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Discussion Starter #1
Like the title says :) Why would I want an 8" 10" oe 12" spring in the 325# range?

Im getting MM coilovers and plan on the 325-350# range for springs. I want a great cornering street car.

Thanks
 

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Like the title says :) Why would I want an 8" 10" oe 12" spring in the 325# range?

Im getting MM coilovers and plan on the 325-350# range for springs. I want a great cornering street car.

Thanks
To answer the question, you will have a gap between the spring and upper spring seat at full rebound with 8" springs. I corrected my gap with some Hypercoil spring helpers. With a 10" you would probably be OK.

That being said I would also question the reasoning to run coil overs on a street car. There are many other things youcan do to improve street manners that would be a more effective use of your money. A good set of dampers along with a set of quality springs goes a long way. From my experience a PHB cures a lot of the handling woes of a Mustang by making it much more predictable.
 

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Discussion Starter #3
I am considering purchasing some used components that include 12" 325# springs - what would be the ups and downs of that length spring?

I hear ya on the street car thing. This is a weekend only car and I started by just wanting more clearance for my longtubes and some weight reduction. I don't trust most of these lightweight suspension setups for street driving. I especially didn't want to go backwards in suspension performance just to save some weight, so I decided to go the MM route :)
 

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You need to look into ride height and spring travel. You want enough spring so you can get full compression without coilbind (unless you are running a NASCAR coilbind setup, which you shouldn't be but you don't want to much spring that you can't set the desired ride height. Generally the stiffer the spring the shorter you can go. For a 325 I'd think 12" would be to long 10" might be a better choice. I also think 325 is going to be to soft especially for a 'great cornering car' street or anything else. At least get 350 and if you don't mind a little hard ride 375's. Spring length is a guess the best way is to setup the car then measure the required spring length, but 10" should get you close.

You are going to either get end play in the coilovers at full droop or you will need to compress the springs to get them installed. That's a pain, so plan for a shorter spring and get helper springs as mentioned above. Coleman racing has them.
 

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Coil-over spring free length

you will have a gap between the spring and upper spring seat at full rebound with 8" springs

With C-C down I was bored and looked through here a bit. I tired, but I just couldn’t leave this technical error alone.

Spring free length in a coil-over application has nothing to do with whether or not the spring will go slack at full droop. I know it seems like it should and lots of people apparently think it does, but it doesn’t.

Car weight, ride height, and spring rate all have an effect. Spring length doesn’t…


Richard P.
 

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With C-C down I was bored and looked through here a bit. I tired, but I just couldn’t leave this technical error alone.

Spring free length in a coil-over application has nothing to do with whether or not the spring will go slack at full droop. I know it seems like it should and lots of people apparently think it does, but it doesn’t.

Car weight, ride height, and spring rate all have an effect. Spring length doesn’t…


Richard P.
You are correct sir. However, having the same car, same setup and same length spring I speak from experience. With the MM coil overs there isn't enough threaded collar to prevent this from happening with an 8 inch spring in his desired rate, even at standard ride height. With a 10 inch spring you can make it work. Had this been a Honda or a Nissan or F-Body he was asking about, then I wouldn't have a clue. I this instance I do have a clue.
 

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That being said I would also question the reasoning to run coil overs on a street car. There are many other things youcan do to improve street manners that would be a more effective use of your money. A good set of dampers along with a set of quality springs goes a long way. From my experience a PHB cures a lot of the handling woes of a Mustang by making it much more predictable.
I actually found that going to coil-overs on my then street-only car made a huge difference in ride quality. I was using the exact same Koni dampers, just went from conventional springs to D&D coilover sleeves. I was pleasantly surprised how much better it felt, despite the fact that I was actually running a higher wheel rate than I had been with the conventional springs.

At that point I was running 10" 300# springs. Now I'm running 8" 425# springs. I would think in the 325# or 350# variety that you'd want 10" (and I agree that 350# or maybe even higher is the way to go).

