[jhein]

The Nitto NT555RII in 285/40/18 specifies a wheel width range of 9.5-11 inches. I can find 18" Rally II style wheels but only up to 9" width. Would that work or no?

[Steve C.]

I posted this info years ago. Some of the info was discounted by a few members.
Therefore take the info with a grain of salt....

https://www.hotrod.com/how-to/wheels...#ixzz3iHWQlclu


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[76TA462]

I am using tubular uppers and lowers with adjustable coil overs. For the rear, Hotchkis sport leaf springs (and their matching tuned shocks) with 1.5 inch drop which I later tweaked to be a 0.75 inch drop for better road clearance. Very happy with the improved handling.

[Steve C.]

Similar here after changes.

On the front I have Global West upper and lower control arms, and have QA1 coilovers with a custom 1-inch sway bar.

Also, and very important, subframe connectors. Mine are custom.

Maybe I'm a bit complacent on my current set up for not making the change to a 18-inch combo
Not being a dedicated 'canyon carver' but it handles surprisingly well for its intended street only use now.

Years ago prior to selling their mono leaf spring and when John Calvert was working closely with Landrum Spring Service they recommended a 300 rate spring on my car.
At the time it was a stiff multi leaf spring for a heavy car and the improved consistency and 60-foot times.
I've had 1.43-1.6 60-ft times depending on the tire and engine combo in use. And with the higher rate spring I don't use a rear sway bar.

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[JLMounce]

Originally Posted by jhein

The Nitto NT555RII in 285/40/18 specifies a wheel width range of 9.5-11 inches. I can find 18" Rally II style wheels but only up to 9" width. Would that work or no?

A 275 is better on a 9” wheel. You likely can fit the 285 on there but it will bulge a little bit on the sidewall.

It sounds like you’re looking at the US Mags off the shelf offerings. They have custom rally II’s available as well. Check their forged wheel line. You can get larger and wider sizes.

The wheels are a better quality forged piece instead of cast, but are of course pricier.

[twilightTA]

I’m running 17”s on my 77 TA. I had the Nitto NT555 tires and switched to Continental ExtremeContact Sports. More expensive but they perform a ton better than the Nittos did.

[JLMounce]

Originally Posted by Steve C.

Similar here after changes.

On the front I have Global West upper and lower control arms, and have QA1 coilovers with a custom 1-inch sway bar.

Also, and very important, subframe connectors. Mine are custom.

Maybe I'm a bit complacent on my current set up for not making the change to a 18-inch combo
Not being a dedicated 'canyon carver' but it handles surprisingly well for its intended street only use now.

Years ago prior to selling their mono leaf spring and when John Calvert was working closely with Landrum Spring Service they recommended a 300 rate spring on my car.
At the time it was a stiff multi leaf spring for a heavy car and the improved consistency and 60-foot times.
I've had 1.43-1.6 60-ft times depending on the tire and engine combo in use. And with the higher rate spring I don't use a rear sway bar.

.

.

I just spoke with Global West today about building a set of their cat5 springs for my car. I was surprised that they still recommended only around 180lbs on the back. Different car of course, my bird carries only 1402 lbs on the rear tires.

In regards to the 18's vs 17's, I made this switch last year and honestly, I don't think I'd go back to 17's at all.

The tire selection is better and actually cheaper now as this size is the mass market. Additionally, by staying the same overall tire height and reducing the sidewall, turn in is noticeably more crisp. I'm down on outright cornering grip, but that's more to do with the change to a touring tire (NT555G2) vs the autocross tire I was running (Falken RT615K+)

Here's how I would characterize the change in turn-in quality. The 18" setup is very direct and immediate. I turn the wheel and the car immediately changes directions. With the 17" wheel and tire setup, It's like the front end followed my steering inputs. I could crank the wheel, then the front would turn. Obviously this is milliseconds, but it feels perceptible behind the wheel.

[twilightTA]

My setup is PTFB GT coil/rear leafs, PTFB subframe connectors, Viking Warrior dual adjustable shocks, PTFB Blade front swaybar, PTFB rear adjustable sway bar, PTFB solid body bushings, SPC upper control arms.

