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I just purchased OEM 18 inch wheels off of ebay for my 2005 STS, which has performance (JE5) brakes and, as most of you are probably aware, the 18 inch wheels were an option (QF8 option).

I'm curious if anyone has experience upgrading from 17 to 18 inch wheels and what was your assessment. By the way, I say "upgrading" loosley because some people may not view larger wheels as an upgrade, especially if ride comfort is sacrificed. To this end, I'm not sure if ride comfort will be sacrificed with larger wheels since, from what I have read, the quality of tires are more important to comfort than upgrading one more inch of wheel diameter.

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....Im curious if anyone has experience upgrading from 17 to 18 inch wheels and what was your assessment. By the way, I say "upgrading" loosley because some people may not view larger wheels as an upgrade, especially if ride comfort is sacrificed.

I have owned and driven many miles on 16" rubber (Seville) and 17" rubber (Deville) and the "ride" was satisfactory in both cases.

Unfortunately, my current ride has 18" rubber and after two hours on the Interstate I am bone weary.


Jim

Drive your car.

Use your cell phone.

CHOOSE ONE !

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If the rolling radius of the tire/wheel combination is within about 2%, your brakes, speedometer and general drivetrain working will be pretty much unaffected. The diameter of the combination is given by

2X((Tire width, mm)*((aspect ratio)/100) + rim diameter

Example: 225-60 16's have an added diameter of 225*0.60 = 135 mm or 5.31 inches. On a 16-inch rim, this gives a diameter of 16 plus twice that, once for each side, or 26.6 inches. This is approximate because both the tire width and the aspect ratio are not very accurate, and things like tread depth are figured into the aspect ratio, often inaccurately. The example is from the 1990's Seville/Eldorado standard tire size, which has an actual measured rolling diameter of 25.9 inches from the Goodyear specs, about a half inch less diameter. But, you can use the formula to see whether different combinations will give you compatible rolling diameters or not. Comparison accuracy is best for comparing the same tire brand and line of tire.

Increasing the rolling diameter may be tempting to avoid reducing the tire aspect too much and help preserve the ride, but it will raise the car, give you longer final gearing, make the speedometer read low, and increase the pedal effort required for the same braking.

Another point is that the strut axis needs to pass through the center of the tire patch to avoid torque steering on braking. This is even more important with FWD cars, of course. If your rolling diameter changes much, this will be affected. There are implications on the rear SLA or multi-link suspension too but I don't think that they are very important compared to the front.

You might be able to preserve the ride if the wheels are light and you reduce the damping rate of the shocks so that the small movements over road roughness that were absorbed by the tire are absorbed in the small suspension movements. If a car is designed for low profile tires, that is what is done, along with reducing spring rates, to keep the ride quality up. The suspension geometry needs to be adjusted to keep handling quality up.


CTS-V_LateralGs_6-2018_tiny.jpg
-- Click Here for CaddyInfo page on "How To" Read Your OBD Codes
-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data
-- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC
Yes, I was Jims_97_ETC before I changed cars.

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Was next to new lacrosse today and it had 235/50/17 tires. I am 98% sure it rides like a pillow. I assume ur 18" combo will use 50 series tires? Tire selection is obviously the key to keeping ride smooth. Going from 55 tires to 50 is almost inconsequential.

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The tire size of the OEM 18" wheels I bought is 235/50/18 (8-inch wide rim) all around.

My current 17" tires are two different sizes:

Front tires are: 235/50/17 (8-inch wide rim)

Rear tires are: 255/45/17 (8.5-inche wide rim)

So looking at the specs of my current tires and the tires on the 18" rims, I'm not sure what to expect since sidewall height with the 18" inch rims are not compromised. And the rear tires of my current 17" rims have lower sidewall height than the tires on the 18" rims.

Another thing is that I will be able to rotate tires since the 18" rims are the same all around.

