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Spring rate tester?

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Old 01-11-06, 09:15 AM
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Spring rate tester?

I want to verify the rates of the stock springs on my FD. I was going to take them to a racer shop but then had an idea. I own a hydraulic press already. Why couldn't I sit a scale on the press, place the spring on top of the scale and then use the press to compress the spring? All I'd have to do is meaure off increments of spring displacement and note the weight shown by the scale for each increment.

Surely someone has done this? Is a bathroom scale for fat people good enough?
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Old 01-11-06, 09:17 AM
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Whoa... All I'd have to do is borrow my buddy's corner weight scales and use one of those. Stay tuned!
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Old 01-11-06, 09:39 AM
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Originally Posted by DamonB
I want to verify the rates of the stock springs on my FD. I was going to take them to a racer shop but then had an idea. I own a hydraulic press already. Why couldn't I sit a scale on the press, place the spring on top of the scale and then use the press to compress the spring? All I'd have to do is meaure off increments of spring displacement and note the weight shown by the scale for each increment.

Surely someone has done this? Is a bathroom scale for fat people good enough?
Damon - you are correct. Really, all you need is a method to measure force and displacement. A while ago, I had tested a few sets of springs using a standard SATEC load-testing machine. My plan was to test all of the available FD springs for spring rate, lateral rate, float displacement, and envelope circles (all of which usually differ from what the manufacturer states). But my project got cut short when I got side tracked with other jobs.

Anyway, couple of points to caution you on. First, you'll want to compress the springs as much as possible to get a good displacement range. Since you want some degree of felxibility, I would suggest using a scale that can measure somewhere around 1500-2000lbs. So a set of corner weight scales could probably be adapted to work (a bathroom scale, even for fat people, probably wouldn't do the trick ).

Second, as you compress the spring, it has a tendancy to "bow" and likes to go shooting across the room (ask me how I know...). So you really should build a small rig to contain the end points and provide some safety against wandering springs (and also to help distribute the load on the scale slightly).

Good luck, let us know how it turns out!


-Rob
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Old 01-11-06, 10:31 AM
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Originally Posted by saxyman990
First, you'll want to compress the springs as much as possible to get a good displacement range. Since you want some degree of felxibility, I would suggest using a scale that can measure somewhere around 1500-2000lbs. So a set of corner weight scales could probably be adapted to work (a bathroom scale, even for fat people, probably wouldn't do the trick ).
Yeah, I figured I want to measure over the complete motion of the suspension. The fat people scale would max out after about an inch and a half

Originally Posted by saxyman990
Second, as you compress the spring, it has a tendancy to "bow" and likes to go shooting across the room (ask me how I know...). So you really should build a small rig to contain the end points and provide some safety against wandering springs (and also to help distribute the load on the scale slightly).
Hadn't thought of that. Thanks. I guess I could attach a couple chains but most of all I need to protect the scale somehow since it isn't mine! Or I could just build simple adaptors and bolt the whole damn strut in there if my press is tall enough. Then the spring can't go anywhere. I'll have to look at that.

Thanks again.
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Old 01-11-06, 01:35 PM
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My racecar builder buddy tells me I should just build it like the real thing. Basically a hydraulic cylinder with a pressure gauge on it. Know the diameter of the piston in the cylinder, read the pressure on the gauge, compute the spring rate. Duh. Cylinders are cheap. I'm going to look for one...

He plans to build a shock dyno that is air powered. If you know the size of the piston in your air cylinder and know the pressure of the air inside the cylinder at all times then all you have to do is measure how long it takes the shock to travel a particular distance; easily done with a simple electronic timer mechanism. This will give you shaft velocity and knowing the pressure and size of your cylinder will tell you how much force it took to acheive that velocity. Keep repeating the test at various air pressures and there's your dyno graph.
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Old 01-11-06, 04:57 PM
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I've done the bathroom scale/press thing to measure springs. You don't need to compress it to coil bind, just take it up to about 200 or so pounds on the scale and meaure the deflection. I made a cup to hold the top of the spring and used a 2x6 to spread the load across the scale.
Also, those shock dynos work great, have one of those too.
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Old 01-13-06, 12:03 PM
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I've got an electronic scale here at work that says it will read up to 400 pounds. I'm going to try measuring my stock springs with that one since all I want to know is if they're soft or not. This way it won't cost money right now.

Looks like for about $50 I could build something from a surplus hydraulic cylinder though. All you need to know is the piston diameter of your cylinder. Fill the cylinder with fluid and screw a pressure gauge into it where the hose would normally have gone. If you had a piston that was 1 square inch in diameter you wouldn't even have to do any conversion from what the gauge reads; PSI would equal spring rate in that case.

Thanks for the help guys.

