Fork spring advice please

[dwalby]

It's a common misconception that adding preload (to reduce sag) results in a firmer ride. Adding preload changes ride height (reduces sag) but cannot change the spring rate of the spring so the ride quality of "softness" or "stiffness" remains the same. The kicker to that is that the ST1300 comes with progressive rate springs. The coils on one end are wound tighter and are easier to compress. Adding preload to a progressive rate spring takes up some of the progression and moves the beginning point of the spring rate further up the curve where it is slightly stiffer. So, adding preload to stock springs can increase stiffness to a small degree but if aftermarket straight rate springs are installed then all preload does is increase ride height and reduces sag.

I don't think its a misconception that adding preload results in a firmer ride, on every bike I've ever owned it did exactly that. What I was never able to figure out for years is what caused the additional firmness/harshness, because I agree with you 100% the spring rate doesn't change, only the position of the spring in the fork relative to the bike. The preload spacer provides an additional force inside the tube so that the lower slider compresses less by that same amount before the spring reaches equilibrium with the weight of the bike/rider. The spring is compressed exactly the same amount with or without the spacer, its just a matter of where the forces on each end of the spring are located. What I finally figured out after buying a newer bike with fully adjustable forks is if you crank up the rebound dampening without changing the spring preload at all you can achieve that exact same firm/harsh response in the fork. Then after I figured that out I read a few articles online that explained how the change in ride height on the older forks had the effect of increasing the rebound dampening, so that's why it feels stiffer and more harsh.

Its a misconception that preloading a progressive wound spring is any different than preloading a constant rate spring for the reason stated above. Even with a progressive spring, the amount the spring compresses is identical in both cases, and the amount of coil binding is identical.

 

[dduelin]

I don't think its a misconception that adding preload results in a firmer ride, on every bike I've ever owned it did exactly that. What I was never able to figure out for years is what caused the additional firmness/harshness, because I agree with you 100% the spring rate doesn't change, only the position of the spring in the fork relative to the bike. The preload spacer provides an additional force inside the tube so that the lower slider compresses less by that same amount before the spring reaches equilibrium with the weight of the bike/rider. The spring is compressed exactly the same amount with or without the spacer, its just a matter of where the forces on each end of the spring are located. What I finally figured out after buying a newer bike with fully adjustable forks is if you crank up the rebound dampening without changing the spring preload at all you can achieve that exact same firm/harsh response in the fork. Then after I figured that out I read a few articles online that explained how the change in ride height on the older forks had the effect of increasing the rebound dampening, so that's why it feels stiffer and more harsh.

Its a misconception that preloading a progressive wound spring is any different than preloading a constant rate spring for the reason stated above. Even with a progressive spring, the amount the spring compresses is identical in both cases, and the amount of coil binding is identical.

At the risk of going off topic I suggest the book Sportbike Suspension Tuning by Trivett. It provides an easily understood explanation of what adding preload does and doesn't do. Adding preload increases the amount of force required to begin compressing the fork/shock. If the weight of the bike and rider doesn't change then the bike rises up the travel because the extra preload isn't offset by any extra weight or compression force so the fork extends by the amount of new preload. The spring "pushes back" against the new force to find equilibrium and raises ride height. In doing so the spring rate isn't changed one bit unless the spring is progressively wound. Race tech springs are not progressive so adding preload does not change stiffness of the spring(s).

 

[dwalby]

the spring rate isn't changed one bit unless the spring is progressively wound.

I think we're basically in agreement, but I disagree with this statement, the spring rate is still the same even with a progressively wound spring.

If we do the math, and ignore things like does the front/rear weight distribution change because of the higher ride height, etc. we are left with the simple equation of how the spring compresses under the weight of the bike/rider. We also assume that the preload force is less than the weight on the fork, so the bottom slider will move under the weight of the bike/rider.

The spring compresses to exactly the amount of travel required for the spring to create an opposing force equal to the weight of the bike/rider. By inserting the preload spacer we're just moving the location of the ends of the spring, it will compress the exact same amount with or without the spacer. See diagrams below.

The first diagram shows a continuous rate spring, compressed with and without a spacer. Note how the spacer simply moves the top of the spring downward, which means the bottom of the spring (lower slider) has to move less relative to the top of the fork tube to achieve the required overall spring compression to counter the weight applied on it.

The other two diagrams show the same thing with progressively wound springs in different stages of their compression curves. Bottom line is the amount of spring compression inside the fork is always the same with or without the spacer, the spacer simply moves the position of the spring relative to the top of the fork tube. That movement is what changes the ride height, the spring doesn't change.

