Air shock to coil - what spring?- Mtbr.com
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  1. #1
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    Air shock to coil - what spring?

    Hi all.

    I'm planning to dump the old fox RP23 and get a DB Coil shock. Is there anyway to convert the fox shock PSI I've been running to come up with an equivalent spring rate for the DB Coil? I would prefer to not have to do trial and error and buy 3 different springs.

    Current set up: Ventana el conquistador, FOx RP23, 200x50, small air can, 285 PSI.
    The Ventana currently has 2.5:1 rockers yielding 5" travel from 2" shock stroke, if that makes a difference. May change back to 2:1 and 4" travel...

    Thanks in advance.
    --Reamer

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  3. #3
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    Air shock to coil - what spring?

    Thanks tor the links. The difficulty comes in determining the correct rider weight to input as the bike is the tandem. Which is why I was trying to work backwards from PSI. I think I'll weigh each end of the bike (with riders in place) and go from there. Fox' link seems to take Total weight and weight bias into account.

    Also, an email to Ventana might be in order.
    --Reamer

  4. #4
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    Getting the correct spring rate is trial & error. I spent countless hours trying to determine the correct rate only to end up with, on the average, 2-3 springs. If you are lucky a Manitou spring (less $) will have the correct rate & free length.

  5. #5
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    Air shock to coil - what spring?

    Thanks, Keen.

    I know there will be some trial and error, but an educated starting place is, well, a good place to start. So I went above and beyond and emailed Sherwood at Ventana. I got a reply within an hour...

    "I use the sum of approximately 82% of Stoker and 39% Captain weights (based on rider position and frame geometry in a spreadsheet) for determining a starting rear spring rate. Based on your numbers, you will need a 650 pound spring for 33% sag and/or a 750 pound spring for 28% sag."

    Was his response. So, it's a start! Tandems are weird beasts, and who better than the creator for some guidance!
    --Reamer

  6. #6
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    Another way to do it: Find the area of the piston head on your shock in inches (maybe email Fox for that). Multiply that by the psi in your shock. That gives you the starting force of an equivalent spring.

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    Air shock to coil - what spring?

    Quote Originally Posted by LarryG View Post
    Another way to do it: Find the area of the piston head on your shock in inches (maybe email Fox for that). Multiply that by the psi in your shock. That gives you the starting force of an equivalent spring.
    Interesting! While I've got a recommended spring rate from the frame manufacturer, I just might give this a try to see what I get.

    Thanks Larry.
    --Reamer

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