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Brake FAQ, mythbusting, and why you may not want a big brake upgrade
Car brake FAQ, mythbusting, and why you may not want a big brake upgrade
This article busts some myths about braking repeated on internet forums and describes basic brake system function. Many people ask for brake caliper upgrades on their street car but do you really want a big brake upgrade? Many racing upgrades like race brakes are of no performance benefit on a street car and often make braking worse - see below for the long explanation and proof.
What exactly do I mean by a street car? As an example, there's no reason a car would ever make more than two panic (0-60 mph) stops in a row during normal driving. Stop-go traffic would be considered light braking since the speeds are not very high and you're not slamming on the brakes. Track use or heavy, repeated braking is not normal street driving, that's track use. A sports car racing through a canyon or a truck towing a boat down a mountain is also not normal street driving. This page also assumes "everything else being equal". A base economy car is not designed to withstand the same type of "normal" driving that a sports car should be, nor is a car with 200,000 miles on the original suspension the same as a brand new car with a fresh suspension.
Common braking complaints
Brakes are squealing:
This is a common complaint. Brake noise is normally caused by a combination of rotors, cold pads, pad shape, and rotor/pad material. Sometimes it's a whisping or grinding noise and sometimes braking causes vibration which sounds like a high pitched screeching noise. Because the properties of pads and rotors change slightly when heated, as you drive, they warm up and sometimes the squeal goes away. It's normal for some performance pads to screech when cold as well. If you are on stock pads and rotors, try an occasional hard brake to change the bias to the front and heat up/wear down the front pads. If you are on performance pads, try less aggressive pads. Some cars use shims or anti-squeal grease (not on the friction surfaces!) to quiet the brakes. You may also notice rust on the rotor faces of a car that isn't driven often. This may cause some noise but will quickly wear off after a few brake applications.
Brake pedal isn't firm or no braking authority:
In any emergency situation with a braking problem, some of your priorities should be to keep control of the car, pump the brakes and brake as needed, steer the car towards a safe spot, and apply the parking brake as hard as possible (without locking the tires) to slow down. The parking brake cable on many German cars actuates the rear calipers with a lever at the caliper. It's not as effective as using the brake pedal but it's something. Some of the newest VWs use an electronic push button parking brake, I'm not sure how effective that is in an emergency situation. Other cars often use rear drum brakes as the parking brake or a shoe inside the rear disc brake. You can try downshifting to use engine braking to slow down the car as well. Do not get distracted from driving when trying to troubleshoot a braking problem, your first priority is to keep control of the car! If you see leaking brake fluid around the wheels or in the footwell near the brake pedal or master cylinder, have the car repaired before driving!
If you have no braking authority and the pedal goes straight to the floor or almost to the floor, the car has lost brake fluid pressure. Pump the brakes and if either the front or rear brake fluid circuit is still good, you should get some braking action on the good circuit. Also apply the parking brake as needed. A common cause of this is a bad brake line near the caliper. Another possible cause is air contamination. If you are in a racing environment, the brake failure is probably due to overheated pads/rotors or boiling brake fluid.
If you have no braking authority but you feel resistance in the pedal, pump the brakes and try to build up pressure and apply the parking brake as needed. Possible causes are bad caliper, master cylinder, or brake pads. If the brake slowly goes to the floor, this is normally a bad master cylinder. Look under the dashboard and at the engine bay firewall for leaking fluid or stains. If the brake pedal feels very stiff and hard but you still have braking authority, press harder on the pedal or pump the brake. This is normally a faulty brake booster caused by a vacuum leak.
Shaking through the brake pedal or vibration when you press the brake pedal:
Something is vibrating and the force is transmitted through the brake pedal. If you are driving very hard or slamming on the brakes, it could be the ABS pump cycling. You can normally hear the pump humming. Vibration could be the pads, caliper, or hub but it's normally a rotor issue. Check for uneven brake deposits (splotches on the rotor face), rotor runout, and hub runout. Take a runout reading with the rotor bolted to the hub. If it needs to be turned (machined on a brake lathe), that's fine as long as it doesn't go below minimum rotor thickness and specs. Because VW rotors are so cheap, it may be better to just buy new rotors. VW rotors are also designed to wear out shortly after the pads are worn out. If it's due to a bad hub, you must have the rotor machined on the car or replace the hub. This process costs more but a flat rotor on a bad hub will not solve vibration issues.
