AutoX wheel rate recommendations wanted for streetable FWD

Car weight: 3,814

Weight Ratio F/R: 63/37

2,402 lbs. front (1201 each front corner)

1,412 lbs. rear (706 each rear corner)

Wheel base 108”

Track width front and rear is a matched 60.9”

Strut front suspension w/12” disc brakes

Rear is a double A-arm IRS with 11” disc brakes

Stock ride height 5.9”

Steering wheel turns to lock: 2.65

Turning circle: 40.35’

Daily Driver (50 miles per day- agressive driving is the norm.)

To be Autocrossed 6 to 12 times per year.

I like a sporty ride quality, but don't want to skate across the "non-smooth" roads in a corner.

FYI: Engine: 4.6 Northstar w/ Sequential TPI

Horsepower: 275 @ 5600

Torque: 300 @ 4000

Compression 10:1

Gear ratio 3.11:1

MPG 18 City, 27 interstate, 24 average

0-60 @ 6.8 seconds

0-80 and back to zero in 13.5 seconds

...and now for the kicker - the car is a 2002 Eldorado ESC... Yep, that's right, its a Cadillac.

You autox ing a early 2000s sts? just saw EL Dorado ESC, should be interesting

Whoa whoa whoa. Are you sure you want to sink money into modding this car, rather than having fun with it and getting something different when you want to get competitive? 'Cuz there's starting off with a not-ideal platform, something I know all about...and then there's THIS

@ GameboyRMH: Whoa, whoa, whoa??? I’m sure similar remarks were made when the first 4 banger CRX showed up in a modified class at the local Pro Solo. And, I bet that driver got the same standing ovation that I did when my best run in a bone stock 1970 Eldorado beat three of the four runs of the slowest 85’ Vette at the 1986 autocross in Fayetteville Arkansas. I joined Grassroots because in my research, I keep getting google hits on some meaty tech discussions about suspensions; and because you guys support each other in the modification of some off-the-wall vehicles. Is Eldorado the exception? Are you familiar with the last generation Eldorado? Or just the 5,000 boats of the mid-70’s? This car is a base model with 136 pound front springs, 275 lb. rear springs and the smallest diameter sway bars offered, yet I can go into an intersection at 30 MPH, turn right, stay in my lane, nail the gas just before apex and rocket out under full throttle without hearing much tire noise.

I feel like with a car of that weight, you have quite a bit of freedom to up your spring rates and not have the ride quality suffer in kind. I've never actually owned/modified the suspension of a car with that kind of mass, but I wouldn't have any qualms about doubling those spring rates.

What's the handling balance like right now?

Funny, i was just thinking the other day about how one of these would do prepped. I think the whole FWD+power+weight thing is going to put a hamper on the party pretty quick. Thats a LOT of weight to be transferring around. Sounds like you have the experience to wheel the biscuit, though. I can beat lots of "faster" cars in an autocross with my junkpile Accord, but its more driver than anything. What class would that thing land in? i'd stick a good tire on it and race in whichever street/stock class it would fit into, first. Being a GM, im sure theres a ton of spring options available, but I'd be more concerned with what shock is available. From there i'd dial in the swaybar, only touching the rear. With that much weight/transfer, I would stay stock up front.

Btw, a defensive stance isnt the right direction to go around here, everyone's here to help whichever way we can.

Is there any way you can break down the suspension and get unsprung weights? The you can choose rates based on frequency.

http://farnorthracing.com/autocross_secrets5.html

farnorthracing said: Of all the claims I've made in this book, this is probably the one that caused the most serious discussion. It's also the one that is the hardest to justify, given that it is a bit of a "magic number" and tire and surface dependant at that. For a slicker or bumpier surface, it could probably drop a tenth of a Hz or so. Grippier tires (like slicks - although sidewall stiffness will play a part here) might jack it up a little. The point here isn't if the car should be 2.2000 Hz or 2.201324 Hz - the major takeaway is that ~2.2 Hz is the ballpark. If the natural frequencies on your Camaro work out to 1.7 Hz front and 3.9 Hz rear... well, you've got a problem there Skippy. (A car like this should be undrivable, all loosey-goosey-can't-put-power-down - unless the front shocks have massive amounts of rebound to compensate....) Yes, it really is that simple. Measure your corner weights, unsprung masses, and motion ratios, and then pick springs that put the front NF at 2.2 Hz and the rear at 2.5 Hz.

@CadVetteStang: Hey I'm not trying to take a dump all over your plans, I just want to make sure you know what you're getting into.

