The general difference with a bypass shock is that the valving becomes position dependent rather than velocity dependent. This means you can have a very 'open' feeling suspension where it is free to absorb more of the minor inflections and 'chop' on dirt roads with more free bleed and lighter initial valving ( it feels similar to the difference when you air down the tires taking the 'edge' off everything ), but adds the capability for the suspension to greatly resist bottoming ( and top outs ) and 'catch' itself better, especially during low shaft speed events like G-outs. This can make the suspension feel much more progressive without having to rely on step up spring rates and/or a lot of bump-stop spring rate without the need for all the additional control over that stored energy on the rebound side with rebound valving changes.
That said, there is a LOT that a good tuner can do with a quality shock that can be custom valved. I'd recommend reaching out to a company like Accutune Offroad if you are going to be spending money for 'good' shocks as they will include their custom valving for 'free' if you purchase shocks from them. Even their first guess remote shock tuning on the Fox 2.0 Performance shocks worked so well for my #LX45 project, it's hard to describe to people but everyone that rides in that vehicle has been VERY impressed vs the investment.
AccuTune Off-Road specializes in pre-tuned shocks and springs from Eibach, Fox, King, PAC & Sway-A-Way. Check out our shock tuning or spring calculations today!
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Beyond that, once you get into shocks that can be rebuilt and valved, an in person tuning session that focuses on your vehicle over a specific type of terrain is the next step.
What most people think of as being a 'really good' performing suspension off-road is just scratching the surface generally speaking.