Please Read: Bilstein 5100 - Preload/Harshenss Explanation

[snivilous]

There are a lot of claims for the 5100 shocks, and I would like to explain two (what I would call) misconceptions. If you see any mistakes or have any questions, please tell me.

1. Preload

People say when you increase the perch setting you are "preloading" the springs. I don't know if the term preload is just thrown around, but regardless it is the wrong term when it is usually used.

A spring is defined by Hooke’s Law:

F= -kx

This means that with a force (F) and constant spring rate (k) a spring will always compress the same amount (x). When you adjust the perch setting on the shock, you are not changing the spring rate or the total force applied, so the spring will always compress the same amount -- moving the perch only changes the position of the spring seat (making the truck taller or lower), not the "preload" on the spring.

2. Perch Harshness

People claim that at higher perch settings the ride is harsher. We've established that the spring rate doesn’t change as the perch setting moves.

The 5100 is a viscous damper meaning its damping is velocity dependent and not position dependent. In that regard, perch setting shouldn't modify the harshness because the velocity of the shock isn’t dependent on perch height.

My assumption of people saying they feel a harsher ride is a result of new shocks, springs, the truck sitting taller, sway bars effective stiffness, etc.

 

[Cartzo]

And that's the way the cookie crumbles...

 

[Dezert4Runner]

Here's my thoughts on your assessment of preload in regards to perch height:

Take a stock spring and put it on the lowest 5100 setting, and it rides better than the same spring on the highest notch (2.5" setting). Just because the spring rate doesn't change, doesn't mean that the effective compression of the spring remains the same. You're reducing the available compression left before the spring bottoms out. You are preloading the available compression of the spring, not increasing it's spring rate. The spring and shock work in unison to give proper dampening, so your ride will be harsher.

Obviously a 600lb+ spring will act differently compressed than a 550lb spring after it's compressed more than it's designed to, otherwise spring rate has little to do with the stiffness after lifting.

 

[snivilous]

Here's my thoughts on your assessment of preload in regards to perch height:

Take a stock spring and put it on the lowest 5100 setting, and it rides better than the same spring on the highest notch (2.5" setting). Just because the spring rate doesn't change, doesn't mean that the effective compression of the spring remains the same. You're reducing the available compression left before the spring bottoms out. You are preloading the available compression of the spring, not increasing it's spring rate. The spring and shock work in unison to give proper dampening, so your ride will be harsher.

Obviously a 600lb+ spring will act differently compressed than a 550lb spring after it's compressed more than it's designed to, otherwise spring rate has little to do with the stiffness after lifting.

You're not reducing the amount the spring can compress. The spring will compress the same amount regardless of the perch position, and the only thing that would possibly limit the compression of the coilover aside from the spring is the shock bottoming out. When you move the perch higher, all you are doing is moving the body of the shock down lower which if anything gives you a longer compression stroke (not that it matters since you hit the bump stops before either the spring or shock bottom out).

So the term "preload" doesn't mean anything, the only possible "preload" is from the weight of the truck which is constant regardless of perch position.

 

[Dezert4Runner]

So you're saying only the shock compresses after the coilover is assembled and the truck's weight is resting on it? I believe that's where my understanding differs. The coil still has a range of motion after being assembled and the weight of the vehicle is resting on it. If this wasn't the case, why would the rear end bottom out on the frame bumpstops on a stock truck when driving too fast through a big dip in the road? The front may be an assembled unit, but the coils still compress with force, just as the shock does.

 

[MTD]

Actually perch height does matter. If you are loading the spring then you are increasing the amount of force required to further deform the spring. So say you preload the spring by 2 units then you just increased the amount of force needed to deform the spring by 2 times. Just graph it you can see it then. It has nothing to do with the dampening mechanism of the 5100. In a sense you just increased the activation energy required to compress the spring.

Correct me If im wrong. It is getting late after all and I have had several pots of coffee.

 

[snivilous]

So you're saying only the shock compresses after the coilover is assembled and the truck's weight is resting on it? I believe that's where my understanding differs. The coil still has a range of motion after being assembled and the weight of the vehicle is resting on it. If this wasn't the case, why would the rear end bottom out on the frame bumpstops on a stock truck when driving too fast through a big dip in the road? The front may be an assembled unit, but the coils still compress with force, just as the shock does.

