Does anybody mind explaining Forks to me?

[Speedy3]

Hey there! Done a lot of searching, but it seems like so many posts are made by people who already know about forks and all the terminology.

Recently got some racetech springs - .9kg/mm of Amazon. These came with a series of washers and a length of pvc. Now, I know you cut the pvc to be your spacer, but what actually goes into the calculations to determine what that spacer length should be? I know the oil is recommended to be about 140mm, if my memory serves me right, but I read oil level/weight had more to do with dampening than preload.

If I'm looking to get 10mm of preload (racetech suggestion on website), how does that translate into spacer length, fork height, et.?

Hate to be obtuse, but at the moment I just see a million variables, and I am struggling to see how they connect to translate to the common phrases like "sag", "preload", "dampening", etc.

Any help is greatly appreciated! Looking to upgrade the stock forks in a 2010 Ninja 250.

[Triple Jim]

The simple answer is that to get 10mm of preload, you need to cut the spacers so that with the washers and spacer and anything else that goes in there in place, the caps are 10mm above where they will end up when they're in the tubes with the retaining rings in place.

Racetech has a good description of how to measure sag. It's the difference between where the suspension sits when topped out (no load) and where it is when the bike is on the ground with you on it in riding position:

http://racetech.com/articles/SuspensionAndSprings.htm

[greg737]

I assume you've already been here: http://faq.ninja250.org/wiki/Install...rs_and_springs

This would be the "first stop, last stop" for an EX250 owner looking to upgrade and sort out his bike's suspension. It's been a while but that's where I went to get it all figured out and I got very good results. The stock EX250 really needs to have both front and rear suspension upgrades, once you do that it's a great bike.

[Motofool]

Some of the weight of your body and your bike rests on both front springs.
When you hit a bump, those springs deform and limit the height that your bike moves up.
The problem is that they will oscillate down and up after that.

A hydraulic damper tries to limit those undesired oscillations via viscous resistance.
In order to fully extend, the spring needs some force to overcome that resistance of the damper: that is the reason of the pre-load.
If your washers and PVC spacer deform your spring 20 mm, then your spring minimum force (max geometrical extension) to push the dampener will be 0.9 x 20 = 18 kg of force.

Besides keeping you comfortable and the bolts in place, the other function of the front suspension is to keep the contact patch of the front tire pushing against the asphalt all the time.
That is really critical when you are cornering hard and big lateral forces are trying to slide your bike from under you.
Additional "weight" is loading the springs even more than when the bike is vertical.
The bike is leaned then, which makes the suspension work harder away from the ideal vertical position.

In that precarious situation, your front suspension should be working at the middle point of its range of movement, in a way that it can equally deal with valleys (extending) and crests (compressing).
That is the reason for which you should adjust the sag of the suspension.
As you cannot do it while cornering, you consider that extra "weight" of cornering and set the static vertical sag to be around 1/3 of the range of movement of the forks.

[InvisiBill]

http://www.ex-500.com/wiki/index.php...ension_Preload is another great reference. The EX500 suspension is very similar to the EX250's, with just some slight differences. There will be some variations in the numbers, but that page should give you lots of good info.

As has already been mentioned, preload is how much you're compressing the spring before the suspension action even comes into play. It's literally just the amount of pre-loading placed on the spring.

Sag is how much the bike "sags" on the suspension. This is a function of the bike/rider weight, spring rate, and suspension settings. You don't want the suspension 100% fully extended in your normal riding state, or there wouldn't be any travel left to account for valleys in the road, like Motofool said. Your neutral position needs to be somewhere in the middle of the travel range.

For the most part, springs are for absorbing movement and oil is for damping that movement.

Originally Posted by sharky nrk

The right rate spring is a required foundation for proper suspension action. It is the primary source of taking your KE (mass of you and the bike moving in relationship to the mass of the earth); storing it as PE and releasing some of it as your KE state changes.

A damper transforms the unwanted KE/PE into thermal energy and spreads it to another media (air). A damper being adjusted up and down to transform more or less of that energy cannot correct a spring that does not have the capability of handling the energy input needed to be stored sufficiently. In the same token but on the opposite side, a spring that does not deflect because it stores and returns KE far to quickly will not allow the tire to track the surface of the road/track and no amount of increase/decrease in damper function is going to change that.

