Plasma Cars are awesome and strong enough for a (flexible-ish) adult to ride. But it relies on front wheel friction to move and the plastic wheels it ships with just aren't all that great. They slip, making it hard to get started, and all four wheels will leave small gouge marks on hardwood floors.
So upgrade the wheels to 84mm inline skate wheels. Upgrading the fronts is trivial. The rears take more work.
I used 84mm hard indoor inline skate wheels. Indoor wheels will degrade faster if used outdoors, but kids don't weigh as much as adults so I think they'll probably hold up just fine. A pack of 10 wheels was about $25 so you can have plenty of spares on hand if you need them.
And you don't need to buy bearings. The Plasma Car uses standard ABEC-7 inline skate wheel bearings. You just pop those out and of the plastic wheels and into your new inline skate wheels.
Step 1: Replace the Fronts
Just remove the one nut holding each front wheel. Pop out the bearings by placing a screwdriver just barely under the back edge of the top bearing and lever up. Insert the bearings into the inline skate wheel, reattach with the nut, and you're good to go.
This alone will be a huge upgrade and you can stop here if you like. But if you keep the plastic back wheels, you'll want to sand them down to remove the hard center line ridge; this is what will gouge your floors.
You might also want to consider the tennis ball mod seen in the photo. The baby front wheels aren't actually wheels--they don't spin. They're really just there to catch you if you turn too sharply and lean out of balance. But those baby front wheels will also mark and gouge the floor. So I removed them and replaced them with the tennis ball.
If you go ahead with replacing the rear wheels, you'll see how I've repurposed those baby wheels to finish off the rear upgrade.
Step 2: Gather Materials for a Rear Axle
A 5/16" threaded rod fits perfectly inside the Plasma Car's standard ABEC-7 wheel bearings. You'll need about 14" of rod. Expect to have to cut the rod down to size.
You'll need the rod plus nuts, lock washers, and fender washers.
You might need some regular small washers just to finish off the spacing at the very end.
I used a Dremel metal cutting wheel to trim the threaded rod.
Step 3: Preassemble and Mark Rod Length
The rear wheels just pull straight out if you push and pull them a bit from side-to-side.
Inline skate wheels are wider than the Plasma Car's rear wheels. So you won't be able to mount them in the wheel housing using the original brackets. That's where the axle design comes in. It has the added bonus of guaranteeing that your rear wheels are perfectly parallel.
With the rear wheels removed you're left with an open U-shaped channel where the mounting brackets had been. Your new rear axle will sit in this channel, but this isn't ideal since there's nothing keeping the axle in there. However, by using washers and bolts we can dial in enough pressure to do a good enough job of keeping the axle in that groove.
On both sides of each wheel housing add a fender washer, lock washer, and nut.
Leave enough room at the end of the rod to attach the baby front wheels and a nut. I used the baby wheels here to act as furniture and parent ankle protectors! I didn't want kids whizzing by with the metal rod or exposed nuts protruding from the outside of the Plasma Car. The baby front wheels are recessed enough to keep the end nut out of harm's way.
Once everything is spaced out you can mark the threaded rod where it needs to be cut down. The rod I got was 24" and I had to cut about 10" off of it.
Step 4: Cut the Rod and Final Tighten
First time I've ever tried to cut a metal rod. Pretty dang awesome! The cut edge will be a bit raw and sharp so take care.
With the rod cut, assemble everything and dial in the tension on either side of the wheel housing. It'll bow in. Just leave enough room so the wheel isn't squeezed and can still spin freely. Cap off the ends with the baby wheels. You may need a washer to push the nut out so that it fully covers the raw edge of the threaded rod.
And that's it! Go ride!