Linear vs Progressive springs - which option for best ride?

[RetroStang]

Hi All -
New to the forum and no luck on my search for an answer, so thought I'd solicit opinions. I have a '69 Mustang fastback with a Heidts rear 4-link suspension installed by the previous owner. The ride is very stiff in the rear, banging over every tar strip and pavement seam. I believe the existing coil springs are too high a rate, and I'm looking to replace them and the shocks, probably with ones from QA1 or RideTech. My question is, which springs will give me a better ride for daily driving and an occasional trip to the track - a progressive spring or linear? Based on what I've been able to research, the weight on the rear axle is around 1425 lbs, and the recommended spring rate is 225 lbs. I can get linear springs in that specific rate, or I can get progressive springs with a 140/250 lbs rate. I'm also thinking an adjustable shock would help in the fine tuning of the ride. Anyone have any experience with a similar setup? Any opinions?

[dontlifttoshift]

There are at least 10 billion things that could cause


banging over every tar strip and pavement seam

but I bet shocks are the largest problem. Take some careful measurements at the current ride height and make sure you are not bottomed out or topped out on shock travel.

To your question, linear springs are the answer.

[TheJDMan]

I agree! A double adjustable shock should be your first step. I don't think a progressive spring will gain you much but I have never run one so I can't speak from experience. I know keeping my 200lb linear springs and changing shocks made a dramatic improvement. Like Donny said, do some careful measuring and make sure your current shocks are not at their travel limits.

[marolf101x]

Both above are correct. . .first, make sure you don’t have a suspension bind. There is the possiblilty that the 4 link bars were over-tightened, so the suspension doesn’t move freely. This is easy to check. . .lift the rear off the ground, place jack stands under the frame, remove the rear coil overs, move the rear und up and down by hand. If you cannot, loosen all the link bolts and try again. We see this all the time where someone tightened the suspension while it was hanging on a lift, or with an air impact gun and it just doesn’t move.

Second, measure to make sure you aren't physically bottoming or topping out the suspension. Most "hard" ride quality issues are from the suspension physically hitting a stop and transferring all that energy into the cab. A suspension that has adequate movement will absorb the road imperfection/irregularity without transferring that energy. . .look up sky hook theory link (https://en.wikipedia.org/wiki/Active_suspension#Skyhook_theory)
Once you know you have ample movement (we shoot for 5" wheel travel) you can move on to the shocks and springs.

Shocks:
I somewhat disagree with Steve above. . .you don't need a double adjustable shock. Most DA's adjust both compression and rebound. You really only need to adjust rebound, and here's why:

Compression is the action of the wheel/tire moving up into the wheel well. As it does the spring is being compressed. Though you get some force from the damper (depending on the damper), it's mainly the springs job to be stiff enough to slow down the wheel/tire and keep the suspension from hitting the bumpstop (or bottoming out on a coil over), but soft enough to not transition too much of that force into the cab.

If you have a shock with a lot of compression force (or a shock that you can turn the knob and add a lot of compression force), then you essentially add spring rate. Now, that depends on the valve stack in the shock. . .but for most of what we are discussing here hitting a bump on a road during regular driving will cause the shock to excerpt force into the suspension, just as if you were running higher spring rates.

Springs:
So my theory has always been to use a spring rate just high enough to mitigate bottoming out the suspension, but light enough that it doesn't jar your teeth out around town. It's a simple math problem . . .Lets use an example with easy math. . .
Let’s say you have a car that weighs 4000lbs, and the weight distribution is 50/50, so you have 2000lbs over the rear wheels or 1000lbs each. . .which means with a motion ratio of 1:1 (each spring holds up equal the amount of corner weight), each spring must hold up 1000lbs at ride height (or installed height).

Spring rate is the measurement of force per inch measured in lbs/in. So if you have a 250lb spring rate that means it will provide 250lbs when compressed one inch. Compress it two inches, and you get 500lbs, compress it three inches and you get 750lbs (assuming this spring is linear).
So, tying into the example above. . .the spring must hold up 1000lbs per corner at ride height (2000lbs rear weight, 1000lbs corner weight).

