Thinking about "downgrading" to 18s. Options? Considerations?

Hello all. The tires my car came with when I bought it have now reached the end of their lifespan, and then some. They are Michelin Pilot Super Sports , and the rear tires went from just about to touch the wear bars, to "holy crap there is cord showing!" in a remarkably short span of time and mileage. The car has since received a new suspension and been realigned, so hopefully the same fate will not befall a new set of tires . My car came with the 19" sport wheel option, and while I do like the way they look, I have always been much more focused on real world aspects. 18" wheels would not only be lighter and perform better, but they would ride sweeter, and offer lower tire replacement costs as an added bonus. The fact that the 19s seem to be a popular upgrade for other owners and would probably not be too difficult to sell for a fair price is another factor driving my interest in this idea. To that end, I've started researching my options. Surprisingly, I am having difficulty finding good threads on the subject using the search function. It seems that virtually all threads must have some reference to wheels in them, as the number of off topic results is well over 95% of searches I have tried so far. Maybe I'm just not familiar enough with how the search algorithms work on this site to properly search for what I'm looking for, but in any case, I thought I would run this past the experts. So, what size and offset should I bee looking for in an 18" wheel? I'm not looking to squeeze an insane amount of rubber in there, but filling out the fender wells slightly would be a nice upgrade in appearance, and a little extra rubber wouldn't hurt if that's the case. to give an idea of usage, I do not track my car, and do not intend to in the future. Most of my fun driving takes place on very bumpy back roads, and I am not interested in feeling the car moving under me etc, I want laser precision. RUB WILL NOT BE TOLERATED. Options that have caught my eye so far include the following. Input on their quality etc is always appreciated. O.Z. Racing Alleggerita HLTs: 8.5 and 10 inch widths front and rear available. Decent price and nicely light, if their listed weights are to be believed. Styling isn't particularly striking, but also not offputting. Forgestar CF5: Available in numerous widths and offsets. Very reasonable price, and would be very east to keep clean which is always a nice bonus. Not 100% sure on the styling though. They look a bit 90s. Also can't find weights listed. Forgestar F14: Same as above on widths and price. May be slightly lighter than the CF5. Styling is fine, if not particularly inspiring. Would be a pain to clean though. HRE FF15: Also 8.5x10". No 11" rear option. Slightly higher price, but still well within reason. Best styling of the bunch in my humble opinion, but might be a bear to clean. No weights listed for the application. HRE has a very good reputation for quality, but this is their new "entry level" wheel which is flow formed rather than forged. So, are there other options I should consider? What would be the best sizing and offset combo on the Forgestars? Tire size options with a given width etc? Any other considerations I may have missed?

You might consider looking for some of these 18" Champion wheels: http://www.planet-9.com/attachments...-and-boxster-chat/118457d1472866314-what-did-you-do-your-987-today-img_5358-jpg

Switched from stock CS 18's to OZ Alleggerita's a couple of years ago. They are as light as advertised. Massive difference in turn in and transition. Noticeable difference.

Those could be a contender. Any info on pricing? Their site just says call, which usually means prepare for sticker shock.

Champion wheels are spectacular. They do come with a hefty sticker. Just for reference, these 19's are listed on ********: https://********.com/forums/parts-m...rums/parts-marketplace/987018-champion-motorsport-19-rs98-wheels-and-tires.html BTW, I will argue the 18's are no "downgrade"

I saw those very wheels and tires listed on ebay last night for $2700 "buy it now". Seems he hasn't been getting any bites for several months, so he has reduced the price. One minor note he has in the description does concern me slightly when it comes to their fitment. He states that they do rub over large bumps, which I absolutely despise. There is almost no more cringe-worthy experience to me that tire rub. It's worse than fingers on the blackboard and Bette Middler's speaking voice. LOL! When it comes to wheels, I just can't really find the value in true forged wheels at the moment. I understand of course that they are empirically superior in strength, etc, but in my experience, flow formed wheels are plenty strong enough for my intended use, just as light, and cost a whole lot less. The difference in cost is such that you could damage and replace 2 or even 3 wheels and still be under the sticker price of the forged wheels. The only wheels I have ever damaged where forged BBS BMW factory rims, and I don't think any wheel could have taken that beating. There was a massive pothole concealed in a small puddle in that case. Fortunately I was able to get the state to pay for it because I was able to prove that the state had been informed of the condition of the road (there was a newspaper article about it with comment from the MVA) and had failed to repair it in a timely manner.

Thanks for the head's up on those. They might not end up being ideal for me, but it's always good to know what's out there.

Well yes you will save some unsprung weight and the tires will be a somewhat cheaper. But IMHO you are making a mistake. Your car looks great as it and a set of tires will be a lot less expensive than wheels and tires . But you pays your money and makes your choice.

I burn through 3-4 sets of tires every year so 18s are the only way to go for me. I am partial to OEM takeoffs which are light, strong, wide enough, and reasonably priced. They can be found often for $800-1000 for a pristine set. Full disclosure: Performance driving is my game and I don't give a rats azz what they look like as long as they do the deed. My car will not be caught dead in a C&C meet or PCA concours.

