CTS-V Suspension Tuning
06-23-2013, 03:31 AM
#1
CTS-V Suspension Tuning All,
I thought I'd start a thread dedicated to CTS-V suspension tuning because:
CTS-V1 Curb Weight:
3850 lbs with a 54/46 weight distribution
Assumed to be 4000 lbs after about $5,000 of common modifications and a 200 lb driver.
CTS-V1 Corner Weight:
1080 lbs front (4000*0.54/2) and 920 lbs (4000*0.46/2) rear.
CTS-V1 Unsprung Corner Weight:
150 lbs front and 140 lbs rear.**
CTS-V1 Sprung Weight:
930 lbs front and 780 lbs rear
Front Shock Measurements:
d1 = 10"
d2 = 17"
theta = 20 degree from vertical
Rear Spring Measurements (for Ground Control / KW Variant 3 users):
d1 = 11"
d2 = 22"
theta = 10 degrees from vertical
Rear Shock Measurements (for k-Sport / LG Coilover users):
d1 = 11"
d2 = 14"
theta = 15 degrees from vertical
The first thing to note is that using the spring perch results in a motion ratio of 2:1, whereas using the shock perch results in a 1.27:1 ratio. This produces an effective spring rate 1.575 times higher. In other words, moving a 500-in lb spring from the spring perch to the shock perch effectively transforms it into a 787 lb spring.
I thought I'd start a thread dedicated to CTS-V suspension tuning because:
- I've read almost every CTS-V suspension-tuning thread on LS1Tech and the Cadillac Forums and felt that there was a lack of clear guidance.
- I've invested about $3500 into my suspension and I hope that, through reading this thread, others will save time and money.
- I appreciate your input and I hope that you guys will provide a sanity check on my calculations and assumptions.
CTS-V1 Curb Weight:
3850 lbs with a 54/46 weight distribution
Assumed to be 4000 lbs after about $5,000 of common modifications and a 200 lb driver.
CTS-V1 Corner Weight:
1080 lbs front (4000*0.54/2) and 920 lbs (4000*0.46/2) rear.
CTS-V1 Unsprung Corner Weight:
150 lbs front and 140 lbs rear.**
CTS-V1 Sprung Weight:
930 lbs front and 780 lbs rear
Front Shock Measurements:
d1 = 10"
d2 = 17"
theta = 20 degree from vertical
Rear Spring Measurements (for Ground Control / KW Variant 3 users):
d1 = 11"
d2 = 22"
theta = 10 degrees from vertical
Rear Shock Measurements (for k-Sport / LG Coilover users):
d1 = 11"
d2 = 14"
theta = 15 degrees from vertical
The first thing to note is that using the spring perch results in a motion ratio of 2:1, whereas using the shock perch results in a 1.27:1 ratio. This produces an effective spring rate 1.575 times higher. In other words, moving a 500-in lb spring from the spring perch to the shock perch effectively transforms it into a 787 lb spring.
**Justification:
Unsprung weight, by and large, consists of wheels, tires, axles, hubs, brake calipers, control arms, shocks, springs, and sway bars. I understand that some of these items contribute a percentage of their mass between sprung and unsprung categories. This is an acknowledged source of error. Also, I have not weighed the control arms, hubs, brake calipers, or axles. Through the process of elimination, however, I feel confident that the following estimate is good enough for our purposes.
Unsprung weight, by and large, consists of wheels, tires, axles, hubs, brake calipers, control arms, shocks, springs, and sway bars. I understand that some of these items contribute a percentage of their mass between sprung and unsprung categories. This is an acknowledged source of error. Also, I have not weighed the control arms, hubs, brake calipers, or axles. Through the process of elimination, however, I feel confident that the following estimate is good enough for our purposes.
- The stock 18x8.5" Speedline wheels (28.5 lbs each) with stock 245/45-18" Goodyear F1 Supercar EMT tires (31 lbs each) = 59.5 lbs per corner.