What I'm saying is if you put the weight of the truck on the coilover, the coil will compress until it's in equilibrium and holding the weight of the truck. At this point you can assume the coil won't move/compress, and moving the perch settings ONLY adjusts how compressed the shock is. Yes, the coil can still move with the suspension (as can the shock), but when you just look at how everything works when the truck is sitting still the coils height is always based on the weight of the truck and the height of the truck/coilover is based on the perch setting. I hope that made sense...

Actually perch height does matter. If you are loading the spring then you are increasing the amount of force required to further deform the spring. So say you preload the spring by 2 units then you just increased the amount of force needed to deform the spring by 2 times. Just graph it you can see it then. It has nothing to do with the dampening mechanism of the 5100. In a sense you just increased the activation energy required to compress the spring.

Correct me If im wrong. It is getting late after all and I have had several pots of coffee.

Yes! I totally agree with what you are saying (minus perch height mattering) and the analogy of the activation energy is very good. The trick with coilovers vs a coil and shock setup, is that when you construct the coilover you have to compress the spring a bit to make it fit (at least with our coilovers). The coil has to be preloaded to fit on the shock, and it's being preloaded by the fact the shock is in tension (at full extension). However, when you install the coilover on the truck and put the weight of the truck on it, you start to compress the coil and that preload is now gone since you've exceeded the "activation energy" as you put it.

Now it is possible that you could have the coil be preloaded enough when the shock is at full extension that it doesn't compress at all with the weight of the truck, however you would always be driving around at full droop then and it obviously would be a very harsh ride, though this never occurs.

 

[MTD]

Yes! I totally agree with what you are saying (minus perch height mattering) and the analogy of the activation energy is very good. The trick with coilovers vs a coil and shock setup, is that when you construct the coilover you have to compress the spring a bit to make it fit (at least with our coilovers). The coil has to be preloaded to fit on the shock, and it's being preloaded by the fact the shock is in tension (at full extension). However, when you install the coilover on the truck and put the weight of the truck on it, you start to compress the coil and that preload is now gone since you've exceeded the "activation energy" as you put it.

Now it is possible that you could have the coil be preloaded enough when the shock is at full extension that it doesn't compress at all with the weight of the truck, however you would always be driving around at full droop then and it obviously would be a very harsh ride, though this never occurs.

But that is what I am getting at with the perch height mattering. It matters in relation to the top plate, because when the truck compress the spring further the higher the perch is on the shock body It will require more force due to the spring already being compressed to a certain length in relation to the top plate when the bump is hit. Thus you are starting farther up on the "spring curve" (forget the technical) and the force of the truck compresses the spring less (lifting the truck) because the delta x of the spring is already farther from xnot.

 

[MTD]

So basically instead of this:

preload-weight of turck= activation force

You should be thinking of it this way:

preload + weight of truck = Activation force

 

[opie74]

There are a lot of claims for the 5100 shocks, and I would like to explain two (what I would call) misconceptions. If you see any mistakes or have any questions, please tell me.

1. Preload

People say when you increase the perch setting you are "preloading" the springs. I don't know if the term preload is just thrown around, but regardless it is the wrong term when it is usually used.

A spring is defined by Hooke’s Law:

F= -kx

This means that with a force (F) and constant spring rate (k) a spring will always compress the same amount (x). When you adjust the perch setting on the shock, you are not changing the spring rate or the total force applied, so the spring will always compress the same amount -- moving the perch only changes the position of the spring seat (making the truck taller or lower), not the "preload" on the spring.

2. Perch Harshness

People claim that at higher perch settings the ride is harsher. We've established that the spring rate doesn’t change as the perch setting moves.

The 5100 is a viscous damper meaning its damping is velocity dependent and not position dependent. In that regard, perch setting shouldn't modify the harshness because the velocity of the shock isn’t dependent on perch height.

My assumption of people saying they feel a harsher ride is a result of new shocks, springs, the truck sitting taller, sway bars effective stiffness, etc.

