Springs - Part 2

In my last post, I mentioned I would be discussing:

1. Figure out the stock spring rates and free lengths.
2. Figure out the compressed length of the springs with the car riding at stock ride height (as described in the owner's manual.)

I'll do part 1 first; I need to get under my car to be able to do part 2.

The information to figure out the stock spring rates and free lengths can be found in one of of the books I purchased last month at Books4Cars.com, The Alfa Romeo Technical Characteristics and Principal Inspection Specifications Manual for the 2000 Berlina, 2000 GT Veloce, and 2000 Spider Veloce. This manual is very nifty, as it provides (as the name suggests) a lot of specs numbers regarding a bunch of aspects of my car. Among these, we can find (guess what): spring free lengths, and compressed lengths for given test loads.

First, free lengths:

The front spring's free length is listed as 313.5mm; the rear spring's free length is listed at 445mm.

Next, spring rates:

The spring rates are not listed directly. However, the manual shows values of length under test loads(i.e. compressed length) for various test loads depending on the kind of stock springs installed on the car. It seems the GTV had five varieties of front springs and three varieties of rear springs installed at the factory (one assumes lucky customers would end up with slightly stiffer springs than less-fortunate brethren.)

For the front springs, the "Length under test load" (i.e. test compressed length) is 200mm. The test loads for the various stock springs are:

- 858.5kg - 868kg (Spring ID no. 43)
- 869kg - 879kg (Spring ID no. 44)
- 880kg - 890kg (Spring ID no. 45)
- 891kg - 901kg (Spring ID no. 46)
- 902kg - 911.5kg (Spring ID no. 47)

For the rear springs, the "Length under test load" is 252mm. The various test loads are:

- 280kg - 285kg (ID no. 18)
- 286kg - 292kg (ID no. 48)
- 293kg - 298kg (ID no. 49)

I suspect these ranges exist due to the variation in tolerances allowed during the manufacture of the springs used in GTVs. The factory probably winds a few hundred spring coils, tests them, and depending on the resulting rates, it stamps a different ID number on them.

How is this useful?

Well, these numbers indicate how much the springs are compressed under a standard load. This information can be used to determine the spring rates values for the stock springs.

First, the front springs.

Difference in length = Lf - Lc
(Lf = Free Length, Lc = Compressed Length for a given load)

Lf - Lc = 313.5mm - 200mm = 113.5mm

From my previous post, the spring constant (k) is defined as:

k = load/Lf-Lc

For the various springs listed above, I'll use the midpoint values for computing the spring rate (k)

Spring ID no. 43, k = 863kg/113.5mm = 7.604 kg/mm = 425.778 lb/in
Spring ID no. 44, k = 874kg/113.5mm = 7.700 kg/mm = 431.205 lb/in
Spring ID no. 45, k = 885kg/113.5mm = 7.797 kg/mm = 436.632 lb/in
Spring ID no. 46, k = 896kg/113.5mm = 7.894 kg/mm = 442.059 lb/in
Spring ID no. 47, k = 907kg/113.5mm = 7.991 kg/mm = 447.486 lb/in


For the rear springs:

Lf - Lc = 445mm - 252mm = 193mm

For the various rear springs listed above:

Spring ID no. 18, k = 282.5kg/193mm = 1.464 kg/mm = 81.965 lb/in
Spring ID no. 48, k = 289kg/193mm = 1.497 kg/mm = 83.851 lb/in
Spring ID no. 49, k = 295.5kg/193mm = 1.531 kg/mm = 85.737 lb/in


So, to summarize, here are the specs for stock springs (using mid-point values):

Front: spring rate = 436.632 lb/in, free length = 12.343 in
Rear: spring rate = 83.851 lb/in, free length = 17.512 in

In metric:

Front: spring rate = 7.797 kg/mm, free length = 313.5mm
Rear: spring rate = 1.497 kg/mm, free length = 445.0mm


An observation: I suspect the folks at Alfa Romeo probably sourced springs with rates of 7.8kg/mm and 1.5kg/mm for the front and rear ends, respectively. If so, we can use these values whenever we talk about stock spring rates. This converts to about 437 lb/in and 84 lb/in for the front and rear springs, respectively. I'll settle on these values when refering to "stock spring rates."

One thing down. Next, we need to figure out the compressed length of the springs when the car is riding at the stock ride height. How is this done? Stay tuned...