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I tend to agree with Rob 930, it appears to me that you load the coils or the TBs.

How do you identify which one is holding the load? Other than when the ride height begins to expand, meaning the coils are taking over the TB. And if so, why to leave the TB in place adding the extra weight, if the coils are doing the work?
Old 02-16-2009, 12:54 PM
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Miguel,

I can answer your second question about why one would leave the torsion bars in place. In my case, it's becasue I don't want to load the shock towers with the full brunt of 650 lb/in from a coil spring. If I felt good about using only coil springs, my T-bars would be gone in a flash. And some people claim to have good luck with full coilover conversions on street cars this way. But there have been reports of structural cracking of the chassis (and I've seen some myself on other 930s). Those shock towers were never intended to carry so much load through that path. Some people reinforce the shock towers (a la early RSR) to alleviate the problem, but I've never been sure that's enough. Because my car is intended for the harshest load case (a hard pounding on the track), I'd rather not subject it to heavy spring loads through the shock tower at all. The "right" way to do it (IMHO) is to install a cage that picks up the rear suspension points and attach the shocks (carrying coil spings) right to the cage. But that's one notch more hard core than I want to go right now. So, the T-bar plus helper spring gets me the stiffness I want without loading the chassis in an unfavorable way. Not an elegant solution, but one that has been shown to work surprisingly well.

The first questions you asked are essentially the same questions I have. More specifically, I'd like to know just how much to "preload" the coil springs when the car is static. If I install the coils so they just contact the perches, then the ride height won't be affected, and theoretically I'll have their full benefit under pure acceleration (squat control). But under cornering or braking, the suspension will lift off the coil springs on one or both sides, which doesn't seem optimal. To improve that, I can first lower the car on the torsion bars (re-index them) a bit first, then bring the car back up to height on the coils, which shares the load between the T-bars and coils under static conditions. Under acceleration, these two setups should yield the same spring rate. But for cornering or braking, the latter solution seems better, to avoid having a sudden increase in spring rate at some point when the coils "engage."

But how much preload is optimal? I'm not sure. And I'm not sure how much it matters -- two springs, acting in parallel, are additive in spring rate. Hence, a 400 lb/in (equivalent) T-bar plus a 250 lb/in coil, will give 650 lb/in, as long as they're both being deflected simultaneously. I would think you'd want keep the coil spring between two limits -- so that it always carries enough load that it won't lift off the perches, and so that it never carries too much load which could cause it to "coil bind" or reach solid height by bottoming out from too much deflection. Other than that, does it matter? I dunno. I'd like to hear what others think

Apologies to the original poster for the slight thread hijack -- I hope this is relevant enough to the general topic of squat control on 930s...

Rob
Old 02-16-2009, 01:49 PM
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Quote:
Originally Posted by Rob 930 View Post
Don,

How do you do this without affecting ride height? If the coil springs are always carrying some of load (street and track), then how does it get stiffer when you dial in more coil spring? Or do you back it off on the street so that no load is carried by the coils?

I have 31 mm rear T-bars and have just installed 250 lb helpers from Rebel racing. I have yet to figure out how much "preload" to give the coils. If I give it any preload at all, it will raise the ride height and affect corner balance which I don't want to do. So I'm planning to re-index the T-bars and lower the car about 1/2 inch or so, and then bring it back up to target height with the coils. Doing this will allow me to corner balance and make very small ride height adjustments easily with the coils. But I don't see how I can voluntarily change the stiffness at the rear for street and track unless I completely disengage the coils from having any contact with the perches, which doesn't sound like a good thing to do.

Rob
Hi Rob - how have you been???? How's the car??

It's actually very easy to set the stiffness. First, roll the car a couple of feet to settle the ride hight. Then, reach up under the car and twist the collars so that the spring is just coming in contact with the spring and top hat. Lock the collar. This gives me the best road ride and very little squat on heavy accel. To adjust for the track, I twist the collar till it comes in contact with the spring and top hat, then add another 3 - 5 turns to preload the spring. This ends up being fairly stiff for the track for me. I once turned it up to about 10 turns and it was too stiff - the rears would break loose too easily. Kinda cool in front of newbie owners, but too expensive and no traction. I also set sway bars to their stiffest and the car handles like its on rails. Sorry it's not more scientific.

Last edited by DonE; 02-16-2009 at 04:54 PM..
Old 02-16-2009, 04:51 PM
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Keep in mind that when talking suspension terms Stiffer is not necessarily better, you need to let the suspension geometry work enough to get the best traction out of it. Looking for that sweet spot is where the fun is. IMO stiff TBs with stiff sway bars setting make the car TOO rigid; feeling good (no squad or rolling...) while in control, BUT when it snaps... it's gone - no recovery. M2c
Old 02-17-2009, 03:25 AM
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Daily driver on the street, running 23/31's with 22mm sways & sport bilstiens on all 4 corners & strut brace & poly-graphite bushings all the way around. Car is lowered 2-3/4" below Euro spec.'s. Works great for me!!
Mark
Old 02-17-2009, 12:01 PM
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Daily driver on the street, running 23/31's with 22mm sways & sport bilstiens on all 4 corners & strut brace & poly-graphite bushings all the way around. Car is lowered 2-3/4" below Euro spec.'s. Works great for me!!
Mark
How did you set up the bump steering and the front shocks spindles in order to get the car that low?

thanks.
Old 02-17-2009, 12:36 PM
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Hey DDDD,
Sorry forgot to note the RSR bump steer kit, which included outer tie rods with lots of spacers (used them all). Geometry ended up perfect.

Stock spindles, but my car was a gray market Euro, so it did not have the thick plastic spacer on the top of the strut tower that American cars often have. Removing that spacer helps with getting the alignment correct when you drop it that low.

Also I did buy custom offset front rims from Fikse to eliminate any possible rubbing issues( that turned out to not be needed, but now I have good clearance both to the washer tank & the inner fender wells).
Mark

Old 02-17-2009, 12:49 PM
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A la low rider... not much suspension compression room there.

It looks good though.
Old 02-17-2009, 12:53 PM
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How dare you associate me with LA, I'm a mick/kraut from the Seattle area!!
Just kidding, no offense to anyone in La.

I've really only had 1 issue with the suspension, when I did scrape the rear sway bar mounts once when I hit a deep dip in the road while doing 110+. Other than that she rides nice(firm), handles great and the suspension soaks up most of what the real world throws at me. However, I do try to avoid most pot holes and associated dips & crests that I can.

Now ground clearance is another issue....speed bumps suck...driveways suck...and don't get me started with cats/squirrels/possums/raccoons..
Hell, I guess that's what I get for wanting a stupid low car!!

Mark
Old 02-17-2009, 01:48 PM
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A stiff rear may make for less squat when accelerating. However note that it will also the rear looser in the corners and more prone to rotating.

With all that weight hanging aft of the rear wheels is the issue more with having the front end lift?

Just a thought, how about raising the spindles on the front struts and /or getting shorter front shocks? This will limit front lift. It will also help restore the front end geometry on a lowered car and allow for more neg camber gain when cornering. This will let the front camber curve better match the camber curve of the rear 930 set up.

Just a thought.
Old 02-17-2009, 03:59 PM
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Guys, I'm new here so be gentle! Here's my two cents

I bought Jerry Brinkley's Turbocharged 3.2 and shipped it back here to the UK.

The car was running Billie HD's at the front and sport rears.

Standard late Carrera TB's (18/25) to which Jerry had added the Rebel Racing Coil over set-up to the rear with 200lb sprigs to control squat. The car was dyno'd over here at 371bhp at the crank (still mapped for your low RON fuel) so it should make around 400 when I have it remapped for our 98RON gas.

Once UK legal it's first drive was 1500 miles to and from Le Mans in France. Several guys in the group drove it including some 911 race and rally drivers from Aus and NZ. We all agreed the car was way too stiff at the back and fairly weird to drive (no offence Jerry). The imbalance made the rear pogo and high speed cornering was interesting to say the least. However, I can confirm the car had zero squat. You dropped the clutch and it went flat towards the horizon.

When we got back I had the car professionally set-up by Francis Tuthill (they build and rally historic 911's). This improved the basic handling, but the poor turn in and pogoing still remained.

Next a trip to Silverstone GP Circuit for a track day. Interesting day. I drove like Miss Daisy. The car handling was unpredictable. No turn it, no traction out of the corners; it was a point a squirt day. In the chicanes the rear would lurch about making the second turn a lottery. There's a brief video link on the forum which you can view, but what it doesn't show was the times I tried to take Club Corner - a fast left/right direction change as most times I didn't make it (straight across the grass and out the other side).

Anyway back home, dropped out the coil overs and replaced the 200's with 100lb springs (still too much IMHO but the lowest I could get off the shelf) + 4lb helpers to keep them engaged at all times. Result - transformation.

The car now turns in and tracks round the bends. The pogoing has all but disappeared. There is now some squat, but 911's are meant to have a little. The steering doesn't go light and the rear follows the front.

I can't comment on what your roads are like or what you're trying to achieve, but on our bumpy windy roads and twisty racetracks, dialling out all the squat made the car horrible to drive.

There's more work to be done and the next plan is to get it to another track where a friend of mines mate will check it out. He builds and races 911 ice driver car so knows his stuff when it comes to setting up 911''s. I may go a little stiffer at the front, nothing extreme, but what I have now is close to the last 930 set-up spring rate wise. Still a little high at the rear though, but that would require some lower rated springs to be custom made and it's probably easier to stiffen the front TB's a little to return the balance.