Another consideration on spring length is wheel/tire clearance. At a given ride height and a given spring rate, the longer the springs you use, the farther down the lower spring perch will be. If you're running 9" wide wheels with fat tires, the lower spring perch might actually interfere with the wheel and/or tire. Perhaps Darius Rudis will chime in with his picture showing that exact problem on his car, which he fixed by going with a shorter spring.
 

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With the MM coil overs there isn't enough threaded collar to prevent this from happening with an 8 inch spring in his desired rate, even at standard ride height.

Huh??? If you run out of threaded collar, you can't get it to the ride height that you want. What does that have to do with the point I was trying to make?

If you use a spring spacer with the 8" coil-over spring so that you can get the ride height you want with the available threaded collar, the fact that it is an 8" spring is still not going to have anything to do with whether or not the spring goes slack at full droop.


Richard P.
 

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Discussion Starter #9
Richard - I can see what Glenn is saying, but I am a little confused by your comments. It seems that your main argument is that by using spring spacers, you can avoid an 8" spring going slack on suspension droop. If that's the case, you might be correct, but that wasn't really the issue being discussed.
 

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Huh??? If you run out of threaded collar, you can't get it to the ride height that you want. What does that have to do with the point I was trying to make?

If you use a spring spacer with the 8" coil-over spring so that you can get the ride height you want with the available threaded collar, the fact that it is an 8" spring is still not going to have anything to do with whether or not the spring goes slack at full droop.


Richard P.
I'm not quite sure what you are driving at. The man asked about using an 8, 10 or 12 inch 325# spring with MM CO's on his 03 Mustang. I said the 8 inch would not be long enough without helpers, that a 10 inch would be a better choice. How you can infer that my response dealt with anything other than that specific application is beyond me. Yes, you can use different length springs if you are willing to purchase additional pieces to make it work. Shorter struts, travel limiters or helper springs are all options. But with stock suspension travel up front, 8 inches is not long enough no matter the spring rate, car weight or ride height without other parts added to the mix.
 

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It seems that your main argument is that by using spring spacers, you can avoid an 8" spring going slack on suspension droop.

What??? My argument is that spring length has nothing to do with a spring going slack at full droop. If an 8" spring goes slack at full droop in a certain application, a 14" spring is also going to go slack at full droop in the same application. A spring spacer also has nothing to do with a spring going slack at full droop.


Richard P.
 

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Understanding spring geometry with respect to available droop and bump travel is not a simple thing. Let me try to explain. I think that some of the issues here are that Richard and Glenn aren't in agreement on vocabulary they are both using.

For this entire explanation I'm assuming that the car has zero caster and zero SAI (steering axis inclination). In other words, the strut/spring are perfectly vertical and act directly at the center of the tire on the ground. Making this assumption will just remove some confusing trigonometry from the discussion, but will not change the conclusions.

Bump travel, the easy one:

Spring deflection under static conditions is governed by the formula:

F=kx

F is the force on the spring.
k is the stiffness of the spring
x is the change in spring length

Example #1) Consider a car with 1,000lbs of sprung weight on each front corner. Using an 8" spring with a rate of 450lbs/in (k). Assume the spring has a coil bind height of 3.95". Coil bind height is the height of the spring when all of its coils are stacked against each other. It can't ever get any shorter than this.

From this we know that the spring has a total available travel of 8"-3.95" = 4.05". When the car is placed on the ground the spring will compress 2.22" because the spring has a rate of 450lbs/in (k) and will push back with a force of 1,000lbs (F) when it is compressed (1,000lbs/450lbs/in) = 2.22" (x). Remember the car is at vertical equilibrium (not moving) so the vertical spring force must equal the vertical force from the car, by definition. We now know that there is (4.05"-2.22") = 1.83" of bump travel available before the spring coil binds.

Of course if the struts were badly designed, we could run out of bump travel before the spring coil binds. But this has nothing to do with the maximum available bump travel that this 8" spring can allow with this weight on it.

Notice that none of this has anything to do with ride height. If the car is too low, it is possible for the tire to hit the fender, the frame rail to hit the ground, etc. before the spring coil binds.

