Tighten the clamp!!
seriously, is there a small tab or something brazed to the bottom of the tube to help hold them in place? you might try a small shim to help it grip better
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk