Cloud9...68

I have also read this thread with great interest, although neither my skill/experience level nor my current use of my car warrant such a serious mod. I'm very impressed with the ingenuity, effort, and integrity of everyone involved on all sides of this journey. Even though I don't own a 944 turbo, this has definitely become my go-to site whenever I'm looking for a trustworthy answer to any technical question with my car.

A couple of quick questions. Approximately how much does the t-bar delete bar raise the control arm attachment points? And doesn't the fact that it does so have the side benefit of introducing some anti-squat geometry?

I can only hope these units will still be available if and when I get to the level where I would benefit from it.

95ONE

CLoud, It raises the Pivot point 1" from stock. That little bit does make a huge difference. One instant advantage you will see is that when the car is lowered to its "sexy" height..- rear wheel just inside the top of the wheel well - The wheel travel won't be going up and FORWARD anymore. It will just be upward due to the new pivot point. No loss of wheel base on that extreme Squat and no more tire rubbing in that one annoying spot at about the 10 o'clock position on the rear wheel well. Most racers know what I'm talking about.

And as said before. I only have parts to make three more of these, and I very highly doubt I will have interest in making any more past that point.

NCLA951

Hi, are there any more of the three available? What do they run? Thinking about changing front geometry and want to match in the rear.

Thanks!

Weston Dillard

After way too long, I finally raced the car again last weekend at Hallett. It was my first time out on Bruce's setup and it seemed to hold up well. I did a lot of curb riding and sliding across rumbles with no issues. I had to unload and head out of town. When I get back I'll recheck alignment and give it a once-over and report back.

West

951and944S

Uhm, well, that's about the best vote of confidence on this I have seen yet.

If you couldn't break it, it's probably pretty solid....

Heard you were at Road Atlanta but never had time to look you up and give a shout.

T

Cloud9...68

West,

Glad to hear the setup held up well. Did you notice any discernible improvement in the way the car handled? And did you make any changes to the car in terms of ride height, spring rates, alignment settings, compression/rebound settings, etc., in response to installing it? Finally, did you level the front control arms as part of the package?

Weston Dillard

I had made a ton of changes since the last run. Rebuilt Motons, Racer's Edge control arms and Bruce's rear, plus I only had one weekend on a new, much stiffer cage (Hanksville). I was just pleased to keep it running all weekend. It felt pretty neutral, but will take some tuning to take advantage of the additional forward bite I'm hoping for. I will run COTA in a month and should learn more then.

West

Cloud9...68

Since Bruce is down to a just a few more of these bars, I'm planning on buying one, not because I strictly "need" it at the moment, but because I don't want to miss out on what might be the last opportunity to correct an inherent limitation with our cars' suspension design in terms of the ability to lower them. But I have a number of questions about the impact of raising the roll center, and thereby shortening the roll couples, and how to optimize the suspension parameters to take the best advantage of the improvements created by installing this bar.

Shortening the car’s roll couples (in front by installing longer ball joint pins to get the A-arm back to parallel with the ground, and in the back by raising the trailing arm attachment points with this bar) on a lowered car creates a number of positive effects:

1) Less weight transfer from the inside to the outside wheel as a result of lowering the center of gravity by being able to lower the car significantly below stock ride height
2) Less tendency to sway due to the shorter lever arm from the shorter roll couples
3) Less camber change under load
4) Less tendency to roll steer (assumes installation of roll steer correcting tie rod connectors)

I'm sure there are others that others can add. My car has a full race set-up from Racers Edge, and as it sits, is very neutral, but I had to take a number of steps from its initial set-up, including softening the rear springs from 1000# to 800# (fronts are 700#), raising the front by a little over 1/2", moving the rear sway bar to full soft, and setting the compression and rebound adjustments on my Moton Clubsports to full soft front and back. The car was very twitchy and bouncy before, not just in my hands, but when driven by the pro's at the track, with a strong tendency to oversteer. It is now very neutral, controllable. and generally feels like a different car, but it bothers me that we had to move everything to the extremes to exorcise the extreme oversteer. Based on Eric K's input after he first installed his bar, the improved rear grip should move the car to a more understeering regime, which will allow me to better center the various suspension parameters. Which leads me to a list of questions for those who are using Bruce’s bar:

1. Since I should be able to lower my car beyond where it is now, would it make sense to reduce the negative camber a little, because of the reduced weight transfer (I’m currently running about -3.3 degrees up front, and -2.4 degrees in back)?
2. What is the “optimum” ride height? I’m currently about 25” in the front, and about 24-1/8” in back, measured from the bottoms of the fenders to the ground. This leaves me with the A-Arms inclined about 5/8” from parallel.
3. Since I’ll have less sway, should I soften the springs a little? If so, just the fronts, just the rears, or both?
4. I currently have the M030 front sway bar. Is an adjustable bar, like a Tarret, a must for fine-tuning purposes (not a problem if it is)?
5. Roughly where should I expect the front and rear sway bar adjustment positions (assuming I upgrade to Tarret front bar) to end up?
6. Same question for the compression/rebound settings.