BBS RS-GT from tire rack are available in 18" and clear easily with no rub. Two piece, forged, and they even clear my girodisc rotors. Sent from my iPhone using Tapatalk

I have both 19" wheels and 18" wheels. The 18" wheels are by far more comfortable on the street. I won't comment on any performance comparison because the 18" wheels do not have the same brand tires as the 19" wheels. I am looking to pick up a set of 17" wheels for track use though.

I have found that Signature Wheels that have forged 18 inch wheels, SV104 for a reasonable price of about $2750 per set. 9.0 inch in front and 10 inch in rear. They say they can customize the offset to fit my needs like an offset pf 50 in front and 35 in the rear. Currently using Porsche 18 inch standard 8 inch 57 offset F and 9 inch 43 offset Rear while using 18 mm spacers F and 23 mm spacers in the rear. Weight is around 19 lbs for both sizes, a little less in front than the rear. You can find this info on their website. I asked about the safety certificate from Germany, which they say they have. Maybe the black tint will work on my black car. These wheels will be fitted with with either RE71R's or Hoosier A7. I am sort of favoring the Bridgeston's as the northwest has a number of rainy days that are attracted to autocross days.

Those look really nice, but their website shows a price of $4500. Does anyone happen to know what offsets would be good for a proper fitment that would fill the wheel arches reasonably well but not result in any rub?

Ivan from Signature Porsche suggested 50 front and 35 rear for aggressive 18 inch SV 104 wheels. I was dealing with Ivan for a end of quarter deal for $2750. Maybe this coming quarter end I can pull the trigger. Currently I have 23 mm spacers on rear with Porsche Cayman 43 offset wheels and and 18 mm front spacers with a Porsche Cayman 57 offset wheel. My wheels do not rub, even with a 30 mm drop from the Ohlins . Cantrell Motor Sports, told me that the negative camber helps avoid wheel rub on top of the wheel arch. I run -1.6 front and -2.1 in rear camber. 18 inch Signature SV 104 are available in 9.0 inch front and 10 inch rear. Please see post #13 for details.

Do you happen to know the rim widths they were using?

My last set of Porsche 18" takeoffs were found for $1k including a decent set of street tires . 8" & 9" widths. I run a 35 sidewall F/R to keep the PSM happy, retain rake and corner balance, and lower the gearing a few % for better corner exit.

Wold running an 18 inch tire with a 40 sidewall cause PSM issues? The tires on the 19s are 35 sidewall, so it seemed appropriate to go to a 40 on the 18s, as that is the factory fitment. Otherwise, it basically defeats one of the main reasons for going down a size.

feeshta- In a pistonheads web thread, CMOOSE, thread title, 2006 Porsche 987 Cayman 3.4 Bore Score Edition, starting on page 14 through the end, has chosen 17 inch wheels and tires for his equipment as his driving style on back country UK roads favor these smaller but comfortable wheels. Take a look at his plight with Bilstein B 12 fitment install issues. You can follow his saga along until finally resolved by fitting H&R front springs along with spring spacers. Its a good read and successful ending using 17 wheels and tires that he purchased for his winter mountain use. Great pictures along the way. He compares the 17's to his standard 18's in very favorable context. I am not sure that would work for me, but it really works for him. He fit the 17's to his Cayman S by using 6 mm wheel spacers to clear his red S brake calipers. I would have used 20 mm square front and back on the Boxsters rims he chose. Hope all is well.

The best autocross setup I'm aware of is 255/35-18 front and 275/35-18 rear. If this is for scca, get a set of 18x8.5 & 18x10 matching XRR 19" offsets. I had a set of forgestars custom made; they're even cheaper than tirerack ozs.

Shorter side walls mean less flex and better tire contact patch, especially when turning and braking fronts, trail braking rear or adding power to the rear to rotate car causing the slip angle to increase (both front and rear). But the cost is less flex for daily driving. This thread was started by feeshta to reduce the harshness of using 19 inch wheels with low profile sidewalls that cause more vibrations within the cabin with Ohlin coilovers for everyday driving. With 18 inch tires , what does 35 sidewalls do to ride comfort vs contact patch control compared to the 40 sidewall tire? Also each manufacture does not have a full range of sizes in both 35 and 40. My upgrade later this year for Autocross will be to 255 front and 275 rear using the Signature 9x18 and 10x18 SV104's and Bridgestone RE 71 R's that is available in my size, thinking 35 sidewall. I will keep my standard Porsche 18 inch wheels with Michelin PSS 235/40 f and 265/40 r for my everday driving. As an offshoot, I can buy a compact compressor and impact wrench to help me change tires quickly. Not sure if a pressure canister will allow me to use the impact gun at the autocross track for changing the sway bar settings as well as adding tire pressure.

The sidewall height is a function of tread width and sidewall ratio. A 255/35-18 has just about exactly the same sidewall height as a 235/40-18. The sidewall angle is different due to the tread width, though; the 35 in this case actually has greater deflection when mounted on a 18x8" and is thus softer than a 235/40-18. The difference is pretty small on a 18x8.5. Similar for the rear.

I went with those 18" OZ wheels and Supersports for my summer tires . Very happy with them, and can feel the weight difference.

