Bicycle Mechanics - cassette/shifter compatibility question

Hi everyone, so I was in the garage with my sis' bf talking bike stuff, and a question came up regarding his gearing setup.

He currently has a 105 rear derailer with tiagra shifters... His current cassette is a 9 speed. He was wondering if he was able to throw on a 10 speed cassette and still have the shifters be able to reach the 10th cog. He wants to be able to drop into a lower gear for climbing, and if anything just have the options for another cog setting.

I also mentioned to him that he would be able to find another 9 speed cassette with a greater tooth range, so I basically have the answer, but for the sake of another opinion we were just wondering if the tiagra shifters would work. Thanks in advance

I do n ot think so. if the brifter is designed for 9spd it willl not shift on 10spd. as for the cassette he should be able to find a wider range what does he have now?

His current cassette is a 9 speed. He was wondering if he was able to throw on a 10 speed cassette and still have the shifters be able to reach the 10th cog.
No. Shifter "speed" must match # cogs. Not only does the 10-speed have more cogs, they are closer together so it wouldn't shift right.

But we're not sure that you're right about a different 9-speed cassette. There are limits to how big you can go. To go really big he might need a MTB derailleur. Or a different crankset.

Unfortunately a 9 speed shifter will only work with a 9 speed cassette. You sisters bf will need to upgrade the shifter, cassette and chain.

Thanks for the replies! I was pretty sure that the speeds depended on the shifters.. right now I believe he has a 12-21.. could be waay off but im sure that he doesn't have a 25. I understand the long and short cage derailer issue, I had to switch to long cage on an old road setup I had to get it to shift to a mega drive cassette. I'll find out more info... I'm glad we have it narrowed down to the number of speeds available for the cassette... In my opinion I think that if he finds a 25t cassette he will be just fine.. he's running a compact crank up front. Thanks again for the info!

You may be OK with a compact crank, 12/25 cassette, and short cage derailleur. You will need one more link in the chain, so just get a new chain for the new cassette.

Get a 12-27. The der. will shift to a 30 without a problem. You will need a long cage if you go to a 34 or put a triple on the front.

You will need a long cage if you go to a 34 or put a triple on the front.
This is kindof correct. The triple and 34T are separate issues as one deals with chainwrap capacity and the other deals with link geometry related to cog diameter. The triple will increase the chain wrap capacity requirement, thus requiring a long cage derailleur. The 34T requires a mountain bike derailleur that can actually travel to the 34T diameter. So you can end up with a short cage mountain derailleur (1x9 using 11-34 cassette) or a long cage road derailleur (triple with 11-21 cassette).