I have to see it again, but I thought Harper hit a bump, lost his back wheel (into the air), thus lost control going to the outside of the curve and then hit the curb / crash hard on his right (hard enough to hurt / break something in his shoulder / collarbone) and hit his head hard but hopefully the helmet took that hit, and next slid down until his head touched the lamp post at relatively low speed. So worst case a concussion + broken bones (+ for sure skin peeled off), best case no concussion and no broken bones and just skin lost.