OK, I'm always hearing that you shouldn't go any larger than 17 inches in dia. for the 2nd gens. Many have given their 2 cents-except with no real data/scientific theory to back up claims. Heck, I've seen the Super Now 2nd gen with a 15/17 combo (front/rear). My Japanese is rusty (trans sux), so I'm not getting answers there.

Would someone place enlighten me? Is it purely down to our roll-center issues with a stock suspension?

~Mike

 

well for one unless you're getting a massive big brake kit. the extra rotational weight is killer.
and 18" rims don't leave a lot of room for tire. i know people that have raced on rubber band tires and they end up destroying their rims from apexing over the curbs or spinning out.

 

Hum, is it? I say that because I've had 18's that were, in fact, lighter than the 5Zigens FNO1RCs that I have on now. Now, where there could have been more weight is at the outer portion of these forged 18" rims, the tires have to be included in that number and the 255/35-18s (Pilot Sport Cups) I had were lighter than the 17" Hankook RS3s.

Not knowing the exact weights of the outer wheel/tire combo keeps me wondering if there is a penalty from running 18"s.

You brought up having less room for tires. I can't agree because you've effectively added 1/2" of diameter to the rolling stock thus increased the distance from the coil spring.

there are heavier cars out there running on 18"s without wheel damage which gets me to thinking that suspension control/ brake-rim heating/rim quality plays the biggest factor in rim damage.

Thanks for the interaction. Not trying to be combative. Just trying to get down to the real facts.

 

18" wheels mean there is 1/2" less room for sidewall than 17".

So unless you want to run 205 width tires on the front, or feel like having some 245/25-18 tires made up...

Don't forget that taller tires bork the suspension geometry too, since the distance from the various mounting points to the contact patch is one of the criteria for determining roll center.

For instance, SA/FB handle horrible when you make the tire diameter larger than 23" without some heavy duty geometry alteration. I don't know what the breakpoint is for FCs but a practical limitation of what fits on a 17" wheel sounds about right, given the stock tire diameters and common tire sizes.

 

On the 2nd gen you have a problem with front tire hitting the unibody on full travel.

Basically, the shorter the front tire the lower you can have your FC and still have some suspension travel left.

 

True, bluetII, but if you do a "plus" fitment, you will see very little change in overall wheel/tire diameter, just width.

So, are you thinking that racers should just go with a smaller wheel/tire diameter to increase shock travel if they don't have enough?

OK, makes sense, but what if you do have enough wheel travel and the wheel/tire diameter is close to factory diameter. what's to keep me from using an 18" up front?

 

bluetII has a point, the car i'm starting with now came with 225/50/16's, and the limit for those is the tire hitting the upper framerail, with the ride height set so that there is 3" between the rail and the tire, the car isn't that low...

granted those are big tires, and the racers generally run smaller tires than stock (225/45/15 is the spec tire size for the IT classes, which is 23")

although i don't see why you couldn't run a bigger wheel as long as the diameter stays sane

 

im using porsche cup slicks(235/18) on my fc without issuses,the main reason is i get them cheap second hand with very little use on them,but ive had no problems in fitting them or in the handing of the car,they measure 640mm in height so i think that makes them about the same height as most 225/50/16 semi slicks(from the info i can get based on the tyres i can get here)i would like to try a set of 15s cos it would improve the gearing and maybe the handling but the 18s are the right price and provide brillant grip so unless my supply runs out im sticking with them!

 

There's nothing stopping you, you're thinking too hard about it. Generally there is no room without compromising sidewall thickness. Period. Heavier cars can do it because those heavier cars typically have more room so they can run the big wheels with reasonable sidewalls.

If you can figure it out othewise then you're unique and can buck the convention. It's not like rx-7 police will come find you for running 18's.

As for weight, ok so you have ONE particular 18" wheel/tire combo that is lighter than another PARTICULAR 17" combo. That's irrelevant. Make your 18" set, a 17" of the same equipment and it'll be lighter. I have some 20" forged, super expensive rays wheels that are lighter than a 15" cinder-block, does that mean 20's are lighter than 15's?

