Tonight's audience are a diverse bunch - gentlemen for whom Saxondale is less satire than documentary predominate, but there are families, grizzled bikers and Mohican-sporting teenagers - yet they are momentarily united in disbelief and consternation when Motörhead perform an acoustic track. Admittedly, the acoustic track is called Whorehouse Blues, somehow manages to contain a widdly-woo guitar solo and, thanks to Lemmy's phlegmy bark, sounds exactly like Motörhead.
But in a fast-changing world, Motörhead's immutability is part of their appeal. Their sound is like a multi-storey car park - it is grey, hard, wilfully ugly and was cast in concrete in the 70s. And now this: acoustic guitars and Lemmy momentarily abandoning his beloved onstage persona - equal parts warty berserker of rock and shoe-in for the next series of Grumpy Old Men - to play a harmonica. A nearby Hells Angel looks like he's going to require CPR. What are Motörhead going to do next? Get a rapper in and go dubstep? No, they play Ace of Spades - grey, hard, wilfully ugly, undeniable in its grunting brilliance.
Alice Cooper is altogether less surprising. His grand-guignol shtick has not changed much since the early 70s, when it was the only form of glam rock acceptable to middle America. The show starts with him stabbing a tuxedo-clad dummy and gets progressively more idiotic from there on in. He plays it heavy on the hits, light on the latterday oeuvre, which seems just as well, given that the latterday oeuvre contains a song with the deeply regrettable title Woman of Mass Distraction. "Some girls wanna get you in bed," its chorus advises, "some girls want a kick in the head." Hang on: what girls actually want you to kick them in the head? Name one. The old ones, however, still work. If the passing years have denuded I'm Eighteen of its bovver-boy menace - it was the song with which Johnny Rotten auditioned for the Sex Pistols - its riff's clunking power has not been eroded by time, nor, for that matter, Cooper's confusing decision to sing it while waving a crutch about.
In any case, songs take second place to showbiz. Cooper is attacked by zombies, threatened by a gun-toting femme fatale and, at one point, scampers around as if trying to evade capture, while searchlights strafe the stage. What he is supposed to be running away from is unclear: a cynic might say his dignity, but it is hard to stay cynical in the face of such relentlessly stupid fun.
· At Cardiff Arena (029-2022 4488) on Friday. Then touring.
