How to pick new suspension springs
TimK
Things have been moving along quickly with the engine removal
procedure. You can see all the photos on the blog at http://civic-ev.blogspot.com.
Any feedback on the removal descriptions is welcome.
I've got a question for the automotive engineers out there
regarding the suspension. How do you determine the optimal spring
size/length/tension for a given weight? Bob Bath used adjustable coil-
overs with new springs. Rob C. just used the front springs from an EX
model to handle the extra weight in the front and cut the old front
springs to use in the back.
My concern is that different batteries might need different spring
sets due to their higher weight. For example, the Hawker Odyssey and
Deka 9A31 batteries have 100 Ahr ratings instead of the 75 Ahr rating
of the Optima YellowTop, but they are also heavier. It looks like you
can order any spring you need from Ground Control (http://www.ground-
control-store.com/index.php). I'm looking for a relatively economic
solution to make this kit open to more people.
In short, how can people pick the right spring height, width and
tension for a given battery weight while maintaining proper clearance
under the fenders? Equations and technical explanations are welcome.
Thanks and cheers,
Tim K
p.s. If this would be a better discussion on one of the Civic specific
forums, please suggest one.
Ian
There is a basic spring calculation, which of course I don't remember but is
in one of my old Mechanical Engineering textbooks downstairs. Luckily there
are a number of online formulas like this one:
http://www.iroczone.com/calcs/chassis.htm
One way to go is to have someone like Ground Control build you a spring.
The hot-rod method to lower a car is to cut the springs, which will lower
the car and stiffen the spring.
Since we want to keep the original ride height the low-buck way is the way
Bob Bath did it, and find other springs that will work. I was thinking that
Integra springs or Accord springs might work, as the Integra is 600 lbs
heavier. I vote for low buck eventually, once we figure out what that is.
Ian
Rob Connelly
Sorry I didn't send this sooner. Attached is a spreadsheet that I made to
help me determine the springs that I had to begin with, the final weight,
and the springs that I needed to get to the proper ride height. Feel free
to attach this to the list, or to reformat it in a way that is more
understandable.
Note that I began by measuring the wire diameter, number of coils, inner
diameter of coils, and the compressed height (with the full load of the
conversion on them.) To get this right, you also need the free length (the
length that the springs are when they are not loaded -- not even on the
struts, so you have to get a spring compressor to remove them, and be
careful!) From all this, the spreadsheet calculates the load on each
spring, and gives you a guess as to the total vehicle weight. The absolute
accuracy of that number is not so important, what is important is that you
use the same number for the next calculations to tell you what the new
spring rates will do to the ride height. In my case, I could measure that
the springs were compressed down to about 8.25" on the front, and 8.5" on
the rear. From my measurements of the height of the wheelwells above the
ground prior to the conversion, I knew that this was about 1.5" too low, so
I set the target loaded height to be about 10", and set about looking for
springs that would get me there. RockAuto.com had the specs on springs for
the front of a Civic EX that miraculously worked just about right (it turns
out their specs were a little off as you can see from the spreadsheet, but
close enough.) With the front taken care of, I decided to take the old
front springs and chop them down (using the spreadsheet as a guide to how
much) until they produced the proper loaded height. The magic number came
in at 12.9" to support the same load as before, but at 9.98" rather than
8.5". So I cut them, installed them and they were right on.
By the way, the calculations are quite sensitive to accurate measurements of
the springs -- especially the wire diameter. Spring rate is a 4th power
function of the wire diameter, and a 3rd power function of the coil
diameter. Be sure to double- and triple-check your measurements, and use
calipers to get numbers with a .001" precision.
Price of one new set of springs from RockAuto.com -- ~$50, so this is the
low-buck approach.
Let me know if you have any questions about any of this.
- Rob
----- Original Message -----
From: "TimK" <tim_k...@yahoo.com>
To: "Civic EV Kit" <civic-...@googlegroups.com>
Sent: Monday, June 30, 2008 1:36 PM
Subject: [CivicEVKit] How to pick new suspension springs
>