21A1806 Hydrolastic Helper Springs 2

Measuring the Spring Tension (Take 1)

I decided that it would be beneficial to measure both the initial tension and spring rate of the 21A1806 extension spring, so as to possibly help future owners of Hydro cars if they had to re-manufacture new springs.

The first ‘take’ of making a fixture to measure the spring parameters was not a total success. It used a pulley system in a 4:1 ratio with a Digital scale reading the values. It was very hard to get an repeatable read due to the ‘stiction‘ of the pulley .

Here is the set up:

Pulley arrangement.
Spring and digital scale.


I created a Spreadsheet to  work out the initial tension and spring rate. Take two of this exercise has this spreadsheet, but here is a excerpt:

Hydrolastic Helper Springs
Spring tension spreadsheet.

 The measured average initial tension was 3.3lb, and spring rate was 32.3lb/in. I haven’t seen any figures published for these springs.

Here is a spring calculator that I entered the spring measurements into. I guessed the material as hard drawn… don’t know if correct, but didn’t make much difference:

Spring Calculator 1
Enter spring type and dimensions.

Spring Calculator 2
Get answers and review any warnings.
Spring Calculator 3
Physical attributes.
Spring Calculator 4
Possible loads.


I think my NOS spring has not lost any significant initial tension, and very closely matches the values from the spring calculator. It will be very interesting to see what the old springs measure, when I get them of the car. My methodology is a bit crude, mainly because the Zenith awning pulleys have poor bearings (or lack thereof) and using a digital weighing scale has readability issues as the value jumps around as well as me not providing an even tension.

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