coil over spring ratings

hey guys, i bought a new car & i'm swapping some parts over from my last car. one of the things i'm swapping are the coil over shocks off the rear. my last car was a ladder bar car & the springs i was using were rated at 130lbs. The new car has a 4 link & the springs on that are rated at 250lbs. The old car was a Dart with subframe connectors & is considerably more weight than the new car which is a tube frame car. now the new car used to run a blown Hemi into the 7.5's, i'm just making it a NA bracket car to run mid 8's. Should i re-use the 250lb springs or will the 130lb'ers be ok. Thge big differences are, less weight than before & changing from a ladder bar to a 4 link. The car did come with a set of 170lb springs in the spares box. I'm keen to hear what you think.

If the old car had 130 lb springs and was heavier than what you have now, I can't see any reason to put in heavier springs. General rule is to use the lightest spring possible to support the car. FYI, my new tube chassis, ladder bar project, I'm starting with 110 lb springs, and may drop lower.

So where does “lightest spring” and coil bind on rebound meet happy ground?

250? On the rear? May as well run solid suspension unless it weighs 5000#. 12" springs? Mounted behind the axle? Weight on the rear axle?

I would be careful using a light spring on a ladder bar car with any power. Light springs can be used on a lower powered 4 link car to store some energy and help apply the tire or on a ladder bar car with decent shocks to control the housing. Obviously you don't want to get into coil bind, but spring rates are something that can used to help get the car to respond the way you want.

Well because I haven't run the car with my power plant, I don't know how much the springs will compress. I was told, from probably not the most reliable source, that ladder bar cars need less spring in the back. I know the previous owner installed the 250lb springs as he tried to control the launch. From how i had my ladder car set up, i think he meeded to go to suspension school because there was a lot of preload on one side. I'm guessing the 170lb springs were on the car before he stepped up to the 250's. So I guess from the answers so far, the 250's are too much so they can go with the old car when I sell her. Does anyone on here have a tube frame car that can tell me what size they have?

My 3000lbs chassis 4-link 1500lbs over rear 850 hp really responded well
when i went from 110 lbs springs to 125 lbs spring.

The lighter spring helps plant the tire. Power helps plant the tire. With a blown Hemi it most likely didn't need much if any stored energy assist to hook. My bet is the shock valving is extremely stiff as well. More than likely with your combo it'll need looser adjustment on the shocks (if they will go loose enough). I'd start with a 95-110 lb spring.
Doug

You have a tube chassis car, you need light springs. Unless you are using thick wall fence pipe, there won't be enough weight to need anywhere near that kind of spring. I have 200 lb. springs in the front of mine, and right now, they are too much to even start compressing my struts. If you can install your springs, run up the adjusters 1/2" past contact, install the shocks, and have them compress to the middle of shock travel, you should be good. My bet is that your springs aren't going to do that.

You can't compare spring rates, even on the same car, unless the attachment points (especially the distance from the wheel) are very close.
A coil-over replacing a shock (in the factory mount positions) is different than a four link with its own mounts. Shorter distance spring > wheel: spring acts heavier.
Yes, this also affects your roll rate, and sway bar size.

True, on an A-arm or independent type suspension. On a 4-link solid beam axle, there is no lever effect. On a ladder, moving the shock closer or farther from the front mounting point and/or away from the rear axle C/L will make a difference in spring rate. Ladder bars are just big A-arms. 98% of the stuff now, that's not 40 years old, is a coilover mounted just behind the axle using a 12" spring. Once you get away from that, things can change.

The placement of the shock between chassis C/L and the wheels will affect the stability and roll characteristics. It also reduces the ability to use spring heights and uneven shock adjustments to tune. I'd prefer to see them kept on the center or even outboard of the ladder/4-link mounts.

On most set-ups, there are maybe 3 common rates (i.e. 95, 110, 130) that fit in the window between not being able to get the shock compressed to ride height and not being able to hold the car up. The choice depends on a lot of factors.

ok so the car is extremely light. i have the diff out at present getting different gears installed. with no diff in the car i can lift the back of the car off the jack stands with a bit of effort. i'll stay with the 130's & just do some tnt to see how well it performs. It does make me wonder why it came with 170's tho. it was built for the 8.5 sec class.

Very nice!

I don't think it has ben mentioned but you need to get the car scaled before you really spend money on springs. I had a set of springs that were to stiff, asked the same questions hear. Got a answer that sounded good, bought that spring rate, nope to light. Bought another and another before I got it right. If your lucky you can sell those extra springs for half what you paid for them. Get it scaled when you get it back together, get the proper springs, get the weight set side to side and ride height adjusted. The car will launch straight also.. Like the spoiler, got any close ups of it?

The car came with a different rear wing, i didn't like it so i made this. It's Zeus clipped on for days i need to look in the trunk but it refuels from a hatch in the middle of the trunk as shown.

this is the front. this was the day i bought it back from the paintshop

Its not that tough. Rear weight minus unsprung rear weight. Rear unsprung weight comprises the full weight of rear tires,wheels,axle w/brakes. Then add half of the weight of the shocks, suspension arms, drive shaft. Run the attached formula for shock location, simple math. I approximate the unsprung by removing the tire/wheels. Then put a scale between a jack and the axle housing. Start to jack it up from full droop. When the jack starts to move, read the scale. This will get you close. That all being said CMcAllister is correct. Not a super wide range that works. Most non tube door cars will be 120-140. Lighter tube cars 90-110. The only reason to use a stiffer spring is when you have a ton of power and the chassis doesn't require any assistance to hit the tire.
Doug