I have a 91 sidekick and I put back seats in it so now the rear end is heavier and wants to bottom out. it has a 4 inch lift on 30s I thought about getting spring helpers or variable rate springs or a polyurethane brace that slips over the spring coil. This is my first sidekick so some guidance would be appreciated. Thanks

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Really need more info to diagnose.. They came with seats in the back from the factory, and the seat is super light.. I couldn't imagine that causing it to bottom out. Do you know what brand of lift is on it, and is it all suspension, or body lift as well?

Either way, something is wrong if just adding a 20lb seat is causing you to bottom out... The amount of fuel in your tank at any given time varies more than 20lb's.

While I agree with Ben, did you put longer shocks on the rear?  Longer shocks will bottom out earlier than shorter ones, and that will shorten their life.