Lancezl1

Has anyone had problems with tire balance on their IS? Ever since I went to an aftermarket rim there's been tire shack between 45 & 55 mph. It's happened with 3 sets of rims; 2 with the stock tires and 1 with 18" rims with 235-40-18 tires. The tire shops can seem to balance them. My IS is a 2003 with 2000 miles. This has been going on since about 700 miles. It didn't do it with the stock rims. Does anyone know if there is a trick to balancing the tires? Thanks.