Understanding Preload

Hi all -

I posted this on another forum, but thought it might interest some here as well...

Over the past five years or so I have gotten very interested in learning about motorcycle suspension and suspension tuning - lots of reading, modding my bike, trying things out at the track, etc.

Even though my overall understanding of suspension tuning is still at a very basic level, I have come to realize that no more than 10% of riders have any idea what a preload adjustment physically does. The erroneous idea that adding spring preload somehow "stiffens" the suspension is incredibly pervasive. It's not just regular riders who labour under this misconception...you read it constantly in motorcycle magazines (Sport Rider is a frequent offender) and on motorcycle websites...guru suspension tuner Dave Moss even gets it wrong quite often when he explains it (although in that case, when pressed, he admits that it is incorrect and just a bit of loose language on his part).

I finally got so tired of trying to explain this to people that I made this figure (attached) to try to help. I offer it here for your inspection and use, along with the following descriptions of parts A to E:

A. Spring is 600 mm long, and has a rate of 1 kg / mm. That is, it compresses 1 mm for each kg of force applied.

B. Spring has been installed. Preload adjuster is wound all the way out. With fork fully extended, the spring is compressed by 15 mm. This is the “installed preload”. Thus, it will take 15 kg of force to start compressing the fork. After that it will compress by 1 mm for each additional kg of force applied.

C. There is now 65 kg of bike + rider weight bearing down on the fork leg (130 kg across both legs). This causes the fork to compress by 50 mm from its fully extended length. That is, the first 15 kg is supported by the 15 mm spring preload, then the additional 50 kg compresses the spring 50 mm. Thus, with this load, the total “sag” in the fork is 50 mm.

D. Since 50 mm is too much sag, the preload adjuster has been wound in 10 mm. This extends the fork by 10 mm, so the sag is now 40 mm. Note that the length of the spring has not changed. Adding preload does not compress the spring, it just extends the fork. This is always true, except if the fork is topped out.

E. With the same preload setting as in D, the weight has been removed from the suspension. Now, there is 25 mm of preload (the 15 mm installed preload shown in B, plus the 10 mm preload added in D). Thus, it will take 25 kg of force to start compressing the fork. After that, it will compress 1 mm for each additional kg of force applied, as always.

Summary: Adding preload does not result in a spring that is more compressed in use, and therefore does not make the suspension feel “stiffer”. The suspension will still compress by the same amount per unit force applied, regardless of the preload setting. Adding preload merely extends the fork so that it has more positive travel available before bottoming.

Unless you have progressive springs.

To me the problem is in the name. Preload adjustment actually has nothing to do with preload unless the damper is topped out as you say (which would be it's own problem). It should simply be called ride height adjustment.

BrianZ said:

Unless you have progressive springs.

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No.

Don't you mean to say that preload doesn't affect spring rate?

Aren't there technically cases where a preloaded spring would indeed result in stiffer feeling suspension? By definition it's initially stiffer - 15kg to begin compression without preload, 25kg to begin compression with. So if you say, crested a hill at sufficient speed to lower the weight on the bike above the initial compression weight, reloading would feel stiffer. Of course in practice it's about ride height and suspension travel

BrianZ said:

Unless you have progressive springs.

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No, even with progressive springs, the springs are operating in the same range (i.e., with the same compression) regarless of the preload setting, since they are supporting the same weight regardless. Notice that in Figures C and D, the spring length is the same. This is also true with progressive springs.

espro said:

Don't you mean to say that preload doesn't affect spring rate?

Aren't there technically cases where a preloaded spring would indeed result in stiffer feeling suspension? By definition it's initially stiffer - 15kg to begin compression without preload, 25kg to begin compression with. So if you say, crested a hill at sufficient speed to lower the weight on the bike above the initial compression weight, reloading would feel stiffer.

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Yes, absolutely! Once the suspension is topped out, then it will take more force to start compressing it again if the preload is high, so that initial process could give the impression of increased stiffness. But only if you top it out. And of course the chances of topping out increase with increased preload, since you have sacrificed negative (expansion) travel.

great explanation, thanks!

fastar1 said:

To me the problem is in the name. Preload adjustment actually has nothing to do with preload unless the damper is topped out as you say (which would be it's own problem). It should simply be called ride height adjustment.

