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Originally posted by Hanserelli: [B]And I can see how lower tires will lower the COG, but I don't think the car's behavior would change, would it? It seems like the car would still roll the same

You're right--the car will still roll the same because the vertical distance between the roll center and cg(the moment arm which causes body roll) has not changed. However, the the vertical distance between the cg and the contact patches(the moment arm that causes weight transfer) HAS changed, decreased for the better. With the shorter moment arm for the weight transfer couple, you have less weight transferred to the outside under cornering, i.e. the inside tires have more loading and can contribute more lateral grip so your outside tires aren't overloaded as easily.

-dave

[This message has been edited by dave shreve (edited December 20, 2001).]

ok, i guess i see what you're saying.....you're saying the tread patch is where the force originates, and the EFFECTS of that force won't be as great if the COG is near the same plane as the tread patch.

Yup, you got it.

-dave

Originally posted by Hanserelli: . . . let's consider a 225/50-R16 tire and a 225/45-R17 tire. They are only 3mm different in overall diameter, so let's say they're equal diameter for the sake of argument. I think that the 17" tire does not have any more rotational inertia than the 16", becuase its tread is at the same distance from the axle. The only thing that's changed is the amount of sidewall. So really, the increase in rotational inertia comes from the 17" wheel, not the tire. . .

Ummm. The 17" tire will have a slightly larger rotational inertia for several reasons. I'll ignore the 3mm if you'll go along with the two tires weighing the same for the purposes of this discussion. The tread on the 225/45-17 (and its steel belting) is almost certainly a little wider than that on the 225/50-16, thereby locating a slightly greater % of the tire weight near the outer diameter. True, the sidewall is shorter and probably lighter, but its average radius is a quarter of an inch larger than that for the sidewalls of the 16's (something like 2.5%). And the bead area (with its steel cords) is at a radius of a little over 8.5" rather than just above 8" (there's that ~6% again). Without specific data from a tire company an overall %difference in rotational inertia between tire sizes would be hard to determine and even harder to defend, but I wouldn't be at all surprised if it was another couple %.

Dave has provided a good evaluation of the difference between the wheel sizes, so all I'm going to add there is to note that the circumference of the 17" rim is roughly 6% larger than that of the 16", making its mass at least 6% higher (assuming equal section thicknesses and widths). Then you do the ~13% math, etc., . . .

I know that these sound like small numbers, but minutely lower accelerations (assuming that you're not everywhere traction-limited) ultimately result in fractionally longer lap times. Unless, of course, the 17's give you bigger gains in the turns than they give up down the straights.

Norm

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seat time is where you find it (semi-retired) weenie CP '79 Malibu, (no longer ST/SP legal) '95 626

But it was my understanding that if you took a 16x7 wheel and a 17x7 wheel, both with 225 tires, the tread width would be the same. Yes, the sidewall SECTION width might be different because of 45 versus 50, but not by much. If you are saying that the 45-series tire would contain stiffer steel belts/sidewall, then I would agree. I guess I'm having a brain haze today because I couldn't quite comprehend your position. But most "Plus One" tire sizes seem to weigh 1-2 pounds less than stock, according to some manufacturer numbers I've seen. So I'm not really concerned about the tire -- it's more the wheel that I'm concerned with.

The conclusion here is that if a 16 and 17 wheel weight the same, the 17 will have a 3-5% greater rotational inertia? Of course, most 17s are 1.5-2 pounds more than the 16 of that model. But my feeling is that that's canceled out by the fact that the tire is 1-2 pounds LESS than stock. So the only thing I have to worry about is the fact that the majority of the rim's mass has now been pushed out 0.5" all the way around its circumference. And THAT'S what's causing the 3-5% increase in rotational inertia, right? Thank you for your patience.

Hey, if I do get 17's, at least the extra 3mm in tire diameter might help improve my ride!! I've heard larger diameter does that. My Probe rides like crap right now, with its H&R Sport springs and Tokico Illuminas, even though I have stock tires right now. The problem is that the Illuminas seem to have too much compression damping and not enough rebound. So in order to get the rebound control up there to where I like it, I have to set them on a high setting which makes the compression damping too stiff. I should have coughed up the extra dough for the Mazdaspeeds (GAB) which are more biased towards rebound control.

I used some Kosei K1 lightweight 16x7.5 with RE71's, and the ride got even worse. I was hoping the lighter unsprung weight would help, but no. The extra 0.5" rim width, along with the RE71s which at size 225 are more like 245, conspired together to destroy my ride.

The 225 number applies to the section width, not the tread width. Tread width generally follows section width but tends to be relatively a larger percentage of section width as the profile drops. True that it's not always the case, but it is so often enough to form the basis for a discussion in the absence of hard dimensional data for specific tires.

What I was getting at was that the distribution of weight in the 225/45-17 is, as a weighted average of the tread, sidewall, and bead areas, further away from the center of rotation that for the 225/50-16. For equal total weight that means that the 17's would have a higher rotational inertia by a few %.

If the 17 is a pound or two lighter than the 16 (probably this weight comes out of the sidewall) then I agree that the tire rotational inertia effect is pretty much a wash leaving you with a net improvement in unsprung weight and suspension response.

I'm under the impression that taller sidewalls rather than larger outer diameters are more responsible for ride comfort. Think about it for a moment, and consider the likely ride characteristics of a (fictitious extreme size for illustration) 225/30-20 tire. That size would be about 9mm larger on the outer diameter than the 225/45-17 and would have only about 2" of visible sidewall (the other 0.66" or so being hidden behind the rim flange).

Again, this sidewall height to ride quality link is a generality, since the actual tire construction also has a lot to do with this. Your evaluation regarding the ride quality of RE71's comes to mind; a stiff tread area that does good things for cornering & handling does so at the expense of ride compliance.

Also a factor is the internal volume of air, since the spring rate of a tire is a function of how much that volume is compressed over a given bump. And (obviously) inflation pressure (which BTW should be kept a bit higher in those 225/45-17's based on the load rating of 90 vs 92 for the 225/50-16's).

Norm

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seat time is where you find it (semi-retired) weenie CP '79 Malibu, (no longer ST/SP legal) '95 626

If Dunlop is to be believed, the tire weight actually goes up! SEe this link.

Originally posted by Potsie: If Dunlop is to be believed, the tire weight actually goes up! SEe this link.

If you are referring to why the 225/45-17 is quoted as weighing more than the 225/50-16, you're right that is surprising, but I think it's just an anomoly. Perhaps Dunlop made the 225/45-17 wider or stiffer than normal in order to be more competitive with other brands, for some reason. If you look at most of their other sizes, generally a tire of the same width but smaller aspect ratio weighs less.

Originally posted by dave shreve: You're right--the car will still roll the same because the vertical distance between the roll center and cg(the moment arm which causes body roll) has not changed. -dave [This message has been edited by dave shreve (edited December 20, 2001).]

That's incorrect. With shorter tires, the line between the instantaneous centre and the tire patch will be at a flatter angle. The roll centre will move up closer to the c.g., hence decreasing the roll moment and chassis roll angle.

All things equal, I'll run a shorter tire any day.

Joe