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Old 05-08-2007, 12:26 PM   #1
Scooby921
Merci Buckets
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Member#: 88606
Join Date: Jun 2005
Chapter/Region: MWSOC
Location: Clarkston
Vehicle:
2011 GMC Sierra
'13 JCW

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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