I've been considering the same thing. My assumption is that I'd have to split the difference between what was stock on a VTX (32 degrees=trail 6.8) and what was desired on the trike (6 degrees). I figured a 4 degree rake (trail 5") would be safe enough on the bike and would lighten the steering on the trike.
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I've read that a trail over 4" is usually considered safe on a bike.
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Here's the RB Racing Rake and Trail Calculator if you want to run your own figures:
RB Racing Rake and Trail Calculator
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Have you run across anyone else who's put a Voyager on a VTX?