Auto_Werks 3.6

I'd be targeting a GT4 similar setup. Searching Rennlist it looks like the GT4 rear is 450lbs, but the front spring is a lot softer. The GT4 also has a pretty big front bar on it. I would be curious to know why they went with soft front springs and a big bar vs the kit you're looking at. They probably spec the Ohlins kit expecting people to use their smaller front bar.

NoSubstitute981

Not even sure what to do with that info…
I’Ve read about some sort of harmonic voodoo thing in regards to suspension tuning. I just want to eliminate the understeer and keep it real on the street. Seems like a no-brainer but obviously everyone has an opinion/experience.

So you say softer spring rates on GT4 front? What did you find?

I just installed the X73 rear bar which, in hand, is substantial, compared to the original hollow bar. Much heavier, beefier.
The Tarett Camber Plates should hopefully be here before the last race (Octocross, October 7th, 8th) I’ve got an appt. for alignment before then. Apparently Tarett is waiting on a specific washer so we’ll see if it works out. I’ve got a couple weeks…
The coilovers will need to wait until next season.

NoSubstitute981

Initial impression after installing the X73 bar is turn-in is crisper and more decisive? Hard to tell on the street. There’s definitely a difference but I can’t quite quantify it it without pushing it on a coarse. Looking at the tire wear on the rear (front I definitely need more camber) I think I’ll ask to max out the rear as well.

Z3papa

My mistake on SSP. Having SST as an option is better having none. You'll find that better dampers comes acceptance of higher spring rates without losing street ability of the car. Sounds like a great pathway.

Abt12

Suspension spring stiffness is normalized across platforms by suspension natural frequency. This takes into account all of the differences in motion ratios, masses, weight balance, etc. and narrows it down to two numbers - front and rear suspension frequency.

Basically, it gives you a really good reference starting point for spring rates, after which you can adjust to your preferences and the specifics of the car. It's a journey though...

DOUGLAP1

What this guy said is right on the money. If you don't want to take the time and effort to learn about motion ratios and how to calculate natural frequencies, then you are just taking a shot in the dark and hoping to hit something when changing springs and shocks.

If you are looking for a non-technical path forward, then the smart thing to do is to start with the simplest step that will address your main problem of understeer, and don't do anything else until you have tested and optimized that one improvement.

Your car has a front suspension design that gains camber in roll (it is a rather poor suspension geometry design, but it is fairly light and relatively cheap to produce ). So, the common sense approach would be to just add a stiffer front roll bar to try to limit the camber gain. In my experience if you increase front roll stiffness "some" you will reduce understeer due to camber gain, but if you go past that sweet spot, understeer will come back hard due to weight transfer.

NoSubstitute981

I’m sure there’s many ways to accomplish the same task. I’m (at the advice of other autocrossers) going to remove traction at the rear (stiffer sway bar) and increase traction at the front (more negative camber using camber plates) Unfortunately winter is on its way so weather permitting I’ll attend the next and last event for the year and see if I’ve made any progress, otherwise it’ll be spring before I can make any real assessment. I had some success at the last event by increasing rear tire pressures. Same as removing traction at the rear. As I stated earlier, I’ve got 245 fronts and 285 rears so much more traction out back proportionately. I’ll move to 255 fronts once these wear out. Initially, I had purchased 991 “V” wheels and 255’s for the front but my race shop wasn’t able to get them beaded so had to go with the 245’s. I’ve since bought different wheels (9” wide front) so won’t have that issue again. At any rate like you and others have said… it’s a journey.

edfishjr

Respectfully: So much sketchy here.
-Your car has ~60% weight at the rear. Do 285s really produce more traction at the rear than the front 245s and cause steady-state understeer? I don't think that's the actual cause. The reality is much more complicated. For instance, a 285 is only 16% wider than a 245 ((285-245)/245) but carries 50% ((60-40)/40) more load. That 285 is working damn hard in a sweeper. Why doesn't that produce oversteer anywhere and everywhere? Because it's a complicated system.
-9" wide front wheels can easily support 275mm wide tires, if they will fit on the car. So, even 255mm is still a little skinny. Will 265s fit? Whatever the actual cause of understeer, more well-supported front tire will help. I've always been able to clearly note the difference in front grip from 255 to 265 to 275 all on the same 8.5" wide wheel. (The King of the Dragon's only remark about a stock mid-engined Ferrari that came to run the Tail Of The Dragon against his modded FD was "Not enough front tire.")
-Adding stiffness front or rear, either with springs or bars, does NOT automatically cause a loss of grip at one end or the other. For instance, adding a stiffer front bar to an otherwise stock Cayman almost always reduces understeer. (Thanks to limiting loss of negative camber.) When we start mixing and matching various changes the effects are unpredictable without either a lot of experience or a complete and verified computer model of the car.
-Your idea to sacrifice rear grip in order to reduce understeer is something that should only be considered as a last resort, or temporary stopgap. More front tire and more static negative camber with the upper mounts I think you bought(?) should solve your problem, even without a stiffer front bar. (Still, add a somewhat stiffer front bar and increase the transient response! If all that produces too much oversteer then fix that.)
-Everyone always seems to forget something about changing springs on Porsches: the stock design uses springs + bumpstops in the corners and the bumpstops add significant rate. If you go to an aftermarket stiffer spring kit with short bumpstops you have no idea how much you are actually increasing the spring rate in a corner, at that end of the car, so using one kit in the front and a different in the rear is risky. As for the spring rate numbers themselves, 8% isn't worth talking about. 20% is not a big jump. You are probably correct that the stock dampers can handle 20% (if it's really 20%) without much bad happening. Optimal? No. But then they aren't optimal stock, either.