 

[dduelin]

I think we're basically in agreement, but I disagree with this statement, the spring rate is still the same even with a progressively wound spring.

If we do the math, and ignore things like does the front/rear weight distribution change because of the higher ride height, etc. we are left with the simple equation of how the spring compresses under the weight of the bike/rider. We also assume that the preload force is less than the weight on the fork, so the bottom slider will move under the weight of the bike/rider.

The spring compresses to exactly the amount of travel required for the spring to create an opposing force equal to the weight of the bike/rider. By inserting the preload spacer we're just moving the location of the ends of the spring, it will compress the exact same amount with or without the spacer. See diagrams below.

The first diagram shows a continuous rate spring, compressed with and without a spacer. Note how the spacer simply moves the top of the spring downward, which means the bottom of the spring (lower slider) has to move less relative to the top of the fork tube to achieve the required overall spring compression to counter the weight applied on it.

The other two diagrams show the same thing with progressively wound springs in different stages of their compression curves. Bottom line is the amount of spring compression inside the fork is always the same with or without the spacer, the spacer simply moves the position of the spring relative to the top of the fork tube. That movement is what changes the ride height, the spring doesn't change.

No, the spacer compresses the soft end of the spring first so the at rest position of the assembly is higher up the rising rate of the progressive spring. In a nutshell that's why virtually all tuners suggest straight rate springs so the addition of preload is linear or almost so disregarding top out springs. Addition of preload to progressive springs is not linear because the spring rate is a curve. The line on a straight rate spring graph is straight. Adding 150 kg to a 15kg/mm spring compresses it 10 mm. To achieve equilibrium the spring extends 10mm. The bottom of the spring against the slider is fixed so the spring pushes against the top cap and reduces sag by that 10 mm as the fork extends 10 mm.

For example the progressive spring rate starts at 10kg/mm and rises to 15kg/mm when the tighter "softer" coils are coilbound. Adding 150 kg of weight compresses the 10kg/mm coils first. The same 150 kg of weight compresses the soft end 15 mm if the soft coils are numerous enough to absorb 15 mm of compression at 10kg/mm. If not the 150 kg takes up the first section of 10kg/mm then starts on the coils stiffer than 10kg/mm but less than and up to 15kg/mm. To achieve the same 10 mm of ride height change the progressive rate spring needs more preload, ie., spacer length, as it starts working against a lower rate.

 

[mlheck]

It's a common misconception that adding preload (to reduce sag) results in a firmer ride.

While you guys argue about spring rates, Less preload does give you what feels like a softer ride. I have the RaceTech Gold caps on my bike and have messed with every adjustment available. Most of the time to my own misfortune. Taking preload off does feel like a softer ride. Now I do understand that preload actually is preloading the spring to cancel out the static weight that the spring must carry. Example: If a spring has a rate of .90kg/mm, then 5mm of preload means that a force greater than 4.5kg is needed before the spring will move, but every additional .90kg will move the spring 1mm after that. As preload is increased a greater force is needed to cause the spring to move, but the spring rate doesn't change. Since we are getting 36mm of sag that means that we are not setting the preload so high that the springs are carrying all of the weight of the bike without moving. So in the Engineering world that means that all forces acting on the springs after the preload amount should have the same result, but it does feel like the spring is softer when you back off preload. I don't the answer to this other than it is more than a mis-conception. I've done it myself to soften the ride.

 

[dwalby]

No, the spacer compresses the soft end of the spring first so the at rest position of the assembly is higher up the rising rate of the progressive spring.

when you put the spacer into the fork tube with no weight on the bike, then yes, you are compressing the softest part of the spring, I will not deny that. But that's not relevant because as soon as you put the weight of the bike onto the forks, the remainder of the spring compresses to create a force equal to the weight of the bike.

1. the weight of the bike hasn't changed
2. the spring rate hasn't changed

so 1 and 2 implies that:

the amount the spring compresses to counter the weight of the bike doesn't change either.

If the spring was compressed by X with no spacer, it has to compress by X with the spacer, that's the only way the forces will be equal in both situations. The spring is the only thing generating a resistive force to the weight of the bike. The spacer generates no force of its own, it simply displaces the spring so that the spring force equation is applied to a different starting location relative to the bottom of the fork tube.

The spacer length of 100mm compresses the spring by 100mm with a force of 90kg (0.9 x 100) and uses up the softest part of the travel. So now when we put the weight of the bike onto the forks the first 90kg has no effect on the lower slider. The next 60kg compresses the spring by 60mm because the rate at that point is 1.0 kg/mm. The spring is still compressed by 160mm inside the tube, the only difference is the lower slider only had to move 60mm before the spring equilibrium was achieved. The coil binding inside the fork is exactly the same in both cases because the spring is compressed by 160mm in both cases. The spacer didn't add more force, it just moved the resistance to the spring force 100mm lower in the tube. Its no better or no worse than the fork cap at resisting the spring force, it just changes the location where that force is applied.

