Stock Bar Rates
Stock Bar Rates
Does anyone know the bar rates of the stock sway bars front and rear? This is the measurement of the strength of the sway bar for those that don't know. Anyone know where I could find this info without taking the measurements myself?(Lack of motivation, might as well see if it's been done already.
)
Thanks.
)Thanks.
I would like to know this as well. Actually, what I really want to know is what MATCHED front and rear bar combos are available out there. Cusco, Whiteline?? Links to vendors would be appreciated.
I found this on a Bimmer forum:
Calculating Sway Bar Rate
- -------------------------
If you're substituting a larger diameter bar that otherwise
has the same dimensions, the rate increase is simply:
(new diameter/old diameter) to fourth power
So when I changed my M5's rear bar from 18mm to 19mm, the
stiffness increased 24%. (19/18)^4 = 1.24
Now, if you want to calculate the actual rate or have adjustable
sway bars (several adjustment holes to change length of lever arm)
and want to figure out the rate increase, you'll need to measure
the bar and use a formula (from Fred Puhn's "How to Make Your Car
Handle").
Formula for sway bar stiffness of a solid steel bar
500,000 D^4
K (lbs/in) = -------------------------------------
(0.4244 x A^2 x B) + (0.2264 x C^3)
B
----------------
A| / \ C
| / \
D - Diameter bar (inches)
B - Length of center section (inches)
A - Length of end perpendicular to B (torque arm - inches)
C - Length of end (inches)
On the E28s, both front and rear bars have nearly a 90 degree angle
between the arms and center section. So I assumed A = C for my
calculations.
B
:----------------:
A : : C (= A)
: :
Example calculation for M5 stock 25mm front bar
D = 25mm = 0.984 inch
A = C = 11.25 inch
B = 31.5 inch
500,00 x 0.984^4
K = ------------------------------------------------ = 233 lb/in
((0.4244 x 11.25^2 x 31.5) + (0.2264 x 11.25^3))
Calculating Sway Bar Rate
- -------------------------
If you're substituting a larger diameter bar that otherwise
has the same dimensions, the rate increase is simply:
(new diameter/old diameter) to fourth power
So when I changed my M5's rear bar from 18mm to 19mm, the
stiffness increased 24%. (19/18)^4 = 1.24
Now, if you want to calculate the actual rate or have adjustable
sway bars (several adjustment holes to change length of lever arm)
and want to figure out the rate increase, you'll need to measure
the bar and use a formula (from Fred Puhn's "How to Make Your Car
Handle").
Formula for sway bar stiffness of a solid steel bar
500,000 D^4
K (lbs/in) = -------------------------------------
(0.4244 x A^2 x B) + (0.2264 x C^3)
B
----------------
A| / \ C
| / \
D - Diameter bar (inches)
B - Length of center section (inches)
A - Length of end perpendicular to B (torque arm - inches)
C - Length of end (inches)
On the E28s, both front and rear bars have nearly a 90 degree angle
between the arms and center section. So I assumed A = C for my
calculations.
B
:----------------:
A : : C (= A)
: :
Example calculation for M5 stock 25mm front bar
D = 25mm = 0.984 inch
A = C = 11.25 inch
B = 31.5 inch
500,00 x 0.984^4
K = ------------------------------------------------ = 233 lb/in
((0.4244 x 11.25^2 x 31.5) + (0.2264 x 11.25^3))
I would say none, because as soon as you change spring rates away from stock, the swaybars will not necessarily "match" up to them. That's why most rear bars are adjustable.
You could run a set of Cusco bars front and rear, but if your spring rates are different that what Cusco used when designing the bars, I would not say they are matched. Cusco, I think, has about the most different sizes of front and rear bars available (2 of each?) for just that reason. . .