SCCAForums.com

Let's say two wheel/tire combinations have the same weight, but one of them is 17" and the other 16". Yes, I already know that the 17" will be slower because a higher percentage of the weight is located further out, but is there a mathematical equation that could say how much of a problem this really is?

It will also lower your effective gear ratio and decrease acceleration besides the weight (assuming you keep the same tire aspect ratio) thereby increasing the overall circumference and diameter of the wheel/tire combimation.

No, the two whhel/tire combinations that I am using in the comparison are the same diameter. THe only difference between the two is that with the 17", a higher PERCENTAGE of its weight is located further out.

Well then it is just a a weight issue. Remember adding weight to the car is additive, adding weight to the suspension is multiplicative and adding rotating mass is to the 4th power!!!! That includes adding mass at a larger diameter, ie. it has a larger lever arm and therefore its effects have a larger moment. Bad Bad Bad.

However, if those larger diameter wheels were Magnesium, then we're talking. Or if using the larger diameter allows you to use a wider and or lower aspect tire then the dissadvantages may be offset to even an advantage.

Rule of thumb is to use the widest tire, with the lowest aspect ratio on the lightest rim that the rules allow or you can afford.

BTW I just happen to have a set of Dymag Special Magnesium 9.5x17 and 11x17, 8.5 #s and 9 #s each, respectively! I saw recently where a vendor could still get them for a mere $3,000 and $3,200 each, respectively!!!!

You will have to pry them from my dead cold fingers!

Jongbloed makes a nice wheel as a cheaper alternative; Aluminum heat treated and hard anodized rims with Magnesium Centers ($750-$800) or you can get BBK Magnesium for about $2,500 each.

How fast do you want to go and how much can you afford? What price speed? I am thinking of replacing my caliper bolts, wheel studs and lug nuts with titanium; just to reduce weight at the wheels.

[This message has been edited by DJWorm (edited November 21, 2001).]

When choosing a wheel/tire package for a class where you have some freedom, you don't choose the package by the wheels. You first choose the best tire that fits within the rules and within your fenders.

Basically, you want the widest tire you can fit in the wheelhouses(or, in STS the widest tire that is happy on legal wheels) and the smallest o.d. that will still give you ~65mph at the top of second. If both of your options meet those two requirements, which you seem to imply they do, then you have to start looking at other things...

Which size is your preferred tire available in? What about other tires that you may want ot try?

Do you like a softer, more forgiving sidewall(16) or a stiffer, more precise sidewall(17)? Your driving style is more important than anything else here--smooth or attack(favors more sidewall)?

What can you find in the way of applicable wheels in each of the respective sizes? Commercially available or custom-made? 16's will be lighter at the same price point. Even if the wheels weighed the same, the moment of inertia is lower for the 16's. 16" tires will typically be cheaper, too.

If you can find no competitive reason to go with the 17's, then you have to decide if you are willing to make competitive sacrifices purely for aesthetics.

-dave

Right, I'm STS, which has a maximum width of 7.5" and maximum tire of 225.....My stock wheel/tire is already 16 x 7" and 225......so the best-perfoming solution is to use step up to a 7.5" width......the problem is, there aren't many 16" rims that are 7.5" except the Kosei K1 which I already used before, and got bored with......and all the others are horribly expensive, such as the CP-035, the Advan, and the Volks......16" is simply not a popular size with the tuner market.......so in order for me to get an affordable, lightweight 7.5" wide rim that isn't Kosei, I have to step up to 17's.

You made an interesting comment about driving style....I'm not good enough to always be smooth all the time.....so I guess you could call me "attack"....are you saying that I should use a softer sidewall in order to make my car less twitchy when I "attack"?


[This message has been edited by Hanserelli (edited November 21, 2001).]

A tire with less sidewall will generally have a smaller effective slip angle. This means you must be smoother with steering inputs in order to keep from blowing through the tire's effective slip angle and causing it to lose traction.

The taller sidewall on the other hand is more compliant and can absorb more thrashing at the wheel before giving up.

If I were in the same shoes, I would without a doubt choose the 16's if I liked the 16" tire selection. It sounds like you've already got some inexpensive, light 16's, but you're tired of them. I'm just not worried enough about aesthetics to give up something on course for them. However, I can understand where you are coming from.

-dave

If you've got a good set of 16" wheels already for running and are only changing because of looks, I'd keep the 16's for competition and get whatever I liked the look of for the street. If you're going to buy the new wheels anyway, go ahead and keep the old ones. You could then put all-weather radials on the new tires and high performance, summer tires on the old rims. This would let you have the best combo available for STS while still being able to drive your car year round.

My own option is slightly different. I'd been running on the stock rims and recently bought Koenig Traffik wheels at Discount Tire. I put my high performance radials on the new rims and put snow tires on the stock rims.

This gives me a good set of wheels to run STS in this year (sorting the suspension out before moving to DSP next year) and also lets me run the car safely during the winter. A nice compromise all around, particularly since I expect to buy some Azenis next spring.


------------------
David L. Burkhead "May I be just half the person
mailto:dburkhuad@home.com my dog thinks I am."
Science Fiction -- Judo -- Space -- Science -- Cars http://www.sff.net/people/dburkhead

[This message has been edited by DBurkhead (edited November 22, 2001).]

