Help me understand Pre-load

[faffi]

I take the liberty to join in. Huzo is correct that a linear spring will compress an equal amount from fully expanded until coil bind. But while that is correct, for each unit of compression, the spring will push back harder. If you load up a spring with 100 lbs, it will try to expand with 100 lbs. On a motorcycle, the spring is usually compressed by anything from 1/4 inch to 4 inches, depending on the use, and is prevented from extending fully. If you increase preload by 100 lbs, another 10 lbs of load will compress the spring the same amount as if it was fully extended without preload. However, you now have 110 lbs of force trying to push back and extend the spring. You will also have less negative wheel travel. Hence the ride will feel firmer to the rider, even if the suspension will deflect the same amount for an extra additional load. Rebound damping will also be under more stress due to the increased preload. So if you add preload, the bike will sit higher and be more likely to top out, be that on the rebound or from riding over dips, both of which can give the sensation of a firmer ride.

Same if you have too little preload, especially on a soft spring, you risk that the bike rides low and bottom out constantly, while having lots of extension available for dips and recoil, causing a bouncy and often uncontrolled ride.

In conclusion: While spring rate does not change by preload on a straight spring, the actual performance of the bike (or car) will definitely be affected in a way you can easily feel.

[SIR REAL ED]

I never have been able to figure out/understand preload/spring rates BUT after reading this thread today I can confidently state that I still can't... 

It's like a Zen koan.

"What is the sound of one hand clapping?"

[SIR REAL ED]

I looked for a video, but everything I found were people talking who were very impressed with the sound of their own voice, and who often used the inaccurate terms during their descriptions.

A very simple illustration of a very simple concept:

https://lifeatlean.com/teach-me-suspension-everything-preload/

[SIR REAL ED]

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.

Aaron D nailed this in the second post of this thread back on March 6, 2020!

[kingoffleece]

Nice discussion on a complex topic.
For those who still don't fully grasp-the ok news is you don't REALLY have to.  What you must do:

1) determine if the shock spring is correct for your weight.  This is done by setting sag.  If the proper range is not achieved it's basically game over until that is corrected.
2)  This is an illustration to help understand.  First, as noted several times here, preload does not "stiffen" a spring.  It changes the "starting point" of where the spring starts to react.  Remember, this is a very BASIC lesson.  Imagine a 12 inch ruler, and a spring of 6 inches.  Along the ruler (vertical) one can adjust any starting point for the spring to engage.  Little preload and the spring starts to move at the 2 inch mark and stops totally at 8.  This is a simple picture to understand the concept.  Add more preload to the spring now.  Nothing changes except now the spring must move to 4 before the spring is asked to work, and will finish at 10 on the ruler.  Again, this is simple but hopefully you get the idea.
3) the jump in understanding is: correct spring is critical to proper suspension.  Without proper sag none of the mother elements are able to be optimized.
4) Understanding this is why the advice to "add preload" when getting bucked out of the seat is usually not even close to the answer.

[Kev m]

Nice discussion on a complex topic.
For those who still don't fully grasp-the ok news is you don't REALLY have to.  What you must do:


2)  This is an illustration to help understand.  First, as noted several times here, preload does not "stiffen" a spring.  It changes the "starting point" of where the spring starts to react. 

I get the rest of your post.

I'm asking if it's semantics on the stiffen part.

By changing the amount of force it takes to further compress the spring does it not "stiffen" the suspension... That is make it feel harder to compress and potentially even keep it topped out depending on the amount of force at work?

Can it not make it feel harder/harsher/transmit more force to the rider... However you want to put it.

[YellowDuck]

IF I DO, then may I ask, what happens when the preload is incorrectly set too high, topping out the shock, and causing the force pushing down on the swingarm (pushing up against the frame) to require more force than the rider to start to move? Does that not increase how much force is felt from hitting a bump? Does that not change the way the frame reacts to said bump? And if so, would that not be a stiffer rear end?

What am I missing?

Yes, but that would be a glitch setting.  In that scenario you have no rider sag at all, which would indicate a spring rate that is WORLDS too high, or a ridiculous amount of preload.  Proper setups have some sag to the suspension just from the bike weight alone, let alone bike + rider.

In the scenario you describe, any force form a bump less than the force needed to move that jammed up suspension past its topped-out condition, would indeed be transmitted directly through the chassis to the rider.

