Thanks folks, I had read all three of those threads before posting. The points GSB addresses are absolutely my concern also. I'm no mech. engineer, so keen to understand exactly what the issue is regarding balance? Is it just front to rear axle, i.e. for a given amount of brake force (retardation) the same front to rear differential (if any) needs to survive a brake upgrade? I'm assuming the ESP and ABS are monitoring rotational speeds relative to one another only, meaning any increased brake retardation on a wheel on the front axle needs to be proportionally matched at the rear. Can't this be achieved by upgrading front and rear? I hate those floating calipers, so a monobloc upgrade is a must.
Does the S3 use any form of brake steer (other than during ESP intervention)?
Basic Brake Balance is front to rear, and is in place to compensate for dynamic weight shift when braking. Discounting the vagaries of ABS/ESP etc for the moment and running the risk that some flat earther will again accuse me of being anti-modification;
Key facts, not open for debate, and proven to be as valid as gravity;
1/The higher the centre of gravity, the more the vehicle will tend to lean on its front axle when you brake. (Dynamic weight transfer)
2/ The limiting factor that dictates ultimate stopping distance is always the tyre.
When braking, the tyre has a point where grip becomes slip, and that point is where you extract the maximum amount of braking effort it can transmit from brakes to road. All other things being equal, that point is very dependant upon how much weight is on the tyre. Dynamic weight transfer puts a larger amount of weight over the front axle when braking, consequently the front brakes can brake harder and do more work than the rear brakes, and this is why they are usually larger and have a higher capacity for heat than rear brakes.
To stop the car in the minimum possible distance, then all four tyres need to reach the maximum point of grip-before-slip at the same time when you apply pressure to the brake pedal. This is perfect brake balance. If the brakes on the rear reach it first and lock up, then you have to much rear balance, and will end up **** first in a lot of hedges. If your front brakes reach maximum grip and lock up before the rears, then you have too much front balance. It's much safer that way, but it means the rear brakes aren't working as hard as they could be, and your braking distance is longer than it should be. The magical sweet spot though, is all four wheels working as hard as they possibly can at the same time.
If you slap a set of big discs on the front, or calipers with more/bigger pistons, then the balance moves forward. The brakes at the front will reach the grip/slip point much earlier than before and consequently the rear brakes won't be working as hard. Remember that the ultimate limit of braking is the tyres, so if your fronts are at 100% deceleration, but your rears aren't even breaking a sweat, your ultimate stopping distance will increase. To regain the balance you need a commensurate improvement in the capacity of the rear brakes. Even then your stopping distance will be no better than it was before, since ultimately it's the tyres that bring you to a stop. If you want to improve further, you need stickier rubber. If that wasn't enough to consider, then know that if you lower a car by any meaningful amount, the dynamic weight transfer reduces, and the fronts carry less weight for a given amount of deceleration. In that case it would be the rear brakes that would need upgrading, not the fronts....
This is with a basic non abs system. With abs the complexity level goes up even more...
Now, I'll be the first to admit my chassis dynamics theory is a bit rusty, but unless VAG have come up with a self aware, self calibrating abs controller, then ESP, brake steer and ABS systems are still very dependent upon being preprogrammed with key data related to how much volume of brake fluid to add / release for a given amount of braking torque change. If you put bigger discs on, you increase braking torque for the same pedal effort and suddenly the calibration of the abs is wrong. If you put bigger calipers on, you drastically alter the amount of fluid required to achieve the same increase or decrease in braking torque and the calibration of the abs is wrong.
These are issues that no BBK vendor has ever adequately explained to me, and I've looked into it for every car I've had since 1993. There's an awful lot of "no, we've never had a problem in testing", but then the testing regimen may have been a quick lap of the local council car park for all I know. The total lack of firm data, or even evidence that vendors even have the slightest clue how brakes or ABS work is frightening, so frightening in fact that I've stuck with the tried and tested method of either leaving it alone, or putting on better tyres & better pads. Both seem more palatable than spending four figures on a load of snake oil that buggers my abs up, and leaves me trying to explain to an insurance loss adjuster / magistrate exactly how these untested and non OEM approved modifications to a critical safety system were justified.
