Thats not to say this holds true for all because you can run a 225mm wide 17" and a 225mm or even 235mm wide 16" but usually 17" = 225mm and 16" = 205mm. The wider tire yeilds better turn in and provides more contact with the road. The 16" with its smaller contact with the road puts the same weight down in a small area which gives it better ability to go through snow. The same by going down to 15". To give you an example think of walking on snow with a high heel shoe and a snow shoe, one will go right to the bottom one will float on top. Weight distribution. If you want to bring the sidewall into it, a 16" usually has more sidewall(55mm) and will give a softer more comfortable ride, a 17" usually has 45mm and will increase the harshness of the ride esp. on bumps.

The most obvious reason people run bigger and bigger tires are looks. In a 400-500hp rear wheel drive car, I can imagine that you need a very large wide tire to put all that power to the group without burning out.

I'm shooting from my hip here, so don't count any of this as fact.

-Jenner
'00 180QX TT Garrett, H&R Sport Springs

on heavy cornering because on heavy cornering, then is less sidewall deflection or side to side movement on a low profile tire (rim stays above tire) so the tire's tread is flatter to the ground.

Whereas on a thicker sidewall, on heavy cornering, the wheel will move towards the outside of the turn because the sidewalls will deflect, the inner sidewall will lift the inner contact patch of the tire, hence reduced traction.

Note that on lower profile tires, because of minimal deflection, when tractio gives, it's gone without much warning. At least on higher sidewalls, you can tell if the car is starting to lean to much and sidewalls start to deflect, so you can compensate for it by letting go of that throttle

with a high profile tire, at the point that the leaning tire DOES let go, it tends to slingshot the car into oversteer (given a rear wheel drive). With soft sidewall tires this becomes downright frightening. Other than that I believe you're entirely correct.

Technically, the 17"s would mean a shorter sidewall, with all the associated benefits, but I wonder if the 17"s are heavier, thus increasing the car's unsprung weight...

This gets more complicated when you factor in the poor camber control of most strut type suspensions (like the front susp of the TT). What happens is that as the car leans, the tire will also lean outward (positive camber) to some extent. In these cases, a low profile (past a certain point) will actually keep the tire *too flat*, and lift the inside edge of the tire under hard cornering. In this case, a bit taller sidewall will give you the compliance you need to keep the tire firmly planted.

The TT probably corners flatly enough that this isn't too big of a problem, but it's something to keep in mind nonetheless. Especially, I'd think, w/18"+

Scott.

That goes out the door on lowered cars using 18"+ wheels that have been lowered where more negative camber occurs as a result front/rear.

... front suspension is able to control the camber when leaning in corners. If it's good enough for Porsche, I believe we too can live with it. ;-)

Not saying macpherson strut suspensions are inherently bad, just that they don't typically have the same level of camber control that say a double-wishbone setup does.

Heck, my M3 has struts, and I'm not complainin.

Just fyi, is all.

Scott
'99 M3/2
'98 A4 30v qm/s
'00 S4 on order
'0x tt on list...

BTW to any interested, I did some measurements on my A4 suspension when it moves around. For every inch it drops, front and rear, Camber gets more negative by 0.5degrees.

For every inch it drops on the front, Toe goes out 3X! it's previous measurement so for example, on a stand still, my Toe is at 0.05 degrees toed out, when the front got compressed an inch, it's 0.15 degrees - actaully adds some understeer.

For the rear, Toe goes in very minimally. For an inh drop, Toe in goes by 1.5 times.

Anyway, your point earlier about the camber going more positive is correct because assuming the car has a 2 inch travel on hard cornering, that's only 1 degree of compensation, but the body will be off more than 1 degree, - heck maybe even 5! so, every added roll of the body is positive camber Yikes!