Example #2) If we use a 10" 450lbs/in (k) spring with a coil bind height of 5.00", then we have 5.00" of total travel available and 2.78" of bump travel available at ride height.

Going to a longer spring, with the same rate, gives you more available spring travel, which is ONLY ever available in bump. In example #1 we had 4.05", in example #2 we had 5.00". This is the advantage to a longer spring. It is important to note that it is possible to build a 10" 450lbs/in spring with LESS total travel available than an 8" 450lbs/in spring. This is a function of using the correct wire gauge, steel alloy and closing and grinding the spring ends properly. Within a given brand/line of springs, a longer spring of the same rate will almost always have more travel available.

Droop travel, the not so easy one:

Let me start with a definition of droop travel. Droop travel is the distance the tire can drop, from ride height, until the spring has lost contact with the upper spring perch. Why not count the travel from that point down to where the strut has no more droop travel? Once the spring has come off of the upper perch, there is virtually zero force pushing the suspension downwards. Only the gas force in the strut or shock (about 40lbs per side) is going to pull the suspension down further. The strut damping forces are going to make that 40lbs take forever to pull the suspension to full strut droop.

In example #1 above, the front suspension had 2.22" of droop travel, because that is how much the spring was compressed. In example #2, the front suspension also had exactly 2.22" of droop travel available also. Huh? From this it should be clear, that the length of the spring has ABSOLUTELY nothing to do with whether it comes loose at full droop or not. The force on the spring and the spring rate determine how much the spring is compressed at ride height. If this amount of compression, 2.22” in the above cases, is less than the amount of droop travel in the strut at that ride height, then the spring will come loose at full droop.

It is possible for someone to build a strut with not enough total travel, and for this to artificially limit the droop travel in the suspension before the spring comes loose. This is essentially what all auto manufacturers do to keep the springs from coming loose. They make sure that the spring always has more available travel than the strut.

Bump resistance: Bonus points.

How much bump travel is needed on a given car to not have it bottom out? Higher wheel rates obviously make it harder to bottom out, but higher car mass makes it easier to bottom out. In addition, more bump travel makes it harder to bottom out. Which is most important? All three are. The formula below gives a normalized parameter which is directly proportional to how hard it will be to bottom out the suspension on a car.

g = Bump Travel/Droop Travel

If g is above 1, the car will be fine when used with a bumpstop. If g is below 1, it may be too easy to bottom out, especially if driven on curbs at a road race track.
 

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What??? My argument is that spring length has nothing to do with a spring going slack at full droop. If an 8" spring goes slack at full droop in a certain application, a 14" spring is also going to go slack at full droop in the same application. A spring spacer also has nothing to do with a spring going slack at full droop.


Richard P.
If I have 12.5" between the upper and lower perch at full droop, then a 10" and a 12" spring will be slack at full droop while a 14" spring will still be in compression. Seems pretty simple to me.

EDIT: Mr. Hidley beat me posting, guess there is a terminology issue.

Now, on a street car though, when is it that you really experience that full droop? The only time my 400# 8" springs become unloaded is when I jack the front end of the car off the ground. If I jack one wheel off the ground the coil spring is still loaded because of the sway bar. So in a driving condition, I would have to be doing some serious off track action or some WRC Rally stuff.

The main considerations in spring length are desired ride height range, coil bind and wheel clearance.
 

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FWIW, my stock ride height car uses 10" springs, which have zero slack at full droop. In fact, one of them has slightly negative slack, so when doing work with them I have to pry the strut upwards slightly to get the ears lined up with the spindle.
 

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Well, Jack gets the math but I disagree with his assertion that the suspension won’t go into full droop because all of the spring preload is gone. The weight of the wheel, tire, spindle, and brake package is more than enough force to keep the suspension moving until the strut tops out. I also disagree that the only time a car would see full droop is when you are jumping it or jacking it up but I don’t really want to fight that battle.

I’ll take Jack’s math and simplify it a bit more with an idealized car. I’ll even work it out with numbers that make the math easier and more obvious. Also following Jack, I’ll include coil-bind information since it’s all the same math and is important to the current discussion.


