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"The labeled rate of the Race Tech springs have been called into question by many sources, although they are still fine quality springs. I rated some 0.85kg/mm labeled springs that I have and they were actually 0.90kg/mm."
"The correct spring is the softest spring available, that is able to support the bike and rider under the hardest of braking/accelerating while still leaving some room for the system to travel if a bumb is encountered in this state."
So Racetech's calculator for fork springs recommended I get 0.83 kg/mm for the street and 0.87 kg/mm for track use. I'm about 210lb without gear. Would you recommend I go in between with the .85 kg/mm or round down to 0.8 kg/mm?

100 Posts
How do you like them? Too stiff, soft, or just right?
i like them. so far they done me good, and for my weight i can set sag and free sag.
even though we play alot with preload,to slow rebound,kinda compromise thing.
im hoping emulators will keep me from having to do that.

230 Posts
Sorry @jrshooter, when my comment made you feel uncomfortable, this was not the meaning.
When I understand you correct then your suspension guy is 'playing' with the air gap when he puts more or less oil into the fork.

Two more things let me add please.
First for your question about the damper rod, you'll better disassemble the fork because this way you also can go and take all measurements.
Since you'll race your bike do the very best and qualified job you can do to have the best setup for your bike.
About the measurements it's important to realize that nearly nobody finds the best setup with the first adjustments, so you always know from where to go.
You'll need many rides to find the best setup with many adjustments and please take the preload in account that also will need different settings until you're really happy.
And second, since the rear wheel is not straight within the swing arm or aligned to the frame you should adjust it straight (the marks are not correct!).

"Things to know about fork oil weight and air gap

With other oil, nothing changes on the immersion of the fork's behaviour.
Basically, about 25% of the spring travel, but at least 25-35 mm should stay out as "negative travel".
In other words, with the rider on it, the fork should be immersed approximately 30 mm (compared to a fully unloaded condition with the front lifted from the floor).
This says nothing about the further spring behavior (even if the "spring rate/constant" and "preload" are always brought back in a mess).
The above-mentioned condition can be achieved by use of a corresponding length of a additional spacer. With the correct length of the spacer the fork is immersed to about 30 mm, in this state, the spring takes up exactly the front wheel weight (a condition that, even with a supposedly rigid rod instead of the spring reach - the rod is also a constant like a "spring", only with an extremely high spring-rate. The main thing is, that the spacer & "spring/rod" are 30 mm shorter than the extended fork).

Now one starts to apply the brakes, and only now the spring constant starts working. With increasing load on the front wheel, the fork dives more, until the two springs absorb exactly the force, which is initiated by the braking process.

If the springs are too soft (to low spring-rate), springs of the fork may completely dive until the fork tubes are at the mechanical end of the dip tube (from there it behaves like a rigid fork and is not really rideable.

If the springs are (extremely) much too hard, the bike bounces too much, the fork acts stubborn, jumps over bumps, etc. and we're back to the rigid, unrideable fork.

The correct value for a good spring lies somewhere between the extremes. However, the "correct" value of a riding (one's) feeling is dependent on the thing, otherwise you could calculate the rate, and it would fit for all the riders about the same weight. But it doesn't fit unfortunately!
This is the reason why serious supplier and refiner of forks/shock absorbers inquire to the weight of the rider, and then, dependent on the bike (weight, head angle, driving behavior, etc.) calculate a spring with a certain rate constant to recommend, as well as the spring and damper system, suitable oil (suitable viscosity).
What are just the predictable optimum or in the vicinity of the rider's perceived optimum (and more, of your experience and opinion, the right value).

Measures when felt from too soft spring: the reduction of the air space in the fork tubes.

Gas (air) has a square progressive characteristic line.

The air gap above the fork oil superimposed in principle, the spring constant of the steel spring in the fork.
Reduced air gap (compared to the manufacturer's instructions) on top of the oil provides increases spring constant/stiffness during compression.

With regard to the damping, even if one has the impression that the fork would dive too far while braking (and the immersion still when pushing), has to do, in first approximation, absolutely nothing with the fork oil viscosity.
Dipping at low speeds oil viscosity almost plays no role. Only when riding and with fast movements.

Unfortunately, there is no mandatory test specification for fork oils, what ensures that each manufacturer to test under different conditions and its values are published (for fork oils there is not an industrial standard such as the SAE viscosity is to be a part of in engine oils. The "success" is that the manufacturer of A gives a 5W output fork oil which is sold from "B" as 30W).

Fork oils should be taken as thin as possible and as thick as necessary. With higher-viscosity oils are used, the breakaway torque will be increased.
In addition, increases in the case of the non-adjustable fork both rebound and compression damping.
The fork is in the case of continuously poor road condition to a low level (it can't rebound fast enough) and is also unlikely to be able to follow road irregularities, it will compare to iron-hard.
And this, in turn, is probably the first "fork duty", because of the inability to follow an uneven road surfaces it suffers like, of course, the foremost task of the front wheel, to lead the bike.

Therefore, to optimize, with the relatively low viscosity of the oil will remain, possibly even thinner, synthetic oils because of the better lubricity prefer (lower breakaway torque!), not to use hard springs (the stronger the damping of the rebound), the oil level experiment, in order to prevent the fork diving until the mechanical end (quadratic dependence of the steel spring overlying air spring from the suspension travel!).
In the case of the mentioned quadratic dependence it is also the problem of the air spring, because when you exceed a certain upper limit of the oil level (the small size of the air chamber above the oil) is the spring rate for small deflections so that the fork is way too hard and unrideable.

Both, air gap and oil weight, are directly dependent on each other.
Wilbers says: Not only is the fork oil for the tune up of importance but also is the setup of the air chamber. Oil is not compressible, but air is. With the air gap becoming smaller, the compressible quantity of material in the fork will be lower. During compression, the oil level rises and the air gap is compressed. A small gap of air is less compressed than a large one. The consequence of this is that the progression of the fork rises at the end of the spring travel and the fork will be stiffer when the air gap is reduced. In the case of a larger air gap the fork is softer."

I've translated this from German and since maybe some things might not be said in the best way please search about this on English spoken websites, I'm sure you'll find many good description.

@ClutchDumper please also read this comment to understand a little bit more about this theme.
And to answer your question, I would go with the 0.8 kg/mm spring instead of the 0.85 but play with the air gap / oil level.

It is very important to understand, that in the case of suspension no setup will ever be right for or fit a different rider!

A good engine tuning will fit every rider, but here also must be said, that a fuel-map made for one region and bike will be worthless in another bike (even from the same model/type) and region!

1 Posts
I know this an older thread but I'm hoping someone can point me in the right direction. I'm considering doing the GSXR 600 rear shock swap. The GSXR shock takes a narrower bolt at the top so how are people mounting it? Are you getting custom spacers made or ?
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