Ducati.org forum banner
1 - 16 of 16 Posts

·
Registered
Joined
·
36 Posts
Discussion Starter · #1 ·
My fork seals are leaking and I know one of the former owners setup the bike for the track. He weighs less then I do and Im thinking he may have changed the springs for his weight.

I am going to have my fork seals & bushings replaced and Im trying to figure out the fork rebuild with springs for my weight to do at the same time. I weight 240-250 without gear depending on how well I eat. Any advice you guys can give me would be great.

According to race tech I need a factory fork spring rate for my showa suspension but that doesnt seem right... Everything I have red says that the factory suspension isnt setup right for my specific weight so that doesnt make sense to me. Hope you guys can help.
 

·
Registered
Joined
·
262 Posts
Racetech springs calculator is telling me 1.09 kg/mm for 250 without gear and street riding. 1.15 kg/mm for track riding. Stock spring rate is 1.00.
 

·
Registered
Joined
·
1,308 Posts
You need to determine how much the suspension sags under your bodyweight alone while in a riding position in full gear. If it is more than 20 mm in the front, or 30 mm in the rear, you need a stiffer spring.
 

·
Registered
Joined
·
30 Posts
You need to determine how much the suspension sags under your bodyweight alone while in a riding position in full gear. If it is more than 20 mm in the front, or 30 mm in the rear, you need a stiffer spring.
You can adjust the preload before you buy more springs, that's provided that there is some adjustment.
 

·
Registered
Joined
·
63 Posts
Hi guys,

For what it's worth, preload alters the sag not ride height. I understand that the ride height is you sitting on the bike (hence after sag impact) but in the real world, when you go over a bump, the amount that the forks extend is fixed by the position they are in the clamps and likewise, the amount the shock extends is fixed by the linkage set-up. Spring preloaded compression (visibly measured by the amount of "sag") simply makes this extension happen quicker or slower.

When you take your bike to a suspension specialist, they check the sag with you on the bike and the first thing they do is adjust the preload to put the sag within acceptable range, Yes? They only change the spring when the preload adjustment cannot get you into the acceptable range (and allow some meaningfull adjustment eachway).

But another indication that preload does not change height is the fact that on a 999S with wizbang Ohlins all round, they also have a height adjuster rod. What might this be used for?

Just my 2 cents (from Australia:))
 

·
Registered
Joined
·
185 Posts
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.


 

·
Registered
Joined
·
63 Posts
Appreciate the scientific explaination of what I "feel" when adjusting the clickers but it's a bit like trying to explain why exposing dried ice to the palm of a blindfolded person, they feel a burning sensation they "feel" as heat.

All good and well the physics, but when any rider (yes, even GP riders) asks their tech for a harder or softer front, the teck will first play with the clickers before resorting to changin out springs.

In reality, the rider is not asking for a ride height adjustment, they are asking for a harder or softer front or rear end.

Pre compressing the spring (pre-load) reduces the amount of travel of the spring has left. In its resting state, a spring has no rebound action, only when it is compressed!! You know, every action has an equal and opposite reaction?

The more preload you add, the more rebound force the spring applies.

Another way to look at it, a road (inatimate object for all intensive purposes), the bike and rider combo weighing in around 240-260kg and then a spring. Will compressing the spring lift the 240k weight higher or create and hold more expansion pressure (spring rate) as it does not have the pressure required to move either the road or 240kg weight?
 

·
Registered
Joined
·
185 Posts
Appreciate the scientific explaination of what I "feel" when adjusting the clickers but it's a bit like trying to explain why exposing dried ice to the palm of a blindfolded person, they feel a burning sensation they "feel" as heat.

All good and well the physics, but when any rider (yes, even GP riders) asks their tech for a harder or softer front, the teck will first play with the clickers before resorting to changin out springs.

In reality, the rider is not asking for a ride height adjustment, they are asking for a harder or softer front or rear end.

Pre compressing the spring (pre-load) reduces the amount of travel of the spring has left. In its resting state, a spring has no rebound action, only when it is compressed!! You know, every action has an equal and opposite reaction?

The more preload you add, the more rebound force the spring applies.

Another way to look at it, a road (inatimate object for all intensive purposes), the bike and rider combo weighing in around 240-260kg and then a spring. Will compressing the spring lift the 240k weight higher or create and hold more expansion pressure (spring rate) as it does not have the pressure required to move either the road or 240kg weight?
Perhaps your getting confused with compression dampening vs preload. For more on preload, read my previous post. Compression dampening will provide a harder or softer front end.
 

·
Registered
Joined
·
262 Posts
Not to muddy the waters at all, but my understanding is that while adding preload on a linear rate spring does not increase the "stiffness", adding preload to a progressive rate spring like a stock shock and linkage effectively does (well our stock shock springs are actually linear, but the linkage makes it progressive). OK, it physically doesn't change the stiffness of the spring -- it's not like it's altering the alloy of the metal. But increasing preload on a progressive rate setup means you are altering how stiff the spring feels to you as you would be adding preload that's taking up the portion of the travel that is lower stiffness, leaving your effective travel as only the more stiff portion of the travel remaining.

