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914World.com _ 914World Garage _ Checking spring rate
Posted by: bondo Jan 8 2006, 09:17 PM
I set the spring with a metal plate on it next to a wall. I made a mark on the wall at the top of the plate. I added 25 lbs, 50 lbs, and 230 lbs (me), and marked the wall accordingly. I then did the math to calculate pounds per inch, and I get around 120 lbs per inch. (for example, it deflected 1.9 inches when I stood on it)
All the measurements started with a nearly completely unloaded spring, which is not the situation it will see in a car. Is the spring effect linear, or do I need to install them in a 914 and see how much it dips when I jump in the rear trunk?
Posted by: SirAndy Jan 8 2006, 10:38 PM
| QUOTE (bondo @ Jan 8 2006, 07:17 PM) |
| or do I need to install them in a 914 and see how much it dips when I jump in the rear trunk? |
find someone with a spring tester ...
rich at HPH has one ...
Andy
Posted by: bondo Jan 9 2006, 10:00 AM
Can anyone at least tell me if I'm way off? Or suggest a closer spring tester? 
Posted by: Borderline Jan 9 2006, 10:09 AM
That's the principle. You should be in the ball park. Make sure the spring is linear...All the coil spacing is the same distance when unloaded. I have a set of springs with 3 coils that are wound very close and actually go solid when installed in the car. That changes the rate. I think they did that so the spring wouldn't fall loose when the car was jacked up as it often is. Good luck.
Bill
Posted by: bondo Jan 9 2006, 10:26 AM
| QUOTE (Borderline @ Jan 9 2006, 09:09 AM) |
| That's the principle. You should be in the ball park. Make sure the spring is linear...All the coil spacing is the same distance when unloaded. I have a set of springs with 3 coils that are wound very close and actually go solid when installed in the car. That changes the rate. I think they did that so the spring wouldn't fall loose when the car was jacked up as it often is. Good luck. Bill |
The coil spacing is even. I was expecting 180, which is why I ask.
Posted by: Aaron Cox Jan 9 2006, 10:35 AM
my afco coils say they are tested this way:
compress spring one inch. now measure how much it takes to deflect the second inch.....
Posted by: bondo Jan 9 2006, 11:01 AM
| QUOTE (Aaron Cox @ Jan 9 2006, 09:35 AM) |
| my afco coils say they are tested this way: compress spring one inch. now measure how much it takes to deflect the second inch..... |
Aha! Ok, I'll try that. Thanks!
Posted by: ppickerell Jan 9 2006, 11:29 AM
Aaron is right. Get the spring under some load at somewhere around 15% of the available travel. Record the load value(L1). Deflect one inch and record the load (L2). Subtract Load 1 from load 2 and multiply by 10. You don't want to measure rate in the first 15 or last 15% of available travel.
Posted by: bondo Jan 9 2006, 11:38 AM
| QUOTE (ppickerell @ Jan 9 2006, 10:29 AM) |
| Aaron is right. Get the spring under some load at somewhere around 15% of the available travel. Record the load value(L1). Deflect one inch and record the load (L2). Subtract Load 1 from load 2 and multiply by 10. You don't want to measure rate in the first 15 or last 15% of available travel. |
Why multiply by 10?
Posted by: blitZ Jan 9 2006, 12:09 PM
| QUOTE (bondo @ Jan 9 2006, 09:38 AM) | ||
Why multiply by 10? |
It's a 10 inch spring?
Posted by: ppickerell Jan 9 2006, 12:27 PM
Ooops, My bad, the springs I make are so f'in small we travel them .100 inches and multiply by 10!
Posted by: bondo Jan 17 2006, 04:17 PM
Ok, I tried it with more weight. I added weight in 50 pound increments, up to 300 lbs. I got more than 2" of compression. It looks like it's pretty linear.. I even graphed it. These are supposed to be 180 lb springs, what do you guys think?
Attached image(s)
Posted by: Mueller Jan 17 2006, 04:31 PM
| QUOTE (bondo @ Jan 17 2006, 03:17 PM) |
| what do you guys think? |
besides the fact that you are a geek???
looks like you are on the right track....
my method (I am not an artist)
Attached image(s)
Posted by: bondo Jan 17 2006, 04:53 PM
| QUOTE (Mueller @ Jan 17 2006, 03:31 PM) | ||
besides the fact that you are a geek??? looks like you are on the right track.... my method (I am not an artist) |
I have a pic to post when I get home.. You WILL laugh. That is what I did, but it wasn't nearly as elegant.
Besides my geekiness, doesn't it look more like a 120-130 lb spring than a 180?
Posted by: Mueller Jan 17 2006, 04:57 PM
| QUOTE (bondo @ Jan 17 2006, 03:53 PM) | ||||
I have a pic to post when I get home.. You WILL laugh. That is what I did, but it wasn't nearly as elegant. Besides my geekiness, doesn't it look more like a 120-130 lb spring than a 180? |
when I tested my springs ( a few different pairs), it seemed that with the correct amount of weight applied, the spring actually compressed a "tad" (1/4" or so) more than 180/1" or 360/2"
Posted by: bondo Jan 17 2006, 05:29 PM
| QUOTE (Mueller @ Jan 17 2006, 03:57 PM) | ||||||
when I tested my springs ( a few different pairs), it seemed that with the correct amount of weight applied, the spring actually compressed a "tad" (1/4" or so) more than 180/1" or 360/2" |
Hmm, 1/4" would put it right in the 180 lb ballpark. Why would 180 lbs per inch really be 180 lb per 1.25 inch?
Posted by: bondo Jan 17 2006, 06:59 PM
I think I may have figured out where the 1/4" comes from.. one of the things you do when calculating spring rate is count the "free coils". Setting the spring on something flat gives it more free coils than when the top and bottom most coils are more supported by the spring saddle and hat. If I can figure out a way to do it, I'll try again using the spring saddle and hat.
Posted by: bondo Jan 17 2006, 07:28 PM
Ok, here's the pic.. this is 300 lbs. Half the pipe is pounded into the ground.
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Posted by: bondo Jan 20 2006, 06:19 PM
Ok, I measured the spring and used http://www.miracerros.com/mustang/springs_calcs.htm formula to calculate the theoretical spring rate. (theoretical because not all alloys that are used for springs are the same) It comes out to 160 lb/inch. It also can't be exactly accurate because I measured the paint, which has some thickness.
Of course I'm not the type to be satisfied by math.. so I rounded up a spring tester.. of sorts. The displacement accuracy is .001.. the force accuracy is 1%. This is the sort of thing that satisfies me. 
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Posted by: bondo Jan 20 2006, 06:22 PM
Here is the graph of the data. (I did the graph a little differently this time.. makes more sense this way.. displacement vs. force) I tested both springs, as a double check. There's no way around it, this is a 140 lb spring.
Attached image(s)
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