[stretch]

Originally Posted by 4banger

Stretch,
Hope you enjoyed your vacation. Where did you go?

Bar Harbor, Maine and Acadia Park for my 5th anniversary. That's where my wife and I honeymooned, so we returned there. It's a great place to relax, eat seafood, hike, eat seafood, bike, and eat seafood. Oh, and blueberry pie is awesome with blueberry beer.

After reading the entire TiC thread from start to finish a few days ago, I realized that all of my calculations have been neglecting the gas pressure in the dampers. It is easy to recompute the numbers but I have no idea what number to use. 75, 100, 150 psi? I'd also need to know the piston diameter, but that is proably somewhere in this thread. What do yout think we should use for the pressure?

Don't know. For twin-tubes, it's low. For Monotubes, it's much higher. But I don't think it's the piston diameter you want, it's the shaft diameter. Bilstein claims to use 380psi (IIRC) but since they can get away with that because the inverted monotube design uses a tiny strut shaft internally. The stock struts might be similar. I think non-inverted monotubes limit themselves to 200psi or so to prevent too much extension force.

I think the Ground Control Koni's felt like they had roughly 60lbs of expansion force (just estimating what it took to compress with my hands). The WRX Koni's are purely hydraulic and have no gas pressure.

Additionally, that gas pressure does go up as the strut compresses, but I don't think it goes up much.

Originally Posted by ButtDyno

FWIW I would try the camber up front first You know you're gonna need that regardless.

Agreed. I've been keeping an eye out for used front camber plates, though honestly I'm not sure how much more I'd like to run on the street. I'm not out to have the fastest car, just a neutral car that does what I want so I can have fun with it.

I did buy Whiteline plates but they didn't work with the Ground Control coilovers, too bad.

I know that my car *can* be neutral with a 27mm front bar and a 17mm rear bar (with appropriate ride heights and alignments). At this exact moment (with higher ride height in the front than before) the car is not where I want it to be, but with the Cobb rear bar that should get here on Wednesday, I expect that I can run "normal" ride heights and still be neutral.

john

Well, it's possible to get a neutral car despite having drastically more roll resistance up front, but then you should ask yourself why isn't the car understeering like it should be. (Then again, you're running pretty high spring rates in the rear, that surely plays its part.)

I'm still surprised that you were able to get the car neutral with so much front roll resistance, but I don't think that makes it ideal. Remember, the coefficient of friction goes down as you shift more weight onto a tire. With all your weight transfer happening at the front of your vehicle (huge front sway), why isn't the front outside tire overwhelmed, causing understeer? My guess is that the contact patch of your front tires is optimized much better than your rear tires via more optimal camber, tire pressures, or other variables. But then again, that's just a guess. There are a lot of variables. Regardless, I suspect you're going to really like the larger rear sway bar.

[ButtDyno]

Dunno if it was ideal, but it sure felt that way. I coned away a trophy at the ProSolo in June, on not-the-tires-to-have (everyone else was on Neovas or RE01Rs) so the car definitely was fast. The rake was the key to the whole thing. But I was only 13.5" in the front (14.25" in the rear) which is lower than I wanted to be. So slowly but surely I'll be switching to a more "normal" setup.

FWIW I really don't think -3 on the street is that bad (with zero toe) but I don't know how many miles you drive a year.

john

[4banger]

Stretch.
I've been slacking. Using your 60lbs of force, here are some new numbers.

STi Konis
Full Soft: 170/388 F/R (lbf/in) 1.37/2.49 Hz
Full Hard: 1082/589 F/R (lbf/in) 3.46/3.07 Hz

I was suprised that it had such a large effect. Now I'd have to say you want to use at least 500/400 rates.

Lets say that you've been running a 300 lbf/in lowering spring that rides the bumb stop. Your actual rate is 600lbf/in. If you swapped over to a 500lbf/in spring that rarely touched the bump stops, would it feel much different that the 600 lbf/in combo? Just going by the numbers, you would think the new setup would feel softer, even though it had much stiffer springs. How do you think it would really feel?

[wacerx]

ok, wow there is alot of good info in here. my one question is: how are most people getting these products? are most people ordering straight through GC or is there another outlet for these? i noticed that the majority of vendors stick to the basic mainstream setups... tein, tanabe, cusco, etc.

[!4KORNRZ!]

They can only be purchased directly...


The reason you're not seeing this Ground Control system elswhere, is due to the fact that they are hand built, one by one, customized to your specifications.

...and continuing customer support.

I don't think you will find that kind of service anywhere else when it comes to an affordable, custom taylored product.

[jagstyle]

Help me make up my mind:

1. Ground Control Koni SA Coilovers
Springs (500/400) and adjusters: $399.00
Koni single adjustables: $159.00 * 4
Custom housings: $152.50 * 4
Total: $1,645

or

2. Racecomp Engineering Tarmac 2 Coilovers (500/400) $1,895



stretch, how are your ground control's working out? What spring rates are you happy with?

[ButtDyno]

jag, what do you use the car for?

[stretch]

Jag, I was happy with them but am now going to try the TiC prototype coilovers. I like to experiment and TiC offered me the very first set. My Ground Control setup is actually for sale here to fund that purchase since TiC's stuff is quite a bit more expensive.

