Level/small lift rabbit hole…

^ He explained it pretty thoroughly. The weight that the suspension is supporting didn't change, so there isn't more force acting on the spring. The higher preload just changed the point at which the spring started to compress. The spring also has a linear force*distance so when you hit a bump (higher than the surface), the spring will compress at the same rate as it did with less preload. The issue is in the other direction as you have less down travel. It doesn't take much before the down travel is used up and it tops out. When that happens and you then recompress, the force will need to exceed the preload before the suspension will even move, and this will feel harsh.

Like I've said a lot of times on here, keep your lift to 2" or less. Using a longer shock to get about half that is also a good idea, and the rest will come from preload. A shock with greater than stock travel is also a good feature; you can add a little droop, and still bottom out on the stock bumpstops before the shock bottoms out.

2" lift is enough for 37s. The guy in that video recently put 37s on his 2nd gen with no lift and no rub, but he used stock (RW) rims and an 11.5" tire, and it fits under the wheel well nicely.

 

Unsurprisingly I'm still not understanding... but I do appreciate the attempt to learn me.
"The higher preload changed the point at which the spring started to compress".
Got it so the 2000lbs that the coil spring is supporting would not compress the coil spring as much as it did before the preloading. Giving the truck a higher stance (lift).

So using arbitrary numbers, lets say it takes 1000ft lbs to compress the spring to the first clip position on the 5100 shock.
The truck sits at stock height.
If you hit a bump that generates 500ft lbs of force, the coil would collapse to a rate of 1500ft lbs and then spring back to the 1000ft lbs preload.

Lets say it takes 1500ft lbs to compress the coil spring to the 3rd clip position on the 5100 shock. The truck sits 2" higher.
You hit the same bump generating 500ft lbs of force, the coil would collapse to a rate of 2000ft lbs?
... and we are saying that adding 500ft lbs of force to a coil that is preloaded to 1500ft lbs will give the same rate of compression to the coil spring as it would to the one preloaded to 1000ft lbs? Same compression speed and compression distance?
and now compressed to 2000ft lbs, wouldn't the coil spring bounce back with 500ft lbs of more force? Harder, faster, ... maybe the shock eats this up?

 

The force needed to compress the spring 1" is the spring rate... and this will not vary from 0" of compression to the max (the coils hit each other). Adding preload doesn't change this, it just means that you used up some of the spring's travel to get a higher ride height.

The force on the spring is the same in both cases, because it's supporting the same weight. The amount that the spring is compressed relative to its free length is the same in both cases.

For instance, our trucks have an unsprung weight of about 1500 lbs on each front wheel, so that is what each front coil supports. I don't know exactly what the leverage ratio is, but say it's around 1.7, so the force on the spring would be 2550 lbs. No matter how you set the preload, or ride height, or if you get different coils, or an aftermarket shock that's longer... that's the force on the spring when the truck is at rest. If we have 600 lb/in coils, and we adjust them with no preload at all, then they'd be compressed 2550/600 or 4.2". If you preload them 1", they'll still be compressed 4.2" when holding up the truck, but now only 3.2" of that is actual wheel motion, and with the leverage ration of 1.7, you got 1.7" of lift by preloading the spring 1".

Let's say the Tundra has 10" of front wheel travel (/1.7 means 5.89" of shock travel), and at stock ride height the preload is set so that it sits right in the middle of its range (5" up, 5" down) with a 600 lb/in coil. That means 5.89/2 or 2.95" of shock compression at stock height. Since I earlier determined that the spring force to hold up the truck is 2550 lbs with a total spring compression of 4.2", then subtracting 2.95" from that gives 1.25" of spring preload.

If you want 2" of lift vs stock (using the same shock), then you add 2/1.7 or 1.18" of preload, for 2.43" of total preload. Note that now instead of 5" of wheel travel in both directions, you have 7" up and 3" down... assuming that the spring can be compressed that much (7"/1.7= 4.12" + 2.43"= 6.55") before the coils hit. Note that for the stock configuration, the spring only needed to compress 5"/1.7 + 1.25"= 4.19".

I hope that helps... and that I didn't screw up somewhere!

Note that these numbers are pretty close to the real Tundra numbers, but not exact. Plus, most aftermarket shocks will be a little longer and have a little more travel than OEM.

 

I'm sure this makes perfect sense to most people ... but can we all just agree that I'm too dumb to understand this?
I lost you at "leverage ratio".
If the weight of the truck compresses the spring 4.2", with or without the extra 1" of preload, how does this "leverage ratio" cause the truck to sit 1.7" higher?

 

The leverage ratio is just the ratio between shock motion and wheel motion. 1" at the shock equals 1.7" at the wheel.

 

Well that I do understand ...
but moving the clip position on the 5100 up and preloading the coil 1" doesn't change the length of the strut ... so there is no "leverage ratio" to factor.
... and we're back to all of this not making sense.

 

Fact: Preloading the coil 1" holds the truck higher (highest clip position on the 5100)... with the strut remaining at the same length.
Fact: There is no way the weight of the truck could compress this preloaded coil spring the same amount as the not-so preloaded coil ...
otherwise the truck would not sit any higher.

So what is giving the truck lift when the spring is preloaded that extra inch? The coil spring HAS to have less give when preloaded.

 

You misunderstood. In the example I gave, it takes 4.2" of spring compression to hold up the truck. Simple math. If you make part of that preload, then the rest will be shock displacement.

The shock won't compress at all until the force is enough to overcome the preload. Then as force increases, the shock will compress at the same 600 lb/in rate.

 

Exactly ... whereas without so much preload, the shock would begin to compress with less force ... aka "a softer ride".
Using motorcycle suspension as an example ... a quote from Penski Shocks website;
"The more preload, the harder the suspension is.
The lower the preload, the softer the suspension is."
https://www.penskeshocks.com/blog/motorcycle-spring-preload-what-it-does-and-why-we-would-adjust-it

It is undeniable. You can not get the same ride quality after preloading the spring. It HAS to be firmer.
I'm sticking with my original thinking... There's 2 ways to lift a truck, a longer strut or a coil spring that compresses less.
one way does not affect ride firmness, the other way does.

 

It is absolutely not firmer... but you also don't get the same ride quality.

The shock will move up and down in response to forces just as easily, no matter where the preload is set... until you reach the limits of the spring, shock, or other suspension component. You are going to get less down travel with a lift, unless all the lift is achieved with a longer shock rather than more preload. When your downtravel is maxed out (shock fully extended) and then forces compressing the shock increase again, it will feel harsh if the preload is higher... because you have to overcome that preload before the shock will move at all.