Brake components and increasing braking performance
Sticky tires should be the first major or supporting step to upgrading braking force or handing in any way. Any increase in braking force is pointless if you do not have enough tire grip. Otherwise, it will only result in the tires locking up and skidding, (with non ABS cars) increasing braking distance through a loss of control. Again, the car can never stop faster than the tires will allow (unless you have reverse thrusters or you just hit a brick wall)! The average car weighs 3000-4500 lbs and the actual contact area per tire with the road is not much larger than your hand. To the right is a thumbnail illustration (click to enlarge) of how the tire footprint changes as the car's balance changes. The other half of tire traction is the braking surface. Snow or loose gravel don't care how good your brakes are, only how good your tires are. Even clean and dry asphalt will have different braking properties depending on the construction, age, wear/tear, and temperature.
A tire with a more aggressive tread compound/pattern will increase how much braking force can be applied to the ground. Don't buy the super cheap tires at your local tire store: they tend to be louder and constructed less sturdy - you get what you pay for! Your car and your safety are riding on the tires so before you upgrade brakes, consider whether or not the tires can handle it. Use a summer tire and if you live in an area which requires you to use winter tires, switch over when needed. All season tires have average performance in summer/winter but summer tires will have better braking performance (except during the winter), everything else being equal. Run flat tires are also at a handling disadvantage compared to non run flats. You can also choose a wider tire on a wider wheel and/or adjust tire air pressure lower. This increases and changes the size and shape of the tire footprint. The drawback is that lower air pressure and wider/more aggressive tires will reduce fuel economy. Because of this, it's best to get a tire that can handle all of the braking force that your ideal brake system will have. For some people, this is the stock brake setup. Some people will choose a big brake kit. See 1000q: mk3 BB kit or 1000q: mk4 BB kit for a chart of available big brake kits and installation procedures.
As a rough theory, each tire should reach the limit of traction at about the same time to maximize straight line braking. This uses all possible traction on all 4 tires. In reality, the stock rear brakes sacrifice some braking performance to prevent them from locking up and skidding, sending the car spinning out of control. This is also to account for differences in car weight and balance from different fuel and cargo loads, a shifting center of gravity, front/rear brake bias, and changes in tire/brake setups.) Read the scanned articles below if you don't believe me. In the second article, every tested car had "brakes strong enough for the [ABS] system to hold its tires on the verge of lockup for at least one full stop." The tested stock Passat even had consistent braking over 25 panic stops from 70-0 mph. Maybe there is something to being tuned for the Autobahn! Under the exact test conditions in the article, that Passat's brakes were not overloaded for it's setup but this doesn't mean that brake performance couldn't be better, just that the brakes are not the limiting factor. With that exact car and identical test conditions, I believe that you could increase tire grip until the brakes or another component became the limiting factor in stopping performance. The stock setup is good in theory because it's better to have 25 consistent stops versus better braking and then having the pedal go to the floor on the last "stop", a condition which will lead you to the site of the crash. In the real world, you will never see 25 panic highway speed stops on the street in a row so there is some room for improvement for most drivers/driving styles with that exact car/test conditions.
The suspension is another major factor in braking performance. A worn suspension changes the way the brakes are supposed to behave by letting the center of gravity move more during braking and changing the balance of the car. VW-Audi suspensions tend to be on the softer side and after some wear and age, get even softer. The VW suspensions in general seemed to be tuned tighter in the 5th generation cars. New struts, strut bearings, mounts, control arm bushings, and sway bar bushings in the front will do a lot to restore the handling and braking bias on your VW. The rears need new mounts and struts. See the FAQ section 1000q for writeups on how to replace these components.
If your car feels like a heaving dinghy in a storm then a big brake kit won't do much. Also note that some brake bias valves on the rear beam axle of mk3 VW are rusted and may not work. It's supposed to change the bias as the car heaves forward during braking. Don't adjust bias by yourself, take it to a VW or brake hydraulic system specialist who has the tools to calibrate it. All mk4 and newer use electronic brake distribution built into the ABS.
Brake Pad choice:
There are too many brands to list so I suggest a normal street pad for street driving. Most TDI drivers would prefer an OEM-level pad since a normal level pad should make less noise. However, OEM type pads can dust more or less than aggressive performance pads, it totally varies by brand and model. In general, the stock VW metallic pads last longer but are noisier than organic pads. See below for an explanation of ceramic pads.
