Coilover springs and spring rates, weight, lift, ride quality and how they relate.

I searched and I didn't find a consolidated thread where information was easily understood so I am going to try and make that thread. I'm not an engineer but I am an aircraft mechanic with fairly good understanding of mechanics and engineering to a point. I will do my best to give you accurate information that is easily understood by any average tacoma owner not just us gearheads. I might also be asking questions to things that aren't clear to me yet and then as things get (hopefully) cleared up by members I will update the post.

THE FOLLOWING INFORMATION IS ONLY WHAT I'VE BEEN ABLE TO FIGURE OUT SO FAR, THERE'S DETAILS THAT HAVE BEEN LEFT OUT FOR SIMPLICITY AND DETAILS THAT I'M JUST UNAWARE OF, IF YOU KNOW BETTER (because you obviously do right?), LET US KNOW.

I started wondering about spring rates because my suspension started sagging. I've lost what seems to be more than inch of lift since installing. I have added some weight recently so I figured it was probably normal but then my tires started rubbing and that's when I decided I had to do something about it. My initial response was to simply crank the preload on my coilovers and I did, I even went a little bit past the manufacturers recommended setting but that was still not enough to get my lift back so I started looking for a solution. In doing so, I've learned some stuff I'm going to try and share here, I know I'm not the only one having this issue.

SPRING RATES
There are an infinite amount of spring rates but on Tacomas most of us will use something in the 500-700lbs rage. What that means is that for any given spring rate, that is the amount of force required to compress that spring 1.00" compressing it 2.00" would require twice the force, etc.
When you buy adjustable coilovers you can adjust the amount of pre-load on the coils which results in most cases in increased ride height, in fact this is one of the main reasons people install coilovers in the first place.
It is "common knowledge" that when you increase preload you will also be stiffening the ride, right? but why? I'm the kind of guy that needs to understand why things are the way they are, I don't usually accept a condition just because it is common knowledge.

At first glance you (and I) would think that by adding preload you are (duh) "pre-loading" or compressing the spring and therefore making it stiffer correct? That sounds right but you have to remember that we are using CONSTANT-RATE springs, which means that no matter how much you compress them (to a point), the spring will always require the same amount extra of force to compress it x distance.

In my situation I was maxed out on preload according to the manufacturer but I also wanted a smoother ride, cranking the preload was was supposed to make the ride stiffer which I was hoping to avoid. So I thought maybe I should just change my spring to a heavier spring rate but I wondered how that would affect ride quality since a heavier spring rate "should" produce a stiffer ride right?

Using my case as an example, I have 650lbs coils which require 650lbs of force to compress 1" but if you compress it 1" it will only take an additional 650lbs to compress it another 1" so why would compressing it via preload make it stiffer?

RIDE QUALITY
When the truck is static, the forces on the spring are balanced, the weight of the truck is compressing it and the spring force pushes against that force equally otherwise there would be movement. So, no matter what pre load the spring is under, it will only require 650lbs to compress it another inch (unless you're at max extension but we're not gonna get into that yet) the spring pre load alone does not make make the ride stiffer.
But that's not all, if pre-load "compresses" the spring how is that force being balanced out since there isn't actually any weight being added to the truck? Short answer is that pre-load doesn't actually compress the spring it just moves it down a bit which results in a slight extension of the overall shock length.
So where does the stiffness come from then?

The result of extending the shock is a change in the position of the suspension components, the shock extends which pushes your control arms away from the truck, effectively lifting the truck. The truck still weighs the same and so the suspension is still holding the same weight its just doing it at a different position.
The position matters, its hard to describe without pictures but the more you lift the truck, the angles of the control arms change and the weight distribution changes from the shock to the arm itself. The steeper the angle the more weight is being supported by the arms instead of the shock which means that now a bump on the road that exerts 650 lbs of force on your suspension now is only applying a portion of that force on your shock and the rest is transfered through the control arms to your chasis making it feel as though your shock was stiffer.

LIFT
Length of the coilover assembly at rest, is what determines how much lift you get, not the spring rate. Spring rate is obviously related because combined with your trucks weight, it determines how much your shock will extend at rest. The only component in the suspension that has a variable length is the shock so when you lift the truck (other than replacing or moving fixed components) the only way to change that is by manipulating the length of that shock. There are a few ways to do that, you can "pre-load" the spring, which moves the spring down and therefore extends the shock. You can also get a higher spring rate which will compress less given the same truck weight and therefore resulting in a longer shock at rest. And last but not least you can get a longer spring which under the same load will compress the same but have a greater overall length.

WEIGHT
Adding weight to the truck means you now have more force on the spring and therefore the spring compresses and you shock shortens, like its supposed to. Spring rate remains unchanged though so even with your added weight it still takes the same amount of force to flex your suspension by any given amount. The weight will make it seem as though your suspension is softer because even though the force required to compress the spring is the same, the force required to move your cab is higher so the suspension will articulate/travel more instead of transfering the bump to the cab. You can compensate for that additional weight by increasing your spring rate but if you overcompensate your ride will be stiffer.

