I was wrong on my ES spring rate, it is 2.04. I would say judging by the sag measurements we took last year on an ES, that it would mean 1.02 each.
Agreed. If it was actually 2.04 each they would feel
much stiffer.
In my perhaps twisted view, once you increase the preload on the progressive spring, you have primarily compressed the weaker part of the spring. I'd think it would compress first, or at least the most, as the higher rate part has a higher resistance to compressing by definition. On a straight rate spring, the rate doesn't change. With either type, there is more weight required to compress as distance compressed increases, which in effect is increasing resistance to sag.
This is one of those "physics things" that's a little hard to wrap your intuition around. Let's look at what happens (ignoring the phenomenon of the fork leg lengths changing due to the top-out springs)
Springs have a particular resistance force to compression based on the wire diameter and coil spacing. In the case of the 3rd Gen A model's dual rate springs each spring is .85 kg/mm in the soft part and 1.02 kg/mm in the stiffer part, for a
total spring rate of 1.7 kg/mm and 2.04 kg.mm. These forces are measured by applying a weight force on the end of the spring and measuring how much the total spring (not just the soft or hard part) compresses overall and allows the fork leg to move.
When you begin compressing the spring from the fork's fully extended length, you will not
only compress the softer coils; the stiffer coils will also compress somewhat, but a much smaller amount than the softer coils do. However, the total resistance of both sets of coils in series felt from the ends of the spring is 0.85 kg/mm. We know that the fork will compress 67.5mm at that .85 kg/mm rate from topped out.
The soft coil part of the spring isn't
physically 67.5mm long. Some of that stroke distance is allowed by the stiffer coils compressing in series with the soft coils, and then there is also a nominal amount of spring preload that was applied from the fully extended length of the fork leg. The spring free length is 345mm. The installed length is 322.8mm, so both springs are pre-compressed (preloaded) by a nominal 22.2mm at the .85 kg/mm spring rate. The kinetic force in the pre-compressed springs will be 37.74 kg total for both legs with the forks fully extended and preloaded.
To make the fork legs begin to move at all from fully extended you will have to apply more than 37.74 kg to overcome the preloaded spring force, and from there it will take 1.7 kg to move each additional mm up to the 67.5mm spring rate change.
The curb weight of a 3rd Gen is 290 kg. I measured the axle weights on my own 3rd Gen and the distrubution is 47.6% front / 52.4% rear. So the forks support 47.6% of 290kg = 138 kg.. Subtract the first 37.74 kg of the preload force, the fork springs would compress from the remaining 100.26 kg force, or 59mm.
So the springs were preloaded 22.2 mm and further compressed 59mm by the bike's static weight (sag), the springs inside would be compressed by a total of 81.2 mm to support the bike's weight.
What happens now when you add 10mm of preload?
The total spring preload would increase from the nominal 22.2mm to 32.2mm. The kinetic force of the preloaded springs would be 32.2 x 1.7kg/mm = 54.74 kg. The first 54.74 kg to overcome the preload leaves 138 kg - 54.74 kg preload = 83.26 kg / 1.7 kg/mm means the static sag would now be 48.9mm.
So in our example, adding 10mm preload the sag went from 59mm to 49mm.; the ride height increased by the preload amount. However, the springs inside with the higher preload had 32.2mm of spring preload plus 48.9mm of sag for the (same) total of 81.2 mm of total spring compression due to the bike's weight. The springs will be compressed by the identical amount. therefore it will not feel stiffer due to the increased preload.
The things that will change slightly is the front to rear weight bias from the raised front ride height, and also the angle of the forks from vertical, but those amounts of change will be very small. The bigger change will be how the steering angles affect how the bike will turn in.
I realize that my example numbers appear not accurate, that the sag values are too large. This is because I did not account for the forks being at an inclined angle from vertical, so the bike's weight is not applied directly to the ends of the springs, but reduced by that vector angle. The force applied to the springs is reduced by that angle, but it will compress the forks at the same rate proportionally.