The addition of the SPC control arms and the Blade bar were at the same time - that’s when the car totally seemed to transform itself - the handling is amazing now after those 2 additions.

I have PTFB lower control arms on order as I need an arm that can accommodate the huge end links on the Blade bar, and the SPC lower arms are just way overpriced at $1200. The SPC arms were a pain to align though and I think I should’ve just stuck with PTFB on those too.

Everything from PTFB has fit absolutely perfectly and performed well on this car.

Agreed on the 285s on 17x9s bulging. I have those my 17x9 year one snowflakes and I am thinking to go back to 275s and “hopefully” fit a 275 on the front when my Contis wear out.

[70GS455]

Nitto has the NT05 street tire in 140 and 200 treadwear

[jhein]

Originally Posted by Steve C.

Also, and very important, subframe connectors.

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I've been waiting for someone to bring that up.

[w72]

There's a lot being tossed around in this thread, all of it with good intentions. Apologies for the lengthy post, hopefully it helps somebody. Any discussion about tires is actually a discussion about tires AND suspension. First rule is to not spend money to make your car worse. I have done a lot of that in the past, I think many of us have.

To state the obvious, 2nd gen cars look pretty bad when the wheel wells aren't full of sidewall. A 26" tire looks just lost in there, and lowering the car to compensate creates a host of other problems. The 15" tires available aren't adequate for decent handling or braking, much less straight line traction unless you make suspension changes that hurt drivability. If your suspension is currently stock, the fat sidewalls of the 15" tires are a big part of how the suspension works, hiding road imperfections and softening all the driver's inputs. You can run 17x9 wheels and the Nitto 555g2 in a 45 or 50 series sidewall and have a pretty good improvement over what you're running now, but the added lateral traction will start to show you the weak links of the stock chassis, suspension, and brakes. They will also ride a bit harsher. Note that if you have worn out bushings (or poly where it doesn't belong), your ride will be harsh no matter what. If you can stomach the looks of a shorter sidewall and a bit less overall diameter, run 18's. Your tire and brake options will be more plentiful. A lot of people stop right there and are perfectly happy, but I would say that welded frame connectors and solid body bushings are a must, especially if power is increased over stock.

It seems that the OP's Firebird isn't intended to be a competitive at the strip, so my advice is to focus on how you will use the car and your budget constraints (if any). Tire and suspension choices are interconnected, and need to be thought about as a single system. You can't expect good results by just slapping drag radials on a pro-touring machine any more than you can expect to run good times at an autocross event by putting a front bar and fat sticky front tires on your drag car. In short, it's a set of compromises that will be different for each use case, you have to keep your eyes and wallet on your end goal.



So what gives? With almost twice the power your car was engineered for, the stock leaf springs can't cope at all. At best, you can choose between spinning and hopping. Aftermarket offerings for traction (slapper bars, Calvert stuff, slider links, etc..) are intended for straight line use. The obvious answer since this is the internet (and it's not my money) is to tell you to spend enough to build a fun bracket car on an "engineered solution" from one of the many aftermarket suppliers. That would typically involve a 4-link setup of some sort. The expense is just massive, and the juice isn't worth the squeeze in my opinion (based of modding several 2nd gen birds over the years).

If you try to stuff a 4-link with good geometry under one of these cars, you'll learn there's not room for an upper link of adequate length or height over the axle if you try to mount the front pickup point in a decent spot. By "decent spot", I mean where the back seat currently resides. The only reinforced spot for the lower link's front attachment is where the front leaf eye went, making that link too long. There also isn't a good reinforced spot to mount a panhard bar or track link, so maybe you band-aid that with a heavy and complex watts link? You'll obviously need a different (or heavily modified) rear housing, and the factory anti-sway bar won't fit anymore. You may also need rear tubs and/or a different fuel tank to fit all this crap back there. Let's not forget that the coil-over shocks/springs are not only pricey, but the threaded shock bodies aren't really meant for long term street use. You're literally counting on some aluminum threads to handle all the forces of spirited driving. Coil-overs offer some advantages, but they belong on race cars that get serviced constantly. Cheap coil-overs don't belong on anything.