Its still called preload. When did preload become a 4 letter word?

From a quick Google search.

Coilover Spring Preload Explained | ZoomSquared

EDIT: Should have read the entire thread prior to posting a reply..

Installing a strut and putting the vehicle weight on it does not negate the fact you preloaded the spring.

 

[Zulch_AW]

A spring is defined by Hooke’s Law:

F= -kx

This means that with a force (F) and constant spring rate (k) a spring will always compress the same amount (x).

Hi,

Physics was a struggle for me but in Hooke's eqn as I understand it; X= the compression of the spring, it is the variable. F is dependent on X and K. K is "built-in" the spring-the springs strength.

F is not constant and a spring will give more, or take more to expand, initially. A spring at 90% compression will take more force to compress another 5% than a relaxed spring to compress 5% of its length. A fn of X.

So by shortening X, by using a higher perch, you have effectively increased F the overall force on the spring. Hence the term pre-load.

So a spring compressed more to begin with will require more F to compress further.

The trick would be seeing how much the weight of the truck compresses the spring at different perches. If it over comes the "pre-load"; no change, if it does not the preload will effect the ride.


Like I said I struggled to get a B in physics so perhaps I am reading it wrong.

Great post though and well thought out. I'm not claiming I'm right just joining the discussion with my interpretation. Please correct if misguided.

Thx,
Zulch

 

[1985taylor1925]

Hi,

Physics was a struggle for me but in Hooke's eqn as I understand it; X= the compression of the spring, it is the variable. F is dependent on X and K. K is "built-in" the spring-the springs strength.

F is not constant and a spring will give more, or take more to expand, initially. A spring at 90% compression will take more force to compress another 5% than a relaxed spring to compress 5% of its length. A fn of X.

So by shortening X, by using a higher perch, you have effectively increased F the overall force on the spring. Hence the term pre-load.

So a spring compressed more to begin with will require more F to compress further.

The trick would be seeing how much the weight of the truck compresses the spring at different perches. If it over comes the "pre-load"; no change, if it does not the preload will effect the ride.


Like I said I struggled to get a B in physics so perhaps I am reading it wrong.

Great post though and well thought out. I'm not claiming I'm right just joining the discussion with my interpretation. Please correct if misguided.

Thx,
Zulch

I never had to take physics as molecular biology substituted it for me degree, but I can follow the concepts and formula here. My question is simply this . . . if adjusting tension on the coil results in a lifted front end under the same amount of weight, than how could preload not be the cause? Seems like it's the only factor unless Im just not understanding how the coilovers work. But simply looking at them, you adjust them by creating more compression on the coil, which results in a lift. The distance between the top and bottom mounting spots of the shock are set, correct? So adjusting the coilover is simply putting more tension on the coil, which changes the amount of force needed to compress it. I think the factor that needs to be considered is that all coils are essentially "progressive" in the sense that more and more force is required (progressively) to fully compress the coil. So with the coilover set with less preload, it could require i.e. 600 lbs to compress it an inch, but after increasing the preload on the coil you might need say 700 lbs to compress it an inch. I know im just throwing numbers out there, but the concept is correct, right?

 

[vincer77]

I think the OP is correct, preloading a linear spring does not cause the spring to be stiffer as each additional unit of deflection requires the same additional force i.e.

delta F = k * deltax

I believe that coilover springs are essentially linear. Our rear springs are progressive as noted by the tapering shape of the coils.

 

[Brilliant]

check assumptions: linear or non-linear springs? F=-kx does not apply for OE springs.

Most complaining of preload and 5100s were using OE springs from what I have seen (but I didnt go looking for complaints either).

 

[Zulch_AW]

I think the OP is correct, preloading a linear spring does not cause the spring to be stiffer as each additional unit of deflection requires the same additional force i.e.

delta F = k * deltax

I believe that coilover springs are essentially linear. Our rear springs are progressive as noted by the tapering shape of the coils.

I think we're using the wrong Hooke's EQN. Another form of Hooke's Law uses the concept of work and is:

work = (.5K)(x^2)

This illustrates how X effects work exponentially and is not linear.

