Chibo

Alright guys, I bought a car from a very cool previous forum member. I'm in the process of doing my due-diligence and figuring out exactly what I'm working with and where to go with it. It is a 2002 Sebring that has been prepped for STR, and I am planning to use it for open trackdays / weekend driving -- mostly in Arizona / SoCal, and I am coming from a background of BMWs (E30s and E46es) and a lot of motorcycle trackdays.

17x9 +63 wheels with 255/45-17 RE71Rs that are wear bar'd minus one that is new and I have a set of Nexen Sur4 255/45-17 to go on as a cheap set of tires to learn. They appear to measure 10.5", where as the RE71R is more like like 10", so this should be fun since the fenders are stock and untouched.

Coilovers are Penske 7500 DAs, and here is where I need some help. From what I can piece together:

Front:
Digressive comp, linear rebound
VS-BP comp stack
VS-FM reb stack
7" 850lb/in springs
Karcepts front swaybar, unknown bar size, one step off full stiff
12.25" center to fender

Rear:
Digressive comp, linear rebound
VS-BP comp stack
VS-E reb stack
Speedway hollow .095 wall rear swaybar set to one step off full soft
7" 500lb/in springs
12" center to fender

The tires are done, so I haven't really played with the car yet, but from what I can tell it is very, very neutral and the rear breaks away very progressively -- requiring some poking to do so. It rides absolutely great for what it is.

It appears to sit very low to me, but I don't want to ruin what I've got here by preloading the springs to add height. Should I leave the height alone?
I also note that it has some reverse rake, which seems opposite to what I've come across in setups around here. Leave it alone if it isn't broke?
How far off of a roadrace setup am I in terms of spring rate ratio, or is the setup on these cars more flexible.

I guess what I'm looking for is for someone to validate what is going on here, and maybe explain a little bit along the lines of what kind of setup changes will be needed in the future.

Thanks guys.

Mike RT4

Handling set ups are pretty subjective, so one man's "cup of tea" can be another man's "poison", so I would throw a set of good tires on and go and do a track day exactly as it is (starting off slow and building up speed progressively) and then adjust the damping up or down to suit what you want. Once you have figured out how it does and doesn't react to how you want it, then you can come report back with some pros and cons for people to help you with. At present without seeing how it handles compared to what YOU want, all you will get is other people's recommendations on how THEY like a car to handle (for example, I like a car that has total front end grip and likes to rotate).

Car Analogy

What swaybars are on the car?

I have a chart I can plug in your spring and bar rates, and it does the math with the motion ratios to give your static front to rear balance. It lists the stock balance for all years and CR as well, so you can compare your static balance with the various iterations of the car stock.

A decent starting point. I suspect you will be close to CR balance, since your front springs are so much stiffer than rear. But the math can work out with surprising results...

DavidNJ

You can call Brian at Karcepts to determine how to determine which bar you have. They differ in wall thickness, which you should be able to measure on the end of bar which should be visible. This is the Karcepts manual: http://www.karcepts.com/index_files/...structions.pdf. Other than the beautiful machined aluminum arms, what makes the Karcepts unique is the arc on the arm that allows easy adjust one side at time.

I'm surprised to see Penske 7500DA for an STR autocross car. The STR is a very sophisticated shock including base valve. In the front, no problem. In the rear, Penske says they don't have enough travel for a lowered car and recommended the 8300 with an external reservoir (the valving at the entry to the reservoir makes it a base valve shock. If ordered, the reservoir could be a piggyback. The stacks are standard Penske shim stacks, explained in this manual: http://www.penskeshocks.com/assets/7300%20Manu.pdf. This the 7500 manual: http://www.penskeshocks.com/assets/T...A)%2008-10.pdf.

It seems very low? it is very low! There has to be something to allow adding camber front and rear, usually an offset balljoint. There may also be "roll center adjusters", basically lowering blocks.

The best way to lower the car, maintain geometry, and work with those shocks could be the Wisefab kit.That will also replace all bushings with spherical bearings.

Chibo

Thanks guys.

Is this spreadsheet a personal one, or is it floating around out there to play with on Google Sheets or something? I'm going to figure out what size the front bar is this weekend.

Is there a rule-of-thumb for proper ride height on the AP1 for a tracked car?
I don't think it has rollcenter adjusters, but in the front it has an offset balljoint. I think the rear is stock, but I'm not 100% sure. I would like to raise it a fair amount, I just don't want to screw it up by preloading the springs, though it would appear that they are not preloaded at all right now (they drop off the hat). I think I'll take the car to Science of Speed next week and see what they have to say.

DavidNJ

"Preloaded the springs"???

I think you have a misconception. The spring will compress depending on the unsprung load on that wheel. A stiff spring with a fair amount of shock travel (typical for a track driven production car) will have excessive droop. For example, assume the car has 6" of wheel travel at OEM ride height it is divided 3" bump, 3" of droop. Then the car is lowered 2". It now has 1" of bump and 5" of droop. Now if spring compressed 3" under static load (e.g. 600lb load and 200lbf/in wheel rate) it would be fully extended when the wheel could still droop another 2". Fortunately, this shouldn't normally occur. It would occur when the car is jacked up or if it did a jump over a bump or sudden dropoff at the track. To prevent the spring from coming loose a helper spring is added. Your car probably has them. They are fully compressed at ride height.

To raise the ride height, jack up the car and start turning the spring perches. Ideally, there should be a ring with roller bearings between the perch and the spring to make this easier.

Other posters with similar cars could give you recommendations for spring rates and ride heights. It appears at first glance your front rates are close to where most track day cars are, but your rears are softer. Hypercoils are available for under $65 in 2.25"x7" and under $55 in 2.50"x7".

Chibo

I think you're right, I'm coming from the motorcycle side where I am adjusting static sag with preload (winding the adjuster to compress the spring at full rest) to achieve a certain area of travel within the shock stroke once you have the right springrate. Obviously, once you get to a certain point actual measured preload stops mattering and you care more about about how the bike actually works (how does it feel on the brakes, how does it feel mid corner, how does it feel driving out of the corner) instead of some numbers on paper. I'm not there with cars yet.

My car does not have helper springs and the springs are not seated at full droop. They are not stupid loose, but I can move them. I'll wind them up until the spring is seated at full droop and see where that leaves me.

I suspect what this boils down to is: "Drive your car instead of screwing with it, you fackin pussy"