So we have a hypothetical 3000 pound race car with 53% of the weight on the front. It’s all nice and balanced so we have about 800 pounds on each front tire. For this hypothetical car, we are going to assume we have coil-overs with a 1:1 ratio. For this example, we are going decouple one front corner from the car and ignore the rest of the car. We will also assume that there is 5” of suspension travel and our baseline ride height will be to have 3” of bump travel and 2” of droop travel.

To start the example, the initial condition will be with the spring perch all the way down with the spring floating freely and the car resting on its bump stop. Since the corner of the car weighs 800#, there is an 800# force compressing the bump stop. We will equip the car with 400 pound per inch springs with an 8” free length. We’ll use Eibach springs just because they have all the important spring data listed in their motorsport catalog on their website.

We can adjust the spring perch up until all the slop is taken out of the assembly. The spring isn’t compressed but it’s not floating freely and the bump stop is still seeing the 800# load. If we adjust the spring perch up one inch from here, the spring is compressed one inch and has a preload of 400 pounds. The bump stop is still seeing a load but it has been reduced by 400 pounds.

If we adjust the spring perch up another inch, the spring has been compressed a total of 2” and has an 800# preload. The car is still touching the bump stop but it isn’t seeing any load.

If we adjust the spring perch up another inch, different things start to happen. The spring does not compress any further. It is still compressed 2” an has a preload of 800 pounds, equal to the corner weight of our hypothetical car. The corner of the car has been raised up one inch off of the bump stop. To get the car to our baseline ride height, we will have to adjust the spring perch an additional 2” to get us three inches off of the bump stop.

Now if we lock down the spring perch and put the suspension through it’s travel, we of course find the spring compresses in bump and extends in droop.

To see if the spring goes slack in droop, we’ll jack up the car until the tire is off of the ground. Since we have 2” of droop travel, the spring has extended 2”. From the calculations above, we find that we had compressed the spring 2” earlier and thus the spring does not go slack in this case but it does loose all of its preload.

With the car back on the ground, we now compress the suspension 3” to full bump. The three inches of bump travel combined with the 2” of spring preload give us a total spring compression of 5”. But wait, the Eibach specifications show that an 8” long 400# coil over spring only has 4.76” of travel before it coil binds. With this setup, we will coil bind before we hit the bump stop (that’s bad).

So now lets change things up to see what happens. If we lower the ride height by one inch, we now have 2” of bump travel and 3” of droop travel available. The corner of the car still weighs 800# and the 400 pound per inch spring is still compressed 2” at this new ride height.

Traveling the 3” in droop we find that the spring has gone slack in it’s mounts by an inch since it was only compressed 2” at ride height. The lower the ride height, the more likely it will be that the spring will go slack at full droop. A helper spring or a keeper spring would be good here.

Going from the new ride height to full compression only takes an additional 2” of travel. So at full bump the spring has compressed only 4” and no longer coil binds. The higher the ride height, the more likely it is that you will coil bind.

So lets switch to a longer 10” spring of the same 400 pound per inch rate and rerun the numbers. The corner of the car still weighs 800 pounds and the spring will still be compressed 2” at whatever ride height we choose.

At the baseline ride height, going to full bump still compresses the spring 5” but since the longer Eibach spring can now compress 5.55” before coil bind we no longer have an issue. Longer springs help protect against coil bind.

Going to full droop at the lowered ride height we find that the suspension still drooped 3” and there was still only 2” of spring compression holding up the 800# corner of the car. The spring still goes slack. The length of the spring has no effect on whether the spring will go slack in it’s perches at full droop. Really. I know intuitively that it seems like it would but it doesn’t.

Lets change some other parameters to see what happens. We will make it a street car with all the crap still in it so that the corner weight is say 1000#. The math still works the same. The 400 pound per inch spring will now be compressed 2.5” at whatever ride height we choose.