- The stock front rotors weigh 25.5 lbs apiece. The rear rotors are about a pound and a half lighter, but internally, our cars have a parking brake shoe that makes up the difference. Quality GS3500 aftermarket rotors (e.g. DRT rotors) are about a pound heavier on each corner. 87 lbs so far per corner.
- The stock FG2 shock body (minus spring) weighs 8.6 lbs (front) and 7.4 lbs (rear), respectively. Each 2.50" ID, 10"-long Eibach spring weighs 6.2 lbs, bringing the assembled weight of each Ground Control/FG2 corner up to 14.8 lbs and 13.6 lbs respectively. 102 lbs front and 100 lbs rear per corner.
- The Hotchkis 2280 front and rear sway bars weigh 9.0 lbs and 7.4 lbs (4.5 lbs and 3.7 lbs per corner). This is probably within two pounds of the stock sway bars. 107 lbs front and 104 lbs rear per corner.
- Finally, we have to guess about the control arms. Since the front has significantly beefier control arms, including that cast vertical section, I'm going to assign 50% more weight to the front control arms, or 24 lbs to the front control arms and 16 lbs to the rear control arms. 131 lbs front and 120 lbs rear.
- The front brake calipers weigh about a pound more than the rear calipers, but this is balanced out by the rear axles, which weigh at least five pounds more than the front axles. Once you factor in the trailing arms (which weigh about five pounds) and hubs, I'm going to assign 19 lbs to the front and 20 lbs to the rear to complete the assembly, bringing us up to 150 lbs front and 140 lbs rear.
Last edited by FuzzyLog1c; 06-23-2013 at 09:56 PM.
06-23-2013, 03:42 AM
#2
Let's run through a couple of typical off-the-shelf configurations, using the Racing Aspirations calculator, plus a couple of interesting modifications by Junior1 and myself:
500/550 Ground Control
Front/Rear Effective Coil Rate: 295.85 and 275 lbs/in
Front/Rear Wheel Rate: 175.06 and 137.5 lbs/in
Front/Rear Wheel Frequency: 81.49 and 78.86 cpm
Front/Rear Natural Frequency: 1.35 and 1.31 Hz
Front/Rear Bias: +3%
574/514 KW Variant 3
Front/Rear Effective Coil Rate: 339.64 and 257 lbs/in
Front/Rear Wheel Rate: 200.97 and 128.5 lbs/in
Front/Rear Wheel Frequency: 87.37 and 76.22 cpm
Front/Rear Natural Frequency: 1.45 and 1.27 Hz
Front/Rear Bias: +14%
700/400 LG Motorsports
Front/Rear Effective Coil Rate: 414.2 and 314.96 lbs/in
Front/Rear Wheel Rate: 245.08 and 248 lbs/in
Front/Rear Wheel Frequency: 96.46 and 105.98 cpm
Front/Rear Natural Frequency: 1.6 and 1.76 Hz
Front/Rear Bias: -10%
900/500 "Junior1" Penske Triple Adjustable config
Front/Rear Effective Coil Rate: 532.54 and 393.7 lbs/in
Front/Rear Wheel Rate: 315.11 and 310 lbs/in
Front/Rear Wheel Frequency: 109.34 and 118.53 cpm
Front/Rear Natural Frequency: 1.82 and 1.97 Hz
Front/Rear Bias: -8%
672/896 "FuzzyLogic" KW Variant 3 config
Front/Rear Effective Coil Rate: 397.63 and 448 lbs/in
Front/Rear Wheel Rate: 235.28 and 224 lbs/in
Front/Rear Wheel Frequency: 94.49 and 100.67 cpm
Front/Rear Natural Frequency: 1.57 and 1.67 Hz
Front/Rear Bias: -6%
The first thing that you need to note about the above numbers is the fact that the standard Ground Control and KW Variant 3 configurations royally effed up their frequency ratios. Generally speaking, you want your rear wheel frequency to be 10-20% higher than the front to prevent pitch and promote a flat ride, which is comfortable. Read more about it (and some of the exceptions to this rule) here courtesy of OptimumG. I can only surmise that both vendors either didn't take measurements, or took measurements from the rear shock perches instead of the spring perches, which is where the car is going to put leverage on the springs.
