[rhudeboye]

I like the look of 18” & 19“ wheels. But there are some nagging facts in the back of my mind that I’d like your input on. I’ve developed a list of things that are affected by that inch or two to see if there is a reasonable offset to the problems they cause.

1. Handling- With a larger tire, axle height rises thus raising the car's center of mass. In turn this changes the way the car was intended to handle.

2. Fuel efficiency- it takes less power to move a smaller tire. So the larger the wheel, the more power/fuel used to move the car.

3. Speed- So larger wheels clearly rotate more slowly. Thus you would loose a step or two when coming off the line. Meaning your 0 -60 time will suffer.

4. Braking- braking distance is sacrificed as it takes more power to stop that larger mass from rotating.

quite a few of you are running 18s so please chime in.



Regards
stukon 17s

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[Anchorman]

A fundamental error in your assumptions is that the rolling radius (or overall wheel/tire diameter) changes - but it doesn't, at least not if you pick your tires correctly. The larger wheel uses lower profile tires, keeping the effect on things like measured speed and centerline distance from the ground, basically unchanged. In terms of braking - the larger wheel generally has a wider tire, therefore greater contact patch - and, potentially, better, not poorer, braking. As for mpg - I couldn't tell the difference, and I keep meticulous records (every tank of gas since I purchased the car 6 years ago, and at various time on 16", 17", and 18" wheels).

There are other, possibly subtler, issues, such as the added (?) stress on A-arms. Again, though, and I can only speak from my experience, I haven't seen this, even after a dozen or so 2-day DE track sessions.

My advice is go for what you like - but I would stay away from some of the larger aftermarket wheels that are quite heavy.

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[Cloud9...68]

I guess the major impact of a larger wheel (assuming, as Anchorman says, you size your tires such that the overall tire diameter stays the same) is that the sidewall becomes shorter. This can either be a benefit (potentially more predictable handling as a resut of less sidewall flex), or a drawback (harsher ride for the same reason). It really depends on what you're looking for in your car's ride/handling compromise.

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[Anchorman]

Ride quality is certainly a (subjective!) issue. My gauge - which is whether or not my wife complains - is that the difference between 17" and 18" is as much related to the specific tire choice (and other things) as it is to the wheel size. Before my last track session, the shocks and struts were "tightened" (by adjustments, not be replacement), and that made a much bigger difference in ride quality than anything I've done with wheels. I will say, though, that when the 16" wheels and snow tires were on the car, the ride was noticeably softer. However, I get no complaints with the current 18" setup, although with the stiffer shock settings, it's as close to harsh as I want to get.

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[rhudeboye]

Touching on point 3 and perhaps 4.
I was watching one of those chop cut shows on Speed channel. I believe it was an Evo they were tuning. They added a larger but lighter rim, put her on the dyno and found that the car was slower. On came an onslaugt of tweaking to get back the power lost. Recently there was an episode of Top Gear where the exact same thing happened. In speaking with a mechanic, I was also told that I can expect to loose a little with a larger wheel. Even though you compensate with less tire as you add a larger wheel your still adding more metal which is heavier. Meaning your turning a greater mass then before.

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[Anchorman]

And, at least with respect to accelerating (and ride comfort as it relates to unsprung weight), that could very well be correct. All I can say is that I'm not racing where 0.1 sec matters, and my butt feels many other things before it feels the difference in wheels (again, other than the 16" with snows). As for braking, the difference isn't so intuitively obvious one way or the other.

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[Ryan]

If 1/10ths matter then stick with smaller wheels, but the looks of say 18"s over the stock 16" is a huge improvement.

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[flash]

weight at the wheels is important - each pound there is effectively the same as about 4 lbs elsewhere - that means that a 2lb change in tire/wheel weight is the same a a 32lb change in the car - that can add up

the thing they likely did not factor in on the EVO test was that the tire they put on the larger rim was probably also larger - tires generally outweigh the rims - you could find the lightest wheel out there and still end up heavier than you started if you choose a heavy tire - they vary as much as 5 lbs in the same size too

i have very light 18" rims for the widths (9 and 10) - they are actually lighter than the factory rims, even though they are much bigger - however my tires are quite heavy - this is something i did not expect - i did not have access to that information when i chose my tires - when i change them to another brand, i will lose almost 4lbs each

another factor is where the mass is located - hub mass does not have as much of an effect as outer mass - the farther out you add weight, the more resistance

there is also a risk of hub and spindle damage - larger and heavier both mean more stress - even if you end up lighter, the larger part, or even just a super sticky tire, can mean more stress on the suspension - this can also wear out bushings faster - these are things to think about

definitely factor in the rim weight, but also consider the tire carefully

18s can be done without huge upgrades (though some will likely need to be done), and on the street can be fine - on the track, i would seriously upgrade some suspension components

19s are absolutely out of the question - not a chance of finding a tire for the rear with the right outside diameter - they are warned against by porsche, and will most certainly be heavier than the suspension would like

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[PorscheDude]

Flash do you have a link for tire weights?

My 18" 3 piece wheels are also lighter than the factory 17's.
I have them dissembled right now and you would be surprised how light the lips and barrels are.
Most of the mass is in the center section, and on the center section most of the mass is at the hub.

I'd imagine a factory 1 piece wheel would have more material at the outer diameter of the wheel as opposed to a true 3 piece wheel.

Everything is a trade off, depends on your objective.

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[flash]

tire rack now has weights for most of the tires on their spec sheets

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[rhudeboye]

<!--quoteo(post=69248:date=Mar 24 2009, 06:54 AM:name=flash)-->QUOTE (flash @ Mar 24 2009, 06:54 AM) <{POST_SNAPBACK}><!--quotec-->another factor is where the mass is located - hub mass does not have as much of an effect as outer mass - the farther out you add weight, the more resistance<!--QuoteEnd--><!--QuoteEEnd-->

That's my understanding as well. Take a yo yo with only 4" of string, attach it to your finger and spin it in a circular motion. It will spin fast and easy. Let the string out to 8" and do the same thing. Its harder to get going and you feel more tension on your hand. 4" - 8" is a big jump but the laws of Centrifugal force doesn't change. I think the seat of the pants feel is misleading do to the low tire wall. On dyno's this seems to always show a lose in speed.

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[flash]

which is why i am trying to sell myself on a set of 17s i found - i am too lazy to clean them though, and may end up compromising by merely reducing tire weight and keeping my wheels

but then there is the sidewall issue - a stiff sidewall is heavy, and a soft one makes for bashed rims

no easy answer for sure, but i'm working on it

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[xrad]

love my 18's. Koni adjustable shocks...

Anyway, ride is much stiffer than stock but not nearly jarring. hard to tell if it is due the lower profile tires...

Does tend to find the grooves..

I would consider offset your most important factor.

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[Fox944]

ride stiffness has a lot to do with which suspension you are running, what Psi you are running in the tires, Tire profile, etc. Several factors. My suspension is really really stiff, but i find it less harsh then a stock MO30 car (and i'm running 18" wheels).

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[PorscheDude]

Also add tire construction to the variables.

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