They're rubbish, don't buy one. Get instead a thing to screw on the valve and use the car pumps, You'll never be more than 5km away from a garage and to get there you can use one of those aerosols with goo. Then there is the CO2 things which although alarming and one use haven't let me down. Or a proper pump under the top tube. And bring money and a mobile. At least a couple of times a year I need to call someone for help. When all else fails.
Mini pumps are the devils handy work.
I got a puncture today.