Ground Control - Dual Rate Spring Configuration
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2004-2007 Cadillac CTS-V Performance Mods Discussion, Ground Control - Dual Rate Spring Configuration in Cadillac CTS-V Series Forum - 2004 - 2007; All-- Looking for a sanity check here. I've been running 8" Eibach 650/700 in-lb linear springs for some time, and ...
  1. #1
    FuzzyLogic is offline Cadillac Owners Connoisseur
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    Ground Control - Dual Rate Spring Configuration

    All--

    Looking for a sanity check here.

    I've been running 8" Eibach 650/700 in-lb linear springs for some time, and while I love the responsiveness of my setup, it's not conducive to long cruises, bad roads, cobblestone roads, or the female sex in general. It's also bad for situations in which you unload the suspension; with such a high spring rates, I have very little preload (about 1.25") keeping the wheels on the ground if the car ever decides to go airborne over a rise. Therefore, I'm considering investing in Eibach 2.50" couplers ($25 apiece) and either 2.50" linear or progressive tender springs (about $95 apiece).



    For those of you who don't know, a tender spring is a small, secondary spring placed in series with a large, primary spring to provide a lower spring rate over a short distance before the tender bottoms out (a condition called coil bind). As best I can tell, this setup provides the predictability and performance of a high-rate linear spring under aggressive driving with a slightly more compliant ride while cruising.



    During the course of my research, I also realized that the fundamental frequency ratio between my front and rear springs is too small. In order to prevent a nose dive condition caused by a bump hitting the front wheels and then traveling to the rear wheels, the fundamental frequency of your suspension is typically 10-20% faster in the rear so that the rear end of the car will "catch up" to the front. Otherwise, the nose of your car will be falling while the rear is still rising, creating a very unsettling feeling. Calculating the optimum front/rear ratio is impossible for all conditions, because it's based on your wheelbase and the speed that you're driving when you hit a given bump. The "normal" 500/550 Ground Control setup has that minimum 10% differential, but when I went up to 650/700, I lost that. Therefore, I'm probably going to move the rear springs to the front and buy a set of 800 lb springs for the rear, giving me a 14.29% differential spring rate. However, those heavier springs are going to exacerbate the ride comfort problem described above.

    So, the equation governing the effective spring rate (ERS) created by the series combination of two springs is Ci = CMxCT/(CM+CT). You can read more about it here. For example, if I paired a 700 in-lb primary spring with a 300 in-lb tender, my effective spring rate (until the tender bottomed out) would be:

    - Ci = 700*300/(700+300) = 210 in-lbs

    It may be a little hard to intuitively understand, until you remember that whatever force you place on the top spring is fully transferred to the spring below it--making them both compress their normal, rated amount. The combination of the two movements give you a lower effective rate.

    So, I'm thinking that I want the absolute minimum amount of mushy travel required for cruising comfort and want my car back on the primary springs the moment I do something even mildly aggressive. Given the rated 3850 lb weight of our cars, that only leaves me with one option, according to Eibach's latest catalog: the 0175.250.1300 tender spring. With an initial rate of 600 in-lbs, a final rate of 1300 in-lbs, 1.75" of total travel, and 1513 total lbs required to put it into coil-bind, this progressive (actually it's stepwise progressive) tender is almost perfect for our CTS-Vs. Math:

    - CTS-V weight = 3850 lbs rated + 18 gallons of gas @ 8.34 lbs each + 200 lbs (me+stuff) = 4200 lbs

    CTS-V weight distribution is 54/46, which would ordinarily lead you to believe that the load on the front corners is 1134 lbs and 966 lbs on the rear corners. But, like most of you, I've shifted weight toward the back a little bit. If you've installed headers, replaced the clutch, installed a new subwoofer, and added soundproofing to get rid of that Magnaflow drone, you've probably moved 14-24 lbs from the front to the back of the car. Therefore:

    - Front Corner Weight: 1120 lbs (53.33%)
    - Rear Corner Weight: 980 lbs (46.67%)


    - 1120 lbs (front) is 74.025% of the 1513 lbs required to reach tender coil bind
    - 980 lbs (rear) is 64.772% of the 1513 lbs required to reach tender coil bind


    If we assume that this progressive tender compresses like a linear spring (a conservative assumption--in reality, it will compress a little more than I'm predicting), here's how much travel I'll be adding (which I can compensate for by spinning my Ground Control coilover rings up):

    - The front tenders will be compressed 74.025% of the way from 3.55" (uncompressed length) to 1.80" (coil bind length), giving me an initial ride height of 2.25". Therefore, they have 0.45" of travel remaining until coil bind. An additional 393 lbs of force is required to lock them out.

    - The rear tenders will be compressed 64.772% of the way from 3.55" to 1.80", giving me an initial ride height of 2.42". Therefore, they have 0.62" of travel remaining until coil bind. An additional 533 lbs of force is required to lock them out.