At the nominal ride height, we now get 5.5” of total spring compression. We still coil bind with an 8” spring and we come very close to coil bind (.050” away) with the 10” spring. You have to be pretty sure of your numbers to run that close to coil bind. Increased weight makes coil bind more likely.

Going to the lowered ride height, we still find that the spring goes slack at full droop but not by as much. A light weight car is more likely to have spring slack issues.

Lets take the light weight race car and swap to 600 pound per inch coil over springs. The corner of the car still weighs 800 pounds but the spring is only compressed 1.33” to hold up this weight at whatever ride height we want.

At nominal ride height, if we go to full bump we get a total spring compression of 4.33”. With an 8” spring having an available travel of 4.29”, we still barely get coil bind. Increasing the spring rate makes coil bind less likely but not by a bunch because the fatter wire of the higher rate spring reduces the available travel before coil bind.

At either ride height, the droop travel easily overcomes the 1.33” preload compression of the spring resulting in the spring going slack at droop. Increasing the spring rate makes it more likely that there will be spring slack problems at droop.

To summarize:
Lighter weight, lower ride height, and stiffer springs (race cars) all make it more likely that the spring will go slack at full droop. Spring free length has nothing to do with this.

Higher weight, higher ride height, and lower spring rates all make it more likely to get coil bind. Since these are all street car qualities, the more like a street car you have, the more you need to pay attention to your spring length.


Hopefully that’s a good enough explanation to allow a few more people to get it.

Richard P.
 

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Discussion Starter #17
Glenn. I think he made an argument where there was really none.

HOWEVER, this is great stuff and it really helps my understanding of different rates and spring heights and how they relate to eachother.

Richard, you expanded far beyond what I thought was a simple question - apparently there was a lot more to it than I realized. I was about to tell you to go wait for CC to come back up because you hadn't backed up your blanket facts with any tech...but then you did :)

Thank you all - I think this is a great thread. I have to imagine that others have the same questions, or maybe simply select the spring that is suggested by the dealer and call it a day. It's cool to know how and why - It will definately help in any potential troubleshooting.

Thank you all!
 

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If I have 12.5" between the upper and lower perch at full droop, then a 10" and a 12" spring will be slack at full droop while a 14" spring will still be in compression. Seems pretty simple to me.

EDIT: Mr. Hidley beat me posting, guess there is a terminology issue.
There's no terminology issue. Assuming each or your 10", 12", and 14" springs are the same rate, the spring perches are going to be different distances apart in each case. By assuming a fixed distance between the perches, you're ignoring the fact that coil-overs have an adjustable lower spring perch, and it's not going to be at the same location on the coil-over sleeve when you have springs that have the same rate but different lengths. Basically, your example doesn't make sense.

Glenn said:
So what does all that have to do with the fact that an 8 inch 325# spring on a Mustang will separate from the seats at full droop?
It has nothing to do with the spring being 8" long. That's Richard's point, which you still seem to be missing. At a given corner weight (say, Jack's 1000#), if I use an 8" 325# spring and a 10" 325# spring, they're both going to compress the same amount: 3.08". To get the same ride height with both springs, the 10" spring's lower perch must be 2" lower. So when I jack the car up, the 8" and 10" springs both have the exact same 3.08" of extension, and if the strut has more droop travel than that, the spring will lose contact with the upper seat.

So yes, you are correct, an 8" 325# spring will lose contact with the upper spring perch at full droop. But so will a 10" 325# spring, and so will a 12" 325# spring. That's all that Richard has been trying to tell you.

Pat
 

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So yes, you are correct, an 8" 325# spring will lose contact with the upper spring perch at full droop. But so will a 10" 325# spring, and so will a 12" 325# spring. That's all that Richard has been trying to tell you.

Pat
Ok, I will put on my dunce cap and go sit in the corner now. The light bulb just popped on. I was looking at the forest instead of the trees.
 

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If it makes you feel any better, Glenn, when I first read Richard's post about a 14" spring not being any different than an 8" spring, I thought, "WTF are you talking about?!" Then I thought for a few seconds and realized, "Oooooooooh. He's right." :)
 
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