That spring needs to compress whatever the suspension designer says. . .in our case, we want 5” overall wheel travel, with 60% of shock stroke available for compression, and 40% for extension. . .so 3” compression travel, 2” extension travel (unless you live in Hazzard County and drive a Mopar you probably won’t be jumping over any “magic” bushes!)


So now let’s look at a couple of spring options:
#1: you could put a 1000lbs/in coil spring in. But it would be compressed only 1 inch when at ride height. When your suspension drooped the required 2” one spring would come unseated, and rattle around. Oh, and the ride quality would be crap as you would add 1000lbs each inch you compressed the spring.

#2: If you use a 250lbs spring, it must compress 4” to achieve 1000lbs force. If the spring were 12” long (free length), it will be roughly 8.0” tall when installed and at ride height.
If your installed height was 8”, and you must have 3” of compression travel, the spring needs to compress to 5”. At full compression the 250lbs spring is providing 1750lbs force.

#3: Let’s throw a 150lbs spring in there for fun. Now you must compress that spring 6.67” to achieve 1000lbs. You add in the required 3” compression travel and the spring is now compressed to 3.67” height. . .which is probably too close to coil bind. However, if it would work, that spring is providing 1450lbs of force. A lot less than the two above.

There are 3 spring rates that will physically hold the car up. However, you can see how you have to do a little math to figure out what rate you actually need (we use wheel frequencies and rates as well).

I realize I’ve not touched on progressive springs, but this has gone on long enough. If you’d like I can discuss those as well.

Bottom line?
1-Make sure you don’t have a bind.
2-Make sure you don’t bottom or top out the coil over
3-use our spring rate calculator to get the correct rate for ride quality (http://www.ridetech.com/tech/spring-rate-calculator/)
4-use the best shock you can get. Make sure it has a very low compression number at low shaft speeds. This will provide good ride quality.

If you have more questions, toss them up here and we’ll address them.

[RetroStang]

Thanks for the great responses. Britt, I really appreciate you taking the time to explain the science behind the choices available. Based on my research of 1425 lbs on the rear axle, what spring rate would you recommend?

So what "careful measurements" should I take? I'm assuming ride height, but what else? Length of spring under it's current load? Free height? Details please.

[marolf101x]

You really should run this through our spring rate calculator yourself so you understand what is going on. . .and you'll have all the information, where I have to make a few assumptions.
Based on what I "think" is correct the calculator shows a 162lbs/in (http://www.ridetech.com/store/coilover-coil-spring-2.5-id-12-free-length.html) rate for daily cruising, 200lbs/in (http://www.ridetech.com/store/coilover-coil-spring-2.5-id-12-free-length.html) rate for performance driving, and 225lbs/in for racing.
**I added links to the springs on our website. This is assuming you use at least a 5" stroke shock, which would utilize a 12" free height spring.***

However, as I stated above spring rate doesn't mean a damn thing if the suspension is bound (link bar bolts too tight), or if the coil overs are bottoming out.

What you want to measure is the shock travel available from ride height. You need 3" for compression and 2" for extension. If you can get to it the easiest way to measure the compression available is to measure how much shock shaft if showing at ride height.

[MonzaRacer]

Dollar for dollar Ridetech has best ride/handling coilovers for the price.
Two things, first pull rear spring/shock and use floor jack to cycle rear suspension from fully extended to fully compressed. if it cycles with no bind then you should match up length and stroke as Britt recommended. For the more enthusiastic driver/autocross bound the price of single vs double adj is no brainer and go for double adj. just cruisin, singles work.
I have helped 7 or 8 people swap in Ridetech coilovers and let me tell you the ride/handling difference was dramatic.. and on car had a set of those uber high end brand remote reservoir race oriented and the handling was a thousand percent better.

[Rod]

Whoa! good write Britt


Both above are correct. . .first, make sure you don’t have a suspension bind. There is the possiblilty that the 4 link bars were over-tightened, so the suspension doesn’t move freely. This is easy to check. . .lift the rear off the ground, place jack stands under the frame, remove the rear coil overs, move the rear und up and down by hand. If you cannot, loosen all the link bolts and try again. We see this all the time where someone tightened the suspension while it was hanging on a lift, or with an air impact gun and it just doesn’t move.

