...whoops...

It sounds like your complaint is the primary ride. I doubt off the shelf coilovers will solve your problem. These are generally aimed at being "uprated" so soft setting isn't necessarily significantly softer than standard. Have a look at the FSD / Special Active. These are similar of softer than stock in the initial primary ride but stiffen up with load. Your other option of course is a custom spec build.

Yes, it seems the special active have been recommended several times. My plan is to first try a smaller wheel with a taller sidewall. I'm going to try a set of indy 500's which supposedly soak up some shocks while retaining a small slip angle when loaded in a turn.

I'm not looking for a huge change, just a bit more comfort over bad sections of road.

Koni Special Active
 

I have Koni Sport (adjustable yellows) on my NA Miata and Koni Special Actives (red) on my '99 986. The Miata is set up for autox and the yellows do that job well. On the street (especially over broken pavement) I prefer the job the red Special Actives do in my 986. And at speed taking corners on smooth pavement, it would be hard to notice much difference between the two.

There was a very detailed article published by TireRack years ago when the Special Actives went by the name of FSD (Frequency Selective Dampening), before they were updated: https://www.tirerack.com/suspension/tests/koni_fsd.jsp

Thank you for this link.
The quote that stands out is " When driving over expansion joints and smaller bumps the intrusion was distant enough to be largely forgotten"

That is what I am looking for.

I'm hoping to swap to the 17" wheels in a week or so with Indy 500 tires (the setup on my 98 986 tiptronic going to my preferred 5 speed 99 986.

The 98 986 tiptronic was gifted to me and was a complete surprise, but I prefer the 5 speed. So I'll likely sell the 98 tiptronic with the arguably more desirable 18" wheels

Ok, let me throw another shrimp on the BarBi...

If I went with 205 45 17 instead of 205 40 17 the Overall diameter is + 3.7% from stock
If I went with 245 45 17 rear instead of 255 40 17 the Overall is + 2.5% over stock.

Given there would be a 1.2% mismatch in RPM per mile compared to the stock OAD, Would this cause a problem with the computer, or ABS?

For the 17" wheelset I want to swap onto the 5 speed car, The rears are currently new 255 40 17 Firestone Indy 500s I just bought on the drive down from Canada, and the front tires are older and need to be replaced anyways.

What if I kept the current (less than 1000 miles on them) 255 40 17 in the rear and added the .2" taller than stock 205 45 17 @ 24.3" OAD vs the 245 45 17 at 24.1" OAD?

Basically this way I'm adding +5 to the sidewall profile, while keeping about the same weight of the tires. This 'should' soften the ride over the bad roads a tiny bit, probably enough to move the needle a tiny bit?? What say you?

The 17" 986 Boxster diameter is 25" front and back and the stock front is 205/50/17. There isn't much room up front between the tire and the lower spring perch but maybe you could fit a 205/55/17 there? I don't know if going larger diameter front messes with the abs. I know going slightly larger in back does not. The front tires you mention all have a shorter sidewall and might roughen the ride vs the stock size.

You are downsizing the wheels from 18” to 17”, right? Then you are essentially adding more sidewall even with the stock tire size. Matching front sizes would be 205/ 50 or 225/ 45 if you are to keep 255/ 40 rear.

45 or 50 is the aspect ratio, so the number alone does NOT determine how thick the sidewall is; it also depends on the tread width. For instance, 255/ 40 tire has 102mm sidewall while 225/ 45 has 101mm.

I just switched from Michelin AS3 to Pilot Sport 4S. Grip is phenomenal and way better as expected, but I’m surprised by forgiving ride quality.

I see my mistake, I was thinking the stock front was a 40 series, not the 50 series that it is.

I thought a 45 series was going to be giving me +5% more sidewall over the stock, but the stock actually, as you pointed out, is a 50 series...

Silly mistake on my part

Thank you for catching my mistake. I thought stock was 40 series, not the 50 that it is...

Since I already have the 255 40s in the rear, I'll match with the 205 50 17 front. The Indy 500's don't come in 55 series for 17s
These won't have quite the same grip as the Pilot 4s, but I'm not competing with this car, and it is not an S, just the 201 HP base model

Mine is a 2.5 too and frankly I feel that 255 rear is overkill, when it is standard size for a 996 with 100 more hp. Stock for 986 was 225 on 16” wheel.

With PS4S, I went for unconventional 215/ 45 and 245/ 40 combo, that is a touch smaller in diameter to help acceleration, together with lighter rotational mass by 2 or 3 pounds per wheel. Maybe gaining 5hp or so? Also with more grip up front (and less in rear), I wanted to reduce understeering and rotate the car better. It drives more livery and planted in corner now, and to my surprise ride is also improved.

How is changing tire & wheel sizing going to let the engine make or lose HP? Have Dyno numbers to show it? An engine is going to make the exact same HP rather it has 12" wheels or 30" wheels. If smaller wheels/tires made more HP then every supercar ever made would come with the smallest wheels possible.

