Understanding spring rates

Stop guessing at what springs to use. Soft or stiff are relative terms that really don’t mean a great deal. Get some base line data for what you have with this online spring calculator and figure out what you really need. In relative terms with front to rear difference the end of the car with softer springs will have more grip. So if you have understeer then stiffen the rear (yes sway bars can alter such dynamics too but it’s best to start with springs).



Thanks Mr Gromsby, once again I appreciate the lesson.
I have punched in the spec to the calculator and have discovered the springs from the Hi-Jet that are in the L60 are massively too stiff at 240 lb/inch.
I am slowly working out what I want and know the standard L60 spring rate is about 80lb.

How do we know What the springs are made out of for this calculator ?

If you have looked at the calculator it gives you the option to choose from a range of materials. It’s up to you to determine what your springs are. Normally 5160, 9260 or some sort of high-alloy low carbon steel. Just use the default in the calc if not sure. Compared to having “no idea” on rate it will provide a ball park figure. If you are really anal and worried that ordering a replacement would be a problem then the calc will at least give your percentage rate diff front to rear (assuming orig springs are factory or from same manufacturer). Then go and order a front and rear set from the same place with a diff percentage split.

You could take one of your springs to a lab that does x ray fluorescence testing and pay for their services.

To know the rate “for sure” I use a friend’s load cell that is set up in a hydro/pneumatic press just for this purpose.

If none of that works for you just go back to the subjective feel of “hmm that feels soft/hard”.

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Here is some more useful information regarding springs and showing other formulas for load, spring rate ect.

Here is a list of OEM springs by moog showing all specs and dimensions, the part numbers can be cross referenced with other OEM spring manufaturers

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Some more useful info on handling below.

It is always interesting seeing on facebok the "did it ‘cause it looks cool bro’ " mods. A little knowledge is dangerous. Just because you find some low springs and or have a cut off wheel on an angle grinder does not mean you should put those S13 springs in. And the huge wheel offsets that look good in a photo are a big step backwards - not just from a performance point of view but also an awful thing even to drive down the shops for milk.



A great rescource for fine tuning suspension, the tables describe how different adjustments affect how the car reacts. Great for Troubleshooting.


For those of you with M100 and L700 twisting beam rear ends. Look into the new Force Vectoring spring design used on the Ford Fiesta ST.



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Very interesting! I’m keen on the Fiesta ST, didn’t know it reverted to a twist beam rear axle.

Watched a couple of YouTube videos on lowering springs, the rear springs are more banana-shaped than the image above. Although the shots weren’t static etc, so hard to see fine detail. I was wondering if the aftermarket options would retain this and it seems the “ST” rear springs from KW have a slight banana shape to them - hopefully retaining the intent of the design but providing a lower height (if that’s what people are after).

Would be interesting if a retrofit added any benefit…hmmm…

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Not sure banana shape is completely necessary. I have seen a Winding design just needs to take side loads. They’d be horrendous if installed 180deg out.


Different idea. Would be interesting too drive one just to feel the rear end.
I would say the banana would be needed to take the load on the angle they use. Quite interesting.
Doesn’t worry me that much :slight_smile: My rear end is a 4 link

Could always retrofit the other car tho :thinking:

Four link or four link with panhard rod or watts linkage?

A quick guide for calculating the required spring rates (the method i used). It focuses on springing the car to a certain frequency.

          (4p² x fr² x m)/1000

The result will be the base spring rate in N/mm

‘p’ = pi (for some reason the pi symbol doesnt show)

‘fr’ = the frequency in Hz

daily= 1Hz
Sporty street = 1.5Hz
Track day (Semi slicks) = 2Hz
Race (Quality semi’s or full slicks) 2.5Hz
Over 2.5Hz sprung frequency is for cars with alot of aero downforce.

‘m’ = the corner weight (minus unsprung weight)

These figures can be used for a rough estimate


              m (front)  = (vehicle weight x 0.315)

              m (rear)= (vehicle weight x 0.175)


                m (front)= (vehicle weight x 0.295)

                m (rear)= (vehicle weight x 0.195)


               m (front)= (vehicle weight x 0.275)

               m (rear)= (vehicle weight x 0.215)

I should also mention the above formula is for a suspension mation ratio of 1:1 (McPherson strut for example) for other suspension types where the motion ratio is more than 1:1 you simply multiply the end result by the motion ratio.


Great info and interesting reading.
My car is cocking the rears at the moment, lucky enough the proflex units on my Charade use the same spring width and lengths as the coilovers on my Escort. I’ve a few different rate spares, now I have a rough baseline I can start changing them out from there.


Like this topic.

Triangulated four link and watts link is the best way imho for a solid axle rear end.
Probably need to screw with the springs on mine, but doesn’t seem too bad at the moment.
Using standard Daihatsu fronts and stock VN wagon springs in the rear. And with 55/45 balance they seem to be reasonable so far :smiley: Tried to spin it and it pulled itself straight the shit of a thing, so seem I might have done the right calculations for the car