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spence88mph's Avatar
 
Join Date: Jan 2007
Location: Melbourne, Australia
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Installing coil over questions

Hi Guys,

So I got some coil overs from Rebel Racing, Clint is a great dude, very helpful, highly recommend, so much suspension knowledge, he even talked me out of getting the more expensive shocks...

So anyway I have written a list of how I think I should tackle the job this weekend. Any feedback would be much appreciated. The car is at stock rear height so looking to drop that too. I'm also having it all aligned and corner balanced once I stall the shocks and the new front suspension Clint is making up for me (raised spindles etc).

1. De-torque rear wheels, chock front wheels.
2. Jack car up via engine crankcase, where do I then put the stands? I always use the torsion bar tubes!
3. Remove wheels
4. Disassemble spring plates
5. Take note of spring plate angle with angle finder
6. Remove torsion bar cover
7. Re-index torsion bar, not sure to what angle (stock 88 carrera bars)
8. Reassemble setting the spring plates in the middle as best as possible.
9. Take car for a drive and see where the suspension settles
10. Install coil overs with the least amount of spring preload
11. Adjust preload until it just starts taking effect on the cars height.

So this is what I imagine, not sure if I should make the car really low then raise it but I guess I would like the torsion bars to be taking most of the load at rest as the shock towers aren't re-enforced...

Thanks guys as always!
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Old 10-28-2009, 04:36 PM
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Why are you keeping the torsion bars if you are installing coil overs?

Are you installing helper springs?

Something is not making sense regarding what you are trying to do.
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Old 10-28-2009, 04:42 PM
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Hey Tim,

Well yeah they're more of a helper spring, I thought it was pretty common practice to run both torsion bars and coilovers, seems a few people on this board do. I'm trying to stiffen up the rear, the car almost drags it's numberplate under boost.

Thanks for the reply.
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Old 10-28-2009, 04:46 PM
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400lb springs in the rear 250lb in the front,no bars. Handles great nice squat no tail dragging.
Old 10-28-2009, 08:09 PM
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Thanks dude, I love Christine but not exactly the answers I was after!
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Old 10-28-2009, 10:35 PM
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OFF THE BOOST PIPE NOW...
 
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Your list is close, but a book or manual would help greatly. Some other thoughts are adjustable spring plates will make the torsion bar adjustment much easier, no need to determine the spring plate angle if your swapping them out for bigger bars ( they will be way off ), you'll need to pull your rockers off to get the torsion bars out, and place the jackstands under the torsion bar tube. Also, use a 2x4 between the jack and engine case.

If your just swapping shocks for coil overs, I'd just throw them on there and then lower the car via the stock spring plates. The shop will need to fine tune the corner weights with the coilovers.

Last edited by A930Rocket; 10-29-2009 at 06:58 PM..
Old 10-29-2009, 06:53 PM
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Me too!

Spence,
I am looking at doing something similar. I have fitted 28mm rear torsion bars Bilstein sport shocks and car is quite low, but I have ocassional problems with rear suspension bottoming out. When this happens at high speed on an uneven and undulating road, the car's reaction can be quite scary Heart stopping, actually!
I know that a large part of the problem is that I am too low, but I like it like that (hate that song) and handling is good, so I really just want to beef up the spring rate towards the end of travel.
See the problem ...



Back to the main plot - I was considering fitting rear coil-overs in conjunction with my 28mm TBs to acheive this. As I see it, a progressive spring rate would be ideal.
Keeping in mind that this is really a road car that sees a few club events during the year, I don't want to go over the top with stiffness (or cost!).
I also don't want to go to the hassle of reinforcing the shock towers so I want to keep my TBs in place.

Once you get your new stuff fitted, let us know what difference it makes.
I may follow you down the same path.

You asked how to support the car ... found this pic of when I was fitting the TBs.
If I am not able to use the TB covers to support the car, I use my stands under the body with a piece of timber between the body and the stand. There is a small flat area where a bit of wood (1-1/2" x 3/4" x 6") sits neatly between A/C or oil lines and the body seam (on my car at least).
To get the TBs in and out, you don't need to take the rocker pannel or the shark fin decal off completely. The panel only needs to have the screws in rear half removed then just spring it away from the body a little. Put a spacer between panel and body just to hold it out far enough while you pull the TB.
Use a box-cutter and split the decal only as far as the the line where the rubber trim covered it. Then you can remove the TB cover plate downwards without destroying the decal. Good luck.



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Old 10-29-2009, 10:31 PM
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Thanks A930 rocket, using stock TBs with 300lb springs, u need to lower the rear, it's at stock (high) height. I cab use stand on the torsion bar covers when I'm removing them!

