adjusting front forks preload

[Rogue]

You guys are very entertaining.

A Spring's RATE is NOT the amount of force required to compress the spring. The spring's rate is the mathematical curve of how much resistance increases as you apply force.

Generally, old springs rates would required increased forces along an exponential curve, meaning the more force you applied the spring would require exponentially more force to continue compression.

The key statement from your citation above is that the spring rate is LINEAR, not exponential. That means that as the spring compressess the amount of force exerted in return increases along a linear progression, not an exponential progression. For your benefit, I have created some compression/force diagrams to illustrate my piont. I suppose I'm hoping you know how to read charts.





As you can see in the charts, the force required to compress a spring DOES IN FACT increaese as a spring is compresssed, what changes from spring to spring is the HOW MUCH FORCE does it require and HOW FAST DOES THAT REQUIREMENT INCREASE as the spring is compressed.

Sorry guys, you can't argue with physics...

No matter how you wish it would work, springs MUST increase resistance as they are compressed, or they wouldn't work.

And to Rogue, thanks for the civil discussion here on this topic.

Preload changes the relative static force a spring is exerting. Using the lever mechanics, if a spring exherts more force on the lever, in this case your swingarm, it will alter how much force that lever is placing against it's object, in this case -- the ground. Ride height changes because the preload adjusted spring has increased the amount of relative static force the swingarm is generating against the ground (thereby raising the vehicle upwards from it)

There is also the fact that under preload, you are shortening the relative spring length, which also could cause a change in ride height, so long as it is not using a lever mechanic (Swingarm = Lever Mechanics, Forks = Direct Mechanics) In forks, you are essentially replacing the area occupied by the spring with an immovable piece of metal, increasing your ride height because your spring is exerheting more static force against the ground.

Ooooh, your charts are lovely.

You're still wrong.

You are increasing the ride height because the top part of the trapped spring is not pushing against a fixed point. The ride height increases because the fork upper extends out of the lower "TELESCOPIC FORKS" the spring does not alter in any way.All you are doing is dialling out the initial slop in the spring due to end coil effects:
END COIL EFFECTS:

The first 20% or so of a spring's travel is at less than the quoted rate of stiffness, due to the end coil effects, as the last 20% is stiffer as the spring becomes coil-bound, HOWEVER - the intervening 60% of the spring's travel is linear and the force required to compress it does not increase.


Once again, explain the difference between linear and progressive springs for me...

[kermitbuns]

Ooooh, your charts are lovely.

You're still wrong.

You are increasing the ride height because the top part of the trapped spring is not pushing against a fixed point. The ride height increases because the fork upper extends out of the lower "TELESCOPIC FORKS" the spring does not alter in any way.All you are doing is dialling out the initial slop in the spring due to end coil effects:
END COIL EFFECTS:

The first 20% or so of a spring's travel is at less than the quoted rate of stiffness, due to the end coil effects, as the last 20% is stiffer as the spring becomes coil-bound, HOWEVER - the intervening 60% of the spring's travel is linear and the force required to compress it does not increase.

The springs you describe defy physics... so I'm not sure where you are getting your information from.

At any rate, I suppose this is now pointless to continue since both of you opposing my view have turned to mockery and general assininity (not even sure if thats a word!).

Once again, explain the difference between linear and progressive springs for me...

Don't be an ass, Linear and Progressive spring RATES. If it is that hard for you to understand you should perhaps start with something more simple, like high-school.

[Rogue]

I think you will find that the information comes from spring manufacturers, who presumably know more about it than you, also progressive and linear springs LOOK different, hence the difference in performance.

http://www.acewirespring.com/compres...fications.html
http://www.tuninglinx.com/html/spring-rate.html
http://www.spring-products.com/capab...s/compression/

[kermitbuns]

I think you will find that the information comes from spring manufacturers, who presumably know more about it than you, also progressive and linear springs LOOK different, hence the difference in performance.

Compression Springs - Specifications Spring Design
What is Spring Rate
spring-products.com: Compression Springs

Thanks for the links. I have read a bit of it and I am still correct. I think maybe we are not talking about the same things.

The SPRING RATES change depending on what part of the spring is compressing.

Again, if the entire middle piece of the spring you described earlier reached enough force to begin to compress it, it would fully compress immediately.

The SPRING RATE (which is what is depicted in my pretty little pictures above) means that the RATE at which the REQUIRED FORCE INCREASES as the spring compresses is the same throughout that piece (IE a LINEAR progression). BTW I'm not talking about linear or progressive springs, i'm talking about linear or exponential spring compression rates, mathematically.

When you say that spring stiffness does not change, I believe you mean to say that spring RATE does not change throughout the spring beyond it's design, in which case we are in agreement.

[Pliskin]

[VIFFER RIDER]

Preload adjustment does not make the forks harder or softer, merely extends the length along the travel where the forks sit . That said, to adjust them you simply turn the top adjuster with a spanner, clock wise to make the fork longer.
Make them both identical, (that's why the lines are there) and use a good fitting spanner so as not to mark the soft aluminium.