I realize there is no set answer to any of these questions, and that I will have to tune everything in through testing, but I’m just looking for inputs for starting points to minimize the amount of fiddling I’ll have to do. I don’t do door-to-door racing (yet), but there is a lot of “friendly competition” at the track where I do weekly sessions with it. The track is very technical, with a lot of tight turns and a fair amount of elevation change, but not much in terms of straights, so it would be nice to be able to keep with up the smaller, lighter cars in the twisties. Also, I think the experience of evaluating the impact of the tuning I will have to do will be a good exercise to go through. Thanks in advance – I’m looking forward to hearing peoples’ real-life experiences with this intriguing mod.

mikey_audiogeek

Are you sure the rear roll centre is raised? I seem to recall there was an erroneous statement made quite a while back which was later corrected. Check with 95ONE directly.
FWIW, raising the outboard pivot point ONLY, lowers the rear RC.

Cheers, Mike

Cloud9...68

Hmmm... I thought raising the rear roll center was the whole point of Bruce's bar. The problem is that lowering these cars lowers the center of gravity, which is good, but it lowers the roll centers more than it does the CG due to the geometry of the suspension design, which is bad, because it increases the length of the roll couples (and therefore the roll axis). This can be corrected in the front by either installing longer ball joint pins, or by raising the A-arm attachment points, to get the A-arms parallel to the ground. However, the creates an imbalance front-to-rear - a tilted roll couple, in other words. Bruce's bar, with its raised trailing arm attachment points, addresses this by raising the rear roll couple, thus leveling the roll axis. If it did the opposite by lowering the rear RC, it would exacerbate an already imbalanced situation.

LOVE New Zealand, by the way. I spent two weeks there with the family in 2014 - wish it could have been two months. Absolutely spectacular country.

Droops83

Hi Andy,

There are numerous threads from the past few years discussing spring rates and wheel rates, etc. It seems like your front springs are quite stiff relative to the rear, which (with all other things equal) would tend induce understeer. But, maybe it works well for your setup/driving style . . . .

As for ride height measurements, using the fender tops as a guide is not very accurate for comparison between different cars (OK for a rough before and after check on the same car, though). I've see 944s and 911s come from the factory with differing fender top heights front and rear! Any subsequent bodywork could make the situation worse.

The 944 and 968 factory manuals stipulate a more exact method of ride height measurement: in the front, measure the height of the heads of the caster block bolts from the ground on each side, and determine how far below the wheel center height each one is (measure both values separately then subtract the bolt head height from the respective wheel center height).

For the rear, it's the same idea but the difference between the torsion bar center and the rear wheel center (use torsion bar end cap). The reason for measuring this way is that the values will be constant regardless of wheel/tire height; you can even perform this measurement with the car on hub stands. More importantly, this value is consistent with respect to control arm geometry. The general rule with any Porsche platform is to not go too far below the "RoW M030" or "RS/GT3" height, or the suspension geometry will be outside Porsche's designed range. If the factory itself made modifications to the roll center to account for ride height changes (like the 993 RS, most GT3s, etc), sit up and take notice, they didn't do it for fun!

Porsche itself never altered the suspension geometry of the 944/968 street car (the above values for the 968 CS are 147 mm +/- 10mm in the front and -37 mm +/- 10mm in the rear. These values are only 20mm lower than the factory US 968 ride height, so one must assume that when going 1" (25.3mm) lower than US ride height or more, you are going outside Porsche's designed range of suspension geometry. Besides the obvious concern of the front ball joint range of motion, roll centers and anti-dive and anti-squat properties are affected . . . .