---

David L. Burkhead
mailto:dburkhuad@comcast.net
"May I be just half the person
my dog thinks I am."
Cold Servings:
Because law and justice are not the same.
http://www.coldservings.com
Updates Wednesdays

Do you drive an RS? The wheel size sounds like you do. 16X7

Check out the SSR competitions. Fairly cheap and really light.

Originally posted by DJWorm: Well then it is just a a weight issue. Remember adding weight to the car is additive, adding weight to the suspension is multiplicative and adding rotating mass is to the 4th power!!!!

If you're going to make scientific sounding claims, be prepared to produce a scientific explanation. Lest your claims be proclaimed, "Bo-o-o-ogus!"

Seriously, someone else quoted this "4th power" thing, and not only does it not make any sense dimensionally (you end up with "pounds squared squared" as a unit, which is not a unit of weight) the person didn't really know where it came from -- some guy he knew had told him that. It smells like an urban legend to me (or maybe a "racer's myth").

Can someone explain what the heck weight to the fourth power is even supposed to mean, or better yet provide a explanation of how it was derived?

In terms of acceleration/deceleration, rotating mass has at most the effect of twice as much mass being carried in the chassis. That's based on the worst case scenario that all the weight is carried at the tread surface of the tire. You can find this 2X factor by calculating the total kinetic energy of the spinning wheel plus the moving car at a given speed. You can read about how to do such calculations in Physics volume 1 by Halliday and Resnick.

From a putely rotational mass point of view, the weight of the tires is much more important than the weight of the wheels...

Dave Shreve's advice makes good sense.

SSR Type C ("competitions") cost about the same as Volk TE37s, and seem to have a quality control problem -- some of them are getting bent on dinky little Miatas. There are 3-4 cases just among the Miata Forum members.

[This message has been edited by Orthonormal (edited December 17, 2001).]

---

John S.
S2000 #42as

Well I run STS, and I live in a sunny area, so I only need one set of rims/tires. My stockers are 16x7. I'm going to go with 17x7.5 because I can't resist the looks. The tires I get will also be lower and lighter than stock too.

Rotational inertia equations take the form 0.5*[mass]*[radius]^2 and you typically integrate to obtain the result. You do need to sum the wheel and tire rotational inertias for a true comparison. At any rate, weight further away from the center of rotation creates more rotational inertia; so 17" wheels otherwise similar to 16's and weighing the same will have more rotational inertia to accelerate. Since the mass of the 17" tires will also be on average further from the center of rotation that rotational inertia for a given tire weight will be greater as well (kind of makes you wonder about all those slammed small-bore imports with 19's & 20's).

The fourth power mix-up has probably resulted from confusing rotational inertia (a length^2 parameter) with moment of inertia (which is a length^4 parameter dealing with structural cross section stiffness).

Norm

------------------
1979 Malibu, 355, M5, some handling mods, a few other interesting things
1995 626 V6, M5
2001 20th Anniversary Maxima, M5

---

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

Originally posted by Norm Peterson: Rotational inertia equations take the form 0.5*[mass]*[radius]^2 and you typically integrate to obtain the result. You do need to sum the wheel and tire rotational inertias for a true comparison. At any rate, weight further away from the center of rotation creates more rotational inertia; so 17" wheels otherwise similar to 16's and weighing the same will have more rotational inertia to accelerate. Since the mass of the 17" tires will also be on average further from the center of rotation that rotational inertia for a given tire weight will be greater as well (kind of makes you wonder about all those slammed small-bore imports with 19's & 20's).

Good post. But 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.

[This message has been edited by Hanserelli (edited December 18, 2001).]

There are two significant factors to consider when the tires are roughly the same weight and overall diameter. First, the rim area of the wheel moves outward 1/2" radially. Doing the math, even if the rim area of the two wheels is roughly the same mass, the rotational inertia goes up by ~13%. When you figure in the rest of the wheel and tire combo, that probably brings that into the neighborhood of a 3-5% increase in rotational inertia, i.e. it's now 3-5% more difficult to accelerate/decelerate your wheel/tire package. Then you have to factor in the added weight of a 17 versus a 16. Of course, the weight increase deal is harder to figure because it depends on the placement, but likely a good bit of it will be in the rim area.

Second, why go with 225/50-16? There's good selection in 225/45-16, which nets a shorter sidewall for better gearing and less sidewall flex(will behave like the 225/45-17). It also gets you a lower rotational inertial and a lower cg by another 1/2" without effecting your suspension geometry(the 225/45 is 1" shorter than the 225/50). Additionally, with the extra 1/2" of fender gap, you could then lower the car another 1/2"(if your setup is height-adjustable), netting you a 1" lower cg.

I guess you need to decide which is more important to you...aesthetics or optimal setup for autocross.

-dave

[This message has been edited by dave shreve (edited December 19, 2001).]

So correct me if I'm wrong, but you're saying that if a 16" and 17" wheel/tire combo have the same weight and diameter, the difference in rotational inertia would only be about 3-5%? That doesn't sound very significant.

And yes I would use 225/45-16 or even 225/40-16 for autocross, but for street use they are ugly because they don't fill the wheel-well as much. Some cars have big wheel-wells that must be filled!!

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 amount, because of two things:
1) The spring stiffness remains the same
2) The COG remains in the same "part" of the body as it did before. All that's been changed is the COG's relation to the ground, not its relation to the body.


[This message has been edited by Hanserelli (edited December 19, 2001).]