That is assuming if the springs are linear type and not progressive like some bikes. Spanner wrench? That is for the rear right (Sounded like if you were talking about the front since you said fork.)? I cant imagine using that for the front since its just a flat head slot. I just used a coil over spanner for the rear and a big flat head screw driver for the fronts.

[Rogue]

Thanks for the links. I have read a bit of it and I am still correct. I think maybe we are not talking about the same things.

The SPRING RATES change depending on what part of the spring is compressing.

ONLY in so much as the ends of a spring are impossible to make behave the same as the centre, due to manufacturing

Again, if the entire middle piece of the spring you described earlier reached enough force to begin to compress it, it would fully compress immediately.


Totally wrong


The SPRING RATE (which is what is depicted in my pretty little pictures above) means that the RATE at which the REQUIRED FORCE INCREASES as the spring compresses is the same throughout that piece (IE a LINEAR progression). BTW I'm not talking about linear or progressive springs, i'm talking about linear or exponential spring compression rates, mathematically.

When you say that spring stiffness does not change, I believe you mean to say that spring RATE does not change throughout the spring beyond it's design, in which case we are in agreement.

Tell you what: read the highlighted parts of this webpage lifted from Sport Rider.....Sport Rider Technicalities-Spring Rate and Preload


Home»Motorcycle Tech Tips»Technicalities: Spring Rate and Preload
Technicalities: Spring Rate and Preload
Beyond nuts and bolts
By Paul Thede


Commonly asked questions like, "What's the difference between spring rate and preload?" and "Can't I make the spring stiffer by cranking in more preload?" require delving a little deeper into the mechanics of spring forces.

A stiffer spring would also start at zero force, but its compression rate would increase at a steeper angle, as in Figure 2. Notice that at 20mm travel, the softer spring requires only 10kg force, while the stiffer spring requires 20kg. The stiffer spring at 1.0kg/mm is both twice as steep and twice as stiff as the softer spring, which is 0.5kg/mm.


Click to View Gallery
Figure 2
Let's back up and define what spring rate and preload really are. "Spring rate" reflects the stiffness of the spring and is measured in kilograms per millimeter or pounds per inch. One of the ways to test spring rate is to first measure the spring's "free length"-the uninstalled length-and then put weights onto it, measuring the resulting compression with the addition of each weight. "Straight-rate" springs maintain a constant rate of compression throughout their travel. If you are testing a straight-rate spring and you plot these points on a graph, you will end up with something that looks like Figure 1.

Now let's take the original spring and install it in the fork. As it's installed, it gets compressed, or preloaded, a small amount. "Preload" (or "preload length") is the distance the spring is compressed from its free length as it's installed with the suspension fully extended.


Click to View Gallery
Figure 3
Just a note here on the difference between preload and preload adjusters: All bikes that I am aware of have preload. Some bikes do not have external preload adjusters, but they do have preload. All forks can have preload adjusted internally by changing spring spacer length, though sometimes it takes special spacers. Forks that have external preload adjustment have preload even when set at the minimum adjustment.

The "preload force" is the initial force the spring exerts on the end of the fork tube with the fork fully extended. Referring now to Figure 3, you can see that when preload is added to the spring, it effectively shifts the curve (line) to the left proportional to the amount of the preload force. In this case, the preload length is 35mm and the resultant preload force is 17.5kg at zero travel. In other words, with this 0.5kg/mm spring and a setting at 35mm preload, you would have to put more than 17.5kg force on the end of the fork tube to create any movement at all.


Click to View Gallery
Figure 4
For a straight-rate spring, the relationship between force, spring rate and travel is described by the equation: F = K x L (or force [F] equals spring rate [K] times length [L]).

When you tighten the adjusting collar on a shock or increase the preload length by tightening the adjuster on the fork, you are indeed increasing the initial force exerted by the springs. This decreases sag, making the bike ride higher. It does not, however, increase the spring rate.



For example, you can achieve a targeted sag on the fork even with a spring that is too light (or soft) if you use a lot of preload. You can also achieve that same sag with a spring that is way too heavy by using very little or no preload. Let's look at just one fork leg on paper. Refer to Figure 4 and notice that the softer spring has 35mm preload and therefore has 17.5kg force at zero travel (fully extended). Let us assume the front end has 30mm of static sag. At 30mm of sag, the total spring force is 32.5kg. This means that each fork spring must push up 32.5kg to create a 30mm sag figure. Any combination of spring rate and preload that gives 32.5kg force at 30mm travel will create the same sag. Notice the stiffer, 1.0kg/mm spring has 2.5mm preload and, at 30mm travel, also creates 32.5kg force. This means they will both have the same sag; however, they will perform totally differently.

The quality of the ride will suffer with a spring that is either too soft or too stiff. The spring with a rate that's too soft will dive and bottom easily because the spring doesn't provide enough additional force as it gets deeper into the travel. The spring that has a very stiff rate will feel harsh, like it's hitting a barrier or very stiff spot.