I think that's right anyways, but I may be wrong, not like it would be the first time.

EDIT: Actually the more I think about it as I progress through my first cup of coffee this morning, the situation I just described wouldn't apply to a linear rate spring with a progressive linkage as the rate of stiffness progression will be the same regardless of how much preload you have on the linear rate spring -- the stiffness progression is dictated solely by the progressive linkage, which is fixed (by "stiffness progression" I just mean how stiff the spring feels to you based on where you are in the progressive spring's progressive travel; note that the actual stiffness of the spring hasn't changed as turning a preload adjuster doesn't make us wizard metallurgists). So the situation I described would only apply to changing preload on a progressive-rate spring. Regardless, a linear rate spring and a progressive linkage like what comes stock on our bikes does increase in "stiffness" felt by the rider as the spring becomes more compressed, but that's independent of the preload adjustment.
 

·
Registered
Joined
·
63 Posts
Hi FunJim,

Not confused mate, preload is just that, the amount of preloaded pressure on the spring via the preload adjuster (clickers).

Damping is the rate at which the fork oil is allowed to flow thereby altering how fast the fork contracts and extends once compressed by a bump (which is also the primary job of the spring).

Preload gets you in the ballpark while damping fine tunes! This is the whole reason we have hi & low damping adjustments!

Progressive springs are more for road use as they are designed for variable rate where as linear springs are more for track uses as they will react the same regardless of speed and surface.

Cheers :)
 

·
Registered
Joined
·
185 Posts
Hi FunJim,

Not confused mate, preload is just that, the amount of preloaded pressure on the spring via the preload adjuster (clickers).

Damping is the rate at which the fork oil is allowed to flow thereby altering how fast the fork contracts and extends once compressed by a bump (which is also the primary job of the spring).

Preload gets you in the ballpark while damping fine tunes! This is the whole reason we have hi & low damping adjustments!

Progressive springs are more for road use as they are designed for variable rate where as linear springs are more for track uses as they will react the same regardless of speed and surface.

Cheers :)
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. If static sag exceeds 30% of total fork travel, springs of a higher rate will be required. If a stuffer "FEEL" is desired, that is achieved with low speed compression dampening.
 

·
Registered
Joined
·
63 Posts
One final go before agreeing to disagree….
Road at one end and the bike with rider at the other end . By adding preload to the spring, will you lift the bike or squish the spring?
Will squishing the spring create an equal and opposite reaction that requires greater force again to squish it further?
The reason adding preload reduces sag is because you have created greater force (reaction) within the spring to counter-act your bodyweight.
Your chart is A grade and what you say is spot on within your variables and constants but this did not take into account the road at the other end of your forks. The bike is creating far greater downward force than the spring can rebound beyond its set rate (sag) level.
Experiment on your shock (you can see both the spring and the plunger shaft). Does winding down the collars compress the spring or extend the shaft out of the assay? Look at how everything is attached and you will note that the only way to adjust the height in this experiment is if the shaft extends from the assay. If it doesn’t, then all you have done is compress the spring and created a greater force that translates to firmer squishing and quicker expanding when the wheel travels over a bump.

A final thought, if changing springs is the way to alter stifness and the damping is how you alter your rebound rate, what is the preload adjuster for?

Please don't asnwer, it's a retohrical question but I hope it sparks some thought from you on the reason the preload adjuster exists on almost every modern fork where as not all of them have damping adjustment......
 

·
Registered
Joined
·
185 Posts
One final go before agreeing to disagree….
Road at one end and the bike with rider at the other end . By adding preload to the spring, will you lift the bike or squish the spring?
Will squishing the spring create an equal and opposite reaction that requires greater force again to squish it further?
The reason adding prehload reduces sag is because you have created greater force (reaction) within the spring to counter-act your bodyweight.
Your chart is A grade and what you say is spot on within your variables and constants but this did not take into account the road at the other end of your forks. The bike is creating far greater downward force than the spring can rebound beyond its set rate (sag) level.
Experiment on your shock (you can see both the spring and the plunger shaft). Does winding down the collars compress the spring or extend the shaft out of the assay? Look at how everything is attached and you will note that the only way to adjust the height in this experiment is if the shaft extends from the assay. If it doesn’t, then all you have done is compress the spring and created a greater force that translates to firmer squishing and quicker expanding when the wheel travels over a bump.

A final thought, if changing springs is the way to alter stifness and the damping is how you alter your rebound rate, what is the preload adjuster for?

Please don't asnwer, it's a retohrical question but I hope it sparks some thought from you on the reason the preload adjuster exists on almost every modern fork where as not all of them have damping adjustment......
Go back to my first post (page 1) and re-read the first paragraph.

All of what you say makes sense ONLY if the forks and shock are fully extended. As in NO sag. As soon as there is sag in the suspension, and there must be, your theory is wrong, but let's agree to disagree instead. OK?
 
1 - 16 of 16 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top