Here's a one that will blow your mind...most (all?) rear suspensions have rising rate geometry, either because of the linkage design, or in a linkageless setup, because of the angle of the swingarm to the shock(s).  Rising rate means that as you compress the suspension, each successive mm of suspension travel requires progressively more force than the previous mm of travel.  Accordingly, if you *add* preload (and thereby extend the suspension), you actually *soften* the suspension, since it is operating in a zone where more swingarm movement is achieved with less force.   

 

[faffi]

To add to preload - if there is no sag from bike weight alone when proper sag with rider is reached, springs are too soft.

I have always been thaught that twin shocks almost always are regressive, because they need less travel as wheel moves upward?

[kingoffleece]

https://www.lesjofors.com/en/technology/insights/motorbike-suspension-springs/

[kingoffleece]

https://www.revzilla.com/common-tread/motorcycle-suspension-vocabulary-explained

[SIR REAL ED]

Here's a one that will blow your mind...most (all?) rear suspensions have rising rate geometry, either because of the linkage design, or in a linkageless setup, because of the angle of the swingarm to the shock(s).  Rising rate means that as you compress the suspension, each successive mm of suspension travel requires progressively more force than the previous mm of travel.  Accordingly, if you *add* preload (and thereby extend the suspension), you actually *soften* the suspension, since it is operating in a zone where more swingarm movement is achieved with less force.   

You are correct that in theory, this could be possible.  It all depends on the progression rate (think leverage ratio) of the rear suspension thruout its travel and how much spring preload is necessary to move the swing arm to a range that has a different leverage ratio.  In practice, I suspect it rarely, if ever occurs.

For example, a 10% increase in preload that results in a 2% increase in leverage ratio is still a stiffer suspension.

It would take a lot of measurements to make the case one way or the other.

[SIR REAL ED]

I have always been thaught that twin shocks almost always are regressive, because they need less travel as wheel moves upward?

It all depends on distances and angles.  I would not apply any "Rule of Thumb" guidelines to any specific motorcycle..

[Kev m]

https://www.revzilla.com/common-tread/motorcycle-suspension-vocabulary-explained

These quotes seem to address the semantics I'm talking about:

Consider a shock that’s been removed from the bike. It’s not supporting any weight and it’s fully extended, yet the spring is still compressed a few millimeters by a threaded collar. That’s preload. And with preload, you’re intentionally compressing the spring a little bit, which changes how much load is required to initiate suspension movement. It also alters the total force necessary to completely compress, or bottom, the suspension piece. What it does not do is change the spring’s rate.

AND

So, adding preload will help prevent your suspension from bottoming by requiring more weight to compress the spring. More preload can give the impression of harder or stiffer suspension, but it’s important to understand that all you’re doing with preload is altering the effective range of the spring stroke used. Preload’s primary purpose, then, is in setting ride height and sag.

AND

Slow and fast don’t refer to the speed you’re traveling on the bike, but rather the speed at which the suspension component is moving. When you hear slow or fast, you know someone is talking about damping. However, when you hear hard or soft, they may be referring to springs or damping since both spring and damping settings can cause a fork or shock to feel soft or hard.

I realize increasing preload isn't going to "stiffen" the damping (rate of the compression or rebound) as controlled by the valving and viscosity of the fluid.

But the spring compresses during suspension travel, and the force needed to compress it increases, would it not feel "stiffer" or "harsher" over bumps at higher preload than softer because you've already created that condition, that need for more force to overcome the spring?

[faffi]

It all depends on distances and angles.  I would not apply any "Rule of Thumb" guidelines to any specific motorcycle..

Spot on. Honda fitted linkages to the twin shocks of the CB1000 big one in order to get a progressive curve similar to that of single shock system with linkages.

Back in the day, BMW and others made the angle of the rear shocks adjustable. By moving the top of the shocks forward, it was said that the rear end was lowered (naturally) as well as softer.

[SIR REAL ED]

Spot on. Honda fitted linkages to the twin shocks of the CB1000 big one in order to get a progressive curve similar to that of single shock system with linkages.

Back in the day, BMW and others made the angle of the rear shocks adjustable. By moving the top of the shocks forward, it was said that the rear end was lowered (naturally) as well as softer.

Waaaaaay back in the day (early 1970's maybe), IIRC, the hot set up for a rising rate rear suspension with dual shocks was supposed to be the distance from the swingarm pivot to the upper shock mount should be equal to the distance from the swingarm pivot to the lower shock mount.

