27.3mm bars are pretty easy to get here but I like the idea of being to adjust ride height without fighting with giant torsion bars. Plus my Bilsteins are starting to leak so I need new shocks anyway.
I ran some sums on spring rates and getting the equivalent of 27.3mm bars with coil springs and got a big surprise how stiff the springs need to be. I couldn't be sure what the spring rates Grant posted were for or what length of lever they used so I started fresh. I used the formula for torsion bar spring rate in the Puhn book and took some measurements, and the stiffness of a
vertical coilover mounted at the shock mount (in addition to the stock TB) needs to be about 230kg/cm to get the same stiffness as a 27.3mm bar. Also due to the actual angle of the shocker(not vertical), the spring actually needs to be
stiffer than 230kg/cm
This seems really odd, given that the available spring rates for the RSR kit are 80-125 kg/cm. So are 27.3mm bars stiffer than I think or is my maths stuffed? Here's the formula and some other stuff I used:
K = 1,178,000 X (D^4 / LA^2)
Where:
- K = spring rate in lbs/in at end of lever
- 1,178,000 = spring constant
- D = diameter of bar in inches
- L = bar length in inches (length of constant diameter not including splines or tapered sections)
- A = Lever length in inches
What I used
D= 0.898" (for stock 22.8mm) or 1.075" (for 27.3mm)
L= 33.27" (845mm)
A= 12.2" (torsion bar centerline to middle of balljoint = 310mm)
To convert
lbs/in to
kg/cm multiply by 1.15
This gives you spring rates at the balljoint, so you then multiply by 1.22 to get the spring rate at the shock mount. (310mm from
A above and 255mm from TB centerline to shock mount)
So did I go wrong? I thought I would end up with a value in the middle of the RSR options (about 100kg/cm).
Scott
