Understanding Preload

[YellowDuck]

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.

Discuss?

[Bob92]

Hidden Assumptions;
1. Your spring is sized properly for the weight of rider+bike, so it is operating in the linear range.
2. Proper amount of oil is in the fork so that the air-spring effect is not happening to early in the travel.

[YellowDuck]

Originally Posted by Bob92

Hidden Assumptions;
1. Your spring is sized properly for the weight of rider+bike, so it is operating in the linear range.
2. Proper amount of oil is in the fork so that the air-spring effect is not happening to early in the travel.

Thanks for the comment. Yes, there are of course subtlties. The basic misconception that I am trying to address is that adding preload results in a more compressed spring, and therefore a stiffer suspension.

Re your point 1...whether it is operating in the linear range or not, so long as there is sag, the spring will be operating in the same range regardless of the preload setting, since it is still supporting the same weight. So, even in that case, adding preload does not change the effective spring rate.

Also, note that in your point 2, if the air gap is insufficient, adding preload would actually *decrease* the effective spring rate, by increasing the air gap at the sag point. So in that case, the suspension becomes less stiff, not more so, with added preload.

Another important detail is that adding front preload without a compensating increase in fork height or rear ride height changes the rake. That geometry change can affect suspension feel as well.

But again, these are more minor details, not the central concept of what preload does and does not do.

[va duc]

Very interesting, looks like the spacers I installed are too long. I only have about 20mm of sag on the machine with the adjusters all the way out. Thanks for the insight.

[John]

Thanks for the explanation, suspension understanding has always been on the confusing side with me. I'm taking my Harley in next week to have new Progressive tube inserts installed, they are called monotubes. I've heard some riders complain they are on the stiff side for the type of riding they do. With the Harley I'm inclined to go softer because I have the Ducati for the fun stuff. I called Progressive about this and they recommended inserting some supplied "c clips", one in each monotube will increase preload slightly and I suppose offer a softer ride. When I heard that I was thinking the oppisite because adding one c clip would compress the spring and the result would be a stiffer ride, not so. Thanks for the clarification.

With the above fork upgrade I didn't think it would affect the ride height since they claim its the same length as OE. Same with the rear shocks; yesterday I installed a set of Suspension Technologies shocks, a relatively new player I decided to give a try. I have no clue how the two work with each other, but mixing and matching is no bigge to begin with, I'll just have to ride the bike and see how it feels then go from there. At this point the ride will likely be more stiff than soft because the shocks are equipped with a HD spring instead of the standard and I'm of average weight, not your typical Harley rider weight. The manufacturer indicated I'm on the edge of the weight specs and if they are too stiff he will change out the spring for $45, thats fair. Removing the Harley shocks is a piece of cake, so that helps. If the weather will clear up, been raining here, I'll ride the bike to see just how stiff the shocks are before installing the fork monotubes. That way I can test the shocks before and after the fork upgrade. I also just installed a tour link stabilizer made by Progressive. This device attaches to the oil pan and frame and is designed to prevent lateral movement of the swingarm, up and down is unaffected. This minor upgrade has paid excellent dividends in ride quality, a very noticible difference in stability.

[YellowDuck]

Originally Posted by va duc

Very interesting, looks like the spacers I installed are too long. I only have about 20mm of sag on the machine with the adjusters all the way out. Thanks for the insight.

20 mm with rider on board, or just the bike weight? 40 mm is a typical number for bike + rider sag.

[va duc]

Originally Posted by YellowDuck

20 mm with rider on board, or just the bike weight? 40 mm is a typical number for bike + rider sag.

Yes, that is with the rider aboard. I tried using a tie wrap on the lower fork legs, I still have 2' of travel. It would appear that I need to reduce the length of the spacers by 20mm or so.

[OldDiavel]

Originally Posted by John

Thanks for the explanation, suspension understanding has always been on the confusing side with me. I'm taking my Harley in next week to have new Progressive tube inserts installed, they are called monotubes. I've heard some riders complain they are on the stiff side for the type of riding they do. With the Harley I'm inclined to go softer because I have the Ducati for the fun stuff. I called Progressive about this and they recommended inserting some supplied "c clips", one in each monotube will increase preload slightly and I suppose offer a softer ride. When I heard that I was thinking the oppisite because adding one c clip would compress the spring and the result would be a stiffer ride, not so. Thanks for the clarification.

With the above fork upgrade I didn't think it would affect the ride height since they claim its the same length as OE. Same with the rear shocks; yesterday I installed a set of Suspension Technologies shocks, a relatively new player I decided to give a try. I have no clue how the two work with each other, but mixing and matching is no bigge to begin with, I'll just have to ride the bike and see how it feels then go from there. At this point the ride will likely be more stiff than soft because the shocks are equipped with a HD spring instead of the standard and I'm of average weight, not your typical Harley rider weight. The manufacturer indicated I'm on the edge of the weight specs and if they are too stiff he will change out the spring for $45, thats fair. Removing the Harley shocks is a piece of cake, so that helps. If the weather will clear up, been raining here, I'll ride the bike to see just how stiff the shocks are before installing the fork monotubes. That way I can test the shocks before and after the fork upgrade. I also just installed a tour link stabilizer made by Progressive. This device attaches to the oil pan and frame and is designed to prevent lateral movement of the swingarm, up and down is unaffected. This minor upgrade has paid excellent dividends in ride quality, a very noticible difference in stability.

I put Progressive Monotubes (w/one C clip) on my 2009 HD Ultra and they really improved the front end handling. I was going to put Progressive 440's on the rear but I sold the Ultra and bought a Ducati Multistrada instead.

[sportbob]

Couldn't agree more Yellow Duck. Had the same arguments 30 years ago while prepping road race sport car suspensions. Preload is, essentially, a ride height adjustment. It only "loads" the spring with the shock fully extended. which it should almost never be with the bike/car off stands.

[999s]

Thank you for getting rid of some of the magic blue smoke!!!!