Help me understand Pre-load

[bad Chad]

I know this will seem like an incredibly basic question to most of you, but for some reason I can't seem to wrap my head around the concept.
Let's stick to only rear shocks, to keep it even more basic. 
Here is what I think know.  Most shocks haver a preload adjustment, and by adding preload we are compressing the spring.   This in turn makes the spring more resistant to compressing more under load, because it has already been partially compressed?   

So a compressed spring is stiffer because it has already used up some of its finite compressibility, and thus this might be a good thing it you are adding more weight to the motorcycle.  Does increasing preload/spring compression raise the ride height too?  Is the only reason to add or subtract preload to better deal with varying weight?

Wouldn't one want to run with as little preload as practical?
 

[Aaron D.]

Real quick-spring rate is measured in force needed to compress a given amount.
Preload does NOT change the rate, only starts the spring down the road of increased force.

So using fake, easy to understand numbers:

Spring rate 25 lbs/inch of compression
Spring with no preload will move 1" when you add 25 lbs to the unladen spring.

With 25 lbs of preload, adding 25 lbs doesn't move it-it only will move 1" if you put 50 lbs on it.
 

[Wayne Orwig]

Preload basically just changes the ride height. Not enough and you bottom out the shock. Too much and you top out the shock. Changing the ride height can also impact handling.
The amount of compression of the spring is determined by the weight being carried.
A normal spring travels X number of inches per every pound added to it. It is linear, for a NORMAL spring. But progressive wound changes that, and the linkages on like a monoshock rear suspension change that relationship.

Or that is my understanding.
 

[Huzo]

When you add preload, you are only raising the perch (the seat in which the spring sits), higher up the shock body.
Indeed if you put the bike on it’s centrestand, the spring will “top out” and will have more load prior to use, but that will only mean it “sags” less when you go to ride off.
The spring will still only have the same load when in use that it had before, your mass has not changed but your ride height will be greater because the shock compressed less to achieve equilibrium.
It is a complete fallacy and always has been, to suggest that winding on pre load “stiffens” the suspension.


There is a lot of material to clarify the myth. Google “spring preload, stiffness myth” or similar and all will be revealed.

[LowRyter]

The general rule of thumb is the static sag should be set so that suspension travel compresses about 1/3 when the rider sits on the bike.

It's much easier to do this where the bike is on a shop stand and someone is measuring the suspension when the rider sits on it.  It can also be done with mounting weights on the bike to mimic the weight of the rider.  There are probably other ways.

That's why when you purchase a new motorcycle, the dealer should set the sag with the rider when the dealer delivers the bike  (I know you Guzzi guys won't spend for a new bike and only buy used).

[Huzo]

So a compressed spring is stiffer because it has already used up some of its finite compressibility, and thus this might be a good thing it you are adding more weight to the motorcycle.

I know that your intention was to add a question mark after that bit.
It’s that part of your post where the misunderstanding exists..
Once the damper has moved off it’s stops, then the pre load is irrelevant to the compressibility. The damper only moves a lesser amount by the time the spring is in equilibrium with the compressive force.
You are correct in your suggestion that you will raise (or lower) the ride height by manipulation of pre load.
Here’s one (of many) explanations.
https://youtu.be/NGGlK2MLOjc

[Cam3512]

So increasing preload (compressing spring) with no weight on the seat will RAISE the ride height?

[80CX100]

When you add preload, you are only raising the perch (the seat in which the spring sits), higher up the shock body.

It is a complete fallacy and always has been, to suggest that winding on pre load “stiffens” the suspension.


There is a lot of material to clarify the myth. Google “spring preload, stiffness myth” or similar and all will be revealed.

Hhmm,    Please keep it in layman terms, because some of the physics can get a little complicated.

      Can you explain to me why, when I first got my CalVin, when it was on the first lightest spring pre load setting,it wallowed around corners like it had a hinge in the middle of the bike, and when I adjusted the preload to the highest setting, it firmed and stiffened up the corner handling a LOT!

       

       Kelly

      Was that not due to the suspension being firmer?

[oldbike54]

Hhmm,    Please keep it in layman terms, because some of the physics can get a little complicated.

      Can you explain to me why, when I first got my CalVin, when it was on the first lightest spring pre load setting,it wallowed around corners like it had a hinge in the middle of the bike, and when I adjusted the preload to the highest setting, it firmed and stiffened up the corner handling a LOT!

       

       Kelly

      Was that not due to the suspension being firmer?

 Actually it is because most of the time suspension is only operating in the first 20% of its travel .

 Dusty

[LowRyter]

So increasing preload (compressing spring) with no weight on the seat will RAISE the ride height?

No.  It just won't compress as much when the rider is on the bike.  So I suppose the bike will stay a little taller with the rider but it will ride very stiff.

[LowRyter]

Hhmm,    Please keep it in layman terms, because some of the physics can get a little complicated.

      Can you explain to me why, when I first got my CalVin, when it was on the first lightest spring pre load setting,it wallowed around corners like it had a hinge in the middle of the bike, and when I adjusted the preload to the highest setting, it firmed and stiffened up the corner handling a LOT!

       

       Kelly

      Was that not due to the suspension being firmer?

It appears that is was set too weak for your weight.  Just a guess. 

The traditional way to do it, is set the static sag on the stand and then adjust the rebounding and dampening as you test ride it.  Test the later in a methodical method, starting from full soft.  Get a feel for each adjustment of separate rides then work them in tandem.  Much easier to count the adjustment from full soft.

[bad Chad]

Hmm, so far it’s about what I was expecting. Several meaningful attempts in various levels of complexity.  I will keep on tying to make the pieces fit, thanks to you all.

[JC85]

So since this thread is conveniently going right now, let me pop in and ask a question that has been on my mind for a while:

Several months ago, a friend and I were out on a ride. Going down US 400, we crossed some railroad tracks. The tracks are
paved and quite smooth, with only a couple of small bumpity bumps as you cross, but as my rear wheel went over the second bump, the bike sprung my skinny butt about 4 to 5 inches off of the seat. At the end of the ride, I told my friend about it, and he said that it sounded like I needed to increase my preload. Is this correct, and if so, can someone explain the basic physics behind it?

Also, thanks for starting this thread, Chad. I'd been thinking about asking about this for a while, but never got around to it.

[Ryan]

Preload allows you to set proper sag. Your dampers work best in the middle third of their stroke, so you use pre-load to pre-compress the spring so when you park your ass on the seat, the ride height settles in the upper part of that middle third.  If your fork allows for it, you should set pre-load there, too. One reason your handling degrades if you leave too much sag in the rear is the steering geometry, weight transfer under throttle and braking, and suspension reaction to mid-corner bumps  are all affected.  So while it doesn't stiffen the spring, increasing pre-load can restore the proper geometry, and keep the shock working where it dampens best. This will "tighten up" the handling and make it feel stiffer. An inch of difference between the lowest and highest perch position may not seem like much, but if your bike has 5 inches of rear wheel travel, you have 1 2/3" of "sweet spot" damping. If your weight causes the bike to sag 3" you are at the bottom of the range. An inch restores the ride height.

[Huzo]

So increasing preload (compressing spring) with no weight on the seat will RAISE the ride height?

Sure will Boss...
With the proviso that you have not hit the top stop...ie, “topping out..”

[Huzo]

Hhmm,    Please keep it in layman terms, because some of the physics can get a little complicated.

      Can you explain to me why, when I first got my CalVin, when it was on the first lightest spring pre load setting,it wallowed around corners like it had a hinge in the middle of the bike, and when I adjusted the preload to the highest setting, it firmed and stiffened up the corner handling a LOT!

       

       Kelly

      Was that not due to the suspension being firmer?

Dunno..
Other than altering the rake due to higher seat position, can’t say.
But here’s the thing.
Put your preload on the lowest setting and ACCURATELY measure the height to a solid point on the bike..(not the bloody seat..)
Then wind your spring preload up to the other end of it’s range and conduct the SAME measurement.
It’ll have raised the point by however much you increased the preload distance.
As long as your damper performs the same way at each part of it’s stroke, then winding on preload has not stiffened the suspension.
Y’all really need to believe this ,or just research it..

[Huzo]

It still is operating in the same range with respect to the spring, it’s just the damper has been extended and is operating in a different part of it’s available stroke..

