coilspring rates vs. torsionbar size

[juse]

Tim;
In our discussion on torsionbar rates some time ago http://www.germanlook.com/Forums/showthread.php?s=&threadid=1292&perpage=15&pagenumber=2
I noticed that your maths overdo mine greatly.
Now I found this article of Porches torsion bar rates converted to inch/lbs
http://tech.rennlist.com/911/pdf/torsionbars.PDF Maybe you can get something out of it with the information you have from the Baja Bug-book.
If you get interested on it let me know of the results.
Justin

[Tim]

Hi Justin,

I put the numbers from the Baja Bug book into the Rennlist formula.

d = bar diameter = 22mm = 0.866in (beetle stock size)
L= torsion bar length = 21.75in (effective length = 20.75in)
A = arm length = 16.5in (torsion bar centre to wheel centre)

The result is 116lb/in

According to the formula from the Baja Bug book, the force to change 1 degree is 35.6lb.

To move 1 inch of the suspension, the torsion bar will be preloaded approximately 3.4 degree. Therefore lb/in is

35.6lb/degree x 3.4degree/in = 121lb/in

Looks like the two formulas are consistent .

Hope that this is not too complicated to read.

Tim

[juse]

So the stock torsion bar rate is around 116-121 lbs/inch?
Thats not very much I think. But I guess its impossible to compare as such linear coil spring to torsion bar strength.
I wanted to know this because I read that if the spring rates are equal front and back, then you might end up driving a rocking chair.
Thanks again Tim, I owe you another one.
Justin

[Tim]

The example which I was using is the torsion bar length of pre-'69 swing axle. The bar length of Supers is 26-9/16in and the spring rate is around 94 lb/in by using the Rennlist formula.

Tim

[juse]

What is your opinion Tim about rating torsion bars; If we decide that the stock rate on super is that 94 lbs/inch, then is it linear or rather progressive?
You might understand my interest on this if you think of the rockingchair effect I was warned of..
Justin

[Tim]

I think a torsion bar spring rate is linear because a linear coil spring is basically a long torison bar wound up into a uniform spiral. In both cases, the load is absorbed by twisting the metal. Correct me if I am wrong.

Tim