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Originally Posted by MountainBikeRoamer
Similarly I wonder about fabbing an arrangement that would put some sort of high-torque linear actuator in place behind each helper spring. (Thus you could, at any time, adjust/vary the amount of "effective shim" behind each spring.)
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That's what an SMB powered top essentially is......they use linak brand linear actuators in place of the helper springs. I took a few pics of that setup when visiting Bmerritts roof install; I'll post them if I can find them.
BTW an automotive shock absorber would likely drop right in place of the helper springs, but the rebound damping is a problem. They are gas filled and can provide a long life with lots of force....perhaps a rebuildable shock...just remove the rebound shims (revalve).
Quote:
Originally Posted by MountainBikeRoamer
I've also wondered about simply finding a slightly-longer spring of the same diameter/coil spec -- and swapping that in. (To create the same total effective length of shim+spring, but without any shim needed.) Longer spring could store more total compressed "boost" energy over a longer effective impulse stroke, might be felt as more effective assist.
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I've thought about this too....not clear where to get the springs. The helpers in my van have clearly been cut down with a grinder, so they are likely long springs commonly used somewhere. The extension springs look very much like extension style garage door springs from a 70's vintage California garage door.
Increasing the spring length while keeping the same outer diameter and wire diameter will lower the spring rate, so the force will increase less per inch of travel.
This may or may not be desirable; I think preloading the helper springs against the lift mech is the way to go, but the plate that keeps them in the tray area needs to be beefed up (don't ask me how I know this..
) since the compression force of the spring will be higher when the top is down.
Quote:
Originally Posted by MountainBikeRoamer
EDIT: OR....what about sleeving a second smaller-diameter spring **inside** the helper spring....? Having to compress two springs together seems like it would certainly increase the total compressed force energy available....?
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This would certainly work....just add the two spring rates together...again that little plate holding the spring in is the weak link. Coincidentally, I've been playing with beefing up that hold-down arrangement on my van very recently.
Quote:
Originally Posted by maxx
On another note, has anyone ever replaced the main or helper springs? Our SMB is 20years old, I wonder if the springs wear out, will it make a difference replacing them with new springs.
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Properly designed springs may take a little set after a some use, but don't wear out/degrade much over time. They are designed operate in their "elastic" deformation region not "plastic" deformation region all the way to being fully compressed.
If they did, there would be a huge business of putting new springs in high mileage autos, or off road motorcycles. *of course lots of people here need springs but it's typically from adding a few thousand pounds of weight to their vans.