Second, measure to make sure you aren't physically bottoming or topping out the suspension. Most "hard" ride quality issues are from the suspension physically hitting a stop and transferring all that energy into the cab. A suspension that has adequate movement will absorb the road imperfection/irregularity without transferring that energy. . .look up sky hook theory link (https://en.wikipedia.org/wiki/Active_suspension#Skyhook_theory)
Once you know you have ample movement (we shoot for 5" wheel travel) you can move on to the shocks and springs.

Shocks:
I somewhat disagree with Steve above. . .you don't need a double adjustable shock. Most DA's adjust both compression and rebound. You really only need to adjust rebound, and here's why:

Compression is the action of the wheel/tire moving up into the wheel well. As it does the spring is being compressed. Though you get some force from the damper (depending on the damper), it's mainly the springs job to be stiff enough to slow down the wheel/tire and keep the suspension from hitting the bumpstop (or bottoming out on a coil over), but soft enough to not transition too much of that force into the cab.

If you have a shock with a lot of compression force (or a shock that you can turn the knob and add a lot of compression force), then you essentially add spring rate. Now, that depends on the valve stack in the shock. . .but for most of what we are discussing here hitting a bump on a road during regular driving will cause the shock to excerpt force into the suspension, just as if you were running higher spring rates.

Springs:
So my theory has always been to use a spring rate just high enough to mitigate bottoming out the suspension, but light enough that it doesn't jar your teeth out around town. It's a simple math problem . . .Lets use an example with easy math. . .
Let’s say you have a car that weighs 4000lbs, and the weight distribution is 50/50, so you have 2000lbs over the rear wheels or 1000lbs each. . .which means with a motion ratio of 1:1 (each spring holds up equal the amount of corner weight), each spring must hold up 1000lbs at ride height (or installed height).

Spring rate is the measurement of force per inch measured in lbs/in. So if you have a 250lb spring rate that means it will provide 250lbs when compressed one inch. Compress it two inches, and you get 500lbs, compress it three inches and you get 750lbs (assuming this spring is linear).
So, tying into the example above. . .the spring must hold up 1000lbs per corner at ride height (2000lbs rear weight, 1000lbs corner weight).

That spring needs to compress whatever the suspension designer says. . .in our case, we want 5” overall wheel travel, with 60% of shock stroke available for compression, and 40% for extension. . .so 3” compression travel, 2” extension travel (unless you live in Hazzard County and drive a Mopar you probably won’t be jumping over any “magic” bushes!)


So now let’s look at a couple of spring options:
#1: you could put a 1000lbs/in coil spring in. But it would be compressed only 1 inch when at ride height. When your suspension drooped the required 2” one spring would come unseated, and rattle around. Oh, and the ride quality would be crap as you would add 1000lbs each inch you compressed the spring.

#2: If you use a 250lbs spring, it must compress 4” to achieve 1000lbs force. If the spring were 12” long (free length), it will be roughly 8.0” tall when installed and at ride height.
If your installed height was 8”, and you must have 3” of compression travel, the spring needs to compress to 5”. At full compression the 250lbs spring is providing 1750lbs force.

#3: Let’s throw a 150lbs spring in there for fun. Now you must compress that spring 6.67” to achieve 1000lbs. You add in the required 3” compression travel and the spring is now compressed to 3.67” height. . .which is probably too close to coil bind. However, if it would work, that spring is providing 1450lbs of force. A lot less than the two above.

There are 3 spring rates that will physically hold the car up. However, you can see how you have to do a little math to figure out what rate you actually need (we use wheel frequencies and rates as well).

I realize I’ve not touched on progressive springs, but this has gone on long enough. If you’d like I can discuss those as well.

Bottom line?
1-Make sure you don’t have a bind.
2-Make sure you don’t bottom or top out the coil over
3-use our spring rate calculator to get the correct rate for ride quality (http://www.ridetech.com/tech/spring-rate-calculator/)
4-use the best shock you can get. Make sure it has a very low compression number at low shaft speeds. This will provide good ride quality.

If you have more questions, toss them up here and we’ll address them.

