Bogie wheels. 20 mm axle hole and 150 mm diameter. These do not fit the ones with black wheels due to the axle hole diameter. For Classic models, Flex has narrower wheels 5310195-84-1
These are the wheels that are on the ones that have ball-bearing wheel axles (up to 2015). The hub is 60 mm long. The Flex models have a 50 mm hub length and they have number 5310195-84-1