Why are Progressive fork springs so DAMNED short???

WHY??!! I'm going to place a call into their customer service, but first want to get everyone's feedback on this phenomena........

Why on earth are Progressive fork springs so damned short? Technically, they don't even come near the minimum spec limits for fork spring length that is published in our manuals. I'm tired of installing these in my bikes only to find out that I need to put a 2" spacer on top of them, and in the case of my current bike, an 81 750 Seca, I had to put a spacer that was about 3" in there!

WTF?? Why don't they just make their springs longer?!

I swear on the next bike I get I'm just going to put a spacer on top of the already existing fork springs! A 29.5" worn out fork spring with a 0.5" spacer has got to have a better feel than a 27" fork spring with a 3" spacer!

Please - someone either confirm my hostility or proove my ignorance!

Chad

 

This is because (and I am by no means a metallurgist nor an engineer) the strength of the material dictates the diameter and length of the spring. My take on this is from this perspective. Imagine that the strength you seek can only be created in a given area of space (about 30 or so mm in diameter) and the metal can only sustain that strength in a given density, pitch, or distance. Your material would limit the length of your spring pitch, the further the pitch, the greater the fatigue factors. If you added additional coils, you could extend the length but you pay a weight penalty and I'm not up on the dynamics on how extra coils would behave (extra weight in motion creates harmonics and additional oscillation). I guess I'm saying that they paid some expensive engineers to create the spring for your application and, since manufacturing is expensive, they chose a given spring rate for the application over the length of the spring (probably because this spring covers more applications than just your beast which makes sense in a business world). They just make up the distance with a spacer because it is a heck of a lot cheaper to buy and modify a tube than make a direct replacement spring. Your thoughts of modifying your existing springs may be the way to go, I just did my FZR's stockers and got great results! I will side with you on the spacers, they are a royal pain to hold in place while trying to screw the tube cap back on, especially if you placed any washers in there to protect the cap and spring. Anywho, my unknowledgable (reads: ignorant) take on the subject. I too would love a learned response.

 

The shorter answer. Maintaining a constant metal quality the longer the spring is the softer it is. This might sound counter intuitive - bear with me.

Say we wanted it to aborb a bump. We can use X for the force of the bump.

The long spring has 29.5". It has therefore (x/29.5) force per inch.

The short spring has 27". It has therefore (x/27) force per inch.

What happens is that the force to deflect the spring increases as it is compressed. It is non-linear. The short spring because it has more force per inch goes up this non-linear curve faster... making it stiffer sooner.

The long spring can travel further (softer) before it goes up the spring's force curve.

Ok - clear as mud. My understanding could be not 100% accurate. The long and short of it is - if you want to make a spring harder cut off a few coils and replace the length with a spacer.

As a spring gets older it gets weaker. This means that the quality of the spring changes. The curve gets flatter and it behaves more softly - ie - more fork travel for the same force.

The shorter spring has MORE force per inch. That means it will TRY to deflect more. However the force it takes to deflect a spring goes up as the

 

To get a better feel on the front end... instead of cutting coils or going with a progressive + spacer what we should really do is find a 29" spring with a harder spring rate.

That would give the most available room for fork travel, yet yeild the benefits of the harder suspension from progressive.

IMHO - the biggest bang for your buck on improving the XJ suspension is not the front, but the rear. The front has very little weight and really doesn't show signs of high travel. The rear on the other hand seems very undersprung, under damped and has a bit too much rebound.

A heavier spring with a thicker oil would go a long way to improving the rear. I wouldn't suggest a spacer on the spring as that would reduce travel, something the rear is lacking as is anyhow. A 1" taller shock with a higher spring rate would be nice. Move the weight onto the front at the same time as smarten the rear up.

 

You scientists are a blessing Woot, how do we survive without you? How far off was I? I'm not blessed with incredible insight and usually have to have things explained twice. You getting all this crc1214? I understand that with greater pitch angle, the softer the ride. This would also mean a greater fatigue factor as well since the same weight is being borne on a more and more obtuse angle joint which weakens it. Comments or suggestions to my understanding please!

 

Well - I'm no rocket scientist... I only spout off what I've learned/heard/know when I get the chance...

I think you're right... I don't know what the shorter length spring would do for a lifespan... I figure if the 29" ones did 20 years then the progressives should easily do 15"

I'm mulling over a post on hindsight thoughts... I worry it would turn into a book of ramblings...

 

What a book it would be Woot.