Now that I understand you know what to expect in modding this mighty barge I'd say you should be looking at massive spring rates - like approaching 4 digits in lb/in. On my Corolla which is nearly 1500lbs lighter, I'm running springs clean over 600lb/in, and this isn't even "seriously hard" by autocross standards.

For autocrossing a car that heavy, you should be looking at over 800lb/in for sure. (Edit: That's assuming your wheel rate:spring rate ratio is in the ballpark of 1:1)

In reply to CadVetteStang:

It would be hard for me to believe that you didn't expect to catch a little grief over this. It's not really the tool for the job. But I understand completely. You like the car and want to make it better. That's what this is all about. There are people, even here on GRM, that will tell you it's an exorcise in futility, use the money you would put into this project and buy a Miata for competition. But those people are missing the point.

I'm not all that up on figuring out spring rates and wheel rates. But it seems to me that with that weight you wouldn't be too far off of what some of the bigger muscle cars are. I mean what would a 69 Chevelle weigh? Pretty similar I would guess. You might be able to poke around some pro touring sites to try and figure out what type of wheel rates those cars are running. I would be more than comfortable at least doubling stock rates at a minimum.

Here was my unconventional auto crosser

Long live the underdog!!!

In reply to GameboyRMH:

I believe he wanted to retain some daily-driving comfort with the increased spring rates. My 1500 lb Civic is stiff on 350lb/in springs...

I didn't mean to get defensive. I am sorry about that... The balance of the car now is understeer. The rear springs are not able to support the car. They were designed as helpers for the air shocks that have failed. An aftermarket company selling a conversion kit has progressive rear springs that start in the mid 300's and end in the low 400 range. My car rides the outside rear bumpstop in a hard corner but still handles much better than my 76 Nova or 72 Mustang ever though about. So a good stock rear spring rate would be 350. I thought about 2X rear rates, but that would be 700. I also thought about 325-400ish for the front.

So, for clarity, is this thing macstruts in the front? What about the rear? How does geometry look through the travel?

High spring rates can be very comfy with shocks worth a damn.

I've ridden in a miata wearing 700lb front, 400lb rear, and the ride was downright comfy.

Everyone is throwing spring rates at you and telling you how it rides, but actual spring rate doesn't matter doesn't matter doesn't matter doesn't matter.

It is the suspension frequency that matters. Actual spring rate is completely irrelevant between two different anythings. You can only compare frequencies.

Follow Far North Racing - dude knows what he's talking aboot. Eh.

Yes, which is why I was asking for wheel rate recommendations and giving the weight of each corner ofthe car. I have not deducted the sprung weight yet to determine unsprung weight because I don't know that info yet.

Yes, strut front suspension. So wheel rate may be close enough to spring weight to be in the same ballpark, but I have to measure it.

IRS rear end that, to me, looks similar to an SVT Cobra as far as spring and shock placement and degrees of vertical orientation... I get massive negative camber when the wheels tuck into the fender well upon compression... I will get pictures of the setup soon.

I am pulling all but the shocks and brake pads frm the local U-Pull-It so my total suspension and brakebuild will cost less than $400 initially. I am on a tight budget and work two jobs. My time to reply and work on the car is limited. I can't afford the money or the time to swap springs over and over. I know that I am pioneering a build that has no reference points. That makes it fun.

I provided the stock rates as a reference point. The 136 rate is a HD spring in the FE3 Touring Coupes. I currently have the FE1 soft ride suspension and do not know that rate. But I do know that the FE3 sport spring will still ride softer than say.....a Taurus.

And for a quick size comparison, here are some photos of this Grand Prix-sized sleeper.
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Coming from Hondas mostly I don't have a ton of experience with the FWD macstrut setup, but my understanding is that most don't have a great camber curve and therefore can benefit from a lot of roll stiffness.

Disclaimer: this is the case with some smaller cars, certainly might not be the case with a big expensive GM.

Based on the factory spring rates, I'm guessing .97 for the front motion ratio and .6 for the rear. That gives us 600lbs/in for a ~2.2hz front, and 1100lbs/in for ~2.5hz in the rear.

In reply to MCarp22:

With that being the case, I wonder what the chances of finding dampers that will work with those rates? Certainly there is nothing off the shelf available for a caddy. How expensive would it be to have some custom ones built to handle that?

Dampers is a major concern, and this is a cheap budget build. By swapping to an earlier body style strut mount, my factory strut will accept a 4” inside diameter RWD coil spring with one square and one tangential end. This makes the unlimited possibilities as many of the Chevy cars and trucks have 3.9” to 4.1” diameter coils. I can cut to fit anything from a 200 lb./inch spring to a 1100 lb./in. coil.