If you don't want to believe me, here is a spring company tech note that agrees with my viewpoint.

http://www.sonicsprings.com/catalog/preload_tech_article.php

 

[dduelin]

when you put the spacer into the fork tube with no weight on the bike, then yes, you are compressing the softest part of the spring, I will not deny that. But that's not relevant because as soon as you put the weight of the bike onto the forks, the remainder of the spring compresses to create a force equal to the weight of the bike.

1. the weight of the bike hasn't changed
2. the spring rate hasn't changed

so 1 and 2 implies that:

the amount the spring compresses to counter the weight of the bike doesn't change either.

If the spring was compressed by X with no spacer, it has to compress by X with the spacer, that's the only way the forces will be equal in both situations. The spring is the only thing generating a resistive force to the weight of the bike. The spacer generates no force of its own, it simply displaces the spring so that the spring force equation is applied to a different starting location relative to the bottom of the fork tube.

The spacer length of 100mm compresses the spring by 100mm with a force of 90kg (0.9 x 100) and uses up the softest part of the travel. So now when we put the weight of the bike onto the forks the first 90kg has no effect on the lower slider. The next 60kg compresses the spring by 60mm because the rate at that point is 1.0 kg/mm. The spring is still compressed by 160mm inside the tube, the only difference is the lower slider only had to move 60mm before the spring equilibrium was achieved. The coil binding inside the fork is exactly the same in both cases because the spring is compressed by 160mm in both cases. The spacer didn't add more force, it just moved the resistance to the spring force 100mm lower in the tube. Its no better or no worse than the fork cap at resisting the spring force, it just changes the location where that force is applied.

If you don't want to believe me, here is a spring company tech note that agrees with my viewpoint.

http://www.sonicsprings.com/catalog/preload_tech_article.php

Your reference doesn't support your contention at all. It says that "...the spring has to compress a certain total amount in order to support the weight of the bike, and it doesn't matter whether or not some of that initial compression is from preload." It does not say the same amount of preload compresses the two types of spring the same amount. It can't possibly because the two springs have different rates. Sonic only sells straight rate springs and somewhere in the site it says that's because the first part of a progressive spring is of little use in actual use and usually coil bound from the get go and that it's hard to match a rebound damping rate to a spring that increases rate (ie. force required to compress it) as the fork compresses.

Let's move on. No one cares about this stuff anyhow except you, me, and Michael Heck and we are apparently all happy with our set ups.

 

[dwalby]

It says that "...the spring has to compress a certain total amount in order to support the weight of the bike, and it doesn't matter whether or not some of that initial compression is from preload." It does not say the same amount of preload compresses the two types of spring the same amount. It can't possibly because the two springs have different rates.

Let's move on. No one cares about this stuff anyhow except you, me, and Michael Heck and we are apparently all happy with our set ups.

Hmmm, Now I'm not sure what you're disagreeing with, I never said preload would have the same effect on two totally different springs. What I said is adding preload and then placing the bike weight on the forks results in the spring rate from that point forward being at the same point in the curve as it would be without the spacer.

edit: Perhaps we may be having a difference in interpretation of what I'm claiming. What I'm saying is my above statement in bold applies to progressive rate springs. Its the same thing as what you quoted above, about how it doesn't matter if some of the initial compression is from preload, the spring still compresses the same amount. So perhaps I didn't write it out clearly enough the other times, but what I'm saying is consistent with the quote above.

But I agree, we're having a discussion that would be better spent over a few cold beers, not on the forum.

 

[tonyp12]

Thank you gentlemen for the detailed and concise debate regarding spring and preload relationship.

 

[broc11]

The technical stuff is a bit over my head, but upgrading the suspension on the ST was the best cash I've invested in her,the hyperpro fork springs,10wt motul synthetic oil set up suits me and the bike perfect. that and the wilbers rear shock, I set her up myself and she rides very good but I'm taking her to a suspension guy soon,so I'll see what he thinks of how she is set up.

 

[habba8]

I can't follow the tech talk. I'm looking for general improvement. I have an 07 1300. I did a rebuild with RaceTech . 95 springs. I weigh 200. Only difference is the springs, which are longer, about 1 1/4 inches.
The oem spacer seems obviously long.
I buttoned her up anyway, I know, too hasty.
There is no sag, no sweet spot, just stiff.