The other thing you should note is that Junior1's car actually weighs 3566 lbs (with driver), due to the fact that he's stripped out the interior, glass, seats, and purchased the prototype carbon fiber doors and trunk that Spectre Werkes built before they went out of business. As a result, his corner weights are about 100 lbs less than ours, and a little bit better biased toward perfect 50/50 distribution (roughly 53/47). As a result, his actual natural frequencies are going to be more what you'd expect from a full-blown racecar: 1.99 Hz front and 2.11 Hz rear.
Since I'm not beholden to the rules and regulations governing Junior1's track events, I can put a heavy engine, supercharger, and intercooler in my car that will further exacerbate the existing weight distribution problem. My spring rates already take this into account: once you add 200 lbs of weight to the front end of the car (100 lbs per corner), the natural frequency of my front suspension drops to 1.49 Hz, which is almost dead-nuts on my 1.50 Hz target for a "sedan racecar" and results in a good -12% bias to prevent pitch and increase ride comfort.
500/550 Ground Control
Front/Rear Effective Coil Rate: 295.85 and 275 lbs/in
Front/Rear Wheel Rate: 175.06 and 137.5 lbs/in
Front/Rear Wheel Frequency: 81.49 and 78.86 cpm
Front/Rear Natural Frequency: 1.35 and 1.31 Hz
Front/Rear Bias: +3%
574/514 KW Variant 3
Front/Rear Effective Coil Rate: 339.64 and 257 lbs/in
Front/Rear Wheel Rate: 200.97 and 128.5 lbs/in
Front/Rear Wheel Frequency: 87.37 and 76.22 cpm
Front/Rear Natural Frequency: 1.45 and 1.27 Hz
Front/Rear Bias: +14%
700/400 LG Motorsports
Front/Rear Effective Coil Rate: 414.2 and 314.96 lbs/in
Front/Rear Wheel Rate: 245.08 and 248 lbs/in
Front/Rear Wheel Frequency: 96.46 and 105.98 cpm
Front/Rear Natural Frequency: 1.6 and 1.76 Hz
Front/Rear Bias: -10%
900/500 "Junior1" Penske Triple Adjustable config
Front/Rear Effective Coil Rate: 532.54 and 393.7 lbs/in
Front/Rear Wheel Rate: 315.11 and 310 lbs/in
Front/Rear Wheel Frequency: 109.34 and 118.53 cpm
Front/Rear Natural Frequency: 1.82 and 1.97 Hz
Front/Rear Bias: -8%
672/896 "FuzzyLogic" KW Variant 3 config
Front/Rear Effective Coil Rate: 397.63 and 448 lbs/in
Front/Rear Wheel Rate: 235.28 and 224 lbs/in
Front/Rear Wheel Frequency: 94.49 and 100.67 cpm
Front/Rear Natural Frequency: 1.57 and 1.67 Hz
Front/Rear Bias: -6%
The first thing that you need to note about the above numbers is the fact that the standard Ground Control and KW Variant 3 configurations royally effed up their frequency ratios. Generally speaking, you want your rear wheel frequency to be 10-20% higher than the front to prevent pitch and promote a flat ride, which is comfortable. Read more about it (and some of the exceptions to this rule) here courtesy of OptimumG. I can only surmise that both vendors either didn't take measurements, or took measurements from the rear shock perches instead of the spring perches, which is where the car is going to put leverage on the springs.
The other thing you should note is that Junior1's car actually weighs 3566 lbs (with driver), due to the fact that he's stripped out the interior, glass, seats, and purchased the prototype carbon fiber doors and trunk that Spectre Werkes built before they went out of business. As a result, his corner weights are about 100 lbs less than ours, and a little bit better biased toward perfect 50/50 distribution (roughly 53/47). As a result, his actual natural frequencies are going to be more what you'd expect from a full-blown racecar: 1.99 Hz front and 2.11 Hz rear.