The oil level, or more precisely the amount of air at the top of the fork, will affect how progressive the fork action is. Oil isn't compressible while air is, so a different amount of air will change how the fork compresses. The fork oil passing through the internals of the fork is all damping function though.


Generally you want to tune your suspension's sag (via preload) rather than tuning the preload number itself. You're more concerned with how the bike sits and moves overall rather than one specific component's measurement. (Though once you find the ideal final setup, it's good to measure the preload length for future reference.)

Note that different spring rates inherently take different amounts of force to compress the same length - using the same 10mm preload setting on two different spring rates will give you differing amounts of force, resulting in different amounts of sag. Preload length measurements are a simple way to state a good starting point for tuning, but they're not good for stating generally-applicable settings. I was told ~3/4" (~18mm) preload to start with on the 500. Also, it's a lot easier to shorten the spacers than to add more length, so I suggest starting longer than needed and trimming them down to find the right length.

[Speedy3]

I've done my best to read and understand the science. I'm simply having trouble conceptualizing what all this means for switching my spring setup.

I have .9 springs from racetech, and a little sheet that I got with them mentioned needing to create a spacer that would bring the length of the whole assembly back to what it was previously.

My only issue with that, is that I feel that would only take me back to the same preload it had (which, from my reading, is typically more than 10mm).

Am I trying to ensure a10mm gap from the bottom of the cap to the top of the spacer-washer when the fork has no load?

[Speedy3]

I have read rojoracing53's sticky over and over, but I simply don't see where he gets the 22mm of measured OEM preload :/

[Motofool]

Originally Posted by Speedy3

I've done my best to read and understand the science. I'm simply having trouble conceptualizing what all this means for switching my spring setup.......

This may help you:
https://www.ninjette.org/forums/showthread.php?t=134557

[Speedy3]

So by twisting that ring, you increase the preload, or, the amount of compression felt by the system solely from its own geometry (level of the top ring).

Translating this to the front forks, I want the whole system to have approximately 10mm of compression with the wheel in the air. Translating to forks with no preload adjusters, I want damping rod, springs, washer, spacer, washer to essentially sit 10mm above where the cap would sit under its circlip, thus having 10mm of preload.

Is this accurate!?

[Motofool]

Originally Posted by Speedy3

....... Is this accurate!?

Yes, it is !!!

The ring in the rear shock tries to keep the geometry of the bike (pitch-wise) for more or less carrying weight by increasing or decreasing rear preload.

In that way, the light beam remains more or less adjusted not to bother upcoming traffic and the steering (rake angle and trial) behaves the same.

[Mohawk]

Couple of tubes at front of bike, holds the wheel on

[Speedy3]

@Motofool

So I am putting my forks back together. I understand the idea of preload, but now to how to obtain said measurement...

For fork oil, one measures without springs and such while the fork is fully compressed (as short as possible). Is this the case for preload? Would I place the springs, spacer, washer and measure my ~10mm preload while the fork is as short as it gets? Or fully extended?

[Motofool]

Spring is fully extended, with no forces acting upon it: that is the unloaded length of the spring.

Spring is assembled inside the telescopic tubes, nothing else to add to the assembly, with no forces acting upon it: that is the pre-loaded length of the spring.

Length of the spacer+washer+ whatever pushes the spring down from the relaxed state to the assembled-unloaded state: that is the difference between the two lengths described above.

[Speedy3]

Awesome!

Well I believe I've stumbled across the last question I'll have until my forks are apart of my bike again!

When it comes to measuring the height of the forks above the triple, what part of the fork tube do you measure? It seems as if the angle the tube sits in the triple will provide different measurements depending on whether you measure the front of the tube or the rear.

I understand that as long as I measure both sides the same, I'll be in good shape, but I want to get this back to stock as close as possible!

[Motofool]

Originally Posted by Speedy3

......When it comes to measuring the height of the forks above the triple, what part of the fork tube do you measure? It seems as if the angle the tube sits in the triple will provide different measurements depending on whether you measure the front of the tube or the rear........

The tubes and the central column are perfectly perpendicular to both trees.

I don't remember how high above the top tree those are set from the factory, but consider these:
- Too high and the horn of the handlebar will not fit over.
- Higher will make your steering lighter (which I like myself).
- Lower will make your steering heavier (and in my experience more prone to undesired oscillations).