Ferch Tbar to Spring Rate formula help needed - G50 bars...

eimkeith · 07-27-2006, 03:34 PM

Posted here from Rennlist (at Jack's suggestion):

Was just admiring/adopting the 911 torsion bar/wheel rate tables and trying to adopt them to G50 rear bar dimensions and must not have as good a grasp on my antique math skills, as I am getting stupid numbers (spring rates with 9 zeroes, lol)

Would anyone be so kind as to run these dimensions for G50 rears?
bar length of 626mm
spline length at each end of 29mm
effective bar length, then, of 568mm

Additionally, what happens for a hollow bar? Stiffer/less stiff per diameter?

Thanks!

eimkeith · 07-28-2006, 02:24 AM

btt for info...

89911 · 05:19 AM

I used this with my 89 and G50 with 31mm bars and it was almost perfect. Are you sure you are using the correct data? Don't worry about additional information and for hollow bars, use the solid bar equivalent.

89911 · 07-28-2006, 05:17 AM

When I calculate mine, I get 19deg.

http://www.rennlight.com/cgi-bin/spring.cgi

eimkeith · 07-28-2006, 07:19 AM

nono - spring rate, not angle...

Wil Ferch · 03:22 AM

It's *essentially* unchanged from the 915 series...

- for both 915 and G50 bars......overall length for rear bar is 626 mm.
- subtracting the spline length, and asuming full spline engagement in the installed condition.....the "effective" length for a 915 bar was said to be 22.9" and with your G50 spline dimensions...it ends up being 22.4". So *theoretically*....it will be a bit stiffer by being shorter. It's only shorter by about 2 % so maybe ..if you're so inclined...add 2% to the wheel rate for a G50 car ( with whatever diameter you're interested in...you didn't say which diameter). The formula is a linear relationship to length. I would think the 18.5" "lever arm" length is unchanged for the G50 cars...as the wheel base did not change.

- Wil

eimkeith · 07-28-2006, 04:16 PM

Thanks! I was interested in the entire range, but have additional curiousity about 31mm hollow bars...

Wil Ferch · 07-29-2006, 03:27 AM

Nuance ...... you need to know if the "31mm hollow bars" means :

- they are really 31mm dia and you want to find the equivalent solid bar rate...likely 29 or 30mm..... OR....

- they are actually bigger than 31mm by a small amount.....to give you the "equivalent" rate that a solid 31mm bar would. Ideally, you would need to know the dia of the "hole" to help figure out the equivalency...in absence of supplier info.

As I said...otherwise the 915 and G50's are about the same.

- Wil

eimkeith · 07-29-2006, 04:37 AM

right on - the hole is the only thing I did not measure, but I can get that dimension Monday morning. The diameter of the bar is 31mm.

How is the total rate calculated with this new information?

Wil Ferch · 07-30-2006, 11:02 AM

Most manufacturers give both "actual" and "equivalent" TB diameters fior their own hollow bars...best to ask them. AFAIK, there are only 2 mfgrs in the US...Weltmeister ( white) aka "Sway-A-Way" or Sanders ( bare).

IIRC..the actual dia was only a fraction of a mm... or as much as 1mm bigger... for the hollow-vs solid bars....others here who've done this can chime in.

- Wil