Is this a byproduct of increased rotational mass?

[Alin]

Rim weights only:
The stock 15" rims on my TDI weigh ~17 lb. (195/65R15 tire mounted on.)
The current 17" rims that I have mounted weigh ~23 lb. (225/45R17 tire mounted on.)

I haven't measured the actual weights with tires on a scale, but let's compare just the rims' weights for the sake of this thread since the combined weight is definitely more than just 6 lb. (It's a rather large difference in exact weight differences because I can lift the stock wheel with mere ease with one hand, and cannot do so with the 17".)

Does the increase in weight of a wheel (rim + tire) affect rolling resistance and engine braking effectiveness?

I've noticed that on an uphill, the car rolls back slower that before. I've noticed that on a downhill, the car actually maintains desired speed while engine braking closer than before (43 MPH at by the bottom of a hill instead of 47 MPH).

I suppose we can expand on this thread as well. Does a rim alone affect body roll/handling, or is it solely dependent on the tires? Let's say the overall diameters and the widths are of similar ratios, but you're comparing a 15" to a 17" rim. Well, this is the closest I can get it:

Would the 195/50R17 wheel handle firmer/stiffer and reduce body roll compared to the 195/65R15 wheel? Is the sidewall ratio the determining factor?

 

[Grinder34]

The biggest difference in TIRE between the 15" and 17" would be sidewall stiffness. With the 17" the sidewall needs to be a lot stiffer to prevent bending your rims on potholes, etc.. Think more runflat-ish.

I can make arguements both ways about raising/lowering rolling resistance.
Argument 1: Stiffer sidewall now takes more energy to deflect for each incremental amount of rotation, therefore more energy is taken out of the system, and the car slows more
Argument 2: Thesidewall will deflect significantly less and take less energy out of the system, slowing the car less.


As for body roll and general ride quality. The 17" will DEFINITELY be less comfy. Thing of the tires as a second set of linear-ish springs for the car's suspension. The tires are much less stiff than the actual springs, and absorb all the small imperfections, such as the feeling of a stone on an otherwise glassy road. With a stiffer sidewall, you're going to feel that stone a little bit more. And that's just on a glassy road. The extra 1" of give on (on each side of the tire) will really help absorb some of the bigger bumps like potholes etc.

 

[Alin]

On the topic of sidewall stiffness, how does the size of the sidewall come into play in regards to harsh impacts that could potentially form bubbles? Let's keep the 195/50R17 vs. 195/65R17 comparison as the example. In a scenario where the 195/50 tire could form a bubble from impact, would the 195/65 avert it? Does that extra cushion of material on the 15" tire help -THAT- much?

Let me ask this: During a pothole impact, is the air inside the tire just compressed? Thus, meaning the less material there is in the overall tire, the less room for the air to compress to? Is this how bubbles are formed?