Rear Torsion Bars

[Southernroadrunner]

I see alot of you folks useing Neuf and Madness rear torsion bars that are all 28mm
I think.Do any of you folks use the Corza Performance torsion bars that is 25mm.I know from looking the stock oem torsion bars are 22mm.Would a 25mm torsion bar be much different from stock?Thoughts on this subject??

[Oramac7891]

I have the neuf. Considering they are less than 30 bucks difference in price, why not just got with the neuf?

[PorcoRosso]

Depends on what you want from the bar. Stock setup with 22mm bars produce a roll couple of about 73% iirc. This means heavy understeer bias. By installing a 28mm rear torsion bar, you reduce roll couple to 47% which is slightly oversteer bias. This is helpful if you want your back end to rotate more around turns. Studies have shown that slight understeer is the most predictable for racing and roll comfort, around 56-63% roll couple. I believe the front and rear neuf sway and torsion bar working together will produce 56% roll couple--a veritable roll couple sweet spot. I've raced with just the neuf rear and slight oversteer bias and it was very managable and fun. Just installed the front sway and looking forward to seeing how to they work together at my next event.

[NSngineer]

You can look up the formula for Torsion Constant, J, but for the same material over the same length, it varies by the square of the radius of the bar. For 22mm. 25mm and 28mm bars: 14x14=121; 12.5x12.5=156.25; 14x14=196. The 28mm bar is 1.62 times or about 38% stiffer than the 22mm bar.
By the seat of my pants, I can tell there's a difference between the 22mm bar I had on our stock 2015 Abarth and the 28mm bar I replaced it with, but I can't really say how much that difference is, or what it means in terms of cornering speeds.
Maybe I can get some sense of it at my next Autocross.

[Tweak]

Still enjoying my RRM "Big Red" 24mm/29mm (if you know the bar you understand the dual sizes) torsion bar.

I definitely like it, car rides on rails and although a 4 wheel slide is possible you need to really make the effort to have it happen.