Cloud9...68

Hi Chris,

Thanks for your inputs – I wasn’t aware of the method described in the workshop manual for measuring ride height – this is very useful information that should make ride height comparison between cars more reliable, which I really appreciate. Here are my car’s ride heights, with my weight equivalent in the drivers seat, and the car on a level garage:

Front left: Caster block bolt head to ground = 116.3 mm, wheel center to ground = 306 mm, delta = 189.7 mm
Front right: Caster block bolt head to ground = 119.3 mm, wheel center to ground = 308 mm, delta = 188.7 mm
Rear left: Torsion bar center to ground = 227 mm, wheel center to ground = 308 mm, delta = 81 mm
Rear right: Torsion bar center to ground = 235 mm, wheel center to ground = 309.6 mm, delta = 74.6 mm

So my car sits over 1.5 inches lower than a 968CS. My ride height was recommended by Karl Poeltl of Racers Edge, who is a very respected figure in the Porsche tuner world. The car recently got a corner balance and alignment, including tuning of my Moton Clubsports on a shock dyno, and the compression/rebound settings optimized, by Bill Dollahite, the owner of Driveway Austin (the track I drive on), a retired racer who has run several teams. As part of the recent work, I softened the rear springs from 1000# to 800 # (kept the fronts at 700#), to eliminate a mid-corner hop at the rear Bill referred to as “porpoising”. I’m sure the softer rear springs helped reduce the car’s oversteer, which likely contributed significantly to the car’s improved manners. It is interesting that Bill still had to push things, particularly the shock settings, to their extremes to bring the car’s cornering attitude from oversteer to neutral, but it drives very nicely now. My wheel rates are 569/324, assuming a F/R motion ratio of 0.92/0.65, and spring angles of 16 degrees front and rear. I see you’re running 500/750# F/R, which gives your car more balanced F/R wheel rates of 406/304.

But even though my car drives very well now, I can’t help but think it would further benefit, at least in theory, from eliminating the tilt in the A-arms (and tie rod connectors) by installing longer ball joint pins and modified tie rod connectors, and balancing this in the rear by raising the trailing arm connection points using Bruce’s torsion bar delete bar. Theoretically, shorter front and rear roll couples should not only improve the car’s cornering capability, it should more importantly make the car more stable and predictable (safer, in other words). There was an excellent article in the November 2016 issue of Grassroots Motorsports in which they took similar steps on the front suspension of a previous generation Mustang which they had lowered, which has a similar strut type set-up as our cars, and this significantly improved that car’s cornering. But having said that, I have to confess that I am the living embodiment of the old saying that a little knowledge is a dangerous thing, so I am a little worried about possible side effects of what on paper looks like a positive series of mods. In other words, I’m wondering how much, if at all, I would have to tweak my current suspension parameters to best work in harmony with the shortening of the F/R roll couples.

mikey_audiogeek

Well the Kokeln T-bar delete definitely lowered the rear RC.
Because of the semi-trailing arm geometry, lowering the rear by 1" only drops the rear RC by 0.5" whereas at the front lowering the car by 1" drops the front RC by about 1.5-2.0".
That's the basic difference between semi-trailing arm and strut suspension.
Since even at stock height the front RC height is much lower than the rear, lowering the car results in a front RC about 5" lower than the rear. Addressing this requires lowering the rear RC and raising the front RC.
Glad you liked NZ!
Cheers, Mike

T&T Racing

The "wheel rate"quoted in this post is not correct.
It is actually the equivalent spring rate.

Wheel rate is spring rate times MR squared or in the case of the front, the wheel rate is 700 *.(.92)^2 or 592 lb/in. The rear wheel rate is 800*(.65)^2 or 338 lb/in. Ratio of rear/front wheel rate is 338/592 or 0.57. This is way towards understeer

IMO, front spring rate of 500 lb/in with MO30 sway bars is a better combination. I have not tested on the track yet but this was a recommendation by Karl to me. I do not have Racer Edge front lower control arms to lower the front of the 944.

In full disclosure, my race car is a 944 2.5L NA.

Are you running a square tire setup or wider tire in rear. Depending on driving style, it can add to understeer at turn in to apex or can help reduce oversteer going to full throttle at apex

Cloud9...68

My wheel rate calculations are correct. You left off the spring inclination angle correction term, which is the cosine of the angle of inclination of the spring from vertical:

http://performancetrends.com/Definitions/Wheel-Rate.htm

I measured the inclination of my front springs to be 16 degrees from vertical, which only reduces the wheel rate by a factor of 0.96, so it's a small correction, and it's applied equally to the front and rear wheel rates (I didn't measure the rear spring angle, but it looks close to the front's, and it's a small correction anyway), so it doesn't change the conclusions.

I'm running a square setup, which may at least partially explain my car's tendency to oversteer. It also makes sense that when I softened my rear springs by 20%, while leaving my front springs unchanged, it reduced my car's tendency to oversteer. Going softer in the front would bring some oversteer back, and I'm happy with it's attitude at the moment, so I don't think I would want to do that without changing something else that would bring it back to neutral.