Bicycle Mechanics - Tiagra & 30 tooth cog

According to Sheldon Brown (http://sheldonbrown.com/harris/derailers-rear.html), most medium range road derailleurs will handle up to a 30 tooth cog on the cassette.

I've decided that my 53 year old knees need lower gears on the occasional 15+% grades I run into so I thought I would convert my cassette (shimano) from 12-13-14-15-17-19-21-23-25 to 12-13-14-15-17-19-21-25-30 giving me a 20% lower bailout gear on the bottom end while keeping the close ratios in the gears I use most. (Yes, this is with a 52-42-30 triple in the front:o ) I found a 30 tooth cog at the local bike co-op and had no trouble tweaking the cassette.

Unfortunately, the tiagra rear derailleur drags on the 30 tooth cog. I've tightened the "B" screw in all the way and even pried the "B" screw flange back an additional 3/8 inch or so, but the derailleur doesn't raise enough. It will raise free by hand, but not by any reasonable use of the "B" screw even if I put a much longer one in.

I've also put a new chain on and adjusted the length to fit around big to big (bypassing derailleur) plus a full link and it didn't help.

Is it time to give up and buy an XT derailleur or is there something else to try?

One trick that you can try is to remove your "B" screw and reinstall it going the other way. That'll give you the additional length of the screw head.

If it was my bike, I'd just stick a mountain bike derailleur on it. If you want to be frugal, a decent used one shouldn't be very hard to find.

One trick that you can try is to remove your "B" screw and reinstall it going the other way. That'll give you the additional length of the screw head.



Or go to the hardware store and get a longer screw with the same thread. I've replaced 'B' screws on a number of Shimano RDs this way to accomodate the 13-30 cassettes on two of my bikes.
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I'd vote for just sticking an mtb derailer on there. As Retro Grouch said, not too expensive.

I'm only 28 but have a 12-25 cassette with a 48/36/26 triple crank on my main road bike. The low gear proves useful in some cases, making otherwise uncomfortable climbs comfortable.

Just to close the loop, I tried dropping down to a 28 tooth cog and putting in a longer hex head "B" screw. It still drags a little. As the new longer "B" screw is screwed in, the flange it's threaded into rotates causing the end of the screw to walk out past the end of the derailleur hanger ledge. Putting the screw in backwards helps a little, but not much.

I guess it's time to buy a MTB derailleur.

Or go to the hardware store and get a longer screw with the same thread. I've replaced 'B' screws on a number of Shimano RDs this way to accomodate the 13-30 cassettes on two of my bikes.
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+1 with an Ultegra RD (same geometry as the Tiagra).