Spring rate???

[dennis68]

Question about spring rates as I am getting ready to order some coils. I am currently running 400lbs springs in the front with 2 coils removed so it’s probably more like 500lbs and 160 lbs coils in the back with 1 coil removed. I know that it’s probably a combination of my poor OE suspension and my hack lowering that has terribly goofed up what was a bad idea already. When going over bumps, especially in off-camber corners, the entire rear end has a tendency to “jump”. I mean it really hops off the ground and around, kind of scary if you are not excepting it. My question is this, what spring rates are we running out there on there street and how do they handle everyday driving? To qualify to answer your car actually has to drive under its own power more than 10 miles a month. I am looking for real world feedback, guys who drive their SCCA cars to/from the track. Is my experience thus far the garbage suspension or does running track type spring rates on the street bring this kind of trade-off?
I worked on a Camaro years ago, it was an 88 IROC that I threw a whole bunch of bolt-ons at (customers money). It got 750lbs front coils and 175 rear coils with Bilstein struts/shocks. On a 245/50R16 GSC it actually didn’t ride harsh at all, not like my bucket which leads me to believe its suspension not coil rate. Thanks guys.

[Salt Racer]

...When going over bumps, especially in off-camber corners, the entire rear end has a tendency to “jump”...

I'd check for suspension bind/bottoming first, then check the shocks. Cheap high-pressure gas charged shocks have tendency to do this as their bump valving is typically way too stiff.

...which leads me to believe its suspension not coil rate...

Spring rates do have effects, but side view geometry and shock bump valving have as much, if not greater, effects (EDIT: at least within the range of streetable spring rates). Soft bump/siff rebound valving is the way to go for street cars.


I don't have an SCCA car (yet), but I'm speaking from my first-hand experience.

My '69 Nova had the following combo at one time - cut stock front coils, PST sta bar, K** shocks, poly bushings, poly subframe mounts, and 235/60R15s. The arse end skipped around a lot, and my then-27-year old body couldn't take its harsh ride, until I did my own version of G-mod (longer SVSA).

My now-few-years-older body can happily live with the ride of my Riviera (800 lbs/in on 0.445 motion ratio, 33mm sta bar, Strange shocks, bunch of rod-ended links, 18s, Recaro SE, and revised geometry). It's pretty stable on rough pavement too.

[spanky the wondermuffin]

although i have begun to trailer my car,i've driven it to many events over the past few years,switching tires,running it around a road course all day or competing in open road racing,then driving home-sometimes 1500 miles one way.and i really don't feel more beat after ten hours on the interstate in it then a crappy midsize rental car.68 camaro,conv,stripped,lots of fiberglass body parts,full load(4 wheels/tires,tools,ez up,ect.).550 front,175 rear.15" travel wheels.however,much as i love the bay area the roads are a bit special.i've bottomed hard a couple times over the last few years down there and would suggest taking that into account.

[chicane67]

An applicable answer to your question would have to render judgement from more input than what you have/have not provided. Have many unanswered questions that need to be laid out and configured for.

I had recently done a 67 Elco that had a meaty SBC power plant, a 2004R and an 8.5" differential. I did use Globals upper control arms and F&R sta bars, but I refered to my judgement on the required spring rates. Now remember, I myself use a 720/320 with a 3/4" sta bar(~front only~) package on my 67 Camaro that was daily driven from the day I got out of high school, raced on multiple tracks, autocrossed and has even seen some SCTA Landspeed driving......but I dont really see one throwing an Elco around like my F-body, so my selection is completely based off of the 67's chassis set-up. But as for the Elco, it went like this:

~Remembering that this answer would change if you had any other engine/drivetrain changes from my friends chassis~

620 front

175 rear

This is a daily vehicle that is used for just about everything you can think of. From towing, to making 1000 mile trips in to go get parts or going to Vegas for the left hand turn stuff at the Bullring. I myself, found it rather comfortable and compliant, even with new Stiners to control the bumps..... I think it is just about right for someone that doesnt drive like me. He's an old guy, but loves his Ducati's and winged sprint cars.

But honestly, there is more to this subject, that your not telling us. Weight and is placement will change the requirements..........

[dennis68]

Thanks Kats and Tommy. Tommy, I have in my possession a whole new front suspension (less the damn round bushing LCA I can't find). I will be running F body spindles, C5 brakes, SCP UCA (no offset for caster gain, 9" version), rod end tie rods, Howe center link and Bilsteins. I'm just getting the coils and finding LCA's and solid bushings before I throw it all together.