[RetroStang]

Finally got around to removing the springs and shocks to test the rear suspension. Good news in that the four link didn't bind at all and appeared to move freely using my jack to raise and lower the differential. Took the following measurements:

- spring (uncompressed) is 11" long
- shock (fully compressed) is 9 3/4" from bolt-hole center to bolt-hole center
- shock (fully extended) is 14 3/8" so about 4 5/8" of shaft

I'm thinking I may have found my problem though (no rebound???)138217

I don't know what the spring rate is, so I'm thinking I should order single adjustable shocks and springs. Any suggestions?

[dontlifttoshift]

That sure looks like 10 1/2" compressed to me.

What is the center to center at ride height?

[RetroStang]

It's an optical illusion because of the camera angle. The shock compressed is 9 3/4 center-to-center. In looking at my measurements earlier, the spring only compressed about 2 1/4 inches, so I'm thinking the spring rate is probably 300 or 350 ft/lbs.

The ideal ride height center-to-center is 13 1/2 inches. When I measured it prior to removing everything, it was just shy of 14 inches, but I thought it was a little too high.

[Peter Mc Mahon]

You said your shock is fully extended at 14 3/8 and it was just shy of 14 when it was in the car? You only have 3/8 of an inch for your shock to extend. Your ride height needs to come way down so your shock can extend. I think the normal is 60% compression and 40% extension or close to that. I think ride height would be in the ball park of 12 1/2". I would change this first and then go from there.

[RetroStang]

You said your shock is fully extended at 14 3/8 and it was just shy of 14 when it was in the car? You only have 3/8 of an inch for your shock to extend. I believe that is why my ride was so bad - the installed springs were too stiff to allow the suspension to work properly and the shock was at the top of it's travel.
Springs:
So my theory has always been to use a spring rate just high enough to mitigate bottoming out the suspension, but light enough that it doesn't jar your teeth out around town.How would I know what is the best spring rate? Using Britt's calculation, I come up with a 12" spring at 170lbs or a 10" spring at 225lbs, to put the car at the correct ride height (there are multiple holes I can use to install the shock in). I don't know what the factory spring rate was for a '69 Mustang, but 170lbs seems kind of low. I don't want to bottom out the suspension when I have 4 people riding in the car (rare, but occasionally have back seat passengers). I'd like a "tied-down" type feeling ride and not "floating", but I don't want to rattle my teeth either...

[marolf101x]

Original spring rates were tossed out the window when the 4-link was installed.
Unless you want to educate yourself on what spring rates are, how to calculate them, what they do to the car, etc, just take our advice and click the part number provided when you put your vehicle weights and measurements into our spring rate calculator.

As I'm not terribly familiar with the Heidts rear kit I don't want to make an assumption and be wrong as I didn't have all the info.

I can tell you the "shock" you show above is about the cheapest thing I've ever seen. Just changing to a good shock with proper stroke will make a HUGE improvement.
We try to use a 5.2" stroke shock on the rear as you want 5" of usable suspension travel.
170-225lbs spring rate sounds about right.
For example our Mustang kit comes standard with a 12" 175lbs spring.

If you have questions, please feel free to ask.

[dontlifttoshift]

You said your shock is fully extended at 14 3/8 and it was just shy of 14 when it was in the car? You only have 3/8 of an inch for your shock to extend. Your ride height needs to come way down so your shock can extend. I think the normal is 60% compression and 40% extension or close to that. I think ride height would be in the ball park of 12 1/2". I would change this first and then go from there.

This.

What hole on the rear end is the shock installed in? Top, middle, or bottom? Would you like to lower or raise the car and if so by how much?

Let's say you have a 13 3/4" ride height at the shock, if you answer the above questions we can get you the exact shock that you need.

[RetroStang]

What hole on the rear end is the shock installed in? Top, middle, or bottom? Would you like to lower or raise the car and if so by how much?
The shock was installed in the bottom hole. Would like the ride height at 13 1/2", if I use the bottom hole again.

[dontlifttoshift]

Here is your part number

Coilover Shock - 4.1" Stroke - HQ Series
Part #: 24149901

That should get you nearly 5" of wheel travel assuming a layover angle of 15 degrees. I would use a 200# spring on that application.

[RetroStang]

Thanks for the info - I really appreciate it!