Although more aggressive performance pads can bite harder (due to friction coefficient) and withstand higher temperatures, this normally doesn't translate into shorter stopping distances on the street. It will change bite and feel but in most cars, the pad is not the limiting factor in stopping. In other words, pads on an otherwise stock car may cause you to feel the brakes are better because they bite harder with less brake pedal travel. A higher friction pad can exert the same torque on the rotor with less pedal force but this initial bite does not necessarily stop the car in a shorter distance. With a less aggressive pad, you just have to press harder on the brake pedal to exert the same torque on the rotor. If you've just read this and don't believe me, read the scanned article at the bottom "Pulp Friction". Again, this article is written with a focus on street use, not track use. For track use you definitely need performance high temp pads due to the high temperatures seen with track use.
Good braking feel is a combination of pedal resistance and travel, different pads change this. You might like how aggressive pads feel, so before buying a big brake kit, try changing the brake pads. If you have cheap aftermarket pads, this could go a long way toward improving braking feel. This shouldn't make the car stop shorter but it should bite harder with less pedal travel and change braking feel. Some aggressive pads that work well for street driving are the Hawk HPS or Axxis metal masters. There are too may brands and types to list, these are just two.
Track or race brake pads:
Never use "race" or "track" high temperature pads unless you are on a track. Race pads are designed only for track use and need to be hot to function well. In addition, the brake dust can become hard if it gets wet and isn't washed off which can damage the wheel finish. I don't like pads advertised for both track and street use like the Hawk HP plus. They stop fine for street use but produce more unnecessary dust and squealing than a street only pad with no increase in street performance. It's always best to use a separate street pad and put in a track pad once you are at the track but this is inconvenient. Like all season tires, they are OK at satisfying contradictory goals but not great at anything.
This is a way of breaking in pad and rotor surfaces to maximize braking force. Although following a strict technique is not mandatory for street use, it is recommended by all brake makers. You probably won't find the technique in your owner's manual so consult your brake manufacturer for the specific procedure. The basic idea is to warm up the brakes and then transfer an initial layer of pad material onto the rotor and heat relax it. This increases braking performance and life.
After the brakes are properly bedded, the pads contact a thin layer of transferred brake pad material instead of the rough surface of the rotor. This adhesion between the pad and transferred material on the rotor is a major source of braking force in addition to the abrasion of a pad scraping the rotor. You can read more about this at the Stoptech site's notes on bedding. As a side note, new tires also have a slippery surface that has to be worn away with normal driving, so new brakes and new tires = less braking force.EM vs. aftermarket rotors, cryo treating, zinc or cadmium coatingOEM rotors are made to a certain standard. Some aftermarket rotors are better and some are worse. Some ebay rotors use low quality iron and won't work well compared to high quality rotors. The difference is weight, balance, machining, longevity, and resistance to cracking. Because of this, I recommend name brand or OEM rotors. I would be careful of buying rotors on ebay since they could be counterfeit. Cryo treating does nothing on a cast iron rotor because it's exposed to wild heating and cooling cycles. It's a marketing ploy. Zinc or cadmium coatings can improve the appearance and cooling of the rotor by reducing rust on the rotor hat and vanes.
Ceramic or carbon rotors and pads:
Carbon ceramic composite rotors are rotors made out of a ceramic resin material. Don't confuse them with ceramic pads or a coating. They are carbon fibers bonded with resin which is baked into silicon carbide. The silicon carbide is where the carbon or ceramic name comes from. To the right is a closeup of a carbon ceramic rotor showing the unique appearance.
You might hear of ceramic coatings on iron rotors - these are marketing ploys. Real carbon ceramic rotors are very expensive: it's about $16,000 for a set of "mass produced" Porsche rotors and a $30,000 option on some high end luxury cars. They don't make the car stop shorter on the street compared to a cast iron rotor although they do have advantages for street use. For $8,000 (factory option price) you get much longer life, less brake dust, 50% less weight (less unsprung weight increases handling and comfort), and they look really cool. Porsche estimates their PCCB rotors could last over 150,000 miles of normal street driving. Most drivers can expect about 1500 track miles out of PCCB. While they're on the second generation of rotors, there's no warranty on the rotors when used with aftermarket pads and there are still occasional complaints of cracks. Why don't they work better on the street? Reread the section above about tires and stopping force.
