21A1806 Hydrolastic Helper Springs 4

Measuring Spring Tension (Take 3)

Ok, finally got around to removing the petrol tank in preparation for putting in the later Rover SPI tank for the fuel injection. I changed over the L/H spring and had time to measure the old one. Here is a picture of the spreadsheet showing there is very little difference between the previously measured R/H one .

Helper Spring L/H, R/H & NOS Spring Rates
Helper Spring L/H, R/H & NOS Spring Rates

Here is the updated Hydrolastic Helper Springs spreadsheet.


As previously mentioned, I feel that my old springs are not that old at all as the difference is roughly 5%. It would be good to get  to measure a known ‘old’ spring to finally put this subject to bed.

21A1806 Hydrolastic Helper Springs 3

Measuring the Spring Tension (Take 2)

After measuring the NOS springs, I decided to the remove the right hand side spring (as it’s the easiest to get off) and profile it and the NOS spring using a different method.

What I did was to mount a Craftright Quick Action clamp in my vice with the movable clamp reversed so that when you moved the grip it went up the clamp rod so that it extended the spring along with the digital scale. Anyway a pictures worth a thousand words so here is some pictures.

Bottom of spring. Note black instead of orange… any ideas folks as to why?
Primitive indicator on top of spring. Don’t knock it, it works!
Top attached to bottom of spring scale.
Top of spring scale attached to movable clamp handle.
Don’t get any ideas about pinching my patented steady method!

I found the new and improved method a lot more repeatable due to the very fine movement offered by the grip handle. If you were wondering what the rope was for at the top, it’s to stop the thing flexing, and giving inaccurate results!


The measured spring rate of the old spring was 29.9lb/in a 5.2% difference. Not a vast difference at all. One thing is the old spring that came of definitely does not look like it is a 47yo spring, so maybe they have been replaced in a former life.

I will be looking at replacing both the front upper suspension arms (yes including the elusive L/H one which I’ve managed to source NOS), as these were repaired after the suspension knuckle wore the holes. The repairer at the time used some sort of epoxy to fill the holes. Unfortunately he didn’t shim the knuckles, with washers, to bring up the ride height. I also suspect the hydro bags have sagged, but other than swapping them for the rears, there is nought I can do.

Finally if anyone wishes to send me an known old spring so I can measure it that would be great.

Here is the Spreadsheet: Hydrolastic Helper Springs

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.

21A1806 Hydrolastic Helper Springs 1

The Search

Around the 23/09/2014 I thought I’d look for two NOS 21A1806 helper springs. The rational was that the Hydrolastic helper springs fitted to the rear end of my car had sagged, causing the car to sits with its bum in the air.

Initial requests on a local forum as to where I’d find these springs, suggested that I’d be hard pressed to find any as they were last used/made around 1973. I don’t like the word no, so I searched the internet high and low. I then remembered that the Italians had the Innocenti Mini.

Luck was on my side and I found two on the Italian EBay site. Money changed hands and I obtained two NOS springs in their original boxes.

Here’s some pictures. Note the black band on end of spring. Shouldn’t it be orange?

Look, original boxes!
Black instead of orange… maybe Italians don’t like orange?
The top shiny bits.
It measures up.

Next thing to do will be to measure both the new springs and also the old ones. Stay tuned.