the very least practical and it adds greatly to safety.
Is brake fade a problem in daily life?
1)Brake fade only occurs if you continuisly brake from high speeds. Factory fitted brake systems is designed and tested to leave plenty margin for any sudden emergency braking, even from the vehicles top speed.
2) As we all know you could experience brake fade if you are f.ex driving down a mountain side.
Does bigger brake pads mean the pads press more or harder against the disc?
1)No.. the clamping force is just distributed over a larger area. You will have less disc and pad wear, and the generated heat is spread over a larger area, but clamping force is the same.
How can you increase clamping force?
1) Press the pedal harder.
2) Increase the master brake cylinder piston bore
3) Change the mechanical pedal input ratio.
q) Is there a maximum for how much clamping force can be applied?
1)Yes. the combined output of the mechanical pedal ratio and master brake cylinder piston bore plus compressabillity of brake lines and brake fluid results in a certain amount of pressure. This pressure is basically what the calipers have to distribute to the pistons. Bigger caliper piston bore means the pressure is spread out over a larger area. Think of yourself standing on either a sewing needle or an oil drum. Even though the pressure is the same, the needle will be pressed into the ground by your weight, while the oil drum will not.
q)Is there a maximum for how much clamping force can be used?
1)Yes the tires is the ultimate limit. How the clamping force is transfered is a different matter. When the tires reach their limit of adhesion they lock up. If you have ABS, and if it works properly, it reacts by decreasing the clamping force to stop the tires locking up. This is the pulsating sensation you get in the pedal when ABS is working.
2)using the example of the Needle and the Oil drum you could also apply too much pressure for any component in the brake system to withstand. F.ex Piston seals, or discs could be stressed too much.
q) If caliper pistons clamp the disc with the same force no matter what the size of the calipers, pads or dics are, what is then the point of bigger calipers with more pistons or bigger discs?
1) Assuming the brake pads, brake fluid, brake lines and brake system as a whole remains stock then:
-installing an 8 piston caliper with bigger pistons on the same size disc will distrubute the available pressure over a bigger surface. This means you will have to press the brake pedal harder and longer then before. Why is that you may ask..This answer comes in two parts..
First when you press the brake pedal, this again moves the master brake cylinder pistons at a certain rate. These pistons "pushes" brake fluid out through the brake lines which then causes the caliper pistons to be pressed outwards. How much fluid is "moved" is determined by the piston bore and the lenght of which the pistons are moved.
a)If there are more caliper pistons then the system is "geared" to by its original design then these extra caliper pistons require more fluid to be pressed "out" To achieve this you have to press the pedal further which again cause the master brake cylinder pistons to move more brake fluid into the calipers.
b) going back again to the needle and the oil drum, you only need very little force to press the needle into the ground, but you need a lot of force to press the oil drum into the ground.
Summation: So what does this mean? Well it means that it is important to balance the calipers to the rest of the brake system. The way this is normally done is to decrease the caliper piston bore proportionally when you add more pistons. Worst case scenario is that you put on a caliper with so many pistones and with such an increased bore that you can press the pedal all the way to the floor without reaching maximum clamping force. Short term negtives is that you can't use Heel/Toe because you have to press the brake pedal down too far. Also that you have to use more leg power then originally necessary to acheive same clamping torque.
what are the potential problems by fitting calipers with oversized pistons and/or more pistons?
Brake performance really relates to tire performance and it's limit of adhesion. No other single component is as important as tire grip. In the end this is what stops the car, not the brakes. Think about a supercar fitted with the best possible tires and brakes doing two brake tests. One in the dry and one on ice. The only difference between the two is how much grip you get from the tires, yet the results are a world apart.
Matching the master cylinder to the rest of the brake system is important in the sense that if you do not do this, and f.ex install multi piston calipers with increased bore there will be a compromise to be paid in terms of how hard you have to press the pedal and how far you have to press it to achieve the same result. If your existing brake system was working without fade, then all things being equal you will not gain anything in terms of performance by adding bigger piston calipers. Lock up will happen when the tire reaches the limit of adhesion. As long as you can reach this with both sizes of pistons, nothing is achieved.
If you add a caliper with the same overall piston surface but this surface is spread out over more pistons and larger pad surface, then you will achieve increased pedal feel and brake control, which in turn should result in better performance, also ABS wise.
