I found some usefull information from vwvortex.com about the downside of "big brake kits" that being the added weight of said kit effects the unsprung weight of the car. This guy has some calculations that prove that those effects are minimal. I copied the page but here is the link.http://forums.vwvort...read?id=2536336
Over the last few years on the Vortex there have been a multitude of posts arguing about the benefits and disadvantages of big brake kits. The most popular “disadvantage” is that a larger diameter rotor means it has a bigger rotational inertia, or moment of inertia. What this means in laymen’s terms is that a, say, because a 12” rotor is bigger, it “saps” more power from your engine than, say, a stock 9.4” set up. This is indeed true. However, how big a factor is it? I was doing some project work on this kind of thing so I decided to diverge a little and do something that might show me and those of you who are interested something pretty interesting. If you couldn't be bothered with the technical explanation, scroll down to "conclusion
". I sort of rushed through this stuff, so if there are calculation errors, please don't kill me, I'm just trying to help, educate and illustrate
The basic equation to calculate the moment of Inertia of a solid (as opposed to “floating” disc and assumed to be a constant-thickness disc) is:
Where m is the mass of your disc, and d is the diameter of your disc.
To interpret this, it means that the moment of Inertia increases with the square of diameter. This looks like a really scary thing! But again, we ask, what does it actually mean?
More useful than the simple moment of Inertia equation for demonstration is the Kinetic Energy (KE) which is required to rotate this disc to a specific speed, due to moment of Inertia. This is given by:
where is angular velocity of the disc, ie. how quickly it is required to spin at a given point. What this means is that the energy required to rotate the disc at that velocity is a factor of the square of velocity, and the square of diameter, as before.
What we will assume:
Car weighs 1100kg
Car has 16” wheels which aren’t coming into the equation
Car is on a flat road
9.4” rotor weighs around 3kg
Decent 12” rotor weighs around 5kg (this is pretty conservative)
If we take a test subject, let’s say a 12” non-floating rotor weighing in at 5kg, we calculate: I
And the energy at a car velocity of 100km/h (62mph) is
KE = 412.175 joules
Since the measure of energy that the engine puts out is simplified to be the energy your car has at a particular speed thanks to the energy the engine transferred, we can actually go and find out exactly how big a difference this EXTREMELY SUPER SIZED OVERKILL TRUCKLOAD MOTHER DISC WILL MAKE!
The graph below shows the ratio of the Kinetic Energy required to reach a vehicle speed of 100km/h starting from 0. The speed of the car signifies the energy that the engine was able to produce from burning fuel (simplified version of Kinetic Energy of the car).
Interpreting this, the curve shows that most of the energy that is ever used to accelerate the rotors is used at the lower speed band, and as you get progressively fast, the rotors rotate quicker and are thus are not as “difficult” to rotate. CONCLUSION
What the data and the graph shows is that most of the energy that is ever used to accelerate the rotors is used at the lower speed band, and as you get progressively fast, the rotors rotate quicker and are thus are not as “difficult” to rotate.
I’ve shown all the rotor energies in the table just for the hell of it – so you can see how the energy required by the rotor goes up exponentially with speed. In this table for TIME SPLITS (not the same as the above graph), the various energies for rotors and energies of the car at various speeds, the ratio of required rotor energy compared to energy from the engine, is the same at all speed splits. This percentage is 0.007494%. That isn’t a hell of a lot!!! So finally there is some evidence that even big brakes aren’t such big factors in “power loss” from the engine! Woot!
A typical 9.4” rotor would have a ratio % value of around 0.00276%. Calculating a percentage ratio of these two ratios, we land up seeing that from changing from a 9.4” rotor to a slightly heavier 12” rotor, you are increasing your energy “wastage” from the engine by 36.78% compared to the original value.
While this may seem sort of high, you have to understand that everything in tuning is a trade-off. I’ve highlighted the negative side-effect of going to bigger brakes, and in my opinion, 36.78% increase is really not much, because you’re looking at heat capacity improvements of up to like 120%.
Also, all the calculations have been done assuming the 12” is of one-piece construction, and weighs 2kg more than the 9.4”. 1.9..16vTurbo weighed his 12” AP’s and the floating rotor was actually lighter than the 9.4” stock rotor!! This brings that 36.78% down quite a margin, and it also reduces your unsprung weight which is great news for cornering performance.
Also, while I have used the term "big brake kit" in this article quite loosely, in automotive aftermarket brake systems, 12" isn't that big and is certainly the maximum diameter I would suggest for a Mk2. I saw an Audi A4 running around with some 14" Wilwoods, and the energy sapping of that size rotor is really going to be bad.
While wheels haven't featured in this, I think I'll do a similar article on energy that wheels require sometime in the future. A lot of people say that the upgrade to bigger brakes means needing bigger wheels and THAT is the big deficit, and I don't agree, because of the mechanics of them, they aren't as influential on inertia as one might think... But that, ladies and gentlemen, is a story for another day... Hope you enjoy, and it isn't too technical :/