I've only seen the spring loaded idler on automobiles, but if it helps it seems that after the initial wear in that will happen very quickly if at all, mine hasn't needed any adjustment, from what I can tell it seems this is due to no wear occurring. It seems a Serpentine belt will last a very, very long time. I have a couple of years and a couple hundred hours on mine and have not adjusted it.
You can also run it looser without any slippage than you can with a V belt, I didn't like how tight I had to keep a V belt with just my stock 80 Amp Alt. I run my serpentine a little looser with a 125 Amp and no slippage.
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