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DeadSkunk UberDork
1/18/18 7:56 p.m.

For your Challenge car just use the set of springs that are the thickest wire or the least number of coils. They'll be the stiffest set. I'm assuming most of the sets came as matched springs and aren't a hodgepodge when I suggest that. Most sets should have front springs about 50% stiffer than the rears.

akylekoz HalfDork
1/19/18 5:33 a.m.

In reply to GTXVette :

Good idea, the load cell and readout are all you need.  Then my kids can still use the press to squish stuff, not really they can use the shop press for that.

GTXVette Dork
1/19/18 12:28 p.m.

Lucky kids,  All we got to Squish was pennies and we had to use a Train for that.

Jumper K Balls
Jumper K Balls PowerDork
1/19/18 12:36 p.m.
Driven5 said:
GameboyRMH said:
Driven5 said:

The equation for spring rate is k=(d^4*G) / (D^3*N*8) where k is the spring rate, d is the wire diameter, G is the modulus of rigidity (~11.2*10^6 psi for steel), D is the diameter of the coil (center to center on the wire), and N is the number of active coils. 

This is exactly the approach I must recommend against. It's just too easy to get wrong. For example, even if you managed to get a set of perfect measurements, you could use this on a chrome-silicon alloy spring and get it all wrong.

Unless you've got something really exotic, I believe there most common spring alloys are all in the same 11.0*10^6 to 11.5*10^6 range. So that should mostly be a non-issue. Accurate dimensions just require a cheap Harbor freight caliper. The least accurate part is the number of active coils, but even that isn't hard to get close enough. I've only done it a couple of times a number of years ago, but don't recall having any problem getting in the right ballpark.

I agree that the formula works well for most common springs. I had a set of mystery 2.5 ID springs and got out the caliper and did the math and wrote the results on the springs in sharpie. I calculated 346 in/lb. I later looked on the ground portion of the bottom of the coil and found a small 350 engraved in it.

akylekoz HalfDork
1/19/18 12:58 p.m.

In reply to GTXVette :

I may still have some of those squished coins, isn't it amazing that they never traveled too far.

After looking at this thing, the only part that makes it big it the 50" upright rods.  If those were removed and later replaced it isn't all that big, just heavy.

Best idea is to just ship the load cell and readout for use in a shop press.  The electric pump/cylinder unit is cool though.


GTXVette Dork
1/19/18 1:24 p.m.

Yes it's Very Cool.  Only someone that squished pennies would know that , I know I lost a Couple but hunting for them WAS part of the Fun.  Yep the Big isn't the problem it's the Heavy.  If you think you can box it up  so not to let the Carrier's DESTROY it ,Michel and I want it .

Thank you. John

akylekoz HalfDork
1/19/18 8:32 p.m.

I’ll get it boxed and weighed next week.  It’s at work.

GTXVette Dork
1/20/18 6:03 a.m.

In reply to akylekoz :

YOU ARE AWESOME !!!!!!!!     When I get it I will try to make It Portable, and bring it where ever I go,   Aka Challenge or other Events.

akylekoz HalfDork
1/25/18 2:39 p.m.

I haven’t forgotten about this, busy week.  Tomorrow I’ll break it down, test it and get a weight.

clshore New Reader
1/25/18 3:23 p.m.

In reply to Driven5 :

A coil spring is just a torsion bar that's been wound around into cylinder.

The spring rate formula given is for a perfect theoretical spring, and ignores a multitude of practical real world factors:

d - Spring wire is NEVER perfectly round, just winding it into a coil distorts the cross section. What about surface effects, rust, paint, surface finish? And since it's raised to the FOURTH power, so any small variation in size, cross section, or measurement is magnified. (a 5% variation yields over 20% difference result when ^4)

G - Note that this 'constant' is specified with leading '~', meaning it's approximate to only 3 digits, but plus/minus how much by actual alloy, by processing of the batch, by operating temperature, etc.

D - Again, how precise is the measurement, and how does the dimension change when the spring is compressed or extended (yes they do that), and it's to the THIRD power, any small variation in measurement is magnified. (a 5% variation yields over 15% result difference when ^3)

N - How do you precisely determine active coils?  Are the ends 'dressed', ie end coil pitch is decreased and ground flat? Do the ends touch the next coil and bind when the spring is compressed? If not dressed, how well do the fitted spring end pockets fit, ie where and to what length does the pigtail end 'bottom out' against the pocket when compressed? Certainly more center coils improve accuracy, but with N <10, expect about 10% variation.

Given all that, I'll take an actual measurement over a theoretical calculation any day.


Driven5 SuperDork
1/25/18 8:31 p.m.

In reply to clshore :

Making mountains out of molehills. 

Sure actual measurements would be ideal. But for all of the people who don't happen to have a handy dandy automotive spring testing setup in their garage, rough calculations are generally going to be more than adequate...And regardless, is pretty much the only other viable alternative anyway.

How many people have measured the exact motion ratios of their suspension through its full range of travel, and the exact actual roll stiffness contributions of their sway bars? For anyone who hasn't, approximate relative numbers should be nearly as useful as exact absolute numbers.

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