One thing that comes to mind for me is that I hope the 18" rims are more pothole proof since I have replace 2 rims due to potholes and this last winter I hit a pothole that bent the left rear rim, which is leaking. Also, another pothole beatup the sidewall of the right front, which has a bulge. The tires I have on are Continential Extreme Contact, which I'm not too impressed by. I don't know if Michelin tires have tougher sidewalls and thus better more pothole proof, but I need to shop around for tires that aren't as susceptible to pothole damage. The problem with Michelin is that I can easily spend about $300 each for the OEM recomended Pilot MXM4 tire.

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I am NOT a fan of big rims! Having said that, I still Think it can not get any worse than what You have already got!

Big rims will give a harder ride and the lower the tire profile gets the more sensitive it gets for potholes.

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I have 18's with 245x50 tires on my 2006...

They were standard on my model.

We have some pretty bad streets here and I have never damaged one of them in a pothole... YET... :)

The DTS has a little over 132,000 on it.

.

.

20120707_112245.jpg


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My wife's Pontiac Grand Am has 50-profile tires and it rides fine, and potholes aren't a problem. I think a lot depends on the load range of the tire and the rim width, and how those match up with the weight of the car to determine how much of a hit it takes to get to the rim. Of course, if the suspension is designed for the low-profile tires that are used, that means that the compliance - the give of the suspension when it hits a pothole - is going to be better by design, and that makes it harder for a pothole to get to the rim, too.

Then, there are potholes, and, there are *potholes*.


CTS-V_LateralGs_6-2018_tiny.jpg
-- Click Here for CaddyInfo page on "How To" Read Your OBD Codes
-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data
-- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC
Yes, I was Jims_97_ETC before I changed cars.

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Would anyone happen to know which lug nuts I should buy for OEM 18" polished aluminum rims. I was told by seller that the size is 12mm 1.50. I'm looking on ebay for a set of 20 but I'm not certain which ones to buy. Some are 19mm and are stated to fit 2005 STS.

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i bought wheels with small covers for the lugnut area and i got new chrome lugnuts but than figured out, i could have used the origional lugnuts. if the lugnuts are the proper thread, length, seat taper angle, and have no fitment issues under cover, IF you have a cover that is, than you may reuse your current lugnuts. do you just want chrome lugnuts?

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i bought wheels with small covers for the lugnut area and i got new chrome lugnuts but than figured out, i could have used the origional lugnuts. if the lugnuts are the proper thread, length, seat taper angle, and have no fitment issues under cover, IF you have a cover that is, than you may reuse your current lugnuts. do you just want chrome lugnuts?

The wheels I purchased will show the lug nuts, so I have to buy the chromed or polished aluminium closed end. My main focus is getting the right size/length. I believe 19mm should be good, but I'm not sure. So I think I'm going to buy 12Mx1.50x19mm.

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I am NOT a fan of big rims! Having said that, I still Think it can not get any worse than what You have already got!

Big rims will give a harder ride and the lower the tire profile gets the more sensitive it gets for potholes.

The 18" OEM wheels are on. Compared to the 17" wheels the ride is better. I know you may not believe it because generally it's believed that bigger wheels automatically mean a less comfortable ride--Not true in my case! The 18" wheels seem to handle bumps better. The wheels also came with Michelin tires, so this is likely to be one reason since Michelin is one of the best, if not the best tires out there. But the ride is that much more noticeably better on bumps, so 18" wheels are fine and look better too. All tires are 235/45/18.

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When I wash my car I'll take a photo. I stand corrected about the size of the tires, which are 255/45/18 and 235/45/18. Obviously, the 255s goes on the rear. Currently I have one of each on front and back, which is why my steering seems a bit awkward. :rolleyes: I put them on at night and thought I saw them advertised as being the same size all around. On Saturday I have to do a brake job, so I'll switch them then. I bought EBC Green Stuff brakes for front and back which get excellent reviews for stopping power and low dust.

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