Calculating out the right spring rate for an independent suspension can seem
pretty daunting. But we're going to show you just how simple it can be.
All right I'm Trevor and I've got Bill with me today and today we're going to discuss how
to appropriately calculate a spring rate for an independent suspension. And this can be a front
independent or rear independent same principle applies there. But when you might be experiencing
this is if you have a you know custom chassis car that you're going to outfit with coil-overs
or you've got an aftermarket tubular front end or even if you're just retrofitting in
some custom mount you know shocks and springs into you know some stock independent suspension
that just doesn't have any direct fit coil-overs for it. So it's a pretty simple procedure really
in calculation but you will need some some known weights and measurements and and Bill and I will
get into that and uh and show you how easy it is. So we got to start out with vehicle weight
and more specifically we need to know you know we're going to go through an example today of a
of a typical front end weight. So I need to know what my vehicle's front end weighs and
I'd say for this scenario let's say my front end weighs 1500 pounds. All right 1500 pound
total weight. Total weight yep I rolled it on a set of scales put some scales under it it's
1500 pounds. And with any suspension we've talked about this before what we need to
know is the amount of weight that is actually being supported by the springs. Okay. So that
is not going to include your wheels tires brakes spindles you know those kinds of
things. So in this scenario I'm going to use a pretty round number that's very common for
a lot of the stuff that we see and what we work on right and that's 150 pounds.
So if we take 1500 minus 150 we get 13.50.
That's how much weight is being supported by the springs. Now there's two springs
so we need to we need to divide that by two because each spring is going to be supporting
half of that. So that's 675 pounds.
Okay. Right? Because we're calculating the spring rate for a single spring
so you know what to buy. Yeah. Right. So the next thing we need to know now
we know essentially how much weight is going to be supported by these springs.
We need to know what length spring you're going to be using and we're in this scenario we're
assuming that you've already worked out your shocks. You've watched our other videos you've
worked out your shock on spring length. Okay. So what spring length are you going to be running
in this particular car? Let's say I'm going to need a 10 inch spring. So our 10 inch spring.
And we want that to compress about 30 percent for a general purpose streetcar
you know high performance kind of thing. Yeah. So times 30 percent
equals three inches. Okay. So that means we want our 10 inch spring to compress three
inches underneath this 675 pounds. Okay. So all we got to do there because spring
rate is pounds per inch we just take our pounds divide by inch right so if we go 675
divided by three inches we get 225
pounds per inch. Okay. Yeah and that is a spring rate then. That's exactly what that is. Okay. So
we've got 225 pounds. Now here's where it gets slightly different for independent suspension.
Yeah. So you've got a control arm here and the control arm is pivoting here
and it's being acted on way out here the shock though is way in the middle so we
need the measurement from the pivot to the ball joint and from the pivot to the shock mount.
So here's our ball joint here's our control arm here's our pivot here's our shock mount.
So let's just assume we got a 16 inch control arm from pivot to ball joint. The shock mount
is right in the middle at eight inches. Okay. What this is saying is that if this is pivoting here
and you're at and you're pushing up or pulling up on this ball joint out here. Yep.
You've got this much leverage on that much spring. Okay. Essentially so we're going to take 16
divided by eight which equals two. And what that really is is a is a ratio
two to one which means that we've got two units of measurement from here to here versus
one unit of measurement from here to here. That makes sense and I should mention in this case
the shock mount happens to be halfway in between the control arm eight inches but if it is further
outboard we need to know Pivot Point to shock mount not ball joint to shock mount so just
so that's clear. The pivot point on the arm to the shock mount would be our our number
that we divide the overall length by. Yep. So in this scenario we've got a two to one ratio
which is actually really common in a lot of the stuff that we see and we work on. Yeah.
So now because we've got two times the leverage on the shock which is what that's saying we just
multiply this by two. So we take this we'll bring it all the way down here. We've got 225
times two
equals 400 150 pounds
per inch. Nice. So that is the spring rate that I would probably I would recommend for that. If
you called up and said hey my front end weights 1500 pounds I have this control arm geometry I
would say Okay 50 or 450 pound springs is going to do you really well in this case. Yeah that
seems really straightforward. Now if your shock mounts further out here and you've got an angle
to the shock you can get into angle correction stuff and we've done that in other videos. You
can go look at our solid axle for a little bit more in-depth explanation of that. It's the same
principle you would just take this number divided by your divided by your angle correction factor.
Now if you wanted a drag race you could go more on your compress or more on your percentage here
you could go you know 35 40 percent and then you'll get a lower rate. Yep. You know things
like that if you're autocrossing you could go let little less on that maybe 20. Yeah this
gets us really close for just about 90 percent of what we're ever going to do with the car. Well I
hope you guys learned how easy it can really be to calculate starting spring rate for an independent
suspension here. And of course if you need any assistance with that we're always happy to help.
Grab that vehicle weight and those control arm measurements and we'll get you on the road.