hi, i have a specialized sirrus hybrid with 700x32c wheels/tires...i CANNOT keep an inner tube inflated. i think my local bike shop is selling me the wrong innertube. ive got 2 of them in front of me. one says 700x25-28. it lasted for about a week and went flat. today they gave me a 700x18-23. it stayed inflated for about an hour. ive checked the wheel and tire multiple times for debris, theres nothing there. are these tubes blowing out because they are the wrong size? ive gone through 3 now.
also, i noticed today when i was changing the tube that the tires are directional. they had them installed backwards. ive put about 500 miles on them backwards. does that matter?
additional info: the front tube is still the original, so im not really prone to getting flats.