Hey Guys I have a set of Koni Coilvers id like to install on rear steel/early arms. any tips on that?
But im trying to find/figure out what springs to go with. Im going to delete the torsion bars, So Ill be running 300lbs FRONT, and was planning on 450 REAR. right now I have 26.8 951 front sway and 18mm rear sways

I know the Diameter is 2.25 but what length (with 1.5-2" lowered)?


and does anyone have 2.25X450lb springs for sale?

 

You can install the Koni shock w/o spring. I am assuming you selected a rear c\o spring rate that is effective spring rate is equivalent to the effective spring rate of the torsion bars. With the shock shaft fully extended, put grease on the shock shaft thick enough so when the car is on the ground, the seal pushes the grease up the shaft to its as rest position, and you can see it.Then remove shock and take measurements from shock static spring collar to threaded adjustable collar with the shock in on ground position to determine compression length. Then decide how much bump and rebound to design for, between 2-3 inches. The rebound plus the compressed spring length is the minimum uncompressed spring length. Subtract the bump travel from the the compressed spring length at static yo determine if there is coil bind. For range of adjustment, The uncompressed spring length should fall to your desire somewhere within the threaded shock body. In rebound, the spring should still be compressed.

 

Curious what Konis do you have?

I have the 30 series on one of mine and iirc there is no way of fitting them to the early steel arm cars. If you convert to aluminum arms you’ll also need hubs off a 1986 car and Racers Edge bottom mounts, you may also need axles. I think the only bolt-on rear coilovers for an early car are made by Spax and Gaz.

As for a suitable spring rate and length, call either Paragon or Ground Control. They both have a lot of experience supplying Koni coilovers for 944’s.

 

You will need longer half-shaft axles for the aluminum trailing arms. If Koni is your option, then contact dcAuto Porsche for the needed Porsche parts. Your spring rate selection is what I would use, front effective spring is 0.91 x 300 = 273; rear effective spring rate with aluminum trailing arms is .56,x 450 = 252. The front and rear effective spring rates should be about equivalent. Your front sway bar and rear sway bar is providing understeer, safe for daily driver while front sway bar is compensating for soft front spring rate, less roll in cornering.

If you want to track the car, then a 19mm rear adjustable sway bar provides the ability to get in the sweet spot of car rotation in slow speed corners.

 

Hrmmmm Yes I have the 30 series...Why won't they fit?

 

FWIW, I have Koni 3012 coilovers in the rear on my S2 with 9" 275# helpers and stock 24mm TB. My calcs are a bit different than T&T Racing as I've been using 0.65^2=0.4225 for the rear multiplier and 0.91^2=0.8281 for the front. So, my 300# front springs are 248# at the wheel, and the rears are 255# (116# + 139# TB). My car is very neutral feeling.

I think 450# rear is a bit low since my equivalent no TB rate is 604#. If this is a street car I would stick with TBs or TB + Helper as driving on the street is very different than on a smooth track. The lower shock mount really takes a beating.

Sorry, I don't know anything about the steel arms.

 

Anybody ever install the KYB's on the rear? I have and had a bit of trouble installing them while the car was on the lift. Seems to sit OK on the ground. Since the suspension travel was more than the extended length of the shocks, we used a trans jack to raise the rear suspension up to the bottom mount of the shock. Any problem with these (insert style) on the fronts? Just trying to get the car on the road before I try to go "exotic" with the shocks.

 

Tom, your calculation is Motion Ratio which is the square of the ratio of spring movement to arm movement at the centerline of the wheel times the spring rate. My calculation is effective spring rate which is ratio spring movement to arm movement at the centerline of the wheel times the spring rate. Rear ratio of 0.65 is probably better than 0.56.

It was my belief that effective spring was relevant to spring selection than Motion Ratio.

As for adding rear helper spring + TB, indexing the two so the effective spring rate is linear is a challenge. Just adding helpers probably end up with no linear effective spring through suspension range of travel.

I agree on rough road single shear mounting bolt on trailing arm will take a beating.
.
Also, the diameter of the sway bars influences spring rate selection. I have a race car with front spring 400# and 25.5 mm MO30 sway bar and 31mm hollow rear torsion bar, effective spring 335 with 19mm sway bar set full hard. Tires all square. Car rotates well in slow speed corners especially Big Bend at Lime Rock.

 

Pretty sure they don’t make a 30 series for an early car, iirc the mounts are different early to late and the only coilovers designed for the early cars are like I said either Spax or Gaz and maybe H&R RSS.

Someone else should chirp in and confirm the difference between the steel and aluminum rear and what parts are interchangeable as I’m speaking from memory and don’t have any early cars now.

 

Thanks. For my calculations I pulled some formulas and information from suspension discussions on Rennlist I found using the search feature. In addtion, per Karl at Racer's Edge, Porsche Motorsports published data on spring rates for the Turbo Cup cars. The coilover helper was a variable rate from 34 to 65 N/mm. The total rate at the wheel was given as 45.4 to 58.5 N/mm. Since the cars ran 25.5mm TB with a rate of 31 N/mm that means the effective coilover helper rate at the wheel varied between 14.4 and 27.5 N/mm, which means the effective percentage was 14.4/34 = 0.424 to 27.5/65 = 0.423, which is pretty darn close to the 0.65^2 = 0.4225 I am using in my calcs.

I agree that TB + Helper may not be ideal, but Porsche ran the Turbo Cup cars that way as well as the 968 M030, so I'm not going to change what's in my S2.

I recently purchased a full set of MCS 2-way adjustable struts and rear coilovers I was going to put on the S2 to replace the Konis, but I think I'll put those on the 951 instead.

 

would be great if someone could clarify. I have both the early konis, and the 30 series, they don't look that different, the 30's obviously have the monoball ends, but I don't see why they couldnt be made to work.

(I actually do have early 951 arms, but this is going on a 931, and I can't have any wider spacing or else I cant fit late porsche wheels, plus id rather keep the steel arms since they have the boxed mount, don't need to worry about the shearing as much, OR need the special bolts)

 

also what length springs in the rear? will 7" be too short?

Also I have done as much reading as I can, and from most of the feedback I have read I think 450 would be about right with 300 fronts (no T-bar)

 

you could swap out the early arms for aluminum arms and spring plates. axles need to change also. Thats what I did.

 

I actually do have all that, but Id rather not.

 

Here's a picture of the first set of Koni 3012s on my S2. I sold these and got the newer version that fits without hitting the body at the top. The newer ones are also threaded all the way down. The bottom adapter is from Racer's Edge. The thread in the control arm is M14x1.5. The helper springs are 275#/in and are 9" long.

One thing to keep in mind regarding rear springs; without torsion bars the springs need to support the weight of the car, so will need to compress a certain amount just to do that, which can affect the length you need.