I built them for my kids so until they break something no upgrades - but don't tell them that
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To be honest the car seems extremely rugged. The first thing I'd expect anyone to want to change is the shocks, but so long as you fill them carefully the kit ones work well enough. I expect others have installed proper turn-buckles too. But unless you're heavily in to racing, with just the bearing set the Monkey is great out of the box.