 

True, bluetII, but if you do a "plus" fitment, you will see very little change in overall wheel/tire diameter, just width.

So, are you thinking that racers should just go with a smaller wheel/tire diameter to increase shock travel if they don't have enough?

OK, makes sense, but what if you do have enough wheel travel and the wheel/tire diameter is close to factory diameter. what's to keep me from using an 18" up front?

Sure, nothing stopping you from running a 35 series sidewall on an 18" rim.

But for the hardcore racer the car would be over an inch lower on 35 series 15" rim and you could lower the car further and still have the same suspension travel, plus the decrease in mass. Hoosier 275/35-15.

If not to lower the car, SuperNow may have run 15 up front 17 rear just to emulate the look of the FCs race cars that dominated GTU/GTO through the 80s. Those race cars ran small diameter front wheel/tires that to keep the hoodline as low as possible.

 

Tasty, with all do respect, I don't think I'm getting too deep-just want to know the answers.

Blue, you're helping me figure this out. So going with the shorter tire/wheel combo takes care of two issues-lowering the car while keeping proper strut/control arm geometry? OK, makes sense. I can visually "see" what you're getting at.

If we're looking at this issue primarily being the suspension geometry/ride height shortcomings, I've got a solution that will solve this issue.

Below is Yawpower's Roll Center Corrector. It's a full-on front suspension upgrade that allows you to, pretty much, have your cake and eat it too.


Roll center's corrected at the spindle (raised) effectively lowering the outer control arm resting location. It allows for @2.5 inches of suspension travel before the control arm sees 180*/horizontal. As we all know, once that arm is past 180*, you start to see positive camber changes.

My issue is I'm doing Time Attack and there are virtually no choices in 15" sizes. 17" is a stretch to get a 285 in the sizes I need and I'm sponsored by Michelin, so i've got to go with what they have.

Any more thoughts on this are appreciated.

 

You asked why people typically only run 17's then refuted the general reasons with very particular thought. Yes that is too deep, for what you asked. I promise you, the reason you don't see the general public (and thus the rule of thumb exists) is not a deep reason.

 

Tasty, thanks. I believe I got my answer from Blue TII.

 

That falls under the category of "suspension geometry alteration" that I mentioned earlier.

Although, it is a ton easier on SA/FBs because the two piece spindle allows you to just put a spacer between the spindle and steering arm/ball joint, no need for a grandiose confabutraption. Basically, he saw what the pre-86 people could do and made something that would work for FCs.

Google turn-in spacers and you'll see what I mean.

 

That suspension looks like a great idea when lowering the 2nd gen in terms of suspension geometry.

As I stated previously, a real problem on the FC is the top of the tire hitting the unibody up front when lowered.







That said, the 285/30-18 PSC is barely taller than stock size for FC. If you add aero to keep air from going under the car instead of lowering the chassis you will be fine.

Before you go nuts spending $$ on FC suspension take a long hard look at how affordable FD shells are and the forged alum double wishbone/multilink goodness that comes with it...

 

I like the way she's looking, Blue TII. Yeah, its incredible what 3rd gen shells are going for. As for the 2nd gen I'm using, i got that for 500 w/ engine. The other parts are basically from peeps selling them because they were no longer SCCA legal.

It's all together and running and I built it for about 6k, so it wasnt too big a hit. That front suspension has been the saving grace. It allows for @ 2.75" of travel with the drop i have. I'll post pics a little later.

Thanks for the help guys. Hey, and wasn't trying to down anything you were saying-just trying to use a little deductive reasoning before I completely threw out the idea of running 18"s. I don't like following the crowd just because someone says so. I'm sure you can understand. ;-)

 

Those are other members cars from "who has tha lowest fc?" thread; though, mine does look like that blue series 4.

 

Last time I emailed Paul he wasn't selling these any more; he sold the design to MazdaTrix... (IIRC)

 

FBs will do that with 195/60-15s, too.

I've seen people cut out that body rail and the inner fender ahead of it in order to be able to use larger tires.