Also consider that your new set of upgrades to the rear suspension will require similar work up front to make the car remotely balanced. You'll be at the mercy of the aftermarket for service parts. The NAPA closest to where you break something definitely doesn't have the weird left hand thread rod end you'll need.

The scary dude in the corner is this: you'll likely be in over $20k before even thinking about brakes. That scary dude's friend is that there are so many adjustment points that you'll probably burn up $3k in tires before even finding a workable baseline setup. So again, what gives? Has anyone already figured this stuff out? Yup. Herb Adams. He had his paws all over this platform before anyone had seen it, and continued to race and optimize them for years. His book "Chassis Engineering" is amazing, and even a casual understanding of it will illustrate why a 4-link is not an appropriate solution for these cars.

To make the leaf spring setup work with more power and better tires, there are some relatively inexpensive things you can do. Relocate the front eye of the leaf spring upward and slightly inward, and use an articulating bearing instead of a bushing in the front of the spring to eliminate binding. Use a leaf spring that is stiffer towards the front to prevent axle wrap. Use the shackle and/or blocks to adjust anti-squat and ride height. You can either run softer springs and stiffer anti-sway bars, or stiffer springs and softer bars, but don't try to do both stiff. With more power, the stronger spring and softer bars are a better bet. Up front, run solid or delrin bushings in the control arms and use an offset shaft (or aftermarket upper arm) to maximize caster.

The setup I have now is by far the most capable, enjoyable, and serviceable I've had. This is a useable but very speedy 467ci street car with A/C that handles crappy pavement but can be autocrossed. It's just a damn joy on curvy two lane roads, and it importantly still looks like a 70's American muscle/sports car with some visible sidewall instead of rubberbands. It's all about compromising towards happiness, and aiming for a well-balanced car that suits your use case.

17x9 snowflakes, Nitto 275/50/17 and 255/50/17. A lot of the chassis/suspension is from PTFB. Subframe connectors (welded in) and aluminum body bushings installed at stock height. I'm using their "comp" springs front and rear. The leaf springs have a bearing in the front eye. I'm using their front leaf spring perch that relocates the front eye and their adjustable shackles, as well as their spring plates. All of it is beefy, and bolts in with no cutting or other modifications. I'm using the stock WS6 bars front and rear with Koni adjustable shocks, full soft rear and a bit firmer in front. I like the geometry of the Detroit speed front upper arms, and have their delrin bushings in stock lower arms. C6 Corvette brakes (not Z51 or Z06) barely fit under 17x9 snowflakes, rotors and calipers are cheap and plentiful, and there are a ton of pad compounds available. Rear brakes are Explorer discs with integral parking brake on a 9" from Moser. If you're keeping the 10 or 12 bolt, there are plenty of affordable disc conversions using OEM parts.

I'd recommend talking with Dave at PTFB about your goals, you can likely get what you're after for far less money and hassle that trying to re-engineer the whole darn car. Cars with his comp setup regularly embarrass much more expensive builds at the track, and I can verify that it still feels like you're driving a classic Trans Am. It's just more capable and focused. I wouldn't say the ride is harsh, but it is firm. Disclosure: I've never gotten a discount, and have no relationship with PTFB aside from being a happy customer.

[jhein]

OK, thanks very much for all the very detailed and thoughtful replies.

Here's my plan. 17" Rally II's with 255/50 front and 275/50 rear. That gives me full wheel well openings and a reasonable sidewall. I'm doing that now. That's obviously not for traction, just handling. Then, either later this year or early next year, I'll replace the rear springs with the PTFB springs and their anti squat brackets and some adjustable shocks. If that doesn't help enough with traction then M&H makes a 275/50/17 DOT drag radial which they claim is also suitable for road racing, so I assume it has a stiffer sidewall. At some point maybe sub frame connectors too...

Thanks again.