ABS works at the Master brake cylinder and not at the caliper pistons. This means that when a wheel locks up and ABS springs into action, it then runs a pre-programmed parameter setting releasing what it thinks is enough pressure to stop wheel lock-up.. In reality it is not enough if you have increased the caliper piston bore and number of pistons to such an expent that the inbuilt "slack" is not enough. ABS thinks it is stopping the wheel from locking up by releasing this pre-programmed amount of brake pressure at the master cylinder, but next time it measures rotational speed of the wheels (happens every 5 millisecond or so) it again detects that the wheel(s) is still locking up, so it again releases what it was told is enough pressure to stop the lock-up. Of course it isn't working, so the system could end up running in a perpetual incorrect loop which manifest itself by locking the wheels.
If you take the GTA brake upgrade as an example. The 305mm discs with 4 pot 34.5mm & 38.5mm piston calipers vs. the 330mm discs also 4 pot calipers with identical piston size but with a larger body to accomodate the larger disc diameter.
-Increased disc diameter improves greatly against fading.
-The larger caliper body and and larger brake pad surface improves brake feel and control.
-The identical piston bore ensures the calibration to the rest of the brake system is intact.
-Pad and disc wear decreases in comparisson to the 305mm system. (all things being equal)
-The bigger 330mm discs are heavier and creates more momentum of inertia. which causes the car to accelerate slower and requires more brake torque to decelerate.
-The heavier calipers and disks infulence handling, especially since it is unsprung mass.
-System runs cooler
how can you increase brake performance on an existing brake system?
There are several way to improve or optimize the existing brake system. First you have to decide what You are going to use the brakes for, and what are You not happy with at the moment. Most of us only want the brakes to work better in everyday life. If that is the case, then there is no reason to spend a lot of money on a big brake system. There is hidden potential in your exitsting brake system, if only You use the right components.
1)Tires - This might come a surprise to many. But Tires is the single most important factor when it comes to stopping the car. Brakes don't stop a vehicle, Tires do. Proper tires with proper pressure is THE key to brake performance. The way to choose a good tire is to read the tests carried out by magazines and so forth. Find the type of tire which suits your purpose and go with it. A good tire is one of the best overall investements you can do for your car.
2)Brake fluid - Make sure your brake fluid is of a good quality and make sure it's fresh. It's a good idea to use a "Race" type with a high boiling point. It's not that expensive, and could make the difference betwen not having to change the brake fluid after a Track day/Thrashing or not. Ordinary brake fluid can easily cook if you upgrade the rest of the brake system and put it to good use.
3) Brake lines - Upgrading the Flexi brake lines or just putting fresh ones can make a huge difference. Non-braided types flexes after years of use. It doesn't cost a lot more to get braided types, and it makes a very positive difference in brake feel. If you feel you have to push the brake pedal too far and too hard, upgrading the flexi brake lines will help a lot.
4) Brake Pads - Now we get to the thing most jump stright to. Choosing the right brake pads is one of the most important elements of the process, but it is also very sensitive to potential mistakes, so choose carefully and bed the new pads in properly. If you chose the wrong pad for your intended purpose, you could be left with an even more useless brake system then before. Most make the mistake of buying a brake pad which is "too Sporty" As a result the brakes don't "bite" properly, they squeal, and the brake discs are chewed up in no time. When choosing the brake pads, read the recommendations of the manufacturer carefully. Brake pads are designed with a specific working enviroment in mind, and putting a race pad on High Street in traffic will not make anyone happy. When you have chosen the Pad, bedding them in properly makes all the difference. Many people overlook this part of the process, and it is just about the biggest mistake possible. Different types of pads has to be bed in differently. Some takes a fair bit of work, some need almost no bedding in. The manufacturer always provide a recommended procedure. Make sure you keep the instructions handy, especially if you are having the pads fitted by a garage.
5) Brake discs - In most cases, there is no need to upgrade brake discs, stock discs will work fine even if you upgraded all the above elements. If you intend to use your car for the occasional Track day, then getting a set of race oriented front discs could be a very worthwhile investment. First thing you should look for is if the disc is heat treated. Secondly the Brand. Thirdly the pattern. Last few years many race teams have stopped using drilled discs. Some even stopped using groved discs. Drilling holes weakens the structure of the disc. Only reason to drill the holes is to dissapate heat. Grooves are there to dissapate gasses and remove glazing which build up during friction. Some of the more modern brake pads no longer produce these gasses, which is the main cause of glazing. Getting a performance brake disc with grooves still makes good sense, plus it looks cool! Some Alfas like f.ex GTA's with 305mm system or 155 Q4 will benefit from drilled front discs.
What are the benefits of Braided brake lines?
1)Increased "brake feel" The brake pedal will feel firmer, making it easier to modulate the brake force.
2)Braided brake lines does not expand as much at OEM lines, therefore they help shorten the lenght of which you need to press the brake pedal.