This isn't $100k a year. Try, $10k at MOST.

You greatly underestimate how much it costs to race karts. $10,000 per year is a reasonable sum for racing at the lower level cadet kart classes, but once Lewis started racing at the highest levels and in all of the major European championships, the costs were easily approaching $100,000 or more per year, but I do believe McLaren financed much of his final years of karting.

By all measures Lewis Hamilton came from a Lower Middle class family, whose dad had to work 3 jobs at once to support his son, that is modest.

According to Wikipedia.

His father told him that he would support his racing career as long as he worked hard at school. Supporting his son became problematic, which caused him to take redundancy from his position as an IT Manager and become a contractor. He was sometimes employed in up to three jobs at a time, while still managing to find enough time to attend all Hamilton's races. He later set up his own computer company as well as working as a full-time manager for Hamilton.

His father was far from wealthy, but he was nowhere near being "lower middle class."