Is the additional stress on the stock brakes from upgraded wheels a result of more diameter or of more weight? If it's weight, would 17" wheels of less weight that stock 15"s not hamper the stock brakes then?

Also, would a narrower wheel help maintain the turning radius?

 

Braking extra rotational weight is offset by the common use of larger tires on 17" wheels. The wheels are harder to stop slightly but they also grip the ground much better, making the stock brakes more effective. But if rotational weight is much greater than the increased traction, you might noticed weaker brakes but that would take a really heavy wheel or narrower than stock tires.

A narrower wheel would only help turning radius if the wider ones were somehow restricting turning motion.

Hope that helps.

 

It's mostly a result of the weight. The diameter of the wheels should remain the same if you got the right sized tire for the larger wheel. 17" wheels of less weight than 15"s will still hamper the stock brakes because the weight is out at the edge of the wheel, and if you remember physics, the further out the weight, the greater the torque on the brakes, hence the more force the braking system has to dissipate. This is why big brake kits are so effective, the larger the rotor, the more torque you get to stop. I've had more experience than most with what larger wheels do to braking. I went from the stock 15"s to 17"s to 19"s. And I can say that the larger wheels will have a noticible effect on braking. When I had 15"s, I was always locking brakes up. But when I got the 17"s, that stopped happening. Instead, I noticed that I had to press harder on the pedal to get the car to stop. It still stopped well due to the nice tires I had, but it required more effort. So I got the 300zx brake conversion and things were cool again, stopping distances went down as did pedal effort. Then I recently got 19"s, and now my brakes are noticably worse. I have to press down harder on the pedal, and sometimes it feels like I'm trying to stop a freight train. It's pretty much all due to the weight of the wheels. Now I'm looking into getting the Stillen big brake package. It's a viscous cycle.