Physics / wheel & tire gurus please step in

[BAMF AE]

So I'm visiting with my family last night and at the dinner table, my brother and I are talking about the new wheel/tire package I got for my ride and told him that I went from a 17/18 to a 18/19 setup and we both started talking about the physical dynamics of the increase in size and the possible ramifications that may occur. Please keep in mind that this was not an argument by any means. My brother had some very good points to share (being the scientific minded person that he is with a Master degree in Science) and I also had some good points to share (no Master's degree here). Now I know everyone has some kind of comment to add, ranging from humorous to serious. I'm not looking to start arguments with any of you all, yet I am looking for those that have pretty intelligent comments to add to our discussion, so if you're truly a wheel/tire guru and pretty good with physics, I would like to hear some of your thoughts on the effects (good or bad) that may result from a 1" increase overall. Some of the things that were already discussed (which I would like to continue with) are the following: -Torque -Fuel Economy -Spent Energy -Speed (overall horsepower) -Revolutions -Momentum -Acceleration (off the block, and WOT) Aside from the above topics, if you feel like you can add more, please feel free to do so..(i.e. something I may have left out or overlooked) I can think of a few people that would probably love to chime in (Y2KVert4me)-Tom who would like this topic as his knowledge has always been quite helpful to me, however I would also like to hear others' thoughts as well. I'm going to send this link to my brothers' work email and based on some of the comments, he may channel through me or just simply chime in on his own so I'm asking that if you do post, please continue to frequently check back! TIA, BAMF AE

[chuck1026]

I switched like you... went to 18/19 CCW's with the "new" C6 EMT's. Rim+tire weight increased approx. 5 lbs per corner.

My rolling diameters are nearly identical, so insignificant change there...

-Torque

Slight decrease as rolling diameter is larger by small percentage (negligable)

-Fuel Economy
-Spent Energy

Similar topic... given larger diameter, negligable improvement.

-Speed (overall horsepower)

Giving up a bit of HP due to unsprung weight increase.

-Revolutions


0.2" increase with rear tire will decrease RPM at given speed...

When the speedometer reads 65, C5 is travelling 65.5 mph.

-Momentum
-Acceleration (off the block, and WOT)

Less sidewall and more unsprung weight will decrease overall performance slightly.


So.... to sum up what I considered when doing this:

Larger wheels and tires look better. Less sidewall will help handling in corners by decreasing sidewall flex. Ride will suffer for same reason. Wider tires will hurt fuel economy because of increase in rolling friction (larger contact patch).

Bling and handling improvement = happier owner. Your results may vary.

Chuck

[WELEPRO]

If done properly the increase in wheel diameter probably woouldn't change much in the way of performance at all.

The overall diameter of your 18/19's are probably very close to your 17/18's. That's generally the objective when plus sizing wheels. This keeps things normal, like speedo readings, mileage, etc.

Depending on what type of wheels you're using, they may be heavier then stock. Increasing unsprung weight. That can have a negative effect on handling.

[DACvette]

Moment of inertia will increase since more mass is added further from the wheels center. This will cause a decrease in acceeleration. Don't know how much of an effect this will have, probably only significant for track use.

[Matt Cramer]

As long as the diameter is the same, the only thing that changes for straight line measurements (acceleration, fuel economy, top speed, etc.) is the weight and how close it is to the edge. Naturally, the less weight and the less it is concentrated at the edge, the better. Usually larger diameter rims manage to be a bit heavier.

The larger rims can have a handling advantage, though, from less tire side wall flex. I'm assuming the tread and general stickiness of the rubber are equal.