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Coilover Theory 101: Coil Spring Comparison

 
Old 09-03-2018, 09:20 AM
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Exclamation Coilover Theory 101: Coil Spring Comparison

Hey guys,

we get asked questions about coilovers, how they work, how they are manufactured, valving theory etc etc on a daily basis, so this gave me an idea; why not make a 101 separated into several, easy-to-read parts?

If you like where I am going with this, please do let me know and what "topics" you would like me to cover, and I'll see what I can do

one of the questions that we get asked quite often by our customers is what is the difference between Swift Springs and the generic Taiwan springs (found on nearly all non-European coilover kits), and what makes them so much more expensive?

to answer these questions, we decided to do a comparison test, and the results are below. What you make of these results, that's for you to decide:

Side-by-side comparison - Swift 7" 10K on the left, generic Taiwan 7" 10K on the right. Notice the difference?


Testing the Spring Rate accuracy on our Intercomp Spring Rate Tester, the Swift is exactly 10K:


And for the generic Taiwan spring, it comes in at 10.25K (15lbs stiffer than rated):


Both are pre-loaded 1" then compressed 1" to get the measurements. Can you say "coil-bind"?

Some have questioned us about the accuracy of the Intercomp Spring Rate Tester, so instead here is a dyno graph of a 100lb sample Swift Spring from our Roehrig Spring Rate Tester, showing the Spring Rate (lbs/in) over an increasing compression force (lbs):


For the weight test, the Swift comes in at 1.1kg:


And the generic Taiwan spring comes in at 1.6kg:


The conclusion from all of these tests? Not only are Swift Springs 50% lighter and extremely accurate (you ask for 10K, you get exactly 10K), they also help reduce coil-bind (i.e the coils hitting each other over large bumps). The Taiwan spring? Maybe after 6 months of driving, it will finally be 10K, then 6 months later, 9K!

I've deliberately made this post short and sweet, so as to encourage questions, so if you have any, please feel free to ask!
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Old 09-03-2018, 10:58 AM
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just looking at the black spring, you can tell it will most likely bind.
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Old 09-03-2018, 11:05 AM
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I'll be doing some tests on older/used springs, as well as other brands, so I can post those up as and when as well, if the community wants me to

We're an official Ohlins service centre, so get quite a lot in to rebuild, although the results would just be the same as either the Eibach or Eibach Pro's that they use/re-brand.
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Old 09-03-2018, 02:51 PM
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Nice, in for more. Data is nice. Data with an explained solid testing methodology is even better.
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Old 09-03-2018, 03:12 PM
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Dumb question, I'm sure, but for us suspension neophytes, how does 557 translate in to a 10k spring?
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Old 09-03-2018, 03:21 PM
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Originally Posted by arbinshire View Post
Dumb question, I'm sure, but for us suspension neophytes, how does 557 translate in to a 10k spring?
Lbs / in vs Kg / mm.

1 lb = .454 kg, and 1 in = 25.4 mm.

Beyond that, it's just math. Several online calculators and charts exist to ease the conversion. Example: https://www.redline360.com/garage/sp...-kgmm-to-lbsin

kg/mm to lbs/in
-----------------------------
16 = 896
15 = 840
14 = 784
13 = 728
12 = 672
11 = 616
10 = 560
9.0 = 504
8.5 = 476
8.0 = 448
7.5 = 420
7.0 = 392
6.5 = 364
6.0 = 336
5.5 = 308
5.0 = 280
4.5 = 252
4.0 = 224
3.0 = 168
2.0 = 112


From the above chart, 557 lbs / in equals 10 kg / mm for all practical purposes.
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Old 09-03-2018, 09:26 PM
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Originally Posted by MFMike View Post
The conclusion from all of these tests?...The Taiwan spring? Maybe after 6 months of driving, it will finally be 10K, then 6 months later, 9K!
It might be true, but that conclusion doesn't follow from the tests you've listed here...
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Old 09-03-2018, 10:52 PM
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Originally Posted by Junkwhale View Post
It might be true, but that conclusion doesn't follow from the tests you've listed here...
That is true, and was a general statement (based on past experiences). I will edit that out if you like as me making a conclusion is not what this thread is about.

