Unsprung weight and rotating mass has a huge effect on how well your tire can stay in contact with the road, how easily you can rotate the tires from a standing stop, and how well you can stop your vehicle.

Everyone has their own priorities but changing from a 15.5 LB 14" wheel to a 20 LB 17" wheel is certainly not a performance enhancement. The car won't laterally accelerate as well, brake as well, or accelerate in a straight line as well as it did with 14" wheels, everything else held equal.

Also, theoretically, even if two wheels were identical in every single way except for diameter, the larger diameter wheel would ALWAYS be harder to turn than a smaller wheel. This means that even though they weigh the same, look the same, and have the same metalurgy, a bigger wheel will still ALWAYS be harder to turn.

I found the following quote to say a lot of things I wanted to convey, but was to lazy to fully type out. I also wanted to add that for performance alone, your primary concerns for wheel choice should be brake cooling, choosing the smallest wheels that will go over your brakes, and strength vs. lightness.

"By: James Rogerson

Noriel, et. al. Here's the skinny in semi-layman terms.
In rotational motion it is not mass alone that counts but mass times the square of the distance from the point (or line) about which the rotation is taking place. The body (wheel and tire) has an AVERAGE mass (m) to radius squared (r^2). This average mr^2 is a point called the 'moment of inertia'. The torque required to stop (or start) the rotating object in a given period of time depends not upon the mass of the object but its moment of inertia. The value of the moment of inertia depends on the distribution of the mass and can be changed without altering the total mass. Practical application: The figure skater spins slowly with arms extended, yet speeds up when their arms are brought close to the body. The rotational energy is the same, yet the moment of Inertia is increased and decreased. Simple, no? If more
is required, let me suggest reading on angular momentum and rotational motion. A good book for the non technical (guys like me) is 'Understanding Physics' by Isaac Asimov (yeah, the same). I've tried to paraphrase his discussion on this subject without the formulae and attendant squiggley lines."

I'm not disagreeing, but nobody has ever factored in the TIRE into this whole equation. Anyone care to elaborate on the effects of the outer part of the rim, the tire, compensating for the smaller inner diameter? I mean sure it sounds perfect saying a lighter 15" rim is way better than a light 17" rim (let alone a heavy 17"), but once you put the tire on the end, wouldn't the 15" tire has more mass than a 17" tire? And isn't the most influencial weight on a rotational system is the weight furthest away from the axis of rotation thus almost cancelling out all factors when choosing a smaller wheel setup?

Well, let's use an example. A 205-50-15 tire mounted on a 15"x7" wheel as opposed to a 205-40-17 tire mounted on a 17"x7" wheel. Let's assume the differences in wheel weight and construction are negligible.

For the 15" wheel you would have a 102.5mm sidewall height which converts to 4.04" sidewall height. Add that onto the 15" wheel diameter and you have a total outer diameter of 19.04".

For the 17" wheel you would have a 82mm sidewall height which converts to 3.23" sidewall height. Add that onto the 17" wheel diameter and you have a total outer diameter of 20.23".

For the sake of comparison I'll add the stock wheel and tire size of 195-60-14. For a stock 14" wheel you would have a 117mm sidewall height which converts to 4.61" sidewall height. Add that onto the 14" wheel diameter and you have a total outer diameter of 18.61".

Using the 14" stock configuration as our base you have 2.31% increase in outer diameter for the 205-50-15 tire and a 8.70% increase in outer diameter for the 205-40-17 tire. This means that you have material further away from the axis which will increase your moment of inertia without considering any differences between tires of those particular sizes.

As for bringing tire weight into the equation, I have found no tire weights to make calculations for among similar tires. With tire weight though, more has to be taken into consideration because bigger tread blocks and stiffer sidewalls will make a tire heavier, but will increase your ultimate grip. It depends what you theorize will make your car faster, less unsprung weight or a higher CF (Bigger tread blocks typically mean more contact patch) with more predictible transient response (Less squirm with bigger tread blocks, and stiffer sidewalls). On a theoretical level, I would have to say I am not qualified to argue that point because there are too many factors involved. I would agree that they should be taken into consideration since most manufacturers use steel belts which equate to being heavy. I think Hoosier uses fiberglass in most of its race tires.