Work being what it takes to compress the spring (or extend it) and to combine force, a force is said to do work when it acts on a body, and there is a displacement of the point of application in the direction of the force.


As 1985taylor1925 points out a compressed/"pre-loaded" spring can result in lift so the work needed to compress it is greater than the force of the 4R at rest; its weight.

Also to be considered is the simple eqn: F=MA. We can determine the force by either of Hooke's Laws, mass=gross weight of 4R + people/gear, acceleration more complicated and only completely explained with vector calculus as we have both gravity, forward direction and speed of vehicle and any sudden motions that will really provide the work in question; ie hitting a big drop.
Dang I wish they gave me problems like this in calc 3 and physics! This is kind of fun!

Still not completely convinced in my own solution/understanding…just how I see it at this point.

Zulch

Edit:

Of course K play a huge role as it's the basic definition of the strength of the spring. It will vary from spring to spring. Typical numbers I found for K on a truck spring were 2.94E4 N/m (i'm gonna double check that…seems high)
Z

Edit2: K is all over place and hard to pin down. I used one formula to get 57N/m a far cry for the 2.94E4 I found elsewhere. But still...

 

[1985taylor1925]

check assumptions: linear or non-linear springs? F=-kx does not apply for OE springs.

Most complaining of preload and 5100s were using OE springs from what I have seen (but I didnt go looking for complaints either).

Why would it not apply to OE coils?

 

[jasonoflivingston]

This is interesting and both schools of thought are correct for different situations. The only time you will increase preload is if the shock absorber is topped out and cannot extend any further (this has been mentioned briefly), thus limiting the distance between the top plate and the perch.

If there is still room for the shock to extend with the perch used, the overall length of the whole assembly will simply increase instead of adding to the preload of the shock, thus leaving the weight of the vehicle as the only source of preload.

It is like the difference between the two types of spacer lifts. a coil spacer will usually achieve the top out of the shock and so preload the coil. A top plate spacer does not preload the coil, but merely extends the length of the strut. The 5100's simply do this at the other end of the strut.

That being said, if you use a long enough coil with the 5100's to achieve top out of the shock on the perch you are using, you will begin to preload the coil and that preload will increase with every perch you move up.

It is all about whether or not you top out the shock. Of course, lifting also changes suspension geometry and can ruin the ride as well, which is often incorrectly attributed to preloading the coil, but is simply a part of lifting a vehicle.

Just my .02

 

[bpowa]

Given Im not going to copy any formula's or advise any technical terms. I dont think most people care for it.

I had my 4runner on the lowest perch on the 5100 with oem front coil.

I now have it on the 3rd/middle notch, and the ride definitely feels a bit more stiffer. Not the Struts fault.

From my experience the spring is slightly "preloaded" when sitting on the third notch. I did have to compress the spring slightly to get the top on. Hence "preload". From my install anything under the 3rd notch with my oem spring is not preloaded.

All said if you have the spring in the higher notch the ride will get stiffer.

It is not sitting in the shocks optimal working position.

 

[1985taylor1925]

It seems that there is confusion with definations, but op's original post is not correct (nothing personal). But adjusting the coil pressure on a set shock absolutely is adjusting preload. It doesnt matter if the coilover is on or off the vehicle, the end result is the same. Adjusting the coil on a coilover is relative to the shock. Whether you adjust the preload before of after installing the coilover on your front end makes no difference.

This guy illustrates everything in a real life example using hookes theory. Lift is achieved by adjusting the preload on a coilover, which absolutely will make the vehicle ride stiffer.

Coilover Spring Preload Explained | ZoomSquared

Edit: I have to take part of that back . . . op you are correct about spring rate not changing. Had to re-read that. However adjusting the coil is adding preload by defination, and that is how lift is achieved. But the stiffer ride must be a result of other factors like changing a arm angles and such as mentioned, not spring rate

 

[MTD]

check assumptions: linear or non-linear springs? F=-kx does not apply for OE springs.

Most complaining of preload and 5100s were using OE springs from what I have seen (but I didnt go looking for complaints either).

I had a PM discussion last night with the OP and over the course of that we were assuming a linear non progressive spring.