    In order to calculate my ERS, I need to estimate the initial spring rate of the tenders (which changes based on preload). Given the CTS-V weight distribution, the initial rates of the preloaded 600/1300 tenders will be:

    - k1 = 600+0.74025(1300-600) = 1105.1 lbs initial rate on the front
    - k2 = 600+0.64772(1300-600) = 1053.4 lbs initial rate on the rear


    Assuming I'm running 700 in-lb springs in the front and 800 in-lb springs in the back, the initial effective spring rate (ERS) of this system will be:

    - 700*k1/(700+k1) = 428.6 in-lbs front
    - 800*k2/(800+k2) = 454.7 in-lbs rear


    Does this sound reasonable to you guys? Could you check my math? I don't want to drop $100 on couplers, $400 on tenders, and $200 on new 800 in-lb rear springs unless this has a good chance of working as I expect it will.

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  3. #2
    ichpen's Avatar
    ichpen is offline Cadillac Owners Connoisseur
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    Re: Ground Control - Dual Rate Spring Configuration

    I'm still reading through this.

    Having run progressive tenders in front (eibach ones) I can comment that the compliance under load is much better than the usual on/off non-progressive springs but it's not going to overcome the inherent jaggedy nature of an overly stiff spring so bear that in mind. You do get better and quicker planting over bumps.

    From a practical perspective I don't know how you're going to add anything out back. The standard 10" spring + ground control or other adjustable perche is already tight. You'd need to go down to at least 8" maybe less to accommodate a stiff progressive tender (which would need presumably at least 3"+). You'll also need a spring compressor to get them on. Bear in mind going with a shorter spring also changes the characteristics so check with eiback (if that's your spring of choice) on the numbers.

    Also from a fear mongering perspective given we don't have a shock running through those rears I'm actually a little worried about spring stacking in general i.e. if airborne could the pieces basically detach. Not basing this on any evidence, just common sense checks.

    I ran the 250-550 tenders btw.

  4. #3
    FuzzyLogic is offline Cadillac Owners Connoisseur
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    Re: Ground Control - Dual Rate Spring Configuration

    I'm already running 8" springs. The tenders are approximately 3.50" long uncompressed. The only way that you'll decouple the rear springs (even with tenders stacked on top) is if you're running the stock end links, which you shouldn't. Unless you like this look when you're braking:



    Why did you select the 0200.250.0550 tenders? They only require 865 lbs to coil bind, meaning that they would be normally closed and only open when you unloaded the suspension. You shouldn't have noticed much of a difference with them, based on the research I've done thus far.

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    ichpen's Avatar
    ichpen is offline Cadillac Owners Connoisseur
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    Re: Ground Control - Dual Rate Spring Configuration

    Purely for rebound, dips etc. to get rid of some of the unsettled feeling when going over rough surfaces. They did not bind fully probably due to the fact that I'm also running much less stiff main springs. But they helped settle the car a bit.

  6. #5
    FuzzyLogic is offline Cadillac Owners Connoisseur
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    Re: Ground Control - Dual Rate Spring Configuration

    Quote Originally Posted by ichpen View Post
    Purely for rebound, dips etc. to get rid of some of the unsettled feeling when going over rough surfaces. They did not bind fully probably due to the fact that I'm also running much less stiff main springs. But they helped settle the car a bit.
    Just got off the phone with Eibach. Spent about 30 minutes with an application engineer. He confirmed that the "load at block height" column in the catalog represents how much weight will put the spring into coil bind. Therefore, your tender springs were definitely normally coil-bound. Because they're coil bound at 854 lbs, it wouldn't take much of an unload to put them to work (given that your corner weight isn't more than 200-300 lbs greater), but the overall effect should be minor. The tender setup that I'm looking at, on the other hand, is almost exactly on the opposite side of the fence. The tenders should be normally active until a load forces them to close.

    By the way, I just found out that by definition, "curb weight" includes fuel. D'oh! Therefore, I should have calculated that the car weighed 4050 lbs, not 4200 lbs.

    - Corrected Front Corner Weight: 1080 lbs (53.33%)
    - Corrected Rear Corner Weight: 945 lbs (46.67%)


    Also, it appears that a bump will transmit force to the springs relative to the amount of unsprung weight on each corner. I don't have the figures in front of me right now, but let's assume that the weight of each wheel is 25 lbs, each tire weighs 30 lbs, and the brake assembly is 35 lbs for a total of 90 lbs of unsprung weight. Therefore, a massive 3G bump would exert 270 lbs on the springs, which is not enough to lock out the tenders.

    Similarly, if we assume that the CTS-V has a center of gravity at 750 mm, perfect 50/50 weight distribution, a track width of 1800 mm, and a weight of 1837.05 kg (4100 lbs), a turn will produce a load transfer of 765 kg (1686 lbs) per G pulled. Therefore, if I need to transfer about 900 lbs to lock out my tenders on one side (based on the effective spring rate above), I'll need to pull a roughly 0.54 G turn (900 lbs / 1686 lbs / G = 0.534 G).

    If I consider that excessive, I can block out the tenders early by using a bump stop to lock them up before they reach their full range of travel. Cool, eh?

    Edit: corrected figures above. Forgot to add the front and the rear effective spring rate. Necessary turn went up from 0.3 G to 0.55 G. I'm almost certainly going to have to add a bump stop to lock out those tenders earlier and prevent "the big squishy" from happening.

  7. #6
    CoSteve is offline Cadillac Owners Member
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    Re: Ground Control - Dual Rate Spring Configuration

    First - interesting read..

    Second, Why don't you find a local scale and weight the vehicle - front/both/rear? That would take away 1 set of estimates.

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