Why are most popular torsion bar combos separated by 6mm?

[dd74]

I've begun to notice some bushing wear on my car, and have used that excuse to freshen the entire suspension. I'll concern myself mostly with bushings and torsion bars. I'll also reinforce the sway bar mounts. With this said, I've long since seen torsion bar upgrades as either 21/27, 22/28 or 23/29/, etc. I understand the thickness of the torsion bars as contingent on the application in which the car will be used. But why does the varying thickness between the front and rear torsion bar sizes always seem to be 6mm? Is there a hoped-for balance between understeer and oversteer that is congruent to each combination having a 6mm difference? Why 6mm and not 5mm, 7mm or even 8mm?

Your thoughts are encouraged.

Thanks.

[Dan Mc Intyre]

Good observation, dd. I'm interested in that answer as well.

Dan

[Tyson Schmidt]

I'm not sure who recommends 23F 29R.

You've only got 3 choices for the front 21,22, and 23. In the rear, they range from 26-33. There are many different combos. Many racers advocate a 10mm differential. So there is definitely no steadfast rule.

Especially when you start running different swaybar combos.

[juanbenae]

if i recall when i up my sc to 21 & 27 that combo represented a consistent spring rate increase over stock, somewhere around 60%. i figured the engineers that designed the car would have a better idea than me over spring rate differences between front and rear. that being the case i went for an increase that would keep the spring rate increases consistent with the original bias front and rear.

[drsimonwong]

My car and many of our club G series 911's run 23/30mm to great success.

Simon

[patkeefe]

I went 21/27 with mine, in a effort to keep the F/R spring rates biased in proportion to the factory settings. This was just an intuitive WAG on my part. I can understand the biasing Tyson refers to for racing, especially when the variance of the sway bar combinations is introduced. I kept the stock sway bars, but I'm starting to realize the importance of the sway bars since I just started autoxing. There is a big picture WRT the intended use of the car which needs to be examined first.

Live and learn.

[David]

Torsional stiffness is determined by bar diameter to the fourth power. You can compare the front to rear ratio of the diameters to the fourth power to compare similar front to rear stiffness. If you do this you will find that any rule of 5 or 6 or whatever mm difference doesn't work. The 23/30 ratio that Simon mentions is very close to 18.4/24 ratio which I think is the original bar size. If you want to correct understeer or oversteer problems you can adjust accordingly.

[David]

[dd74]

Originally posted by 125shifter
Torsional stiffness is determined by bar diameter to the fourth power. You can compare the front to rear ratio of the diameters to the fourth power to compare similar front to rear stiffness. If you do this you will find that any rule of 5 or 6 or whatever mm difference doesn't work. The 23/30 ratio that Simon mentions is very close to 18.4/24 ratio which I think is the original bar size. If you want to correct understeer or oversteer problems you can adjust accordingly.

So in that case, 21/28 should as well be close to the stock ratio. Or 22/29, correct?

[David]

OK, here are all the combos I think would be used:

[dd74]

Interesting - I currently have "stock turbo" w/o the turbo car, of course.

I love oversteer, so maybe I'll stick with that combination...

But it looks like - if I use, for example, 28mm bars - I would, to keep the turbo ratio, use 20mm front bars to keep the same level of oversteer.

Does this make sense?

[patkeefe]

there is some more info in the Pelican tech info center, and a couple more websites (I used the info for my WAG)

Nice info by David, BTW

[ted]

Originally posted by Tyson Schmidt
There are many different combos. Many racers advocate a 10mm differential. So there is definitely no steadfast rule.

Especially when you start running different swaybar combos.

With a 2400lb RS clone and R rated tires my 21/30 combo with custom valved shocks and Tarret ARBs was fun on the track.

[ted]

Originally posted by patkeefe
I went 21/27 with mine, in a effort to keep the F/R spring rates biased in proportion to the factory settings. This was just an intuitive WAG on my part.

My intuitive WAG sez that might just be duplicating an under-steer factory bias. I prefered the 21/30 combo too. It still lifted ever so slightly the front tire on hard cornering with R rated tires.

[}{arlequin]

Tyson, maybe it's secret, "evil"-only info, but if not, then I'm interested in what combo you are (or will be) running in your early car....

[patkeefe]

Ted:
I must admit to my ignorance of chassis stiffness and the torsional loads placed on the car during cornering when I redid my suspension last year. I have a 2600 lb SC, and I definitely notice a bit of understeer in the autox. I have to consider that my Yokahama tires are not exactly the softest compound out there (the wear rating is 385R/425F), in contrast to Yokahama 048's, which are 60. So, I suppose, simplistically, that the tires contribute to the understeer. It is a good ride for the street, which was my intent. I'm still trying to learn about cornering dynamics.

Looking at the above charts, I notice the 930 has much stiffer rear bars, I would guess due to the static weight, and the squat factor when accelerating. I'd like to see some commentary on that.

I can't answer the original question posed by dd74, but my thoughts were encouraged. I find these threads to be good learning tools. Lots of smart and experienced guys on this board.

[dd74]

Originally posted by patkeefe
Looking at the above charts, I notice the 930 has much stiffer rear bars, I would guess due to the static weight, and the squat factor when accelerating. I'd like to see some commentary on that.

Just to interject, I have the 930's torsion bar setup on my car, which essentially has SC dimensions, though much lighter than a stock SC. It's very crisp and loves to turn "in." Oversteer is very much the car's nature with these bars. What I'm after is a bit more stiffness, but more or less the same ratio of stiffness as I have now. I don't want to re-learn how to drive this car. In fact, I would - and who knows - may - leave the torsion bars alone, and just get new bushings. But while I'm in there, I may as well consider the option of an upgrade.

[ted]

Originally posted by dd74
Just to interject, I have the 930's torsion bar setup on my car, which essentially has SC dimensions, though much lighter than a stock SC. It's very crisp and loves to turn "in." Over-steer is very much the car's nature with these bars.

I had the same combo for 2 years when I did a lot of AXing.
Worked great in the parking lot, though I found it too soft in the big track sweepers in 4th or 5th.

FYI My car also has an 80% LSD and that will add some push too... also it had an elevated rear wing and front spoiler/splitter on faster tracks too...

No one set up its right for everyone, whatever improves your lap time or provides the most comfort on the street, your choice.

[Vintage911Racer]

I am running 23/30 in my 911 track only car. 1950lbs. I love the way it handles. Very predictable and controlable.

Many are running different set ups (smaller front bar) but there cars do not handle as well as mine.

[Tyson Schmidt]

Originally posted by }{arlequin
Tyson, maybe it's secret, "evil"-only info, but if not, then I'm interested in what combo you are (or will be) running in your early car....

I'm running 22F 30R.



The charts are interesting. I suppose they are a nice way to get a rough idea. But there are just way too many variables to pinpoint the right combo based on a chart.