I'd recommend softer spring rates unless you're going for a track only setup. Consider 400/350 as a very aggressive daily drive. Softer than that, and I'd switch to the WRX Koni's in the rear, which can be adjusted softer than Ground Control's custom valving. Heck, I'd recommend them anyway simply because you can find the housings for almost free on Nasioc. (If you do that, the Koni/GC value is unbeatable.)

I'd run those springs rates only with Ground Control's camber/caster plates to get a bit more bump travel.

It's a good setup, for sure. I autocrossed just once on my setup before removing them, and while I had a lot of understeer (courtesy of a huge front sway bar, promptly removed afterwards) I still had the fastest time on street tires. Car control was excellent even at my daily driver settings. The setup has a good highway ride (thank you, low friction twin-tube struts) and good car control all at the same adjustment. This is what a good strut should do. Your ride quality at slow speeds is more related to your spring rates, which is why I don't recommend going too stiff.

I haven't tried RCE's stuff so I can't compare them. Obviously the RCE stuff is more expensive but gives you independent bump adjustability. I personally think adjustability is overrated, as I tend to "set and forget" my dampers.

[jagstyle]

Thanks for the information. I'll be reconsidering the lower 400/350 rates, but what about 4banger's calculation in post #198 saying to use at least 500/400 rates?

Also, what spring length do you recommend?

Anyone know how much these coilovers weigh? Is there weight reduction compared to stock?

[stretch]

I think 4banger's original calculations were correct (post #187), not the revised ones. I guess I missed that post and never responded to it.

The gas pressure he took into account in post #198 is not a damping pressure; it's constant extension pressure. I'm not sure I know all the effects it'd have on the calculations, but it's somewhat akin to removing 60lbs of sprung weight per corner. You'd need less spring then to get the same spring frequency, not more... but overall it's not going to make any significant difference.

I'd go with the longest springs you can run at your desired ride height. For most people I think that'll be 8" springs front, 9" rear.

[4banger]

After having an awesome 13 hour day at work I only had a few minutes to take a look at this problem. The first thing I noticed was that I was using a total weight of 3400 lbs, which was a hold over from when I was helping BigSky with his calcs and he has lighter WRX.

Subtracting 60 lbs from that corners unsprung weight was exactly what I did to arrive at those numbers. The spring rate also goes down, like you expect Stretch, but the critical damping number is a function of both spring rate and weight. There are also some square roots thrown in there so I can't say for sure how they should interact. Maybe tomorrow I will double check everything and try and make a graph to show how the three behave together.

Here are some more numbers to munch on, using the 3600 lbs total weight for the rear on full soft for STi Konis:

660 lbs unsprung weight - 2.26 hz - 353 lbf/in spring rate - 65% crit. damped
600 lbs unsprung weight - 2.49 hz - 388 lbf/in spring rate - 65% crit. damped

Originally Posted by stretch

I think 4banger's original calculations were correct (post #187), not the revised ones. I guess I missed that post and never responded to it.

The gas pressure he took into account in post #198 is not a damping pressure; it's constant extension pressure. I'm not sure I know all the effects it'd have on the calculations, but it's somewhat akin to removing 60lbs of sprung weight per corner. You'd need less spring then to get the same spring frequency, not more... but overall it's not going to make any significant difference.

I'd go with the longest springs you can run at your desired ride height. For most people I think that'll be 8" springs front, 9" rear.

[4banger]

I updated my numbers in post 198 and I still think that you'd want to go with 500/400 f/r as a minimum.

I did an experiment in Excel to try and see if anything interesting happens to your 65% critically damped spring rate as weight changes. I was dissapointed to find that it only a linear relationship. As weight decreases, the 65% critical spring rate/frequency increases linearly. For the rear springs, a 100 lb decrease in total vehicle weight leads t a 0.1 increase in frequency.

Unfortunately, since damping rate really isn't a variable the frequency isn't something that you can choose. You get whatever it turns out to be. Because of this there aren't any other variables to compare.

[stretch]

Interesting. It's funny how Mark@GC says these struts work well on a variety of rates (all of which he's tried), but we calculate them to be much too stiff for those rates. I wonder if the strut that got dyno'd wasn't actually at full soft?

I was thinking of changing that in the Excel spreadsheet to a graph of "% critically damped" versus "piston speed". It seems like that'd be much more useful.

[4banger]

Or maybe his but dyno needs to be recalibrated.
j/k. There were quite a few odd things about the graph for the rears so it wouldn't suprise me if something wasn't right. For example, the front adjusts both the rebound and compression, but on the rear the change in compression is very minimal. Also, the range of adjustment is much less than the fronts. You can run nearly twice the spring rate in the front than you can in the rear.

I don't think a graph of % critically damped vs pistion speed would be very revealing. It would also be a lot of work to generate with any percsion. The damping rate is just the slope of the graph and you can tell by looking at it that is is fairly constant. For other dampers with a sharper difference between high and low speed rates the comparison of % critcal and speed would be a lot more interesting.

Originally Posted by stretch

Interesting. It's funny how Mark@GC says these struts work well on a variety of rates (all of which he's tried), but we calculate them to be much too stiff for those rates. I wonder if the strut that got dyno'd wasn't actually at full soft?

I was thinking of changing that in the Excel spreadsheet to a graph of "% critically damped" versus "piston speed". It seems like that'd be much more useful.

[jagstyle]

Originally Posted by 4banger

I updated my numbers in post 198 and I still think that you'd want to go with 500/400 f/r as a minimum.

500/400 on a track ready daily driver?