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Old 01-13-06, 12:32 PM
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Originally Posted by jgrewe
...You don't need to compress it to coil bind, just take it up to about 200 or so pounds on the scale and meaure the deflection...
Sorry, but I don't agree with this statement at all. In order to properly test a spring, you need to measure it over as much displacement as possible (usually this is the working displacement of the spring, not necessarily to the point of coil bind). Almost all automotive springs, even if they're listed as "linear," show progressive properties. The spring rate given by the manufacturer is usually just the median spring rate measure over it's working displacement, or the rate indicated by a "best fit" line.

If you only measure the first 200 lbs it's highly probably that you will be measuring the area of the curve that has the largest deviation from the listed rate. In other words, it will not give you very useful data.

Let me see if I can dig up some of my old data to show as an example...

-Rob
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Old 01-13-06, 12:54 PM
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Originally Posted by saxyman990
In order to properly test a spring, you need to measure it over as much displacement as possible (usually this is the working displacement of the spring, not necessarily to the point of coil bind). Almost all automotive springs, even if they're listed as "linear," show progressive properties.
I just finished reading about measuring springs and what I read supports that. I wouldn't have guessed measuring springs had so many pitfalls!

To minimize all these problems my goal will be to fit the entire shock/spring assembly in my press. That way the spring is supported just as it is in the car and the free length of the assembly is controlled by the shock. I'll measure as much of the range of travel as my scale will allow.

http://www.stockcarproducts.com/spgtech.htm

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Old 01-13-06, 01:20 PM
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Thanks for that info, I knew that the wheel rate changes with suspension movement but not the spring. I use the press to check my math on the spring dimensions. Maybe I should have sombody else test the springs on the press and not tell them what number I got by doing the math! I may have gotten the number I figured and told myself how great I am at math and measuring with a dial caliper only to be off

Damon, let us know if you do some testing and find that the rate changes enough to matter given the steps in rates of springs offered(10,20,25lb increments)

I've had them come out within a couple pounds between the math way and the squish them/measure them method.
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Old 01-13-06, 02:58 PM
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Originally Posted by jgrewe
Damon, let us know if you do some testing and find that the rate changes enough to matter given the steps in rates of springs offered(10,20,25lb increments)
I will. Hopefully I can get it done this weekend if I finish my other chores in time

My problem now is that I'm measuring stock springs and so I can't say exactly how they were rated to start with. I know what Mazda said the rates are but after reading the above article it seems you could quite easily get different numbers depending on how exactly you measure the rate! Hopefully I can fit the entire shock in the press. I'm pretty sure I can make it work.

All I want to know right now is if my original stock springs have gotten soft. I know they're sagging a little (which is fine; low is good) but if their rates are low then I'm going to replace them.

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Old 01-16-06, 08:34 AM
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Results

I verified my spring rates this weekend. These springs are original and have approximately 140,000 miles on them. They've seen over 100 autox events and probably a dozen track events.

My limitation in this test was that my scale only read to just over 400 pounds. This determined over what range of motion I could test the springs. I built a simple rig in my hydraulic press with the scale rests on the platform and the spring above. The press then displaces the spring and the scale outputs the weight. I rigged a dial indicator to the rig so I knew how much I had displaced the spring.

Here are shots of the rig with the entire left rear shock and spring installed and ready to begin. Not wanting to harm the mounting ears of the shock I rested the shock body on a scrap piece of steel laid across the press plates which are resting on the scale. The vertical perforated steel post behind the press was merely the mount for my dial indicator. My dial indicator only measures 1" at a time and luckily the post is perforated every inch as well. After each inch of travel I'd just move the indicator down the post and re-zero it before continuing.

Keeping the assembly completely plumb and centered under the press is a necessity for good readings.
Attached Thumbnails
Spring rate tester?-rate1.jpg   Spring rate tester?-rate2.jpg   Spring rate tester?-rate3.jpg  

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Old 01-16-06, 08:42 AM
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With the entire rear shock assembly in the rig I read:

displacement/weight

left rear:
.25"/370#
.5"/415#

right rear:
.25"/368#
.5"/415#

Unfortunately my scale only reads to 400#. Since the spring is preloaded so much when installed on the shock I couldn't get enough readings to be useful IMO. The two rear springs did appear to be well matched so I removed one spring from the shock and tested it alone while resting in the stock spring seats.

The spring is carefully assembled in the seats and the press preloads the spring just a tiny fraction to remove any free play. At that point I zero the scale and the pointer and begin measuring at .25" increments.

Here's that pic.
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Spring rate tester?-rate4.jpg  
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Old 01-16-06, 08:54 AM
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measuring the rear spring alone:

displacement/weight

.25"/32#
.5"/68#
.75"/105#
1"/141#
1.25"/181#
1.5"/223#
1.75"/264#
2"/309#
2.25"/355#

If you compute the rate over these different ranges you'll find my spring rate rises some. This is due to the preload changing is the spring is compressed towards its actual installed free length on the shock. If I choose the highest reading (most preload, resembles closest to stock) I find a rate of 174# for this spring. The stock rear spring rate is 193#. I've seen lots of different numbers but if you compute the rate mathematically using the data in the FSM it comes out to 193# so that's what I believe.

