Bicycles are remarkable machines. They can be as reliable as a good car. And the most efficient transportation. But they’re built light with their parts exposed.
Before your hop on a bike and ride off, give it a quick look over.
Are the wheels in straight? Are they true? Do they not wobble to-and-fro? Are the spokes ok? Are the wheels firmly attached to the bicycle? Are the axle nuts tight? Or are the quick release levers closed correctly? The final tightening comes with a swing of the lever, not a turn of the screw. They should begin to get tight with the lever straight out from the bike, or a bit closer. And take the force of your palm to close.
Do the tires have anything sticking in them? Better to find out now than the flat later. Do the tires have enough air?
Do the wheels spin freely? Is there enough room between the brake pads? Do they rub? Or is there too much room between the pads? Are they worn down? Do the brakes spring open when you release the levers? Do the brakes not wobble?
Do the brake levers work? Are they at least an inch from the handlebar when the pads engage? Do the brakes lock? Best if the brake levers cannot touch the bar.
Do the brake cables move freely? Do they have any damage? Does the cable housing have any damage? Or do the hoses have any damage or leaks?
Is everything firmly attached to the bicycle?
The wheels must be properly mounted to the bicycle frame. Misalignment can result in problems with shifting and bike handling. If the wheel is not securely mounted in the dropouts, it may come out when the bike is ridden, possibly causing injury to the rider. — Wheel Removal and Installation from Park Tool