A NOTTINGHAM cyclist sharing his security secrets with the rest of his pressure group advocates this: insert a nail in the chain closest to the bottom arch of the chain wheel. This gives the thief enough leverage for just one revolution of thrust before he or she and the bike part company.

Booby traps, disengaging brakes and other designed-to-deter guerrilla techniques are coming out of the closet as cyclists grow increasingly frustrated with the apparently insoluble problem of bike theft.

Even the most expensive, sophisticated U-locks are eventually beaten. One of the latest is a pounds 45 Kryptonite steel alloy which claims to withstand assaults by thieves using 122cm bolt croppers and hacksaws, while thwarting leverage attempts. But Liz Marriott, of the London Cycling Campaign, says there seems to be no lock which cannot ultimately be overcome.

The 7,000 members of the LCC are among the most vulnerable. London is the worst place in the country to own a bike. While national figures for theft doubled between 1988 and 1991 - and are still rising - London emerged as the black hole. Even cycling university cities like Oxford and Cambridge have nothing like the same scale of theft. Insuring a pounds 400 mountain bike in London through the LCC now costs pounds 80 a year.

Mountain bikes attract a pounds 15 additional premium, being at the top of the thieves' preferred list, but outside London it is still possible to insure a cycle worth up to pounds 150 for pounds 15 a year.

The magazine New Cyclist reviews practically every new lock on the market - among the latest is a 3ft 6in flexible link designed for the US motorcycle market, weighing just under 3lb and costing pounds 70 - but its columnist, Richard Ballantine, has had enough. When he bought a 'new' cycle for his daughter he made sure it was truly undesirable, from its old- fashioned second-hand frame to its vanilla colour. Disguising a desirable machine is now as effective as locking it. One courier with Actus Couriers in Bristol has painted a very classy pounds 1,000 mountain bike matt black, removed all its steal-me-why-not stickers and concealed its appeal.

The London Cycling Campaign advises an insurance policy and a good U-lock. 'If you make cycle security so complicated people get fed up and just get on a bus,' says Ms Marriott.