Lionheart

I’ve got a Shimano 105 drivetrain that is 9 speed with a 12-27 cassette. I want to swap out this cassette for one with more teeth. Would a Deore XT CS-M770 11-34 cassette be compatible with my hub? (I think it’s made for mountain biking.)
Thanks in advance

alcjphil

It is more about whether your derailleur can handle a 34 tooth large cog. I doubt that it can. You will probably have to purchase a 9 speed mountain bike rear derailleur to accomplish this feat

bldegle2

Rear derailleur long cage or short cage presently?

alcjphil

Doesn't really matter. Long or short cage, 105 9 speed rear derailleurs were not designed for much more than a 28 tooth large cog. 34 teeth is going to be too much

bldegle2

Thanks for the update on the 105. I did some further checking with shimano specs, 30 tops maybe with b stop cranked...

3alarmer 

.
...is there some specific reason you can't accomplish lower gearing by getting a smaller chainring (or rings) ? It's usually cheaper and way less work than switching both rear derailleur and cogset.

mitchmellow62

The 11-34 cassette will fit your hub, but the responses of the rest are mostly true. I don't agree that changing to smaller chainrings will be cheaper or easier. If your 105 crankset is a double it is likely 39-52 or 53. It has a 130 mm bcd and the smallest ring that will fit is a 38. That does almost nothing to lower your gearing, which I assume is your goal. Also, the combination of 52 or 53 ring with an 11 tooth cog produces a gear that I would never use because it is insanely high. With a 39 chainring and the current 27 large cog your lowest gear is 38.4 gear inches with 700c x 28 mm tires(gear inches are a tool for comparison; the number is equivalent to directly pedaling a wheel of that size, as on an old high wheel bicycle). With the new cassette that you are considering, the lowest gear (39/34) would be 30.5 gear inches with a 700c x 28 mm tire. That's close to a 20% reduction. There were also 11-30 and 11-32 9 speed cassettes. A 39/30 combination would result in 34.6 gear inches and about a 10% reduction.

In your situation I would:
1. Install the 11-34 wide range cassette.
2. Look for a decent used long cage derailleur with enough capacity or install a derailleur extender with the current derailleur (like this: https://www.wolftoothcomponents.com/products/roadlink )
3. Replace the large chainring with a 48 tooth ring. Your current assumed situation produces a 52/12 or 53/12 top gear. That's 115.3 or 117.5 gear inches with 28 mm tires. A 48/11 will produce a top gear of 116.5 gear inches. In addition, the new combination will give you evenly spaced gears with no overlaps.

As you can probably tell, studying gearing can become addictive for the anal retentive, lol.

Lionheart

Thanks to every body for their insight! I’m repurposing a Specialized Tri Cross Comp. I was trying to get it to where I can spin up a rather lengthy and steep hill, which would allow me to commute.

I was already planning to swap out my small ring at the same time I changed the cassette. It has a 48/38 FSA crank, 110 BCD. I want to swap out the small ring to get it to a 48/34. It looks doable from what I can see for only the cost of the ring, maybe $40. I will start here.

For the cassette I wanted to change from my current 12/27 to the 11/34. I was only thinking about the splines, like if this cassette would fit the hub, and I forgot to take into account the contact with the rear derailleur. The RD is a short cage like on a road bike, its a Shimano 105 9spd.

So you think if I choose to change the cassette (About $50), then I also must either: change to a long cage RD, maybe $40-80, or try the wolf tooth RoadLink, for about $30 and keep my existing RD.

Since I also didn’t want to spend a bunch on this I might start with the small ring and see how much it helps.

3alarmer 

...that's what I would do. If that's enough, then you're done. If it's not, you can start on the other stuff.

ChrisAlbertson

It might be simpler to buy a smaller chain wheel.

You can test your current derailer. Shift to the 27T cog then pull the chain up by 7 links and see if the derailer has enough range to allow for 7 more teeth in the system. If not going downsizing the chain wheel might be cheaper. It all depends on what you have.

BTW, any derailer that can handle 9 speeds or more can work. So a new long-cadge 11-speed 105 could work. They are conservatively spec'd. If it says it can do 30T, it can actually do a couple more.