The outward “push” experienced by the riders is m⍵²r and the gravitational pull is mg where m is the mass of the riders. Coefficient of friction µ acts at right angles to the push and the frictional force is µm⍵²r opposing mg. If a rider is to remain pinned to the steel wall µm⍵²r>mg so µ⍵²r>g and µ>g/(⍵²r). So ⍵²>g/(µr).
The minimum value of µ is 0.4 and the maximum is 1, covering kinetic and static friction. g=9.8㎨ approx, r=2.5m. Therefore ⍵²>9.8/(0.4×2.5), ⍵²>9.8 and ⍵>3.13 revs per sec or 187.8 rpm.