Do I really have to go through all those measurements?
Really?
Can't I just remove a small amount (one inch) of preload to get a general improvement.

I had to ask

 

[970mike]

I can't follow the tech talk. I'm looking for general improvement. I have an 07 1300. I did a rebuild with RaceTech . 95 springs. I weigh 200. Only difference is the springs, which are longer, about 1 1/4 inches.
The oem spacer seems obviously long.
I buttoned her up anyway, I know, too hasty.
There is no sag, no sweet spot, just stiff.

Do I really have to go through all those measurements?
Really?
Can't I just remove a small amount (one inch) of preload to get a general improvement.

I had to ask

You do have to setup the front and rear end right or you will never have the proper ride and handling. Sorry to tell you the bad news.

 

[habba8]

Ugh.....

 

[broc11]

You need to have the static sag at about 1/3 of total suspension travel from what I remember, I went with hyperpros instructions and it worked out perfect,tie a cable tie around the fork leg,it's a good way to get your measurement correct.

 

[Reginald]

I can't follow the tech talk. I'm looking for general improvement. I have an 07 1300. I did a rebuild with RaceTech . 95 springs. I weigh 200. Only difference is the springs, which are longer, about 1 1/4 inches.
The oem spacer seems obviously long.
I buttoned her up anyway, I know, too hasty.
There is no sag, no sweet spot, just stiff.

Do I really have to go through all those measurements?
Really?
Can't I just remove a small amount (one inch) of preload to get a general improvement.

I had to ask

Previous advice is correct. You need to set it up right. By you're description it sounds like you didn't adjust for sag correctly. Using the stock spacer, washer, and Race Tech spring is too long a stack and might result in raising the front end. If I remember right it will cause the bike to handle less responsively; ie turn ins will take longer. Too short results in too much sag and turn in are too sharp and can cause drifting down the highway.

Between my second and third adjustment I took a week to get back to it. I wrote it up so others could learn from my error in starting from too short a spacer. Here's what I did: https://www.st-owners.com/forums/showthread.php?136656-Setting-Race-Tech-Fork-Spring-Static-Sag.

 

[mlheck]

What you did is like throwing a bag of flour, a couple eggs, and some milk in the oven and hope you get a cake.

It will be worth the effort to learn the correct way to set it up and do it.

 

[roadriderg]

I can't follow the tech talk. I'm looking for general improvement. I have an 07 1300. I did a rebuild with RaceTech . 95 springs. I weigh 200. Only difference is the springs, which are longer, about 1 1/4 inches.
The oem spacer seems obviously long.
I buttoned her up anyway, I know, too hasty.
There is no sag, no sweet spot, just stiff.

Do I really have to go through all those measurements?
Really?
Can't I just remove a small amount (one inch) of preload to get a general improvement.

I had to ask

I did the race tech .95 and you defiantly need to cut down the spacer to get the overall stack maybe 5mm longer than the original stack. I am surprised you could get the cap on with that setup.

When I get home I have the cutoff from my stock spacer. I will measure it and this should give you a good starting point. From there on you can add spacers to get the correct rider sag. Also it is important that you have to correct level of oil in the forks and measured correctly without the spring or spacer in.

Gary

 

[dduelin]

I can't follow the tech talk. I'm looking for general improvement. I have an 07 1300. I did a rebuild with RaceTech . 95 springs. I weigh 200. Only difference is the springs, which are longer, about 1 1/4 inches.
The oem spacer seems obviously long.
I buttoned her up anyway, I know, too hasty.
There is no sag, no sweet spot, just stiff.

Do I really have to go through all those measurements?
Really?
Can't I just remove a small amount (one inch) of preload to get a general improvement.

I had to ask

What oil did you fill with and how high? Everything can affect the end result.

 

[roadriderg]

I can't follow the tech talk. I'm looking for general improvement. I have an 07 1300. I did a rebuild with RaceTech . 95 springs. I weigh 200. Only difference is the springs, which are longer, about 1 1/4 inches.
The oem spacer seems obviously long.
I buttoned her up anyway, I know, too hasty.
There is no sag, no sweet spot, just stiff.

Do I really have to go through all those measurements?
Really?
Can't I just remove a small amount (one inch) of preload to get a general improvement.

I had to ask

I cut off exactly 1" off my stock metal spacer and had to add 3 washers to get the correct rider sag. You could get a length of pvc pipe and cut it 1" shorter than your stock spacer. Check your oil level. Make sure when you put the cap on you thread it all the way down to the rod then tighten the lock nut. Do this and you should be very close to correct. Check the many post her on oil and oil level as this has a big effect on for performance.