Engine does not produce more power, but less rotational mass reduces power loss so effectively more power to move the car.

Simple math is every 1 pound of rotational mass reduction equals 7 lbs static weight reduction, and I figure 100 lbs static weight loss is like 10 hp gain. I’m going after weight reduction, with a 5-lb lithium iron battery, etc.

Same with the underdrive pulley, it does not make the engine produce more hp but reduce parasitic loss from belt-driven engine accessories, which effectively helps the car accelerates faster. I did feel a bit more get up and go.

It’s a simple physics, and dyno does not measure how fast the car goes anyway.

For racing they change gear ratios for the same effect… and many other reasons to keep the tire size as large as it is….

Yes, I believe this is what it's called 'sprung weight'..?
.

"Rotational mass"
Mass--regular, ordinary, linear mass--is a measure of inertia, of how easy or difficult it is to put an object in motion or to bring it to rest.

*lighter wheels/tires will result in faster acceleration and deceleration

"sprung and unsprung weight"
In simple terms, a vehicle’s sprung weight is supported by springs of one kind of or another (coiled springs, air springs, Belleville springs). Unsprung weight moves up and down with the wheels as they travel over bumps, potholes, and other obstructions. So unsprung weight includes the wheels, tires, brake assemblies, differential, solid drive axles, hub motors, and anything directly connected to the wheels is unsprung mass. This still leaves the majority of the vehicle’s components weight as sprung weight—the chassis, motor, transmission, body, and interior, as well as the passengers and cargo.

Rotational mass and sprung weight are two different things as already noted in another reply, but glad you mentioned it. So much trade-offs with larger wheels and tires for more ‘look’ over function, sacrificing performances in terms of acceleration, deceleration/ braking, handling and ride. Of course you can upsize them when you have overwhelming power to manage, I’m just a minimalist when it comes to my 986, less is more.

Yes, you were 100% correct, this is "sprung" weight because it is part of the suspension that interacts with the chassis via springs.

I just roughly measured the sidewalls on the 18s and 17s. The 17s have approx 10mm more sidewall height over the 18s. I was only able to get a relatively rough measurement with the caliper but it was something like 70 mm versus 82 mm. Then I test drove the car and the difference is substantial over the sharp little bumps. So I think I'm going to swap Wheels and that might be enough for me to get what I want.

The reason for the delay was the Tiptronic car was stored at my other house which is an Airbnb and I did not have access to it while it was rented. I have too many cars so I am storing a couple cars over there. As soon as the registration comes back from Sacramento DMV, delayed because this car was just re imported from canada. I will put the tiptronic car up for sale. I'm definitely keeping the five-speed and I'm really enjoying this little gem.

Success ;0)~

I made the time today and swapped from the 18s to the 17s with new Indy 500s 255 40 17 rear
and Michelin Pilot Sports in the front at 205 50 17.

The rear Indy 500s are new. Probably at the end of the summer I'll buy a matching set for the front.

I didn't think to weigh the differences until I got to the rear wheels/tires which were 9lbs lighter each. That is quite a bit of rotational inertia. The car is a bit zippier with the lighter tires. And the car that now has the 18s is a bit slower :0( Pressures are 35 rear and 29 front.

I achieved the amount of bad road shock reduction I was hoping for. So I don't have to change the shocks or do anything else. I'm happy about that.

I prefer the look of the larger wheels, but not over the extra comfort and a slightly faster acceleration.

:cheers: Tire/ wheel as rotational mass is NOT sprung weight, it is unsprung. Wheel hub and brake caliper are also unsprung weight, but not rotational mass as they are static, obviously.

On the other hand, flywheel is a heavy rotational mass, and it is sprung weight as the entire engine and transmission.

Anyway, lighter wheels/ tires reduce not just rotational mass to help acceleration (as if you gain more power) and braking but also unsprung weight so that suspension reacts quicker to maintain grip. Basic, simple and relatively cheap way to get more out of the car.

IMO you're really chasing minutiae here. At the end of the day, the 986 chassis isn't super rigid, and it acts like a giant, undamped spring. It's not going to ride like a pillow no matter what you do.

Porsche made a good compromise between ride and handling, to the standards of a quarter century ago. Attempting to deviate from this is going to be a tough battle.

Yes, Koni reds vs fresh B6s might ride slightly better, but they're not going transform the car.
Yes, 17s vs 18s might ride slightly better, but they're not going to transform the car.
Yes, reducing unsprung weight will improve ride. But unless you're going from 50lb 20" chrome spinners to 17" OEM wheels you're unlikely for it to register at all on the butt-o-meter.

My car rides fine most of the time (US M030 / fresh Koni yellows / stiff sidewall tires on 18s) but I'll grit my teeth over particularly broken pavement every once in a while. My modern sporty sedan over the same road would absorb it far better.