Factory through-body sway bar sizes and decisions
 

My car had Smart Racing sway bars on it that I removed because they were crazy stiff and transmitted vibration. I'd like to put some softer sways on that use rubber bushings to isolate noise and vibration.

My car is an '84 M491 with a 3.6 that is something of a fair weather daily driver. The suspension is stock 930—stock shocks, stock torsion bars (19mm front, 26mm rear). Corner weights are as follows (37/63 weight distribution):

FL 519
FR 490
RL 863
RR 847

Based on 930 sway bar diameters noted in this thread, I'm inclined to do 22mm bars up front, and 20mm bars in the rear.

Everyone tells me that the through-body sways are superior to underbody, and since my car already had through-body sways installed, I don't think swapping them for other through-body suckers will be more involved than a few bolts.

Some questions:

1) Are there any aftermarket sway bars that use rubber throughout?
2) Did Porsche ever make a 22mm through-body sway bar?
3) Should I favor through-bodiness or the more appropriate diameter?
4) Should I make a size adjustment since my car is ass-heavier than a stock 930?
5) Does this minutiae matter on a street car?

You'll get various opinions on this thread, but your sway bar inclinations sound fine if the drop link arms are adjustable (long arm = less roll stiffness). Adjust to taste, application and driving style.

Most aftermarket sway bars use higher density polyurethane. Not sure about rubber bushing options. Others can contribute that info.

Favor the through-body sway bars. Under-chassis bars don't offer adjustments for roll stiffness nor compensate for corner heights for proper corner balancing.

Consider increasing the torsion bar size to something on the order of 21F and 28R. + or –.

Larger torsion bars add firmness to the ride but don't affect ride harshness as much as larger sway bars. Install with appropriate shocks.

For a street cruiser, stock suspension should be fine, but then again, why is there a 3.6 back there. :-)

MHO,
Sherwood

The car (lowered to 200mm up front and ~50mm in the rear) is already too harsh for my tastes with the stock torsion bars, so I'm not gonna go up there. I'm loathe to add stiffness with sways, but I also feel like I'm missing out on some lateral motion control and high-speed stability running without sways.

Would it make sense to use smaller sway bars? I'm iffy on the purpose of sways. They reduce roll, but are also used to dial in understeer and oversteer? How does one size these things for a car that only sees the limits of tire adhesion during surprising moments?

Uhh. 200mm is approx. 7-7/8" (to or from). That's quite a bit and probably a typo as well. Otherwise, that explains the harsh ride. Re-measure?

50mm is almost 2". That sounds reasonable from the factory height setting.

Your setup is somewhat incomplete without knowing the current setup. This we know:
Torsion bars, 19mm/F, 26m/R (factory size)
Corner weights: Good
Shocks: Frt/Rear?
Ride Height: Frt/Rear?
Tire size: Frt/Rear?
Sway bar size: Frt/Rear?

Installing smaller sway bars (or increasing arm distance, pivot to drop link connection) will decrease the ride harshness, as will substituting hard bushings for more compliant rubber.

Sounds to me like you need to determine what is causing your car ride so harshly.

When a 911 is lowered it will tend to bounce of the bump stops, that will rattle your teeth.

What torsion arm bushings do you have? Those will cause harshness if they are stock and totally warn out or do you have hard bushings?

What shocks do you have? Stiff shocks?

You may want to drive around with out your sway bars on just to see what those are adding to your problem.

If you want the best ride get it raised up to stock level, new rubber bushings, stock or custom or adjustable shocks, and stock size sways.

I am not sure about factory options, but I have some perspective to offer.

The Jerry Woods Motorsports (JWM)/Smart Racing Products (SRP) bars were developed by some of my friends here locally. I have one of their front bars on my race car. They offered a variety of diameters, and had some helpful information in the SRP catalog and data sheets. I can try to post that info here if it would be helpful. JWM still sell the bars, and may sell smaller sizes than what you have.

If you change to a different through body bar, you may have to remove the fuel tank or fuel cell and install different bearing hardware. Depends on the brand and other variables. So a change may be more complex than just a few bolts.

Most aftermarket ARBs can be adjusted to be quite a bit stiffer than most factory ARBs. That is, many can add substantially more wheel rate than the factory choices do. Back in the day, folks took advantage of this fact to add wheel rate without changing torsion bars. I am sure that goes on today.

I have never had an issue with a variety of Porsche street, street/track or full race cars where the ARB(s) transmitted untoward vibrations. I have trouble envisioning how this could occur, but of course a lot depends on what you call vibration.

I have experienced many situations with cars that use ARBs to add too much wheel rate instead of changing tbars and/or springs, and have kind of harsh ride quality as one of the consequences.

Like JWM/SRP, Tarett sell some ARBs that offer a choice of diameters/wheel rate ranges. I spoke to the head guy at Tarett today, and like the guys at JWM, he is an excellent and highly experienced resource.

