The diameter of the coil on my son's 72 1.7 is 10mm. The diameter of the coil on my (formerly heavily auto-x'd) 70 six is 12mm. Was original 10mm and how much of any upgrade was the bump to 12mm? Technically speaking, how are the springs rated?

In the 1973 dealer information introduction guide, it tells about the new springs have 3 green stripes so you could tell early version from the new late ones. Do not remember the diameter information

I think most folks refer to spring rate vs coil diameter. iirc, stock 1.7 was 70# rear springs and the stock 6 was closer to 100#.

More complicated than you might think.

Broadly speaking springs are generally rated by two parameters:

1) Spring Rate -- as measured in pounds of force per inch of spring comporession.
2) Check Load -- this will the # of lbs force at some particular compressed length.

There are a ton of other factors that affect these parameters:

Free length
# of coils in the spring
Diamter of the coils
Max compressed length before coils clash
Type of steel used, and how it is heat treated
Peened or not peened
Etc.

If you order a set of 914 specific springs, most parameters have been managed by the spring supplier and you are left to choose the spring rate.

Typical rates for 914's vary from about 90 lb/in to 140 lbs/in.

Check load has been set by the spring supplier based on the nominal corner loading of a stock 914 to yeild a nominal ride height. If you have a massively heavier or lighter than stock set up, you will have to manage check load (ride height) in addition to spring rate.

If you want a much deeper dive into suspension design and how spring rates affect ride frequency and damper tuning, here is a good guide. It is aimed at college students participating in Formula SAE design competitions.

http://www.kaztechnologies.com/wp-content/...im-Kasprzak.pdf

@Superhawk996 - nice article on the topic! Thx for posting.

- Tony