When asked how he could be leading bike rides well into his eighties, B.M. Shirar always replied, "I eat a banana every day." B.M. died nine years ago, but his bicycle shop -- the nation's oldest, according to B.M.'s grandson and the current proprietor, James Turner -- is still pedaling strong at 88. B.M. and his father opened Shirar's at Franklin and Main, in the Magnolia Brewery Building, with a horse-drawn wagonload of tools and spare parts brought over from Hempstead. Their first customers were a group of delivery boys. Today there's not much having to do with bikes that you can't get or find out about at Shirar's, which, come to think of it, is what you ought to be able to say about any bike shop worth its spokes. But there's something about a place that started selling bicycles before there were automobiles and whose founder was able to pass his love for bicycles down the line to a fourth generation. Maybe because it's always been family-owned, Shirar's understands and rewards loyalty: Shirar's gives first priority and $15 discounts on tune-up and overhaul services to bikes bought at the shop.