Skip Wolfe

I think the confusion come from the fact the the spring rate of a torsion bar increases as the square of the radius. So the increase of spring rate as it relates to increasing bar thickness is not linear, however the spring rate of the bar itself is.

Oddjob

For one reason - PCA Club Racing requires that the stock spring type must be maintained for stock class cars. So technically, the torsion bars need to be there. How or why any coilovers are allowed within those rules, Im not sure - but they are.

So, there are two schools of thought within the 944 Club Racing crowd. You either run the t-bars in conjunction with the coilover "helper" springs, or you "retain" the t-bars but make their effective rate = zero (grind the splines off one end, machine them down to pencil thin, etc - which is really not the intent of the rule and therefore questionably legal).

If you leave the t-bars functional, they need to be re-indexed so the rear ride height is not too high when adding the coilovers. And the coil needs to have enough preload that its not clanking or becoming free when the suspension is unloaded (when cornering, braking, or hitting bumps). Now, at what orientation/angle do you re-index them at?

One belief is to index the bars so they are approx neutral at ride height. Van mentioned a couple reasons for this above - additive when loaded, subtractive when unloaded. The thought is that the rear end will not pop up as much under braking, since the torsion bars are resisting motion in that direction. Ive talked to some guys that are absolutely convinced this is the best way to go. A local p-car shop that specializes in setting up 944s for club racing, does it this way. And its very hard to refute their results; their cars are very successful in some of the large venue nation-wide events.

The factory ran t-bars with "helper" coils on the 944 Turbo Cups/Club Sports, and the 944S and S2 Club Sports. I have seen two different sources of information that specify 9 deg or 18 deg preload on the t-bars for the motorsport cars. Im assuming that that means that the bars are twisting 9 / 18 degrees when the car is stationary on the ground - 9 deg sounds possible, 18 seems too much, I cant envision the car compressing the rear suspension enough to twist the bar that much, especially in conjuction with a higher rate coilover spring.

Ive run two cars coilovers only, and did convert one back to running the t-bars w/ the coils (I did index them to be approx neutral at ride height). Honestly, there is not a very dramatic difference in handling characteristics, at least not as much as you may expect. Its subtle. But note: my experience with the two setups may be somewhat thrown off because the effective spring rates were not equal before and after - so its not an exact apples to apples test.

Oddjob

I would most likely be running w/o t-bars, if I was not trying to comply with the class rules I run under. It certainly doesnt save any weight by leaving them in...

I agree that the factory method may not be the best, but for the sake of cordial argument - I would also offer that if it was problematic or reduced performance, they probably would have figured that out sometime during the early Turbo Cup series, and I doubt they would have retained a faulty setup as late as the '91 S2 Club Sports.

I have not seen or heard of enough failures of the factory or aftermarket shock mounts/bolts to be overly concerned about that type of failure, with or without torsion bars. I do think there may be some issue with play in the spring plate pivot joint if the t-bars are removed. Certainly the stock rubber bushings would likely be considered inadequate. I am not certain if the delrin bushings are good enough either. The elephant racing bearings probably are. Kelly Moss used to machine tapered roller bearings into the torsion bar carrier and carrier flanges - that was a trick setup and would definitely work very well w/o t-bars.

2bridges

If you spend time around other race cars of any type you will be hard pressed to find any car running coil over that is mounted unsupported on each side with a through bolt setup.

As my automotive interests have changed I have worked on a number of dirt track and drag cars, street rods, etc...... until entering 944 world I have never seen anybody run a coil over mounted in this manner.

Common sense and basic physics tell you there is a fundamental design flaw.

Surprises me the "gurus" buy into and recommend setup with such apparent design issues.
no insults - just makes you wonder

M758

T-bar rates increase by the 4th power of diameter. The actual effective spring rates increase by about 25lbs to 75lbs per additional mm in thickness. Paragon products has chart with this data.

Chris White

Its not optimum - but it also does not fail....

Oddjob

Lower rear shock (full coilover) mount on 964s are the same single bolt design, into a cast alum. trailing arm - and they've got substantially more weight on the rear wheels than a 944. The Cups, RSRs, and Turbos had a very sucessful pro racing career in the early 90's.

blodstrupmoen

Didn`t Porsche run Coilovers on their 924/44 Le-mans cars aswell

ENGINEERMAN

Hi all:

I was one of the guys who recently wrote about my coil over conversion and chose to retain the torsion bars based on the advise of the well known midwest 944 specialist referred to earlier (Bob Vaiu at Auto Edge if we are talking about the same guy). Odd Job is right AE has demonstrated excellent results at the track and having dealt with Bob several times can testify that he knows his stuff with these cars. I am an engineer and know this to be true from a technical perspective but I also know that these systems are very complex and relying on the experiences of someone who has done the time consuming and expensive R&D is a generally a good idea.