If I go all the way back to the original two measurements I made with the spring assembled on the shock I measured a 45# change in rate over .25". If I extrapolate that out I get a rate of 180#. I'm more tempted to believe the 180# number even though it's only one measurement over a quarter inch because the spring is already preloaded and this is how it's installed on the car. My numbers with the spring free show the rate rising as the spring is further and further preloaded so I actually believe they agree with the trend.

If my results are true than the rear springs have lost 7% of their rate.

Last edited by DamonB; 01-16-06 at 09:00 AM.
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Old 01-16-06, 09:05 AM
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I repeated the same tests with the front springs.

Spring assembled on shock:

.25"/337#
.5"/390#

If I extrapolate those numbers I get a rate of 212#. Using the data from the FSM and computing the rate mathematically they should be 270#. That's a loss in rate of over 21% !

With the spring free I measured:
.25"/48#
.5"/101#
.75"/156#
1"/212#
1.25"/270#
1.5"/328#
1.75"/387#

If you compute the rates with the spring free you again see the rate rising as the spring is further preloaded.
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Old 01-16-06, 09:19 AM
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What did I learn from all of this? First read this article at Stock Car Products. It explains everything and more about springs and their measurement. My results are exactly as that article said they would be.

If you are really going to take the time to measure a spring the only way to get accurate results for how it will act on the car is to first preload the spring to it's installed height on the shock. If you're one of those guys whose springs rattle in the seats when the car is jacked up then your springs are not preloaded and you don't have to worry about anything. My results prove that if you try this with a spring that is heavily preloaded when installed your results won't be reliable! Because of this I would be leary of directly comparing the published rates of stock type springs to 2.5" closed and ground racecar type springs. Because of all the preload in most stock systems your installed spring rate could easily be higher than what you think it is.

If my results are to be believed my front springs have lost quite a bit of rate. At this point I'm going to buy new stock springs for the front and I'll rate those before installing them to see if they measure higher than the ones I have now.

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Old 05-22-06, 03:49 PM
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I know I said I'd test the new springs before I put them on the car but I didn't do that due to lack of time. I put fresh springs on the front and rear and drove on them the first time at the ProSolo this past weekend. The old springs were most certainly shot. The car is much better now but it took me a few runs to readjust to it. On my first run into a fast slalom I found myself thinking "Holy sh*t. This is not my car!"

I have always heard about old springs sagging but had never bought into the idea that they could lose so much rate too. I have certainly changed my mind. The old springs had 140,000 miles on them.
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Old 05-22-06, 04:54 PM
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so no more pictures of your car riding the bumpstops on hard corners???
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Old 05-22-06, 05:34 PM
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I've done this as well. Plot your points on a graph. Get a straight edge out and you'll quickly see whether they're linear or not. Your rears for example are not quite linear but they are once you get around 150#. After that they're around 175#/in. That's what I would call 'em anyway. And of course you'd really want to do this a couple of times to verify that the set up is repeatable. And how accurate is that scale anyway?

I've tried what you've done as well. Except my scale only went to 250# so I didn't get enough data.

I'm fortunate enough to have access to some calibrated equipment at my workplace though. I've found Eibach springs to be pretty darn close to their advertised rating. Others..... well...... example: I bought some 150# rear springs for my FB. I measured them and found them to be about 170#. Another example: I bought some no name springs from Summit. Advertised as 300# and measured as 330#.
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Old 05-23-06, 08:52 AM
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Originally Posted by 7racer
so no more pictures of your car riding the bumpstops on hard corners???
It still does that, it just takes a little longer to get there I have some great pics from the Pro I need to post. A very nice photog was out there.
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Old 05-23-06, 08:57 AM
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Originally Posted by slowautoxr
And of course you'd really want to do this a couple of times to verify that the set up is repeatable. And how accurate is that scale anyway?
For the first spring I did repeat the test 3 times and got numbers within a pound or two each time so I'm confident the test was at least repeatable. The fact that the measurements wandered that slight amount I'm sure was due to the fact that I had the spring in its rubber mounts when measuring it.

I can't vouch for how accurate the scale is other than it says it's "certified"; whatever that means.
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Old 05-19-17, 01:11 PM
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homemadetools guy here, not a spammer (PROOF: 350+ forums that know us)

Just a note that we featured DamonB's clever spring rate tester in our HomemadeTools.net newsletter: HomemadeTools.net May 18, 2017

We know RX7CLub as a good source of homemade tools. We've got 13 homemade tools listed from you guys so far, each one of course fully credited to its builder and linked to its original discussion. Here they all are: Homemade Tools from rx7club.com - HomemadeTools.net

I hope we've been able to send you lots of traffic. Cheers
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