I bought new Bontrager shoes and a Shimano SPD pedals, plus cloths. I did't changed the saddle yet. It good within 30-40km, but after... I have an old Raleigh bike also and I changed the original saddle on that bike with a new one (ergonomic one). No change. I think the problem is with me

Is there any huge differences between my system and the 105 or Ultegra? I read a lot about those systems and about mine, but I have no experiences. I know that the 105 and Ultegra systems are better than my, but how much? I read that the 10 speed Tiagra is nearly as good as the 105. I'm not a racer, I bike only for my fun so I could feel something difference If I use an Ultegra 6700 shifter? Or a 6800 brake?