[Huzo]

No.  It just won't compress as much when the rider is on the bike.  So I suppose the bike will stay a little taller with the rider but it will ride very stiff.

One of us is going to wish you hadn’t said  that..
We may need to come back to reply #9..

[Huzo]

No.  It just won't compress as much when the rider is on the bike.  So I suppose the bike will stay a little taller with the rider but it will ride very stiff.

So LR..
If winding on preload has compressed the spring, why does the top of the spring not just extend outwards to achieve the same state as before, given that the damper has not topped out...Careful now...

[keener]

Preload allows you to set proper sag. Your dampers work best in the middle third of their stroke, so you use pre-load to pre-compress the spring so when you park your ass on the seat, the ride height settles in the upper part of that middle third.  If your fork allows for it, you should set pre-load there, too. One reason your handling degrades if you leave too much sag in the rear is the steering geometry, weight transfer under throttle and braking, and suspension reaction to mid-corner bumps  are all affected.  So while it doesn't stiffen the spring, increasing pre-load can restore the proper geometry, and keep the shock working where it dampens best. This will "tighten up" the handling and make it feel stiffer. An inch of difference between the lowest and highest perch position may not seem like much, but if your bike has 5 inches of rear wheel travel, you have 1 2/3" of "sweet spot" damping. If your weight causes the bike to sag 3" you are at the bottom of the range. An inch restores the ride height.

this is the fact  .............Ryan ....  preload simply means you are preloading the spring

[oldbike54]

It still is operating in the same range with respect to the spring, it’s just the damper has been extended and is operating in a different part of it’s available stroke..

 That's what I said .

 Oh , as for altering rake , do the math , and inch of rear ride height won't really change the rake numbers enough to matter .

 Dusty

[wirespokes]

There are two things going on here - actually three - spring rate (preload is a subheading), compression damping and rebound damping.

Like Ryan said - preload is the way to set sag. Why do you want to be part way into the suspension (normal is 1/3rd of the way down from the top)?

Compression damping (like when the wheel runs over a rock) forces the wheel up and compresses the shock. You want as much travel as possible to absorb that movement.

But some movement of the suspension is needed for depressions, like pot holes. That's the importance of sag. The wheel needs to be able to drop into the depression rather than taking the whole bike with it. Sag guarantees there's shock travel available for pot holes.

JC85 - What happened in your case was the first bump compressed the shock. When the second bump came soon after, the shock hadn't rebounded, so it bottomed out and transferred the force directly to the frame. One of two things happened - the sag was too great (more than a 1/3rd of the way into its travel) or the rebound is too stiff and wouldn't allow the shocks to rebound quickly enough.

[Tom H]

So since this thread is conveniently going right now, let me pop in and ask a question that has been on my mind for a while:

Several months ago, a friend and I were out on a ride. Going down US 400, we crossed some railroad tracks. The tracks are
paved and quite smooth, with only a couple of small bumpity bumps as you cross, but as my rear wheel went over the second bump, the bike sprung my skinny butt about 4 to 5 inches off of the seat. At the end of the ride, I told my friend about it, and he said that it sounded like I needed to increase my preload. Is this correct, and if so, can someone explain the basic physics behind it?

Also, thanks for starting this thread, Chad. I'd been thinking about asking about this for a while, but never got around to it.

I had the same problem with my stock EVT shocks over a RR track. Adding preload would only make the problem worse.

I am not a shock expert. With my EVT I believe there was not enough rebound dampening. It compressed fine, but when it came time for the spring to expand, it came "flying" back to full length rather than gradually coming back, This is why I came off the seat, I believe.

I changed to a Koni set and the problem was gone.

Tom

[Tom H]

Wirespokes, your reply makes sense for the RR tracks.

But, at least in my case I felt as though the shock was coming up too fast and tossing me. I felt compression, but I never felt a "ummm.. a bang" to kick me off the seat like the shock was still fully compressed. More like a spring tossed me up. Over the same track, my stock shocks on my HD never gave me the problem, neither did my Eldo with Koni.

I'm still learning suspension.