For one, the design of the 944 rear suspension is intended to use the torsion bars as a structural member at the spring plate bushing. Removing the TB really requires that a reinformcement be incorporated, e.g. a roller bearing replaces the bushing. This costs money!

Second, the bolt that is holds the shock is only carrying the load of the shock loading with TB's but has to carry the entire weight of the car PLUS shock loadings without the TB's further requiring additional beefing up with special hardened adapters that are avaialble. This costs additional money. Additionally, IF that bolt fails you have a catastrophic collapse of the rear wheel into the wheel well. I wouldn't want that to happen at 130, 140, or 150 MPH. Leaving hte TB's in, even when set to neutral at ride height, provides some measure of additional protection and this helps me sleep better at night since I have experienced a catastrophic failure of a bolt one time at 130 mph while catching a ride in a freinds car and NEVER want that to happen again (I am lucky to be alive)!

Regarding the dynamic benefits, again I chose to rely on good advise from an experienced tuner vs spending many hours (that I quite honestly don't have) to try to figure out what's best on my own. The dynamics under braking are in fact different with TB's combined with Coil overs vs coil overs alone since you get an "unloading" force with TB's when they go past neutral that you do not get with pure coil overs. Is this a better characteristic? I simply do not have the time, money or interest in finding out. I chose to go on the advice of Bob that that is a good way to do it. He has done the time consuming and expensive R&D and I trust his judgement. With my budget and intents for the car (mostly street driving with a couple DE's and autocrosses per year) that was what he recommended.

I will say that the car rides and handles like it's on rails. The improvement over stock is phenomenal. I will add that Bob did recommend that if I could afford it that going with double adjustable Koni 3012's was a good choice if I was really going to get serious about track performance since they did have the added adjustabillity and could be tuned better to particular track conditions but that the Escort Cup Bilsteins were a reasonable (if not the most modern) set up if I did not want to spend the extra dough.

That's my 2 cents worth.

Chris White

Interesting – so on some items you are willing to accept the advice (conventional wisdom) of others and on some issues you feel the need to review the engineering.
In reality the single sheer bolt mount does not fail. Its not an elegant solution but it has been field proven….a lot.
The TB delete has also been shown to work fine – no compression / deformation of the TB bushing – in fact there is less loading there than the original TB design.
As for the loading of the shock mount – the impulse loading does not change significantly with the coil overs – the impulse loading goes through the shock either way (TB or coil over) and the impulse loading is a lot more than the static loading.
BTW – if you set the TB to a neutral position the and the coil over mount fails the wheel will collapse all the way into the well and lock up.
Are we debating about a huge difference in handling…nope. I have seen some pretty odd setups handle fairly well (and some theoretically great set ups handle like pigs). The key is that you can get linear and expected results with a TB delete and it makes tuning easier than leaving the TBs in – especially corner balancing.
It sounds like you are defending the way you have chosen because it is the way you have chosen. I will agree that it is the less expensive and least amount of effort (and it does in fact work) – but it is not the optimal answer for tuning the system.
BTW – for personal experience – the 3012 are a true pain in the butt to adjust ( you have to take them off the car to adjust). In fact they are enough of a pain that most folks that have them do not adjust them for changing conditions….which is what an adjustable shock is made for!

Skip Wolfe

Class rules are certainly another reason over the structural concerns.

I didn't say it was problematic or not-optimal, just more complex and difficult to tune, and certainly not better. So why go with a setup that is more complex and does not offer any benefits? IMHO, the Escort Cup series was there to promote the 951 line so I would guess they did not want to make significant design alteration on the racecars as it could bring into question the design on the street cars, and the expense to change the structural design of the street car to switch away from tbars is not something that Porsche wanted to do at that stage in the evolution of the line. Same goes for the S2 Club Sports, the coilovers do give some performance advantages in changing ride height/corner balance and was really the only option as they would not redesign the whole rear tub/suspension for a limited run car.

That is correct - my bad.

Well said Chris (the whole post and not just this quote). I just was discussing this with Eric Steinel who has a ton of 944 race prep experience. I was explaining how I decided to go with Bilstein Escort Cups to start out on the racecar rather than a set of double adj Ledas I could get a good deal on. I really don’t know enough about how to tune them to gain the benefit out of them, and the Bilsteins eliminate a variable in shaking down a new racecar. Eric agreed that there are very few people who know how to properly tune shocks/suspensions for the track, and that most drivers just adapt their driving styles to deal with the setup they have.

So can a tbar setup work well - absolutely. Would the average track driver be able to tell the difference between a well set-up torsion bar only from a well set-up coil over + tbars from a well set coil over only - probably not. However, what the coilover only setup gives you is an elimination of a variable and flexibility. Flexibility to easily tune the suspension by changing spring rates, ride height, etc.