Multiplied by 4 rotors, you can lose an average of 38 lbs of unsprung weight which is very significant and noticeable. Such a big difference in weight improves both handling and comfort. In theory, this should also result in shorter stopping and greater fuel economy due to less rotating mass but in reality, the difference is well within the normal variations in fuel economy, road/tire surface, driver behavior, etc., so don't expect to see any difference there. For track use, the advantages of ceramic rotors are more apparent. The reduction in unsprung weight becomes more noticeable and they are much more resistant to warping and the high heat levels seen at the track. Because of their high price, you won't see them on cheap street cars anytime soon.
Although the huge weight reduction is excellent, my opinion is that PCCB isn't worth stretching for financially since most drivers will not see a noticeable increase in braking ability. There is a handling and comfort benefit but reducing wheel weight is a much better value. For 99% of drivers, spending that money on driving lessons will give far better results than fancy brakes. If you can afford it I would definitely get it but a better value for reducing weight are lightweight 2 piece rotors (see below) and the lightest wheels you can find.
Although ceramic pads are not "fake", beware of the term carbon or ceramic thrown around as a marketing term. If a single carbon ceramic rotor costs over $5,500 and a set of Porsche ceramic brake pads costs $600, how can I buy a set of four "ceramic" brake pads for $40 online? One look at the Porsche pads vs. other types makes it obvious which is which. Ferrari just introduced the first true carbon-ceramic brake pads on a street car on the 2011 599. So far, the best advantage of switching to "common" ceramic branded pads is that they dust less.
Drilled and slotted rotors:
I recommend blank faced rotors for all street cars. They cost less, perform better, and are less likely to warp/crack.
I do not recommend slotted or drilled rotors because they are not the determining factor in maximum braking performance. Before you dismiss that statement, slotted and drilled rotors DO change the coefficient of friction and brake pedal feel but this does not necessarily mean the car will stop shorter. Slotted and drilled rotors do clean overheated and glazed brake pads but this is not a factor in any way for street cars. While they'll change how the pedal feels, the tradeoff is how efficently they turn brake pads into brake dust with no increase in stopping performance. (On track cars, overheated pads are more common and could be helpful). For more on this, see the section below "the truth about big brake kits for the street" and read the scanned articles from reliable sources below.
The holes in drilled rotors were originally used to vent gas build up from hot brake pads during racing, a problem that you won't see with the materials used in modern brake pads. Drilled holes also don't have any noticeable effect on rotor weight. Using a 2 piece rotor is a much more effective way to reduce unsprung and rotating mass than drilling holes. If you just want to change the look of the rotor, slotted rotors are less susceptible to cracking than drilled.
Localized uneven heating of the material along the edge of the holes vs. the rest of the rotor also causes cracking from thermal stress. Although the holes might be chamfered (edge of the hole is smoothed) on the outer rotor face, they are not chamfered on the inner face. If someone says to buy only drilled rotors which have the holes cast, this is misinformation. All holes in rotors are CNC drilled and machined. There are rumors of Porsche casting the holes but this is a myth. If they did, they are the only manufacturer to make their own brakes like this and they did a really good job making cast holes look machined. Nobody has ever proved it and there is a cash reward for proving that Porsche casts the holes. To this day, it's unclaimed so I hope you too can help this myth die. Casting and precisely machining the small holes adds difficulty and an expense not reflected in the rotor price. Lastly, machining the holes after casting would reduce the benefit of casting holes.
The argument that they are for racing is also false - no aftermarket big brake kit vendor recommends a drilled rotor for heavy track use. Go to a race track and see what the race cars are using. Depending on what type of event you attend, you may see cars with drilled rotors but a race team can afford to go through a few sets each season and they might have certain parts on their car due to sponsorship or regulations. If drilled rotors were that great, you would expect to see them on the landing gear of jet aircraft where weight and performance are critical but they use blank faced rotors too. If rotors saw extreme heat as in just came off the track, not just came off the highway, keep driving for a bit to get some cooling air flow during the cool down lap. This will help avoid slow cooling and warping. You should never see the limit of a quality rotor during normal street use so just stick with what came with your car. Once you go to track tires, higher speeds, and extreme use, the car is not seeing "normal street use".
