Okay, I'm not gonna say tear apart your frame or anything, clearly you don't wanna do that.. But For the sake of a wooden frame.. I see NO wood joinery! As someone who's built a few tables/work benches, and his own bed frame that doesn't squeak [Kinky

] A simple hand saw and some practice pieces you could make something like Finger tenons..
hidden dovetail + wedge joinery. The "connecting pieces" at the front of your last posted picture, Those are no good for a chair to sit on, never less something moving.
I'm stubborn. Now I'm not gonna call you stubborn, but there is a better way to do your frame construction without the need of road rash, hell I say go for a wooden frame.. I thought about it with an electric cart, but the "Friend" didn't deliver on the power chair he promised.
But Please, PLEASE use some joinery.. and don't use bolts in your wood frame construction that are to hard, Use something softer, so the bolts will bend inside of the wood, not splinter the wood.. I would say B5, SAE Grade 5. They will bend before they break.
Also look at boat building for making wood framing stronger. Some of the better made boats are still made with wood ribs. [not large ships like freights, but personal house boats and even yachts] Look into it, might help with the construction of your kart. Trust me, its well worth the brain space!
Also for safety splintering issues. Some titebond and old white cotton t-shirts. Can make a strong enough "backing" as they call it in bow building. to keep the wood from splintering! and add strength, at a cheap price. Go to a thrift store and pickup 100% cotten t-shirts. Doesn't matter what color if you get a 2" brush, and a gallon of paint!
Pointless Story EDIT: I had an engineering after school class/thing back when I was in middle school, Built a crane out of knex [I had A LOT! of knex back then!] that could lift 400lbs. Then we had a contest to build a balsa wood frame 8 inches tall, and weigh like 3 ounces or something like that, Held 103LB's. another contest to build a cardboard boat under 1 LB that could carry 5LB's for 25 yards. I built all of these with great passion. I made 2 records, [the balsa wood frame, and the boat] the teachers were amazed at my skills and artwork I put into these things. The kid who always got in trouble in school, could actually do something amazing.. Sadly, I got a D on my next report card and got in trouble one to many times, and they kicked their best player out. Because they couldn't make exceptions... I was a stubborn kid, while I knew what I was doing, I didn't like people trying to force their ideas and "This is how you should do it" ways on me.
What I'm trying to get at here, is the idea may be GOOD in your head, but its BAD in reality.. So don't get kicked out of your own gokart.