Well, not getting the kind of info I was hoping for, and having this mind that when I don't understand something, I study it to death, 'till I figure it out. So, I've been doing some research into the whole suspension thingie...
Here is some of what I've discovered..
First thing I didn't understand about suspension was this "Spring Rate" thingie. Oh I understood that a 200lb spring meant that for every 200lb loaded onto the spring, it compressed by 1 inch. But, why could two very different looking springs have the same spring rate, while two almost identical springs have very different spring rates?
Well it turns out that the Spring Rate can be calculated by the following formula:
Spring Rate (lbs/in) = (modulus of spring steel x wire diameter^2) / ( 8 x number of active coils x mean coil diameter^3)
Where:
1. The modulus of spring steel = 11250000 lbs/sq.in.
2. The ^ means "raised to the power of" the following number. (^2 = sq., ^3 = cubed, etc.)
3. Active coils means ones that can move, eliminating much of the top and bottom coils. Usually is a fractional number (eg: 4.3, 5.25, etc.)
4. Mean coil diameter is the diameter along the centreline of the coil. Inside Diameter(ID) plus wire diameter or Outside Diameter(OD) minus wire diameter.
Note, the length of the spring has nothing to do with the Spring Rate calculation. It does however impact two aspects of the suspension, the distance to coil bind (coils touching each other), and preload. I'll talk about preload later. And if your suspension coil binds, you have serious suspension design or other problems
So, I promptly went out the garage and measured my TR7 springs (inches) and plugged them in with the values I measured.
          #     wire     coil   spring
       coils   dia     ID     rateÂ
Front   5.5    0.5      5      175
Rear    4.5   0.465     5      157
Hmmm, that's strange. The rear is fairly close to the stated stock rate. What's going on with the front? Fought with that for a few days messing around with formulae, straining my mind, and just doing a lot of soul searching. Then I suddenly remembered that I had installed a set of TSI aftermarket springs 5 or 6 years ago. Duh! Still had a piece of the old springs in a drawer somewhere. Found and measured its diameter, and using the same number of the rest, plugged it in and got 91 lbs/in. Hey, this thing actually seems to work.
So then I wanted to try some others. I bought a cheap eBay Miata coilover lowering kit similar to Leon's conversion.
I didn't know what the rates were (no one in the street rod crowd seems to publish them, strange), but I knew I could use the spring perches, if nothing else. Here is what I calculated:
           #     wire    coil   spring
         coils   dia     ID     rateÂ
Front   4     0.435    2.5     498
Rear    4     0.390    2.5     337
Afterwards I realized the numbers printed on each spring included the spring rates- 500lbs & 350lbs. Cool, but way too short and stiff for me to use for general road use.
FYI - Spring specs use either a metric or US code, in the format of "FreeLength.InnerDiameter.SpringRate".
So, metric 200.64.67 = US 0700.250.0375 Which means a spring that is 7" (200 mm) Long, 2.5" (64mm) ID, with a 375 lb/in (67N/mm) Spring Rate. Amazing what you can learn on the internet on a cold winter night.
Then I checked into other coilover spring types.
Dual-rate: Basically two springs, one short soft spring and one longer firm spring with a ring to hold them aligned. Starts off with soft suspension for regular road use, but if you start to push it, the soft ones bottom out and the firmer ones take over, giving you stiffer suspension.
Progressive: A single rate spring, wound so that about 1/4 to 1/3 of the coils are progressively closer together than the rest. As the spring compresses, the narrow gaps close first, effectively reducing the number of coils, thereby increasing the effective spring rate. Performs similar to the dual-rate (soft to stiff), but much more gradual and no harsh change over.
Beehive: This is a bit similar to the Progressive, but allows for a much greater range of suspension progression. Rather than gradually reducing the spacing of the coils, a beehive spring gradually reduces the diameter of the coils. Some even do both. Jeeze, wouldn't want to try to figure out how they calculate the spring rates on them!
I think that's enough for tonite. Tomorrow’s lesson, Spring Preload and what it means to you....
Martin
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