If your car sees track use, brake ducting is a good second step (track pads would be the first step), not drilling holes in rotors. Use temperature sensitive paint to test rotor temperatures to determine ducting and brake system needs. Don't bother using an infrared heat gun unless you can aim it out the window while braking. Pictured right and below are examples of OEM front brake ducting on a Porsche and my personal mk5 VW Jetta.
Here is an example of simple aftermarket brake ducts. The duct attaches to the lower control arm and guides air into the brake rotor. Some performance cars even have plastic ducts attached to the control arm from the factory.
Some late 80's early 90's wheels were even designed to extract air and cool the brakes. The wheel spokes were designed as fan shapes. As the wheel spun, the fan shaped spokes would draw air out. These aren't used anymore because the wheels are directional (can't be switched left vs. right) and modern styles are more open designs that show more of the brake.
What direction should the drill holes or exterior slots point?
It doesn't matter if the holes/slots point forward or backwards, it's all the same. Depending on the manufacturer, the holes/slots may point front/back, straight/curved, it does not make a difference. Don't confuse the slots cut into the rotor face with the internal vanes because internal vane direction does matter!
What direction should the internal vanes point?
Virtually all modern cars use vented front rotors. Most OEM parts use straight vanes so they can use reduce manufacturing costs and use the same rotor on both left and right sides. Better rotors use internal directional vanes for better cooling. Remember, the drilled holes or slots on the outside face of the rotor are not directional but the internal vanes sandwiched between the rotor faces must always vent air out. In other words, accounting for the direction of rotation, the vanes should always push air out from the hub and "lean" towards the rear of the car. Again, brake ducting is a major factor in cooling brakes so consider that as well. Below is an example of internal vanes direction vs. external slot direction.
If the vanes are straight, they are omni directional and can be mounted on either the driver or pass side.
Why you may want aluminum hat or 2 piece rotors:
If you are serious about reducing unsprung weight, I recommend 2 piece rotors or floating/hat rotors because they are much more cost effective than carbon ceramic rotors. I doubt anyone is spending $20,000 for CC brakes for their street driven VW! These are rotors with a ring shaped braking surface bolted or pinned to an aluminum "hat" hub. It's more resistant to warping than a 1 piece rotor because the hat can expand and contract independently of the rotor and is much lighter due to the aluminum hat. Bolted rotors are much quieter but don't allow for as much expansion during heavy racing use as pinned/floating rotors. Some pinned rotors use belleville spring washers or spring clips to reduce noise but bolted rotors for street use are guaranteed to be the quietest. Even though pinned rotors for street cars come with springs or clips, some just make a lot of noise that is not acceptable on a street car. If you are heavily racing your car, you should be using pinned floating rotors since they allow for greater expansion between the rotor and hat metals.
Reducing rotor weight will noticeably increase comfort/handling and positively effect mileage, acceleration, and braking performance because you are reducing rotating mass and unsprung weight. The only negative is the high cost: they can be as much as three to four times as much for the initial setup. However, once the rotor ring is worn out, you can reuse the hat with a replacement rotor ring, which could make the lifetime cost cheaper - just keep telling yourself/partner that !
Here is an example of a Wilwood bolted 2 piece rotor. The bolts are safety wired to avoid backing out. The black piece is the aluminum hat. Even though it's larger in diameter than the cast iron rotor on the right, it's about 5 lbs lighter and cools better due to the internal vane structure. If you look closely you can see the directional vanes on the 2 piece rotor. While lighter wheels are also a priority for reducing unsprung weight, wheels aren't a replacement item.
Why some calipers are forward or front of the rotor and some are rear or aft
The position of calipers on the rotor is a result of routing brake lines safely, steering and suspension geometry, and hub/caliper design. Other factors are road grime, production costs, and cooling. There is no difference in stopping power because the rotor is a circle! See this article for more details: www.roadandtrack.com answers caliper position. Ferraris and Lambos use two rear calipers on the 360 - one fixed 4 piston for main braking and a floating caliper for the parking brake. There are even a few cars whose rear calipers are forward of the rotor on one side and aft on the other. Front calipers are generally on the same side because of the steering rack.
