Briefly, I am looking at the Speedway Engineering 1.25" OD hollow bars. 36" long, .120" or .188" wall thickness.
What I am looking for is what anyone is currently using and why.
Arm style and length?
Distance from the lower control arm pivot to the new ARB attachment point? (to figure out motion ratio)
Any way to make this style of bar adjustable?
Here are my notes from my current 27mm bar:
Berlina/105/115: 105/115 Alfa front ARB on my Berlina is 1.085”Ø and has an effective length of 30.75” and the arms are 11.0” long. Motion ratio is .54
Rate at mounting LCA is 443 lbs/in. Rate at the wheel is 239 lbs. /in.
Direct numerical comparisons can be made with some straightforward maths.
Bar diameter raised to the 4th power gives an arbitrary number that can be compared to another bar diameter also raised to the 4th power.
Tho these numbers will only be directly comparable if the ARB is of the same design and the spring section of the bar is the same length.
If the spring section of the ARB is made longer, it gets softer, so a percentage difference of length should give the correct numerical comparison.
The same should apply to different arm lengths. Shorter arms will have less leverage over the spring section of the ARB, so will effectively make the ARB stiffer. So a percentage comparison should again be applicable.
And given that you'd probably attach the ARB to the standard position on the LCA's, the motion ratio will stay the same so doesn't need to be allowed for in the calculations.
Converting everything to metric numbers:
27^4 = 5314401 units
1.25" x 0.120" wall thickness
= 31.75^4 - 25.654^4
=1016190 - 433132 = 583058 units
The hollow bar section is about 15% longer so, if my maths is correct, should be about 15% softer. So the new ARB would have a calculated value of 495599 units
A measurement of the new arm lengths would be needed to complete the calculations. Again the percentage difference would need to be applied to the 495599 units
to be able to compare it to the original ARB.
As you can see, the longer ARB spring section is already softer than the 27mm bar in the original position, so the arms would need to be considerably shorter to get the ARB stiffness back up to the 27mm bars value.