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Tire sizing for Cup 1 wheels

 
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jacobroufa  



Joined: 18 Nov 2016
Posts: 511
Location: Belvidere, IL

PostPosted: Tue Dec 25, 2018 5:50 am    Post subject: Tire sizing for Cup 1 wheels Reply with quote

So I recently acquired another 924, this one a Turbo. You might recognize it as Woody, formerly one of Ideola's Garage collection. http://924board.org/viewtopic.php?t=41290

It doesn't need a whole lot this year to start enjoying so I'm planning out my expenditures for 2019 and one of the most pressing things is the tires -- they are dry and the sidewalls are pretty cracked up. It's sitting on square 7" x 16" Cup 1 wheels and currently shod with 205/55R16 up front and 225/55R16 in the rear.

With square wheel size should I keep the staggered tire setup? Does it gain me anything? This will be mostly street driven, maybe with some light autocross. I've researched a bit and the Continental ExtremeContact Sport has come up as a top-rated tire. I have had good experience with Continentals for years and am inclined to get them, however they do not come in the same size as is currently fit to the car.

For the 205 section tire, Continental offers a 45mm sidewall (83W rating) or 55mm sidewall (91W rating). For the 225 section tire with 16" wheel they only offer a 50mm sidewall (92W rating).

So... should I go with a square 205 setup with a 55mm sidewall? Square setup 225/50? If I go for a staggered setup again, which front tire would you recommend? Shorter (45mm) or taller (55mm) than the rear (50mm)? Does it make a difference?

Sorry for all the questions, but I've not really bought sport tires before, only all seasons, and also never had a staggered setup before. I'm mostly wondering if it's worth it, and what y'all recommend. Also if there's a different tire that anyone has more experience with and could recommend as a good fit, I'm all ears.
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Cedric  



Joined: 27 Aug 2004
Posts: 2476
Location: Sweden

PostPosted: Tue Dec 25, 2018 7:58 pm    Post subject: Reply with quote

Square setup is almost always preferable. Regardless of what sizes you use. The car is best balanced, and you can swap around tyres to even out tyre wear.

The sidewall number is a %, so 205/55 and 205/50 have approx same sidewall height. Imo opinion those two have 112mm sidewall which I think looks best on our cars, purely looks. For performance, lower will give better gearing and sharper steering, but look bit to modern

My choice on those wheels would be a square 205 tyre setup, with some kind of high performance tyre.
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TJC  



Joined: 04 Apr 2010
Posts: 828
Location: Central-ish Arizona, USA

PostPosted: Wed Dec 26, 2018 2:45 am    Post subject: Reply with quote

When looking at aspect ratios one must keep in mind the diameter of the tire as well as sidewall size. If you stick with a staggered setup a 205/55 and a 225/50 are compatible in diameters. If you go for square I would gravitate toward the 205/55 as you will probably not run into any major clearance issues on the front and wouldn't be "over-tiring" the car as in stock form it doesn't really need any wider tire from a performance standpoint. I would most definitely stay away from the 45 series tire as in my opinion it just looks wrong, there is no significant performance increase for your driving style intentions and the ride quality will suffer.
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jacobroufa  



Joined: 18 Nov 2016
Posts: 511
Location: Belvidere, IL

PostPosted: Fri Dec 28, 2018 3:07 am    Post subject: Reply with quote

Thanks Cedric and TJC! Looks like I will be going with a 205/55 tire on all four wheels.

I agree a smaller sidewall on a vintage car does not look right, and as a bonus a period look also provides a much better ride comfort! The roads around me are not the greatest and comparing my daily driver Audi on 16s with a 225/65 to my wife's Passat on 18s with 245/40 is night and day. My car is so much more composed and supple on the road, and while her car handles well and also rides well it is noticeably more jarring -- you can feel every crack and crevice.

So, for a performance/balance standpoint with a 924/931, square setup is preferable... why is it that many of the more performant Porsche cars shipped with staggered wheels and that is also a setup a lot of folks end up moving to? Is it that once more power is added the extra rear grip helps compensate? Does that not then run into the same issues of balance and even wear?
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Cedric  



Joined: 27 Aug 2004
Posts: 2476
Location: Sweden

PostPosted: Fri Dec 28, 2018 9:00 pm    Post subject: Reply with quote

If you don't have excessive amounts of hp a good measure is to have the tyre ratio front/rear similar to your weight distribution. A square setup is much more common on race cars, because having narrower tyres up front is crap if you push the car hard with trail braking etc. Most cars that aren't that powerful like 944s etc have it mostly as a bling factor, just like drilled discs some people just like the looks, even though it's not a rational choice.

If you have adjustable sway bars for example you can dial the balance back fairly neutral. If you don't then you will of course get more understeer with smaller front wheels.

If you have alot of hp then you can start overheating the rears or have problems getting the power down. But if you look at really fast race cars they always have ridiculous wide front tyres, often they have as much tyre as you can fit, and fitting wides at the fronts is always more challenging. Like the porsche 919 le mans car, probably the fastest sports car of them all, 310mm wide square setup
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Carrera RSR  



Joined: 08 Jan 2010
Posts: 2287
Location: Somerset, UK

PostPosted: Sat Dec 29, 2018 7:22 am    Post subject: Reply with quote

Currently shopping tyres myself. Currently have 195 50 15 R tyres on 7x16 Fuchs. They measure 201mm wide so half way between a road spec 195 and 205 tyre.

I did have briefly 205 45 T1Rs on the Fuchs but the sidewall was far too low profile. Id say 97 to 107mm is about right on 16 on a 924

Looking at this pic the rears look far too balloon like for the car. Too wide for 7 and too tall a side profile. Looks over tyred. The sidewalls balloon out too far and are too rounded. Great for avoiding scratched wheels when parking. Not good for looks


Personally Id go with 195 50 or 205 50 all round on your wheels. Especially if looking to close the arch gap with lowering. 55s if not as per Cedrics comments
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