Stainless steel braided brake lines:
These can add braking feel but shouldn't change stopping distance on the street. The reason stainless steel braided brake lines are used in a racing environment is because it protects the internal line from track debris and it helps control line expansion. This firms up pedal feel by removing compliance but it probably won't make your car stop shorter. I recommend leaving the stock brake lines alone on a street car because they should last the life of a the car assuming regular brake fluid changes. While the stock "rubber" brake lines do expand very very slightly, steel brake lines expand much less, close to zero. The difference isn't noticeable on a street car.
Some SS lines can have a slightly shorter life compared to rubber lines. Although the difference is insignificant, improper installation, twisting of the line, or dirt getting into the steel braids can damage a SS brake line. A track car which has the brake lines changed every 2 years and doesn't see a lot of mileage won't see the limit of a SS line but a 16 year old street car with 300,000 miles might. Note - rubber brake lines should also be replaced on high mileage or old cars and if you track the car it should have relatively new brake lines! Broken metal braids can chafe the internal tubing and can cause a line break and loss of braking force. Plastic coatings or covers on the braid can help prevent chafing.
Always use name brand stainless steel brake lines and make sure the braids or lines are not twisted, and never ever buy ebay cheapo brake lines. Many of those are known to be non-DOT approved (or counterfeit) and prone to failure. A failure may be sudden and will cause you to lose braking force, probably resulting in an accident. Your safety, your life, and the lives of bystanders are worth more than the savings from buying cheap brake lines. Stoptech, goodrich, willwood, brembo, baer, ATE, or other brand name SS braided brake lines are more reliable than cheapo brake lines. Don't buy these lines off ebay because they could be counterfeit items. For the US, stainless brake lines must be tested to DOT FMVSS 106. New cars sold in the US must meet or exceed NHTSA Federal Motor Vehicle Safety Standard 106 and SAE standard J1401 for volumetric expansion, whip test, and burst/working pressure.
On a side note, a brake master cylinder brace can help reduce compliance on some cars. On some cars, the firewall (where the brake master cylinder is mounted) flexes under heavy braking. A brace stops this movement but its usefulness depends on the car and intended application. A modern car shouldn't need one for normal street driving and it shouldn't have an effect on stopping distance, only pedal travel and feel.
Why you need to change your brake fluid:
You have to change your brake fluid every 2 years or as needed because it breaks down with heat, age, and absorbs water. Modern VWs use DOT 4 fluid. Always use freshly opened bottles of fluid instead of old bottles because it may have already absorbed moisture from the air in the bottle and brake fluid is cheap! Moisture can also condense in lines and calipers. DOT 3, 4 brake fluid is designed to absorb water so that it doesn't pool in the low spots and cause rust. Old fluid can even cause damage to the caliper and rust the line, causing sudden brake failure. Water in the brake fluid significantly lowers the boiling point of the brake fluid as well. Track cars (not autocross or aggressive driving) should be using racing brake fluid and bleeding the brake fluid before (if not done in the last 2 weeks) and/or after each track session. Not using race brake fluid and a brake setup sufficient for a racing environment can result in total loss of braking authority!
The truth about big brake kits for the street
For street use, some cars can stop shorter from adding big brake kit but some cars will actually lose braking performance! The engineers who designed your car made the brake system according to the day-day changing weight and balance of the car (fuel load, passengers and cargo, etc), suspension settings, and the stock tires. While you could stop shorter if all four tires used all available traction, the rear tires sacrifice a little braking performance to keep from locking up and sending the car into a skid. This also accounts for differences in car weight and balance from different fuel and cargo loads, a shifting center of gravity, front/rear brake bias, and changes in tire/brake setups. With all mk4 and newer VW, the electronic brake distribution in the ABS supposedly applies the rear brakes first under light braking to reduce dive. Front brakes are bigger than the rears because under heavy braking, the car's weight shifts onto the front tires and gives them more traction.
Changing any brake/tire/suspension component from stock will change how the brakes behave. For example, a big brake kit on the front will increase braking at the front tire but this bias and weight shift doesn't let you take advantage of unrealized grip at the rear tires. This could result in longer stopping distances. In some cases, a medium big brake kit will give better braking than the large big brake kit just due to balance! All good brake kits increase braking torque without negatively effecting the bias. Good brake kits should keep about stock pedal travel and be matched to the brake master cylinder. Upsetting the balance of the car also changes braking behavior during turns too. Combined with how well the engineers designed the stock braking system and considering economic considerations with a mass produced car, you might gain performance or you might lose performance with a kit. A sports car might be already tuned from the factory for optimum braking performance. A softly tuned car might have considerable room for improvement over the factory setup.