The rear will be an owner fabbed 3-link with PHB arrangement. C5 rear brakes as well. The 3-link as I have it laid out should provide 101%AS, 9" RRCH, and a 51" SVSA. Not too bad for my first attempt a totally from scratch build (ASSuming it all works out) in a 2-car garage.

Wheel/tire selection is 17X11 with 0 offset and 285/40R17 rear--17X9.5 with 0 offset and 265/40R17 front.

Honestly, no more to the story. I did my time in the minitruck thing years ago, including a 1980 Mazda with the top leaf flipped to provide 0 suspension (can't get any lower though), so I can handle harsh. I have found that harsh is not typically the fast way around corners, especially when there is a bump in the middle (ask me how I know). I want very firm, but compliant. I am 80% confident my current situation (totally stock with a 4" drop) is my own doing and even with softer springs would still handle like a dookie sandwich. Just thinking out loud I guess. So Tommy thinks about 620/175, not far off from what I was going with. I actually figured 800/175 (no, I didn't already order them), not sure on shocks yet. I have the Bilsteins but don't have any personal experience with them.

Before I get flamed for the list, I am responding (maybe overboard) to a request for more information. Thanks again

but I dont really see one throwing an Elco around like my F-body, so my selection is completely based off of the 67's chassis set-up. But as for the Elco, it went like this:

Oh I fully intend to throw it around like an F body.

[chicane67]

My bad Dennis.....I should have inquired about your power plant (materials construction included), transmission and weight modifications. That is what would be important to me, so that I may possibly interject a concise direction for consideration......

I dig your suspension direction. Learning from those amoungst us in the past (related) suspension threads has been priceless and shed light onto all involved. I am greatful for what we have collectively in our threads for personality. Its been like 'Miracle Grow' for the masses in concerns to understanding vehicle dynamics as of late.

But, I do know that any real opinion on what front spring rate to consider, will be ultimately determined by your power plant, its measured power output and other possible weight distribution issues. If you could include such items, I would like to ponder a possible solution.

Please include your Bilstein part numbers, that information is invaluable to some of the calculations.

Drive it like an F-Body......grab it by the wheel and start slapping some rears....uh....I mean gears. sorry....sometime a distracting woman IS a good thing. I gotta go........

[dennis68]

The powertrain as it sits right now is a 350-400HP 360" sm blk/ Tremec TKO/ 12 bolt 3.08.

Future plans are an LS1, that may be a year or more in the future though. When I drop in the LS1 Iplan on setting back a couple inches (every little bit helps).

[Salt Racer]

Tom,
Where did you run your car at (probably El Mirage)? How fast did you go?

Sorry, off topic but I just had to ask.


Dennis,
You probably could drop down RRCH a little bit, assuming roll axis doesn't go to oversteer direction. P/U has lower localized CG on arse end. And due to long rear overhang (larger polar moment), it's better to let the chassis take a set more gradually. Note: the latter is just my theory - I could be wrong.

[Norm Peterson]

Question about spring rates as I am getting ready to order some coils. . . . . My question is this, what spring rates are we running out there on there street and how do they handle everyday driving? To qualify to answer your car actually has to drive under its own power more than 10 miles a month. I am looking for real world feedback, guys who drive their SCCA cars to/from the track. .

The Malibu in my sig is driven to/from events (~80 miles each way for some) and has been daily driven and taken trips.

Around 3625 lbs with me in it (175-ish)
Approximately 53/47 distribution (as weighed, SBC w/alum heads, Tremec 3550)

Geometric roll centers are about -1.4" F and 16.7" R. I know, too low up front and way too high in the rear, but I didn't put them there.

Springs - 639 F, 167 R. Front motion ratio is roughly similar to the '88 F-body.

Sta-bars - 32 mm F, 22 mm R. Front bar has poly chassis bracket bushings and modified poly endlink bushings. Rear bar is control arm mounted.

Bilstien shocks (that would benefit from revalving)

Poly control arm bushings up front, modified poly bushings in the rear lowers, poly body:frame bushings.

To me, the ride is firm-ish but entirely liveable on a daily basis. But it's a bit on the soft side as regards auto-x, especially through slaloms. And although my wife drives it or rides in it on occasion, she prefers not to (ride quality isn't the reason).

Norm

[nitrovette]

Im now a firm believer in bilstiens,i had the exact thing your describing and yes it get s scary,but ive switched to bilstiens and it completely changed the way it drives and handles now,definitely worth the extra bucks.I did the same thing as you cut 2 coils from my 450's and i have a single 360 rear spring and it was horrible ,but once i swapped to the bilstiens it was night and day,cheap shocks sucks.