Equipment to measure spring rates

[James Doulton]

I have recently attempted to measure the spring rates of my Legerra while the springs were fitted to the car. In essence, I did this by putting the car on multiple sets of bathroom scales and jacking the car up a few millimetres at a time, measuring the wheel position with respect to the bodywork, and noting the readings on the scales. This seemed to give credible results which showed that my front suspension is stiffer than my rear. However, I measured the wire thickness on the two sets of springs and the rear ones are thicker, plus the leverage ratio is greater on the front, so it looks like my measurements are rubbish. So I am interested in a better method. How have other people managed this? Do you have pictures? 

[Mark Bagnall]

I'm happy to run dimensions through my spring calculator if you're interested in comparing with any measurements. The important dimensions are the wire diameter (measured with a micrometer or vernier) the overall diameter of the coils and then the number of turns in the spring (or a photo).

[2edvaw06pgbav]

I used a hydraulic press, a set of bathroom scales and a tape measure. The spring rate is defined as the weight required to compress the spring 1 inch. Simply put the scales on the press with the spring on them, zero the scales then use the press to compress the spring 1 inch. Read off the weight from the scales.

[Dave Adams]

i have some pictures .....possibly on here as well. i built a "A" frame like device with bathroom scales at the bottom and a bottle jack, the spring sits in a cup with a 10mm threaded rod up the centre, i will dig out some pictures and post on here again.

[James Doulton]

My two spring sizes are: front 8.25mm diameter, rear 8.95mm diameter. Both are 10 coils and 1.9" internal diameter (48.26mm). 

[Mark Bagnall]

Just run some numbers.  The rears are around 216 lbf/in and the fronts 161 lbf/in.

FYI the things that effect the accuracy of these calcs;

• The form of the ends of coil springs vary between "open" and "closed & ground" which change the number of active coils so, for example, the front springs could be between 194 and 242 depending on the end form.
• The wire diameter has a big effect on stiffness, so any problem with measurement like difficult access or corrosion masking the steel size will impact the calculated stiffness.
• The calculations I use are, from experience, pretty good but, for example, a spring manufacturer would have more accurate calculations for their materials and process.

[James Doulton]

Cheers Mark.

I have finally come up with a simple and safe way to measure the spring rate, seeing as I do not have a hydraulic press handy. I will use a spring of known rate and a long threaded rod. Firstly I measure the free length of the two springs. Then I will put the known spring and the unknown spring end to end with the threaded rod down the middle of them with a couple of end plates for the rod to pass through (taken of an old pair of dampers. Pop on a couple of nuts, one on each end of the rod, and tighten them. When I have compressed the combination by a couple of inches, I measure the respective lengths of the two compressed springs with a tape measure. By calculating how much each spring has compressed, and knowing the rate of one spring, I can calculate the rate of the other one because the rates will be inversely proportional to the compressions. I hope to try that tomorrow, when I have finished fitting my new radiator.

[James Doulton]

Well my simple and safe way to measure the spring rate didn't turn out to be quite so simple nor as safe. I did successfully measure my existing rear spring and got a figure of 166lb/in. That seems a little low compared to the calculations. Do springs change as they age?

The method didn't quite work as well as expected, I needed to stop the springs bending as that made it difficult to measure. Then when I went to undo the tensioning bolt the whole assembly 'exploded' because the whole threaded shaft was rotating and it came off the nut at the other end which was in my vice!

[James Doulton]

Yes, they are beautiful aren't they.

I got a pair of new Gaz dampers today from Rally Design (a good tip Richard) they had them in stock so I got them in 24 hours.

I have chosen 140/95B12 dampers for my Legerra, which are 2" longer than my existing Spax and give me an extra inch of travel. I chose 10" springs of 200lb/in. This choice was the result of lots of calculations in order to prevent the car from hitting the bump stops going over some of my local obstacles - I measured the ramps on the 'plateau' zebra crossings by measuring off photos (the ramps rise 90mm over 1500mm which is almost exactly 1 degree). I modelled the suspension travel using Excel and this confirmed that I am currently hitting the rear bump stops, but with my longer travel and different springs then I won't.

The object of the exercise is not to speed over zebra crossings, of course, but to make the car comfortable for normal driving. The local zebra crossing was a place that I knew the car didn't cope with well. There are many other occasions where it is beaten but I can't remember where. 

[James Doulton]

The rear coil-overs are pretty close to vertical, close enough to ignore any differences as far as I am concerned. My fronts are also fairly upright too and they bear on the suspension arm just outside of the top strut joint, so they are also pretty close to direct as well. (I had thought that the front leverage ratio was around 1.4, but now that I have just looked at it again I can see that it is about 1).