There is a suspension 101 web page (I can find the link if needed). I never fully understood it, but I understood enough to be dangerous

I can't find the page to link. Search for Suspension101eBook. pdf

Tom

[JC85]

Wirespokes, your reply makes sense for the RR tracks.

But, at least in my case I felt as though the shock was coming up too fast and tossing me. I felt compression, but I never felt a "ummm.. a bang" to kick me off the seat like the shock was still fully compressed. More like a spring tossed me up. Over the same track, my stock shocks on my HD never gave me the problem, neither did my Eldo with Koni.

I'm still learning suspension.

There is a suspension 101 web page (I can find the link if needed). I never fully understood it, but I understood enough to be dangerous

Tom

That's how it seemed to me in this instance. I have Koni shocks on my Eldo, but they're adjusted at the top/least amount of preload setting. I'm going to take the advice in this thread to adjust them properly. Thanks, everyone!

[Huzo]

Guys.
Please just stick to the original question. The waters get muddied too easily.
An example..
If you look at a shock that is loaded with the damper at mid travel, the spring is subjected to a compressive force of say 150 kg, and the length eye to eye might be say, 300 mm.
The shock will have moved in or out and come to rest where the upward force of the spring = the downward force of the mass.
Surely that’s clear..
Now if you wind the collar upwards at the bottom (generally), you’re attempting to compress the spring, but the top is free to extend outwards so the SPRING is the same length ( the load has not changed), but the top has moved up.
All you have changed is the dimension of the shock eye to eye (ride height), but the spring is in the same condition and length so it will perform identically, it’s just moved upwards.
If you like, all you’ve done is increased the distance from the bottom of the spring perch, to the lower eye.

[oldbike54]

Guys.
Please just stick to the original question. The waters get muddied too easily.
An example..
If you look at a shock that is loaded with the damper at mid travel, the spring is subjected to a compressive force of say 150 kg, and the length eye to eye might be say, 300 mm.
The shock will have moved in or out and come to rest where the upward force of the spring = the downward force of the mass.
Surely that’s clear..
Now if you wind the collar upwards at the bottom (generally), you’re attempting to compress the spring, but the top is free to extend outwards so the SPRING is the same length ( the load has not changed), but the top has moved up.
All you have changed is the dimension of the shock eye to eye (ride height), but the spring is in the same condition and length so it will perform identically, it’s just moved upwards.
If you like, all you’ve done is increased the distance from the bottom of the spring perch, to the lower eye.

 Peter , mate , if the spring is free to move upward , then increasing preload would have zero effect .

 Dusty

[LowRyter]

So LR..
If winding on preload has compressed the spring, why does the top of the spring not just extend outwards to achieve the same state as before, given that the damper has not topped out...Careful now...

point taken but fractions vs actual ride height adjustment.

[blu guzz]

for the rail road example, the rider is pointing to an issue with almost all bikes.  most are over sprung and under damped.  the maker does not know whether the buyer will weigh 90 lbs or 400+, 2 up and loaded.  therefore, they select a compromise.  to save expenses, they usually choose suspension with only a rear preload adjustment.  if by luck, you happen to fit the size that the  maker has guessed, then you might get a decent ride.  however, for 98% of the riders, that wont be the case.  as such, preload will only make a minor difference.  it seems to me that the europeans must choose 225 lbs single rider or thereabout as their optimum.  also, stock shocks tend to be on the cheaper, lower quality side.
when you order a custom shock from ohlins, you supply them with your weight, riding style and weather you travel 2 up regularly.  they then choose the correct spring and set the damping for you and your style.  but all of this comes with a pretty hefty price.
so, to the op, you should experiment, but don't expect perfection.

[wirespokes]

It compressed fine, but when it came time for the spring to expand, it came "flying" back to full length rather than gradually coming back, This is why I came off the seat, I believe.

If that was the case, then why wouldn't that happen on the first bump? Why would it take two?

All the pre-load is doing is keeping the suspension above mid way in its travel. No need getting into spring formulae unless ordering new ones - if the shocks are compressed too far, add preload. Not enough, reduce preload.

This is important because the shocks, besides absorbing bumps and making a smoother ride, are keeping the tires in contact with the pavement. When the suspension isn't set up correctly, the wheels bounce around and go out of contact with the road and control is reduced.