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Not so; ride height is something else.

The name "preload" implies the load that is applied to the spring with the suspension topped out. The name is correct.

Ride height affects the relative position of the available fork travel limits (topped out to bottomed out), with respect to the bike itself.

Now, changing the preload affects the height that the bike will stabilize at with the normal applied load, but it is different from changing the ride height.

See the projection of the top of the forks above the top of the triple clamp (and in this case, the handlebar holders)? Sliding the forks up or down through the triple clamps affects the ride height without changing the preload and has no effect on the sag.



And in this case (damper rod forks), see the length of these spacers ...



Those affect the amount of preload on the spring and therefore, the amount of rider-aboard sag that the suspension will have. In the case of fancier forks that have external preload adjustments, turning that adjustment screw has exactly the same effect that changing the length of this spacer has, it's just a whole lot easier.

And before anyone inquires why ... the two home-made spacers are intentionally slightly different lengths because the actual free lengths of the fork springs were a couple millimeters different due to production tolerances, and this equalized that out so that the free length of spring + spacer, and therefore the actual applied preload on each side, is as close to the same as I could get it.

When changing ride height by sliding forks up through triple clamps, it's important to make sure that at the mechanical travel limit of maximum fork compression, the fender cannot collide with anything. This is easy with the fork caps off and the spacers and springs out. Support the front of the bike up by some other method and lift the wheel as far up as it will go. This photo is taken looking straight up at the bottom of the radiator and the back of the front fender to confirm that there are several millimeters of clearance, which is necessary to allow for deflections that will occur under heavy braking or suspension loads. This is with the forks already slid up as shown in the first photo.

There is another thing that both ride height (sliding forks up or down through triple clamps) and preload changes - the "rider-aboard nominal ride height".

This is what affects the rake and trail of the steering when you are rolling down the road straight ahead, and that in turn, is what affects initial turn-in response.

If you add 10mm of preload without changing anything else, the fork springs stay the same length because they have the same load on them (as explained in the first post) but it will essentially raise up the front of the bike as a whole when the rider is aboard. That will change the steering response.

In the case of the bike in the photos above, with my weight on it, initial turn-in felt okay but the front end had excessive sag (57mm!). What was done, was to slide the forks up through the triple clamps by 15mm, which is as much as I felt comfortable with while maintaining adequate clearance to the radiator and the bottom of the upper fairing at full bump travel, and installing spacers that were 15mm longer than stock. This kept the rider-aboard nominal ride height, and therefore the steering geometry with the bike rolling down the road with me on it, the same as before to preserve steering response. But there is now 15mm more compression travel available before getting into the hydraulic cushion (very high compression damping) as the fork approaches the travel limit and of course 15mm before actually hitting the mechanical compression travel limit. This should give better compliance when leaned waaaay over or when braking hard.

Of course, it lost 15mm of rebound travel, but you don't need 57mm of rebound travel available.

Total fork travel in this case is 120mm. Nominal rider-aboard sag should be somewhere near a third of that (now 42mm - close enough), not almost half of that (as it originally was).

What Brian said! 'Zactly.

Brian P said:

There is another thing that both ride height (sliding forks up or down through triple clamps) and preload changes - the "rider-aboard nominal ride height".

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Who sets up a bike for "rider-not-aboard nominal ride height"?

That's why preload is a misnomer, it only describes the process of adjusting ride height while the wheel is in the air, not the consequent change in suspension behaviour when it's being ridden.

And yes, there are several ways of adjusting ride height. Typically on the front you would slide the forks up or down in the yokes, as you showed. In the rear, you typically turn the adjuting ring on the spring, though different linkages can also change the ride height. The difference between the two is that by adjusting the spring you are changing the resting point of the suspension relative to the damper travel, but it's easier than changing linkages. So you will have more bump/less rebound or vice-versa, along with the ride height change. Still, this is not preload. Unless you ride the bike with the damper topped out, you can't change preload. There is no preload to change.

fastar1 said:

Who sets up a bike for "rider-not-aboard nominal ride height"?