But the only thing I have to put it on are stock grade aftermarket struts. My car has 80,000 miles on it and the struts are good and strong. The best replacement I have found is the Monroe Sensa – Trac. The limiting factor to a strut swap is the sway bar end link mounting tab. I am afraid that welding the tab to a different strut will destroy the new strut. … if I go with a super stiff spring, I will have to add a shock that connects to the strut, turns with it, and is yet anchored to the inner fender mounting point by the upper strut mount or saddled onto the sway bar end. I am working on both designs, but don’t have the budget for it now. .. I think if I attach a shock to the sway bar, I would deactivate the bar by creating extra twist forces on it….. so for now, I need a spring that I can use with a stock strut…. I have located a donor car with a 325 lb./rate that is 3” shorter than my stock spring. All I have to do is cut the pigtail off to get the tangential end and I should have a lowering spring with a ballpark rate of 350#.

Or I could cut down a set of 360# Caprice Classic springs and get in the 500ish range. It is a shame that there is no quick change front strut or I would make road springs and track springs….

For the rear, (which as you know is priority in FWD autocrossing), I have already pulled a 489# spring that is the same length as my stock rear springs and same diameter of 3.5”.. depending on cut, can be a 550# or 611# spring rate. Have not installed it yet. If I have to cut more out of it to reduce ride height it could be a 650ish rate or more. … or if that is too stiff, I can cut down 400# Mustang front springs into a 500ish rate.

The rear shocks from an SVT Cobra will fit and that creates freedom to dial it in eventually with adjustable shocks. But for now, Sensa-Tracs for a GT Mustang will have to do.

Again, this is why I am after recommended WHEEL rates based on the weight and weight distribution of the car. I still have to measure spring and shock ratios and compare them with the Cobra IRS to get a good picture of how to match the setup.

So, I am back to wheel rates….. I think a Civic has 70% of its weight up front. If I could get the weight and front/rear ratio data on a FWD car that is proportioned like mine, I could do the math for the overall weight increase and apply it to get the proportionate wheel rate. THEN, I could figure what springs I need to create that wheel rate.

For now, I can only assume that the stock wheel rates are balanced for the car (taking into account the 300ish or 350ish rear spring rate on a non-air suspension car) and increase the rear to a slightly greater proportion. Example: what if I increase the rates by 2X? That would be 650 to 700 lb. rear springs and 272# front springs.

The Hotchkis performance spring rate for the front strut on the newer Camaros are 210#. They don’t have as much weight up front as I do, (and the rear drive characteristics can use a little squat with a big bar) but considering that rate, the 325# springs I found, don’t look too soft …. (I think)…. But again, wheel rates…. I need to start with wheel rates.

And with frequency, 1Hz being the factory ideal, what is the ride quality like in the 2Hz range?

BTW: thank you everyone for the help. Cody

CadVetteStang wrote: I think a Civic has 70% of its weight up front.

It's more like 65%

MCarp22 wrote:

CadVetteStang wrote: I think a Civic has 70% of its weight up front.

It's more like 65%

In that case, my 63% is close enough to forget the extra steps and apply the same wheel rate percentage of a streetable Civic to the extra total weight of my car as a basis to start from. I know it won't be exact because of the different suspension geometries, but worth a look. The Grand Prix GTP would be another good car to study.

I just found lowering springs for a Grand Prix GTP; the ad says compatible with OEM shocks and the front rate for the strut springs are 450 lbs./in. so..... maybe 350 could be too light. the spring motion ratio of these two cars should be very close and the Grand Prix is a little lighter on the front end than the Eldo.

Here's a chart of front and rear ride frequencies, corner weights, unsprung weights, sprung rates, and ride rates for factory suspension setups on various cars:

Note that ride rate is not the same as wheel rate. Ride Rate (Kr) = Kw * Kt / (Kw + Kw), where Kw is the wheel rate and Kt is the tire spring rate.

(From this Powerpoint presentation on suspension design from FSAE Lead Design Judge, Steve Lyman from DaimlerChrysler

@ series8217, Thank you; I have only been able to glance at the info in a quick summery due to my work schedule, but if I am interpreting correctly, I should keep the percentage of increase in spring rate front to rear as equal as possible to maintain “flat ride”.

3X increase in Rear spring rate = 1,050

3X increase in Front spring rate = 408

2X increase in rear = 700

2X increase in front = 272

That should maintain flat ride.

However, a 2X increase in rear = 700

And a 3X increase in front = 408

Could cause pitch problems with pitch over bumps....

Am I on the right track here?