Since I'm not beholden to the rules and regulations governing Junior1's track events, I can put a heavy engine, supercharger, and intercooler in my car that will further exacerbate the existing weight distribution problem. My spring rates already take this into account: once you add 200 lbs of weight to the front end of the car (100 lbs per corner), the natural frequency of my front suspension drops to 1.49 Hz, which is almost dead-nuts on my 1.50 Hz target for a "sedan racecar" and results in a good -12% bias to prevent pitch and increase ride comfort.
Last edited by FuzzyLog1c; 06-23-2013 at 04:13 PM.
06-23-2013, 04:29 AM
#3
At this point, I'd like to invite any and all comments and questions you may have.
In general, I was not surprised by the results of this exercise. Ever since I upgraded to the KW Variant 3, I noticed how much I had to use the variable low-speed valving settings on the shocks to effectively "band-aid" the effect of the soft rear springs included with the kit. Having experimented with about $800 worth of Eibach springs and the Ground Control kit, I knew from previous experience that I needed at least 100 (and preferrably 150) lbs/in worth of additional rate in the rear to make the car respond crisply to inputs.
I'd also like to note that one look at the wheel rates blows any worries about damaging the KW Variant 3 with a high rear spring rate out of the water. According to AAIIIC, KW once stated that "our shocks can only handle about 200 in-lbs more spring than the kit includes." Looking at the effective wheel coil rate of the front suspension (339.64 lbs/in with the 574 lbs/in spring), 200 additional lbs/in (i.e.: a 774 lbs/in spring) would present an effective coil rate of 457.98 lbs/in to the shock. By the same rationale, the 896 in-lb spring with the rear geometry presents an effective coil rate of 448 lbs/in to the rear shocks, which is less than 457.98 lbs/in.
In general, I was not surprised by the results of this exercise. Ever since I upgraded to the KW Variant 3, I noticed how much I had to use the variable low-speed valving settings on the shocks to effectively "band-aid" the effect of the soft rear springs included with the kit. Having experimented with about $800 worth of Eibach springs and the Ground Control kit, I knew from previous experience that I needed at least 100 (and preferrably 150) lbs/in worth of additional rate in the rear to make the car respond crisply to inputs.
I'd also like to note that one look at the wheel rates blows any worries about damaging the KW Variant 3 with a high rear spring rate out of the water. According to AAIIIC, KW once stated that "our shocks can only handle about 200 in-lbs more spring than the kit includes." Looking at the effective wheel coil rate of the front suspension (339.64 lbs/in with the 574 lbs/in spring), 200 additional lbs/in (i.e.: a 774 lbs/in spring) would present an effective coil rate of 457.98 lbs/in to the shock. By the same rationale, the 896 in-lb spring with the rear geometry presents an effective coil rate of 448 lbs/in to the rear shocks, which is less than 457.98 lbs/in.
Last edited by FuzzyLog1c; 06-23-2013 at 04:44 AM.
06-23-2013, 03:44 PM
#7
Very well laid out thread. I am still putting miles on my LG setup. it was great in the canyons, but im still trying to get a feel for what i like and what i would want to improve upon.
This week i have to drive my v so i should have more feedback.
This week i have to drive my v so i should have more feedback.
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06-23-2013, 04:46 PM
#8
It's kind of like the three versions of every movie: there's the movie that the screenwriter wrote, the movie that the director shot, and the movie that the editor produced. If anyone screws up during that process, the final product could be ruined.
06-23-2013, 06:23 PM
#9
06-23-2013, 10:10 PM
#12
This thread reminds me of how terrible I am at math. Thanks for that. 
Guess I have some remedial learning to do, this thread deserves a sticky... hugely useful info here.
Guess I have some remedial learning to do, this thread deserves a sticky... hugely useful info here.
06-25-2013, 06:21 PM
#13
I dont want to muck up your thread, but it seems my LG coilovers might be suffering a bit of coil binding on its progressive springs. I will have to visually inspect the coils this week and maybe setup a test to check if that is the case with these.
Has anyone else had some coil binding occur with their progressive springs? Anyone have this happen with their LG or similar coilover setup? I have read a bit about people swapping springs if need be, i will have to contact LG to see what spring rates i have and what other options are out there for me.