That's why preload is a misnomer, it only describes the process of adjusting ride height while the wheel is in the air, not the consequent change in suspension behaviour when it's being ridden.

And yes, there are several ways of adjusting ride height. Typically on the front you would slide the forks up or down in the yokes, as you showed. In the rear, you typically turn the adjuting ring on the spring, though different linkages can also change the ride height. The difference between the two is that by adjusting the spring you are changing the resting point of the suspension relative to the damper travel, but it's easier than changing linkages. So you will have more bump/less rebound or vice-versa, along with the ride height change. Still, this is not preload. Unless you ride the bike with the damper topped out, you can't change preload. There is no preload to change.

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I don't have a problem with the term "preload". Winding it on DOES change the installed preload of the spring when the suspension is topped out (there is more spring preload in Figure E than Figure B). People just need to understand that it refers to the spring tension when - and only when - the suspension is fully extended. AND understand that in any other condition (non-zero sag), preload has NO effect on spring tension.


Regarding the rear shock, turning the rings is analagous to turning the preload adjuster on the forks, not changing the front fork height in the triples. Turning the rings down increases the installed (topped out) spring preload on the shock. Of course it also increases ride height by extending the rear suspension and - here is the surprise - because of the progressive swingarm linkage on all modern bikes, will tend to make the rear suspension action *softer*.

Many bikes and aftermarket shocks have a rear ride height adjustment that is independent of rear preload. On my Penske it is just an adjustment to lengthen one end of the shock so that it is longer, without moving it in its travel. But of course that does change the swingarm angle.

fastar1 said:

The difference between the two is that by adjusting the spring you are changing the resting point of the suspension relative to the damper travel, but it's easier than changing linkages. So you will have more bump/less rebound or vice-versa, along with the ride height change. Still, this is not preload. Unless you ride the bike with the damper topped out, you can't change preload. There is no preload to change.

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This is pretty much completely wrong.

With the forks or shock fully extended the spring is still compressed from its free length. There most certainly is preload on the spring. You are not just changing the resting position, you are changing the entire range through which your suspension operates. I'm also not sure why you think there would be compression/rebound changes as a result of a preload change.

Can you still get pregnant from preload?

caboose483 said:

This is pretty much completely wrong.

With the forks or shock fully extended the spring is still compressed from its free length. There most certainly is preload on the spring. You are not just changing the resting position, you are changing the entire range through which your suspension operates.

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I thought that's what I said!

I'm also not sure why you think there would be compression/rebound changes as a result of a preload change.

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When I said bump/rebound I was referring to travel, not damping.

Obviously, I need to improve the clarity of my posts.

fastar1 said:

I thought that's what I said!

When I said bump/rebound I was referring to travel, not damping.

Obviously, I need to improve the clarity of my posts.

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In that case I don't know what you're talking about.

kellen said:

can you still get pregnant from preload?

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yes

And I only now got kellen's joke. Give it time I guess

Ride height on the rear is typically not adjusted by the threaded spring collars. Those adjust ... preload! and therefore sag. They are analogous to the threaded preload adjusters on front forks or the length of spacers above fork springs on forks that don't have external adjustments.

Ride height (not preload!) on the rear is adjusted by changing any of the following depending on the bike:
- The overall (uncompressed) center-to-center length of the rear shock. Some shocks that have built-in ride height adjustments allow this to be changed, but this adjustment is NOT on the spring seats, it's on the shock rod itself.
- Center-to-center length of rear suspension "dogbones", or equivalent modifications that change the geometry of the linkage in some way.
- Thickness of spacers at the upper shock mount, between the top of the shock linkage and the chassis.

Adjusting ride height and adjusting preload are two separate things!

And it's true that nobody directly adjusts "preload" with a target value for the "preload". One adjusts the "preload" in order to get the "sag" correct. Then one adjusts ride height front and rear to get the geometry of the bike correct so that it steers properly and has proper grip coming out of corners (mostly rear) and has sufficient cornering clearance.

Measure the sag, compare with target value, adjust preload, re-check. Ride the bike to observe steering feel to establish if ride-height changes are warranted. Then you adjust the RIDE HEIGHT - not the preload! - in order to get the steering response correct.