April 14, thanks to all who replied. If you all can help me understand, because I am confused with conflicting information. Progressive vs non-progressive, valve vs no valve, spacer with stock springs vs new springs. Oil weight 20 or 15 or 10, but generally the idea I got is start at 10.
Can anyone recommend a manufacturer of springs? Race Tech? or another brand? Hyper Pro or some other brand?
Thanks Terry / Pittsburgh
Well I'm new to my old ST1100, but not to fork modifications or suspension in general, so take what I say in that context. I can perhaps explain things, but I have no personal knowledge of what will work with the ST. I have installed emulators in damper rod forks before (86 VFR750F), and have re-sprung and re-valved my last three bikes with cartridge forks (RF900, VFR800 and VTR1000F).
Springs first: the function of the spring is to hold the motorcycle at the correct attitude for steering geometry, and to absorb bump energy allowing the wheel to ride up/over or down/through road imperfections. The combination of spring rate (stiffness, usually measured in kg/mm) and preload is what determines these properties.
A straight rate spring has the same stiffness through its travel, whereas a progressive spring starts soft and has increasing stiffness as it compresses. If you look at a progressive spring you?ll see the coils are very close together at one end and further apart at the other, basically as the spring compresses the tight coils touch and stop compressing further, so the rest of the spring becomes stiffer.
In a fork however you also need to consider the effect of the air pocket over the oil. This gets compressed as the fork travels and adds its own spring effect. You can make the effect bigger or smaller by making the air gap smaller or bigger (by changing the oil height). The air pocket is a naturally progressive spring, and can be used in conjunction with straight rate springs to give a progressive effect. If you use straight rate springs, you should also use a smaller air gap than with a progressive spring.
If the spring rate is too low, the fork will bottom occasionally which is bad for braking traction. If the rate is too high the ride quality will suffer as the fork is unable to travel far enough to absorb bump energy, transferring more force to the chassis.
From my limited research, Racetech seem to suggest lighter springs for the ST (0.9 kg/mm) than Sonic (1.1-1.2 kg/mm). My experience with my VFR and VTR suggest that 0.85-0.9 is correct for these lighter bikes, so I?m leaning towards the heavier Sonic springs.
The spacer used with a spring will determine the spring preload and that governs the normal ride height/fork travel (usually termed sag) in the absence of other forces. A typical target value for sag with the bike loaded and stationary is ? to 1/3 of the total travel. You need the sag to allow the wheel to drop into dips and holes and maintain traction.
For any given set of springs, the spacer length (if any) should be tailored for the length of the spring and to preload the springs enough to get the correct ride height. The stock springs use a high preload to get a reasonable ride height as they are a lower rate progressive spring. A stiffer spring will need less preload to get to the same ride height.
Damping is a whole other matter, and the ST along with the GL1800 use a hybrid damping set-up consisting of a damper rod in the left leg plus an antidive unit, and a conventional cartridge damper in the right leg.
Compression damping slows the rate of fork dive when hitting bumps, and rebound damping stops the fork extending too fast afterwards and causing chassis oscillations. Too little of either damping causes vagueness and loss of chassis control, too much starts to introduce harshness.
My initial examination suggests the left leg actually has little or no rebound damping, and that this is solely produced in the right leg.
The damper rod design produces damping force by sending oil through an orifice (literally, a hole drilled in the damper rod). Damping force produced through an orifice increases at the oil velocity squared, so the damper rod fork can feel slushy on soft small bumps but become harsh on a large sharper bump. The cartridge force overcomes this by combining orifice damping with a blow-off system of sprung shims; as the oil pressure in the damper rises quickly, the shims stacks open to bleed the excess away, avoiding the harsh response. The shims are simply thin washers of spring steel, and they are pushed off their seats by the oil pressure. The damper cartridge has two valves, one for rebound, the other for compression, and you can adjust the damping ?feel? by changing the individual shim stacks to make them softer or stiffer, just like leaf springs in an older car.
The Gold Valve emulator is intended to make a damper rod fork work more like a cartridge, and includes a blow-off valve that can be tailored by changing the spring and preload. Emulators are easy to install, you need to drill extra holes in the damper rod so the rod doesn?t produce damping force, and then the emulator is dropped onto the top of the rod and held in place by the fork spring.
Gold Valves for the right leg cartridge replace existing cartridge parts. Typically the valve bodies that feed the shim stacks are much greater than in the OEM part, so they flow fluid more easily and can thus be tuned for a firm but supple ride. Racetech provide specific recommendations for the shim stack recipe for compression and rebound and these are usually very close to optimum.
Gold Valve installation involves disassembling the damper cartridge, a bit daunting the first time but doesn?t require anything other than simple hand tools and paying attention to assembly order and cleanliness.
Spring rate and damping do need to go hand in hand to get the best result. You can offset deficiencies in weak springs for example by increasing the compression damping to reduce bottoming, but that is not the optimum solution.
Oil viscosity will impact on the damping forces generated as a more viscous oil will flow more slowly through orifices, producing more damping force. The oil weight will affect both compression and rebound, so as long as you wanted more of both then a higher weight oil is a good thing, but if you only wanted more rebound and no more compression damping, a higher weight oil will make the fork action harsher on bumps.
At the end of the day, you need to ask yourself what you want to do with the bike, and where you want it to excel, and also what you are prepared to invest in terms of time, money and effort.
For me, I normally ride on twisty, bumpy country roads, so I tend to err on the side of control over comfort. I?m leaning towards 1.2 kg/mm Sonic springs, modifying the right side cartridge with some high-flow pistons and stiffer shim stacks, and (hopefully) installing a similar cartridge into the left fork as well.
However the simplest path to follow is to try thicker oil in the first instance, if that works for you then great. I wouldn?t go to 15 or 20 weight as I suspect that will feel really harsh, 10 weight would be my suggested starting point (based on how my unmodified, ageing ST feels to me). You could try a different weight oil in the left and right legs, 5W to keep the compression damping supple in the left leg, 10W in the right for some more rebound control.
If you have issues with fork dive and bottoming still, in the first instance try raising the oil height in increments of 10mm, after that then new springs would be the next option.
Anyway, that?s just my opinion, and I remind you of the provisos that I made at the start! Best of luck
Terry, in Auckland.
. The stock springs are not the problem, low level or cruddy fork oil is.
to what TerryS said above.