A few more measurements will show if your spring rates are in the ballpark. Set the sag to standard settings (see Technicalities, August '95) and then measure the "free sag." "Free sag" is the amount the bike settles under its own weight. Use exactly the same procedure as when checking static sag, but without the rider on board. Street and roadrace bikes require 0 to 5mm of free sag on the rear, but should not "top out" hard. "Topping out" occurs when the suspension extends to its limit. It should barely have enough force to top out without the rider on board. If it takes a lot of force to compress it at all, you can bet it needs a different spring. On the front, expect to see 5 to 10mm of free sag.

When the static sag is correct and the free sag is less than the minimum recommended (e.g., it tops out hard), you need a heavier spring rate with less preload. A lighter spring is recommended when the free sag is more than the maximum recommended.

Most bikes, but not all, are set up with fork springs that are too soft for aggressive riding. Keep in mind that personal preference, conditions and type of riding come into play when setting up suspension. Racers generally use higher spring rates with less preload than street riders. When in doubt, consult a good suspension tuner.

Static sag for a particular bike and rider combination is dependent only on spring rate and preload. Springs are position sensitive and only care where they are in their travel, not how fast they're being compressed. Damping settings, on the other hand, are dynamic forces. In other words, damping forces only occur when there's vertical suspension movement. This means they do not affect static sag, as sag is measured when there is no movement.



The preload adjustment does NOT make the spring stiffer, it merely brings the range of the spring in use from "including the soggy bit AND the part which is rated " to "only the rated bit"....

[barton664]

...............

[Paul47]

I was reading this thread, and thought I might try to add some information. I suspect folks are just either miscommunicating, or are getting confused with extraneous factors. When you study Physics, the first thing you try to do is remove all the extraneous factors so you are left with the stuff you want to know about.

We can do that in this case too. Just imagine a spring on the ground, vertically oriented, with a weight on top of it. The weight compresses the spring some.

The equation for a spring is F=kx, that is, the force compressing the spring is equal to a constant (k) times the distance of compression (x). A stiff spring has a large k, a weak spring has a small k.

Now, imagine the spring in our example is 20" long when not compressed, and is 15" long with 100 lbs of weight on it. Then the amount the spring is compressed is 5 inches, and k is 100lbs/5 inches or 20lb per inch (20 lb would compress it 1", 40 lb would compress it 2", and so on). And let's imagine the weight on this spring, just like the weight of a motorcycle on a fork, is 100lb. That means the length of the spring will be 15", and in our case the weight will be 15" above the ground.

But, what if we don't want the weight 15" above the ground; instead we want it at 16"? For example, in a motorcycle, say the sag is taking up too much of our available fork extension. There are 2 ways to get it up to 16". First, just put a 1" spacer below the spring (or above it - doesn't matter). This is "preload". We've just raised the weight by an inch, which is equivalent to using less of the available fork travel. Notice the spring is still compressed 15"!

The other way is to use a stiffer spring. Assuming a spring that is 20" unloaded, we want it to compress only 4", not 5", for that 100lb weight. That means the spring constant has to be 100/4 = 25 lb per inch. That would set the height of the weight at 16", the spring would be 16" long when compressed, and there would be no preload spacer.

So sag is affected by either preload or spring rate (or weight upon the spring). Amount of spring compression however, is only affected by spring rate (or the weight upon the spring) - NOT by preload. Add all the preload you want, within reason, and your spring compression will always end up the same for the same load it is bearing. Adding preload does not affect how "stiff" the spring feels, since it does not change the amount of compression of a given spring; you have to change the spring rate to do that. Adding preload only affects sag.

I think progressive springs are just like stacking two linear springs of different rates. Both will compress for a given load, the strong spring a little and the weak spring a lot; the apparent spring rate of the combination will be actually less than the rate of the weaker spring. When the weak spring binds (full compression), the apparent rate will change to the rate of the stronger spring.

[squirrelman]

Read Andrew Trevitt's book about sporbike suspension tuning, please.....

[betarace]

here are two files (PDF) that I find very helpful on all my bikes re: setting up suspension
Download suspension.pdf from Sendspace.com - send big files the easy way
Download SuspensionGuide-02.jpg from Sendspace.com - send big files the easy way

[02 VFR Rider]

Incorrect. Preload only adjusts the ride height and has very, very little, or nothing, to do with the overall spring rate, that is fact. To obtain stiffer spring rate you must buy stiffer springs. I will just pretend that I didn't read the rest....

Guys he is correct, don't compare a bathroom scale with a fork spring PLEASE.
springs are rated and that is a set # when you set the preload ( like adding weight to the spring ) the spring will compress less with added weight overall then it would w/ no preload, because it is being compressed by the preload adjuster.
so say I have preload set at something that say ='s 20lbs, all that means is that I would need to add 20lbs to the forks to get them to start to compress.
There are many great books on the subject.