more t/bar stuff - anyone got experience/comments on 21/31 combo?

[dickster]

ted,

thats a damn fine idea

it never occured to me that the tub might be twisted/damaged

i'll see if i can get it on some scales.

thanks for all your input guys.

i know i'm jumping the gun going for these mods that are way beyond my ability.......

[dickster]

well, i went with 22/31's in the end and very pleased that i did.

however, having chnaged the rear bushes for neatrix at the same time, the left/right handling problem remains

although i have now spotted that the left side tie rod bush is very worn, i am hoping turbo ones will fix it........................

[chrisp]

good call on the 22/31. It was the right decision...as you found out.

Chew the fat with some of the p-car shops that PCA race at the front and they'll lead you in the direction of a good bilstein valving spec for those bars.

911's are difficult to get the roll stiffness correct in the rear. That's why the trend has been towards bigger t-bars. More t-bar, less sway bar.

Also as tire technology advances then spring rate goes up. The first big jump was when the Hoosiers came out and the second was recently when the Michelins started becoming popular. They respond well to this higher spring rate.

Keep evaluating your stuff on the track and not the street.

[Jeff Alton]

I am joining this one a little late but here goes. I currently have 21/27 t-bars and 22/21 sways and Koni sports all around. I wanted a little more rear t-bar and thought about 28 or 29. Now I think I should look at 30 or 31?!?! I am "pretty competent" behind the wheel and the car is more for autox than track, but it will see the track 5 or 6 times this year.

Should I go with 29 or 30 and see what happens? I don't mind spending the cash to do it twice more to get it right. Some one will buy the bars that I do not end up using .... right .......guys...... someone will won't they......
oh ya, Wil, I like your idea of 7's all the way around.
Chuck, You are suspiciously missing from this thread, I value your advice!

Thanks, Jeff

[dd74]

Hmmm...I'm going to deviate from the idea of increasing torsion bar sizes by asking how many out there now know how to drive their car at consistent speed with the torsions they currently have on their cars?

It would seem that the best way to find out if one needs larger torsions is to effectively use up all the "spring" their current torsions offer - and at speed.

My idea is to take what you have for your suspension, corner balance and align it, then drive the daylights out of it until you are certain you've surpassed its ability. Only then will larger torsions, tires and such truly matter.

With that said, Dickster, I believe did the right thing: he figured out he wanted more oversteer, and installed appropriately-weighted bars to insure that oversteer.

In my opinion, even if you're not a racer, you will learn a huge amount about your car if you take it to a track and drive it hard in its current condition. As my car did with me, I found it surpasses my driving ability, showing me that the mostly stock setup I have on it now is more then adequate until I "earn" the necessity to change torsion bars or anything else.

Until then, there is an infinite amount of things one can do to their car - as I've found out. Tire pressure increase or decrease, ride height adjustment via the springplates or torsion bar indexing, toe in/toe out adjustments, and camber/caster changes - to name a few - can make a big difference. If at all, heck, you'll spend less than what it costs to r/r torsion bars you may not need.

[Wil Ferch]

I agree with dd74.
Dickster...you and I have had lengthy discussions on this topic (others...see archives)...and if you look at the equivalent spring rates I published for the various bars...I *still* think you went a bit overboard on the rear bar size. The fact you say that it's fine might be good as to how it feels on the street, but I will guess that as you near the absolute cornering power of the car ( like on the track...or an aggressive street avoidance maneuver)....you will find the car hard to balance on this new edge.
Summary of my previous points.....upgrades are typically quoted at 21/27 and 22/28. This maintains the fr/rr spring rate balance of the stock car with smaller bars. Typically too much low speed understeer as Porsche intended for "legal" purposes. The suggestion then is to go 21/28 ( or even 29)....or 22/29 ( even 30)....but IMHO....31 is pushing too far toward high speed oversteer...noting that at low speed it may indeed feel neutral and better. Let's remember that the spring rate goes up to the FOURTH power of diameter...a big stiffness increase for a small diameter increase.

Then again..if it works for you.....OK ... just make sure it's still OK at some extreme driving conditions.

---Wil Ferch

[gumba]

I run 22/30 t/bars, 27mm f&r sway bars & re-valved shocks on our 2525 lb. '73 targa track car w/3.2. A race engineer friend of mine implied the 30mm rears were too stiff. The problem being the front tires floated over road irregularities causing less grip. He suggested I try 28mm w/245 rears or 29mm w/275 rears. I guess it depends on how you like the car to feel.

[Thorsten S.]