Don't consider it a downgrade. I have a 987.2 on KW's. My car came on 18" Cayman S wheels (factory upgrade) in the 235/265 combo. I switched to 19" Forgestar wheels in 19x9, 19x10 in 245/265 combo. After one season, I switched to 18" 3-pce BBS wheels in 18x9, 18x10 in 255/285 combo. The 18's feel more agile and have a quicker steering response, despite the wider tire and likely equal weight. IMO, for street; 18x9, 18x10 with 245/40/18 and 275/40/18 is the perfect combo. A little bit of lowering and some extra camber up front to get the ride in check and you'll be on rails even more than you already are!

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It's not a secret, race teams have known for many years that the most optimal setup is generally the smallest possible diameter wheel that clears the brakes and a pneumatic tire on top of that: for Formula 1, that's a 13" wheel. This is largely because a pneumatic tire is lighter than a metal wheel, but also interaction with the suspension. The tire sidewall contributes to the spring rate but is undamped, which factors into the suspension design when combined with springs and dampers. (Stiffly sprung cars are often quicker with soft sidewall tires while soft sprung cars like ours favor stiffer sidewalls.) F1 design takes this all into account. Formula 1 cars also weigh half of what ours do yet have way more rubber (305 f and 405 r IIRC) have enormous downforce. I'm not sure why there is a misconception that taller wheels are better; the Lambos, Ferraris, and other supercars that started the trend only did it to clear the brakes. It's since become a fad and the norm, but tall wheels on cars with small brakes was never about performance, only about appearance. Tire availability is also a factor when it comes to autocross and time trial. 17" would likely work better on our cars than 18" or 19" but we're limited to mass produced tires that don't come any wider than 255mm in 17", therefore 18" becomes a near necessity. Blame people that favor looks over performance I guess; you can get 335/35ZR17 Hoosiers (marketed to people that actually care about performance), but nothing streetable in the right sizes unless you go to 18".

The "misconception" is because in street tires most guys want to improve responsiveness in the car's handling and not reduce the O.D. to maintain an accurate speedometer. Plus 1 and plus 2 sizing does frequently improve handling with street tires. The wheel makers and car manufacturers respond to the consumer accordingly. This still doesn't answer the question. Why do F1 tires have huge sidewalls capable of providing 5 g's of grip braking and 3 g's of lateral grip but street tire sidewalls would fold over at half that side load even though they have half the sidewall? Following the logic of a smaller sidewall is better, they could spec and provide a 35% profile tire on 13" wheels, there is no reason why they can't. The teams have redesigned their suspension in the past 3 years to run the new radial tires, so saying that Pirelli had to supply a tire with a huge sidewall to work with the chassis in F1 would not be the answer.

Sidewalls don't really work the way most people think. They sidewall portion near the ground is only a small % of the force holding the car off the ground in F1 tires - it's the sidewall above the tire, held in tension, that's providing most of the offset force. It's not visually obvious, but I've measured my 18" Porsche tires as stretching about a half inch out of round, just sitting static in my garage; it stretches even more under dynamic loads. F1 tires run as low as 21psi, but there are so many square inches involved that net overall forces are still really high. This includes pressure on the sidewall holding it against the rim. Now, this changes a bit once you get to ridiculous rubber band tires on giant wheels, but we're talking F1 here.

It's the air that supports the car, not the sidewall. The sidewall flex in the F1 cars is many times greater than the flex allowed by our 35% profile tires . That is why I keep wondering what Pirelli and the other manufacturers aren't telling us. By the way, your street tires can not stretch because the steel belts prevent the circumference from changing; it can distort, but not stretch. If your tires ever stretch, they will separate immediately.

You've just answered your own question: visualize how the air pressure is acting on the inside of the tire. And yes, I'm well aware of what is happening inside the tire carcass with respect to strain and elastic deformation (both are occurring), but I try to stick to layman terms and simple descriptions on public forums as not everyone here has a PhD in engineering. Speaking of, if you haven't read Paul Haney's book on racing tire dynamics, it's a great read.

I am getting the itch to finally upgrade my wheels and tires to 18 inch forged Signature wheels, 9.0 front and 10.0 in the rear. My question is using Bridgestone RE71R's with 35 and 255 front and 275 rear, is this the best combo for Autocross as far as side wall stability? I am going with a 50 wheel offset in front and a 35 wheel offset in the rear. Currently, I am using stock 18 inch Porsche wheels with a 57 offset in front and a 18 mm spacer and 43 offset rear and a 23 mm spacer. By using the calculator below without using the spacers, the new wheels will be just a few mm outside my current stance. I have Ohlin coilovers so in front, the spring perch should not be a fitment issue and in the rear I may have to use some plastic ties to pull the brake line from the tire side wall. Turn in at lock in front should clear the inside fender wall. If not I can add a few mm's of spacers to push the wheel out. And lastly if needed I could have my fenders rolled. In the end of this addition, I would like the suspension stance to be as wide as possible by putting as much weight on the outside tires as possible.

https://www.discounttire.com/learn/offset-backspace Not quite. Do the math after reading the above link.