FWIW I love the car. Jerry did a great job and I'm building on his good base.

Cheers.
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Old 02-19-2009, 04:38 PM
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Again, a stiff rear is a prescription for big handling problems. It will actually take traction away from the inside rear wheel in a turn and make the car rotate faster than is should.

The back of a 930 has over 50% more weight on the wheels than the front. Thus it is ok for the front to be stiff and lift a wheel as this transfers traction to the rear. To have the rear be stiff makes the rear very loose. To stiff a front is not as much of an issue on the track but will make the car push in tight corners like on autoX.

Sorry but putting coil over helpers on the back w/o stiffing up the front to match is a cosmetic solution and a poor idea. It dose not even help drag race starts as it dose the same thing, take traction away from one rear wheel.

For reference the RUF yellow bird twin turbo just had turbo size 26mm TBs out back. However I bet the fronts where properly modified.

Want to reduce the front lift, try raising the front spindles. This will also improve handling.

When a 930 is lowered it takes some of the anti squat geometry out the back and adds travel to the front so it has more travel to lift.

Just my two cents worth.
Old 02-19-2009, 06:10 PM
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Waaaaaay too general of advice, senor.
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Old 02-19-2009, 06:15 PM
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Craig ???

I must have left out the part about how lowering the front of the car screws up roll center and camber curve. Oh, and how the 930's increased front wheel base effectively lowers the front spring rate of the torsion bars where the rear dose not.
Old 02-19-2009, 06:29 PM
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Bellow is the apx spring rates of 911 torsion bars:

Front: lbs/inch % of stock
18.8 110
21 173 157%
22 210 191%
23 250 227%
24 290 264%
Rear
24.1 122
25 140 115%
26 165 135%
27 191 157%
28 221 181%
29 254 208%
30 294 241%
31 332 272%
33 427 350%

Adding 200 lb helpers to the rear increases the rate from about 140 to 340 lbs per inch putting things way out of ballance.

Good combos migh be 22/29, 23/31's with stock sway bars. A max torsion bar set up would be 24/31'a with 27/22 sway bars. 400/600 coil overs would be a step up from that.

Sorry, I got carryed away. Check with a real expert.
Old 02-19-2009, 06:37 PM
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Sorry max torsion bar is 24/33 w 27f/22r sways.
Old 02-19-2009, 06:38 PM
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To be honest if I was living the States, with quick and cost effective access to uprated TB's I would be running 22/29 with no coilovers. I reckon that's plenty for a track oriented, street legal car on UK roads.

Any stiffer than that and I'd be looking at full seam welding and a welded in cage to stiffen the chassis in line with the suspension. As the 911 prep companies over here have said "Just stiffening up the running gear without addressing the chassis is not good engineering."

Anyway, next business trip across the pond I'll be placing a forward order on PP to meet me at my hotel..... assuming the gets off its arse of course!
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Old 02-20-2009, 07:32 AM
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Sorry max torsion bar is 24/33 w 27f/22r sways.
Why the imbalance between front and rear sway bar sizes? Wouldn't you want equal or more rear bar?
Just wanted to see ur line of reasoning on this.
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Old 02-20-2009, 02:38 PM
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Lets double check as I am more up on the normal Carrera.

On a stock 930 set up is a 18.8f and 26r for 110lbs/165lbs or 40%/60% front rear spring rate distribution.

A 33mm rear is 427 lbs and to match stock we would want a 285lb front spring. Well, I was off a little for the turbo. It seems a 33 rear might match up with a 24mm front very well. I guess you might be able to run this set up with close to equal size sway bars. Such a set up would be about 264% or 2.64 times stiffer than a stock turbo.

It was not until recently that we even had access to a 24mm front torsion bar. With the 24 mm front one really must run the hard Poly Bronze bushings or the bars will quickly interfere with front torsion tube and become comprimized.

To redo, if one wanted to match the turbos 40/60% spring rate distribuition the natural matches would be:

21/29
22/30 or 22/31
23/31 or 23/33
24/33

A normal 911 run spring rates that are closer to 45% front and 55% rear thus typical matches are:
21/27
22/28 or 22/29
23/30
24/31

24/33 needs a relatively larger front sway bar to more closely match the factory balance. (This is where the 27f/22rear came from.)

There are exceptions to the above. If one is running autoX stiffing up the rear will let the car rotate faster (less rear bite). Stiffing up the front makes the car faster in big sweepers (inside rear tire works harder).

I would very this will an real expert.
Old 02-20-2009, 03:47 PM
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911st is on the money... ++1

When talking suspension, stiffer doesn't necessarily mean better. You must let the suspension work to get the most out of it.
Old 02-23-2009, 03:41 AM
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