Auto_Werks 3.6

Your ideas about bump stop tuning are a crutch at best. Street class ideas should stay in street class.

Below is my video from this year's finger lakes national tour. Probably the best finish I've ever had at a national tour; top 10% event pax both days, with one day being rain. This car had bone stock base S suspension plus 981 rear sway bar. Skinny well supported tires on 8.5 and 10 class legal wheels. If you want a car that drives like this, look into working with your rear bar. If you want a car that has a peaky front axle that won't let the car rotate, looking into trying to increase the front roll stiffness to "find grip".

One thing that I've noticed over and over again is that people who talk about ride frequency and motion ratio the most, end up with cars that don't quite work right. You can't make a car with a spreadsheet and have it pop off the page working correctly without tuning. John V recently made some comments about how his mazda is finally starting to come together, and how it didn't match the initial calculations. Tim white took the rear bar off his 981 and put the stiff bar back on the front, and I've never seen an 981 look as bad as his did at nationals. He drove around a car that wasn't working very well and still trophied, props to him. And Brian Mcgreevy's car looked beyond upset over the large bump in the west course, after all the talk of bump stop tuning and special shocks the car doesn't work. I truly felt compassion for him because it sucks to put all that effort into a car and not be making progress.

to the OP, Unfortunately you're probably going to have to do some tuning on the car. You can pick your initial spring rates, but match them with the damper manufacturer. Most of these tuning items are small changes on the clock, but big changes in feeling. You can make the car more fun to drive, easier to drive, and you will find time from being comfortable and consistent.

edfishjr

[QUOTE=Auto_Werks 3.6

[i]Your ideas about bump stop tuning are a crutch at best. Street class ideas should stay in street class.

I don't remember recommending bumpstop tuning to the OP. I meant to caution him that the stock design includes bumpstops that are integral to cornering performance, making it difficult to compare aftermarket kits that don't use tall bumpstops to what Porsche supplies, especially if he intends to mix aftermarket with stock. I don't remember you or anyone else mentioning that.

You can't make a car with a spreadsheet and have it pop off the page working correctly without tuning.

I also don't remember saying that you could.

dps214

I rode in a car with ohlins on it this weekend. It definitely didn't feel like too much front stiffness, even with the stiffer gt4 front bar and less power (base model with cheap headers and a basic tune). I think that's the way to go; it might not be the best setup possible, but it's pretty good and just works without needing a bunch of tuning and setup iterations.

NoSubstitute981

Nice run!

I agree about the rear bar. The car needs to rotate and that’s how it’s done. At least on these cars.

The surface there looks to be fairly smooth. Ours is very bumpy in comparison which is why I started this thread. My thinking is/was some slightly stiffer springs and a bit lower that stock (20mm) might be a good choice for the surface we race on. Hopefully I’m on the right track with the rear bar and the front camber plates for now. I should be able to attain at least 2 to 2.2 negative. I’m maxed at 1.3 now.

To answer a previous question about tires… for starters, just because the sidewall states it’s a 285 doesn’t mean it is. For example The 295 P Zero tires I have for daily use don’t have as wide of tread patch as the RE71RS 285’s. It was asked if a 265 would fit in front? I believe it would but depending on which tire. Currently Bridgestone doesn’t offer a 265 for a 19” wheel. I would be willing to bet however that the 255 they do offer is as wide as most 265 tires out there. Heck, even the two 245 sizes, one being a 35 series and the other a 40 series that Bridgestone offers, have different width contact patch’s.

edfishjr

We are typically only thinking about the top autocross tires, which all run wide. (P-Zeros don't count.)

David Borden

I had the Ohlin Road and Track on a 718 Cayman S and it was fantastic suspension. I bought it specifically for autocross and it transformed the car. Despite much stiller spring rates, the car was smoother than the stock suspension. I ran that with an adjustable rear bar and the car was fantastic. I removed the suspension before selling the car for a GT3 and still have the kit on the shelf. It's the same part number as the 981 cars as well but my existing 981S has X73 which is very good as well but just not adjustable. One of the local guys was VERY fast on a 981S with X73 alone.

I agree with everyone else, do not piece a system together unless you really understand what you are doing with regards to damping and spring rates.

David

NoSubstitute981

Thanks, that’s great info. Do you happen to know what the minimum amount the car can be lowered with the Ohlins R&T? I’d prefer not to go more than 20mm lower and may even stock ride height.