I have replaced my front tube now about 5 times and I am completely baffled as to what is causing this. Excel rim, 21" rim strip, Maxxis IT front tire, Trackside heavy duty rim lock (new), tube inflated to 12.5. I ride strictly MX. This is a fresh tube properly lined up prior to a 2-3 hour moto session. The wheel was excessively cleaned prior to installation. I am wondering if maybe the notches that the tire bead sits on needs to be roughed up or hit with a dremel. The rim itself is from an '08 bike. The previous tire, a M403 never slipped. The bead is good on the Maxxis tire.