I second the Specialized Armadillo. The 23cc weighs 378 grams and it rides pretty stiff, but it's almost rocket-proof. I put a pair on Shannon's old bike in combination with some lightweight Velomax wheels to reduce the weight impact. I don't recall her getting a flat in a year or so of riding. I used a pair myself for about 6 months with no flats. I think they're a good choice for training.