Here's a picture to better illustrate my point about "preload" not compressing the springs...
Without adding real weight to the truck theres noway to compress the springs, the so called pre-load only changes the point at which the springs are seating at in the shock body, if the shock shaft can still extend, it will do so, allowing the spring to maintain its length and therefore not adding any extra "pre-load compression" but indeed providing lift.



MY SOLUTION TO MY SITUATION... (I think)
I want more lift but I also want a softer ride, I can get a softer ride by lowering my spring rate but doing that alone will mean that I will lose some lift, (less spring rate means the spring will compress more and therefore shorten) so I have to compensate for that and the way to do so while maintaining a lower spring rate is getting a longer spring. I am ordering springs and I will update this post once I install them.

I know there's a lot that I left out, suspension is a dynamic system with a bunch of variables. I didn't even mention shock valving, travel limits, axle angles, momentum, etc. When lifting your truck there are many many things to consider if you want to do it right. I just wanted to put some information out there that it took me a bit to find and understand. I'll try and find some useful charts and pictures to make things easier to understand too.

 

Nice. I look forward to seeing what happens with the next set. Thanks.

 

It would make it "stiffer" because constraining a coil spring with preload only stores energy, it doesn't use it. So 1" of preload on 650lb spring requires 650lb of force and that energy is stored when it's compressed 1". Compressing it 1" further will require an additional 650lbs, or a total of 1300lbs of force. So say you preload the springs 1" for more lift and then hit a 1" bump, 1300lbs will be needed to absorb that bump without transferring it to the chassis and whatever isn't absorbed by displacement of the spring will transfer to the chassis and be felt as a "stiffer" ride. This is obviously over simplified and I used a linear rate of 650lb per inch, but it gets the idea across. I'm not saying that is how much force is actually required on amy specific vehicle.


Also, shocks don't carry any weight. They simply control or resist the movement of the spring. A longer shock on the same spring will not determine lift as your explanation claims.

Lift is a function of suspension attach points, spring rate and spring length. Shocks hand nothing to do with lift aside from longer ones may be needed for a lift.

 

you are correct I used wrong wording and I will correct it, when I said longer shocks I meant longer coilover but even that is not entirely accurate, the at-rest length is what changes.
Same for the weight, the shock won't hold the truck up but the coilover assembly does, if the assembly is longer at rest than any other assembly then it will result in a bit of lift

 

And thank you for pointing it out BTW

 

Sure, but coilovers are just a spring and a shock. The spring holds the weight. The shock could be literally any shock that will fit and it would have zero effect on height/lift.

 

This I have a problem with... how does it "store" the energy? I get that when you compress a spring you are storing energy in it, but in order to compress a spring you have to be holding it on both ends and there's nothing holding the truck at its height other than weight and if weight doesnt change how could you possibly store an additional X amount of energy on the spring without adding weight to the truck?

 

Adding preload does the same thing as adding weight. The shock will only extend so far, once it's topped out, preload is the extra compression of the spring against the stop. For the spring to compress further, the force on it needs to exceed the spring rate for that much displacement. So 1" of preload will need the 1" spring rate force + whatever additional amount of force is required to absorb the bump.

 

Only if the shock is fully extended though... otherwise it just extends a bit more doesn't it? and for our shocks to be fully extended you would be sitting at like 6 iches of lift, so at any normal situation where we are looking to get about 3" of lift most, tacoma shocks will still have a few inches of downtravel

 

I'm obviously not very good at explaining this... Hopefully someone with a better way to describe/explain this will do so.

 

Here's a picture to better illustrate my point about "preload" not compressing the springs...
Without adding real weight to the truck theres noway to compress the springs, the so called pre-load only changes the point at which the springs are seating at in the shock body, if the shock shaft can still extend, it will do so, allowing the spring to maintain its length and therefore not adding any extra "pre-load compression" but indeed providing lift.

 

https://accutuneoffroad.com/articles/

@AccuTune Offroad

 

https://accutuneoffroad.com/articles/tacoma-4runner-coilover-preload-faq/

This might help you.

You are right, you aren't compressing the spring anymore, but you now have less down travel on the shock.

 

Yes that actually does help, thank you! and it confirms what I had been thinking since the beggining, adding what is commonly called "pre-load" does not alone cause a harsher ride...

 

I understand that if a spring has little to no stored energy, and it hits a pothole; the downward force that sends the tire to the bottom of the hole is gravity, but if there is preload, then it is gravity on top of the stored energy making it feel much harsher.

Am I wrong?

 

The harsh ride usually comes from the shock bottoming out because there is literally no down travel left.

IIRC Accutune recommends no more than 21-21.5" from top of the strut to the middle of the bottom eyelet.

 

Except for some minor things and nitpicks about terminology, I think, FreshMexicanTaco, that you have a good understanding of preload.

 

I dont think you are...

 

I had so much in my head that I know I must have messed something up I just needed to put it somewhere so I can start cleaning it up and making sense of it.

 

Besides decreased down-travel as you said, I'd like to add that increased spring compression at full droop might also affect ride quality when preload is increased. It's a trade off. I installed extended length shocks to offset these trade-offs.