I have no idea why I would remember that.   I would expect that may only be true for swingarms that are at an angle of X degrees (?) below horizontal with no load on the rear suspension.

Angles and distances.......

[Huzo]

Well…
I can recall a few years ago on this forum, you could safely say…?
“The arse end of my Gazunthapede was way too soft, so I cranked up the preload to make it harder…”
I don’t think anyone would dare say it now in all seriousness….
Mission accomplished…

[YellowDuck]

You are correct that in theory, this could be possible.  It all depends on the progression rate (think leverage ratio) of the rear suspension thruout its travel and how much spring preload is necessary to move the swing arm to a range that has a different leverage ratio.  In practice, I suspect it rarely, if ever occurs.

For example, a 10% increase in preload that results in a 2% increase in leverage ratio is still a stiffer suspension.

It would take a lot of measurements to make the case one way or the other.

Why would it be stiffer?  Adding preload does not make the suspension stiffer, except in the fully topped-out position.  Read the rest of this thread - most people commenting know what they are talking about.

[kingoffleece]

https://www.amazon.com/MOTORCYCLE-HANDLING-CHASSIS-DESIGN-science/dp/8493328669

[faffi]

Why would it be stiffer?  Adding preload does not make the suspension stiffer, except in the fully topped-out position.  Read the rest of this thread - most people commenting know what they are talking about.

Progressive springs become stiffer with more preload, as one example.

Many progressive linkage suspension systems can become SOFTER with more preload, whether spring is progressive or straight, because it will move the wheel/swingarm/linkages/what-do-you-call-it into an angle/range with less progressive action. Yes, it will eventually become stiffer as wheel travel increase, but can be more comfortable during normal riding AND provide more resistance against bottoming. I did just that on my previous XT600Z, and experienced a very welcome improvement in suppleness together with no more bottoming.

I would also challenge everybody with straight wound springs to play with preload and report back what they observe while riding, with both less and more preload.

[YellowDuck]

Progressive springs become stiffer with more preload, as one example.

Nope.  The spring will be compressed exactly as much as it needs to be to support the weight of bike and rider.  That is true whether it is a linear rate spring or a progressive one.  And the *total* amount it needs to compress doesn't change with preload - preload just determines how much of the spring compression is already there when the shock is topped out.  (Again, ignoring geometry / linkage ratio changes associated with ride height.)

Sorry, not backing down on this.  You are just reiterating the #1 misconception about what preload adjustment does.  Adding preload doe not stiffen the suspension, unless you add so much that the top-out springs are engaged.

I suggest reading this: https://www.promecha.net/sag-and-preload

and this: https://www.promecha.net/leverage-and-linkages

Hugh

[Huzo]

Nope.  The spring will be compressed exactly as much as it needs to be to support the weight of bike and rider.  That is true whether it is a linear rate spring or a progressive one.  And the *total* amount it needs to compress doesn't change with preload - preload just determines how much of the spring compression is already there when the shock is topped out.  (Again, ignoring geometry / linkage ratio changes associated with ride height.)

Sorry, not backing down on this.  You are just reiterating the #1 misconception about what preload adjustment does.  Adding preload doe not stiffen the suspension, unless you add so much that the top-out springs are engaged.

I suggest reading this: https://www.promecha.net/sag-and-preload

and this: https://www.promecha.net/leverage-and-linkages

Hugh

You Sir, are 100% correct.
The people who hold the opposite view are unwittingly just regurgitating what has been passed on to them over the journey.
As you say..
The PROGRESSIVE spring will compress to the the length that it needs to support the load.
Equal and opposite forces.
It will behave EXACTLY the same in real world use regardless of preload. Again people must understand that once the damper rod comes off the bump stop at the top of stroke, it is absolutely irrelevant how much load it HAD to begin with, it just sunk less to achieve equilibrium.
Thus giving the impression that it is “stiffer”.
The mis use of terminology and the muddying of the waters with half baked and erroneous anecdotal explanations, is what drags it all into the mire.
ALL THE PRELOAD DOES AND EVER HAS DONE, IS TO MOVE THE DAMPER ROD TO AN EXTENDED OR CONTRACTED LEGTH, WHICH ALTERS THE EYE TO EYE LENGTH OF THE SHOCK.
The preload adjustment just moves the bump stop/s away from their limits.
Yours is the best response of the thread.