Skip Wolfe

I would be a lot more concerned about your use of geometry correcting 19mm ball joint pins on stock control arms. I have seen failed control arms happen first hand and rather than the wheel burying itself in the wheel well, the wheel leaves the car all together. I am a mechanical engineer as well and consequently know you understand the fact that you increased the moment arm by going with a longer pin, which puts an increased strain on a cast aluminum socket in the stock arms. Plus the longer pins will have even a smaller range of motion and consequently more prone to binding which is one of the principle causes of failure in the aluminum arms.

As I proof this post, it looks like I am picking on you which is not my intent. My intent is just to show that there are a lot of weak points in the 944's (or any car for the matter) when setting it up for the racetrack, and that you need evaluate the whole system. As Chris stated in his post, the impulse loading does not change significantly with the coilovers, and the lower shock mounts, while less than optimal in single shear, have proven to be able to deal with the punishment, and is a cheap and easy part to replace on a PM program.

I agree that this is a good thread.

Chris White

Skip makes a good point -just remember - if you have an adjustable shock ith 24 settigns - 23 of them are wrong (!) and the odds of the 'monte carlo' method of chassis set up will not be in your favor!!!

If you do not have a bunch of experience with the car and track driving then one of the 'canned' suspension set ups is a much better choice.

ENGINEERMAN

Chris and Skip:

As far as I am concerned this is a friendly exchange and I have no other intent. Skip, I have a thick skin and don't take your comments in the wrong way. I will be the first to admit that I don't know it all (engineer or not).

With that said regarding the rears set up maybe the ultimate reason that I went with the TB/coil over combination is that intuitively I liked the approach and it ws the lowest cost. Please correct me if I do not understand correctly the loading situation. In the stock set up, as I understand it the torsion bar is carrying the entire weight of the rear end of the car and the bolt that attaches the shock is only carrying the loads imparted by the shock absorber itself (impulse/impact). If you convert to coil overs alone the shock bolts carry 100% of the weight of the car PLUS the shock loads, correct?

If that is correct (and I think it is) I liked the idea of reduced loads on the shock bolts (however small) and at least some support from the torsion bar in the case of a failure. Those considerations combined with the advise that I recieved from a well known 944 suspension expert who told me that the combined system works well AND finally the fact that it was a lower cost approach. I'm not saying that it is the ultimate but it seemed to me to be both the lowest cost approach with the highest level of safety.

Regarding safety, Skip you suggested that going with geometry correcting (longer) ball joints (see photos) increases the loading of the control arm and the socket (in tension). Are you sure? When you lower the car from stock ride height, if you keep the stock length ball joints you not only increase te potential for binding of the ball joint due to the increased angle of the ball joint releative to the control arm but you also shorten the distance from the top shock mount to the ball joint socket resulting in an INCREASE in the tension that the control arm is in. But if you add longer ball joints you essentially add the length back that was reduced by lowering the car and as such the length from the top perch of the strut shock to the ball joint socket of the control arm essentially is the same as stock, i.e. the forces remain the same on the control arm as stock (a better situation). NOW, the reason for increasing the diameter of the ball joint stem from 17 to 19mm is because there IS increased bending load on the shaft since it is longer but in fact the stress situation on the control arm is better with the GC kit.

Boring the pinch joint of the spindle out to 19 mm DOES weaken that area but it is cast steel and I was told that Rennbay had never seen a failure in that area (I did not try to calculate the stress levels in that area since that is a VERY complicated affair). In summary I do not think that the actual stress in the control arm increases due to the GC Ball joints when lowering a car. Unless my logic is completely off (it would'nt be the first time) in a lowered car they actually reduce stress in the control arm (including the ball joint socket area) than if the stock length ball joints are used. I think that as you said that control arm ball joint failures in lowered track cars are a result of either:

a) The ball joint binding which can generate HUGE loads, or

b) The increased loads from track use with super sticky R compound tires are just more than an aluminum control are can take for the long term and not from the longer GC ball joints which actually reduce stress levels vs. stock length.

But not due to the use of a GC ball joint.

Am I completely wet here? Please check my logic and let me know what you think. I even drew up a force vector diagram to check my logic and I think I'm right.

With that said I am concerned by what you said and do not want to take any chances (especially if the real issues is b), that the aluminum is just not up to the types of stresses found in track driving on sticky tires for extended periods of time. Remember that aluminum does not have a fatigue limit, it is just a matter of time before it fails and increasing the loads just speeds the time to failure).

My use on the track will amount to a couple of DE's per year at BIR and a few auto crosses. I was a passenger in a 993 last year at the track where the inner control arm bolt broke at 130 mph and I never want to have that happen again (I'm lucky to be alive).

If I am wrong, I will freely admit it. Am I better going to chrom moly Charley Arms just to make sure?

Comments appreciated.

Thanks!

ENGINEERMAN

Sorry: For some reason the photos were not attached in my previous e-mail.