This was post that started this thread, and as I haven't heard it discussed here I that it would be appropriate to post it here.

I'm not disagreeing, but nobody has ever factored in the TIRE into this whole equation. Anyone care to elaborate on the effects of the outer part of the rim, the tire, compensating for the smaller inner diameter? I mean sure it sounds perfect saying a lighter 15" rim is way better than a light 17" rim (let alone a heavy 17"), but once you put the tire on the end, wouldn't the 15" tire has more mass than a 17" tire? And isn't the most influencial weight on a rotational system is the weight furthest away from the axis of rotation thus almost cancelling out all factors when choosing a smaller wheel setup?

Here is my initial answer, but it still leaves some things to be answered.

Well, let's use an example. A 205-50-15 tire mounted on a 15"x7" wheel as opposed to a 205-40-17 tire mounted on a 17"x7" wheel. Let's assume the differences in wheel weight and construction are negligible.

For the 15" wheel you would have a 102.5mm sidewall height which converts to 4.04" sidewall height. Add that onto the 15" wheel diameter and you have a total outer diameter of 19.04".

For the 17" wheel you would have a 82mm sidewall height which converts to 3.23" sidewall height. Add that onto the 17" wheel diameter and you have a total outer diameter of 20.23".

For the sake of comparison I'll add the stock wheel and tire size of 195-60-14. For a stock 14" wheel you would have a 117mm sidewall height which converts to 4.61" sidewall height. Add that onto the 14" wheel diameter and you have a total outer diameter of 18.61".

Using the 14" stock configuration as our base you have 2.31% increase in outer diameter for the 205-50-15 tire and a 8.70% increase in outer diameter for the 205-40-17 tire. This means that you have material further away from the axis which will increase your moment of inertia without considering any differences between tires of those particular sizes.

As for bringing tire weight into the equation, I have found no tire weights to make calculations for among similar tires. With tire weight though, more has to be taken into consideration because bigger tread blocks and stiffer sidewalls will make a tire heavier, but will increase your ultimate grip. It depends what you theorize will make your car faster, less unsprung weight or a higher CF (Bigger tread blocks typically mean more contact patch) with more predictible transient response (Less squirm with bigger tread blocks, and stiffer sidewalls). On a theoretical level, I would have to say I am not qualified to argue that point because there are too many factors involved. I would agree that they should be taken into consideration since most manufacturers use steel belts which equate to being heavy. I think Hoosier uses fiberglass in most of its race tires.

All things kept equal, including tire construction and diameter, then the only diff between a 15" wheel/tire and a 14" wheel/tire would be the 1" of sidewall/tire on the 14" setup where metal would go on the 15" wheel setup. I'm guessing that the 1" of tire is lighter than 1" of metal in its place, so then that would make the 14" wheel/tire have a smaller moment of inertia.

When you change the diameter of the wheel, not only do you change the moment of inertia, you change your final drive ratio slightly, smaller diameter = increased final drive, which increases the effects of gearing. It makes it hard to compare unless everything is kept equal... but in the real world, its hard to find tire sizes that have near equal diameters.

Unless your interest in primarily academic the subject is pretty much a waste of time.

For the 15" wheel you would have a 102.5mm sidewall height which converts to 4.04" sidewall height. Add that onto the 15" wheel diameter and you have a total outer diameter of 19.04".

For the 17" wheel you would have a 82mm sidewall height which converts to 3.23" sidewall height. Add that onto the 17" wheel diameter and you have a total outer diameter of 20.23".

For the sake of comparison I'll add the stock wheel and tire size of 195-60-14. For a stock 14" wheel you would have a 117mm sidewall height which converts to 4.61" sidewall height. Add that onto the 14" wheel diameter and you have a total outer diameter of 18.61".

[Modified by mojoGSR92, 4:55 PM 4/8/2002]