[JLMounce]

Do the subframe connectors at the same time you do the springs. These cars with lots of torque tear themselves apart without subframe connectors. The rear end is trying to lift the body off of the front subframe and you start pulling the body mounts through the subframe holes. This is especially true, if your subframe has typical amounts of corrosion and wear at the body bushing mounts.

The more power and the more traction you have, the worse this gets. After seeing what was happening with my car before I added the subframe bushings, this is in my opinion a critical item.

[w72]

I would definitely start with solid body bushings and subframe connectors. The PTFB spring bucket moves the leaf inboard, I don't know if the 275/50/17 has enough clearance without it, I'm sure someone else has tried it.

[jhein]

Well, that's it then. Connectors and bushings it will be. Thanks.

[twilightTA]

@w72 do you have some pics of your 255 and 275 setup. Contemplating that next time around. I’m a bit surprised you don’t have clearance issues though with your Comp springs.

Is that the sphero bushing you have at the front leaf eye? I was worried initially that may be too harsh. How do you like it?

[w72]

Originally Posted by twilightTA

@w72 do you have some pics of your 255 and 275 setup. Contemplating that next time around. I’m a bit surprised you don’t have clearance issues though with your Comp springs.

Is that the sphero bushing you have at the front leaf eye? I was worried initially that may be too harsh. How do you like it?

Yes, the front eye has the spher-o-bushing. It's kind of misnamed, it acts like a bearing or "johnny joint", similar to the cat5 spring. It doesn't add harshness to the ride, if anything it improves it because the bounciness and unpredictable deflection of the rubber bushing is eliminated. When that bushing twists or compresses, it adds spring rate in a non-linear fashion and when it unloads, it does so suddenly. The bearing is able to move freely. It probably is noisier, but there's no way I can hear that over the intake/exhaust noise. I expect it to need grease and/or service more frequently than a rubber bushing.

I remain convinced that when people spend their way into suspension complaints, they blame everything except the poly bushings they use in every available spot. For the 70-81 F cars, poly is only useful for end links and the rear eye of the leaf springs. Rubber is useful for anti-sway bar chassis mounts. If your car was 100% stock with all new parts, adding subframe connectors and solid body mounts *and doing nothing else* would make the car smoother and more predictable, not harsher. The only downside is that your ass might feel a few more vibrations from the motor. Whether that's good or bad is a topic for a different website.

I don't have any clearance issues on my '78, but note that I'm using different upper control arms and the front eye of the leaf springs are slighly inboard of the stock position. The caliper brackets I'm using up front reduce the track width by a whopping 0.03" per side, which is likely of no consequence.

I'm very happy with how it performs, but everyone's expectations are different. These cars were pretty cushy from the factory, but that simply doesn't work with double the power and more traction in every direction. I will say that it rides a lot nicer and is more predictable than my autocross prepped 4th gen bird while offering similar performance. I don't plan to make any changes until after 5k miles. I have the option to lower the rear another bit by using the other set of holes in the shackles, which I plan to try just for the sake of stance even though it would be a move in the wrong direction on paper. I may go to a nicer double adjustable rear shock at some point, the Konis are only rebound adjustable with fixed valving for compression.

I tried resizing pics on my phone, they were terrible. I'll get some uploaded asap

[w72]

In the first three pics, the car isn't sitting on totally level ground, and the springs hadn't taken a set yet.. The 4th pic shows it on a fairly flat driveway with some miles on it.

[w72]

For comparison, this is the same car in the same spot with 100% stock WS6 suspension, 225/70/15 front and 255/60/15 rear tires.

[VCho455]

Originally Posted by JLMounce

Do the subframe connectors at the same time you do the springs. These cars with lots of torque tear themselves apart without subframe connectors. The rear end is trying to lift the body off of the front subframe and you start pulling the body mounts through the subframe holes. This is especially true, if your subframe has typical amounts of corrosion and wear at the body bushing mounts.

The more power and the more traction you have, the worse this gets. After seeing what was happening with my car before I added the subframe bushings, this is in my opinion a critical item.

Yep! All true.

I too am a fan of Dave at PTFB. A real gentleman with a ton of knowledge.