These Cars cars are junk. They break so easily. The axles are thin, flimsy pieces of metal, the paint jobs are terrible, and they drive smoothly for two weeks before they start stuttering and sliding.
For comparison, we have a few hand-me-down Hot Wheels (edit: Matchbox, not HW) cars made in England in the 70s that have been abused on-and-off for 20 years, and they still roll like they were just made. Newer ones aren't as sturdy or as heavy as those 50-year-old ones, but they're still much more nicely built than the Cars ones.
All I can say is: thank goodness the Cars phase is over.