SRAM Red group with Shimano Cassette?
I just finished building my new bike and I'm using full Red except the cassette. I'm currently using a used DA7800 cassette that my buddy gave me. Turns out the 4 large cogs are worn out because it's not grabbing when I use it. I decided to buy an ultegra 6700 to replace it and had my mechanic replace it. He then told me that it's not shifting to the largest cog because the gear ratio of SRAM and Shimano is a bit different. Ive read on the forum that people uses the same setup with no problem. He suggested me that I better get the SRAM cassette and just return the ultegra cassette to avoid the trouble.
My setup is 53/39 and 12-25 with brand new KMC chain. When he is shifting it on 53 and 25, thats when it's skipping and just goes back to 23. I'm thinking could it be because of the cross chain? Or I should be able to use all the gear regardless of the setup?
My mechanic didnt charged me after doing all of this btw Would like to hear some input.