Trying to benchmark may seem daunting, but it is at least theoretically possible for you to connect with someone close enough with a car that is close enough so that you can compare ride quality. Back in the olden days, the PCA network was an excellent resource for this.

Full disclosure... my race car is a '75 with a 3.6. No rubber-- all solid mounts and joints. I drive it on the street occasionally, wing, numbers and all. The ride quality is harsh and the clutch is a PITA. So obviously a car like mine would not be a good benchmarking candidate. But again, there may be an opportunity fo ryou to find a close car, and speak with the owner and/or shop and maybe take a ride.

Good luck.

SmartRacing offered a range of ARB sizes. Why not look for a smaller size? They're all interchangeable.
What sizes do you have?
My car has 22/28 torsion bars which offer enough roll stiffness over stock that I can run with the ARB bars disconnected for maximum comfort.

More
 

Craig and/or Jerry were thorough enough to provide this information in the SRP catalogue, manual and other product literature:


http://forums.pelicanparts.com/uploa...1502934196.png


Ira at Tarett may have similar information available.

I use the good info above along with the "Sway Away" website and their calculator to figure out the total front and rear spring rates generated by the various arm length settings on my 25 mm Tarret ARB's with 31R/24F T-bars.

You know...That 200mm does not seem right. I have a piece of paper from SpeedSport Tuning that says 200mm, so I sent them an email. The car looks to be about an inch lower than stock and handles wonderfully, even without having any sways on the car.

http://ensaster.com/pics/vehicles/sm...-breakdown.jpg

I guess my first question is: Should I even bother putting sways on the car? It handles wonderfully, and I don't want to increase the slight harshness that is there...It just seems weird to be running without a significant suspension component.

Tires are 245/45/17 up front, 275/40/17 in the rear. Shocks are stock 930. Before the car was lowered (no sways), it wasn't harsh at all. It was lowered not from a good stock setup, but from a wild guess when I redid my rear sprig plate bushings and eyeballed the front the height. It's almost a thousand bucks to change the ride height in this car, so I'm not inclined to do it until I make other major weight changes or get new tires.

With regard to vibration, any time a rubber bushing is removed between the wheels and the chassis, some amount of vibration is introduced. How much certainly varies, but I'm expending great energy to minimize NVH, and I'd rather not introduce anything else that could contribute. I don't believe there are any adjustable sway bars that use rubber bushings similar to the stock setup, and I don't think any stock setups were adjustable. If there are, I'll probably just give those a whirl...

Note: I'm not trying to soften my ride, I'm just trying to avoid stiffening it further. I'm surprised to learn that sway bars have a greater effect than torsion bars in this area.

Did you know that on earlier long hood 911s sway bars were optional? My '71 E did not have sway bars when I got it. It didn't even have the rear mounts. So no rule you have to have them, I guess.

Of course the newer cars are much heavier.

There are a variety of stock sway bar sizes available over the years. You could install some of the smaller sized bars and adjust if needed by changing sizes.

Are you set on using a through chassis front bar?

Some guys only install a front sway. If you are not tracking the car, you are probably not concerned about the ultimate in chassis balance. A front only sway can work to reduce roll, but will be safe since it will induce under steer, not over steer.

Only you can decide if you want to add sway bars. You've already said the car drives well so I guess you don't need them.

Leave them off, increase spring rate via torsion bars as that will help with cornering without the super harshness that sway bars can create. How are the shocks?

All 911s are tail-happy at the limit. While your normal driving may not reach that threshold, any emergency maneuvers will get you there in a hurry. Wet conditions bring oversteer closer to real-world scenarios.

To move oversteer further up the scale, I'd suggest installing a front through-bar for ease of R&R and disconnection, adjustment and bar size options - at a minimum, a 16mm front bar with adjustable arms. If conditions permit, practice emergency maneuvers in a wet parking lot to get an idea of what happens to your car at the limit of adhesion.

Sherwood

Eibach has completed the development of a new underbody sway bar system that uses the existing mounting points of the later 911 such as the SCs and 3.2s. They are stiffer and thicker than the stock bars of the 86 to 89 Carreras and designed to target the performance oriented street use car, although I don't doubt they'll also work well for occasional track use as I think they are somewhere around 22-24 mm diameter. They are also adjustable. These eliminate the need to drill through the body and modify the front A-arms to install the earlier style bars, something many owners are reluctant to do with the increased value of these cars, and eliminate the NVH of solid mounts.


Their official kick off is October 14th but these new bars may be something to look onto.

im really surprised that your bars are effecting the so much. i driven a 930 with thickest smart bars and maxed out TB and i didnt notice a huge effect on the ride quality.

when john had your car it was a pretty track focus car. are the torsion bars mounts or pillow blocks solid? you mentioned dropping the ride height affected the ride... wrong bump stops in your set up with the lowered ride height?

i have stock bars on my M491, if you want i would luv to trade you bars and + some.

i wished i would have bought your car! it had all the goodies i needed!

regards pf