There are few things that test matrimonial relations more than the unauthorized acquisition of extra motorcycles. Women have special motorcycle detection sensors, and the mere arrival of unknown pickup truck can trigger an unannounced SWAT-style raid on the garage.
Attempting to deceive your partner is doomed to unpleasant and possible life-changing failure. Especially if the motorcycle purchase consumes the housekeeping or holiday budget and distracts you from essential DIY/in-law visiting duties.
However, don't despair. Diplomacy does work and if the nuclear superpowers can disarm then it's also possible for permission to be granted for the purchase of a new motorcycle.
If you want to buy more motorcycles in defiance of a 'No more motorcycles' directive you need to... a) have the directive rescinded, and b) ensure that you don't give your partner any reason to regret that decision. Or c) wonder whether she hasn't got a point. Crucially, getting the 'No more motorcycles' directive lifted has to be viewed as a longterm strategy, not something you can achieve by this Friday.
Hiding a 'secret' motorcycle, or lending it to a friend to look after may be tempting, but it will either cause the worst of all battles when your partner does find out, or it's so well hidden that you never see the motorcycle anyway.