have a look at the inner face of the chainstays, see if there are marks where the tyre has rubbed, I am sure it is the loose wheel nut allowing the wheel to move under load and the tyre then rubs on the frame, causing a lot of friction.
@adroit started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
have a look at the inner face of the chainstays, see if there are marks where the tyre has rubbed, I am sure it is the loose wheel nut allowing the wheel to move under load and the tyre then rubs on the frame, causing a lot of friction.