Audi never seemed to fit any TT's, spare the ALMS editions, with particularly flattering wheels in my opinion. It's no surprise that people constantly swap their wheels for something more appealing. Here I will address some common points about choosing wheels for this car.
Things to know when buying wheels:
-Offset (ET 38/35/etc)
-Centerbore (usually around 70mm)
-Bolt pattern (5 x 100, 5 x 112, 5 x 130)
-Big brake compatibility
This determines the overall look of your new setup. Most stock wheels are 17's and it seems like a majority of people run 18's while some run 19's. With 17's or 18's you will be left with a hideous wheelgap. With 19's, it's not as bad, but still could use lowering.
Diameter also affects things like ride and handling. The bigger your wheel, the less tire sidewall you ride on. Therefore, your ride will be rougher, but most likely your handling will be more responsive. I found my car's handling to be incredible with riding 1.5" of sidewall on my 19" wheels with a slight tire stretch to firm up the sidewall even more.
This is useful for determining the width of tires and also used for choosing spacers. Wheel width is measured on the inside of the bead so the whole wheel in reality is slightly wider than the given dimension. For instantce, a 225/55/17 tire will still fit on a 7.5" wide rim without bulging even though 225mm (the tread width, designated above) is really about 8.6". A 225mm tread tire on an 8.5" wide rim will usually produce a moderate stretch effect.
Stock offset varies, mine was +38, meaning the mounting hub on the wheel is 38mm towards the outside of the car away from the center of the wheel. Other stock wheels may have different offsets. A smaller offset will move the wheel outwards whereas a larger offset will move it inwards. Most aftermarket wheels with offsets close to stock will sit deep in the wheel well, making for a non-aggressive look. To correct this, spacers are used. Use this to determine the size spacers you need for your wheels:
Make sure the spacers you buy are hubcentric to both the hub and the wheel. Also, make sure to buy longer wheelbolts for mounting your spacers; a rule of thumb is take 28mm (stock lug bolt length) + the thickness of your spacer to get the length of lugbolt you need. For example, a 15mm spacer would require a 43mm lugbolt. If you can't find exactly the number you need, feel free to overshoot by a few mm. My front hubs easily swallowed all but 2mm of my entire stock 28mm lugnut. I bought my spacers and bolts off ECS Tuning.
Also, if you are installing spacers yourself, make sure you torque them properly and retorque them after driving on them for a few hundred miles. One should also apply a thin layer of grease between the spacer and hub as well as between the spacer and wheel to prevent the spacer from seizing onto the wheel or hub.
Most people aren't aware of centerbore and its importance. Today, most wheels are hubcentric; that is, they are centered on the hub by the circular protrusion of metal in the middle of the hub in the middle of the lugs. This takes stress off the lugs and ensures a perfectly centered wheel. The TT uses a 57.1mm hub center to mount wheels on...unfortunately, this is not a standard. Therefore, manufacturers make the centerbore, that is the hole in the middle of the wheel that mounts to the hub, fairly large so that it may be adapted to all kinds of cars. So before one can mount a wheel, they need to buy hub centering rings to match the centerbore to the hub center. For example, my AT Italia wheels have a 67mm centerbore so I have hub centering rings that adapt them to a 57.1mm centerbore to match my hub. Hub rings run around $20-$30 a set depending on whether they are plastic or aluminum.
The TT uses a 5 x 100 bolt pattern. Unfortunately, many newer VAG cars use 5 x 112 and just about all Porsches use 5 x 130. One can buy adapters, but they are usually in the neighborhood of $300-$400 for a full set including bolts. Wheels can also be redrilled into a different bolt pattern, but generally one should just stick with 5 x 100.
Big brake compatibility:
Not all wheels will clear big brake kits. If you plan on upgrading to one, verify compatibility
People are always very concerned with how much wheels weigh. "Unsprung weight," which includes the weight of the wheels and brake rotors along with other components of the car that are not held up by the suspension, play a big factor in the performance of a car. It's easy to see why; it's harder to make a heavier object spin and it's harder to make a heavier object stop spinning. Stock wheels are quite heavy for what they are; factory six spokes weigh around 28lbs each, factory 18's weigh a little more than that. This is really embarassing considering most 19" wheels don't even weigh that much. Another consideration in unsprung weight is weight of the tires; tire weights can vary a lot.
Overall, I haven't noticed too much of a difference going from stock 17's to lighter 16's to 19's that weigh about the same as stock 17's. Overall the 19's handle best for me since they have small, firm sidewalls. The 19s did, however, noticeably slow the car down at highway speed.
That's it for Part 1. Part 2 will address some more specifics.