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Discussion Starter · #1 ·
Info gleaned from many sources on the trifive forums has shown a lot of confusion relative to the effect and use of springs for the front end.

Addressing the stock geometry shown in the "1955 Chevrolet Passenger Car Shop Manual, Front Suspension 3-2" which has a cross section of the front suspension I scaled it to full size and determined:
1. The stock spring compresses .61" for every inch the lower ball joint & wheel moves up (or the body moves down).
2.The spring height at the design ride height is 12.0"
3. The bump stop will contact when the suspension moves up 3.4". Or considering the spring, when it is compressed .61x3.4= 2.0".

Vehicle weights vary between different models but an example given by ChevyNut in one of his very informative posts is:
A Nomad with a LS1 engine and 4L60E trans weighted 3686 with 935lb on each front corner and 908 on each rear corner. We do not know other info on the configuration such as passenger load, chassis mods that effect weight etc but this is a good starting point.

Now we need to determine the sprung weight that each spring will need to support. The front unsprung weight consists of the wheels, tires, spindles brakes, and portions (usually about half) of the control arms,springs, roll bar attachments. As there are many choices in these items, lets use 100lb as a good starting point for the unsprung weight on each front corner.

I have not found the stock spring rate specs but each model had to have a spring that would give the desired ride comfort and provide enough initial support to set the car at the design ride height and not go into coil bind before the suspension made its full travel.
Another source on the trifive site had data for two Moog springs;
p/n638 with a rate of 328lb/in and a free height of 16.44"
p/n656 315lb/in, 17.35"
To support the front corner of the car with 935-100=835lb the spring must support 835/.61=1370lb. The p/n638 spring must compress 1370/328= 4.2 inches to a height of 16.44-4.2= 12.2". Wow, this is just about the design geometry of 12 inches.[/SIZE]
The other Moog spring with a lower spring rate but longer free length calculates to a compression of 1370/315= 4.4" and a compressed length of 17.35-4.4= 13".this spring would have our car in this example sitting 1/.61=1.6" too high. We could cut an inch off this spring and the car would sit at design height and have a softer ride.
If we knew the number of coils and the coil diameter of the springs being considered we could check to make sure that the fully compressed spring does not go into coil bind before it reaches full bump.

This is pretty important stuff when you are looking at coilovers as the must do the same job with different geometry. I'd like to address this in another post. I did a lot of design work when I modified the Talbo to coilovers and created spreadsheets to evaluate different configurations. Perhaps I can modify them to work as a tool for the trifive configurations.

The importance of choosing a configuration that will meet your car needs is needed too. Addressing what I think is the major use, driving a safe comfortable, good handling car we would look at the desired natural frequency of the front and rear suspension systems. One source is
and using his chart for my guess** of a desired natural frequency of 1.35 hz or 80 cycles per min we see that to support our example 835lb corner weight we would need a vertical spring rate at the wheel of 150lb/in.
The first Moog spring above has a 328x.61=200 lb wheel rate which would be about 93cpm or 1.55hz The second Moog spring has a wheel rare of 315x.61~190lb/in and would give the system a natural frequency of 90cpm or 1.5 hz.
**My guess is gleaned from what modern cars seem to be set up as. I believe the rates are higher for the original trifive's because they did not have the benefits of the very good shock damping technology and anti roll bars that we can use today.

This is just a starting point for a discussion that can make our rides ride better. I have made some assumptions and using scaled geometry probably has some errors, but the gist of this is that this front spring stuff is not a mystery but can be figured out. I welcome comments and criticism and encourage us to get some hard data, such as vehicle weights (total, corners, loaded and unloaded, and unsprung),spring data I have tried to get aftermarket upper & lower control arm geometry with spring position and available spring length geometry from several vendors to no avail so far.

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Interesting post. I'm anxious to see where you take this - right now you've just attempted to describe what's there.

Your scaling of the shop manual figure agrees pretty much with GM published information, as well as the Moog springs.

From GM published data for 55 models, the "standard" spring rate at the spring is 311 lb/in, and the spring rate at the wheel is 109 lb/in. This gives a spring rate ratio of 2.85, and a force/displacement ratio of 1.69. You calculated the inverse at .61, which inverted is 1.64. Or another way GM's equivalent of your 0.61 is is 0.59.

Optional spring rate for 55 models is listed as 338 lb/in. So these compare well to the Moog 656 at 315 lb/in and the 638 at 328 lb/in. Not enough to quibble about.

I've found that the free length of the lighter 55-56 springs is about 17". I think that GM measured free length differently than overall maximum to maximum or wire center to wire center at the max point, and Moog may have a different method too. So I'm not real sure your ride height calc's are on the money.

There were actually several springs available for 55s other than the two mentioned, and even more for 56-57.

The GM data that I have is from the book "Chevrolet 55-56 Restoration Guide" by Aregood. Other spring data is in the assembly manuals for the 3 years.

I have not gone through your frequency calculations or studied your link, but will do so if the discussion progresses.

As far as aftermarket control arm geometry, as far as I know it's mostly stock except for caster mods. Unless you move the inner pivots, the only real mod you can do to the geometry is a taller spindle. Jim Meyer Racing does have a bolt on setup with rather short control arms, so those are probably theoretically worse but I don't know how bad. Generally to see different and improved geometry, you're going to have a full frame clip (like a C4 conversion) or a complete aftermarket frame (like the AME).
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