Gary_in_NJ wrote on 08/08/17 at 05:30:08:The shock is an air shock for a mountain bike. I don't have the specs for the one I have but spring rate is variable. They can be pressurized to make them stiffer and I think they are inherently progressive. With zero pressure it takes about 100 pounds to travel the first inch. And it's got two adjustments for damping, one for compression and one for rebound rate.
I estimate the stock front spring rate to be about 130 pounds per inch by measuring travel and the weight on the front wheel. So I figure to match that I need to be in the 260 pounds/inch on my shock. I'm not sure the pressure it will take to achieve but that's well within the typical spring rates advertised for spring based MTB shocks.
For this version to go with the stock frame I'm not changing the rake. I plan to run the stock wheel/axle/brake and fork length. If I like the way it rides I may change things up when I do the frame build.
The OEM spring are a dual rate progressive spring with an initial rate of 19 lbs-in ( 0.34kg/mm) from its static position to 4 inches and then 23 lbs-in (0.41 kg/mm) from 4 to 7 inches.
On my cafe racer I'm using linear rate springs that are 40 lbs-in (0.70kg/m) each. I think you need to recalculate your spring rates as 260 lbs-in will be non-compliant.
Also, I would also caution you on using a shock that is "air sprung". If the bladder fails at speed the bike will become unstable. A traditional coil would be a safer choice.
Finally, I don't believe you want a dual rate spring for this application. Given the geometry of your linkage, your spring will inherently load-up during compression.
I'm new to motorcycling and the savage so any input is appreciated.
It's not entirely clear but I think stock spring rates are for individual springs? So there are two in play when installed in the forks, that would double the spring rate making it effectively about 40 lb/in measured at the end of the forks.
I think I made a mistake in my stock fork spring rate, it looks like I might have missed any spring preload when installed in the fork. Is there a pre-load of the springs when installed in the forks?
When I mentioned 260 lb in that's rate I estimate needing at the spring, the mechanical advantage in the shock linkage will decrease the rate to about 1/2 that when measured relative to fork travel. If the lowest spring rate of the air shock is 100 lb/in that turns into about 50 lb/in at the end of the fork which should put me in ballpark to match stock.
The fact that I can change the spring rate is part of why I like the air spring. They make coil spring shocks that are physically interchangeable with air springs the only thing is the spring rate is fixed. If there's something about the performance of the air shock I dislike I can switch to a coil spring once I have a better idea of the best spring rate.
I've tried to find info on failure modes for air shocks and it sounds like slow air or oil leakage is typical, rapid and sudden failure rare. So assuming it goes flat I'd end up at the extreme travel up with no suspension. The fork geometry actually increases the trail in this position. I'd imagine it's a rough ride but I'm not sure what would make it unstable?
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