Really though, if you're not running huge brakes, larger wheels mainly just get you shorter sidewalls, which isn't a good thing unless you race on pool tables.

 

Yes, Paul did sell the design to Dave @ Mazdatrix. Unfortunately, SCCA banned the design, effectively killing all future sales.

Big brakes are 14"s and in need of 18" wheels to work.

Funny, Peejay, but Michelin spent TONs of money convincing POC/PCA racers that a stiffer sidewall tire (Sport Cups) with a softer sprung suspension would work better on the typically beat-up racetracks within the US! LOL, imagine telling "broke" Porsche racers that they'd need to revalve their coilovers and purchase new, softer springs-it didnt work!

 

Im into drift.... So u guys will see where im coming from. Im running 18x9.5-1 with 225 35 18s. The car looks amazing and im dumped with my car unfortunately scrapes on the frame under hard cornering. Im running 25 mm front fenders and I trimmed them a centimeter for ultimate lowness. The car handles alot slower compared to running 17x9s. Honestly id recommend 17x9s assuming ur on a fc setup

 

Advantages of small wheels (15-16):
-Rotational inertia
-Cheap tires in shorter overall diameter

Advantages of big-ish wheels (17-18)
-Room for big brakes
-Much greater tire selection (at least with serious widths)
-Bling
-Less sidewall roll
-Overall heights are shorter for the width (mostly an 18 vs. 17 comment).

Case in point on the diameter thing.... Tires wider than 275s in 17s are a fairly barren wasteland particularly in the 285 to 315 my widebody lets me run. My quicky Tire Rack search just to get a feel for things turned up:

0 options 285/35r17
3 options 295/35r17 (25.1 diameter, Kuhmo Esta XS plus a couple of hoosiers)
0 options 315/30r17
12 options 315/35r17 (25.7" diameter would be fine for the rear)
0 options 335/30r17
7 options 335/35r17 (26.2" diameter is getting pretty dang tall)

Compare that to 18s and the widths/diameters are better.

23 options 285/30r18 (24.8" diameter)
16 options 295/30r18 (25.0" diameter)
10 options 315/30r18 (25.5" diameter)
10 options 335/30r18 (25.9" diameter)

So to the OP's original question, "shouldn't" is relative term... It's all compromises and 18's work great if you have a reason to do it.

 

I just downsized my tire sizes to get shorter lighter tires as an experiment.

Old setup- 275/40-17 rear, 255/40-17 front (24.92" diameter)

New setup- 255/40-17 rear, 235/40-17 front (24.40" diameter)



On my setup the 235 up front allowed me to get a little more camber by pulling the wheel/tire toward the strut (not shown in this pic), so I am not sure I will lose much front grip in the end.

6" ground clearance at front lip and 2.5" suspension travel up front before tire hits body.

I might mess around with 22" tall front tires on my 16" rims to get the car lower.

 

17's are the compromise size.

You have to think about Ride height & Sidewall size mainly.
The other items like weight and geometry can be changed.

17's offer decent sized wheels and reasonable sidewall size.

I say reasonable sidewall because running 20/25 sidewalls on a street car will usually result in you buying new wheels after you bash them in a pothole. I run 235/40/17's as a precaution and it's paid off so far. It's a decent size sidewall too so it's still stiff compared to monster truck 50+ sidewalls.
The sidewall acts like a small shock against instant impacts. Stiffening it up increases performance but you risk damaging rims as a cost.

Then there's ride height like shown above with Bucky's car. Lowering the car reduces overhead travel before the wheel comes in contact with the frame. Changing sidewall can help with this but then you come into what I said above.

I rubbed upperframe rail while racing with the setup below.
17x8 235/45/17
Ksport kontrol pro 8 or 9k spring
1-2 inches off ground

 

1-2 inches off ground

Uh, that looks about like same ride height as my car once you remove the low hanging fiberglass. Look at the centers of the wheels and the wheel gap in front with our same size tires.

Not that that is a bad thing; it keeps your suspension in proper geometry and gives you more travel while keeping air from under your car. And it looks cool.