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coil spring rate


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I have a coil, 7.5 turns, 291 mm (11 1/2 in) long, wire dia. 10 mm, and I.D 70 mm (2 3/4 in).

It has stamped JW on it.

It was fitted to that most respected of British supercars: the Triumph Dolomite :rolleyes:

Now I am guessing Rover/Triumph/BL/Whoever would not be making their own coil springs but buy them from a spring manufacturer so in theory there is a whole plethora of springs in this size (coil diameter).

The JW is either the manufacturer or more likely a code for the spring rate.

Does anyone know of a website - spring makers etc. that can give the spring rate for this spring, ideally using the 2 letter code? Or a website with spring data that I can use to extrapolate the rate?

Alternatively, is there a formula that calculates spring rates - I think I have all the data except for the material; but let's assume it is regular springy steel stuff.


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Assuming its not progressive

Measure the height of the spring

Add a known weight

Measure again

Do basic maths

For the known weight, you might find yourself and the bathroom scales appropriate.

If you can find a fat burd to help you out - your results will be more accurate, and you might have some fun ;)

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yeah, I'ver done the standing on them - not as easy as you may think as it is very instable (used a full herry can each side to support it).

anyways, I get between 125 and 150 lbs/in; I would like to know what they are meant to be though.

another way was to put it on top of scales on a drill press and measure that way.


now to give a bit more of the picture.

The second test was to measure the other spring fitted to car in free air and "when on the ground" (i.e. jack up under axle until chassis is hust off the stand).

Free length = 291

Fitted length = 250 (I have thought how can measure this better)

Compress by = 41

Change in length when loaded 15 mm

Total change in length = 56 mm

using 150 lb/ins, supported weight = 150 kg

using 125 lb/ins, = 125 kg

From SVA, rear axle = 282 kg.

Or 141 kg per side.

Now english axle, brakes, shafts etc is gonna be about 50 kg, wheel + tyre is 15 kg (my estimates) or about 40 kg per side.

So adding the unsprung mass, then I get 190 or 165 (cf measured 141).

close but still a few kgs out.

The point of all this is to see what is needed to get the rear end lower. There is a rubber buffer between bulkhead and top of spring shroud with 2 shaped washers that I reckon removing will give me about 25 mm - it is bad idea to do that as the top of the spring will move against the bulkhead but I could fit a thinner rubber buffer? (or I could cut the top of the mount and use a plate held by bolts that can be raised/lowered to adjust ride height)

Basically I think the set up in shock/spring is good enough so no need for new springs or shocks.


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