hey guys. im wanting to lower my A3 1.8T 1999 and been told by a garage that i have to lower one end more than the other. i.e buying a
Weitec 50/30mm Ultra GT Kit - A3 8L
theres also a 30mm all round kit and a 40mm kit all round

whats the best one to get?

 

Here are what you need to determine when lowering a car. You will need to go on line to find a good discussion of suspension geometry to understand this. There are two axes that are critical to good handling. One is the roll axis. You will need to be able to draw out your suspension to scale after measuring critical distances. There is software to assist you. Once you have determined the front and rear suspension roll centers you draw a line through these to determine the roll axis. This is the line around which the car's body will roll in a corner.
Next you have to determine the "Mike Axis", this is an imaginary line through the car's center of mass. A good rough estimate is this line will run through the engine at crank shaft level in the front and will be at the bottom of the trunk in the rear.
With these two lines estimated, there will be a distance between the two. The greater the distance, the more tendancy there is for the car to roll. On most production cars, the Mike Axis is nose high too, in other words the center of mass is higher in front than at the rear. You can measure this distance by crawling under the car with a tape measure and measuring the distance from the bottom of the trunk to the ground and measuring the distance from the big seam in the engine block where the oil pan attaches to the ground, or from the center of the harmonic balancer to the ground depending on your under hood accessability.
The optimum lowering strategy is to lower the car such that the Mike Axis is dead level with the ground and the distance from the Roll Axis and the Mike Axis is minimized.
On most Audi suspenions lowering the front will bring the center of mass closer to the roll center, but this is not a given. Some other Mac strut cars when lowered will actually increase the distance between the roll center and center of mass, so to be certain you have to measure, draw and study how suspension geometry works or buy software that does this for you.
On my two wheel drive 1988 Audi 90 I used settings from Johan Abt, a former race tuner for Audi from the glory days of rallying. He sets a 90 such that the Mike Axis is level to the ground at 25cm. The front will be noticeeably lower than the rear. A good rule of thumb is that the lower control arms should be level to the ground. Don't go much lower than this or you begin to introduce a lot of bump steer and you will run out of travel. With the Mike Axis level to the ground there is zero brake dive, and at least on my 90 the roll center is now so close to the center of mass that there is almost no body roll in hard corners. No need for anti-roll bars. Perfect.