We will be testing different rates, different brands and different degrees of mileage, which will provide more data for you guys to ponder over. Again, any conclusions from the data provided in this thread is done by you, not by me; I'm just providing the data (which is still under question whether you guys want me to continue or not)
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Old 09-04-2018, 12:17 AM
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These are really good posts Mike, keep them up mate.

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Old 09-04-2018, 10:04 AM
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Great Thread. Thanks for doing this. Perhaps some comparison between the various springs used between the DIY bilstein crowd. A lot of these builds have everything from "ebay specials" to summit brand, qa1, hyperco and even swift springs. Would probably be helpful for us in determining which springs to go with there.
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Old 09-04-2018, 10:10 AM
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Originally Posted by sometorque View Post
Great Thread. Thanks for doing this. Perhaps some comparison between the various springs used between the DIY bilstein crowd. A lot of these builds have everything from "ebay specials" to summit brand, qa1, hyperco and even swift springs. Would probably be helpful for us in determining which springs to go with there.
Yup, that's the plan
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Old 09-04-2018, 10:47 AM
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I use Summit springs. If they prove to be 15% off from advertised spring rates they will still be more accurate than my driving, lol.
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Old 09-04-2018, 11:52 AM
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That was a 15lb difference which is a 2.7% difference. But you raise a very good point, exactly at what point does the margin of error result in a slower race car or at least noticable handing issues?
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Old 09-04-2018, 12:23 PM
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The point that hopefully was being made here was that it isn't about the actual difference in this particular test, but more about "consistency". Of course, I would need to run more tests to prove that point. It's coming, I just need to find the spare time to do it.

If the spring rate is consistent between springs (which Swift themselves guarantee), then you don't have to worry about your dampers/springs not being matched. Basically, you have 4 springs on your vehicle and if they are all slightly different rates, that is never a good thing. Also, your dampers are valved to your vehicle setup and spring rate (well, they should be anyway. That is what I'm planning on writing up in the 2nd part of the 101)
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Old 09-04-2018, 12:57 PM
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That's awesome that you'll be doing some of the other brands. (I have summit brand springs on my budget bilsteins because I was being as cheap as possible on my daily) I'm looking forward to your results and how they all measure up. I wasn't very surprised but Swift being right on the spring rate, I was surprised by the .5kg lighter, that's a pretty decent difference
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Old 09-04-2018, 01:07 PM
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While you're educating, why not delve into why the swift is fewer coils and the taiwan is more coils. Is it material? wire diameter (hard to see a difference, but paint thickness may be different too)? or both?
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Old 09-04-2018, 01:40 PM
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Originally Posted by y8s View Post
While you're educating, why not delve into why the swift is fewer coils and the taiwan is more coils. Is it material? wire diameter (hard to see a difference, but paint thickness may be different too)? or both?
I'm not educating here; I'm simply providing data (gathered in my spare time i.e lunch break) and presenting it to the community. I did make my own quick conclusion (which in hindsight, I shouldn't have done now), however, how you interpret that data is up to you.

Sorry if that wasn't the reply you wanted. I've just been blasted on another forum, accusing me of blatantly advertising Swift Springs...
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Old 09-04-2018, 01:50 PM
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Plenty of ways to skin a cat



k= spring rate
d=spring wire diameter
G=shear modulus
D= Diameter of spring
N=number of active coils
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Old 09-04-2018, 02:19 PM
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Originally Posted by Scaxx View Post
Plenty of ways to skin a cat



k= spring rate
d=spring wire diameter
G=shear modulus
D= Diameter of spring
N=number of active coils
So from that formula, with "k", "d" and "D", being equal, in order to get the same "k", a spring with a lower "G" will require a higher number of "N"? Or the other way around?
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Old 09-04-2018, 02:47 PM
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Originally Posted by MFMike View Post
So from that formula, with "k", "d" and "D", being equal, in order to get the same "k", a spring with a lower "G" will require a higher number of "N"? Or the other way around?
Lower shear modulus (lower G) = less torsionally stiff wire = fewer wraps to match spring rate (lower N), all else being equal.
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