FuzzyLog1c

iTrader: (4)

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.

Onefast V

iTrader: (3)

To use natural frequency as a guide for suspension tuning you need to know your wheel rate front and rear which is a function of spring location and spring angle. Don't use the spring rate for determining frequencies etc. Look at the bella vista coilover bible its great info. It's originated from 4wheel but the application of theory is the same.

You need to corner weigh your car so you know the corner sprung and corner unsprung weight.

http://www.pirate4x4.com/tech/billavista/coilovers/Part_1/

FuzzyLog1c

iTrader: (4)

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.

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.

AAIIIC

iTrader: (19)

Concur.

You're a much better man than I, Fuzzy. I've never been motivated enough to try to calculate all this stuff, even though I know it matters and can make a huge difference in both the handling and the performance of the car.

garrettg

iTrader: (1)

I have similar interest in trying to soften the V a touch for daily driving and keep somewhat close to stock ride height for curbs and car washes. I have the KW V3 setup which already comes with linear tender springs on the front. I have not adjusted the shocks(need to look into this) from default settings but wanted to get some ideas on springs since this thread talks about dual rates.

KW V3 springs provided in kit
Front tender spring 114 lb/in 80mm length id 60mm
Front main spring 571 lb/in 200mm length id 70mm
Rear springs 514 lb/in 250mm length id 62mm(60mm works but very tight, 65mm/2.5in might work)

All options for softer ride from the spring:
firmer tender springs - They are nearly fully compressed just based on preload to get the front bumper at a reasonable DD height

longer tender springs - doubt this would help without increasing rate

firmer and longer tender springs - may depend on what I can find for tender springs

Softer main front springs - Go down 2 steps to 448 lb/in, still more than stock and closer to 10% front to default rear difference?

softer and longer main front spring - If go down to 448 may need longer spring to prevent coil bind on main spring

NIKDSC5

iTrader: (10)

This is the thread I like to read from a distance and....wait....haha There is so much going into the whole concept that I cannot wait for someone else to figure it out, and then I copy them.

Seriously though, lots of info/math/things to consider with this.

FuzzyLog1c

iTrader: (4)

In retrospect, the 600/1300 progressive tender springs were wrong for this car. I'll have more details later, but suffice it to say that they needed less rate. At this point, I'm looking at doing a KW Variant 3 group buy. I'll still keep the springs I've got, because they should be compatible, but as far as a budget competitor goes, this idea was (mostly) a bust due to a lack of appropriate spring options.

I8URSVT

iTrader: (2)

fuzzy if you are going to order springs and such and use formulas, I would take your car to weigh-station first. That way you can get accurate weights for each corner.

Just a thought, would hate to see you unhappy with the springs because some little thing like that (not that I think your #'s are off, just a way to "verify" these figures).

FuzzyLog1c

iTrader: (4)

The problem with the tenders is that they have WAY too much load at block height. When I ordered them, Ground Control misquoted their load by 500 lbs (50% wrong). Sucks, considering their $100 apiece cost.

I8URSVT

iTrader: (2)

didnt realize that this wasnt a new thread

I need to do something with my suspension, I'm getting tired of the 4x4 stance but I'm scared I'll scrape speed bumps