Effect of changing torsion bars compared to sway bars?
 

Is there an easy way to decide how much roll stiffness you will get from a bigger torsion bar as opposed to a bigger sway bar? Intuition suggests that the sway bar would produce more anti-roll for a given change in strength than the same change in a torsion bar. Can anyone offer an explanation?

That's the question: which offers more roll stiffness per % of size change.

Fatter torsion bars create a higher "wheel-rate" ( think "spring rate"...but measured as forces acting on the wheel), so the thicker bars will help cornering AND braking/acceleration motions ( pitch and dive).

Common convention for other cars says adding fatter anti-roll bars doesn't affect ride, whereas adding fatter suspension springs does. Common convention for our 911's tells us the opposite.

Because the "spring" effect of either is based on diameter ( "D" to the fourth power, quite sensitive to diameter changes).... AND....also the length of the lever arm the operates on either of these types of springs ( anti-roll bar or torsion bar).....it is not a simple matter to say, "Im going from "X" to "y" and expect such-and-such change.....not that cut-n-dry.

Sorry can't give you %%'s but I can give you my seat of pants feel:

I have uped my sways to the '86-89 thicker front and rears. It did improve the sway but I gained some looseness when I hit corners to quickly. I was also worried with the OEM '85 T-bars it may stress the sway bar mounts too much and snap the mount welds. Like many have experienced here.

With the combination of Strut Bar, Thicker Sways and Larger T-bars 22/29, she rides like she is on rails. I do not worry about snapping the Rear Sway Bar Mounts. I also learned how to control her thru the quick corners.

Caveat, albeit stiffer but I carry alot of stuff in my daily so it isn't that bad. At the track when I unload her, she is pretty stiff.
Jim

Wil,
Looking thru old notes I found a nice piece someone sent me a long time ago. I'm sorry to say I don't know who did it. Perhaps it was you? Or perhaps if he sees this he will chime in? MYSTERY SOLVED. BURGERMEISTER DID IT.
In any event, if you look at the items highlighted in orange by me, you will find my summary of this author's view that the front sway bar contributes 37% of the total front roll stiffness and the rear sway bar 23% of the total rear roll stiffness... given all of his assumptions including size of the various bars etc. Does that sound like it's in the ball park? I hope you can read these photos of the author's spreadsheet. I can send you or anyone else who is interested the Excel file if you want.


http://forums.pelicanparts.com/uploa...1300651216.jpghttp://forums.pelicanparts.com/uploa...1300651413.jpg

Where is Fleigler?

I suspect that was Burgermeister who sent that through to you. Based on much of the work he did I can estimate the following:

Increasing T-bars 3mm diameter front and rear = approximately .6 degrees less lean at 1 G cornering.
Stabilizer bar diameters would need to be increased by roughly 2.5 mm front and rear to have the same -.6 degrees per G effect.
Swapping out the bushings with something much stiffer could also have a similar effect.
Tire compliance is much smaller- going from 16s to 18s would probably only get you in the range of .2 degrees.

A stock car starts out rolling about 4 degrees per G. Many race car setup books tell you to aim for roughly 2 degrees total roll on cars such as ours.

This is before studying the spreadsheet posted above:

Well, by my calcs, the motion ratio for the front anti-sway bar* (*=aftermarket) means it twists about 108.4% of the torsion bar for a given wheel displacement, so the motion ratios are pretty close.

If the sway bar is measured in # per degree, that usually means that half of that degree comes from one wheel, and half from the other. The torsion bar being measured in # per degree means that all of that degree comes from one wheel. This means that for a given wheel displacement, the sway bar contributes twice as much force as the torsion bar, or that the front sway bar contributes 2/3 of the front roll stiffness.

Torsional stiffness # per radian is JG/L
J=(pi/2)r^4
G is about 11600000psi
effective L for the sway bar is about 36 inches
effective length for the torsion bar is about 22 inches, IIRC.

For actual stock front carrera suspension diameters I have the stab bar contributing half of the front roll stiffness, 40% of the rear roll stiffness, but those are not from my own calculations/ measurements. Bushing rates and other variables have a big effect on the actual contribution beyond the bars themselves.

It is truly amazing how much and in such detail some of you know about these cars. There cannot be many issues that have not been covered on this board.

Yeah, but it boils down to some seat-of-the-pants decisions. Sure, if you have data from a track and want to do the math, you can tune the car that way.

Or, you can decide on how the car is going to be driven and ask someone who has set up dozens (like Chuck Moreland or Dave B from TRE) for some practical guidance.

You will need to know the car, the weight, the ride height, the track width, the wheel sizes and what tires. Then you need to know the type of roads or track surfaces where the car will be used.

Personally, I'd rather make the tires stay on the surface rather than over focusing on body roll, whether treating it with t-bars or sway bars. Remember what Mark Donohue supposedly said, use heavier spring rates or heavier sway bars; you don't need both.

Of course that would be relative to what you start with.

Apples and oranges. Both ARB (sway bars) and T-bars (spring rate) influence body roll but the underlying reasons for the effects are far from similar.

Milt, I was thinking the same thing, give the experts my parameters that I want to run and let them sort it out. That then is the starting point to fine tune. Most of the heavy lifting has been done. It is a wonderful science.

Agreed. Unless you are very sophisticated you are far more likely to get yourself in trouble figuring it all out entirely on your own. Understanding the issues, however, can be very useful when working through the various options.

Max,
Had a chance to study the chart above yet? The still anonymous author of that work, possibly Burgermeister, suggested the front roll bars contribute 37% of the antiroll. If I understand you correctly, each side individually contributes twice that number or 74%...again, given his assumptions about sway and torsion bar sizes etc. So you and he are in the same ball park?
Jim

i like your last comment.

skid pads and lap times are a good way of tuning.

there are a lot of good books out there. its an old book, but start with "how to make your car handle"

You realize that the chart you're referring to has after market 24mm front and 33mm rear torsion bars, but stock swaybar sizes? That's why you're getting so little contribution from the swaybars- was that the intent?

Change the Tbars to the stock 19 and 25 and see what results...

fwiw, the early RUF 911s went to larger swaybars only. ...for Nurbergring.

What bugs me about these discussions is that rarely does anyone talk about --what should be an obvious bit of the discussion- That is; what are the road/track conditions, and the speed you intend to move over it?

That is, the whole idea of suspension is to balance the needs of the car/driver against the height variations of any given piece of road. ... a slight dip at 30mph, can become an abrupt hit at 130.... unless the suspension is tuned to those conditions.

..airport cone-killing autocross does not have the same requirements as rally racing.

edit, looks like Milt was on top of this.

Good points all. I agree that the only real way to find out what you want is to drive the car on the track and check results and "feel." When I started this thread though I didn't give a fig about real world conditions, wanting only to understand the relative effects of a change in the sway bar vs a change in the torsion bar. So far, the answer seems to be that the sway bars typically do 2/3 to 3/4 of the job, torsion bars the rest.

No. Sway bars are 50% to 40% of the job, torsion the rest (on a stock car).

I see where you got 2/3 from Flieger (though I think he's overestimating), no idea where you got 3/4 from.