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Old 05-08-2007, 12:26 PM   #1
Scooby921
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Default Springing options for coilovers - attacking stroke and droop travel

One of these days I'll be purchasing a new set of coilovers for my car. I've already sorted out where they are coming from. My problem is figuring out what I want to do for springs. For those who will want to know...the car is a daily driver that gets used for autocrossing on the weekends. Its prepped for STX and will be running national level events. I want to see how many trailered cars I can beat at Nationals in September!

The coilovers will be coming from Zzyzx once I have everything figured out. I do know that the front springs will be between 500 and 600 lb/in, and the rear springs will be between 750 and 850 lb/in. I'm looking for a little help with choosing what length of spring I use, whether or not I should use a tender spring, and what can be done to increase droop travel in the rear so it doesn't tripod through corners quite so much.

The things I don't know revolve around the damper and suspension travel. I don't know what the stock travel is on an Impreza. I don't know how much that changes when you lower the car and add stiffer springs. I know the Koni 8611 damper has between 5.47" and 6.22" of stroke, but I don't know which model Zzyzx uses (I have sent a message to them for some help). The big issue I see is comparative stroke between the spring and the damper. A standard 8" spring gives you a little over 4" of stroke for the stiffnesses I'm looking at. A 10" spring only gets you another inch. That leaves at least 1" of damper / suspension travel on the table. Is that a bad thing? Does the car need that extra travel? Can I benefit from adjusting the spring setup to use that extra travel?

Sticking with the idea that maximizing suspension stroke is the best option, I've come up with the following idea for a setup:


Front:
Main spring - 8" 550lb/in (Eibach / Swift)
Tender/assist spring - 2.68" 280lb/in (Swift)
Static spring compression - 2.25" (approximation...if my math is any good)
Spring stroke - 5.92"

Rear:
Main spring - 8" 800lb/in (Eibach / Swift)
Tender/assist spring - 2.68" 224lb/in (Swift)
Static spring compression - 1.82" (again...if my math is good)
Spring stroke - 5.72"


To me the front looks pretty good. The static load of the body should bottom out the tender spring and not interfere with the linear response of the main spring under compression. The rear is where I have concerns. If weight transfer unloads the corner and displacement is more than 2", I'm pulling a wheel off the ground. So how do I get more droop travel? Is there a simple way to do this?

My thought is to add a 3rd spring in the rear. Just a simple helper spring with a very soft rate (45lb/in) which is only there keep the other springs in contact with the topmount. Setup the main two springs such that at full extension of the damper and full compression of the helper spring they are an inch below the topmount. When uncompressed, the helper spring will keep the springs in contact with the topmount at full extension. Under vehicle weight the helper spring is bottomed out and its effect on the ride height and compressed height of the suspension is negligible. When weight transfers and that corner is unloaded you have gained an extra inch worth of droop travel to keep that wheel in contact with the ground.

So what does the suspension-knowledgable crowd think? Am I way off and need to go read some suspension books again, or might this be a reasonable idea?
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Last edited by Scooby921; 05-08-2007 at 01:32 PM.
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Old 05-08-2007, 12:44 PM   #2
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Any reason why you want to run a tender spring rather than just a helper spring? Tuning dampers for a tender spring setup can be a bit tricky as you basically have a progressive setup.
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Old 05-08-2007, 12:56 PM   #3
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Any reason why you want to run a tender spring rather than just a helper spring? Tuning dampers for a tender spring setup can be a bit tricky as you basically have a progressive setup.
My idea behind the tender was the length and stiffness. A helper spring is primarily there to keep the main spring in contact with the topmount. The main spring still absorbs nearly all of the weight of the body and results in less total stroke. My thought / assumption is that the tender spring will absorb the majority of the body weight and leave more stroke and suspension travel to the main spring. If the tender is completely compressed at static weight, then the majority of compression travel of the suspension is simply the linear rate of the main spring.

But that is if my assumptions are correct. Maybe the tender is going to function way different than I expect and I do end up with a progressive rate. I suppose that isn't too bad for the daily driver, but it isn't what I want for racing.
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Old 05-08-2007, 07:19 PM   #4
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Have you actually contaced Zyzzx for their recommendation? They've been autoxing this setup for years now and I'm sure they have very specific recommendations for you application. No sense reinventing the wheel if they've done the homework.

regardless of whether or not you are using a tender or just a main spring and helper, stroke will be taken up by the weight of the car. Just because you have a tender on there doesn't mean you will magically keep the car from compressing the damper. How stiff your overall spring rate is will determine how much travel you will lose from just the weight of the car, and naturally, what ride height you set the car at.

Last edited by Arnie; 05-08-2007 at 07:26 PM.
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Old 05-09-2007, 01:10 PM   #5
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I understand that a tender isn't going to work some magic and suddenly make things exactly how I want them. I suppose I just don't fully understand what the different spring setups are going to do for me. I was hoping someone like Myles would find his way in here and either tell me I'm way off or that I have half a clue.

Broken down, my thought is this...
A helper spring is really soft. Say its 80lb/in and has a stroke of 1.5". So when the car is at static ride height, that spring is bottomed out and absorbs 120lbs of the corner weight. That means the other ~700lbs is go into the main spring. A 550lb/in main spring is going to compress 1.25" under that load. Static compression of the suspension is then 2.75". With a useable stroke of 4.1" for the main springs minus the 1.25" static compression, you end up with 2.85" of compression travel before you bottom out the spring. You also can't forget that you've taken up 2.75" of the damper's total stroke.

If I use a tender springs with a stiffer rate (280lb/in) and a 1.61" stroke I now only have 370lbs that are eaten up by the main spring. So the main spring is now compressed 0.67". With the same spring stroke, I now have 3.43" of compression travel.


Does that make sense or am I way off in left field? I know its not quite that simple. Springs in series are a softer total rate, and the tender isn't going to compress 100% before the main starts to compress. But the differences in stiffness means the tender should take most of that static compression. With the right stiffness tender it should see 100% compression under static load. Then its just a case of fine tuning main spring length to achieve the desired stroke.




All of that said, it really depends on where the suspension should be sitting in terms of required compression and rebound travel. How much travel do I want / need? Do I want to set it up such that static ride height provides equal compression and bump travel, or do I bias it one way or the other?

Last edited by Scooby921; 05-09-2007 at 01:21 PM.
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Old 05-09-2007, 03:34 PM   #6
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What are you trying to achieve for spring characteristics? With those main springs and tender spring rates and lengths, especially the rears, it almost looks like you want to use the main spring rate only for movement below static ride height and the combined spring rate for movement above.
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Old 05-10-2007, 09:00 AM   #7
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Originally Posted by PossumK View Post
What are you trying to achieve for spring characteristics? With those main springs and tender spring rates and lengths, especially the rears, it almost looks like you want to use the main spring rate only for movement below static ride height and the combined spring rate for movement above.
Hmm...I hadn't given that any thought. I am concerned with setting it up such that the majority of the travel occurs in the linear rate of the main spring. Does having a significantly softer spring rate above static ride height create a problem? More things I don't know.
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Old 05-10-2007, 11:41 AM   #8
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If you fully compress the helper/tender spring at static ride heigha s you mentioned, then all of the corner's weight is being transferred to your main spring and your helper/tender spring has become a spacer.

Tony
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