But like I said i will have to visually confirm on my vehicle that this is going on before i go any further.
Has anyone else had some coil binding occur with their progressive springs? Anyone have this happen with their LG or similar coilover setup? I have read a bit about people swapping springs if need be, i will have to contact LG to see what spring rates i have and what other options are out there for me.
But like I said i will have to visually confirm on my vehicle that this is going on before i go any further.
06-25-2013, 08:37 PM
#14
You don't have progressive springs. You may want to consider a set of torsion release bearings or thrust sheets, however.
06-25-2013, 09:54 PM
#16
And if I did know exactly what they were, I'd have to make a pretty sweeping assumption about the way that progressive springs ramp with respect to shock deflection.
06-26-2013, 01:29 AM
#18
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Great thread idea. I hate that I haven't been able to get track times since my suspension upgrades. Now that I'm moving, once I do make it to the track I won't have good before/after info.
The one math class I've taken over the course of my undergrad and graduate work doesn't help me much here.
I have some corner weight info from my car if it helps at all here (if not, I can remove this post as I don't want to clutter the thread too much).
No current corner weights after roll bar/seat install.
So it appears there is a slight passenger side weight bias (since I was in the car for these weights). Also, at my current ride height (about 14" from fender lip to center of hub all around), I was getting a slight rub only on the passenger side on dips in the highway (285 tires on TD wheels) with a bit of luggage in the trunk and my 100 lbs girlfriend in the passenger seat.
The one math class I've taken over the course of my undergrad and graduate work doesn't help me much here.
I have some corner weight info from my car if it helps at all here (if not, I can remove this post as I don't want to clutter the thread too much).
I don't think that I posted this info in this thread, but I corner weighed my car at the track early last season.
Quote:
Originally Posted by UnsafeAtAnySpd View Post
I figured some of you would like to know these numbers. These were taken with me in the car (about 168 lbs), my helmet in the passenger seat, about a half tank of gas, a few trunk interior pieces removed, some dynamat back there, Kooks headers, hi-flow cats, Corsa catback, sunroof, rear strut tower brace, BMR front brace, Optima redtop in stock location.
LF 1114 1084 RF
LR 924 896 RR
Front 2198
Rear 1820
Total 4018
I went ahead and weighed it again yesterday. Changes are the clutch/flywheel, wheels/tires, rear seat removed, maybe a quarter tank of gas less (if that) and I've gained about 20 lbs...
3886 lbs!
Quote:
Originally Posted by UnsafeAtAnySpd View Post
I figured some of you would like to know these numbers. These were taken with me in the car (about 168 lbs), my helmet in the passenger seat, about a half tank of gas, a few trunk interior pieces removed, some dynamat back there, Kooks headers, hi-flow cats, Corsa catback, sunroof, rear strut tower brace, BMR front brace, Optima redtop in stock location.
LF 1114 1084 RF
LR 924 896 RR
Front 2198
Rear 1820
Total 4018
I went ahead and weighed it again yesterday. Changes are the clutch/flywheel, wheels/tires, rear seat removed, maybe a quarter tank of gas less (if that) and I've gained about 20 lbs...
3886 lbs!
So it appears there is a slight passenger side weight bias (since I was in the car for these weights). Also, at my current ride height (about 14" from fender lip to center of hub all around), I was getting a slight rub only on the passenger side on dips in the highway (285 tires on TD wheels) with a bit of luggage in the trunk and my 100 lbs girlfriend in the passenger seat.
07-20-2013, 12:35 AM
#20
Guess what time it is? If you're interested in upgrading your rear upper shock mounts (RSMs), you may want to check out the following thread.
Banski Motorsports Prototype Spherical CTS-V RSMs
Swift Z70-203-120 springs (672 in-lbs) and Swift Z60-228-160 springs (896 in-lbs)
Banski Motorsports Prototype Spherical CTS-V RSMs
Swift Z70-203-120 springs (672 in-lbs) and Swift Z60-228-160 springs (896 in-lbs)
Last edited by FuzzyLog1c; 07-20-2013 at 12:42 AM.