Bill you rock thanks heaps for the info!! I'll let you know how I go.
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Old 10-30-2009, 05:09 AM
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I believe a good starting point for a sport suspension might be:

22/28 for and early 911,
22/29 for SC/Carrera's,
22/30's for 930's.

I am still wondering about using coil over's only on the rear without reinforcements. One of our owners here found cracking at where the stock cross member meets up with the sides on a stock suspension 3.0 Turbo. His shop said this was typical.

Some have eluded that some or all of the 3.3 Turbos have been reinforced and ok without reinforcement but that the non-turbos do need to be reinforced.

I do not know what to believe.

I would think that larger torsion bars even to the point of running smaller sway bars would be a good choice for 930's. Moving spring rate from the sways to the springs could keep about the same total wheel rate in a corner but increase the squat and pitching under acceleration and braking.

Just a note, a stock 26mm rear torsion bar is equal to about 175# in wheel spring rate. A 33mm rear torsion bar is equal to about 425# per inch or about a 500# or so coil over because of the difference in where the shock attaches to the rear arm.

To complicate things, on a stock 930 I suspect the torsion bars are about 60% of the wheel spring rate and the sways are most of the rest. Thus, if you double your torsion bar spring rate, you are not doubling your effective wheel rate.
Old 10-30-2009, 09:21 AM
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I also will be doing this as a winter project but am looking at 300# springs in the front, and 550# springs in the rear and removing the torsion bars completely. As well, I am also doing Poly-bronze bushings for the front and 935 spring plate set in the rear, rear monoballs, and some low friction control arm mounts... All said I think it will be a nice set up for street and track.

I will be interested to see how this goes for you as I will be checking back for any gotchas when I start mine. Good luck!!!
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Old 10-30-2009, 09:24 AM
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Thanks everyone for the feedback, apologies to A930rocket, yes you can use the torsion bar tube, I was thinking you meant the covers...cause I'm a fool.

So the whole process was pretty straight forward, once you crack the 20 or so year old nuts and bolts the rest is straight forward.. I did notice that unless you give the springs about an inch preload the car bonces around like the truck with leaf springs I never owned. I think my coils might be rubbing in some spots. I am so sore from being under the car I will leave the adjustments til next weekend.

The result is exactly what I was after, car stays very level under boost but I only have around 360hp so I would imagine you high HP freaks would need stiffer springs. I went for 300lb they feel great, doesn't really feel any harsher than before.
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Old 10-31-2009, 10:57 PM
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Spence,

Just a thought. If you find that by stiffening the rear with coilovers your car over-steers significantly more which is what one would expect unless the front is stiffened to match.

You might try running without a rear sway bar and better yet, add a much stronger front sway or increase the spring rate up front to.

In my mind our suspension's have three types of wheel spring rates.

The first is one where a wheel on only one side hits a bump or pot hole. This spring rate is from a combination of the spring and sway bar acting together. I like to call this "in bump:.

The second is the squat and pitch that comes from acceleration and braking. This is the spring rate only from the springs as the sway bars do not do anything.

The third is "in sway" where the springs and are active and both sides of the sway bar being worked with one wheel going up and one going down.

If my numbers are correct you doubled your spring rate as it relates to squat which is what you wanted.

However, you have moved you sway rates from about 53/47% front/rear (19/25 w 22/21)combined spring rate to about 32/68%. This should make for a tail happy set up.

By removing your rear sway bar you will keep the squat improvement you wanted and move to about a 43/57% balance. A stock 930 is about 53/47% ft/rr so you should still be on the tail happy side but not as much.

On top of this, removing your rear sway bar should also benifit you acceleration traction in a drag race type.
Old 11-01-2009, 08:09 AM
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Playing with some more numbers and thinking of the issue with a 930 squatting from so much HP I have come up with a set up that might be interesting.

In the rear one could run 33mm rear torsion bars with no sway. This would be equal in spring rate to about a 30 to 31 mm rear torsion bar and stock sway bar combination but would be 30% stiffer in squat. It is also about 140% stiffer in squat than stock over a stock 26mm 930 rear torsion bar.

In the front one could run 22mm torsion bars with an adjustable aftermarket sway bar. This will stiffen up the front's lift under acceleration about 125% and allow for tunning of the front to rear balance.

Again, such a set up should run about like a 22/30 or 22/31 with sways but have less squat.

Further it would squat about 10-12% less than adding 300# helpers to the rear but should handle much better.
Old 11-01-2009, 08:28 AM
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