Hi all together,

this is my first post in this forum.
I read all your opinions about the torsion bar diameters front and rear an how to adjust them, but nobody was talking about a correlation of the weight of the front end and the rear end and the diameter of the torsion bars.
The stock 911 ever had a 19 mm (o.k.it is 18,8) front torsion bar- even the last 89 turbo. But the rear diameter was increasing over the years from 23mm(?) to 27mm at the 1989 5 speed turbo (I dont know if this car was sold in USA ).
The most increase of weight over all the years was in the rear(for example 915 gear to G50, turbo (where the engine is more than 1 inch further in the rear of the car) , etc.).
When we look at the Carrera with 915 gear box (car weight 1160kg), we have 24,1 mm torsion bars. With the havier G50 (car weight 1210 kg) we have 25mm.
The turbo with 4 speed gear box (weight?) had 26mm, the 5 speed (car weight 1350 kg and more) 27mm.
The sway bars were always adjusted to the torsion bar changes. But the cars ever had a built in understeer character- a forgiving set up.
When they changed the rear tb from 24,1 to 25mm, they increased the front sb from 20 to 22 and the rear from 18 to 21mm- and the car is 50 kg heavier with the most weight increase in the rear.
We know that the Carreras have 40% of weight in the front and 60% in the rear ( my example, a 1987 Carrera G50: 519 kg front (full gas tank)and 734kg rear, which is 1,41).
When I look at Wils List I see: Front Spring rate for 19 mm=110, and rear spring rate for 25 mm=140. 140/110=1,27 The car does understeer.
But there was a sport suspension from the special wishes programm of the factory in the 80s with 26mm turbo torsion bars in the rear (and Bilstein shocks and adjusted sway bars).
26 mm =165. 165/110 = 1,5! The car is more neutral.
We know that the turbos have a front/rear weight relation which is more like 35% to 65%. For example 530 kg front and 820 kg rear- this is 820/530=1,5. With the built in understeer we have 165 (for26mm)/110=1,5 or 191 (27mm) /110= 1,72. But with the 27mm tb the sway bar was reduced from 20mm to 18mm.
What I want to show is, that, in my opinion, the torsion bar diameter depends more on the front/rear weight distribution then on the equal spring rate. For a street car I think much more than the stock 19 mm is to much- especially on german autobahn with 150 mph. Last summer I was at RUF factory and talked about tbs and sbs. They recommended (or example for the Yellow Bird) tbs: 20/29 , sb 23/20. The RUF cars are street cars- no race cars. But with reinforced chassis.
Since two weeks I have tbs 19/27 on my 3,2 Carrera.
Why? I didnt like that the car squatted in corners in the rear (and lifted the front wheel oposite a little bit).
Now this seems to be o.k, but I feel that I have to change the sbs (perhaps less diameter in the rear or more in front-or both), because it has more oversteer. But I will test this in the next months.
I hope this was not to boring.

Thorsten

[Wil Ferch]

Thorsten:
Your reasoning is sound.
Turbos usually are 37-38% front....62-63% rear.
If you adjust *only* for the weight difference...and keep everything else the same...then you didn't change the low speed understeering characterisitc that Porsche built in originally. For a given wieght, you'll see that the "common" 21/27 and 22/28 combinations fairly well match the fr/rr balance that Porsche had originally. For each of these, many have gone stronger in the rear TB size to dial out a bit of understeer .
Another thing to consider is that the stiffness of the TB should not be compared simply to the fr/rr weight distribution. The "leverage" affect of the suspension arm that acts on the spring... must be taken into consideration. For example, a 50/50 weight distribution car might come with equal spring rates front and rear only if the "A" arm length that acts on the spring is the same front and rear. If the "lever arm" is longer on one end..it will make that end softer...and higher rate springs will be needed ....quite independent of the fr/rr weight distribution.
One other thing....for our 911's....whatever you end up with will have different low speed vs high speed characterisitics. A low speed car that understeers terribly might be nice and neutral at high speed...because at high speed you use less steering angle and have more optimum tire contact patch to the ground. At low speed there is a great deal of steering angle for the same cornering load...and generally more understeer.
Thanks for sharing and commenting...welcome aboard.
----Wil Ferch
Ich spreche Deutsch...aber bloss ein bischen !

[Thorsten S.]

Hi Wil,

ich schreibe trotzdem hier in Englisch weiter ;-).
Yes, I agree with you and the TB rate does NOT ONLY depend on the weight- but it is ONE reason (some engineers from the factory told me). And for race cars from the factory they had combinations like 22/28, when only torsion bars were allowed by the rules. O.K.
In this thread was a question about a street car. As I wrote in my last post, Porsche had a sport suspension from the special wishes programm for 3,2 Carrera with 19 mm and 26 mm for cars until 1986.
When I look at the 968, there was a M030 sport suspension (968 CS), too, where they added coil springs in the rear to the stock tbs.
And for the 964 there was a M030 too, where only the rear coil springs where 40 % stiffer than stock (and a sway bar in front, which was 1mm bigger than stock + shocks- I think this was the suspension of the RS America).
Here in Germany I know nobody, who is driving bigger tbs in front on the street. I think its because they do not response very well on bumps and waves at high speed.
So you are right: It makes sense, to change the original balance with stiffer rear bars, to dial out a bit of understeer.

