A question:
Do you set the sag with a full tank of gas, 1/2 tank, or empty?
The guy who does my suspension set's the preload on the shock spring at 5mm, and as it turns out this gives a static height of 35mm (with 1/2 tank of gas).
With me on it, in all my regalia, tool belt, full camel back etc.... The (amount of spring preload can be varied from 4 to 6mm to suit) This works out to about 4" of sag or 101 mm's, or just a little less. I would consider my bike to be stable at speed. Considering the above mentioned 115mm of sag for the KTM's, that number would 36% of the total travel for the advertised rear wheel travel for the Husaberg's 320mm. "Conventional" wisdom as put forth by the aforementioned web page is 33%, or 105mm of sag. Just don't have so little sag that the spring has no preload on it what so ever when the bike is on the stand. And as also mentioned earlier, if the static measurement is too little after setting the sag with you on it, the spring is too soft and visa versa.
I'm sure that you all saw the post of FC550 and the telescoping shock needle offered by Race Tech. For those of you who do not know, the PDS shock has two pistons on the shaft, one is for speed sensitivity, the other for position sensitivity. One of the ways the shock knows where it is in it's travel is that there is a needle attached to the upper shock body that extends down into the shock body. This needle is designed to go into the shock shaft, thereby cutting off oil flow down the shaft. So in essence the needle becomes a valve of sorts and shuts off a bypass for the oil, thusly forcing the oil to flow through the shim stacks of two pistons. The timing of the needles insertion into the bypass, the shape of the needle which can vary the amount of bypass cut off, has been changing every year since the inception of the PDS system. This bypass cutoff also has to be coordinated with the spring rate, especially in light of the fact that for so many years the PDS system came with an rising rate spring. Believe it or not, [sic] there has not been good coordination between the use of the rising rate springs and the shape of the needles. For example, my 01 has a needle that is a long tapered affair so there is a gradual cut off of the bypass. This did not lend itself to the use of a rising rate spring, as there would be a spike in spring rate at the same moment there was a dramatic increase of hydraulic damping effect. Most suspension tuners were then putting on straight rate springs like my 01 has to achieve a more progressive effect. So, now, the shock needle is a short stubby affair to radically increase bottom resistance right at the end, but, at least KTM is doing this, now they are installing straight rate springs from the factory. This results in a softer ride, and will have a tendency to bottom, and to counter act this a suspension tuner may try and compensate with stiffer valving which would produce a rougher ride over the small stuff but be good at resisting bottoming. When in fact they should be using the rising rate spring (sounds like linkage doesn't it?) to have a more progressive damping effect, to better coordiante with design of the needle.
There are other fators that cause headshake though, and can can be mistaken for a geometry headshake and that is that the suspension can be too stiff in the front causing the front wheel to deflect when hitting bumps at speed. This has happened to me in the past, and was easily cured with a couple of clicks less compression damping in the front, and the difference was huge.
As we all know, the PDS system is extremely sensitive to ride height. I have experimented with more pre load on my bike, which produced quick turning, and a nervous head, and with less pre load which resulted in a more stable head but a noticable push with a more conscious effort to keep weight over the front end to keep it turning the way I like. Also, one needs to keep in mind the high speed compression adjuster on the newer bikes, which consequently has an effect on ride height when hitting successive bumps, can you say whoops?
After my experimentation on my 04 here is what I have come up with: First, start with the fork tubes set at the first line under the fork caps.
Second, my suspesion tuner knows what he is doing, after all that's what I paid him the big bucks for, and I leave the spring set at 5mm of preload.
Third, if the bike is shaking it's head a bit, try softening the fork compression a couple of clicks, that usually takes care of that problem.
Fourth if the bike is not turning as well as I'd like it, then raise the fork tubes one line and try it, this will transfer more weight to the front, and thusly slightly steepen the head angle at the same time, so a small adjustment makes a big difference here. Another effect of this weight transfer will be to smooth out the front end under high speed chop.
Fifth, if you feel the bike is riding a bit high in the back after getting the steering down, try backing off the high speed adjuster on the shock a 1/4 turn. This will allow the rear of the bike to setttle a bit more in pounding terrain.
Sixth, you can also adjust the slow speed adjuster in the rear to help with the slow speed suppleness.
However, keep in mind that each one of these adjustments needs to be made seperately and tested to check the outcome. Suspension tuning is no different than carb tuning and making more than one adjustment at a time will muddy the waters, IE you will become confused, and not truly understand the effect of the changes you are making.
Another thing to keep in mind is that when you adjust a compression clicker on a shock or a fork, you are effecting the overall oil flow in the fork or shock. Even though each individual clicker is designated for comp or reb, the adjustment in one will affect the other to a degree. So the order of the day is to make an adjustment, and carefullly test those results to make sure you are not correcting one problem and making another.
For instance, one might feel the rear is too stiff, so one keeps backing off of the compression clickers to get a softer ride. So, in doing so one is allowing the suspension to travel further through it's stroke, so now the suspension is compressed more and wants to return faster since it's A) compressed more and B) has less damping via the change in the compression adjuster b/c the oil in the resevoir will be able to return to the shock faster, thus having an effect on the rebound damping circuit. The end result will be that the bike might now start to kick you in the back side. Another example would be that if the spring was too soft in the first place, and now the bike is riding too low in the travel and is repeatedly coming into the more progressive portion of the damping caused by the position sensitive nature of the PDS system, so if this was the case, decreasing the amount of daming through the slow speed, or high speed comp damping clicker would only make it worse.
So at this point is it the fact there is not enough rebound damping? Or are you allowing the bike to travel too far through the stroke because you backed off so far on the clickers to try and overcome an incorrect shim stack setting (bad valving) and have now created this kicking problem?
One needs to keep in mind that clickers have a limited range of operation and cannot overcome an initally bad valving job, or incorrect spring rates.
And for those of you who are thinking of buying the adjustable preload adjusters for your forks BEWARE. Incorporated into the WP forks is a top out spring. The top out spring is just what it sounds like, it's the spring that helps keep the forks from clunking when you pop a wheelie, or when the front wheel suddenly comes off the ground. There is a delicate balance between how much fork spring preload there is, and not affecting the top out spring. IE preload the main fork springs too much, and you will start to compress the top out spring. You can see how the top out spring works by putting your bike on a stand to where the front end is suspended, now pull down on the front wheel, and you'll notice that the front suspension extends, and when you let go, it compresses. It is critical that this movement not be changed to keep the suspension working properly, as this will help keep the front wheel in contact with the ground over uneven terrain.