This doesn't mean that braking feel is unchanged because like brake pads, changing the brake setup changes pedal feel, modulation, travel, and lowers rotor/caliper/pad/hub temperatures. Unfortunately, brake feel does not equal shorter braking distance - the weight shift, brake bias, etc., are effected by changing the brake setup.
Does this mean that big brake kits are a scam? Absolutely not! Different model cars have different limiting factors in braking. Doing nothing other than adding a big brake kit might shorten braking distances if the brakes were the limiting factor. Stickier tires, suspension changes, adjustments in the weight and balance of the car, all change the behavior of the braking setup. Some cars have smaller rotors with undersized thermal capacity. As a car ages or as you replace worn out parts, changing the setup, you might be able to take advantage of a big brake kit. A big brake kit is actually needed on many cars if they're taken to the track! A big brake kit has greater rotor and pad mass to absorb the additional heat, pad compounds that can handle high heat, etc. Stock Volkswagen and Audi brakes are fine for street/autocross, but can heat up pretty quickly when really pushed. Again, this article is written with a focus on street use but I've seen TDI perform very well at the track on stock rotors! Driver braking technique is also at play here. So would your car benefit from a big brake kit? It changes by car model, individual car setup, and your intended use, so the correct answer is "it depends".
For example, Eurocar did a braking test of the New Beetle. After 5 full back to back 60-0 mph stops, the brakes were overheated. Switching to the mk4 Golf R32 brakes kept the brakes cool after many stops. Brake ducting can help but if you have 2.slow or TDI engine New Beetle, then a big brake kit is highly suggested if you want to track the car. Ultimately, if you are making 5 full panic stops in a row during normal street driving, the problem is 100% driver behavior. Again, this doesn't include towing (the New Beetle wasn't designed to tow heavy loads) or driving down long downgrades (you should downshift for engine braking), etc. See 1000q: mk3 big brake upgrade chart or 1000q: mark iv big brake upgrade charts to see big brake kits for your VW, both OEM and aftermarket.
To put kits in perspective for the street, let's say a well designed big brake kit can reduce panic braking (single hard stop, not repeated stops) distances by 5 feet. This would be very good for a big brake kit on an otherwise stock car/stock tires, etc. 5 feet could be the difference between an accident and avoiding an accident...but cars don't get faulted for average brakes; drivers do get faulted for following too closely! For street driving, increasing the distance between traffic and driver behavior, everything else being equal, are the primary factors in avoiding accidents. More time and distance between cars also reduces stone chips, driver stress, and increases mileage because you don't have to tap the brakes as often.
In addition, driver behavior during an emergency braking situation is not to immediately slam on maximum braking. Most drivers will apply the brakes medium, then think "Oh sho!T, this guy means it!", then slam on the brakes. In this case, a big brake kit won't save you if you weren't braking hard in the first place. And if you just stopped that hard, the guy behind you couldn't and just hit you from behind! In either situation you shouldn't be following so closely anyways. If you skipped the first section where tires are often the limiting factor, read it again and read the scanned articles below. The mk5 Passat made 25 panic stops from 70-0 with consistent performance. Again, this doesn't mean that brake performance couldn't be improved or should be improved, it's just that the exact Passat tested under those exact conditions showed that tires were the limiting factor, not the brakes.
External links and Additional reading
http://www.sportcompactcarweb.com/tech/0710_sccp_point_brake_tech/index.html - case where a well designed big brake kit on a modified car resulted in worse braking
http://www.zeckhausen.com/Testing_Brakes.htm - test between stock, stock plus, and aftermarket brakes
http://www.europeancarweb.com/tech/0402ec_autotech_r32_brake_conversion/index.html - conversion of stock New Beetle brakes to Golf R32 Brakes and test
http://forums.corvetteforum.com/showthread.php?t=1034158 - compilation of posts about brakes from a Corvette forum
http://corner-carvers.com/forums/showthread.php?t=21273 - if you can prove Porsche casts the holes in their drilled rotor, this guy has a cash reward for you
Pulp Friction - 6 thumbnails, click for larger view -
scanned from Grassroots Motorsports and mkiv.com
The Power To Stop - scanned from Car and Driver Aug 2008