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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Technical Forums > Suspension | Brakes | Chassis > Coilover Theory 101: Coil Spring Comparison



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      09-03-2018, 09:12 AM   #1
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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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      09-03-2018, 10:49 AM   #2
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I would like to see a re-test after 6 months of use of both springs and see how they 'settle'.

If you have less travel to bump stops or shock bottoming than distance for coil bind, it doesn't matter.

It would be nice to measure the length to coil bind. It probably is not even an issue as suspension travel will be less.
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      09-03-2018, 10:57 AM   #3
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Quote:
Originally Posted by CobraMarty View Post
I would like to see a re-test after 6 months of use of both springs and see how they 'settle'.

If you have less travel to bump stops or shock bottoming than distance for coil bind, it doesn't matter.

It would be nice to measure the length to coil bind. It probably is not even an issue as suspension travel will be less.
Yes, we will be doing some back-to-back testing on some used springs as well And also other brands

Provided all things equal (the length and rate of the spring), the more sprint stroke you have, then the less chance of coil-bind. In the comparison image above, that was compressed only 2" in total

I have all the spring stroke figures etc for the Swift Springs. If you need a figure for a certain rate/length, let me know
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      09-03-2018, 01:21 PM   #4
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awesome. I'm so tired of all the kids recommending cheap garbage "BC/Fortune Auto/etc" coilovers (all basically the same thing with a different sticker on the box). You can hardly even buy decent struts/shocks for what some of those kits cost - but they probably don't know what coil bind is anyway, and don't care as long as it's low.

Marty, good spring steel doesn't really settle significantly. That's pretty much the reason they use steel and not whatever is in those cheap knockoff springs..
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      09-03-2018, 01:58 PM   #5
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Quote:
Originally Posted by hassmaschine View Post
awesome. I'm so tired of all the kids recommending cheap garbage "BC/Fortune Auto/etc" coilovers (all basically the same thing with a different sticker on the box). You can hardly even buy decent struts/shocks for what some of those kits cost - but they probably don't know what coil bind is anyway, and don't care as long as it's low.

Marty, good spring steel doesn't really settle significantly. That's pretty much the reason they use steel and not whatever is in those cheap knockoff springs..
To be fair, I wouldn't lump Fortune Auto in with BC etc. They do have their own Roehrig Shock Dyno, so do have the ability to custom-valve their own dampers, unlike all the budget Taiwan stuff kicking around. Just don't use their generic springs though; upgrade to the Swift or Hyperco that they offer.

If the community decides they want it, the next part in this 101 was going to be valving theory, where I will go into detail on how a damper functions, how the valving affects it, and post up all the dyno graphs that I've accumulated over the years.
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      09-03-2018, 05:57 PM   #6
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My Eibachs settled about 1/2-3/4" over abut 5 years. Now they are just where I wanted them originally and the car sits perfectly.
Now if I only had adjustable perches for the rear springs.
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      09-03-2018, 10:08 PM   #7
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Quote:
Originally Posted by CobraMarty View Post
My Eibachs settled about 1/2-3/4" over abut 5 years. Now they are just where I wanted them originally and the car sits perfectly.
Now if I only had adjustable perches for the rear springs.
Do you have the regular Eibach's, or their Pro's?
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      09-04-2018, 03:02 PM   #8
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Quote:
Originally Posted by CobraMarty View Post
My Eibachs settled about 1/2-3/4" over abut 5 years. Now they are just where I wanted them originally and the car sits perfectly.
Now if I only had adjustable perches for the rear springs.
It is more likely that your strut mounts degraded over time, that's a very common cause of ride height loss when nothing has been changed.
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      09-04-2018, 06:39 PM   #9
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Great thread! Really appreciate the info backed up with testing.

Would certainly love to hear your thoughts on valving, etc.
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      09-04-2018, 08:19 PM   #10
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Eibach Pro springs and Bilstein B8 shocks and UUC F+R sway bars.
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      09-06-2018, 09:21 PM   #11
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I used the ycw spring rate calculator on your new website and got completely different results then when I do the calculations by hand. Can you cover relativity of spring rates and spring rate theory? E.g. what is spring frequency and how does it relate the vehicles weight and suspension geometry to the effective wheel rate?
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      09-07-2018, 02:24 AM   #12
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The calculator doesn't factor in the tire spring rate, which can vary a lot. What equations/numbers did you use?
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