How do they determine what’s an acceptable stopping distance for a car? And would doubling the size of the brakes on my car halve its stopping distance, or cut it by some smaller number?
I don’t know about who determines what’s acceptable, but I would guess that doubling the size of the brakes would have little or no effect. The limiting factor in terms of stopping quickly is surely the friction between the tyres and the road, rather than that applied by the brakes. So, wider tyres might have more effect.
I seem to recall from school physics that increasing the size of the tyres may not be terribly effective; there would be more rubber in contact with the road, but the weight of the car is spread over a greater surface area which means that there is less ‘grip’ per unit area, you could overcome this by increasing the weight of the car, but then it’s harder to stop anyway and so on…
You could make the tyres of a more ‘grippy’ material, for instance softer rubber will better deform to fit the texture of the road surface, but it will wear away much quicker.
The brakes, tires, and suspension work as a team in stopping a car. There needs to be a reasonable balance among them to get good results. Figuring out this balance is what automotive design engineers do. While you might be able to get improvement by changing tires, brake components, etc., it would be a trial and error process. Could get costly and possibly even dangerous.
Installing larger brakes would almost certainly not have an appreciable effect on stopping distances. As GaryT mentions, the stopping distance of a car is impacted by a variety of interacting systems, and arbitrarily increasing rotor or pad size xould actually increase stopping distances.
What bigger brakes will generally do, however, is reduce fade, because they dissipate heat more effectively.
Unless you regularly drive your car under race track conditions, brake upgrades are simply not worth it, and can seriously compromise the saftey of your car.
Here’s some more info:
One theory concerning the distance between you and the car ahead is that you be 3 seconds behind. As the car ahead passes a telephone pole or other marker, count [ul][li]one thousand one []one thousand two []one thousand three[/ul][/li]The big problem that I have found is that if you maintain that distance in heavy traffic, other cars keep changing lanes and decreasing the distance and if kept up you might as well just pull over. 
This quote, taken from the first article that racekarl linked, seems to cut to the heart of the matter:
Here’s another article: Grassroots Motorsports on brakes Sorry I wasn’t able to find a more readable transcription; I know they’re out there but this was the best I could find with a net search unless you want a PDF file.
Here’s a short summary of some of the important factors in stopping distances:
-
Tires. These are the biggest factor if everything else is working right. Some tires are made of stickier rubber than others. Wide tires have an advantage because the contact patch is shorter and in contact with the road for less time for an equal amount of tire pressure, so the tires do not heat up as much as skinny tires under hard braking.
-
Brake pad material. Some pads work well when both hot or cold, while others start fading as they heat up, and if you run road-race brake pads on the street, you’ll find that they don’t bite very well until they are good and hot - not exactly what you’d want for a normal commute! The “organic” pads are the cheapest, and mostly made from paper and sawdust glued together - not exactly a material that works well for repeated hard stops, but it does have the advantage of not scraping the metal off your rotors. Semi-metallic pads work a bit better, while the carbon / metal pads are often considered good choices for hard use on the street.
-
The brake’s ability to absorb heat. This mostly depends on how much metal you’ve got there, but also depends a bit on how well cooled it is. A big rotor won’t heat up as quickly as a small one will. So if your brakes have a tendancy to build up heat and lose effectiveness, larger rotors might cure this. Some people will also use various devices to cool the brakes, ranging from mounting fans on the wheels (seriously!) to ducts to exotic water-cooling systems.
-
To a lesser extent, the suspension - it’s possible for some designs to put more weight on the front tires while braking, heating up your brakes more.
All of those changes have definite trade-offs. Sticky tires are more expensive and wear out faster, usually. Pads that stop faster frequently cause more wear and brake dust. Brakes that absorb more heat are heavier, which can affect handling. And a suspension designed to minimize weight transfer may provide a harsher ride. I wouldn’t suggest changing the suspension unless you really know what you’re doing, but the others can be experimented with a bit more easily.
Doubling the size of the brakes would definitely be excessive in most cases. That would either mean you have saucer sized rotors to start with, or would have to upgrade to rotors the size of garbage can lids. I’ve seen a pretty serious debate on one car discussion mailing list about whether increasing brake size on one particular car from around 10 5/8" to 11 3/4" would be worth the extra weight.
Okay if tires are the most important part, then why do (or did) car makers offer different sized brakes on the same car? The brake size varied with the engine size. Why not simply slap the largest size brakes on the car and be done with it? It’d save inventory costs for the car makers and auto parts places.
Do (or did) they really?
Some GM and Ford models had optional heavy-duty brakes. I’m not aware that engine size was a determining factor, in fact I’m often asked whether the vehicle has heavy-duty brakes when the engine size is known. I would expect them to be part of a towing package.
Yup. I’ve had several different cars where the brake size increased with the engine size. IOW, you had to know the engine size in order to get the correct brakes for the car. It was a real bitch to discover this when I owned a car that someone had shoehorned a V-8 into and the car was originally a 6 banger. There were three different possible sizes for the brakes on that car (and three different size engines available when it was made), and it was a pain just finding parts for it, muchless checking to make sure that the ones I got were the right ones.
Contrary to popular opinion, the area of contact between any surfaces, does not ahve any bearing on the frcition that is generated between two surfaces.
In other words, having fatter tires does not improve grip on the road. However it may improve stability and that is whole different story.
Cite: http://hyperphysics.phy-astr.gsu.edu/hbase/frict3.html#are
To answer fletbert’s question:
Gary T is right about trucks having larger optional brakes to go with their towing capacity.
On cars, as I mentioned, the larger brakes on certain models have very little to do with stopping distance but much more to do with fade resistance.
A car with a larger engine is 1) heavier and 2) theoretically operated at higher speed. A heavier car with the same tires as the base model will generate more heat in its brakes for a comparable stop (remember that the physics behind how brakes work is that they convert kinetic energy into heat). So the larger brakes will dissipate the heat more effectively to prevent brake fade. Heat is the #1 enemy of street car brakes.