Viele Grüße

Thorsten

[dickster]

Thorsten,

Thanks for the post.

So from what you are saying, your setup 19/27 tb at 1.73 is actually more agressive (towards oversteer) than my 22/31 1.58 (near neutral?) setup?

This ignores the fact that the stiffness in mine is probably better suited to track than street. But then that is always going to be a compromise. i wanted to be able to have a blast on the track, so then running stiffer makes sense?

Granted my original plan to go with 21/31 at 1.91 would have been more oversteer orientated than you have found yours to be.

[Wil Ferch]

Dickster:
If the spring rate equivalents you're using are correct ( I don't have my list in front of me now)...then what you're saying is true...the 19/27 combo has dialed out more understeer than your 22/31 combo.

What isn't addressed is:
- Thorsten's point that there is a supplemental component ( sway bar sizes)...that also come into play. He admits he may be too aggressive and will find out later...to be fixed, presumably by different sway-bar sizes.
- the overall effect of running "stiff" ( overall) vs "soft" ( overall)....regardless of balance. There is a view largely held in Germany that some suspension compliance is necesary. In the States, we seem to feel that going stiff all around is an OK compromise... a lot depends upon the roads you drive on...and how dry weather / wet weather friendly , etc, etc...the set up is. I presume the stiff set up if over-done can cause wheels not to follow road irregularities well.In addition, overly stiff will also result in rather "quick" transitions at the limit...like "snap" oversteer instead of a more gentle oversteer with softer springs.

There is a lot to this...no simple answers... the suspension guru's kept by race teams surely earn their keep !!

---Wil Ferch

[Magnus Rostadmo]

Hi !
I have an -86 Carrera. I`m rolling on 17" RUF`s 8+9 with 205/255.
I have dialed out some of the understeer caused by my wheel set-up by adjusting my Koni-Sports.
Now I wonder if going to 26 mm rear without any other changes will make it handle well on the road and at DE`s as well.
I don`t want it to oversteer, but as it is now with stock 25mm it handles like a fat pig with absolutely no turn in at highway speeds if my shocks aren`t adjusted minimum half stiff at rear and full soft front.
I somehow imagine that this setup must be great for a car with this wheels (to broad at the rear) that is mostly used on twisty highways and some DE`s.

[Thorsten S.]

Thanks Wil for the quick answer to dicksters questions. You have been faster than I, but you wrote all things that I would have written.
Another short story I heard last summer, which was told to me by a former Porsche factory driver: In the early 70s they tested the 2,4S and the 2,7 RS on some bumpy streets in the black forest (not on the plain race track). They beted which car was faster. The stiffer RS or the softer 2,4S- and the 2,4S was faster!
Magnus: Do you surely have the 25mm rear torsion bar (as you wrote) and the 22mm/20mm Sway bars? This change was made in 1986- thats why I ask.
Perpaps you know this link: http://www.9mracing.co.uk/html/modules.php?op=modload&name=NinemeisterContent&file=index&req=viewarticle&articleid=19&topic=7&articlename=Tuning%20Guide:%20911%20Carrera%2083-89


Thorsten

[dickster]

Thorsten

Yes, i can understand that. On bumpy roads a stiff setup will struggle - that makes sense. But if you use a car on both street and track you cannot get the ideal setup for both. i just thought it would be more sensible to do insane speeds on the track rather than the street

[Wil Ferch]

Dickster is absolutely right to keep highly lethal speeds on the track
Magnus...I don't think you'll help under/oversteer simply by shock settings, although it might feel you're doing something along these lines. Shocks should simply damp the springs. You're running unusually ( vs stock) front/ rear aspect ratios ( 205/255 ....vs 205/225 stock ) and that combo will tend to give you a lot of understeer. Try running 225's in the front if you can...or something like a 225 or 245 in the back. You can try 26 mm torsion bars in the back ( up from stock 1986 25's)...but 27 might suit you better with that wheel/tire combo. Either one won't get you into trouble...in my opinion.
---Wil Ferch

[dickster]

well, thorsten's other thread and a road trip prompted me to resurect (sp?) this one.

currently i have the 22/31 bars fitted, with neatrix bushings, and bilstein sport shocks - all other parts are oe and mostly very worn. i have set ride height and front toe only, still to set rear camber (currently around 4 degrees!) and toe.

with the car in this state i went on a 350 mile non-stop, round trip today, mostly motorway (freeway?) miles. i got back dissappointed that the trip had ended! the car is a joy on those types of roads, very solid, confidence inspiring handling, and not too harsh. i have to admit that the setup is a bit harsh on small bumpy back roads, but if you do alot of m/way miles its perfect - imho.

the steering feels "darty" at the moment, but i have gone to 25" in the front and need to fit the turbo tie rods that i have and a bump steer kit that i'll get soon.

got to sort the rear camber quickly as the tyres were warm on the inside and pretty cool on the outside - this will no doubt result in rapid wear that is not too good for my wallet!